An understanding of mitochondria and its role in targeting nanocarriers for diagnosis and treatment of cancer.

Introduction

Cancer is a group of disease in which abnormal cells grow and divide in a rapid way without any control. In addition, these abnormal cells are also responsible for destruction of normal body tissues [1] . According to the prevailing theory of cancer, the normal cells convert into the cancerous cells, which eventually lead to abnormal cellular, molecular and biochemical networking of normal cells. These cancerous cells also spread or metastasized from a primary location to a secondary location in the body [2] . There are some common risk factors also which promotes unwanted growth of cells like tobacco-alcohol consumption, being overweight, exposure of chemicals and other substances, poor diet, etc. Aging and family history also act as a risk factor of cancer [3] . Till date, majority of cancer patients are being treated by the surgery, radiation therapy and chemotherapy. Generally, cancer tissue is removed by the surgery and then radiation therapy is given followed by chemotherapy. But these treatment strategies possess potential drawback due to their non-selectivity, which produces harmful effects on normal tissues too. Hence, to overcome these serious drawbacks there is need of novel targeting treatment approaches [1] .

Mitochondria are the powerhouse of cells responsible for the generation of adenosine triphosphates (ATPs), which is prerequisite for the proliferation as well as other proper cellular function. Mitochondria are comprised of different types of protein that have crucial role in the induction as well as prevention of cell death. Therefore, mitochondria play a critical role in determining the fate of a cell [4] . Also, for the growth of cancer cells, functional mitochondria are the most important component because it produces the Warburg effects or aerobic glycolysis which helps in mitochondrial dysfunction that produce a favorable environment for the cancerous cells. This effect also confirms unbalanced mitochondrial metabolism due to the mitochondrial DNA (mtDNA) mutation, creates an adaptable environment for cancer cells and increases tumorigenesis. This Mitochondrial metabolism dysfunction enhances the production of mitochondrial reactive oxygen species (ROS) and alters the cellular redox activities. Thus, by changing the functions of transcriptional factors such as HIFα and FOS-JUN, they change the gene expression and stimulate cancer cell proliferation [5] .

Since past few years, the use of different types of nanocarriers for tumor targeting has emerged as a newer approach for cancer treatment. Nanocarriers possess larger surface area with an unique electronic, mechanical, photonic and magnetic properties due to their nanosize range. To overcome the limitation of the conventional drug delivery system, specialised nanocarriers have been designed to achieve targeted cancer therapy by imparting unique properties [1] . Cancer cell are able to create multidrug resistance which can be successfully overcomed by using the multifunctional nanocarriers as a targeted drug delivery carrier [6] . Nanoparticles (NPs) drug delivery has been used to reduce the side effect and improve pharmacokinetics, and have longer circulation of half life.

Role of mitochondria in tumorigenesis

Mitochondria play an important role in tumorigenesis which was observed by Otto Heinrich Warburg in the year 1924. He found that there was a difference in the way that glucose was metabolised by cancer cells as compared to normal cells. In case of normal cells, in presence of oxygen, the glucose is metabolized by glycolysis in cytoplasm to form pyruvate followed by its oxidation in mitochondria via tricarboxylic acid cycle (TCA). This metabolism leads to the production of carbon dioxide, NADH [reduced form of nicotinamide adenine dinucleotide (NAD + ), which is used as a fuel for ATP production by oxidative phosphorylation (OXPHOS)] and minimum amount of lactate. In anaerobic conditions, glucose is metabolized via glycolysis into pyruvate, due to absence of oxygen pyruvate further cannot be oxidized, so it converted into lactate. This results in the production of a large amount of lactate inside the cell. In case of cancer cells, irrespective of oxygen availability most of the cells metabolize glucose by glycolysis and leads to overproduction of lactate, this is called as “aerobic glycolysis”. Apart from this, the intermediates produced in glycolytic pathway act as a raw material for the biosynthesis of proteins, lipids and nucleotides, which further helps in the proliferation of cancer cell [7] . It is observed that mitochondria play a major role in maintaining microenvironment in tumor cells more acidic than normal cells, which is very important for cell proliferation. The main reason for this acidic environment is the enhanced metabolism of glucose into lactic acid via aerobic glycolysis. This lactic acid releases H + ions inside the cell and then these H + ions diffuse outside the cell and lead to a decrease in pH of the microenvironment [8] .

Targeting mitochondria via nanocarriers is an appropriate approach for cancer treatment. These nanocarriers targets mitochondria by using different fate and mechanisms which includes macropinocytosis, caveolae-mediated endocytosis, phagocytosis, clathrin-mediated endocytosis, membrane fusion, receptor and protein-mediated, mitochondrial transmembrane-potential mediated accumulation, glutathione (GSH)-triggered triphenylphosphine (TPP) exposure, ROS-accelerated cellular internalization, hydrophobic-peptide-mediated [9] , etc. Yamada et al. prepared a liposome-based carrier for the delivery of macromolecular cargo into an internal part of mitochondria via membrane fusion. They identified a liposome-based carrier which endorses its fusion with the mitochondrial membrane as well as releases the cargo into an internal part of the mitochondrial membrane [10] . In one such study, Chan et al. demonstrated dual ligand functionalized nanodiamonds (ND) for the treatment of cancer which circumvents the drug resistance. They reported that multifunctional NDs not only successfully distinguished normal cells and cancer cells but could also reach to mitochondria via a receptor and protein mediated targeting mechanism and successfully deliver the drug to cancer cell plasma membrane [11] . In another study by Zhou et al., they have synthesized paclitaxel liposomes conjugated with D-α-tocopheryl poly-ethylene glycol 1000 succinate-triphenylphosphine as mitochondrial targeting molecule to treat the multidrug resistant lung cancer via mitochondrial transmembrane-potential mediated accumulation [12] . Mkandawire et al. prepared conjugated gold nanoparticle with 3rd generation dendrimers for the induction of apoptosis in human cancer cells by targeting mitochondria via a specialised form of phagocytosis and caveolae-mediated endocytosis [13] . Xiang et al. developed resveratrol-loaded dual function titanium disulphide nanosheet which targeted and accumulated in mitochondria for photothermal triggered tumor chemotherapy via endocytosis [14] .

Barriers in targeting mitochondria for cancer treatment

Targeting to mitochondria directly is very difficult due to its complex structure and many cellular barriers. Mitochondria possesess complex structure which includes mitochondrial outer membrane (MOM), mitochondrial inner membrane (MIM), intermembrane space and matrix. The crossing of MOM barrier is fully dependent upon the concentration, thus molecule can pass through passive diffusion [15] . The main barrier for the targeting mitochondria for treatment is,deficiency to reach therapeutics in the mitochondrial matrix owing to lack of their structural components. The major hurdles for the small molecule crossing through membranes are strong electronegative potential, which is −160 to–180 mV and presence of unusual phospholipid cardiolipin that exist across the membrane [16] . Apart from MOM and MIM, cytomembrane and cytoplasm are the main obstacles for targeting mitochondria. Cytoplasmic aqueous phase packed with vast macromolecular species (DNA, RNA, etc.), ions and small molecules and highly viscous cytosol retard the diffusion of targeting molecule to diffuse inside cells [17] .

Mitochondrial targets for cancer therapy

There are various functions that a mitochondrion performs in association with channels, enzymes and proteins, energy production in the form of ATP, destruction of ROS, proper glucose metabolism, etc. In Fig. 1 , different mitochondrial targets along with the drugs reported to bind with these targets are shown. All these functions are important for the cell survival and growth ( Fig. 2 ). In cancer therapy, targeting these channels, enzymes and proteins associated with mitochondria can lead to destruction of cancer cells. Fig. 1 Different mitochondrial targets along with the drug for their targeting. Fig 1 Fig. 2 Overall summary of role of mitochondrial factors in cancer progression and cancer inhibition. Fig 2

Targeting HKIII

Cancer cells largely depend upon aerobic glycolysis for their energy production in the form of ATP. Hexokinase (HK) is an enzyme that is responsible for catalyzing the first step of glycolysis, i.e. from glucose to glucose-6-phosphate (G6P). There are four isoforms of hexokinase, namely HKI, HKII, HKIII and HKIV. Out of these four isoforms, HKII is highly overexpressed in the cancer cells [18] . HKII binds to voltage-dependent anion channel 1 (VDAC1) present on the MOM, which facilitates the interaction of VDAC1 with adenine nucleotide translocase on the MIM ( Fig. 3 ). This interaction favours the coupling of aerobic glycolysis with OXPHOS, which creates a working relationship between the two metabolic processes. This increases the rate of glycolysis by allowing HKII to exchange ADP for ATP from the mitochondria [19] . Hence, targeting HKII could lead to stopping of aerobic glycolysis and further resulting in cancer cell death. Fig. 3 Regulation of apoptosis by HKII and VDAC. Right: During unfavorable conditions that exists in tumor microenvironment, caspase-mediated induction of apoptosis would be enabled by the activation of the MPTP because of release of HKII from VDAC. This pore complex is responsible for the transfer of caspase activator cytochrome c (located within the inter-membrane space). Bcl-2-related proteins (Bax and Bak) would likely overcome effects of the MPTP inhibiting protein Bcl-X L and helps in the release of cytochrome c. Left: On conjugation of HKII with VDAC, opening of the MPTP is inhibited. This causes inhibition of conjugation of VDACs to Bax and Bak, and the cytochrome c is then retained inside the mitochondria. Thus, HK II helps guarantee a highly malignant tumor's proliferation, and its bypass from cell death, under conditions that would otherwise favor this process. Fig 3

There are some examples of glucose analogs that used to target HKII, namely 2-deoxy-D-glucose (2-DG) and 3-Bromopyruvate (3-BPA) [20] . 2-DG competes with glucose for its entry into the cell, and once gets entered into the cell, it undergoes phosphorylation by HKII to 2-deoxyglucose-phosphate (2-DG-P) which cannot be used in further steps of glycolysis. Hence, 2-DG-P gets accumulated inside the cells, resulting in product inhibition of HKII [21] . The binding of HKII to mitochondria is important to prevent the initiation of apoptosis. 3-BPA acts as an alkylating agent and it covalently modifies sulphydryl groups of HKII. This modification leads to dissociation of HKII from mitochondria which trigger apoptosis and further cell death [20] . In a study carried out by Bao et al. showed that out of 13 steroidal derivative isolated from the mushrooms of Ganoderma sinense , (22E,24R)−6β-methoxyergosta-7,9(11),22-triene-3β,5α-diol was assumed to have a high HKII binding ability and was validated by in vitro microscale thermophoresis, enzyme inhibition and cell-based assays. It was the first natural compound that is reported to possess HKII inhibitory activity for cancer treatment [22] . Methyl jasmonate, an anticancer agent had shown a good binding affinity for HKII. It binds to HKII and detaches it from VDAC1, leading to mitochondrial permeability transition pore (mPTP) formation, through which cytochrome c comes out from mitochondria into cytosol. In the cytosol, cytochrome c triggers caspase activation and cell destruction [23] .

Targeting ion channels

Ion channels are very helpful for any cell as they involve in the exchange of ions, fatty acids, cholesterol, etc., and ultimately maintain the homeostasis of the cell. These ion channels are broadly classified into two classes, namely channels present on MOM and channels present on MIM.

Ion channels of mitochondrial outer membrane

There are various ion channels that are present on MOM which are responsible for the entry of all the nutrients as well as the exit of many apoptosis mediators. Ion channels present on MOM are translocase of the outer membrane that is responsible for protein transport, mitochondrial apoptosis-induced channel that facilitates cytochrome c release and VDAC that transfers metabolite. These three channels help in the formation of mPTP. This mPTP facilitates the release of cytochrome c that further leads to apoptosis [24] . From the above-mentioned ion channels of MOM, VDAC is mostly targeted for cancer treatment. VDACs are also called as mitochondrial porins. These channels present on the MOM help in the transport of ions (Cl − , K + , Na + , and Ca 2+ ), ATP/ADP and NAD + /NADH, fatty acids, cholesterol, ROS between inner mitochondrial membrane and cytosol through MOM [25] . There are three isoforms of VDAC that has been identified in mammals, namely VDAC1, VDAC2 and VDAC3 [26] . VDAC1 is abundantly present in the mammalian cell mitochondria and it plays a central role in mitochondrial apoptosis. It interacts with apoptosis regulatory proteins such as members of the B-cell lymphoma 2 (Bcl-2), Bcl-xL (anti-apoptotic proteins), apoptosis-suppressing glycolytic enzyme HK, etc. which inhibits the release of pro-apoptotic proteins [cytochrome c, apoptosis-inducing factor, second mitochondria–derived activator of caspases/direct IAP-binding protein (SMAC/Diablo)] to cytosol and helps in the survival of cell [19] . If the binding of apoptosis regulating proteins to VDAC1 is prevented, then it will lead to an increase in the MOM permeabilization. This will further help the pro-apoptotic proteins to cross MOM and enter into the cytoplasm where they will lead to apoptosis of cells [25] .

There are various compounds known as pro-apoptotic compounds that act on VDAC1 or HKII and inhibit their interaction or help in their dissociation, leading to cell death [27] . VDAC1 based peptides were synthesized namely Antp-LP4 and N-terminal-Antp, that selectively kill peripheral blood mononuclear cells obtained from chronic lymphocytic leukemia patients, but does not do any harm to normal cells obtained from healthy donors [28] . The same group of researchers had also synthesized one more cell-penetrating peptide to improve specificity, targeting and cellular stability, named as R-Tf-D-LP4. This peptide contains a transferrin receptor internalization sequence (Tf) that enhances the targeting of a peptide to cancer cells as there is an overexpression of transferrin receptors on cancer cells [29] . A study carried out by Yuan et al. observed that the endostatin causes the opening of mitochondrial permeability transition pore (mPTP) via VDAC1 and this leads to release of cytochrome c into the cytoplasm further causing a cascade of apoptosis and ending up with cell death [30] .

Ion channels of mitochondrial inner membrane

MIM forms the surroundings of the matrix and is also allows the formation of an electrochemical gradient that is responsible for ATP generation. There are many ion channels that are present on the MIM which are responsible for the exchange of ions, the exit of ATP and cytochrome c from mitochondria to the cytoplasm [31] . These channels help in maintaining homeostasis of mitochondria and also regulates apoptotic cascade by regulating the release of cytochrome c in the cytoplasm. These ion channels are discussed below in more detail.

Mitochondrial permeability transition pore (mPTP).

mPTP is a pore that is commonly believed to be responsible for increasing the MIM permeabilization usually referred to as mitochondrial permeability transition (MPT) [32] . MOM has a highly complex structure consisting of VDAC and some proteins like HKII, while MIM consisting of adenine nucleotide translocase (ANT) and cyclophilin D (CyP-D) [33] . These channels get opened, when there is a high calcium ion concentration inside mitochondria and also at a time when there is ROS production inside mitochondria. Opening of mPTP causes depolarization, further leading to decrease of mitochondrial membrane potential that affects ion homeostasis, respiratory chain complexes activity and ATP production [27] . Binding of HKII with VDAC prevents the formation of mPTP in two different ways. First, the binding of HKII to VDAC causes conformational changes in VDAC, which further leads to conformational changes in ANT. These changes are not suitable for the formation of mPTP but it creates a passage through which ATP can be transferred. Second, binding of HKII to VDAC prevents the binding of Bax and other pro-apoptotic proteins to VDAC. Hence, VDAC cannot form its oligomeric structure which is necessary for mPTP activation [33] . Prevention of binding of HKII and other pro-apoptotic proteins to VDAC is necessary for the formation of mPTP which will lead to cell death. Furthermore, the reduced form of CyP-D closes mPTP pore, whereas oxidized from is responsible for its opening. In a study carried out by Floda et al. showed that auranofin, an oxidizing agent, increases the oxidation CyP-D in human leukemic T cells [34] .

Mitochondrial calcium uniporter (MCU) complex.

Mitochondrial intra-cellular calcium ion (Ca 2+ ) level plays a very noteworthy role in pathophysiological conditions. Normal Ca 2+ levels are responsible for the proper regulation of the TCA cycle, stimulating oxidative phosphorylation, whereas high Ca 2+ causes apoptotic cell death by the opening of Mptp [35] . Once the Ca 2+ ions released from the endoplasmic reticulum into the cytosol, it can enter the mitochondria by crossing two membranes. It enters mitochondria through VDAC1 that are present on MOM, then it enters inside the mitochondrial matrix with the help of a complex containing pore-forming subunits MCU and MCUb (MCU isoforms ‘a’ and ‘b’) along with regulatory proteins like mitochondrial calcium uptake protein 1 (MICU1), MICU2, and EMRE (essential MCU regulator). MICU1 acts as an activator, whereas MICU2 acts as an inhibitor of MCU [27] . In normal conditions, both MICU1 and MICU2 acts as a heterodimer and are attached to MCU thereby it regulates Ca 2+ entry inside the mitochondria. As cytosolic Ca 2+ level increases, this induces conformational changes in the dimer and MICU2 gets detached, leaving MCU-MICU1 complex. This complex gets stabilized by EMRE, and further, it allows the entry of Ca 2+ ions inside mitochondria [27] .

Ca 2+ is very essential for the proper functioning of mitochondria for ATP generation. A high amount of Ca 2+ ions because of the upregulation of MCU complex, leads to apoptosis; whereas, low Ca 2+ levels resulting from downregulation of MCU complex, causes inefficient energy production. Hence, a balance should be maintained for the proper functioning of mitochondria. Therefore, the upregulation and downregulation of the MCU complex can be a possible way to eliminate cancer cells [36] . Bortezomib treatment for multiple myeloma causes an increase in the expression of MCU complexes, more Ca 2+ levels in mitochondria followed by more ROS production and changes in the membrane potential [37] .

Voltage-dependent potassium channels.

Voltage-dependent potassium channels (Kv) consist of a six-transmembrane helix and a pore. There are total 12 families of Kv channels from Kv1 to Kv12, and each family has many isoforms. Amongst all the isoforms, Kv1.3 is most abundant in T lymphocytes, central nervous system, epithelial cells, kidneys, etc. [38] . Their presence on the plasma membrane governs cell proliferation; whereas when they are present on MIM, they are involved in apoptosis due to their ability to interact with pro-apoptotic Bcl-2 associated X (Bax) in lymphocytes [39] . Kv1.3 is overexpressed in cancer cells as compared to normal cells and by which they help the cancer cell in proliferation, migration and metastasis [27] . Inhibiting these channels will help in eliminating the cancer cells. Mostly permeant inhibitors (Psora-4, PAP-1 and clofazimine) were studied, which have the ability to cross the plasma membrane and have an effect on the MIM Kv1.3 channel. The inhibition of MIM Kv1.3 leads to excessive ROS production, followed by the opening of permeability transition pore, leading to depolarization of MIM, swelling of mitochondria, the release of cytochrome c and finally apoptosis [27] . In a study carried out by Zaccagnino et al., it was observed that clofazimine was able to kill pancreatic ductal adenocarcinoma (PDAC) cells in vitro and also it was helpful in reducing the tumor size in vivo in a severe combines immunodeficient (SCID) mouse orthotopic xenograft PDAC model by 50% [40] .

Ca 2+ -dependent channels of mim.

There are certain ion channels located on MIM, which are regulated by Ca 2+ ions and are referred to as Ca 2+ -dependent channels. One such channel is Ca 2+ -dependent potassium channels (K Ca ), which are situated on the plasma membrane of both excitable and non-excitable cells [27] . Based upon their conductance, they are classified into three families: big-conductance potassium channel (BK Ca ; K Ca 1.1 or Slo1 or MaxiK), intermediate-conductance potassium channel (IK Ca ; K Ca 3.1 or SK4 or IK1) and small-conductance potassium channels (SK Ca : K Ca 2.1, 2.2, 2.3 or SK1, 2, 3) [41] . BK Ca (K Ca 1.1) is highly expressed in human glioma cells. In a study carried out by Debska-Vielhaber et al. on LN229 human glioma cells, it was shown that, CGS7184 (ethyl 1-[[(4-chlorophenyl)amino]oxo]−2–hydroxy-6-trifluoromethyl-1H-indole-3-carboxylate) opens BK Ca (K Ca 1.1) channel. This causes increased cellular respiration, followed by mitochondrial membrane depolarization and further leads to cell death [42] .

Targeting Bcl-2

Bcl-2 is a protein, belonging to the anti-apoptotic B-cell lymphoma 2 family, which contribute in tumor initiation, progression, apoptosis prevention and lack of response to chemotherapy [43] . Based upon their structure and function, Bcl-2 can be categorized as anti-apoptotic and pro-apoptotic members. Anti-apoptotic members contain four Bcl-2 homology domains (BH1, BH2, BH3, BH4) that includes Bcl-2 (the founding member), Bcl-X L , Bcl-w, Bcl-2-related protein A1 (Bfl-1/A1), myeloid cell leukemia 1 (MCL-1), and BCLB/Boo protein [43] . The pro-apoptotic members can be further divided into two subfamilies: the multi-domain pro-apoptotic ‘effectors’ (such as BAK, BAX and BOK) and those members which are known as ‘BH3-only proteins’ (BAD, BID, BIK, BIM, BMF, HRK, PUMA, and NOXA), as they possess only the short BH3 domain [44] , as shown in Fig. 4 . Both pro- and anti-apoptotic proteins regulate apoptotic cell death. These proteins have a direct effect over the MOM permeabilization (MOMP). Activation of BAK/BAX leads to oligomerization of BAK/BAX, which forms a pore and thus, increases mitochondrial permeability leading to a cascade of events that eventually leads to apoptosis. Bcl-2 like anti-apoptotic proteins inhibit the activation of BAK/BAX, which prevents apoptosis; whereas BH3-only proteins encourage oligomerization of BAK/BAX, leading to apoptosis [45] . In the case of cancer, anti-apoptotic proteins are increased; whereas pro-apoptotic proteins are found to be decreased. Recently, BH-3 mimetic compounds were developed and tested for their anticancer activity. ABT-263 (navitoclax) is an orally active BH-3 mimetic compound that selectively inhibits Bcl-2 and showed good results in clinical studies [19] . Venetoclax, by selectively targeting Bcl-2, was orally active against patients with relapsed small lymphocytic lymphoma (SLL) or chronic lymphocytic leukemia (CLL) [46] . Fig. 4 Different domains of B-cell lymphoma 2 family. Mammalian Bcl-2 proteins are categorised into three subclasses based on their function and the number of Bcl-2 homology (BH) domains: anti-apoptotic proteins, pro-apoptotic and BH-3 only proteins. Most of the members possess a C-terminal hydrophobic transmembrane domain (TMD) that can anchor proteins in the mitochondrial outer membrane. Fig. 4

Targeting electron transport chain complexes

Electron transport chain (ETC) acts as a central player for energy production by oxidative phosphorylation in mitochondria. ETC consists of five complexes namely CI, CII, CIII, CIV and CV that are embedded in the MIM. These complexes are responsible for energy production in the form of ATP. CI, CIII and CIV help in transportation of electron from NADH and succinate to molecular oxygen that forms water molecule. CI and CII transfer the electron to CIII via electron carrier's ubiquinone (UbQ), followed by its transfer to cytochrome c. Cytochrome c transfers these electrons to CIV, which converts molecular oxygen into water molecules with the help of these electrons. During this process, energy is generated which is stored in the form of electrostatic and proton gradient across IMM. CV (the F1FO-ATPase) uses this energy for the conversion of ADP and inorganic phosphate into ATP, which provides the cell with its fundamental energy substrate [47] . Along with ATP, ETC also produces ROS, plus CI and CIII are primary sites for superoxide generation, which is responsible for maintaining cellular signaling. Higher levels of ROS and superoxide leads to apoptosis [47] . By inhibiting CI, CII and CIII, apoptosis can be induced leading to cancer cell death. A study carried out by Andrzejewski et al. showed that metformin which was used for the treatment of type II diabetes had also shown good anticancer activity by acting on ETC CI of mitochondria [48] . CII and CIII were inhibited by sorafenib (Nexavar Ⓡ ) along with inhibition of ATP synthase, causing activation of serine–threonine protein kinase PINK1-perkin pathway, further leading to Parkin-mediated apoptosis and cell death [49] .

Oxidative stress targeting

OXPHOS is one of the ATP producing mechanism in human beings. This mechanism is also responsible for controlling ROS in the mitochondria. Hence, it regulates apoptotic cell death. Targeting OXPHOS will lead to increased ROS generation plus decreased ATP production which ultimately initiates the cascade mechanism of apoptosis leading to cell death [50] . Targeting is done by using pro-oxidant agents that induce oxidative stress by generating ROS. Hu et al. demonstrated that rotenone causes activation of NOX-2 complex leading to excessive ROS generation through PI3K/Akt/mTORC1 signaling pathway, leading to cell death [51] .

mtDNA

mtDNA is double-stranded, circular DNA. There are more than 1000 mitochondrial proteins present, out of which, 13 are encoded by mtDNA and remaining ones are encoded by nDNA (nuclear genome). These 13 proteins are associated with the respiratory chain, so increasing or decreasing the expression of mtDNA in tumor cells can interfere with energy production. Hence, mtDNA can be a good target for treating cancer. Vitamin K3 causes the inhibition of mtDNA polymerase γ, leading to cytotoxicity in cancer cells [52] . Table 1 . Briefly summarizes different mitochondrial targets used for therapeutics targeting. Table 1 Briefly summarizes different mitochondrial targets used for therapeutics targeting. Table 1 Target Name Name of compound Mechanism of Action In vitro model In vivo model Indication Ref. HKII miR-143 Knockdown of HKII reduced glucose consumption leading to apoptosis OECM-1, Tca8113 Xenografted athymic nude mice Oral squamous cell carcinoma [53] miR-218 Downregulation of HKII and apoptosis U87, U251 and HEK-293T Female NOD/SCID mice Glioma [54] 2-deoxy-D-glucose (2-DG) Product inhibition of HKII LNCaP and P69 cells – Prostate cancer [21] 3-bromopyruvate Alkylating agent Human leukemia cells lines (HL-60) and lymphoma cell line – Leukemia [55] Methyl jasmonate Detaches HKII from VDAC CT-26 murine colon carcinoma, B16 murine melanoma and Molt-4 human R-lymphoblastic leukemia cell lines Wistar female rats and ICR male mice Colon cancer [23] Ion channels ∆9-tetrahydrocannabinol (THC) Targeting VDAC, leading to apoptosis MCF-7 and MDA-MB-231 human breast carcinoma, DU-145 human prostate carcinoma, CaCo-2 human colorectal carcinoma and ASG human gastric adenocarcinoma Xenografted Charles River athymic mice Human breast carcinoma [56] Cisplatin Targeting VDAC, leading to apoptosis PCI-13 and PCI-51 – Head, neck and cervix carcinoma [57] R-Tf-D-LP4 peptide Metabolic disruption and apoptosis induction by interfering in anti-apoptotic proteins activity U-251MG, U-118MG, U-87MG, PANC1, T3M4, HTB-72, K-562, H358, T-Rex-293, PBMCs, MDKC, HuH-7 Glioblastoma xenografted mouse model Glioblastoma, lung and breast cancer [29] CGS7184 Opening of calcium dependent potassium channel leading to membrane depolarization LN229 human gliomacells – Brain tumors [42] Bortezomib Overexpression of MCU complexes leading to more calcium ion entry into mitochondria resulting into apoptosis Multiple myeloma (MM) cell lines – Breast cancer [37] Clofazimine Inhibition of potassium channel Kv1.3 Jurkat, CTLL-2 and K562 Orthotopic melanoma B16F10 mouse model Hepatocellular carcinoma [58] PC-1, Bx PC-3, Capan-1, Mia PaCa-2 and Panc-1 SCID beige mice Pancreatic cancer [40] Psora-4 Inhibition of potassium channel Kv1.3 Jurkat, CTLL-2 and K562 Orthotopic melanoma B16F10 mouse model Malignant tumor like melanoma [58] B-CLL – Chronic lymphocytic leukemia [59] PAP-1 Inhibition of potassium channel Kv1.3 B-CLL – Chronic lymphocytic leukemia [59] Auranofin Oxidation of cyclophilin D, leading to opening to mPTP pore Leukemic lymphoid CEM-R cells, Human epithelioid cervix carcinoma HeLa cells Mitochondria isolated from rat heart mitochondria – [34] Arbutin Apoptosis by overexpressing VDAC1 A375 human malignant melanoma cells – Melanoma [60] Bcl-2 Gossypol Simultaneous inhibition of Bcl-2, Bcl-X L , Bcl-W and MCL1 HT-29 human colon carcinoma – Colon cancer [61] Obatoclax Disruption of the interaction between BAK and MCL1. Induces the formation of an active Bak/Bax complex U937, HL-60, KG1 and OCI-AML3 acute myeloid leukaemia cell – Acute myeloid leukemia, [62] ABT-199 Specificity for Bcl-2 and no affinity for Bcl-X L . Sparing platelets by Bcl-2-dependent tumors HeLa Female C.B-17 SCID mice Non-Hodgkin's lymphoma, Acute lymphoid leukemia [63] ABT-263 Orally bioavailable Bad-like BH-3 mimetic. Stimulation of Bax translocation, cytochrome c release and subsequent apoptosis Human tumor cell lines C.B-17 SCID-BG mice and C.B-17 SCID mice small cell lung cancer, leukemia and lymphoma [64] ABT-737 Dissociation of the Bcl-2/BAX complex and BAK-dependent, BIM-independent activation of the intrinsic apoptotic pathway U937, HL-60, KG-, NB4 and OCI-AML3 cells Xenografted SCID mice model Acute myeloid leukemia [65] Venetoclax Inhibits Bcl-2 – Phase 2 clinical trial with 53 patients Chronic lymphocytic leukemia and small lymphocytic leukemia [46] Electron Transport Chain Complex Tamoxifen Inhibition of complex I, leading to excessive cellular hydrogen peroxide production – Male Wistar rats Breast cancer [66] Honokiol (HKL) Disruption of mitochondrial respiration by affecting complexes I, II, and V A375 (B R AF V600E ), SKMEL-28 (B R AF V600E ), and MeWo (wild type B R AF) human malignant melanoma cell lines – Skin cancer [67] Lonidamine Inhibition of complex II leading to ROS generation DB-1 melanoma cell line, HepG2 human liver cancer cell lines – Skin cancer [68] Metformin Inhibition of complex I NT2196, NMuMG Wild type male C57BL6J mice – [48] α-Tocopheryl succinate Inhibition of complex II MCF7 human breast cancer cells, NeuTL murine breast cancer cell Balb/c nu/nu mice Breast cancer [69] Sorafenib (nexavar) Inhibition of complex II and III HEK293T cells, HeLa cells – Liver cancer [49] Oxidative stress Resveratrol ROS production and hyperpolarization of mitochondrial membrane, leading to apoptosis SW620 colon cancer cell line – Colon cancer [70] Parthenolide ROS overproduction Primary human AML cells and blast crisis CML (bcCML) cells NOD/SCID, NOD.CB17-prdkdc scid/J mice Acute myelogenous leukemia [71] Rotenone NOX-2 complex leading to excessive ROS generation Human lung cancer A549 cells – Lung cancer [51] mtDNA Etoposide mtDNA topoisomerase II Jurkat T- lymphocytes Male Harlan Sprague-Dawley rats – [72] Vitamin K3 Inhibition of DNA polymerase γ HCT116 colon carcinoma cell lines – Colon cancer [52] Fialuridine Inhibition of DNA polymerase γ HepG2 cells (human hepatoblastoma) – Liver cancer [73]

Nanocarriers for mitochondrial targeting in diagnosis and treatment of cancer

NPs have received much attention in recent years, especially for treatment of cancer due to their capability of enhancing safety, pharmacokinetic profile and bioavailability of chemotherapeutic agents [74] . The nanocarriers employed for mitochondrial targeting are enlisted in Table 2 . Table 2 The nanocarriers employed for mitochondrial targeting. Table 2 Formulations Composition Therapeutic molecule Targeting ligand Cell line studies Animal studies Ref. Liposomes SPC, DSPE, Chol 5-Flurouracil Transferrin HT-29, Hu02 – [75] DOPE, SM, DC—Chol Mitochondrial DNA vector (pCMV-mtLuc (CGG)), KLA peptide D[KLAKLAK] 2 (Cellular uptake and Mitochondrial targeted device) C2C12 – [76] SPC, Chol, DOPE DTX EphA 10, TPP MCF-7, HepG2, A549 Female BALB/c nude mice [77] DSPE-PEG2000, DPPC, DSPE-PEG2000-NH2, Chol Chlorin e6, IR780 iodide TPP HeLa Female BALB/c nude mice [78] Polymeric nanocarriers CD, PEG, OEGMA, MGMA DOX – MCF-7, HeLa Kunming mice [79] PLGA, FA, PEG 2000, DSPE, Trans Retinoic acid, Cisplatin Celastrol TPP MDA-MB-231, MCF-7 Female BABL/c nude mice [80] PF127 Heptamethine cyanine dye me-IR825 – HeLa, A549, MCF-7, Hep G2, COS-7, L02, AT II, RGC-5, U14 Nude mice [81] HA, D-α-Tocopherol succinate Lapatinib TPP MDA-MB-231 MDA-MB-231 tumor xenograft murine models [82] PEG, Chol, DSPE DOX – B16F10, 4T1, OVCAR-8, NCI/ADR-RES Female C57BL/6 mice [83] HA Cisplatin-prodrug, Ce6 TPP B16F10 B16F10 tumor-bearing mice [84] HPMA, Gelatine DOX GA HepG2, HepG2/ADR Male BALB/c nude mice [85] Prodrugs M-PEG, DSPE, N, N′ -bis(octadecyl)-L-glutamic diamide Artesunate TPP A549, A549/DDP (cisplatin resistant), 7404, 7404/DDP (cisplatin resistant), MCF7, 4T1 – [86] PDMA, Cyclic Arg-Gly-Asp CPT TPP 4T1, MCF-7, U87 Female BALB/c mice [87] PEG, Methacrylate, OEGMA, Dextran-Br CPT TPP MCF-7, 4T1, HeLa female BABL/C mice [88] PEG2000, 1,8-naphthalimide Chlorambucil, DOX TPP HeLa, MCF-7, HL-7702, MCF-7 – [89] HA Cisplatin, Ce6 TPP B16F10 B16F10 tumor bearing mice [84] Carbon based materials PEG, CNT-ABT737 – – A549, NHFB – [90] SWCNTs, PAMAM ICG – HeLa – [91] GO DOX GA HepG2, L02, H9c2 HepG2-bearing nude mice [92] GO, PEG DOX Hypericin MDA-MB-231, MCF-7, MCF-10A female BALC/c nude mice [93] GO, β-CD, PEG – Transferrin, Mitochondrion-targeting peptide, poly lysine, azobenzene A549 S180-tumor-bearing mice [94] C-dots Citric acid, m-aminophenol – Rho HeLa Zebrafish [95] APTMS, DAMO, AEEA, APTES, MAPS, TPEDA – – HeLa, MDA-MB-231, Hep G2, U14, MCF-7, A549, L02, COS-7, RGC-5, AT II – [96] 1,3,6-Trinitropyrene – TPP, PEI Hela – [97] Peptide drug delivery HPMA SS20 peptide (MA-GG-FDRFK) HeLa, RAW264.7 BALB/c mice, BALB/c nude mice [98] PEG Protoporphyrin IX, Peptide (rFxrFxrFxr) – 4T1, COS7 BALB/c mice [99] PEG, PGA DOX, Peptide (KGFRWR) – SMMC 7721 Male Sprague Dawley rats, ICR mice [100] PEG, Chol DOX, HER-2 Peptide (YCDGFYACYMDV) DQA MCF-7, MCF-7/ADR Female nude mice [101] Abbreviations - AEMA: 2-aminoethyl methacrylate, AEEA: 3-[2-(2- amino ethyl amino ) ethyl amino ]propyl-trimethoxy silane, AuNPs: gold nanoparticles, APTMS: (3-aminopropyl) trimethoxy silane, APTES: (3-aminopropyl) triethoxy silane, Chol: cholesterol, CD: cyclodextrin, CNTs: carbon nanotubes, Ce6: chlorin e6, CPT: camptothecin, DAPI: 4′,6-diamidino-2-phenylindole, DAMO: n-[3-(trimethoxy silyl)propyl]ethylenediamine, DIPEA: diisopropyl ethyl amine, DMPC: dimyristoyl phosphatidyl choline, DOPE: 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, DPPC: 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DSPE: Distearoyl phosphatidyl ethanolamine, DSPC: 1,2-distearoyl-sn-glycero-3-phosphocholine, DTX: docetaxel, DQA: dequalinium, GO: graphene Oxide, ICG: indocyanine green, MWCNTs: Multi walled CNTs, MAPS: n-methyl-3- amino propyl trimethoxy silane, MGMA: ethyl glycinate methyacrylamide, M-PEG: methoxy polyethene glycol, OEGMA: poly(ethylene glycol) methyl ether methacrylate, PCL: poly-ε-caprolactone, PDMA: poly dimethyl acrylamide, PDA: polydopamine, PEI: polyethyleneimine, PGA: poly- γ-glutamic acid, PNIPAM: poly(n-isopropylacrylamide), PAM: polyacrylamide, PAMAM- polyamidoamine, PTX: paclitaxel, PF127: pluronic F127, PPI: poly(propylene imine), PTCDA: perylene tetracarboxylic anhydride, SWCNTs- single walled CNTs, Rho: rhodamine, SM: sphingomyelin, SPC: soya phosphatidyl choline, TEA: triethylamine, TPEDA: n-[(3-trimethoxy silyl)propyl]ethylenediamine triacetic acid trisodium salt, TPP-7: n-heptyl TPP, TPP-9: n-nonyl TPP, TPP-10: n-Decyl TPP, TPP-12: n-dodecyl TPP, TPP-14: n-tetradecyl TPP.

Carbon based materials

Carbon based nanomaterial such as carbon nanotubes (CNTs), graphene derivative and carbon dots (C-dots) have attracted significant attention over past decade because of their fascinating performance in biomedical field such as targeted drug delivery, chemo-photothermal therapy and real time bioimaging. Carbon based material have some unique physiochemical properties such as biocompatibility, supramolecular π-π stacking, high adsorption capacity and excellent photoluminescence [102] .These materials are widely employed for targeting to mitochondria to kill cancer cells. The chemically functionalized CNTs show promising tumor-targeted accumulation, exhibit low toxicity, and good biocompatibility [103] . One of the early examples of mitochondrial targeting CNTs is peptide-based multi-walled CNTs (MWCNTs). The mitochondrial targeting sequence (MTS) functionalized MWCNTs were able to undergo selective accumulation in mitochondria of macrophages and Hela cell lines which was confirmed by different microscopic techniques such as confocal laser scanning microscopy, transmission electron microscopy, and fluorescent microscopy. The obtained results indicated that no significant cytotoxicity was produced by MTS functionalized MWCNTs, which further showed their promising utilization potential as a nanocarrier [104] .

Recently, Wu et al. fabricated mitochondrial-targeted near-infrared (NIR) light activable multitasking nanographene (GT/IR820/DP-CPG) for in situ trigger efficient triple punch approach for photodynamic therapy in cancer. They designed strategy based upon the modification of TPP on graphene which can serve as a carrier to deliver IR820 (photosensitive dye) to mitochondria. The photoactive compound generates ROS and photothermal heat to kill cancer cells by inducing mitochondrial collapse and cell apoptosis after irradiation of NIR laser. After the administration of immunostimulatory conjugate DP-CpG in vivo, they will significantly enhance the secretion of proinflammatory cytokine like interleukin-6, tumor necrotic factor α, interferon γ and remarkably increases immunogenicity of tumour. In vivo results demonstrated that GT/IR820/DP-CpG shows significant inhibition of tumor growth (tumor inhibition rate approximately 88%) which resulted from the combination of the photothermal effect of IR 820 and immunostimulatory activity of DP-CpG ( Fig. 5 ) [105] . Fig. 5 In vivo PTT effect of GT/IR820/DP-CpG in EMT6 tumor bearing mouse model. (A) Thermal images of mouse treated with saline, GT/IR820 and GT/IR820/DP-CpG before and after laser light irradiation. The represented circle shows size of the tumor. (B) Isolated tumor at the end of the treatment. (C) Tumor growth curve of different groups at the end of the treatment. (D) Images of the tumor weight after the treatment. (E) Comparative body weight of different groups of tumor bearing mouse after the treatment. (F) Haematoxylin and Eosin staining of different treatment group. (G) TUNEL staining images of tumor slices isolated form mice treated with saline (control group), saline plus laser (control +laser), GT, IR820, DP-CpG, GT/IR820+ laser and GT/IR820/DP-CpG. Fig. 5

Liposomes

The liposomes are enclosed globular vesicles made up of lipid bilayer with internal aqueous core region. Both hydrophobic and hydrophilic drugs can be loaded in lipid bilayers and aqueous core region respectively. Biswas et al. fabricated novel PEGylated phosphatidylethanolamine (PEG-PE) conjugate with TPP attached with distal end of PEG block (TPP-PEG-PE). These conjugates were incorporated into the liposomal lipid bilayer and these modified liposomes were studied for their cytotoxicity, mitochondrial targeting capability and efficient delivery of paclitaxel (PTX) to cancer cell. The PTX loaded PEG-PE-TPP liposomes (TPP-PEG-L-PTX) showed higher PTX mediated cytotoxicity and anti-tumor action in both in vitro and in vivo models as compared to PTX loaded unmodified liposomes (PL-PTX) ( Fig. 6 )> [106] . Boddapati et al. formulated targeted liposomes using stearyl TPP (STPP) bromide by film rehydration method followed by ultrasonication. By encapsulating rhodamine- PE dye, these liposomes were tracked in BT 20 mammalian breast cancer cell lines. From the obtained results, they found that liposomes were selectively co-localized in mitochondria [107] . Utilizing mitochondrial capability of STPP, Malhi et al. developed novel DOX loaded liposome for mitochondrial targeting with surface-functionalized with the dual ligand, folic acid (FA) for selective cancer targeting and TPP cation for mitochondrial targeting. The cytotoxicity, ROS generation study, and cellular uptake study of DOX loaded liposome were carried out by using KB cell. These dual-targeted liposomes produce more cytotoxicity and ROS generation as compared to non-targeted ones [108] . Fig. 6 Cytotoxicity of different formulations of PTX in HeLa and 4T1 cells. (A and B) HeLa and 4T1 cells were incubated with PL-PTX and TPP-PEG-PTX with different concentration of PTX at 24 h. (C and D) indicates cells were treated with 650 ng PTX in different formulations at 48 h. Fig. 6

Polymeric nanocarriers

The polymeric nanocarriers made up from biocompatible and biodegradable polymer can serve as the most promising drug delivery system. By using an engineered composition of the hydrophilic and hydrophobic block, these polymers can be used to encapsulate both water-insoluble and water-soluble drugs to improve the bioavailability, retention time and aqueous solubility [109] . The polymeric NPs have many advantages such as higher stability and more controlled payload release. The most widely used biocompatible and biodegradable hydrophobic polymeric blocks are poly (lactic acid), poly(glycolic acid), and their co-polymer poly(lactic-co-glycolic acid) and polycaprolactone. Most widely used, Food and Drug Administration (FDA) approved hydrophilic block polymer is PEG. The PEG has some unique features such as it prolongs retention time in body and allow further functionalization with drug or targeting moiety [110] .

Liu et al. designed selective mitochondrial targeting NPs system to overcome multi drug resistance (MDR) through enhanced ROS generation. Initially, doxorubicin (DOX) was conjugated to glycyrrhetinic acid (GA) and then attached to N-(−2-hydroxypropyl) methyl acrylamide (HPMA) polymer backbone via hydrazine bond (P-DOX-GA). Furthermore, (P-DOX-GA) was grafted on the surface of gelatine NPs (GNPs). The GNP-P-DOX-GA would stay stable in blood circulation and then be degraded by extracellular tumor metal matrix protease 2 (MMP2) thereby releasing small size P-DOX-GA. The released P-DOX-GA was internalized by tumor cell through GA mediation and delivered to lysosome, DOX-GA get separated from HPMA polymer through hydrolysis of hydrazine bond at lower lysosomal pH. The separated DOX-GA could efficiently target mitochondria. As compared to the non-GA modified carrier, GNP-P-DOX-GA demonstrates nearly 4 times increased cellular uptake nearly 8.8 times mitochondrial targeting, and approximately 3 times increased ROS production level, thus causing significant decrease efflux rate in drug-resistant HgpG2/ADR cells. It also showed improved in vivo antitumor activity on HgpG2/ADR heterotopic tumor nude mice. GNP-P-DOX-GA based drug delivery system would be a promising approach for Anti MDR treatment and further enhanced ROS generation by mitochondrial targeting will be an effective approach to overcome tumor MDR [85] .

Inorganic nanoparticles

The inorganic NPs possess an advantage due to their smaller and more uniform particle size as compared to organic NPs. Till date, number of inorganic materials have been used to prepare mitochondrial targeting NPs. Hydroxyapatite (HAP) [Ca 10 (PO 4 ) 6 (OH) 2 ] is one of the main constituents of mammalian hard tissue, which shows biocompatibility and high drug loading capacity. Most importantly, HAP NPs enter mitochondria of tumor cells and induces apoptosis via changing mitochondrial membrane potential and leakage of cytochrome c [111] . Sun et al. synthesized rod-shaped HAP NPs (approximately 10 nm in width and 50 nm in length) by aqueous precipitation method. The cellular uptake of HAP NPs takes place via caveolae-mediated endocytosis in both lung cancer cell lines (A459) and normal bronchial epithelial cells (16 HBE). But importantly, more NPs are uptaken by A459 cell and caused sustain increase in intracellular calcium in contrast to the transitory increase in normal cells (16 HBE). This further caused increased cellular uptake and increased intracellular calcium which resulted in nearly 40% tumor growth inhibition in a lung cancer nude mice model without any side effects [112] . Ma et al. designed TPP functionalized gold NPs (TPP-Au) for selective mitochondrial targeting. They found that TPP-Au NPs selectively accumulated in mitochondria of tumor cells, and subsequent interparticle plasmonic coupling effect was responsible for hyperthermia in mitochondria and subsequent antitumor effect. But in normal cells, negligible amount of accumulation of TPP-Au NPs takes place, which is not sufficient to reach threshold for plasmonic effect as well as hyperthermia. As a consequence of this, irradiation would not damage normal tissue. The in vivo study demonstrated that an increase in temperature after irradiation was found approximately 4 times greater than normal tissue, which proves that TPP-Au NPs shows high selectivity for cancer therapy with fewer side effects [113] ( Fig. 7 ). Fig. 7 (A) PTT imaging of tumor tissue at different time point after mouse treated with PBS, Lys Au and TPP-Au; PDT images of adjacent normal tissues and tumor tissue, when mice were treated with TPP-Au; whereas A represent adjacent normal tissue and T represents tumor tissue. (B) Demonstrates increased temperature of various groups after increasing laser irradiation time. (C) Schematic illustration of laser irradiation technique. This provides the boundary between adjacent normal tissue and tumor tissue. (D) Increased temperature of tumor tissue in comparison to normal tissue with increased laser irradiation time. (E) Represents the temperature of tumor tissue and normal tissue after the 10 min laser light irradiation. Fig. 7

Dendrimers

Dendrimers are a hyperbranched, star-shaped macromolecule that are in nanometre in size range. It exhibits some exclusive features such as high aqueous solubility and drug encapsulation capacity, biodegradability, low toxicity, high retention time, specificity and allows surface modification. Biswas et al. reported, for the first time, novel mitochondrial targeted 5th generation poly(amidoamine) (PAMAM) dendrimer (G-(5)-D). To achieve this goal, initially, the fraction of cationic charge of G-(5)-D was neutralized by partial acetylation of primary amine groups (G-(5)-D-Ac) and at last TPP was conjugated on the surface of G-(5)-D-Ac via acid amine coupling reaction (G-(5)-D-Ac-TPP) [114] . Wang et al. fabricated TPP modified dendrimer as a polymeric vector for efficient gene delivery to mitochondria. The TPP conjugated dendrimer could effectively target mitochondria and showed a higher amount of gene transfection capacity and low cytotoxicity in HeLa and COS-7 as compared to commercially available transfection reagent SupreFect and comparable efficiency to Lipofectamine 2000 [115] . MDR is a major leading problem in the failure of PTX chemotherapy. To avoid this problem, Ma et al. fabricated glucose transporter-mediated and MMP-2 triggered mitochondrial-targeted conjugate. The conjugate composed of PAMAM dendrimer core co-modified with TPP via amido bond and PTX disulphide bond and further conjugation was done with long circulating PEG layer via MMP sensitive peptide (GPLGIAGQ). It was reported that the conjugate was able to reach to mitochondria via TPP followed pathway and releases PTX by the breakdown of the disulphide bond. The conjugate selectively accumulates in mitochondria and shows higher cytotoxicity in MCF-7/ADR cell, which significantly reverse of MDR of MCF-7/ADR cells [116] .

Peptides and their role in mitochondrial targeting

Mitochondrial targeted peptides are small biomolecules consist of numerous amino acids ranging from 2 to 50, and that are linked by a peptide bond. Apart from this, functional modification with a targeting moiety, the therapeutic peptide demonstrates better targetability and therapeutic efficacy [117] . Peptide shows faster elimination rate because of their rapid renal clearance due to enzymatic degradation. They have some significant advantages such as small size, easy to synthesize and modify, good tumor targeting and penetrating capacity and biocompatibility. They can be used as a directly cytotoxic agent through several mechanisms or can specifically act as a carrier for the various cytotoxic agent by targeting specific organ [118] . Bae et al. reported MTS-hybrid cationic oligopeptide (MTS-H 3 R 9 ), which shows double roles of mitochondrial targeting carrier along with anti-tumor activity. MTS-H 3 R 9 demonstrates significant cell-penetrating and internalization activity, efficient endosomal escape tendency and improved mitochondrial targeting ability in the HeLa cell line as compared to unmodified MTS. Most importantly, MTS-H 3 R 9 causes cell death by a higher amount of ROS generation and a decrease in mitochondrial membrane potential as well as an increase in anticancer activity in 3D spheroid and 2D cell culture model. MTS-H 3 R 9 provides dual potential both as a carrier for targeted drug delivery and anti-cancer agent for cancer treatment [119] .

Prodrug as a promising strategy for treatment of cancer via mitochondrial specific drug delivery system

Prodrugs are bio reversible derivative of a drug molecule that must undergo enzymatic and chemical transformation into active parent molecule with the objective to overcome the physiochemical, biopharmaceutical and pharmacokinetic barriers. These barriers include solubility, stability, permeability, pre systemic metabolism, and targeting limitation [120] . It is estimated that about 10% of drugs approved worldwide can be classified as prodrugs. Prodrugs are chemically modified versions of pharmacologically active agents that must undergo in vivo metabolic bioconversion to release active drugs. Non-toxic prodrugs that are activated by tumor microenvironment along with photodynamic therapy show synergistic anticancer activity to achieve this goal [121] . Wang et al. designed a mitochondrial-targeted camptothecin (CPT) polyprodrugs system for tumor-specific drug delivery. The system consists of an amphiphilic polyprodrug of dextran P (OEGMA-co-CPT-co-TPP) (DCT). The disulfide bond in polyprodrug is cleaved by intracellular reductants such as glutathione and leads to the release of active drugs. CPT molecule in polyprodrug system shows enhanced DNA damage and mitochondrial apoptosis which further accelerate apoptosis and cell death as compared to non-targeted prodrug. The mitochondrial-targeted polyprodrug demonstrates increased therapeutic efficacy by inhibiting tumor growth and promoting the cancer cell apoptosis ( Fig. 8 ) [88] . To avoid the problems associated with polymeric drug conjugate, Chen et al. fabricated mitochondrial-targeted NPs based on the modification of long aliphatic chain methoxy–polyethylene glycol-distearyl phosphatidylethanolamine (mPEG-DSPE) with artesunate-N, N′-bis(octadecyl)-Lglutamic diamide (ART-LGC12) prodrug and this fabricated prodrug self-assembled to the NPs with further surface modification of NPs facilitated by TPP (NPs-TPP). NPs-TPP demonstrates significant enhancement of cytotoxicity compared with free ART in various cancer cell line including cisplatin-resistant cancer cells [122] . Fig. 8 Demonstrates in vitro release behaviour, cell viability and apoptosis. CPT release curve of (A) DC and (B) DCT in PBS with different DTT concentrations (0, 2 µM, 2 mM). The cell viability assay of (C) HeLa cell line, (D) MCF-7 cells and (E) 4T1 cells incubated with free CPT, DC micelles and DCT micelles at different concentration for 72 h. (F) Dead and live cell staining of HeLa cells incubated with free CPT, DC micelles and DCT micelles with CPT concentration 30 µg/ml for 24 h at 37 °C. Untreated cells were used as control group. Fig. 8

Immunotherapy based approaches in mitochondrial targeting

Cancer immunotherapy is quickly growing to the fourth most important cancer therapy after chemotherapy, surgery and radiation therapy. It is also referred to as biologic therapy, which boosts the body's natural defence system to target and eradicate cancer cells thus resulting in durable anticancer action, reduce metastasis and recurrence of cancer. In contrast to other cancer treatment, immunotherapy primarily aims to prevent metastatic spread of cancer and to improve the quality of life of patients [123] . Approaches that are applied in immunotherapy are based on stimulation or complementation of immunity system via plethora of compound such as lymphokines, vaccines and antibodies [124] . Marrache et al. reported that stimulation of host immune response can be done by using light activated NPs containing zinc phthalocyanine (ZnPc) as photosensitizer. The NPs (T-ZnPc-NPs) were fabricated by encapsulation of ZnPc into biodegradable polymer i.e. poly (D, L lactide-co-glycolide)-b-poly(ethylene glycol)-TPP (PLGA-b-PEG-TPP). The in vitro results demonstrated that upon 660 nm laser irradiation, T-ZnPc-NPs generate tumour antigen form breast cancer cells to activate dendritic cell to produce high level of interferon- gamma. The remarkable ex vivo dendritic cell stimulation ability of this tumour cell supernatant was responsible for interleukins (IL)−12 and IL-18 autocrine effect. These ex-vivo studies suggested that secreted compound from T-ZnPc-NPs treated cancer cells also might be useful as cancer vaccines [125] .

The checkpoint blockade-based cancer immunotherapy has recently emerged as promising approach for cancer treatment. But its clinical application has been obstructed by poor tumor penetration of nanocarriers and activation of immune response. To overcome these obstacles, Dai et al. designed tumor acidity-responsive, mitochondrial nanocomplex co-loaded with programmed cell death ligand 1(PD-L1)-blocked siRNA and photosensitizer for synergistic combination of PDT and immunotherapy [126] .

Stimuli-responsive mitochondrial targeting for diagnosis and treatment of cancer

Stimuli responsive drug delivery system releases the drug only when they are triggered by certain stimuli like ultrasound, light, external magnetic field, pH or enzyme. These systems help in delivering the drug at tumor site and also avoid its leakage from nanocarrier. These systems possess theranostic function (both therapeutic as well as diagnostic function) [127] .

Ultrasound mediated mitochondrial targeting

Ultrasound is used as a stimulus to release the drug from nanocarriers because it possesses minimal invasiveness along with deep tissue penetration as well as it is cheaper and non-radioactive. Also, it does not cause any harm to neighboring cells [128] . Sonodynamic therapy is one of the approaches, which is based on ultrasound waves. In sonodynamic therapy, when a high energy wave is bombarded on sonosensitizers (agents that get activated by ultrasound), they get activated and then they transfer energy to nearby oxygen molecules that ultimately lead to ROS generation [129] . Zhang et al. carried out a study by taking IR780 as sonosensitizer and preparing its nanodroplets. In vitro studies demonstrated that IR780-nano droplets had shown mitochondrial targeting, and cancer cell death was observed due to apoptosis induced by overproduction of ROS in mitochondria [130] .

Redox triggering as a theranostic approach for mitochondrial targeting

For the survival as well as for proliferation of cancer cell, it is very much necessary for them to regulate their microenvironmental conditions like low pH, high level of GSH and ROS, and some more special enzymes which are not present in normal cells [131] . A high level of GSH in tumor tissue is responsible for providing a reduced environment. In the case of disulphide bond containing redox triggered drug delivery system (nano-drug delivery system), the disulphide bond gets reduced to sulphydryl groups by GSH present in tumor cells, leading to bond breakage and release of drug from nanocarrier [132] . Zhou et al. prepared PTX loaded lipid-polymer hybrid NPs that contain PLGA, TPP-containing amphiphilic polymer (C 18 -PEG 2000 -TPP) and reduction-responsive amphiphilic polymer (DLPE-S-S mPEG 4000 ). Once the PTX-PLGA/CPT/DSSP gets entered into the cancer cell, PEG 4000 gets removed because of reduced conditions provided by GSH leading to enhanced targeting towards mitochondria and apoptotic cell death [133] .

Magnetically delivering diagnostic and therapeutic agent to mitochondria

As it is widely known, providing elevated temperature to tumor cells makes them sensitive towards chemo- and radiation therapy leading to a decrease in the viability of tumor cells. Also, there should be a localization of drug at the tumor site in order to avoid side effects on normal cells and more specific destruction of tumor cells [134] . To fulfil this purpose, magnetic NPs were used for targeting as well as for drug delivery by producing hyperthermia at the targeted site by external application of alternating magnetic field [135] . Shah et al. developed magnetic core-shell nanoparticles (MCNP) for the delivery of amphiphilic tail-anchoring peptide (ATAP), a pro-apoptotic peptide, used for targeting mitochondria in cancer treatment. Targeting of MCNP-ATAP to malignant brain and metastatic breast cancer cells was done by applying the external magnetic field as well as by attaching internalizing RGD (a tumor targeting peptide), and hyperthermia was induced by an alternating magnetic field, overall concluding to synergistic cell death. MCNPs based hyperthermia causes inactivation of anti-apoptotic Bcl-2 inhibition leading to overproduction of pro-apoptotic Bcl-2 proteins, leading to cell death by apoptosis; whereas, MCNP-ATAP increasing permeability across MOM leading to dysfunction of mitochondria [136] .

Light responsive mitochondrial targeting

Light triggered stimuli include photothermal, photodynamic and radiation therapy. In photothermal therapy, photosensitizers are first localized to the targeted area then bombarded by near-infrared energy. Photosensitizers converts this NIR energy into heat, leading to an increase in the temperature of above 42 °C that causes cancer cell death [137] . In photodynamic therapy, photosensitizers gets excited when light is bombarded on them in presence of oxygen, a photodynamic reaction takes place that can produce singlet oxygen ( 1 O 2 ) and ROS that are highly cytotoxic leading to cancer cell death [138] . Li et al. prepared X-ray responsive nanoradiosensitizers that unlocks a cascade of reactions leading to cancer cell death. They have modified titanium dioxide NPs (TiO 2 NPs) with gold NPs (Au NPs) and TPP was attached to the surface of TiO 2 for targeted delivery of nanoradiosensitizers to cancer cells mitochondria. On X-ray exposure, these nanoradiosensitizers produce ROS and leading to a cascade of apoptosis. The use of nanoradiosensitizers in radiotherapy decreases overall radiation exposure, treatment time and side effects associated with radiation therapy [139] .

pH mediated targeting to mitochondria

Different tissues and organs have different pH in the body. By taking this into consideration, nanocarrier drug delivery systems can be developed that will release the drug only when they will reach the targeted site having a particular pH. pH-sensitive polymers can be used to release the drug at the targeted site. If pH of the targeted site is acidic then the pH of polymer should be alkaline so that, the polymer can dissolve in the acidic environment of targeted site and release the drug vice versa in case where the targeted site pH is alkaline the polymer will be acidic in nature [140] . The pH of mitochondria in tumor cells is weakly alkaline and it is around 8.0 [141] . The solubility of an alkaline drug is high in an acidic environment and solubility of an acidic drug remains high in an alkaline environment. Hence, an acidic drug can be incorporated into the nanocarriers for targeting to the mitochondria [142] .

Enzyme triggered mitochondrial targeting

Enzymes are biocatalysts that carry out catalysis of several metabolic as well as biological processes. Apart from catalyzing these processes, they also act as a stimulus for nanocarriers to release the drug at the targeted site [143] . There is an overexpression of CD44 receptors in case of cancer cells as compared to normal cells and these receptors interact with several ligands (hyaluronic acid (HA), collagens, osteopontin, etc.) that are present in extracellular matrix leading to enhancement of cancer cell migration and metastasis. Also, there is an overexpression of hyaluronidase enzyme that causes degradation of HA in tumor cells acidic environment [144] . By considering this, Naz et al. prepared mesoporous silica nanoparticles (MSN) and attached TPP to it for mitochondrial targeting. Then DOX was loaded into it followed by capping it with HA. Here, HA was used as capping agent to prevent DOX from pre-leakage before it reaches to the targeted site as well as to increase CD44 targeting. The prepared MSN-DPH (consist of DOX loaded, TPP attached, HA capped mesoporous silica nanoparticles) acts as an enzyme responsive nano-drug delivery system. When this enters inside the cancer cells, it specifically targets mitochondria because of presence of TPP and hyaluronidase enzyme and causes degradation of HA that leads to release of DOX and eventually cancer cell death [145] .

Future perspective and regulatory constraints

Since the last decade, nanomedicine has contributed majorly in the field of biomedical applications. However, while developing nanomedicines, proper regulations are also very essential to control the quality. FDA and European Medicines Agency (EMA) are the major watchful regulatory bodies that are responsible for introducing and controlling the quality and safety of nanomedicines. The number of new drug applications for nanomedicines is getting increased day by day due to their advantages over the conventional dosage forms. Moreover, there are many products available in the market like liposomes, nanocrystals, polymeric nanoparticles, nanoemulsions, polymeric protein conjugates as well as many are in the phases of the clinical trial [146] . Doxil Ⓡ is a classic example of the first FDA approved liposomal nanomedicine containing the DOX that was introduced in the market for the treatment of ovarian cancer, breast cancer and Kaposi's sarcoma in the year of 1995 [147] . EMA also has released some papers regarding the nanomedicines along intravenous liposome, block copolymer micelles, surface coating and iron-based colloidal nanomedicines. FDA has established a Nanotechnology Interest Group at each centre and office, which reports to the Office of Commissioner. This group majorly focuses on the need for central resources to gain basic data such as safety, pharmacokinetics, biodistribution and toxicity of nanoparticles besides raising key questions for the regulation of nanotechnology which can further smoothen the regulatory process [148] . The major problem associated with nanomedicine is about manufacturing and scale up due to new material properties and technology. Considering the fact that most of literature discussed in the preceding section for mitochondrial targeting talks about formulation strategies which are not the conventional procedure used in formulation development, an entirely different set of keys in process parameter monitoring is needed, that to, tailor-made for each delivery system. This further necessitates the need of having expert people who can identify the loopholes and lacunas in current regulatory norms before they are extrapolated to newer materials used in NPs used for cancer targeting. The need of monitoring the effects of these NPs over prolonged administration may pose newer challenges and probably newer vigilance norms and regulations. However, quality by design approach is providing some space to identify the critical points that affects product and process with implementation of on line/at line/in line process analytical tools which detects defect at the site [149] .

Conclusion

Mitochondria play a key role in different cellular functions. In fact, it is responsible for providing the energy in the form of ATP needed to carry out all biological processes. Moreover, the growth of neoplastic tissue is often supported by anaerobic hyperactive glycolytic machinery. A thorough characterization study of the metabolic process involved in tumour has clearly indicated the dependency of tumorigenic cell populations on mitochondrial respiration compared to their dependency on glycolysis, and its depletion resulting in markedly limited tumorigenic potential. Hence targeting the mitochondria appears a promising strategy which can be used to easily kill the cancer cells by choking the much-needed energy supplement, essential for their growth. There are many reported mechanisms by which mitochondrial targeting can be achieved, however before that, it becomes essential to have a thorough understanding of the molecular mechanisms and types of factors present in mitochondria that are involved in tumorigenesis. The two most commonly used approaches for mitochondrial drug delivery: direct conjugation of the targeting ligand to drugs and attachment of the targeting ligand to a nanocarrier, have their own advantages and limitations. In the case of direct conjugation of the targeting ligand to a drug (eg. TPP based conjugates which exhibit strong mitochondrial targeting ability), many aspects should be considered, including the type of linkage and the type of targeting ligand. Despite the fact that direct drug-targeting ligand conjugation is simple and easy to control, and the drugs can readily reach the mitochondria; there is equally good probability that the conjugation procedure can diminish the therapeutic effects and solubility of the drugs. Apart from this, nanomedicine mediated targeting to mitochondria is significantly improving since the last decade due to their numerous advantages like higher surface area, smaller particle size (helps in enhanced permeability and retention mediated targeting as cancer blood vessel possesses fenestration, which helps in entry of NPs in tumor tissue from blood vessels), ease in functionalization and many more. Furthermore, attachment of targeting ligand to NPs really helps in accurate and effective mitochondrial targeting that will reduce the toxicity associated with conventional anti-cancer therapy. In the case of the nanocarrier system, there is lesser concern of loss of therapeutic effect as most of the approaches tackle the physical interaction and solubility issue very well, but optimization still pose a major bottleneck due to the use of many different compositions to prepare the nanocarrier, which may hinder mitochondrial delivery. In this review we have summarized all the mitochondrial factors that have role in cancer induction and inhibition including strategies to achieve targeting via direct or indirect methods specifically highlighting the different nanocarriers that have been employed for the treatment of cancer using mitochondria as a target. Though most of these studies are in initial stages, the day is not far when these advancements in cancer delivery can be moved from bench side to bed side provided exhaustive further in-depth studies are carried out.

Conflicts of interest

There is no conflict of interest and disclosures associated with the manuscript

References

1 Rajani C. Borisa P. Karanwad T. Borade Y. Patel V. Rajpoot K. Cancer-targeted chemotherapy: emerging role of the folate anchored dendrimer as drug delivery nanocarrier Chauhan A Kulhari H Pharmaceutical applications of dendrimers 2020 Elsevier 151 198

2 Hejmadi M. Introduction to cancer biology 2009 Bookboon

3 Kalyane D. Raval N. Maheshwari R. Tambe V. Kalia K. Tekade R.K. Employment of enhanced permeability and retention effect (EPR): nanoparticle-based precision tools for targeting of therapeutic and diagnostic agent in cancer Mater Sci Eng C 98 2019 1252 1276 10.1016/j.msec.2019.01.066 30813007

4 Dong L.F. Neuzil J. Mitochondria in cancer: why mitochondria are a good target for cancer therapy Prog Mol Biol Transl Sci 127 2014 211 227 25149219 10.1016/B978-0-12-394625-6.00008-8

5 Wallace D.C. Mitochondria and cancer Nat Rev Cancer 12 2012 685 23001348 10.1038/nrc3365 PMC4371788

6 Jabr-Milane L.S. van Vlerken L.E. Yadav S. Amiji M.M. Multi-functional nanocarriers to overcome tumor drug resistance Cancer Treat Rev 34 2008 592 602 18538481 10.1016/j.ctrv.2008.04.003 PMC2585991

7 Vander Heiden M.G. Cantley L.C. Thompson C.B. Understanding the Warburg effect: the metabolic requirements of cell proliferation Science 324 2009 1029 1033 19460998 10.1126/science.1160809 PMC2849637

8 Tannock I.F. Rotin D. Acid pH in tumors and its potential for therapeutic exploitation Cancer Res 49 1989 4373 4384 2545340

9 Battogtokh G. Cho Y.Y. Lee J.Y. Lee H.S. Kang H.C. Mitochondrial-targeting anticancer agent conjugates and nanocarrier systems for cancer treatment Front Pharmacol 9 2018 922 30174604 10.3389/fphar.2018.00922 PMC6107715

10 Yamada Y. Akita H. Kamiya H. Kogure K. Yamamoto T. Shinohara Y. MITO-Porter: a liposome-based carrier system for delivery of macromolecules into mitochondria via membrane fusion Biochim Biophys Acta (BBA)-Biomembr 1778 2008 423 432 10.1016/j.bbamem.2007.11.002 18054323

11 Chan M.S. Liu L.S. Leung H.M. Lo P.K. Cancer-cell-specific mitochondria-targeted drug delivery by dual-ligand-functionalized nanodiamonds circumvent drug resistance ACS Appl Mater Interfaces 9 2017 11780 11789 28291330 10.1021/acsami.6b15954

12 Zhou J. Zhao W.Y. Ma X. Ju R.J. Li X.Y. Li N. The anticancer efficacy of paclitaxel liposomes modified with mitochondrial targeting conjugate in resistant lung cancer Biomaterials 34 2013 3626 3638 23422592 10.1016/j.biomaterials.2013.01.078

13 Mkandawire M.M. Lakatos M. Springer A. Clemens A. Appelhans D. Krause-Buchholz U. Induction of apoptosis in human cancer cells by targeting mitochondria with gold nanoparticles Nanoscale 7 2015 10634 10640 26022234 10.1039/c5nr01483b

14 Xiang S. Zhang K. Yang G. Gao D. Zeng C. He M. Mitochondria-targeted and resveratrol-loaded dual-function titanium disulfide nanosheets for photothermal-triggered tumor chemotherapy Nanoscale Res Lett 14 2019 211 31227943 10.1186/s11671-019-3044-5 PMC6588667

15 Wang Z. Guo W. Kuang X. Hou S. Liu H. Nanopreparations for mitochondria targeting drug delivery system: current strategies and future prospective Asian J Pharm Sci 12 2017 498 508 32104363 10.1016/j.ajps.2017.05.006 PMC7032167

16 Pathak R.K. Kolishetti N. Dhar S. Targeted nanoparticles in mitochondrial medicine Wiley Interdiscip Rev: Nanomed Nanobiotechnol 7 3 2015 315 329 25348382 10.1002/wnan.1305 PMC4397104

17 Xu Z. Chen X. Sun Z. Li C. Jiang B. Recent progress on mitochondrial targeted cancer therapy based on inorganic nanomaterials Mater Today Chem 12 2019 240 260

18 Siu M.K.Y. Jiang Y.X. Wang J.J. Leung T.H.Y. Han C.Y. Tsang B.K. Hexokinase 2 regulates ovarian cancer cell migration, invasion and stemness via FAK/ERK1/2/MMP9/NANOG/SOX9 signaling cascades Cancers 11 2019 813 10.3390/cancers11060813 PMC6627345 31212816

19 Nguyen C. Pandey S. Exploiting mitochondrial vulnerabilities to trigger apoptosis selectively in cancer cells Cancers 11 2019 916 10.3390/cancers11070916 PMC6678564 31261935

20 Seo M. Crochet R.B. Lee Y.H. Targeting altered metabolism—Emerging cancer therapeutic strategies Neidle S Cancer drug design discovery Second ed 2014 Academic Press San Diego 427 448

21 Ben Sahra I Laurent K. Giuliano S. Larbret F. Ponzio G. Gounon P. Targeting cancer cell metabolism: the combination of metformin and 2-deoxyglucose induces p53-dependent apoptosis in prostate cancer cells Cancer Res 70 2010 2465 2475 20215500 10.1158/0008-5472.CAN-09-2782

22 Bao F. Yang K. Wu C. Gao S. Wang P. Chen L. New natural inhibitors of hexokinase 2 (HK2): steroids from Ganoderma sinense Fitoterapia 125 2018 123 129 29305912 10.1016/j.fitote.2018.01.001

23 Goldin N. Arzoine L. Heyfets A. Israelson A. Zaslavsky Z. Bravman T. Methyl jasmonate binds to and detaches mitochondria-bound hexokinase Oncogene 27 2008 4636 18408762 10.1038/onc.2008.108

24 O'Rourke B. Mitochondrial ion channels Ann Rev Physiol 69 2007 19 49 17059356 10.1146/annurev.physiol.69.031905.163804 PMC2712118

25 Magrì A. Reina S. De Pinto V. VDAC1 as pharmacological target in cancer and neurodegeneration: focus on its role in apoptosis Front Chem 6 2018 108 29682501 10.3389/fchem.2018.00108 PMC5897536

26 Shoshan-Barmatz V. Krelin Y. Shteinfer-Kuzmine A. Arif T. Voltage-dependent anion channel 1 as an emerging drug target for novel anti-cancer therapeutics Front Oncol 7 2017 154 28824871 10.3389/fonc.2017.00154 PMC5534932

27 Bachmann M. Costa R. Peruzzo R. Prosdocimi E. Checchetto V. Leanza L. Targeting mitochondrial ion channels to fight cancer Int J Mol Sci 19 2018 2060 10.3390/ijms19072060 PMC6073807 30011966

28 Prezma T. Shteinfer A. Admoni L. Raviv Z. Sela I. Levi I. VDAC1-based peptides: novel pro-apoptotic agents and potential therapeutics for B-cell chronic lymphocytic leukemia Cell Death Dis 4 2013 e809 24052077 10.1038/cddis.2013.316 PMC3789174

29 Shteinfer-Kuzmine A. Amsalem Z. Arif T. Zooravlov A. Shoshan-Barmatz V. Selective induction of cancer cell death by VDAC1-based peptides and their potential use in cancer therapy Mol Oncol 12 2018 1077 1103 29698587 10.1002/1878-0261.12313 PMC6026870

30 Yuan S. Fu Y. Wang X. Shi H. Huang Y. Song X. Voltage-dependent anion channel 1 is involved in endostatin-induced endothelial cell apoptosis FASEB J 22 2008 2809 2820 18381814 10.1096/fj.08-107417

31 Leanza L. Zoratti M. Gulbins E. Szabo I. Mitochondrial ion channels as oncological targets Oncogene 33 2014 5569 5581 24469031 10.1038/onc.2013.578

32 Bonora M. Pinton P. The mitochondrial permeability transition pore and cancer: molecular mechanisms involved in cell death Front Oncol 4 2014 302 25478322 10.3389/fonc.2014.00302 PMC4235083

33 Suh D.H. Kim M.K. Kim H.S. Chung H.H. Song Y.S. Mitochondrial permeability transition pore as a selective target for anti-cancer therapy Front Oncol 3 2013 41 23483560 10.3389/fonc.2013.00041 PMC3592197

34 Folda A. Citta A. Scalcon V. Calì T. Zonta F. Scutari G. Mitochondrial thioredoxin system as a modulator of cyclophilin D redox state Sci Rep 6 2016 23071 26975474 10.1038/srep23071 PMC4791683

35 Mammucari C. Gherardi G. Rizzuto R. Structure, activity regulation, and role of the mitochondrial calcium uniporter in health and disease Front Oncol 7 2017 139 28740830 10.3389/fonc.2017.00139 PMC5502327

36 Rizzuto R. De Stefani D. Raffaello A. Mammucari C. Mitochondria as sensors and regulators of calcium signalling Nat Rev Mol Cell Biol 13 2012 566 22850819 10.1038/nrm3412

37 Vultur A. Gibhardt C.S. Stanisz H. Bogeski I. The role of the mitochondrial calcium uniporter (MCU) complex in cancer Pflügers Arch J Physiol 470 2018 1149 1163 29926229 10.1007/s00424-018-2162-8

38 Leanza L. Venturini E. Kadow S. Carpinteiro A. Gulbins E. Becker K.A. Targeting a mitochondrial potassium channel to fight cancer Cell Calcium 58 2015 131 138 25443654 10.1016/j.ceca.2014.09.006

39 Szabó I. Bock J. Grassmé H. Soddemann M. Wilker B. Lang F. Mitochondrial potassium channel Kv1. 3 mediates Bax-induced apoptosis in lymphocytes Proc Natl Acad Sci 105 2008 14861 14866 18818304 10.1073/pnas.0804236105 PMC2567458

40 Zaccagnino A. Managò A. Leanza L. Gontarewitz A. Linder B. Azzolini M. Tumor-reducing effect of the clinically used drug clofazimine in a SCID mouse model of pancreatic ductal adenocarcinoma Oncotarget 8 2017 38276 27542263 10.18632/oncotarget.11299 PMC5503532

41 Gueguinou M. Chantome A. Fromont G. Bougnoux P. Vandier C. Potier-Cartereau M. KCa and Ca 2+ channels: the complex thought Biochim Biophys Acta (BBA)-Mol Cell Res 1843 2014 2322 2333 10.1016/j.bbamcr.2014.02.019 24613282

42 Debska-Vielhaber G. Godlewski M.M. Kicinska A. Skalska J. Kulawiak B. Piwonska M. Large-conductance K+ channel openers induce death of human glioma cells J Physiol Pharmacol 60 2009 27 36 20065494

43 Perini G.F. Ribeiro G.N. Neto J.V.P. Campos L.T. Hamerschlak N. BCL-2 as therapeutic target for hematological malignancies J Hematol Oncol 11 2018 65 29747654 10.1186/s13045-018-0608-2 PMC5946445

44 Thomas S. Quinn B.A. Das S.K. Dash R. Emdad L. Dasgupta S. Targeting the Bcl-2 family for cancer therapy Expert Opin Ther Targets 17 2013 61 75 23173842 10.1517/14728222.2013.733001 PMC3955095

45 Campbell K.J. Tait S.W.G. Targeting BCL-2 regulated apoptosis in cancer Open Biol 8 2018 180002 10.1098/rsob.180002 PMC5990650 29769323

46 Roberts A.W. Davids M.S. Pagel J.M. Kahl B.S. Puvvada S.D. Gerecitano J.F. Targeting BCL2 with venetoclax in relapsed chronic lymphocytic leukemia N Engl J Med 374 2016 311 322 26639348 10.1056/NEJMoa1513257 PMC7107002

47 Rohlena J. Dong L. Neuzil J. Targeting the mitochondrial electron transport chain complexes for the induction of apoptosis and cancer treatment Curr Pharm Biotechnol 14 2013 377 389 22201598 10.2174/1389201011314030011

48 Andrzejewski S. Gravel S.P. Pollak M. St-Pierre J. Metformin directly acts on mitochondria to alter cellular bioenergetics Cancer Metab 2 2014 12 25184038 10.1186/2049-3002-2-12 PMC4147388

49 Zhang C. Liu Z. Bunker E. Ramirez A. Lee S. Peng Y. Sorafenib targets the mitochondrial electron transport chain complexes and ATP synthase to activate the PINK1–Parkin pathway and modulate cellular drug response J Biol Chem 292 2017 15105 15120 28673964 10.1074/jbc.M117.783175 PMC5592685

50 Bhatti J.S. Bhatti G.K. Reddy P.H. Mitochondrial dysfunction and oxidative stress in metabolic disorders—A step towards mitochondria based therapeutic strategies Biochim Biophys Acta (BBA)-Mol Basis Dis 1863 2017 1066 1077 10.1016/j.bbadis.2016.11.010 PMC5423868 27836629

51 Hu W. Tian H. Yue W. Li L. Li S. Gao C. Rotenone induces apoptosis in human lung cancer cells by regulating autophagic flux IUBMB Life 68 2016 388 393 27015848 10.1002/iub.1493

52 Sasaki R. Suzuki Y. Yonezawa Y. Ota Y. Okamoto Y. Demizu Y. DNA polymerase γ inhibition by vitamin K3 induces mitochondria-mediated cytotoxicity in human cancer cells Cancer Sci 99 2008 1040 1048 18312466 10.1111/j.1349-7006.2008.00771.x PMC11159526

53 Sun X. Zhang L. MicroRNA-143 suppresses oral squamous cell carcinoma cell growth, invasion and glucose metabolism through targeting hexokinase 2 Biosci Rep 37 2017 10.1042/BSR20160404 PMC5463264 28174335

54 Liu H. Liu N. Cheng Y. Jin W. Zhang P. Wang X. Hexokinase 2 (HK2), the tumor promoter in glioma, is downregulated by miR-218/Bmi1 pathway PLoS One 12 2017 e0189353 10.1371/journal.pone.0189353 PMC5722312 29220380

55 Chen Z. Zhang H. Lu W. Huang P. Role of mitochondria-associated hexokinase II in cancer cell death induced by 3-bromopyruvate Biochim Biophys Acta - Bioenerg 1787 2009 553 560 10.1016/j.bbabio.2009.03.003 PMC2731236 19285479

56 Ligresti A. Moriello A.S. Starowicz K. Matias I. Pisanti S. De Petrocellis L. Antitumor activity of plant cannabinoids with emphasis on the effect of cannabidiol on human breast carcinoma J Pharmacol Exp Ther 318 2006 1375 1387 16728591 10.1124/jpet.106.105247

57 Yang Z. Schumaker L.M. Egorin M.J. Zuhowski E.G. Guo Z. Cullen K.J. Cisplatin preferentially binds mitochondrial DNA and voltage-dependent anion channel protein in the mitochondrial membrane of head and neck squamous cell carcinoma: possible role in apoptosis Clin Cancer Res 12 2006 5817 5825 17020989 10.1158/1078-0432.CCR-06-1037

58 Leanza L. Henry B. Sassi N. Zoratti M. Chandy K.G. Gulbins E. Inhibitors of mitochondrial Kv1. 3 channels induce Bax/Bak-independent death of cancer cells EMBO Mol Med 4 2012 577 593 22496117 10.1002/emmm.201200235 PMC3407946

59 Leanza L. Trentin L. Becker K.A. Frezzato F. Zoratti M. Semenzato G. Clofazimine, Psora-4 and PAP-1, inhibitors of the potassium channel Kv1. 3, as a new and selective therapeutic strategy in chronic lymphocytic leukemia Leukemia 27 2013 1782 23426166 10.1038/leu.2013.56

60 Nawarak J. Huang-Liu R. Kao S.H. Liao H.H. Sinchaikul S. Chen S.T. Proteomics analysis of A375 human malignant melanoma cells in response to arbutin treatment Biochim Biophys Acta (BBA)-Proteins Proteom 1794 2009 159 167 10.1016/j.bbapap.2008.09.023 18996230

61 Zhang M. Liu H. Guo R. Ling Y. Wu X. Li B. Molecular mechanism of gossypol-induced cell growth inhibition and cell death of HT-29 human colon carcinoma cells Biochem Pharmacol 66 2003 93 103 12818369 10.1016/s0006-2952(03)00248-x

62 Konopleva M. Watt J. Contractor R. Tsao T. Harris D. Estrov Z. Mechanisms of antileukemic activity of the novel Bcl-2 homology domain-3 mimetic GX15-070 (obatoclax) Cancer Res 68 2008 3413 3420 18451169 10.1158/0008-5472.CAN-07-1919 PMC4096127

63 Souers A.J. Leverson J.D. Boghaert E.R. Ackler S.L. Catron N.D. Chen J. ABT-199, a potent and selective Bcl-2 inhibitor, achieves antitumor activity while sparing platelets Nat Med 19 2013 202 23291630 10.1038/nm.3048

64 Tse C. Shoemaker A.R. Adickes J. Anderson M.G. Chen J. Jin S. ABT-263: a potent and orally bioavailable Bcl-2 family inhibitor Cancer Res 68 2008 3421 3428 18451170 10.1158/0008-5472.CAN-07-5836

65 Konopleva M. Contractor R. Tsao T. Samudio I. Ruvolo P.P. Kitada S. Mechanisms of apoptosis sensitivity and resistance to the BH3 mimetic ABT-737 in acute myeloid leukemia Cancer Cell 10 2006 375 388 17097560 10.1016/j.ccr.2006.10.006

66 Moreira P.I. Custódio J. Moreno A. Oliveira C.R. Santos M.S. Tamoxifen and estradiol interact with the flavin mononucleotide site of complex I leading to mitochondrial failure J Biol Chem 281 2006 10143 10152 16410252 10.1074/jbc.M510249200

67 Trotta A.P. Gelles J.D. Serasinghe M.N. Loi P. Arbiser J.L. Chipuk J.E. Disruption of mitochondrial electron transport chain function potentiates the pro-apoptotic effects of MAPK inhibition J Biol Chem 292 2017 11727 11739 28546431 10.1074/jbc.M117.786442 PMC5512068

68 Guo L. Shestov A.A. Worth A.J. Nath K. Nelson D.S. Leeper D.B. Inhibition of mitochondrial complex II by the anticancer agent lonidamine J Biol Chem 291 2016 42 57 26521302 10.1074/jbc.M115.697516 PMC4697178

69 Dong L.F. Low P. Dyason J.C. Wang X.F. Prochazka L. Witting P.K. α-Tocopheryl succinate induces apoptosis by targeting ubiquinone-binding sites in mitochondrial respiratory complex II Oncogene 27 2008 4324 18372923 10.1038/onc.2008.69 PMC2668987

70 del Mar Blanquer-Rosselló M. Hernández-López R. Roca P. Oliver J. Valle A. Resveratrol induces mitochondrial respiration and apoptosis in SW620 colon cancer cells Biochim Biophys Acta (BBA)-Gen Subj 1861 2017 431 440 10.1016/j.bbagen.2016.10.009 27760368

71 Guzman M.L. Rossi R.M. Karnischky L. Li X. Peterson D.R. Howard D.S. The sesquiterpene lactone parthenolide induces apoptosis of human acute myelogenous leukemia stem and progenitor cells Blood 105 2005 4163 4169 15687234 10.1182/blood-2004-10-4135 PMC1895029

72 Robertson J.D. Gogvadze V. Zhivotovsky B. Orrenius S. Distinct pathways for stimulation of cytochrome c release by etoposide J Biol Chem 275 2000 32438 32443 10961984 10.1074/jbc.C000518200

73 Lewis W. Levine E.S. Griniuviene B. Tankersley K.O. Colacino J.M. Sommadossi J.P. Fialuridine and its metabolites inhibit DNA polymerase gamma at sites of multiple adjacent analog incorporation, decrease mtDNA abundance, and cause mitochondrial structural defects in cultured hepatoblasts Proc Natl Acad Sci 93 1996 3592 3597 8622980 10.1073/pnas.93.8.3592 PMC39655

74 Kumari P. Ghosh B. Biswas S. Nanocarriers for cancer-targeted drug delivery J Drug Target 24 2016 179 191 26061298 10.3109/1061186X.2015.1051049

75 Moghimipour E. Rezaei M. Ramezani Z. Kouchak M. Amini M. Angali K.A. Transferrin targeted liposomal 5-fluorouracil induced apoptosis via mitochondria signaling pathway in cancer cells Life Sci 194 2018 104 110 29275107 10.1016/j.lfs.2017.12.026

76 Katayama T. Kinugawa S. Takada S. Furihata T. Fukushima A. Yokota T. A mitochondrial delivery system using liposome-based nanocarriers that target myoblast cells Mitochondrion 49 2019 66 72 31326598 10.1016/j.mito.2019.07.005

77 Zhang J. Yang C. Pan S. Shi M. Li J. Hu H. Eph A10-modified pH-sensitive liposomes loaded with novel triphenylphosphine–docetaxel conjugate possess hierarchical targetability and sufficient antitumor effect both in vitro and in vivo Drug Deliv 25 2018 723 737 29513049 10.1080/10717544.2018.1446475 PMC6058733

78 Wang S. Guo F. Ji Y. Yu M. Wang J. Li N. Dual-mode imaging guided multifunctional Theranosomes with mitochondria targeting for Photothermally controlled and enhanced photodynamic therapy in vitro and in vivo Mol Pharm 15 2018 3318 3331 29975536 10.1021/acs.molpharmaceut.8b00351

79 Bai S. Zhang X. Ma X. Chen J. Chen Q. Shi X. Acid-active supramolecular anticancer nanoparticles based on cyclodextrin polyrotaxanes damaging both mitochondria and nuclei of tumor cells Biomater Sci 6 2018 3126 3138 30362476 10.1039/c8bm01020j

80 You C.Q. Wu H.S. Gao Z.G. Sun K. Chen F.H. Tao W.A. Subcellular co-delivery of two different site-oriented payloads based on multistage targeted polymeric nanoparticles for enhanced cancer therapy J Mater Chem B 6 2018 6752 6766 32254692 10.1039/c8tb02230e

81 Pan G.Y. Jia H.R. Zhu Y.X. Sun W. Cheng X.T. Wu F.G. Cyanine-containing polymeric nanoparticles with imaging/therapy-switchable capability for mitochondria-targeted cancer theranostics ACS Appl Nano Mater 1 2018 2885 2897

82 Lee S.Y. Cho H.J. Mitochondria targeting and destabilizing hyaluronic acid derivative-based nanoparticles for the delivery of lapatinib to triple-negative breast cancer Biomacromolecules 20 2018 835 845 30566834 10.1021/acs.biomac.8b01449

83 Xi J. Li M. Jing B. An M. Yu C. Pinnock C.B. Long-circulating amphiphilic doxorubicin for tumor mitochondria-specific targeting ACS Appl Mater Interfaces 10 2018 43482 43492 30479120 10.1021/acsami.8b17399 PMC6893847

84 Wei G. Wang Y. Huang X. Yang G. Zhao J. Zhou S. Induction of mitochondrial apoptosis for cancer therapy via dual-targeted cascade-responsive multifunctional micelles J Mater Chem B 6 2018 8137 8147 32254933 10.1039/c8tb02159g

85 Liu Y. Zhou Z. Lin X. Xiong X. Zhou R. Zhou M. Enhanced reactive oxygen species generation by mitochondria targeting of anticancer drug to overcome tumor multidrug resistance Biomacromolecules 20 2019 3755 3766 31465208 10.1021/acs.biomac.9b00800

86 Chen Z. Kang X. Wu Y. Xiao H. Cai X. Sheng S. A mitochondria targeting artesunate prodrug-loaded nanoparticle exerting anticancer activity via iron-mediated generation of the reactive oxygen species Chem Commun 55 2019 4781 4784 10.1039/c9cc00531e 30957121

87 Zhang W. Hu X. Shen Q. Xing D. Mitochondria-specific drug release and reactive oxygen species burst induced by polyprodrug nanoreactors can enhance chemotherapy Nat Commun 10 2019 1704 30979885 10.1038/s41467-019-09566-3 PMC6461692

88 Wang Y. Zhang T. Hou C. Zu M. Lu Y. Ma X. Mitochondria-specific anticancer drug delivery based on reduction-activated polyprodrug for enhancing the therapeutic effect of breast cancer chemotherapy ACS Appl Mater Interfaces 11 2019 29330 29340 31329411 10.1021/acsami.9b10211

89 Liu B.Y. Yang X.L. Xing X. Li J. Liu Y.H. Wang N. Trackable water-soluble prodrug micelles capable of rapid mitochondrial-targeting and alkaline pH-responsive drug release for highly improved anticancer efficacy ACS Macro Lett 8 2019 719 723 10.1021/acsmacrolett.9b00121 35619529

90 Kim S.W. Kyung Lee Y. Yeon Lee J. Hee Hong J. Khang D. PEGylated anticancer-carbon nanotubes complex targeting mitochondria of lung cancer cells Nanotechnology 28 2017 465102 10.1088/1361-6528/aa8c31 29053471

91 Liu Y. Li H. Xie J. Zhou M. Huang H. Lu H. Facile construction of mitochondria-targeting nanoparticles for enhanced phototherapeutic effects Biomater Sci 5 2017 1022 1031 28352857 10.1039/c6bm00878j

92 Zhang C. Liu Z. Zheng Y. Geng Y. Han C. Shi Y. Glycyrrhetinic acid functionalized graphene oxide for mitochondria targeting and cancer treatment in vivo Small 14 2018 1703306 10.1002/smll.201703306 29205852

93 Han C. Zhang C. Ma T. Zhang C. Luo J. Xu X. Hypericin-functionalized graphene oxide for enhanced mitochondria-targeting and synergistic anticancer effect Acta Biomater 77 2018 268 281 30006311 10.1016/j.actbio.2018.07.018

94 Zhang B. Yu Q. Zhang Y.-.M. Liu Y. Two-dimensional supramolecular assemblies based on β-cyclodextrin-grafted graphene oxide for mitochondrial dysfunction and photothermal therapy Chem Commun 55 2019 12200 12203 10.1039/c9cc05727g 31549132

95 Geng X. Sun Y. Li Z. Yang R. Zhao Y. Guo Y. Retrosynthesis of tunable fluorescent carbon dots for precise long-term mitochondrial tracking Small 15 2019 1901517 10.1002/smll.201901517 31165584

96 Gao G. Jiang Y.W. Yang J. Wu F.G. Mitochondria-targetable carbon quantum dots for differentiating cancerous cells from normal cells Nanoscale 9 2017 18368 18378 29143843 10.1039/c7nr06764j

97 Fan Z. Nie Y. Wei Y. Zhao J. Liao X. Zhang J. Facile and large-scale synthesis of graphene quantum dots for selective targeting and imaging of cell nucleus and mitochondria Mater Sci Eng C 103 2019 109824 10.1016/j.msec.2019.109824 31349464

98 Liu Y. Li Q. Xiong X. Zhou Z. Improved mitochondrial targeting effect of HPMA copolymer by SS20 peptide mediation and nonendocytosis pathway J Pept Sci 25 2019 e3144 30588703 10.1002/psc.3144

99 Cheng H. Zheng R.R. Fan G.L. Fan J.H. Zhao L.P. Jiang X.Y. Mitochondria and plasma membrane dual-targeted chimeric peptide for single-agent synergistic photodynamic therapy Biomaterials 188 2019 1 11 30312907 10.1016/j.biomaterials.2018.10.005

100 Chen R. Wang J. Qian C. Ji Y. Zhu C. Wu L. From nanofibers to nanorods: nanostructure of peptide-drug conjugates regulated by polypeptide-PEG derivative and enhanced antitumor effect Adv Funct Mater 29 2019 1806058

101 Shi M. Zhang J. Li X. Pan S. Li J. Yang C. Mitochondria-targeted delivery of doxorubicin to enhance antitumor activity with HER-2 peptide-mediated multifunctional pH-sensitive DQAsomes Int J Nanomed 13 2018 4209 4226 10.2147/IJN.S163858 PMC6054761 30140154

102 Zhao Q. Lin Y. Han N. Li X. Geng H. Wang X. Mesoporous carbon nanomaterials in drug delivery and biomedical application Drug Deliv 24 2017 94 107 29124979 10.1080/10717544.2017.1399300 PMC8812584

103 Liu Z. Chen K. Davis C. Sherlock S. Cao Q. Chen X. Drug delivery with carbon nanotubes for in vivo cancer treatment Cancer Res 68 2008 6652 6660 18701489 10.1158/0008-5472.CAN-08-1468 PMC2562710

104 Battigelli A. Russier J. Venturelli E. Fabbro C. Petronilli V. Bernardi P. Peptide-based carbon nanotubes for mitochondrial targeting Nanoscale 5 2013 9110 9117 23903095 10.1039/c3nr02694a

105 Wu C. Wang L. Tian Y. Guan X. Liu Q. Li S. “Triple-punch” anticancer strategy mediated by near-infrared photosensitizer/CpG oligonucleotides dual-dressed and mitochondria-targeted nanographene ACS Appl Mater Interfaces 10 2018 6942 6955 29400948 10.1021/acsami.7b18896

106 Biswas S. Dodwadkar N.S. Deshpande P.P. Torchilin V.P. Liposomes loaded with paclitaxel and modified with novel triphenylphosphonium-PEG-PE conjugate possess low toxicity, target mitochondria and demonstrate enhanced antitumor effects in vitro and in vivo J Control Rel 159 2012 393 402 10.1016/j.jconrel.2012.01.009 PMC3348446 22286008

107 Boddapati S V. Tongcharoensirikul P. Hanson R.N. D'Souza G.G.M. Torchilin V.P. Weissig V. Mitochondriotropic liposomes J Liposome Res 15 2005 49 58 16194927 10.1081/lpr-64958

108 Malhi S.S. Budhiraja A. Arora S. Chaudhari K.R. Nepali K. Kumar R. Intracellular delivery of redox cycler-doxorubicin to the mitochondria of cancer cell by folate receptor targeted mitocancerotropic liposomes Int J Pharm 432 2012 63 74 22531856 10.1016/j.ijpharm.2012.04.030

109 Kumari A. Yadav S.K. Yadav S.C. Biodegradable polymeric nanoparticles based drug delivery systems Colloids Surfaces B Biointerfaces 75 2010 1 18 19782542 10.1016/j.colsurfb.2009.09.001

110 Pathak R.K. Kolishetti N. Dhar S. Targeted nanoparticles in mitochondrial medicine Wiley Interdiscip Rev Nanomed Nanobiotechnol 7 2015 315 329 25348382 10.1002/wnan.1305 PMC4397104

111 Xu J. Xu P. Li Z. Huang J. Yang Z. Oxidative stress and apoptosis induced by hydroxyapatite nanoparticles in C6 cells J Biomed Mater Res Part A 100 2012 738 745 10.1002/jbm.a.33270 22213249

112 Sun Y. Chen Y. Ma X. Yuan Y. Liu C. Kohn J. Mitochondria-targeted hydroxyapatite nanoparticles for selective growth inhibition of lung cancer in vitro and in vivo ACS Appl Mater Interfaces 8 2016 25680 25690 27602785 10.1021/acsami.6b06094

113 Ma Z. Han K. Dai X. Han H. Precisely striking tumors without adjacent normal tissue damage via mitochondria-templated accumulation ACS Nano 12 2018 6252 6262 29791136 10.1021/acsnano.8b03212

114 Biswas S. Dodwadkar N.S. Piroyan A. Torchilin V.P. Surface conjugation of triphenylphosphonium to target poly (amidoamine) dendrimers to mitochondria Biomaterials 33 2012 4773 4782 22469294 10.1016/j.biomaterials.2012.03.032 PMC3725283

115 Wang X. Shao N. Zhang Q. Cheng Y. Mitochondrial targeting dendrimer allows efficient and safe gene delivery J Mater Chem B 2 2014 2546 2553 32261422 10.1039/c3tb21348j

116 Ma P. Chen J. Bi X. Li Z. Gao X. Li H. Overcoming multidrug resistance through the GLUT1-mediated and enzyme-triggered mitochondrial targeting conjugate with redox-sensitive paclitaxel release ACS Appl Mater Interfaces 10 2018 12351 12363 29569435 10.1021/acsami.7b18437

117 Hoye A.T. Davoren J.E. Wipf P. Fink M.P. Kagan V.E. Targeting mitochondria Acc Chem Res 41 2008 87 97 18193822 10.1021/ar700135m

118 Farsinejad S. Gheisary Z. Ebrahimi Samani S. Alizadeh A.M. Mitochondrial targeted peptides for cancer therapy Tumor Biol 36 2015 5715 5725 10.1007/s13277-015-3719-1 26142734

119 Bae Y. Joo C. Kim G.Y. Ko K.S. Huh K.M. Han J. Cationic oligopeptide-functionalized mitochondria targeting sequence show mitochondria targeting and anticancer activity Macromol Res 27 2019 1071 1080

120 Rautio J. Kumpulainen H. Heimbach T. Oliyai R. Oh D. Järvinen T. Prodrugs: design and clinical applications Nat Rev Drug Discov 7 2008 255 270 18219308 10.1038/nrd2468

121 Liu H.W. Hu X.X. Li K. Liu Y. Rong Q. Zhu L. A mitochondrial-targeted prodrug for NIR imaging guided and synergetic NIR photodynamic-chemo cancer therapy Chem Sci 8 2017 7689 7695 29619164 10.1039/c7sc03454g PMC5861986

122 Wang W. Liu J. Feng W. Du S. Ge R. Li J. Targeting mitochondria with Au–Ag@ polydopamine nanoparticles for papillary thyroid cancer therapy Biomater Sci 7 2019 1052 1063 30628592 10.1039/c8bm01414k

123 Mellman I. Coukos G. Dranoff G. Cancer immunotherapy comes of age Nature 480 2011 480 489 22193102 10.1038/nature10673 PMC3967235

124 Velcheti V. Schalper K. Basic Overview of current immunotherapy approaches in cancer Am Soc Clin Oncol Educ B 24 2016 298 308 10.1200/EDBK_156572 27249709

125 Marrache S. Tundup S. Harn D.A. Dhar S. Ex vivo programming of dendritic cells by mitochondria-targeted nanoparticles to produce interferon-gamma for cancer immunotherapy ACS Nano 7 2013 7392 7402 23899410 10.1021/nn403158n

126 Dai L. Li K. Li M. Zhao X. Luo Z. Lu L. Size/charge changeable acidity-responsive micelleplex for photodynamic-improved PD-L1 immunotherapy with enhanced tumor penetration Adv Funct Mater 28 2018 1707249

127 Li L. Yang W.W. Xu D.G. Stimuli-responsive nanoscale drug delivery systems for cancer therapy J Drug Target 27 2019 423 433 30173577 10.1080/1061186X.2018.1519029

128 Boissenot T. Bordat A. Fattal E. Tsapis N. Ultrasound-triggered drug delivery for cancer treatment using drug delivery systems: from theoretical considerations to practical applications J Control Rel 241 2016 144 163 10.1016/j.jconrel.2016.09.026 27667179

129 Chen H. Zhou X. Gao Y. Zheng B. Tang F. Huang J. Recent progress in development of new sonosensitizers for sonodynamic cancer therapy Drug Discov Today 19 2014 502 509 24486324 10.1016/j.drudis.2014.01.010

130 Zhang L. Yi H. Song J. Huang J. Yang K. Tan B. Mitochondria-targeted and ultrasound-activated nanodroplets for enhanced deep-penetration sonodynamic cancer therapy ACS Appl Mater Interfaces 11 2019 9355 9366 30734551 10.1021/acsami.8b21968

131 Li R. Peng F. Cai J. Yang D. Zhang P. Redox dual-stimuli responsive drug delivery systems for improving tumor-targeting ability and reducing adverse side effects Asian J Pharm Sci 2019 10.1016/j.ajps.2019.06.003 PMC7327776 32636949

132 Liu J. Liang H. Li M. Luo Z. Zhang J. Guo X. Tumor acidity activating multifunctional nanoplatform for NIR-mediated multiple enhanced photodynamic and photothermal tumor therapy Biomaterials 157 2018 107 124 29268142 10.1016/j.biomaterials.2017.12.003

133 Zhou W. Yu H. Zhang L.J. Wu B. Wang C.X. Wang Q. Redox-triggered activation of nanocarriers for mitochondria-targeting cancer chemotherapy Nanoscale 9 2017 17044 17053 29083424 10.1039/c7nr06130g

134 Giustini A.J. Petryk A.A. Cassim S.M. Tate J.A. Baker I. Hoopes P.J. Magnetic nanoparticle hyperthermia in cancer treatment Nano Life 1 2010 10 10.1142/S1793984410000067 PMC3859910 24348868

135 Pankhurst Q.A. Connolly J. Jones S.K. Dobson J. Applications of magnetic nanoparticles in biomedicine J Phys D Appl Phys 36 2003 R167

136 Shah B.P. Pasquale N. De G. Tan T. Ma J. Lee K.-.B. Core–shell nanoparticle-based peptide therapeutics and combined hyperthermia for enhanced cancer cell apoptosis ACS Nano 8 2014 9379 9387 25133971 10.1021/nn503431x PMC4174096

137 Liu D. Ma L. An Y. Li Y. Liu Y. Wang L. Thermoresponsive nanogel-encapsulated PEDOT and HSP70 inhibitor for improving the depth of the photothermal therapeutic effect Adv Funct Mater 26 2016 4749 4759

138 Rao V. Han H.S. Lee H. Jeon S. Jung D.W. Lee J. ROS-responsive mesoporous silica nanoparticles for MR imaging-guided photodynamically maneuvered chemotherapy Nanoscale 10 2018 9616 9627 29756137 10.1039/c8nr00888d

139 Shi M. Zhang J. Li X. Pan S. Li J. Yang C. Mitochondria-targeted delivery of doxorubicin to enhance antitumor activity with HER-2 peptide-mediated multifunctional pH-sensitive DQAsomes Int J Nanomed 13 2018 4209 10.2147/IJN.S163858 PMC6054761 30140154

140 Goyal A.K. Rath G. Faujdar C. Malik B. Application and perspective of pH-responsive nano drug delivery systems Mohapatra SS Ranjan S Dasgupta N Applications of targeted nano drugs and delivery systems 2019 Elsevier 15 33

141 Llopis J. McCaffery J.M. Miyawaki A. Farquhar M.G. Tsien R.Y. Measurement of cytosolic, mitochondrial, and Golgi pH in single living cells with green fluorescent proteins Proc Natl Acad Sci 95 1998 6803 9618493 10.1073/pnas.95.12.6803 PMC22642

142 Tan Y. Zhu Y. Zhao Y. Wen L. Meng T. Liu X. Mitochondrial alkaline pH-responsive drug release mediated by Celastrol loaded glycolipid-like micelles for cancer therapy Biomaterials 154 2018 169 181 29128845 10.1016/j.biomaterials.2017.07.036

143 Hu Q. Katti P.S. Gu Z. Enzyme-responsive nanomaterials for controlled drug delivery Nanoscale 6 2014 12273 12286 25251024 10.1039/c4nr04249b PMC4425417

144 Senbanjo L.T. Chellaiah M.A. CD44: a multifunctional cell surface adhesion receptor is a regulator of progression and metastasis of cancer cells Front Cell Dev Biol 5 2017 18 28326306 10.3389/fcell.2017.00018 PMC5339222

145 Naz S. Wang M. Han Y. Hu B. Teng L. Zhou J. Enzyme-responsive mesoporous silica nanoparticles for tumor cells and mitochondria multistage-targeted drug delivery Int J Nanomedicine 14 2019 2533 31114189 10.2147/IJN.S202210 PMC6497824

146 Vitorino C.V. Nanomedicine: principles, properties and regulatory issues Front Chem 6 2018 360 30177965 10.3389/fchem.2018.00360 PMC6109690

147 Bulbake U. Doppalapudi S. Kommineni N. Khan W. Liposomal formulations in clinical use: an updated review Pharmaceutics 9 2017 12 10.3390/pharmaceutics9020012 PMC5489929 28346375

148 Jain K.K. Ethical and regulatory aspects of nanomedicine The handbook of nanomedicine 2017 Humana Press New York 605 620

149 Sainz V. Conniot J. Matos A.I. Peres C. Zupanǒiǒ E. Moura L. Regulatory aspects on nanomedicines Biochem Biophys Res Commun 468 2015 504 510 26260323 10.1016/j.bbrc.2015.08.023

Acknowledgement

The corresponding author would like acknowledge the Department of Science and Technology and SERB ( INSPIRE Grant no: IFA-LSBM-13 and EMR/2016/007966/HS) for project funds. All other authors are grateful to the Ministry of Chemicals and Fertilizer for providing fellowship as allowing the conduct research on cancer at NIPER Ahmedabad.