1. Introduction
Bone metastasis is commonly associated with significant clinical complications which cause severe decrease in quality of life due to pain, skeletal-related events, such as pathologic fractures and spinal cord compression, and reduced patient mobility [ 1 , 2 ]. Furthermore, the presence of bone metastases is associated with a poor prognosis and reduced overall survival in both breast cancer and prostate cancer [ 3 , 4 , 5 , 6 , 7 ]. It is a recurring complication especially in prostate cancer, where bone is the primary site of metastatic disease [ 8 ]. In metastatic castration-resistant prostate cancer (mCRPC), bone metastases dominate the clinical picture of the disease, and bone metastasis has been shown to affect up to 90% of patients with mCRPC [ 8 , 9 , 10 ]. Bone metastasis is also highly frequent in breast, thyroid, and bladder cancer, as well as non-small-cell lung cancer and renal cell carcinoma [ 11 , 12 ]. In metastatic breast cancer, for example, ~71% of patients have been found to develop bone metastases [ 11 ]. Additionally, multiple myeloma is clinically characterized by the infiltration of the bone marrow with differentiated plasma cells that cause lytic lesions in large bones and vertebrae.
Bone metastases form as a result of disseminated tumor cells (DTCs) spreading to bone tissue and persisting there until progressing into metastatic growths. The metastatic process is initiated by activation of DTCs during bone remodeling and progresses into a vicious cycle of self-amplification. In the vicious cycle, factors secreted by cancer cells induce the activation of osteoclasts to break down bone and osteoblasts to form excessive new bone. This ultimately leads to a further release of growth factors and, consequently, added tumor growth. Breaking this vicious cycle has been proposed as a central strategy in the treatment of bone metastasis. Bisphosphonates and inhibitors of receptor activator of nuclear factor kappa-Β ligand (RANKL) have been used to prevent bone resorption, but new strategies are also needed to improve the quality of life and life expectancy of patients with cancers involving bone metastases.
Radium-223 belongs to the class of TATs. It receives its targeting properties from being an osteomimetic that homes into bone tissue, not only due to its physical similarity with calcium but also through active incorporation by osteoblasts [ 13 , 14 ]. It emits high-energy alpha particles that cause double-stranded DNA breaks (DSBs) eliciting cell death which is confined to the cancer cells and the surrounding tumor microenvironment due to the short range of the radiation, thus limiting toxic side effects. As an added benefit, the toxic effects of alpha radiation on the tumor microenvironment disrupt the activity of both osteoblasts and osteoclasts helping to break the vicious cycle of metastasis by interrupting the cross talk between cancer cells and osteoblast/osteoclasts [ 15 , 16 ]. New, promising therapy combination strategies are also emerging in conjunction with radium-223 therapy. Radium-223, which acts at a molecular level via the induction of difficult-to-repair, clustered DNA double strand breaks, shows potential of enhanced efficacy in combination with DNA damage repair inhibitors. Immuno-oncology treatments such as checkpoint inhibitors have provided an immense breakthrough in cancer therapy. However, widespread adoption of immune checkpoint therapy to treat cancers is obstructed by the low response rate in some cancer types [ 17 ]. In this setting, radium-223 treatment may enhance the activity of immuno-oncological treatments by various mechanisms.
In addition to the use of drug combination strategies to improve antitumor efficacy and increase survival, the use of the bone supportive agents such as denosumab and bisphosphonates, which are approved in mCRPC and other cancers that frequently result in the development of bone metastases, may also help improve the bone health safety profile of treatment strategies containing radium-223 [ 18 , 19 ]. The aim of this review is first, to elucidate the mode of action of radium-223 in breaking the vicious cycle of bone metastasis and second, to discuss how to utilize this unique feature in combination with novel emerging cancer treatments.
2. Disseminated Tumor Cells and Dormancy in Bone Metastasis
Bone metastases originate from DTCs that have left the primary tumor and entered the circulation, reaching the bone marrow and persisting in the new environment, with some of them eventually progressing to develop metastases. DTCs are known to harbor stem cell-like properties, such as low proliferative activity, resistance to apoptosis, unlimited ability for self-renewal, and differentiation [ 20 ]. The presence of DTCs is an independent factor of poor prognosis [ 21 , 22 , 23 ], and DTCs are often found in the bone marrow of patients with breast and prostate cancer at a very early phase [ 21 , 22 , 24 ]. Thus, it is vital to understand what makes tumor cells dormant, what wakes them up and induces their growth into metastases, and how cancer treatments affect this process.
It has been demonstrated that increased bone remodeling increases the number of bone metastases in both breast and prostate cancer mouse models [ 25 , 26 ]. It seems that the prevalence of bone metastases can be explained by the increased activation of the DTCs present, rather than by an increased number of cancer cells homing to bone tissues. Quite interestingly, the longest clinical tumor recurrence times, a decade and more, also coincide with the cycle of renewal of all bone in the skeleton, taking 10–20 years [ 27 ], underlining the relevance of remodeling in the formation of metastases.
One of the early events in a new bone remodeling cycle is the formation of new blood vessels in the remodeling site. Angiogenesis is necessary for normal bone remodeling, bone growth, and fracture repair, and it has been demonstrated that while stable endothelium supports tumor dormancy, activated sprouting endothelium in fact induces tumor growth [ 28 ]. Thrombospondin 1 binding on the stable endothelium has been described to be crucial for dormancy, whereas transforming growth factor β1 (TGF-β1) and periostin expression induces tumor growth near the sprouting vessels [ 29 ]. Accelerated bone turnover by parathyroid hormone (PTH) is also known to induce expansion of the hematopoietic stem cell (HSC) niche [ 30 ] which offers a safe haven for dormant tumor cells [ 31 ]. Thus, suppressing the level of bone turnover, that is, suppressing the activity of the bone-resorbing osteoclasts and the bone-forming osteoblasts, should be a viable strategy for keeping the DTCs in a dormant state, resulting in the prevention of bone metastases.
3. The Tumor Microenvironment in Bone Metastasis
The bone microenvironment is intricately involved in the formation of metastases, being involved in the process even before tumor cells leave the primary tumor. Facilitators of metastasis, including monocytes (macrophages), mesenchymal stem cells, and immature myeloid cells, are expanded in and mobilized from the bone marrow and guided to the primary tumor by cytokines secreted by the tumor [ 32 , 33 ]. Subsequently, through education by tumor cells, these cells become tumor-associated macrophages (TAMs), cancer-associated fibroblasts (CAFs), and myeloid-derived suppressor cells (MDSCs), respectively. These in turn assist in tumor growth and the metastatic process by inducing immune evasion, angiogenesis, epithelial-to-mesenchymal transition, and invasion [ 34 , 35 ]. In bone, the invading tumor cells induce changes in both osteoclasts and osteoblasts as well as their precursors. Depending on the activated cell types, tumor growth in bone results in either an osteolytic (bone resorbing), osteoblastic (bone forming), or a mixed bone lesion. In breast cancer, these lesions are mostly osteolytic (~75%), while the majority of prostate cancer bone metastases are osteoblastic [ 36 ]. It should be noted, however, that in most patients both lesion types are present, and at the cellular level, both bone resorption and formation is induced by tumor cells [ 37 ].
Bone metastases progress as a result of a vicious cycle of self-amplification, where factors secreted by the cancer cells induce the breakdown of bone or the formation of new bone, leading to a release of growth factors, further supporting tumor growth. Here, the osteolytic and osteoblastic cycles are presented separately for clarity, despite the fact that in a clinical setting both processes often co-exist in the same patient and even in the same metastatic site.
3.1. The Vicious Cycle of Osteolytic Bone Metastasis
In healthy bone tissue, bone formation and bone resorption are in homeostasis. In osteolytic metastases, resorption and formation become uncoupled due to factors secreted by cancer cells. These factors are able to induce osteoclast formation both directly and indirectly through osteoblasts ( Figure 1 ). Parathyroid hormone related peptide (PTHrP), which leads to increased local bone resorption, and TGF-β1, which is a ubiquitous growth factor in the bone microenvironment, were the first molecular players identified in cancer-induced osteolysis. Breast cancer cells produce and secrete PTHrP, which increases bone resorption indirectly by binding to the PTH receptor in osteoblasts [ 38 ]. Osteoblasts in turn secrete less osteoprotegerin (OPG), which reduces bone resorption, as well as more RANKL, which binds to the RANK on osteoclasts and induces differentiation and subsequently bone resorption. This results in an imbalance with excessive resorption-inducing signal molecules compared to formation-inducing molecules. Additional factors secreted by cancer cells, such as hypoxia-induced lysyl oxidase (LOX), colony stimulating factor 1 (CSF-1), tumor necrosis factor α (TNFα), vascular cell adhesion molecule 1 (VCAM-1), matrix metalloproteinase 1 (MMP1), Jagged 1, and interleukins 8 and 11, also induce the activation of osteoclasts, some of them directly, independently of RANK, enhancing the imbalance [ 27 , 32 , 39 , 40 , 41 ].
As the newly activated osteoclasts resorb bone, growth factors such as TGF-β, insulin-like growth factor 1 (IGF-1), and bone morphogenetic proteins (BMPs) that have been deposited in bone are released. They enhance the growth of cancer cells and the subsequent secretion of osteoclast activation factors. In addition to its effects in bone, TGF-β is also important in a plethora of cellular functions that can promote tumor formation, such as epithelial-to-mesenchymal transition, invasion, angiogenesis, and immune tolerance [ 42 ]. Moreover, the calcium released from bone as a result of resorption can act as a growth factor in cancer cells that express the calcium-sensing receptor [ 43 , 44 ]. Furthermore, immune cells are known to play an important part in the vicious cycle. Activated T cells express RANKL and are able to induce the differentiation of osteoclasts in vitro without any supporting chemokines [ 45 ]. TGF-β in turn inhibits the proliferation of T cells, helping tumor cells to evade their effects [ 46 ].
3.2. The Vicious Cycle of Osteoblastic Bone Metastasis
While most breast cancers are primarily osteolytic, prostate cancers tend to be osteoblastic, forming new, unorganized, and weak bone, or a clear mixture of the two types ( Figure 2 ). One constitutive factor behind the osteoblastic reaction seems to be prostate-specific antigen (PSA). PSA is a serine protease and cleaves many proteins important in bone metabolism, releasing, e.g., IGF-1 and TGF-β2 from their binding proteins to stimulate osteoblasts [ 47 ], as well as osteoblast-inactivating PTHrP [ 48 ]. Additionally, the induction of PSA through a transgene in a PSA-negative prostate cancer cell line has been shown to shift the bone reaction from osteolytic to osteoblastic [ 49 ]. However, in osteoblastic metastasis of prostate cancer, strong osteoclastic resorption is also typical, and bone resorption marker levels in patients with prostate cancer can be even higher than in patients with osteolytic breast cancer [ 50 , 51 ]. Osteoblasts need a resorbed bone surface onto which new bone can be formed and thus require an increased number of bone multicellular units, in addition to increased osteoblast number and activity within one bone multicellular unit, in order to achieve the vast amount of new, pathological bone observed in osteoblastic metastases.
Prostate cancer cells stimulate mesenchymal stem cells (MSCs) toward osteoblastic differentiation by secreting endothelin 1 (ET-1), wingless and Int1 proteins (Wnts), BMPs, TGFs, fibroblast growth factors (FGFs), and platelet-derived growth factors (PDGFs), leading to an increased number of osteoblasts. These factors also induce angiogenesis, expanding the HSC niche that prostate cancer cells can inhabit [ 27 ]. Interestingly, ET-1 has been found to be secreted also from breast cancer cells that produce osteoblastic lesions, underlining its overall significance in the osteoblastic lesion type [ 52 ].
The osteoblasts, in addition to forming new bone tissue, secrete RANKL, which induces osteoclast differentiation and bone resorption. This, as described above in the context of osteolytic metastasis, releases TGF-β, IGF-1, and BMPs, as well as calcium, which enhances the growth of cancer cells and drives the cycle forward.
4. Bone Supportive Agents
4.1. Bisphosphonates
Bisphosphonates are pyrophosphate analogs that bind to hydroxyapatite and inhibit bone resorption. Their mode of action is the inhibition of the mevalonate pathway in osteoclasts which disrupts the intracellular enzymatic functions needed for bone resorption [ 53 ]. Bisphosphonates are currently used as the main treatment for osteoporosis, and also for patients with bone metastases. Bisphosphonates have very few side-effects and are efficient in decreasing pain and protecting bone, thus preventing skeletal-related events [ 54 ]. In combination with radium-223, abiraterone, and prednisone/prednisolone, a lower fracture rate has been observed in patients receiving bisphosphonates or the RANKL inhibitor denosumab [ 55 ].
Two large clinical trials, AZURE and ABCSG-12, have been performed to study the efficacy of zoledronic acid in preventing breast cancer bone metastasis. The studies concluded that zoledronic acid increases disease-free survival in postmenopausal women and in women over 40 but not in premenopausal women [ 56 , 57 ]. Interestingly, patients with estrogen receptor-positive (ER+) cancer treated with goserelin, a bone resorption-inducing gonadotropin-releasing hormone agonist, benefitted from zoledronic acid treatment in ABCSG-12 [ 58 ], while ER+ patients that were not given goserelin in AZURE did not [ 56 ]. Thus, these results suggest that inhibiting increased bone resorption can benefit patients in risk of developing bone metastasis, namely by disrupting the osteolytic metastasis cycle or by preventing cancer cells either from finding refuge in the bone metastatic niche or awakening from dormancy. This is supported by a recent study demonstrating increased distant disease-free survival in postmenopausal patients with breast cancer on aromatase inhibitor therapy with oral osteoporosis treatment [ 59 ].
4.2. RANKL Inhibitor
The critical importance of the RANK/RANKL/OPG pathway in bone resorption led to the development of the fully human RANKL antibody denosumab, which is approved for the treatment of both osteoporosis and bone support of breast and prostate cancer. It has proven to be very effective in inhibiting bone resorption and preventing skeletal-related events in patients with breast and prostate cancer with bone metastases [ 60 , 61 ]. Similarly, RANKL inhibition has been shown to prevent the occurrence of bone metastases in patients with prostate cancer [ 62 ]. When used as an adjuvant supportive treatment in patients with breast cancer receiving aromatase inhibition therapy, denosumab seems to increase time to first fracture as well as improving disease-free survival [ 63 , 64 ]. Thus, the inhibition of the vicious metastasis cycle in bone tissue at a single point, such as RANKL signaling in the case of denosumab, seems to be an effective strategy in reducing the deleterious effects of bone metastases.
5. Mechanism of Action and Combination Potential of Radium-223
Radium-223 is a targeted alpha therapy (TAT) compound. TAT refers to a class of radioactive alpha emitting isotopes with short-range, high linear energy transfer (LET) of radiation with the potential of carrying vector conjugates [ 65 , 66 ]. TATs induce complex, difficult to repair DSBs that result in cytotoxicity [ 67 , 68 ], and there are no known resistance mechanisms towards alpha particles. Although TATs are delivered systemically, the short range (less than 100 µm/10 cell diameters) [ 69 ] and high LET delivers an end-organ scatter of alpha particles limited to cancer cells and the tumor microenvironment with less damage to surrounding normal tissues outside the alpha emission path range [ 65 , 70 , 71 ].
The biochemical properties of specific TAT compounds determine the feasibility of conjugation to a vector, as well as determining which of three possible targeting strategies the TAT compound exhibits: 1. self-targeting (physiologic integration by mimicking a natural essential ion); 2. passive targeting (accumulation in tumor areas with leaky vasculature); 3. active-induced targeting (specific ligand-receptor interactions between small molecule/peptide/antibody-labeled radionuclide and target cells) [ 72 ].
Radium-223 is a calcium-mimetic isotope that has been shown to incorporate into sites of increased bone turnover and osteoblastic activity due to both physical properties and active incorporation by osteoblasts, making it a self-targeting TAT [ 13 , 14 , 16 , 70 , 73 ]. Compared to beta radiation, for example, alpha radiation has a significantly shorter range and higher linear energy transfer which allows the treatment of metastatic tumors with minimal toxic effects to surrounding noncancerous tissue [ 74 ]. The shorter range of radiation results in milder myelotoxicity [ 13 , 75 ], and the higher LET leads to stronger induction of DSBs in tumor cells [ 76 ]. Therefore, radium-223 has become an appealing choice for the treatment of bone metastasis. For a summary of the most important nonclinical studies reviewed here, please see Table 1 .
Radium-223 is the first-in-class commercially available TAT approved for the treatment of patients with mCRPC with bone metastases. In addition to increasing overall survival in patients with mCRPC, radium-223 increases time to first symptomatic skeletal related event (SSE) and decreases the number of SSEs [ 85 , 86 ]. While radium-223 has shown signs of biological activity in advanced breast cancer patients with bone-dominant disease in early clinical trials [ 87 , 88 ], it remains unclear if this could ultimately translate into a survival advantage.
Preclinical studies have recently demonstrated that radium-223 is indeed incorporated into the bone matrix where it inhibits the proliferation of breast cancer cells and the differentiation of osteoblasts and osteoclasts. In an established osteolytic breast cancer bone metastasis setting, radium-223 prevented tumor-induced cachexia and decreased osteolysis by 56% and tumor growth by 43% [ 15 ]. Radium-223 also induced DSBs in cancer cells in vivo and extended survival as a monotherapy and in combination with zoledronic acid. Similar positive results have been demonstrated in the syngeneic 5TGM1 mouse multiple myeloma model, where radium-223 decreased osteolytic lesion area [ 79 ], and in the MX-1 osteolytic breast cancer bone metastasis model in rats, where radium-223 increased symptom-free survival [ 13 ].
The binding of radium-223 to bone tissue has been demonstrated to occur due to active incorporation by osteoblasts in addition to the passive binding of radium-223 as a calcium mimetic to hydroxyapatite [ 15 ]. Additionally, the differentiation of osteoclasts was shown to be inhibited dose-dependently by radium-233. This effect is driven either by direct disruption of osteoclast function by high energy alpha particles emitted by radium-223 deposited near them in areas of active bone remodeling or by inhibiting RANKL secretion by osteoblasts, or the combination of both of these mechanisms. Interestingly, radium-223 has been found to be deposited in the midst of prostate cancer growths in mice xenograft models [ 16 ], suggesting a direct effect on tumor metastasis. However, the extent and nature of co-localization with tumor cells remains to be elucidated.
Radium-223 also inhibits the vicious cycle of osteoblastic metastasis. In the LuCaP 58 patient-derived prostate cancer xenograft model, radium-223 was deposited in the intratumoral bone matrix and DNA DSBs were induced in cancer cells within 24 h after treatment [ 16 ]. This resulted in the inhibition of tumor-induced osteoblastic bone growth and the preservation of normal bone architecture, leading to reduced bone volume. These effects in tumor-bearing bone occurred through radium-223-induced suppression of abnormal bone metabolic activity as evidenced by a decreased number of osteoblasts and osteoclasts and a reduced level of the bone formation marker PINP. Furthermore, the treatment with radium-223 resulted in lower PSA values and reduced total tissue and tumor areas compared to the vehicle control, indicating that radium-223 constrains prostate cancer growth in bone. Similar findings were also obtained in the LNCaP prostate cancer xenograft model. Preclinical data, however, indicate that radium-223 may need to be combined with other agents to affect larger bone metastasis areas, especially in tumor types with weaker osteoblastic reactions [ 82 ].
The mode of action of radium-223 results in a very favorable risk-benefit profile, especially in terms of limited hematotoxicity. After intravenous injection, radium-223 is rapidly cleared from the bloodstream. Thereafter, radium-223 homes into bone tissue where the short range of emitted alpha particles largely limits the effects of radium-223 to the cells of the bone microenvironment, while excess radium-223 is excreted via the gut. In animal studies, no signs of bone marrow toxicity or body weight loss have been observed with therapeutic doses of radium-223 [ 13 ], and even high doses have not been shown to inactivate hematopoietic cells [ 77 ]. In good agreement with this, the favorable safety profile in clinical trials [ 89 , 90 ] is likely the result of its targeting properties, its mode of excretion, and the short range of the alpha particles, minimizing unwanted radiation to cells beyond the bone microenvironment, the bone marrow cells surrounding bone metastases, and the cells of the gastrointestinal epithelial layer [ 82 , 91 ]. In a retrospective analysis of 1021 patients treated with radium-223 dichloride alone, less than 10% of patients showed one of the most frequently observed adverse events related to radium-223 treatment, such as anemia, thrombocytopenia, fatigue, diarrhea, vomiting, and bone pain. Notably, most of these adverse events were also observed in placebo-treated patients in a randomized phase 3 trial [ 92 ]. The risk of hematological side effects may increase in combination with hematotoxic chemotherapeutics [ 55 , 93 , 94 ]. The potentially negative effects on bone health which may occur especially in certain combination regimens with 2nd generation anti-androgens in patients with disease mediated impairment of general bone health can be diminished by concomitant treatment with bisphosphonates or denosumab [ 19 , 55 ]. Specifically, the combination of radium-223 with abiraterone and prednisone/prednisolone is currently contraindicated in the European Union due to an increased fracture risk. In general, however, combination therapies with radium-223 are broadly feasible in the clinical setting.
5.1. Radium-223 Therapy in Combination with DNA Damage Repair Inhibitors
The ability of radium-223 to home into bone tissue and cause DNA DSBs to disrupt the function of both cancer and bone cells has led to considerable interest in using radium-233 in combination with DNA damage repair (DDR) inhibitors. The DDR is the name used collectively for a series of complex signaling pathways that secure the integrity of the genome in eukaryotic cells [ 95 ]. This combination is hypothesized to provide clinically relevant synergy in the induction of cancer cell death due to the mode of action of the two therapies—radium-223 treatment induces DSBs in the bone microenvironment which are then not repaired due to DDR treatment. Recent results with the inhibition of ATR (ataxia telangiectasia and Rad3-related) kinase, which is activated by a broad spectrum of DNA damages, including DSBs, in combination with radium-223 treatment have been promising. In the LNCaP intratibially injected prostate cancer model mimicking CRPC with bone metastases, combination treatment with radium-223 and an ATR inhibitor increased intratumoral levels of DNA damage and decreased total tumor lesion area and PSA values suggesting the potential of disrupting the vicious cycle of bone metastasis [ 80 ]. A recent in vitro study has additionally shown that ATR inhibition potentiates the inhibitory effect of radium-223 radiation on cell survival [ 83 ]. Radium-223 treatment has also been used together with the inhibition of poly (ADP-ribose) polymerase (PARP), which has been shown as monotherapy to be an effective strategy to treat cancers with certain types of DNA repair deficiencies [ 96 ]. This concept is further supported by an increase in antitumor efficacy observed when combining either ATR or PARP inhibition with actively-targeted TAT in the form of a mesothelin (MSLN)-targeted thorium conjugate (MSLN-TTC) [ 81 ]. Currently, a combination treatment with radium-223 and niraparib, an orally active small molecule PARP inhibitor, is also under evaluation in patients with CRPC with bone metastases ( NCT03076203 ).
5.2. Radium-223 Therapy in Combination with Immuno-Oncological Treatments
Radium-223 may also provide benefits when combined with immuno-oncological treatments. A recent preclinical study has demonstrated that radium-223 treatment enhances the sensitivity of various carcinomas to lysis by antigen-specific cytotoxic T lymphocytes through calreticulin-mediated immunogenic modulation [ 78 ]. This mode of action may well also inhibit the formation of bone metastases. Radiotherapy in general has been demonstrated to induce the release of danger-associated molecular patterns (DAMPs), such as CRT, HMGB 1 , and secreted ATP, which elicit antigen-presenting cells to activate CD8 + T cells to induce immunogenic cell death [ 97 , 98 ]. Radiotherapy is also known to upregulate major histocompatibility complex class I (MHC-I) expression [ 99 , 100 ], which is commonly downregulated in cancerous tissue [ 101 ] and associated with poor survival [ 102 , 103 ]. The increase in MHC-I expression together with novel proteins produced by immune cells in response to radiation [ 104 ] ultimately leads to the expansion of the T cell repertoire. Actively-targeted TAT in the form of mesothelin (MSLN)-targeted thorium conjugate MSLN-TTC has also been shown to induce immunogenic cell death and secretion of pro-inflammatory cytokines in vitro [ 84 ]. In addition to directly enhancing the function of the immune system, osteomimetic radionuclides, such as radium-223, target osteoclasts, which have recently been shown to have immunosuppressive properties [ 105 ]. With this in mind, the safety and tolerability of the immune checkpoint inhibitor atezolizumab, a fully humanized and monoclonal PD-L1 antibody, is currently being studied in combination with radium-223 in patients with mCRPC which has progressed following treatment with an androgen pathway inhibitor ( NCT02814669 ). The combination of radium-223 treatment with immune checkpoint blockade may also give rise to additional benefits outside the tumor microenvironment, as recent reports have called attention to the abscopal effect of combining immune checkpoint inhibition and irradiation, that is, the occurrence of systemic antitumor effects resulting in the regression of tumors outside of the irradiated area [ 106 ]. As PSA appears to be a constitutive factor in the osteoblastic reaction in bone metastatic prostate cancer, this abscopal effect may bring an added benefit to breaking the vicious cycle of metastasis.
6. Conclusions
Targeted alpha therapy with radium-223 provides a two-pronged approach to disrupting the vicious cycle of bone metastasis regardless of its type or tissue of origin, according to preclinical studies. Due to its bone-homing characteristics, radium-223 disrupts not only the activity of cancer cells by eliciting cancer cell death through DNA DSBs, but also the function of both osteoblasts and osteoclasts, thereby simultaneously breaking at least two links of the metastasis cycle.
The targeted nature of radium-223 and its short alpha radiation range cause relatively few side effects, providing a strong rationale for its use in combination treatments. Accordingly, studies related to combining radium-223 treatment with therapies of various modes of action, such as DNA damage repair inhibitors and immune checkpoint inhibitors, are currently under way. These therapy combinations have the potential to significantly improve on the current effectiveness of the treatment of bone metastatic CRPC and bone metastasis from additional tumor types.
Acknowledgments
Aurexel Life Sciences Ltd. ( www.aurexel.com ) is acknowledged for editorial support.
Author Contributions
All authors contributed to the writing, review, and editing of this article.
Funding
This research received no external funding. Editorial support was funded by Bayer AG.
Conflicts of Interest
M.I.S. holds ownership interest (including patents) in Pharmatest Services Ltd. A.S. holds ownership interest (including patents) in Bayer AG. S.-M.K. holds ownership interest in Aurexel Life Sciences Ltd.
References
1. Smith M.R. Skeletal-related events in prostate cancer Clin. Adv. Hematol. Oncol. H O 2012 10 535 536 23073052
2. Coleman R.E. The role of bone markers in metastatic bone disease Cancer Treat. Rev. 2006 32 Suppl. 1 1 2 10.1016/S0305-7372(06)80001-0 16680830
3. Cetin K. Christiansen C.F. Svaerke C. Jacobsen J.B. Sorensen H.T. Survival in patients with breast cancer with bone metastasis: A Danish population-based cohort study on the prognostic impact of initial stage of disease at breast cancer diagnosis and length of the bone metastasis-free interval BMJ Open 2015 5 e007702 10.1136/bmjopen-2015-007702 25926150 PMC4420974
4. Som A. Tu S.M. Liu J. Wang X. Qiao W. Logothetis C. Corn P.G. Response in bone turnover markers during therapy predicts overall survival in patients with metastatic prostate cancer: Analysis of three clinical trials Br. J. Cancer 2012 107 1547 1553 10.1038/bjc.2012.436 23033003 PMC3493771
5. Tait C. Moore D. Hodgson C. Brown M. Morris T. Growcott J. Malone M. Hughes A. Renehan A. Clarke N.W. Quantification of skeletal metastases in castrate-resistant prostate cancer predicts progression-free and overall survival BJU Int. 2014 114 E70 E73 10.1111/bju.12717 24589330
6. Fizazi K. Massard C. Smith M. Rader M. Brown J. Milecki P. Shore N. Oudard S. Karsh L. Carducci M. Bone-related Parameters are the Main Prognostic Factors for Overall Survival in Men with Bone Metastases from Castration-resistant Prostate Cancer Eur. Urol. 2015 68 42 50 10.1016/j.eururo.2014.10.001 25449207
7. Gobbini E. Ezzalfani M. Dieras V. Bachelot T. Brain E. Debled M. Jacot W. Mouret-Reynier M.A. Goncalves A. Dalenc F. Time trends of overall survival among metastatic breast cancer patients in the real-life ESME cohort Eur. J. Cancer 2018 96 17 24 10.1016/j.ejca.2018.03.015 29660596
8. Pezaro C. Omlin A. Lorente D. Rodrigues D.N. Ferraldeschi R. Bianchini D. Mukherji D. Riisnaes R. Altavilla A. Crespo M. Visceral disease in castration-resistant prostate cancer Eur. Urol. 2014 65 270 273 10.1016/j.eururo.2013.10.055 24295792 PMC4881819
9. Tannock I.F. de Wit R. Berry W.R. Horti J. Pluzanska A. Chi K.N. Oudard S. Theodore C. James N.D. Turesson I. Docetaxel plus prednisone or mitoxantrone plus prednisone for advanced prostate cancer N. Engl. J. Med. 2004 351 1502 1512 10.1056/NEJMoa040720 15470213
10. Freedland S.J. Richhariya A. Wang H. Chung K. Shore N.D. Treatment patterns in patients with prostate cancer and bone metastasis among US community-based urology group practices Urology 2012 80 293 298 10.1016/j.urology.2012.04.007 22748612
11. Savci-Heijink C.D. Halfwerk H. Hooijer G.K. Horlings H.M. Wesseling J. van de Vijver M.J. Retrospective analysis of metastatic behaviour of breast cancer subtypes Breast Cancer Res. Treat. 2015 150 547 557 10.1007/s10549-015-3352-0 25820592 PMC4383810
12. Coleman R.E. Skeletal complications of malignancy Cancer 1997 80 1588 1594 10.1002/(SICI)1097-0142(19971015)80:8+<1588::AID-CNCR9>3.0.CO;2-G 9362426
13. Henriksen G. Breistol K. Bruland O.S. Fodstad O. Larsen R.H. Significant antitumor effect from bone-seeking, alpha-particle-emitting (223)Ra demonstrated in an experimental skeletal metastases model Cancer Res. 2002 62 3120 3125 12036923
14. Salmon P.L. Onischuk Y.N. Bondarenko O.A. Lanyon L.E. Alpha-particle doses to cells of the bone remodeling cycle from alpha-particle-emitting bone-seekers: Indications of an antiresorptive effect of actinides Radiat. Res. 1999 152 S43 S47 10.2307/3580112 10564935
15. Suominen M.I. Rissanen J.P. Kakonen R. Fagerlund K.M. Alhoniemi E. Mumberg D. Ziegelbauer K. Halleen J.M. Kakonen S.M. Scholz A. Survival benefit with radium-223 dichloride in a mouse model of breast cancer bone metastasis J. Natl. Cancer Inst. 2013 105 908 916 10.1093/jnci/djt116 23682134
16. Suominen M.I. Fagerlund K.M. Rissanen J.P. Konkol Y.M. Morko J.P. Peng Z. Alhoniemi E.J. Laine S.K. Corey E. Mumberg D. Radium-223 Inhibits Osseous Prostate Cancer Growth by Dual Targeting of Cancer Cells and Bone Microenvironment in Mouse Models Clin. Cancer Res. 2017 23 4335 4346 10.1158/1078-0432.CCR-16-2955 28364014 PMC5540794
17. Darvin P. Toor S.M. Sasidharan Nair V. Elkord E. Immune checkpoint inhibitors: Recent progress and potential biomarkers Exp. Mol. Med. 2018 50 165 10.1038/s12276-018-0191-1 30546008 PMC6292890
18. Smith M.R. Parker C.C. Tombal B.F. Miller K. Saad F. Shen J. Zhang A. Kornacker M. Higano C.S. ERA 223: A phase 3 trial of radium-223 dichloride in combination with abiraterone acetate and prednisone in the treatment of asymptomatic or mildly symptomatic chemotherapy-naïve patients with bone-predominant metastatic castration-resistant prostate cancer J. Clin. Oncol. 2016 34 TPS5088 10.1200/JCO.2016.34.15_suppl.TPS5088
19. Tombal B.F. Loriot Y. Saad F. McDermott R.S. Elliott T. Rodriguez-Vida A. Nole F. Fournier B. Collette L. Gillessen S. Decreased fracture rate by mandating bone-protecting agents in the EORTC 1333/PEACE III trial comparing enzalutamide and Ra223 versus enzalutamide alone: An interim safety analysis Proceedings of the 2019 ASCO Annual Meeting Chicago, IL, USA 31 May–4 June 2019
20. Bao B. Ahmad A. Azmi A.S. Ali S. Sarkar F.H. Overview of cancer stem cells (CSCs) and mechanisms of their regulation: Implications for cancer therapy Curr. Protoc. Pharmacol. 2013 61 14 25 10.1002/0471141755.ph1425s61 PMC3733496 23744710
21. Morgan T.M. Lange P.H. Porter M.P. Lin D.W. Ellis W.J. Gallaher I.S. Vessella R.L. Disseminated tumor cells in prostate cancer patients after radical prostatectomy and without evidence of disease predicts biochemical recurrence Clin. Cancer Res. 2009 15 677 683 10.1158/1078-0432.CCR-08-1754 19147774 PMC3162324
22. Braun S. Vogl F.D. Naume B. Janni W. Osborne M.P. Coombes R.C. Schlimok G. Diel I.J. Gerber B. Gebauer G. A pooled analysis of bone marrow micrometastasis in breast cancer N. Engl. J. Med. 2005 353 793 802 10.1056/NEJMoa050434 16120859
23. Hartkopf A.D. Stefanescu D. Wallwiener M. Hahn M. Becker S. Solomayer E.F. Fehm T.N. Brucker S.Y. Taran F.A. Tumor cell dissemination to the bone marrow and blood is associated with poor outcome in patients with metastatic breast cancer Breast Cancer Res. Treat. 2014 147 345 351 10.1007/s10549-014-3113-5 25151295
24. Sanger N. Effenberger K.E. Riethdorf S. Van Haasteren V. Gauwerky J. Wiegratz I. Strebhardt K. Kaufmann M. Pantel K. Disseminated tumor cells in the bone marrow of patients with ductal carcinoma in situ Int. J. Cancer 2011 129 2522 2526 10.1002/ijc.25895 21207426
25. Ottewell P.D. Wang N. Brown H.K. Reeves K.J. Fowles C.A. Croucher P.I. Eaton C.L. Holen I. Zoledronic acid has differential antitumor activity in the pre- and postmenopausal bone microenvironment in vivo Clin. Cancer Res. 2014 20 2922 2932 10.1158/1078-0432.CCR-13-1246 24687923 PMC4040234
26. Ottewell P.D. Wang N. Meek J. Fowles C.A. Croucher P.I. Eaton C.L. Holen I. Castration-induced bone loss triggers growth of disseminated prostate cancer cells in bone Endocr. Relat. Cancer 2014 21 769 781 10.1530/ERC-14-0199 25052474
27. Hensel J. Thalmann G.N. Biology of Bone Metastases in Prostate Cancer Urology 2016 92 6 13 10.1016/j.urology.2015.12.039 26768714
28. Ghajar C.M. Peinado H. Mori H. Matei I.R. Evason K.J. Brazier H. Almeida D. Koller A. Hajjar K.A. Stainier D.Y. The perivascular niche regulates breast tumour dormancy Nat. Cell Biol. 2013 15 807 817 10.1038/ncb2767 23728425 PMC3826912
29. Ghajar C.M. Disseminated Tumor Cell Dormancy by distant microenvironments (abstract) Proceedings of the 107th Annual Meeting of American Association of Cancer Research New Orleans, LA, USA 16–20 April 2016
30. Calvi L.M. Osteoblastic activation in the hematopoietic stem cell niche Ann. N. Y. Acad. Sci. 2006 1068 477 488 10.1196/annals.1346.021 16831945
31. Shiozawa Y. Pienta K.J. Taichman R.S. Hematopoietic stem cell niche is a potential therapeutic target for bone metastatic tumors Clin. Cancer Res. 2011 17 5553 5558 10.1158/1078-0432.CCR-10-2505 21676926 PMC3593121
32. Ren G. Esposito M. Kang Y. Bone metastasis and the metastatic niche J. Mol. Med. 2015 93 1203 1212 10.1007/s00109-015-1329-4 26275789 PMC4636917
33. Giles A.J. Reid C.M. Evans J.D. Murgai M. Vicioso Y. Highfill S.L. Kasai M. Vahdat L. Mackall C.L. Lyden D. Activation of Hematopoietic Stem/Progenitor Cells Promotes Immunosuppression Within the Pre-metastatic Niche Cancer Res. 2016 76 1335 1347 10.1158/0008-5472.CAN-15-0204 26719537 PMC4794356
34. Giannoni E. Bianchini F. Calorini L. Chiarugi P. Cancer associated fibroblasts exploit reactive oxygen species through a proinflammatory signature leading to epithelial mesenchymal transition and stemness Antioxid. Redox Signal. 2011 14 2361 2371 10.1089/ars.2010.3727 21235356
35. Barcellos-de-Souza P. Comito G. Pons-Segura C. Taddei M.L. Gori V. Becherucci V. Bambi F. Margheri F. Laurenzana A. Del Rosso M. Mesenchymal Stem Cells are Recruited and Activated into Carcinoma-Associated Fibroblasts by Prostate Cancer Microenvironment-Derived TGF-beta1 Stem Cells 2016 34 2536 2547 10.1002/stem.2412 27300750
36. Mundy G.R. Metastasis to bone: Causes, consequences and therapeutic opportunities Nat. Rev. Cancer 2002 2 584 593 10.1038/nrc867 12154351
37. Guise T.A. Mohammad K.S. Clines G. Stebbins E.G. Wong D.H. Higgins L.S. Vessella R. Corey E. Padalecki S. Suva L. Basic mechanisms responsible for osteolytic and osteoblastic bone metastases Clin. Cancer Res. 2006 12 6213s 6216s 10.1158/1078-0432.CCR-06-1007 17062703
38. Guise T.A. Yin J.J. Taylor S.D. Kumagai Y. Dallas M. Boyce B.F. Yoneda T. Mundy G.R. Evidence for a causal role of parathyroid hormone-related protein in the pathogenesis of human breast cancer-mediated osteolysis J. Clin. Investig. 1996 98 1544 1549 10.1172/JCI118947 8833902 PMC507586
39. Morgan H. Tumber A. Hill P.A. Breast cancer cells induce osteoclast formation by stimulating host IL-11 production and downregulating granulocyte/macrophage colony-stimulating factor Int. J. Cancer 2004 109 653 660 10.1002/ijc.20056 14999770
40. Sotiriou C. Lacroix M. Lespagnard L. Larsimont D. Paesmans M. Body J.J. Interleukins-6 and -11 expression in primary breast cancer and subsequent development of bone metastases Cancer Lett. 2001 169 87 95 10.1016/S0304-3835(01)00524-9 11410329
41. Cox T.R. Rumney R.M.H. Schoof E.M. Perryman L. Hoye A.M. Agrawal A. Bird D. Latif N.A. Forrest H. Evans H.R. The hypoxic cancer secretome induces pre-metastatic bone lesions through lysyl oxidase Nature 2015 522 106 110 10.1038/nature14492 26017313 PMC4961239
42. Juarez P. Guise T.A. TGF-beta in cancer and bone: Implications for treatment of bone metastases Bone 2011 48 23 29 10.1016/j.bone.2010.08.004 20699127
43. Saidak Z. Boudot C. Abdoune R. Petit L. Brazier M. Mentaverri R. Kamel S. Extracellular calcium promotes the migration of breast cancer cells through the activation of the calcium sensing receptor Exp. Cell Res. 2009 315 2072 2080 10.1016/j.yexcr.2009.03.003 19285978
44. Kim W. Takyar F.M. Swan K. Jeong J. VanHouten J. Sullivan C. Dann P. Yu H. Fiaschi-Taesch N. Chang W. Calcium-Sensing Receptor Promotes Breast Cancer by Stimulating Intracrine Actions of Parathyroid Hormone-Related Protein Cancer Res. 2016 76 5348 5360 10.1158/0008-5472.CAN-15-2614 27450451 PMC5026591
45. Silva M.J. Kajiya M. AlShwaimi E. Sasaki H. Hong J. Ok P. Rezende T.M. Pagonis T.C. White R.R. Paster B.J. Bacteria-reactive immune response may induce RANKL-expressing T cells in the mouse periapical bone loss lesion J. Endod. 2012 38 346 350 10.1016/j.joen.2011.12.029 22341072 PMC3291477
46. D’Amico L. Roato I. The Impact of Immune System in Regulating Bone Metastasis Formation by Osteotropic Tumors J. Immunol. Res. 2015 2015 143526 10.1155/2015/143526 26064994 PMC4433688
47. Rehault S. Monget P. Mazerbourg S. Tremblay R. Gutman N. Gauthier F. Moreau T. Insulin-like growth factor binding proteins (IGFBPs) as potential physiological substrates for human kallikreins hK2 and hK3 Eur. J. Biochem. 2001 268 2960 2968 10.1046/j.1432-1327.2001.02185.x 11358513
48. Cramer S.D. Chen Z. Peehl D.M. Prostate specific antigen cleaves parathyroid hormone-related protein in the PTH-like domain: Inactivation of PTHrP-stimulated cAMP accumulation in mouse osteoblasts J. Urol. 1996 156 526 531 10.1016/S0022-5347(01)65919-6 8683730
49. Cumming A.P. Hopmans S.N. Vukmirovic-Popovic S. Duivenvoorden W.C. PSA affects prostate cancer cell invasion in vitro and induces an osteoblastic phenotype in bone in vivo Prostate Cancer Prostatic Dis. 2011 14 286 294 10.1038/pcan.2011.34 21826098
50. Mountzios G. Terpos E. Syrigos K. Papadimitriou C. Papadopoulos G. Bamias A. Mavrikakis M. Dimopoulos M.A. Markers of bone remodeling and skeletal morbidity in patients with solid tumors metastatic to the skeleton receiving the biphosphonate zoledronic acid Transl. Res. 2010 155 247 255 10.1016/j.trsl.2010.01.002 20403580
51. Roato I. D’Amelio P. Gorassini E. Grimaldi A. Bonello L. Fiori C. Delsedime L. Tizzani A. De Libero A. Isaia G. Osteoclasts are active in bone forming metastases of prostate cancer patients PLoS ONE 2008 3 e3627 10.1371/journal.pone.0003627 18978943 PMC2574033
52. Yin J.J. Mohammad K.S. Kakonen S.M. Harris S. Wu-Wong J.R. Wessale J.L. Padley R.J. Garrett I.R. Chirgwin J.M. Guise T.A. A causal role for endothelin-1 in the pathogenesis of osteoblastic bone metastases Proc. Natl. Acad. Sci. USA 2003 100 10954 10959 10.1073/pnas.1830978100 12941866 PMC196909
53. Coxon F.P. Thompson K. Roelofs A.J. Ebetino F.H. Rogers M.J. Visualizing mineral binding and uptake of bisphosphonate by osteoclasts and non-resorbing cells Bone 2008 42 848 860 10.1016/j.bone.2007.12.225 18325866
54. Kanis J.A. McCloskey E.V. Johansson H. Cooper C. Rizzoli R. Reginster J.Y. European guidance for the diagnosis and management of osteoporosis in postmenopausal women Osteoporos. Int. 2013 24 23 57 10.1007/s00198-012-2074-y 23079689 PMC3587294
55. Smith M. Parker C. Saad F. Miller K. Tombal B. Ng Q.S. Boegemann M. Matveev V. Piulats J.M. Zucca L.E. Addition of radium-223 to abiraterone acetate and prednisone or prednisolone in patients with castration-resistant prostate cancer and bone metastases (ERA 223): A randomised, double-blind, placebo-controlled, phase 3 trial Lancet Oncol. 2019 20 408 419 10.1016/S1470-2045(18)30860-X 30738780
56. Coleman R. Cameron D. Dodwell D. Bell R. Wilson C. Rathbone E. Keane M. Gil M. Burkinshaw R. Grieve R. Adjuvant zoledronic acid in patients with early breast cancer: Final efficacy analysis of the AZURE (BIG 01/04) randomised open-label phase 3 trial Lancet Oncol. 2014 15 997 1006 10.1016/S1470-2045(14)70302-X 25035292
57. Gnant M. Mlineritsch B. Stoeger H. Luschin-Ebengreuth G. Heck D. Menzel C. Jakesz R. Seifert M. Hubalek M. Pristauz G. Adjuvant endocrine therapy plus zoledronic acid in premenopausal women with early-stage breast cancer: 62-month follow-up from the ABCSG-12 randomised trial Lancet Oncol. 2011 12 631 641 10.1016/S1470-2045(11)70122-X 21641868
58. Wilson C. Hinsley S. Marshall H. Cameron D. Bell R. Dodwell D. Coleman R.E. Reproductive hormone analyses and effects of adjuvant zoledronic acid in early breast cancer—An AZURE (BIG 01/04) sub-study J. Bone Oncol. 2017 9 48 54 10.1016/j.jbo.2016.10.005 29234591 PMC5715229
59. Lipton A. Chapman J.W. Leitzel K. Garg A. Pritchard K.I. Ingle J.N. Budd G.T. Ellis M.J. Sledge G.W. Rabaglio M. Osteoporosis therapy and outcomes for postmenopausal patients with hormone receptor-positive breast cancer: NCIC CTG MA.27 Cancer 2017 123 2444 2451 10.1002/cncr.30682 28464211
60. Fizazi K. Carducci M. Smith M. Damiao R. Brown J. Karsh L. Milecki P. Shore N. Rader M. Wang H. Denosumab versus zoledronic acid for treatment of bone metastases in men with castration-resistant prostate cancer: A randomised, double-blind study Lancet 2011 377 813 822 10.1016/S0140-6736(10)62344-6 21353695 PMC3090685
61. Lipton A. Fizazi K. Stopeck A.T. Henry D.H. Brown J.E. Yardley D.A. Richardson G.E. Siena S. Maroto P. Clemens M. Superiority of denosumab to zoledronic acid for prevention of skeletal-related events: A combined analysis of 3 pivotal, randomised, phase 3 trials Eur. J. Cancer 2012 48 3082 3092 10.1016/j.ejca.2012.08.002 22975218
62. Smith M.R. Saad F. Oudard S. Shore N. Fizazi K. Sieber P. Tombal B. Damiao R. Marx G. Miller K. Denosumab and bone metastasis-free survival in men with nonmetastatic castration-resistant prostate cancer: Exploratory analyses by baseline prostate-specific antigen doubling time J. Clin. Oncol. 2013 31 3800 3806 10.1200/JCO.2012.44.6716 24043751 PMC3795889
63. Gnant M. Pfeiler G. Dubsky P.C. Hubalek M. Greil R. Jakesz R. Wette V. Balic M. Haslbauer F. Melbinger E. Adjuvant denosumab in breast cancer (ABCSG-18): A multicentre, randomised, double-blind, placebo-controlled trial Lancet 2015 386 433 443 10.1016/S0140-6736(15)60995-3 26040499
64. Gnant M. Pfeiler G. Steger G.G. Egle D. Greil R. Fitzal F. Wette V. Balic M. Haslbauer F. Melbinger-Zeinitzer E. Adjuvant denosumab in postmenopausal patients with hormone receptor-positive breast cancer (ABCSG-18): Disease-free survival results from a randomised, double-blind, placebo-controlled, phase 3 trial Lancet Oncol. 2019 20 339 351 10.1016/S1470-2045(18)30862-3 30795951
65. Brechbiel M.W. Targeted alpha-therapy: Past, present, future? Dalton Trans. 2007 4918 4928 10.1039/b704726f 17992276 PMC2408446
66. Targeted Alpha Therapy Working G. Parker C. Lewington V. Shore N. Kratochwil C. Levy M. Linden O. Noordzij W. Park J. Saad F. Targeted Alpha Therapy, an Emerging Class of Cancer Agents: A Review JAMA Oncol. 2018 4 1765 1772 10.1001/jamaoncol.2018.4044 30326033
67. Sgouros G. Roeske J.C. McDevitt M.R. Palm S. Allen B.J. Fisher D.R. Brill A.B. Song H. Howell R.W. Akabani G. MIRD Pamphlet No. 22 (abridged): Radiobiology and dosimetry of alpha-particle emitters for targeted radionuclide therapy J. Nucl. Med. 2010 51 311 328 10.2967/jnumed.108.058651 20080889 PMC5680544
68. Nevedomskaya E. Baumgart S.J. Haendler B. Recent Advances in Prostate Cancer Treatment and Drug Discovery Int. J. Mol. Sci. 2018 19 1359 10.3390/ijms19051359 PMC5983695 29734647
69. Du Y. Carrio I. De Vincentis G. Fanti S. Ilhan H. Mommsen C. Nitzsche E. Sundram F. Vogel W. Oyen W. Practical recommendations for radium-223 treatment of metastatic castration-resistant prostate cancer Eur. J. Nucl. Med. Mol. Imaging 2017 44 1671 1678 10.1007/s00259-017-3756-7 28631036 PMC5537325
70. Marques I.A. Neves A.R. Abrantes A.M. Pires A.S. Tavares-da-Silva E. Figueiredo A. Botelho M.F. Targeted alpha therapy using Radium-223: From physics to biological effects Cancer Treat. Rev. 2018 68 47 54 10.1016/j.ctrv.2018.05.011 29859504
71. Dekempeneer Y. Keyaerts M. Krasniqi A. Puttemans J. Muyldermans S. Lahoutte T. D’Huyvetter M. Devoogdt N. Targeted alpha therapy using short-lived alpha-particles and the promise of nanobodies as targeting vehicle Expert Opin. Biol. Ther. 2016 16 1035 1047 10.1080/14712598.2016.1185412 27145158 PMC4940885
72. Kozempel J. Mokhodoeva O. Vlk M. Progress in Targeted Alpha-Particle Therapy. What We Learned about Recoils Release from In Vivo Generators Molecules 2018 23 581 10.3390/molecules23030581 PMC6017877 29510568
73. Jadvar H. Quinn D.I. Targeted alpha-particle therapy of bone metastases in prostate cancer Clin. Nucl. Med. 2013 38 966 971 10.1097/RLU.0000000000000290 24212441 PMC3874447
74. Pouget J.P. Navarro-Teulon I. Bardies M. Chouin N. Cartron G. Pelegrin A. Azria D. Clinical radioimmunotherapy--the role of radiobiology Nat. Rev. Clin. Oncol. 2011 8 720 734 10.1038/nrclinonc.2011.160 22064461
75. Baczyk M. Radioisotope therapy of bone metastases Nucl. Med. Rev. 2011 14 96 104 10.5603/NMR.2011.00023 22219150
76. Kassis A.I. Adelstein S.J. Radiobiologic principles in radionuclide therapy J. Nucl. Med. 2005 46 Suppl. 1 4S 12S 15653646
77. Larsen R.H. Saxtorph H. Skydsgaard M. Borrebaek J. Jonasdottir T.J. Bruland O.S. Klastrup S. Harling R. Ramdahl T. Radiotoxicity of the alpha-emitting bone-seeker 223Ra injected intravenously into mice: Histology, clinical chemistry and hematology In Vivo 2006 20 325 331 16724665
78. Malamas A.S. Gameiro S.R. Knudson K.M. Hodge J.W. Sublethal exposure to alpha radiation (223Ra dichloride) enhances various carcinomas’ sensitivity to lysis by antigen-specific cytotoxic T lymphocytes through calreticulin-mediated immunogenic modulation Oncotarget 2016 7 86937 86947 10.18632/oncotarget.13520 27893426 PMC5341329
79. Suominen M.I. Kähkönen T. Mäki-Jouppila J. Rissanen J.P. Luostarinen A. Fagerlund K.M. Huhtinen A. Sjöholm B. Alhoniemi E. Siemeiste G. Ra-223 and bortezomib combination in a MM model Proceedings of the American Association for Cancer Research Annual Meeting 2017 Washington, DC, USA 1–5 April 2017 AACR Philadelphia, PA, USA 2017
80. Wengner A.M. Siemeister G. Luecking U. Lefranc J. Scholz A. Suominen M. Meyer K. Lagkadinou E. Mumberg D. Synergistic in vivo activity of the ATR inhibitor BAY 1895344 in combination with the targeted alpha therapy radium-223 dichloride in a preclinical tumor model mimicking bone metastatic castration-resistant prostate cancer (mCRPC) Proceedings of the American Association for Cancer Research Annual Meeting 2018 Chicago, IL, USA 14–18 April 2018 AACR Philadelphia, PA, USA 2018
81. Wickstroem K. Hagemann U.B. Cruciani V. Wengner A.M. Kristian A. Ellingsen C. Siemeister G. Bjerke R.M. Karlsson J. Ryan O.B. Synergistic effect of mesothelin (MSLN)-TTC in combination with DNA damage response inhibitors Proceedings of the American Association for Cancer Research Annual Meeting 2018 Chicago, IL, USA 14–18 April 2018 AACR Philadelphia, PA, USA 2018
82. Dondossola E. Casarin S. Paindelli C. De-Juan-Pardo E.M. Hutmacher D.W. Logothetis C.J. Friedl P. Radium 223-mediated zonal cytotoxicity of prostate cancer in bone J. Natl. Cancer Inst. 2019 10.1093/jnci/djz007 PMC6792112 30657953
83. Bannik K. Zitzmann-Kolbe S. Scholz A. Jarke S. Jarzombek M. Sutter A. Siemeister G. Mumberg D. Radium-223 α-particle radiation: Characterization of the in vitro effects on cancer cells in monotherapy and in combination with DNA repair inhibitors Proceedings of the American Association for Cancer Research Annual Meeting 2019 Atlanta, GA, USA 29 March–3 April 2019 AACR Philadelphia, PA, USA 2019
84. Hagemann U.B. Zitzmann-Kolbe S. Kristian A. Sperl C. Schatz C.A. Bjerke R.M. Cuthbertson A.S. Hennekes H. Ziegelbauer K. Mumberg D. MSLN-TTC (BAY 2287411) demonstrates increased activity in comparison to standard of care chemotherapy in models of acquired drug resistance Proceedings of the American Association for Cancer Research Annual Meeting 2019 Atlanta, GA, USA 29 March–3 April 2019 AACR Philadelphia, PA, USA 2019
85. Hoskin P. Sartor O. O’Sullivan J.M. Johannessen D.C. Helle S.I. Logue J. Bottomley D. Nilsson S. Vogelzang N.J. Fang F. Efficacy and safety of radium-223 dichloride in patients with castration-resistant prostate cancer and symptomatic bone metastases, with or without previous docetaxel use: A prespecified subgroup analysis from the randomised, double-blind, phase 3 ALSYMPCA trial Lancet Oncol. 2014 15 1397 1406 10.1016/S1470-2045(14)70474-7 25439694
86. Wissing M.D. van Leeuwen F.W. van der Pluijm G. Gelderblom H. Radium-223 chloride: Extending life in prostate cancer patients by treating bone metastases Clin. Cancer Res. 2013 19 5822 5827 10.1158/1078-0432.CCR-13-1896 24052017
87. Coleman R. Aksnes A.K. Naume B. Garcia C. Jerusalem G. Piccart M. Vobecky N. Thuresson M. Flamen P. A phase IIa, nonrandomized study of radium-223 dichloride in advanced breast cancer patients with bone-dominant disease Breast Cancer Res. Treat. 2014 145 411 418 10.1007/s10549-014-2939-1 24728613 PMC4025174
88. Takalkar A. Adams S. Subbiah V. Radium-223 dichloride bone-targeted alpha particle therapy for hormone-refractory breast cancer metastatic to bone Exp. Hematol. Oncol. 2014 3 23 10.1186/2162-3619-3-23 25243101 PMC4168986
89. Nilsson S. Larsen R.H. Fossa S.D. Balteskard L. Borch K.W. Westlin J.E. Salberg G. Bruland O.S. First clinical experience with alpha-emitting radium-223 in the treatment of skeletal metastases Clin. Cancer Res. 2005 11 4451 4459 10.1158/1078-0432.CCR-04-2244 15958630
90. Nilsson S. Franzen L. Parker C. Tyrrell C. Blom R. Tennvall J. Lennernas B. Petersson U. Johannessen D.C. Sokal M. Two-year survival follow-up of the randomized, double-blind, placebo-controlled phase II study of radium-223 chloride in patients with castration-resistant prostate cancer and bone metastases Clin. Genitourin. Cancer 2013 11 20 26 10.1016/j.clgc.2012.07.002 23021204
91. Elgqvist J. Targeted alpha therapy: Part I Curr. Radiopharm. 2011 4 176 10.2174/1874471011104030176 22201706
92. Parker C. Nilsson S. Heinrich D. Helle S.I. O’Sullivan J.M. Fossa S.D. Chodacki A. Wiechno P. Logue J. Seke M. Alpha emitter radium-223 and survival in metastatic prostate cancer N. Engl. J. Med. 2013 369 213 223 10.1056/NEJMoa1213755 23863050
93. Morris M.J. Loriot Y. Sweeney C.J. Fizazi K. Ryan C.J. Shevrin D.H. Antonarakis E.S. Pandit-Taskar N. Deandreis D. Jacene H.A. Radium-223 in combination with docetaxel in patients with castration-resistant prostate cancer and bone metastases: A phase 1 dose escalation/randomised phase 2a trial Eur. J. Cancer 2019 114 107 116 10.1016/j.ejca.2019.04.007 31082669 PMC7474951
94. Soldatos T.G. Iakovou I. Sachpekidis C. Retrospective Toxicological Profiling of Radium-223 Dichloride for the Treatment of Bone Metastases in Prostate Cancer Using Adverse Event Data Medicina 2019 55 149 10.3390/medicina55050149 PMC6572036 31100964
95. Ciccia A. Elledge S.J. The DNA damage response: Making it safe to play with knives Mol. Cell 2010 40 179 204 10.1016/j.molcel.2010.09.019 20965415 PMC2988877
96. Bouwman P. Jonkers J. The effects of deregulated DNA damage signalling on cancer chemotherapy response and resistance Nat. Rev. Cancer 2012 12 587 598 10.1038/nrc3342 22918414
97. Hernandez C. Huebener P. Schwabe R.F. Damage-associated molecular patterns in cancer: A double-edged sword Oncogene 2016 35 5931 5941 10.1038/onc.2016.104 27086930 PMC5119456
98. Wu Q. Allouch A. Martins I. Brenner C. Modjtahedi N. Deutsch E. Perfettini J.L. Modulating Both Tumor Cell Death and Innate Immunity Is Essential for Improving Radiation Therapy Effectiveness Front. Immunol. 2017 8 613 10.3389/fimmu.2017.00613 28603525 PMC5445662
99. Wan S. Pestka S. Jubin R.G. Lyu Y.L. Tsai Y.C. Liu L.F. Chemotherapeutics and radiation stimulate MHC class I expression through elevated interferon-beta signaling in breast cancer cells PLoS ONE 2012 7 e32542 10.1371/journal.pone.0032542 22396773 PMC3291570
100. Wang X. Schoenhals J.E. Li A. Valdecanas D.R. Ye H. Zang F. Tang C. Tang M. Liu C.G. Liu X. Suppression of Type I IFN Signaling in Tumors Mediates Resistance to Anti-PD-1 Treatment That Can Be Overcome by Radiotherapy Cancer Res. 2017 77 839 850 10.1158/0008-5472.CAN-15-3142 27821490 PMC5875182
101. Garcia-Lora A. Martinez M. Algarra I. Gaforio J.J. Garrido F. MHC class I-deficient metastatic tumor variants immunoselected by T lymphocytes originate from the coordinated downregulation of APM components Int. J. Cancer 2003 106 521 527 10.1002/ijc.11241 12845647
102. Turcotte S. Katz S.C. Shia J. Jarnagin W.R. Kingham T.P. Allen P.J. Fong Y. D’Angelica M.I. DeMatteo R.P. Tumor MHC class I expression improves the prognostic value of T-cell density in resected colorectal liver metastases Cancer Immunol. Res. 2014 2 530 537 10.1158/2326-6066.CIR-13-0180 24894090 PMC4048875
103. Goeppert B. Frauenschuh L. Zucknick M. Roessler S. Mehrabi A. Hafezi M. Stenzinger A. Warth A. Pathil A. Renner M. Major histocompatibility complex class I expression impacts on patient survival and type and density of immune cells in biliary tract cancer Br. J. Cancer 2015 113 1343 1349 10.1038/bjc.2015.337 26461054 PMC4815783
104. Reits E.A. Hodge J.W. Herberts C.A. Groothuis T.A. Chakraborty M. Wansley E.K. Camphausen K. Luiten R.M. de Ru A.H. Neijssen J. Radiation modulates the peptide repertoire, enhances MHC class I expression, and induces successful antitumor immunotherapy J. Exp. Med. 2006 203 1259 1271 10.1084/jem.20052494 16636135 PMC3212727
105. Tai Y.T. Cho S.F. Anderson K.C. Osteoclast Immunosuppressive Effects in Multiple Myeloma: Role of Programmed Cell Death Ligand 1 Front. Immunol. 2018 9 1822 10.3389/fimmu.2018.01822 30147691 PMC6095980
106. Rodriguez-Ruiz M.E. Rodriguez I. Leaman O. Lopez-Campos F. Montero A. Conde A.J. Aristu J.J. Lara P. Calvo F.M. Melero I. Immune mechanisms mediating abscopal effects in radioimmunotherapy Pharmacol. Ther. 2019 196 195 203 10.1016/j.pharmthera.2018.12.002 30529041