1. Introduction
The regulation of eukaryotic cell division involves complex mechanisms, with cyclins and cyclin-dependent kinase (CDK) complexes playing a pivotal role [ 1 ]. Since the seminal discovery of cell cycle regulation by Tim Hunt, Paul Nurse, and Leland H. Hartwell—work that garnered a Nobel Prize—20 CDKs have been identified. CDK1-7 and CDK14-18 are primarily associated with cell cycle regulation, while CDK7-13, CDK19, and CDK20 are involved in transcription [ 2 , 3 , 4 , 5 ]. Owing to their critical roles in cell cycle progression, cellular transcription, and apoptotic pathways, CDKs have emerged as significant targets in anticancer drug development.
Over the past three decades, there has been an intensive search for small molecules that target CDKs. The first-generation compound, Alvocidib (Flavopiridol, Compound 1 , Figure 1 ), demonstrated inhibition towards multiple CDKs, with IC 50 values against CDK1, CDK2, CDK4, CDK5, and CDK9 of 30 nM, 170 nM, 100 nM, 170 nM, and 20 nM, respectively. Selliciclib (Roscovitine or CYC202, Compound 2 , Figure 1 ) shows IC 50 values against CDK1, CDK2, CDK5, CDK7, and CDK9 of 330 nM, 220 nM, 270 nM, 800 nM, and 230 nM, respectively. However, subsequent preclinical studies revealed limitations like low efficacy and high toxicity. This is due to off-targets and poor pharmacokinetics [ 6 ].
The second-generation CDK inhibitors, such as Milciclib (Compound 3 , Figure 1 ), with IC 50 values against CDK1, CDK2, CDK4, CDK5, and CDK7 of 53 nM, 45 nM, 140 nM, 170 nM, and 150 nM; SNS-032 (Compound 4 , Figure 1 ), with IC 50 values against CDK2, CDK7, and CDK9 of 48 nM, 62 nM, and 4 nM; Nu6140 (Compound 5 , Figure 1 ), with IC 50 values against CDK2, CDK4 and CDK5 of 410 nM, 850 nM, and 75 nM; Purvalanol A (Compound 6 , Figure 1 ), a selective CDK2 inhibitor, with the IC 50 values of 4 nM; Dinaciclib (Compound 7 , Figure 1 ), with IC 50 values against CDK2, CDK5, and CDK9 of 3, 1, 1, and 4 nM, all have their own flaws.
The advent of third-generation CDK inhibitors marked a notable improvement in selectivity, activity, and toxicity. Selective CDK4/6 inhibitors, in particular, have achieved remarkable success in clinical applications, notably in advanced-stage ER-positive breast cancer treatments. Four drugs from this class have been approved by the FDA, with three for cancer and one for myeloprotection [ 7 , 8 , 9 , 10 ]. Current research is focused on enhancing the selectivity of CDK inhibitors and addressing drug resistance. There are many excellent reviews on pan-inhibitors, and this review concentrates on selective CDK4/6 inhibitors [ 11 , 12 , 13 , 14 , 15 ].
2. The Biological Rationale for Targeting CDK4/6
The classical and best-documented function of CDK4/6 in cell proliferation is that cyclin D1-CDK4/6 phosphorylates the retinoblastoma protein (RB1) and RB-like proteins (RBL1 and RBL2), impacting the G1-S phase transition ( Figure 2 ). The unphosphorylated RB1 interacts with E2F transcription family members, blocking their activity and repressing transcription essential for S phase entry. Phosphorylated RB1, on the other hand, releases E2F, which promotes transcription of cyclin E that associates with CDK2 to further phosphorylate RB1, resulting in the facilitation of the S phase entry. This process is intricately regulated by external signals and mitogenic signaling, with aberrations in the CDK4/6-RB-E2F axis observed in various cancers [ 16 , 17 ].
However, there are some debates surrounding this model. For instance, RB1 is monophosphorylated during the G1 phase and becomes inactivated in the late G1 phase by cyclin E-CDK2, which hyperphosphorylates RB1 on multiple residues. Further, the phosphorylation of RB1 by cyclin D-CDK4/6 is crucial for normal cell-cycle progression, highlighting the need for more research in this area to elucidate the biological function of the CDK4/6-RB axis [ 18 ].
CDK4/6 also influences cell cycle progression through kinase-independent mechanisms. For instance, the Cyclin-dependent kinase inhibitor 1/kinase inhibitory protein (CIP/KIP) protein family, including p21 CIP1 , p27 kIP1 , and p57 kIP2 , binds cyclin E-CDK2 and suppresses its activity ( Figure 3 ). Upregulation of cyclin D and the formation of cyclin D-CDK4/6 complexes, which competitively bind CIP/KIP, leads to redistribution of CIP/KIP, thus activating cyclin E-CDK2 and promoting the G1-S transition.
Beyond facilitating the G1-S transition, Cyclin D-CDK4/6 promotes tumor progression through various pathways. For example, it phosphorylates and stabilizes transcription factor FOXM1, which promotes cell-cycle progression and protects cancer cells from entering senescence [ 19 ]. Cyclin D-CDK4 also phosphorylates SMAD3 and inhibits its transcriptional activity, which disables the anti-proliferative ability of growth factor beta (TGF–β) [ 20 ]. Cyclin D-CDK4/6 phosphorylates and inactivates tuberous sclerosis complex (TSC2), a negative regulator of rapamycin complex 1 (mTORC1), which subsequently activates mTORC1. CDK6 binds the promoter region of the FMS-like tyrosine kinase 3 (FLT3) gene and the promoter of proviral integration of molony murine leukemia virus 1 (PIM1) pro-oncogenic kinase, stimulating their expression. Treatment of FLT3-mutant leukemic cells with a CDK4/6 inhibitor decreased expression of FLT3 and PIM1, which induced cell cycle arrest and apoptosis [ 21 ]. The research on Cyclin D-CDK4/6 promoting tumor progression has also been well reviewed [ 22 ]. These diverse roles of CDK4/6 in tumor progression underscore their potential as targets in cancer therapy.
3. The Overview of CDK4/6 Sites
All CDKs possess a dual-leaf structure, with the N-terminal comprising β-sheet elements and the C-terminal formed by α-helices. The N-terminal leaf contains G-ring inhibitory components, while the C-terminal leaf is characterized by activating fragments and phosphorylation sites (serine or threonine, referred to as the T-loop). CDK4 is located on chromosome 12q14.1, consisting of a narrow region approximately 3.2 kbp in length. CDK6, mapped to human chromosome 7q21.2, encodes a cytoplasmic protein comprising 326 amino acids and weighing about 37 kDa. The structural and functional similarities between CDK4 and CDK6, with 71% homology in amino acids, facilitate the design of compounds as CDK4/6 inhibitors [ 23 ].
The cocrystallization study of Ribociclib and CDK6 (PDB ID: 5l2t) suggests that the 2-aminopyrimidine moiety of Ribociclib forms hydrogen bonds with VAL101, and the pyrrole ring interacts with PHE98, potentially underpinning its inhibitory effect ( Figure 4 ). Additionally, Ribociclib’s piperazine ring, extending outside the protein cavity, seems to play a role in modulating hydrosolubility and selectivity against other CDKs.
4. Approved CDK4/6 Inhibitors for Marketing
The third generation of CDK inhibitors, characterized by enhanced selectivity, reduced side effects, and improved pharmacokinetic properties, has shown significant potential in cancer therapy, particularly in breast cancer. Palbociclib (Ibrance/PD-0332991, Compound 8 , Figure 5 ), developed by Pfizer, was the first FDA-approved CDK4/6 inhibitor, marking a milestone in the development of these drugs. It exhibits IC 50 values against CDK4/6 of 11 and 9 nM, respectively [ 7 ]. Palbociclib received accelerated approval in the US in February 2015 for first-line treatment of advanced or metastatic ER-positive, HER2-negative breast cancer in postmenopausal women. It has shown efficacy in reducing the proliferation of ER-positive breast cancer cell lines in vitro by blocking Rb phosphorylation, causing G1 phase arrest [ 24 ]. Studies also indicate that combined therapy with palbociclib and antiestrogen agents (e.g., letrozole and fulvestrant) leads to a more significant reduction in phosphorylated Rb levels, E2F and FoxM1 levels, and downstream target gene expression. In the xenotransplantation model of ER-positive breast cancer derived from patients, palbociclib plus anti-estrogen letrozole (compared with any drug alone) has a greater inhibitory effect on Rb phosphorylation, downstream signal transduction, and tumor growth [ 25 ].
Ribociclib (Compound 9 , Figure 5 ), developed by Novartis, is another oral, small-molecule inhibitor of CDK4/6, approved in the USA in March 2017 [ 8 ]. It displays IC 50 values against CDK4/6 of 10 nM and 39 nM, respectively. Patient-derived xenograft models of ER-positive breast cancer have demonstrated the enhanced efficacy of ribociclib in combination with antiestrogen agents (letrozole or fulvestrant) and, in some cases, further improvement when combined with a phosphatidylinositol 3-kinase (PI3K) inhibitor [ 26 ]. The antitumor effects of ribociclib have also been demonstrated in vitro in leukemia cells.
Abemaciclib (Verzenio/LY2835219, Compound 10 , Figure 5 ), an oral inhibitor of CDK4/6 developed by Eli Lilly, was approved in the USA on 28 September 2017 [ 9 ]. It is indicated in combination with fulvestrant for the treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced or metastatic breast cancer, in combination with fulvestrant in women with disease progression following endocrine therapy, and as a monotherapy in adult patients with disease progression following endocrine therapy and prior chemotherapy in the metastatic setting [ 27 , 28 , 29 ]. It shows IC 50 values against CDK4 of 2 nM.
Trilaciclib (G1T28, Compound 11 , Figure 5 ), developed by G1 Therapeutics (formerly G-Zero Therapeutics), is a transient inhibitor of CDK4/6 with IC 50 values of 1 nM and 4 nM. Trilaciclib induces a transient, reversible G1 cell cycle arrest of proliferating hematopoietic stem and progenitor cells (HSPCs) in bone marrow, protecting them from chemotherapy (myeloprotection) [ 10 ]. On 12 February 2021, trilaciclib received its first approval in the USA to decrease the incidence of chemotherapy-induced myelosuppression in adult patients when administered prior to a platinum/etoposide-containing regimen or topotecan-containing regimen for extensive-stage small cell lung cancer (ES-SCLC) [ 30 , 31 ].
Dalpiciclib (SHR6390, Compound 12 , Figure 5 ), approved by the National Drug Administration (NMPA) on 31 December 2021, shows IC 50 values against CDK4/6 of 12.4 and 9.9 nM [ 32 ]. It exhibits effective antiproliferative activity against various human RB-positive tumor cells, specifically inducing G1 phase arrest and cell aging while reducing the level of Ser780-phosphorylated RB protein.
Birociclib (XZP-3287, Compound 13 , Figure 5 ), developed by Xuanzhu Biotechnology Co., Ltd., received the NMPA approval in 2022. It is indicated for locally advanced or metastatic adult breast cancer patients with hormone receptor (HR) positive and human epidermal growth factor receptor 2 (Her2) negative who have received two or more endocrine treatments and one chemotherapy in the metastatic setting and disease progression [ 33 ].
5. Synthesized CDK4/6 Inhibitors
5.1. Structures with 8-Alkyl-2-(arylamino)pyrido [2,3-d]pyrimidin-7(8H)-one
In 2000, Mark Barvian et al. synthesized a series of pyrimidine-7-ketone compounds. The most potent compound, Compound 14 ( Figure 6 ), exhibited a CDK4 inhibition IC 50 of 0.004 µM, albeit with moderate selectivity against CDK1/B, CDK2/A, CDK2/E, CDK4/D, and FGFr (IC 50 values of 0.079, 0.015, 0.020, 0.004, and 0.051 µM, respectively) [ 34 ]. Enhancements in selectivity were achieved by introducing a 5-methyl group and replacing cyclohexyl with cyclopentyl on the pyrido [2,3- d ]pyrimidin-7(8 H )-one moiety, resulting in Compound 15 ( Figure 6 ), which showed a CDK4/D IC 50 of 14 nM and >5 µM for CDK1/B, CDK2/A, and CDK2/E [ 35 ]. Substitutes in six positions of pyrido [2,3- d ]pyrimidin-7-ones have been explored. The introduction of bromine, iodine, acetyl, methoxyformyl, and ethoxyformyl all have improved selectivity (Compounds 16 and 17 , Figure 6 ). Compounds 16 and 17 exhibit IC 50 values against CDK4 of 4 and 2 nM, respectively [ 36 ]. Compound 18 ( Figure 6 ) inhibits CDK4/6 with IC 50 values of 3.9 and 9.8 nM [ 37 ]. In 2021, Huifang Shan et al. designed and synthesized a series of covalent CDK4/6 inhibitors targeting Thr107 amino acids based on palbociclib scaffolds. The optimized Compound 19 ( Figure 6 ) exhibited strong in vitro anticancer activity against CDK4/6. It had inhibitory activity against CDK4/6, with IC 50 values of 14 ± 1.01 nM and 6.1 ± 0.32 nM [ 38 ]. In 2022, Huan He et al. reported pterin-7(8 H )-one derivatives as CDK4/6 inhibitors, with the most promising Compound L2 ( 20 , Figure 6 ) exhibiting significant inhibitory activity with CDK4 and CDK6 IC 50 values of 16.7 and 30.5 nM [ 39 ]. Based on the above, we may infer that the selectivity of compounds in this class against CDK4/6 has been improved by introducing a small group at the fifth or sixth position on pyrido [2,3- d ]pyrimidin-7(8 H )-one moiety, such as methyl, halogen, acetyl, etc.
5.2. Structures with a 2-Aminopyrimidin-Containing Tricycle Moiety
Exploring tricycle moieties based on pyrido [2,3- d ] pyrimidin-7(8 H )-one, Hui Zhao et al. in 2018 reported selective 4,5-dihydro-1 H -pyrazolo [4,3- h ]quinazoline derivatives. Compound 21 ( Figure 7 ) selectively inhibited CDK4/6 with IC 50 values of 0.01/0.026 µM [ 40 ]. In 2020, Chen Shi et al. synthesized a new series of imidazole [1′,2′:1,6]pyrido [2,3- d ]pyrimidin derivatives as CDK4/6 inhibitors. Compound 22 ( Figure 7 ) was found to selectively inhibit CDK4/6, with IC 50 values of 0.8 nM and 2.0 nM. In addition, the bioavailability has improved. Some other tricycle-moiety-containing compounds have been reported. Compound 23 ( Figure 7 ) showed IC 50 values for CDK4 inhibition of 26.5 nM. Compound 24 ( Figure 7 ) was found to selectively inhibit CDK4/6, with IC 50 values of 2.2 and 2.5 nM [ 13 , 41 ].
In 2014, Zhihong Li et al. synthesized pyrido [4’,3’:4,5]pyrrolo [2,3- d ]pyrimidine derivatives, with Compound 25 ( Figure 7 ) inhibiting CDK4 and FLT3 (IC 50 values of 3 and 1 nM) and exhibiting favorable pharmacokinetics and bioavailability [ 42 ]. The structure–activity relationships (SARs) of these compounds were explored, with some ( 26 – 29 , Figure 7 ) showing strong selectivity towards CDK4 (IC 50 values against CDK4: 2, 3, 7, and 2 nM) [ 43 ]. In 2020, Xingpeng Shi et al. designed a series of pyrrolidone [2,3- d ]pyrimidine with 6-aniline carbonyl substituted derivatives, one of which (Compound 30 , Figure 7 ) was found to selectively inhibit CDK4/6 with IC 50 values of 20.5 and 52.3 nM [ 44 ]. Based on these compounds, we may infer 9 H -pyrido [4’,3’:4,5]pyrrolo [2,3- d ]pyrimidine and 6a,7-dihydroimidazo- [1’,2’:1,6]pyrido [2,3- d ]pyrimidine is a good replacement for pyrido [2,3- d ]pyrimidin 7(8 H )-one without impairment in potency or selectivity.
5.3. Structures with 2-(Arylamino)-4-aryl Pyrimidin
Meanwhile, 2-(arylamino)-4-aryl pyrimidin has been explored for the ATP binding site of other kinases; this moiety has been tried for CDK4/6. In 2000, through high-throughput screening, Gloria A. Breault et al. identified a 4,6-bisanilino pyrimidine core structure, with optimized Compound 31 ( Figure 8 ) exhibiting IC 50 values against CDK2/4 of 200 and 10 nM [ 45 , 46 ]. Xin Jie Chu et al. reported 2,4-diamino-5-ketopyrimidine 6 as a new class of ATP-competitive inhibitors targeting CDK families, with Compound 32 ( Figure 8 ) showing the highest inhibitory activity against CDK4 (IC 50 :1 nM) but also potent against CDK1/2 (Ki = 1/3 nM) [ 47 ]. In 2003, Malcolm Anderson et al. discovered Compound 33 ( Figure 8 ), inhibiting CDK4 with an IC 50 of 0.15 μM [ 48 ]. Timothy P. Heffron et al. designed and synthesized a CDK4/6 inhibitor containing an alkaline spirozane structure ( 34 , Figure 8 ), an inhibitor of the cyclin-dependent kinase CDK4/6. It has the strongest inhibitory activity against CDK4/6, with IC 50 values being less than 0.3 and 1.6 nM [ 49 ]. Other aryl groups have been introduced to the four positions of the core structure of 2-(arylamino)-4-aryl pyrimidin, including pyrrole, pyrazol, thiazole, pyrazolo [1,5- b ]pyridazine, and 1 H -pyrrolo [2,3- b ]pyridine. Compound 35 ( Figure 8 ) had inhibitory activity against CDK2/4, with IC 50 values of 0.03 and 0.12 μM [ 50 ]. Compound 36 ( Figure 8 ) had inhibitory activity against CDK2/4, with both IC 50 values being 0.3 nM [ 51 ]. Compound 37 ( Figure 8 ) exhibited an inhibitory effect against CDK1/4, with IC 50 values of 71 and 5 nM [ 52 ]. Compound 38 ( Figure 8 ) was found to selectively inhibit CDK4/6, with IC 50 values of 4 nM and 30 nM. Furthermore, it has good oral bioavailability [ 53 ]. Compounds 39 ~ 41 ( Figure 8 ) are highly selective CDK4/6 inhibitors. Compound 39 exhibits inhibition activity against CDK4/CDK6, with IC 50 values of 10 nM and 1.67 µM. Compound 40 exhibits inhibition activity against CDK4/6, with IC 50 values of 7 and 42 nM [ 54 ]. Compound 41 shows IC 50 values of CDK4/6 at 5 and 50 nM [ 55 ]. In 2022, Kai Yuan et al. reported Compound 42 , which had inhibitory activity against CDK4/6 with IC 50 values of 22 and 10 nM; besides, it had favorable bioavailability and slow clearance with a t1/2 value of more than 24 h in Sprague-Dawley (SD) rats [ 56 ].
In 2016, Lei Yin et al. reported a series of CDK4/6 inhibitors with notable blood-brain barrier permeability aimed at treating glioblastoma multiforme (GBM). Compound 43 ( Figure 9 ) exhibited good pharmacological characteristics and significant penetration of the blood-brain barrier. It inhibited CDK4/cyclin D1 and CDK6/cyclin D3, with IC 50 values of 3 nM and 1 nM, respectively [ 57 ]. Another highly selective CDK4/6 inhibitor, Compound 44 ( Figure 9 ), exhibited IC 50 values of 7.4 and 0.9 nM [ 58 ]. Compound 45 ( Figure 9 ) showed high selectivity against CDK1/4, with IC 50 values of 1180 and 1.4 nM [ 59 ]. Compound 46 ( Figure 9 ), notable for its high potency and selectivity towards CDK4/6 (IC 50 : 0.710/1.10 nM), also displayed anti-proliferative activity, excellent metabolic properties, and favorable pharmacokinetics [ 60 ]. Zhi Huang et al. reported novel inhibitors targeting both CDK4 and VEGFR2, with Compound 47 ( Figure 9 ) inhibiting CDK4 and VEGFR2 at 1 µM by 97% and 95%, respectively [ 61 ]. In 2022, Xiaoxing Wu et al. identified a series of SHP2 and CDK4 dual inhibitors, including Compound 48 ( Figure 9 ) with notable SHP2 (IC 50 = 4.3 nM) and CDK4 (IC 50 = 18.2 nM) inhibitory activity, for triple-negative breast cancer (TNBC) treatment [ 62 ]. In 2023, Junyu Xu et al. reported a series of novel pyrimidin-2-amine compounds for the treatment of glioblastoma (GBM), among which LH20 ( 49 , Figure 9 ) can inhibit the activity of CDK4 and CDK6 and reduce the phosphorylation of Rb [ 63 ]. Based on the above, we may infer that groups at four positions on pyrimidine have a great influence on selectivity. Usually, the bulk groups have good selectivity; for example, bicycle and tricycle groups are better than the monocycle moiety.
5.4. Structures Containing 5-Amino-3-arylindeno[1,2-c]pyrazol-4(2H)-one
In 2001, David A. Nugiel et al. [ 64 ] identified Compound 50 ( Figure 10 ) via high-throughput screening and optimization, which showed inhibition against CDK2/4 with IC 50 values of 0.27 and 0.45 µM. Compound 51 ( Figure 10 ) showed inhibition against CDK2/4, with IC 50 values of 0.018 and 0.012 µM [ 65 ]. The inhibitory activity of Compound 52 ( Figure 10 ) against CDK2/4 with IC 50 values was 9 and 5 nM [ 66 ]. Compound 53 ( Figure 10 ) inhibited CDK2/4, with pIC 50 values of 8.222 and 8.523 nM [ 67 ]. Compound 54 ( Figure 10 ) has an inhibitory effect against CDK4, with IC 50 values of 0.011 µM. All these compounds have poor selectivity toward CDK2 [ 68 ].
5.5. Structures Containing 2-Aminothiazole
In 2004, Raj N. Misra et al. synthesized a series of N -arylaminothiazole compounds, with Compounds 55 and 56 ( Figure 11 ) exhibiting IC 50 values against CDK4 of 9 and 26 nM [ 69 ]. Compound 57 ( Figure 11 ) had an inhibitory effect against CDK1/2/4 with IC 50 values of 480, 48, and 925 nM, respectively [ 70 ]. Metabolic and pharmacokinetic studies showed that it had a plasma half-life of 5–7 h in three species and exhibited moderately low protein binding in mouse (69%) and human (63%) serum. Oral administration showed 100%, 31%, and 28% bioavailability in mice, rats, and dogs, respectively.
5.6. Structures with 8-Alkylamino-quinolin
In 2010, Mark A. Klein et al. identified a series of 8-alkylamino-quinolin compounds based on a pharmacophore established from the peptide p16 INK4a (a type of Cip). Compound 58 – 60 ( Figure 12 ) showed inhibition against CDK4, with IC 50 values of 160, 179, and 128 μM, respectively [ 71 ].
5.7. Structures Containing (Z)-4-(Aminomethylene)isoquinoline-1,3(2H,4H)-dione
In 2008, Hwei Ru Tsou et al. designed and synthesized a series of 4-(phenylaminomethylene)isoquinoline-1,3(2 H ,4 H )-diketone compounds, which selectively inhibit CDK4 over CDK2 and CDK1. The IC 50 value of Compound 61 ( Figure 13 ) against CDK4 was 27 nM [ 72 ]. In 2009, Hwei Ru Tsou et al. designed and synthesized a series of 4-(benzylaminomethylene)isoquinoline-1,3-(2 H ,4 H )-dione and 4-[(pyridylmethyl) aminomethylene]isoquinoline-1,3-(2 H , 4 H )dione derivatives, which effectively and selectively inhibit CDK4 over CDK2 and CDK1. The IC 50 values of Compounds 62 and 63 ( Figure 13 ) against CDK4 were both 2 nM, while the IC 50 values against CDK1/2 were 23.3/18.3 and 2.5/1.1 nM [ 73 ].
5.8. Structures Derived from High-Throughput Screening and Natural Products
In 2000, Chung Kyu Ryu et al. reported 5-arylamino-2-methyl-4,7-dioxobenzothia- zole as an inhibitor of cyclin-dependent kinase 4 (CDK4) and a cytotoxic drug. The IC 50 value of Compound 64 ( Figure 14 ) for CDK4 was 3 µM versus 200 µM for CDK2 [ 74 ]. Compound 65 ( Figure 14 ) was identified based on high-throughput screening and exhibited CDK1/2/4 inhibition with IC 50 values of 95, 97, and 3.6 µM [ 75 ]. Compound 66 ( Figure 14 ) was identified based on high-throughput screening and optimization. It exhibited inhibition against CDK1/2/4, with IC 50 values of 0.2, 0.019, and 0.024 µM [ 76 ]. Compound 67 ( Figure 14 ) had an inhibitory effect against CDK4, with a pIC 50 value of 7.717 µM [ 77 ]. Compound 68 ( Figure 14 ) showed an inhibitory effect against CDK4, with IC 50 values of 0.05 µM [ 78 ]. Compound 69 ( Figure 14 ), derived from Fascaplysin, exhibited inhibition activity against CDK2/4, with IC 50 values of 521 and 6.2 µM [ 79 ]. Compound 70 ( Figure 14 ) showed potent cytotoxic activity, and the antitumor mechanism was probed by a docking study with CDK4 [ 80 ]. Compound 71 ( Figure 14 ) had good CDK4 inhibition activity, with IC 50 values of 1.26 µM [ 81 ]. In 2023, Mohamed M. Saleh et al. identified Compound 72 ( Figure 14 ), which inhibits CDK4/6 with IC 50 values against CDK4/6 of 95 nM and 184 nM, respectively. In addition, it inhibited other kinases, such as EGFR, c-Met, and B-Raf [ 82 ].
5.9. Other Structures
Compound 73 ( Figure 15 ) is a highly selective CDK4/6 inhibitor with IC 50 values of 9.2 and 7.8 nM, which were found through screening the Merck sample repository and further optimization [ 83 ]. In 2015, Takao Horiuchi et al. reported a series of CDK inhibitors, including Compound 74 ( Figure 15 ), which showed CDK2/4 inhibition with IC 50 values of 880 and 22 nM [ 84 ]. Compounds 75 and 76 ( Figure 15 ) exhibited inhibition activity against CDK6, with IC 50 values of 115.38 nM and 726.25 nM, respectively. Moreover, they increased the levels of bax and p53 and decreased the levels of bcl-2 [ 85 ].
6. Natural Product Inhibiting CDK4/6
In 1997, Jun’ichi et al. reported the isolation and identification of Konbu’acidin A ( 77 , Figure 16 ), which exhibits inhibition activity against CDK4 with an IC 50 of 20 µg/mL [ 86 ]. It is noteworthy that there are two guanidine moieties in the molecule. In 2020, Abdel Nasser B. Singab reported the isolation of Pulchranin A ( 78 , Figure 16 ), which also contains a guanidine moiety. The IC 50 values of inhibiting CDK1, CDK2, and CDK4 are 9.82, 15.6, and 2.7 µg/mL, respectively [ 87 ]. In 2003, Doriano Fabbro et al. reported the isolation of siphonodictyal C ( 79 , Figure 16 ) and halistanol sulfate ( 80 , Figure 16 ) from sponge in Micronesia and inhibition of CDK4/cyclin D1 with 9 and 9.5 µg/mL (IC 50 ), respectively. Staurosporie ( 81 , Figure 16 ), an alkaloid from the Streptomyces strain, shows potent CDK4 inhibition activity with an IC 50 value of 59 nM [ 88 , 89 ]. In 1994, L. M. Ballas reported the isolation of Arcyriaflavin A ( 82 , Figure 16 ) from the marine ascidian ( Eudistoma sp.), which has a similar aglycone to Staurosporine. It exhibits inhibition of CDK4/cyclin D1 with an IC 50 value of 140 nM [ 90 ].
Indirubin ( 83 , Figure 16 ) is the active ingredient of traditional Chinese medicine (i.e., Danggui Longhui Wan), which is used to treat chronic diseases. It interacts with CDK2′s ATP-binding site [ 91 ]. Compound 84 ( Figure 16 ), the synthetic analogy of Indirubin, shows potent cytotoxicity against different cancer cell lines. It arrests cells in the G1 phase and is supposed to bind to CDK4 [ 92 ]. In 2000, Karsten Schaumann reported four CDK4/cyclin D1 inhibitors produced by the genus Microsphaeropsis isolated from the Mediterranean sponge Aplysina aerophoba, of which one betaenone derivative ( 85 , Figure 16 ) and three 1,3,6,8-tetrahydroxyanthraquinone congeners ( 86 – 88 , Figure 16 ). The IC 50 values of CDK4/cyclin D1 inhibition activities are 11.5, 43.5, 22.5, and 37.5 µM, respectively [ 93 ]. In 2000, Byoung-Mog Kwon et al. found that 2′-hydroxycinnamaldehyde ( 89 , Figure 16 ), isolated from the stem bark of Cinnamonum cassia Blume, showed inhibition activity against CDK4/cyclin D1, with an IC 50 of 35 µM [ 94 ]. The potent and highly specific CDK4/cyclin D1 inhibitor, Fascaplysin ( 90 , Figure 16 ), isolated from the sponge Fascaplysinopsis sp, exhibits CDK4/cyclin D1 inhibition with an IC 50 of 0.35 µM [ 95 ].
Flavonoids are widespread in some traditional Chinese herbs, as well as in some fruit and vegetables. Tangeretin ( 91 , Figure 17 ) is a polymethoxylated flavone that is concentrated in the peel of citrus. It exhibits inhibition activity against CDK2 and CDK4, with IC 50 values of 26.5 µM and 19.2 µM, respectively [ 96 , 97 ]. In 2005, Jung Han Yoon Park et al. reported that Fisetin ( 92 , Figure 17 ), an ingredient in some fruits and vegetables, inhibits CDK4 activity [ 98 ]. Rajesh Agarwal reported that Silibinin ( 93 , Figure 17 ), isolated from milk thistle, shows potent CDK2 and CDK4 inhibition activity [ 99 ]. In 2013, Umashankar Vetrivel et al. identified Linarin ( 94 , Figure 17 ) as a potential CDK4 inhibitor via virtual screening [ 100 ]. Quercitrin ( 95 , Figure 17 ), isolated from Brownea grandiceps Jacq, also exhibits CDK4 inhibition activity with an IC 50 of 3.22 µM [ 101 ]. In 2020, Ehab M. Mostafa reported that Myricetin ( 96 , Figure 17 ) was isolated from Scorzonera tortuosissima. Boiss inhibits CDK4/cyclin D1 with an IC 50 value of 3.16 µM [ 102 ].
7. PROTAC Targeting CDK4/6 and Cyclins
Recently, PROTAC, which degrades CDK4/6, has been considered a strategy to overcome drug resistance. In addition, degradation of CDK4/6 also eliminates other functions of CDK4/6 besides kinase activity, which benefits cancer therapy.
In 2019, Amarnath Natarajan et al. reported Palbociclib-based PROTAC ( 97 , Figure 18 ) selectively degraded CDK6 while sparing CDK4. It degrades CDK6 as early as 4 h and achieves complete degradation after 8 h at 0.5 µM [ 103 ]. Kevin reported Pal-pom ( 98 , Figure 18 ), based on Palbociclib, degraded both CDK4 and CDK6, and degraded CDK4 more efficiently than CDK6 with DC 50 values of 12.9 and 34.1 nM, respectively (treatment at concentrations of 0.3 µM for 18 h), while the dissociation constants (Kd) for CDK4/6 are 0.9 and 0.2 µM [ 104 ]. In 2020, Andrew B. Benowitz reported that Palbociclib-based PROTACs ( 99 , Figure 18 ) degraded CDK4/6 with high binding affinity and degradation potency (pIC 50 for CDK4/6: 8.5/8.1, pDC 50 for CDK4/6: 8.0/9.1). Moreover, PROTACs ( 100 , 101 , Figure 18 ) recruiting von Hippel Lindau (VHL) and cellular inhibitor of apoptosis protein (cIAP) ligase have also been prepared, and they showed good degradation potency towards CDK4/6 [ 105 ]. Jan Kronke reported systematic research on the structure–activity relationship (SAR) of the linker in the PROTAC targeting CDK4/6. The potent PROTACs ( 102 , 103 , Figure 18 ) were found, with 85%/92.3% and 88%/97.1% degradation of CDK4/6 at 0.1 µM based on different ligases. In addition, the PROTAC ( 104 , Figure 18 ) of CDK6 selective degradation has also been found, with 35%/98.6% degradation of CDK4/6 at 0.1 µM [ 106 ].
In 2022, Nathanael S. Gray et al. reported a triple degrader ( 105 , Figure 19 ) that consists of palbociclib which could degrade CDK4/6 and Helios [ 107 ]. In 2022, Jian Jin reported a palbociclib-based PROTAC ( 106 , Figure 19 ) degraded cyclin D1 via recruiting the CDK4/6-cyclin D1 complex to the VHL ligase, which was considered as an undruggable protein [ 108 ]. Recently, the highly CDK4/6 degraders ( 107 , 108 , Figure 19 ) recruiting heat shock protein 90 (HSP90) and DDB1-and CUL4-associated factor 16 (DCAF16) have been reported [ 109 , 110 ].
8. CDK4/6 Inhibitors in Clinical Research
In recent years, CDK4/6 inhibitors have been developed rapidly, and some have gradually entered clinical trials. In this section, we list some drugs that have already entered clinical research.
G1T38 (Compound 109 , Figure 20 , Table 1 ), developed by G1 Therapeutics ((North Carolina, USA)), is a novel potent CDK4/6 inhibitor with good selectivity and oral bioavailability. At present, it is in a Phase-II-stage clinical study. G1T38 reduced RB phosphorylation, blocked cells in the G1 phase, and inhibited cell proliferation in various CDK4/6-dependent oncogenic cell lines, including breast, melanoma, leukemia, and lymphoma cells. In addition, G1T38 accumulates in mouse xenograft tumors but in plasma, with less neutropenia. All these good pharmacokinetic and pharmacodynamic properties make G1T38 a continuous, daily oral anti-tumor agent [ 111 ]. Ebvaciclib (Compound 110 , Figure 20 , Table 1 ), developed by Pfizer (New York, NY, USA), is a CDK2/4/6 inhibitor. In November 2018, the in vitro and in vivo data of Ebvaciclib were first released at the 30th AACR Annual Conference held in Dublin, Ireland. Ebvaciclib has a higher binding affinity for CDK2, 4, 5, and 6, which is 40 times higher than that of CDK1 and CDK9. In March 2018, a Phase-I/II-trial treated patients with HR-positive, HER2-negative breast cancer, metastatic triple-negative breast cancer, or advanced cisplatin-resistant epithelial ovarian cancer/fallopian tube cancer [ 112 ].
MM-D37K ( Table 1 ) is a synthetic peptide composed of p16 INK4a (a specific inhibitor of cyclin D-CDK4 and CDK6) and cell penetrating peptide (CPP)-Antp (Penetratin). It is a non-ATP-competitive CDK4/6 inhibitor that is in Phase-II-stage clinical research. The merit of MM-D37K over the existing ATP-competitive CDK inhibitors will be explored, benefiting the development of next-generation CDK inhibitors [ 113 ].
BPI-16350 ( Table 1 ), developed by Beida Pharmaceutical (Hangzhou, China), is in a Phase-III-stage clinical study. It is applied to treat locally advanced, recurrent, or metastatic breast cancer that has progressed HR+/HER2−, combined with Fluvastatin [ 114 ]. RGT-419B ( Table 1 ), developed by Shanghai Qilu Ruige Pharmaceutical Research and Development Co., Ltd. (Shanghai, China), is a potent CDK2/4/6 inhibitor that is in Phase I clinical research. The informed use of RGT-419B is for the unmet medical needs of patients with refractory or recurrent disease after previous treatment and patients with advanced/metastatic breast cancer to improve the survival rate and quality of life [ 115 ]. FCN-437c ( Table 1 ) is developed by Fuchuang Pharmaceuticals (Chongqing, China), a subsidiary of Fosun Pharmaceuticals. On 23 January 2019, a Phase I clinical trial was conducted in the United States. In September 2020, the Phase II clinical study was conducted in China for patients with ER+/HER2− advanced breast cancer (excluding Hong Kong, Macao, and Taiwan) [ 116 ]. TY-302 ( Table 1 ), developed by Zhengzhou Taiji Hongnuo Pharmaceutical Co., Ltd. (Zhengzhou, China), is a potent and highly selective oral CDK4/6 inhibitor. It was in Phase I clinical trials in December 2019 [ 117 ]. TQB3616 ( Table 1 ), developed by Zhengda Tianqing Pharmaceutical (Lianyungang, China), is in Phase III clinical study. It is applied to patients with breast cancer and lung cancer, including HR+/HER2− late/metastatic breast cancer and ER+/HER2+ late/metastatic breast cancer [ 118 ]. BEBT-209 ( Table 1 ), developed by Guangzhou Beibeite Pharmaceutical Co., Ltd. (Guangzhou, China), is in Phase II clinical study for patients with advanced breast cancer. Unlike the already-marketed CDK4/6 anti-tumor inhibitors, BEBT-209 improves the selectivity of CDK4 over CDK6, which is expected to reduce the hematological and immunosuppressive toxicity caused by CDK6 activity inhibition [ 119 ]. FLX925 (Compound 111 , Figure 20 , Table 1 ), developed by Amgen (FLX BIO) (California, USA), is in Phase I clinical study. FLX925 selectively acts on FLT3 and CDK4/6, and its current indication is acute myeloid leukemia (AML) [ 120 ]. P-276-00 (Compound 112 , Figure 20 , Table 1 ), developed by Piramal (Mumbai, India), is in phase II clinical studies for advanced refractory neoplasms and multiple myeloma. It also has inhibitory activity against TNF-α. It also has anti-inflammatory activity, and its first indication for application is for the treatment of mucositis caused by severe radiation in patients with head and neck cancer [ 121 ]. SPH4366 ( Table 1 ), developed by Shanghai Pharmaceutical Group (Shanghai, China), is in Phase II/III clinical study. It is used for advanced solid tumor, local, or metastatic breast cancer [ 122 ].
ETH-155008 ( Table 1 ), developed by Shengke Pharmaceutical (Suzhou, China), is in Phase I clinical study. It is a PIM3 and CDK4/6 dual-target inhibitor. ETH-155008 further blocks tumor cells in the G1 phase, to some extent avoiding the development of CDK4/6 inhibitor resistance via a synergistic effect [ 123 ]. Narazaciclib ( ON123300 ) (Compound 113 , Figure 20 , Table 1 ), developed by Onconova Therapeutics Inc. (Newtown, CT, USA), is in Phase I/II clinical study. It has multi-kinase inhibition activity, including CDK4 (IC 50 = 3.9 nM), CDK6 (IC 50 = 9.82 nM), Ark5 (IC 50 = 5 nM), PDGFR β (IC 50 = 26 nM), FGFR1 (IC 50 = 26 nM), RET (IC 50 = 9.2 nM), and FYN (IC 50 = 11 nM), with good blood-brain barrier penetration. Narazaciclib inhibits Akt phosphorylation and activates Erk in brain tumors [ 124 ]. CGT-1967 ( Table 1 ), developed by Suzhou Shengshi Taike (Suzhou, China), is in Phase I clinical study in China. The informed use is for the treatment of patients with acute myeloid leukemia (AML) [ 125 ].
Auceliciclib ( Table 1 ), developed by Ancentra Therapeutics Pty Ltd. (Adelaide, Australia), is currently in phase I/II clinical study [ 126 ]. XH-30002 ( Table 1 ), developed by Shanghai Xunhe Pharmaceutical Technology Co., Ltd. (Shanghai, China), is in Phase I clinical trial. Its intended use is to treat advanced solid tumors, including colorectal cancer, breast cancer, ovarian cancer, etc. [ 127 ]. HS-10342 ( Table 1 ), developed for Hansen Pharmaceutical (Shanghai, China), is in a Phase I clinical trial. Its intended use is for the treatment of patients with advanced breast cancer with ER+/HER2− [ 128 ]. QHRD110 ( Table 1 ), developed by Changzhou Qianhong Biochemical Pharmaceutical (Changzhou, China), is in phase I clinical trials [ 129 ]. NUV-422 ( Table 1 ) is a novel inhibitor of the cyclin-dependent kinase CDK2/4/6, developed by Nuvation Bio. (New York, NY, USA). It is a Phase II clinical study for patients with HR+/HER2− advanced breast cancer [ 130 ].
9. Summary and Prospect
Recently, selective CDK4/6 inhibitors have shown clinical success, particularly in treating advanced-stage estrogen receptor ER+/HER2− breast cancer. Herein, we mainly review the mechanism of action and the progress of CDK4/6 inhibitors. These compounds have been categorized based on molecule similarity and origin. In addition, the proteolysis targeting chimers (PROTACs) targeting CDK4/6 have been reviewed.
However, in the clinic, some patients develop primary or acquired drug resistance. For example, about 20% of breast cancer patients receiving CDK4/6 inhibitor treatment have no response to treatment [ 131 ]. These patients already have genetic mutations in their tumor cells, allowing them to avoid the effects of CDK4/6 inhibitors and continue to proliferate in the presence of drugs. Thus far, many primary resistance mechanisms have been identified, all of which seem to involve activation of the cyclin D-CDK4/6-Rb pathway [ 132 ].
In addition, the activation of cyclin D-CDK4/6-Rb, activation of other proliferation pathways, changes in the tumor microenvironment, and regulation of tumor metabolism may also lead to the emergence of acquired drug resistance [ 132 ]. Within 2 years of starting treatment with CDK4/6 inhibitors in the PALOMA-2 study, over 30% of enrolled patients developed resistance to palbociclib [ 133 ]. Furthermore, 40 months later, over 70% of patients in the combination of palbociclib and letrozole group in this study experienced tumor progression during treatment. As prolonged exposure to CDK4/6 inhibitors continues, more and more patients have developed drug resistance; ultimately, all patients receiving CDK4/6 inhibitor treatment will develop acquired resistance [ 132 ].
The coming research may involve the following aspects: First, the more potent and selective inhibitors that overcome drug resistance according to the mechanism of drug resistance. Secondly, the dual or multiple targeting inhibitors may have been researched for their synergetic effects or myeloprotection. Finally, the PROTACs may have been tried for anti-resistance based on the action mechanism of PROTAC and the non-enzymatic function of CDK4/6, which benefits cancer therapy.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
Author Contributions
Data collection, H.W. and J.Q.; writing—original draft preparation, H.W.; writing—review and editing H.W., J.B., Y.K., Z.G., T.L., Y.Z., J.Q. and J.W.; project administration, J.Q.; funding acquisition J.Q. and J.W. All authors have read and agreed to the published version of the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
References
1. Fisher R.P. A cell cycle regulator branches out: A key G1 kinase targets transcription to drive cell cycle commitment Science 2021 374 263 264 10.1126/science.abm2010 34648332
2. Michowski W. Chick J.M. Chu C. Kolodziejczyk A. Wang Y. Suski J.M. Abraham B. Anders L. Day D. Dunkl L.M. Cdk1 Controls Global Epigenetic Landscape in Embryonic Stem Cells Mol. Cell 2020 78 459 476.e13 10.1016/j.molcel.2020.03.010 32240602 PMC7214218
3. Zhang L. Li X. DEAD-box RNA helicases in cell cycle control and clinical therapy Cells 2021 10 1540 10.3390/cells10061540 34207140 PMC8234093
4. Li B. Li A. You Z. Xu J. Zhu S. Epigenetic silencing of CDKN1A and CDKN2B by SNHG1 promotes the cell cycle, migration and epithelial-mesenchymal transition progression of hepatocellular carcinoma Cell Death Dis. 2020 11 823 10.1038/s41419-020-03031-6 33009370 PMC7532449
5. Zhao H. Li S. Wang G. Zhao W. Zhang D. Wang F. Li W. Sun L. Study of the mechanism by which dinaciclib induces apoptosis and cell cycle arrest of lymphoma Raji cells through a CDK1-involved pathway Cancer Med. 2019 8 4348 4358 10.1002/cam4.2324 31207099 PMC6675732
6. Lapenna S. Giordano A. Cell cycle kinases as therapeutic targets for cancer Nat. Rev. Drug Discov. 2009 8 547 566 10.1038/nrd2907 19568282
7. Pfizer Inc. IBRANCE® (Palbociclib): US Prescribing Information 2015 Available online: http://www.accessdata.fda.gov. (accessed on 12 February 2015)
8. Xiang R. Fan Y. Li Y. Guo Q. Huang Z. Heterocyclic Derivative with CDK4/6 and HDAC Inhibitory Activity CN106831780 13 June 2017
9. Eli Lilly and Company VERZENIOTM (Abemaciclib): US Prescribing Information 2017 Available online: https://www.fda.gov. (accessed on 17 October 2017)
10. Dhillon S. Trilaciclib: First Approval Drugs 2021 81 867 874 10.1007/s40265-021-01508-y 33861388
11. Mariaule G. Belmont P. Cyclin-dependent kinase inhibitors as marketed anticancer drugs: Where are we now? A short survey Molecules 2014 19 14366 10.3390/molecules190914366 25215591 PMC6271685
12. Cheng W. Yang Z. Wang S. Li Y. Wei H. Tian X. Kan Q. Recent development of CDK inhibitors: An overview of CDK/inhibitor co-crystal structures Eur. J. Med. Chem. 2019 164 615 639 10.1016/j.ejmech.2019.01.003 30639897
13. Ammazzalorso A. Agamennone M. De Filippis B. Fantacuzzi M. Development of CDK4/6 inhibitors: A five years update Molecules 2021 26 1488 10.3390/molecules26051488 33803309 PMC7967197
14. Susanti N.M.P. Tjahjono D.H. Cyclin-Dependent Kinase 4 and 6 Inhibitors in Cell Cycle Dysregulation for Breast Cancer Treatment Molecules 2021 26 4462 10.3390/molecules26154462 34361615 PMC8348313
15. Yuan K. Wang X. Dong H. Min W. Hao H. Yang P. Selective inhibition of CDK4/6: A safe and effective strategy for developing anticancer drugs Acta Pharm. Sin. B 2021 11 30 54 10.1016/j.apsb.2020.05.001 33532179 PMC7838032
16. LaBaer J. Garrett M.D. Stevenson L.F. Slingerland J.M. Sandhu C. Chou H.S. Fattaey A. Harlow E. New functional activities for the p21 family of CDK inhibitors Genes Dev. 1997 11 847 862 10.1101/gad.11.7.847 9106657
17. Cheng M. Olivier P. Diehl J.A. Fero M. Roussel M.F. Roberts J.M. Sherr C.J. The p21Cip1 and p27Kip1 CDK ‘inhibitors’ are essential activators of cyclin D-dependent kinases in murine fibroblasts EMBO J. 1999 18 1571 1583 10.1093/emboj/18.6.1571 10075928 PMC1171245
18. Anders L. Ke N. Hydbring P. Choi Y.J. Widlund H.R. Chick J.M. Zhai H. Vidal M. Gygi S.P. Braun P. Systematic Screen for CDK4/6 Substrates Links FOXM1 Phosphorylation to Senescence Suppression in Cancer Cells Cancer Cell 2011 20 620 634 10.1016/j.ccr.2011.10.001 22094256 PMC3237683
19. Matsushime H. Ewen M.E. Strom D.K. Kato J.Y. Hanks S.K. Roussel M.F. Sherr C.J. Identification and properties of an atypical catalytic subunit (p34PSK-J3/cdk4) for mammalian D type G1 cyclins Cell 1992 71 323 10.1016/0092-8674(92)90360-O 1423597
20. Matsuura I. Denissova N.G. Wang G. He D. Long J. Liu F. Cyclin-dependent kinases regulate the antiproliferative function of Smads Nature 2004 430 226 231 10.1038/nature02650 15241418
21. Uras I.Z. Walter G.J. Scheicher R. Bellutti F. Prchal-Murphy M. Tigan A.S. Valent P. Heidel F.H. Kubicek S. Scholl C. Palbociclib treatment of FLT3-ITD+ AML cells uncovers a kinase-dependent transcriptional regulation of FLT3 and PIM1 by CDK6 Blood 2016 127 2890 2902 10.1182/blood-2015-11-683581 27099147 PMC4920675
22. Otto T. Sicinski P. Cell cycle proteins as promising targets in cancer therapy Nat. Rev. Cancer 2017 17 93 115 10.1038/nrc.2016.138 28127048 PMC5345933
23. Kalra S. Joshi G. Munshi A. Kumar R. Structural insights of cyclin dependent kinases: Implications in design of selective inhibitors Eur. J. Med. Chem. 2017 142 424 458 10.1016/j.ejmech.2017.08.071 28911822
24. Finn R.S. Dering J. Conklin D. Kalous O. Cohen D.J. Desai A. Ginther C. Atefi M. Chen I. Fowst C. PD 0332991, a selective cyclin D kinase 4/6 inhibitor, preferentially inhibits proliferation of luminal estrogen receptor-positive human breast cancer cell lines in vitro Breast Cancer Res. 2009 11 R77 10.1186/bcr2419 19874578 PMC2790859
25. Lee N.V. Yuan J. Eisele K. Abstract LB-136: Mechanistic exploration of combined CDK4/6 and ER inhibition in ER-positive breast cancer Cancer Res. 2014 74 (Suppl. 19) LB-136 10.1158/1538-7445.AM2014-LB-136
26. O’Brien N.T. Emmanuelle E.D. Ayala R. In vivo efficacy of combined targeting of CDK4/6, ER and PI3K signaling in ER+ breast cancer Cancer Res. 2014 74 (Suppl. 19) 4756 10.1158/1538-7445.AM2014-4756
27. Eli Lilly and Company FDA Grants Priority Review for Potential New Indication for Lilly’s VerzenioTM (Abemaciclib) As Initial Treatment of Advanced Breast Cancer [Media Release] Available online: https://www.lilly.com (accessed on 12 October 2017)
28. US FDA FDA Approves New Treatment for Certain Advanced or Metastatic Breast Cancers [Media Release] 2017 Available online: https://www.fda.gov/news-events/press-announcements/fda-approves-new-treatment-certain-advanced-or-metastatic-breast-cancers (accessed on 28 September 2017)
29. Finn R.S. Aleshin A. Slamon D.J. Targeting the cyclin-dependent kinases (CDK) 4/6 in estrogen receptor-positive breast cancers Breast Cancer Res. 2016 18 1 11 10.1186/s13058-015-0661-5 26857361 PMC4746893
30. US FDA FDA Approves Drug to Reduce Bone Marrow Suppression Caused by Chemotherapy Media Release U.S. FDA Washington, DC, USA 2021
31. G1 Therapeutics COSELATM (Trilaciclib): US Prescribing Information 2021 Available online: https://www.g1therapeutics.com/consela/pi/ (accessed on 25 February 2021)
32. Zhang H. Yan S. Zhan Y. Ma S. Bian Y. Li S. Tian J. Li G. Zhong D. Diao X. A mass balance study of [14C] SHR6390 (dalpiciclib), a selective and potent CDK4/6 inhibitor in humans Front. Pharmacol. 2023 14 1116073 10.3389/fphar.2023.1116073 37063263 PMC10102643
33. Kan Q. Tian X. Zhang X. Yang Z. Du Y. Cheng W. Yuan Y. Wang S. Pyrimidine Derivative Useful in Treatment of Cancer and Its Preparation CN108299312 20 June 2018
34. Barvian M. Boschelli D. Cossrow J. Dobrusin E. Fattaey A. Fritsch A. Fry D. Harvey P. Keller P. Garrett M. Pyrido[2,3-d]pyrimidin-7-one Inhibitors of Cyclin-Dependent Kinases J. Med. Chem. 2000 43 4606 4616 10.1021/jm000271k 11101352
35. Bathini Y. Singh I. Harvey P.J. Keller P.R. Singh R. Micetich R.G. Fry D.W. Dobrusin E.M. Toogood P.L. 2-Aminoquinazoline inhibitors of cyclin-dependent kinases Bioorg. Med. Chem. Lett. 2005 15 3881 3885 10.1016/j.bmcl.2005.05.131 15993068
36. VanderWel S.N. Harvey P.J. McNamara D.J. Repine J.T. Keller P.R. Quin J. III Booth R.J. Elliott W.L. Dobrusin E.M. Fry D.W. Pyrido[2,3-d]pyrimidin-7-ones as Specific Inhibitors of Cyclin-Dependent Kinase 4 J. Med. Chem. 2005 48 2371 2387 10.1021/jm049355+ 15801830
37. Sanchez-Martinez C. Gelbert L.M. Lallena M.J. de Dios A. Cyclin dependent kinase (CDK) inhibitors as anticancer drugs Bioorg. Med. Chem. Lett. 2015 25 3420 3435 10.1016/j.bmcl.2015.05.100 26115571
38. Shan H. Ma X. Yan G. Luo M. Zhong X. Lan S. Yang J. Liu Y. Pu C. Tong Y. Discovery of a novel covalent CDK4/6 inhibitor based on palbociclib scaffold Eur. J. Med. Chem. 2021 219 113432 10.1016/j.ejmech.2021.113432 33857728
39. He H. Liu Q. Chen L. Wang J. Yuan Y. Li H. Qian X. Zhao Z. Chen Z. Design, synthesis and biological evaluation of pteridine-7(8H)-one derivatives as potent and selective CDK4/6 inhibitors Bioorg. Med. Chem. Lett. 2022 76 128991 10.1016/j.bmcl.2022.128991 36130661
40. Zhao H. Hu X. Cao K. Zhang Y. Zhao K. Tang C. Feng B. Synthesis and SAR of 4,5-dihydro-1H-pyrazolo[4,3-h]quinazoline derivatives as potent and selective CDK4/6 inhibitors Eur. J. Med. Chem. 2018 157 935 945 10.1016/j.ejmech.2018.08.043 30165341
41. Shi C. Wang Q. Liao X. Ge H. Huo G. Zhang L. Chen N. Zhai X. Hong Y. Wang L. Discovery of a novel series of imidazo[1′,2′:1,6]pyrido[2,3-d]pyrimidin derivatives as potent cyclin-dependent kinase 4/6 inhibitors Eur. J. Med. Chem. 2020 193 112239 10.1016/j.ejmech.2020.112239 32200202
42. Li Z. Wang X. Eksterowicz J. Gribble M.W. Alba G.Q. Ayres M. Carlson T.J. Chen A. Chen X. Cho R. Discovery of AMG 925, a FLT3 and CDK4 Dual Kinase Inhibitor with Preferential Affinity for the Activated State of FLT3 J. Med. Chem. 2014 57 3430 3449 10.1021/jm500118j 24641103
43. Sanchez-Martinez C. Lallena M.J. Sanfeliciano S.G. de Dios A. Cyclin dependent kinase (CDK) inhibitors as anticancer drugs: Recent advances (2015–2019) Bioorg. Med. Chem. Lett. 2019 29 126637 10.1016/j.bmcl.2019.126637 31477350
44. Shi X. Quan Y. Wang Y. Wang Y. Li Y. Design, synthesis, and biological evaluation of 2,6,7-substituted pyrrolo[2,3-d]pyrimidines as cyclin dependent kinase inhibitor in pancreatic cancer cells Bioorg. Med. Chem. Lett. 2021 33 127725 10.1016/j.bmcl.2020.127725 33316409
45. Beattie J.F. Breault G.A. Ellston R.P.A. Green S. Jewsbury P.J. Midgley C.J. Naven R.T. Minshull C.A. Pauptit R.A. Tucker J.A. Cyclin-dependent kinase 4 inhibitors as a treatment for cancer. Part 1: Identification and optimization of substituted 4,6-Bis anilino pyrimidines Bioorg. Med. Chem. Lett. 2003 13 2955 2960 10.1016/S0960-894X(03)00202-6 12941311
46. Breault G.A. Ellston R.P.A. Green S. James S.R. Jewsbury P.J. Midgley C.J. Pauptit R.A. Minshull C.A. Tucker J.A. Pease J.E. Cyclin-dependent kinase 4 inhibitors as a treatment for cancer. Part 2: Identification and optimization of substituted 2,4-bis anilino pyrimidines Bioorg. Med. Chem. Lett. 2003 13 2961 2966 10.1016/S0960-894X(03)00203-8 12941312
47. Chu X.-J. DePinto W. Bartkovitz D. So S.-S. Vu B.T. Packman K. Lukacs C. Ding Q. Jiang N. Wang K. Discovery of [4-Amino-2-(1-methanesulfonylpiperidin-4-ylamino)pyrimidin-5-yl](2,3-difluoro-6- methoxyphenyl)methanone (R547), A Potent and Selective Cyclin-Dependent Kinase Inhibitor with Significant in Vivo Antitumor Activity J. Med. Chem. 2006 49 6549 6560 10.1021/jm0606138 17064073
48. Anderson M. Beattie J.F. Breault G.A. Breed J. Byth K.F. Culshaw J.D. Ellston R.P.A. Green S. Minshull C.A. Norman R.A. Imidazo[1,2-a]pyridines: A potent and selective class of cyclin-Dependent kinase inhibitors identified through structure-Based hybridization Bioorg. Med. Chem. Lett. 2003 13 3021 3026 10.1016/S0960-894X(03)00638-3 12941325
49. Bronner S.M. Merrick K.A. Murray J. Salphati L. Moffat J.G. Pang J. Sneeringer C.J. Dompe N. Cyr P. Purkey H. Design of a brain-penetrant CDK4/6 inhibitor for glioblastoma Bioorg. Med. Chem. Lett. 2019 29 2294 2301 10.1016/j.bmcl.2019.06.021 31307887
50. Wang S. Wood G. Meades C. Griffiths G. Midgley C. McNae I. McInnes C. Anderson S. Jackson W. Mezna M. Synthesis and biological activity of 2-anilino-4-(1 H -pyrrol-3-yl)pyrimidine CDK inhibitors Bioorg. Med. Chem. Lett. 2004 14 4237 4240 10.1016/j.bmcl.2004.06.012 15261277
51. Stevens K.L. Reno M.J. Alberti J.B. Price D.J. Kane-Carson L.S. Knick V.B. Shewchuk L.M. Hassell A.M. Veal J.M. Davis S.T. Synthesis and evaluation of pyrazolo[1,5-b]pyridazines as selective cyclin dependent kinase inhibitors Bioorg. Med. Chem. Lett. 2008 18 5758 5762 10.1016/j.bmcl.2008.09.069 18835709
52. Cho Y.-S. Borland M. Brain C. Chen C.H.T. Cheng H. Chopra R. Chung K. Groarke J. He G. Hou Y. 4-(Pyrazol-4-yl)-pyrimidines as Selective Inhibitors of Cyclin-Dependent Kinase 4/6 J. Med. Chem. 2010 53 7938 7957 10.1021/jm100571n 21038853
53. Tadesse S. Yu M. Mekonnen L.B. Lam F. Islam S. Tomusange K. Rahaman M.H. Noll B. Basnet S.K.C. Teo T. Highly Potent, Selective, and Orally Bioavailable 4-Thiazol- N -(pyridin-2-yl)pyrimidin-2-amine Cyclin-Dependent Kinases 4 and 6 Inhibitors as Anticancer Drug Candidates: Design, Synthesis, and Evaluation J. Med. Chem. 2017 60 1892 1915 10.1021/acs.jmedchem.6b01670 28156111
54. Tadesse S. Zhu G. Mekonnen L.B. Lenjisa J.L. Yu M. Brown M.P. Wang S. A novel series of N -(pyridin-2-yl)-4-(thiazol-5-yl)pyrimidin-2-amines as highly potent CDK4/6 inhibitors Future Med. Chem. 2017 9 1495 1506 10.4155/fmc-2017-0076 28795589
55. Tadesse S. Bantie L. Tomusange K. Yu M. Islam S. Bykovska N. Noll B. Zhu G. Li P. Lam F. Discovery and pharmacological characterization of a novel series of highly selective inhibitors of cyclin-dependent kinases 4 and 6 as anticancer agents Br. J. Pharmacol. 2018 175 2399 2413 10.1111/bph.13974 28800675 PMC5980294
56. Yuan K. Kuang W. Chen W. Ji M. Min W. Zhu Y. Hou Y. Wang X. Li J. Wang L. Discovery of novel and orally bioavailable CDK 4/6 inhibitors with high kinome selectivity, low toxicity and long-acting stability for the treatment of multiple myeloma Eur. J. Med. Chem. 2022 228 114024 10.1016/j.ejmech.2021.114024 34875521
57. Yin L. Li H. Liu W. Yao Z. Cheng Z. Zhang H. Zou H. A highly potent CDK4/6 inhibitor was rationally designed to overcome blood brain barrier in gliobastoma therapy Eur. J. Med. Chem. 2018 144 1 28 10.1016/j.ejmech.2017.12.003 29247857
58. Wang Y. Liu W.-J. Yin L. Li H. Chen Z.-H. Zhu D.-X. Song X.-Q. Cheng Z.-Z. Song P. Wang Z. Design and synthesis of 4-(2,3-dihydro-1H-benzo[d]pyrrolo[1,2-a]imidazol-7-yl)- N -(5-(piperazin-1-ylmethyl)pyridine-2-yl)pyrimidin-2-amine as a highly potent and selective cyclin-dependent kinases 4 and 6 inhibitors and the discovery of structure-activity relationships Bioorg. Med. Chem. Lett. 2018 28 974 978 29429832 10.1016/j.bmcl.2017.12.068
59. Zha C. Deng W. Fu Y. Tang S. Lan X. Ye Y. Su Y. Jiang L. Chen Y. Huang Y. Design, synthesis and biological evaluation of tetrahydronaphthyridine derivatives as bioavailable CDK4/6 inhibitors for cancer therapy Eur. J. Med. Chem. 2018 148 140 153 10.1016/j.ejmech.2018.02.022 29459274
60. Shi C. Wang Q. Liao X. Ge H. Huo G. Zhang L. Chen N. Zhai X. Hong Y. Wang L. Discovery of 6-(2-(dimethylamino)ethyl)- N -(5-fluoro-4-(4-fluoro-1-isopropyl-2-methyl-1 H -benzo[d]imidazole-6-yl)pyrimidin-2-yl)-5,6,7,8-tetrahydro-1,6-naphthyridin-2-amine as a highly potent cyclin-dependent kinase 4/6 inhibitor for treatment of cancer Eur. J. Med. Chem. 2019 178 352 364 10.1016/j.ejmech.2019.06.005 31200237
61. Huang Z. Zhao B. Qin Z. Li Y. Wang T. Zhou W. Zheng J. Yang S. Shi Y. Fan Y. Novel dual inhibitors targeting CDK4 and VEGFR2 synergistically suppressed cancer progression and angiogenesis Eur. J. Med. Chem. 2019 181 111541 10.1016/j.ejmech.2019.07.044 31382120
62. Chen X. Shu C. Li W. Hou Q. Luo G. Yang K. Wu X. Discovery of a Novel Src Homology-2 Domain Containing Protein Tyrosine Phosphatase-2 (SHP2) and Cyclin-Dependent Kinase 4 (CDK4) Dual Inhibitor for the Treatment of Triple-Negative Breast Cancer J. Med. Chem. 2022 65 6729 6747 10.1021/acs.jmedchem.2c00063 35447031
63. Wang Y. Li Y. Liu D. Zheng D. Li X. Li C. Huang C. Wang Y. Wang X. Li Q. A Potential Anti-Glioblastoma Compound LH20 Induces Apoptosis and Arrest of Human Glioblastoma Cells via CDK4/6 Inhibition Molecules 2023 28 5047 10.3390/molecules28135047 37446710 PMC10343613
64. Nugiel D.A. Etzkorn A.-M. Vidwans A. Benfield P.A. Boisclair M. Burton C.R. Cox S. Czerniak P.M. Doleniak D. Seitz S.P. Indenopyrazoles as Novel Cyclin Dependent Kinase (CDK) Inhibitors J. Med. Chem. 2001 44 1334 1336 10.1021/jm0100032 11311054
65. Nugiel D.A. Vidwans A. Etzkorn A.-M. Rossi K.A. Benfield P.A. Burton C.R. Cox S. Doleniak D. Seitz S.P. Synthesis and Evaluation of Indenopyrazoles as Cyclin-Dependent Kinase Inhibitors. 2. Probing the Indeno Ring Substituent Pattern J. Med. Chem. 2002 45 5224 5232 10.1021/jm020171+ 12431050
66. Nugiel D.A. Vidwans A. Dzierba C.D. Parallel synthesis of acylsemicarbazide libraries: Preparation of potent cyclin dependent kinase (cdk) inhibitors Bioorg. Med. Chem. Lett. 2004 14 5489 5491 10.1016/j.bmcl.2004.09.023 15482910
67. Singh S.K. Dessalew N. Bharatam P.V. 3D-QSAR CoMFA study on indenopyrazole derivatives as cyclin dependent kinase 4 (CDK4) and cyclin dependent kinase 2 (CDK2) inhibitors Eur. J. Med. Chem. 2006 41 1310 1319 10.1016/j.ejmech.2006.06.010 16890327
68. Honma T. Hayashi K. Aoyama T. Hashimoto N. Machida T. Fukasawa K. Iwama T. Ikeura C. Ikuta M. Suzuki-Takahashi I. Structure-Based Generation of a New Class of Potent Cdk4 Inhibitors: New de Novo Design Strategy and Library Design J. Med. Chem. 2001 44 4615 4627 10.1021/jm0103256 11741479
69. Misra R.N. Xiao H.-y. Williams D.K. Kim K.S. Lu S. Keller K.A. Mulheron J.G. Batorsky R. Tokarski J.S. Sack J.S. Synthesis and biological activity of N-aryl-2-aminothiazoles: Potent pan inhibitors of cyclin-dependent kinases Bioorg. Med. Chem. Lett. 2004 14 2973 2977 10.1016/j.bmcl.2004.02.105 15125971
70. Misra R.N. Xiao H.-y. Kim K.S. Lu S. Han W.-C. Barbosa S.A. Hunt J.T. Rawlins D.B. Shan W. Ahmed S.Z. N -(cycloalkylamino)acyl-2-aminothiazole inhibitors of cyclin-dependent kinase 2. N -[5-[[[5-(1,1-dimethylethyl)-2-oxazolyl]methyl]thio]-2-thiazolyl]-4- piperidinecarboxamide (BMS-387032), a highly efficacious and selective antitumor agent J. Med. Chem. 2004 47 1719 1728 10.1021/jm0305568 15027863
71. Klein M.A. Mayo K.H. Kratzke R.A. p16INK4a Peptide mimetics identified via virtual screening Bioorg. Med. Chem. Lett. 2010 20 403 405 10.1016/j.bmcl.2009.10.046 19914831
72. Tsou H.-R. Otteng M. Tran T. Floyd M.B. Jr. Reich M. Birnberg G. Kutterer K. Ayral-Kaloustian S. Ravi M. Nilakantan R. 4-(Phenylaminomethylene)isoquinoline-1,3(2 H ,4 H )-diones as Potent and Selective Inhibitors of the Cyclin-Dependent Kinase 4 (CDK4) J. Med. Chem. 2008 51 3507 3525 10.1021/jm800072z 18494457
73. Tsou H.-R. Liu X. Birnberg G. Kaplan J. Otteng M. Tran T. Kutterer K. Tang Z. Suayan R. Zask A. Discovery of 4-(Benzylaminomethylene)isoquinoline-1,3-(2 H ,4 H )-diones and 4-[(Pyridylmethyl)aminomethylene]isoquinoline-1,3-(2 H ,4 H )-diones as Potent and Selective Inhibitors of the Cyclin-Dependent Kinase 4 J. Med. Chem. 2009 52 2289 2310 10.1021/jm801026e 19317452
74. Ryu C.-K. Kang H.-Y. Lee S.K. Nam K.A. Hong C.Y. Ko W.-G. Lee B.-H. 5-arylamino-2-methyl-4,7-dioxobenzothiazoles as inhibitors of cyclin-dependent kinase 4 and cytotoxic agents Bioorg. Med. Chem. Lett. 2000 10 461 464 10.1016/S0960-894X(00)00014-7 10743948
75. Carini D.J. Kaltenbach R.F. Liu J. Benfield P.A. Boylan J. Boisclair M. Brizuela L. Burton C.R. Cox S. Grafstrom R. Identification of selective inhibitors of cyclin dependent kinase 4 Bioorg. Med. Chem. Lett. 2001 11 2209 2211 10.1016/S0960-894X(01)00416-4 11514172
76. Markwalder J.A. Arnone M.R. Benfield P.A. Boisclair M. Burton C.R. Chang C.-H. Cox S.S. Czerniak P.M. Dean C.L. Doleniak D. Synthesis and Biological Evaluation of 1-Aryl-4,5-dihydro-1H-pyrazolo[3,4-d]pyrimidin-4-one Inhibitors of Cyclin-Dependent Kinases J. Med. Chem. 2004 47 5894 5911 10.1021/jm020455u 15537345
77. Dessalew N. Bharatam P.V. 3D-QSAR and molecular docking study on bisarylmaleimide series as glycogen synthase kinase 3, cyclin dependent kinase 2 and cyclin dependent kinase 4 inhibitors: An insight into the criteria for selectivity Eur. J. Med. Chem. 2007 42 1014 1027 10.1016/j.ejmech.2007.01.010 17335939
78. Horiuchi T. Chiba J. Uoto K. Soga T. Discovery of novel thieno[2,3-d]pyrimidin-4-yl hydrazone-based inhibitors of Cyclin D1-CDK4: Synthesis, biological evaluation, and structure-activity relationships Bioorg. Med. Chem. Lett. 2009 19 305 308 10.1016/j.bmcl.2008.11.090 19091560
79. Mahale S. Bharate S.B. Manda S. Joshi P. Bharate S.S. Jenkins P.R. Vishwakarma R.A. Chaudhuri B. Biphenyl-4-carboxylic Acid [2-(1 H -Indol-3-yl)-ethyl]-methylamide (CA224), a Nonplanar Analogue of Fascaplysin, Inhibits Cdk4 and Tubulin Polymerization: Evaluation of in Vitro and in Vivo Anticancer Activity J. Med. Chem. 2014 57 9658 9672 10.1021/jm5014743 25368960
80. Kamal A. Mahesh R. Nayak V.L. Babu K.S. Kumar G.B. Shaik A.B. Kapure J.S. Alarifi A. Discovery of pyrrolospirooxindole derivatives as novel cyclin dependent kinase 4 (CDK4) inhibitors by catalyst-free, green approach Eur. J. Med. Chem. 2016 108 476 485 10.1016/j.ejmech.2015.11.046 26708114
81. Al-Warhi T. El Kerdawy A.M. Aljaeed N. Ismael O.E. Ayyad R.R. Eldehna W.M. Abdel-Aziz H.A. Al-Ansary G.H. Synthesis, biological evaluation and in silico studies of certain oxindole-indole conjugates as anticancer CDK inhibitors Molecules 2020 25 2031 10.3390/molecules25092031 32349307 PMC7248897
82. Saleh M.M. El-Moselhy T. El-Bastawissy E. Ibrahim M.A.A. Sayed S.M. Hegazy M.-E.F. Efferth T. Jaragh L.A. Sidhom P.A. The mystery of titan hunter: Rationalized striking of the MAPK pathway via Newly synthesized 6-Indolylpyridone-3-Carbonitrile derivatives Eur. J. Med. Chem. 2023 259 115675 10.1016/j.ejmech.2023.115675 37506545
83. Shimamura T. Shibata J. Kurihara H. Mita T. Otsuki S. Sagara T. Hirai H. Iwasawa Y. Identification of potent 5-pyrimidinyl-2-aminothiazole CDK4, 6 inhibitors with significant selectivity over CDK1, 2, 5, 7, and 9 Bioorg. Med. Chem. Lett. 2006 16 3751 3754 10.1016/j.bmcl.2006.04.048 16682184
84. Horiuchi T. Takeda Y. Haginoya N. Miyazaki M. Nagata M. Kitagawa M. Akahane K. Uoto K. Discovery of novel thieno[2,3-d]pyrimidin-4-yl hydrazone-based cyclin-dependent kinase 4 inhibitors: Synthesis, biological evaluation and structure-activity relationships Chem. Pharm. Bull. 2011 59 991 1002 10.1248/cpb.59.991 21804244
85. Abbas S.E.S. George R.F. Samir E.M. Aref M.M.A. Abdel-Aziz H.A. Synthesis and anticancer activity of some pyrido[2,3-d]pyrimidine derivatives as apoptosis inducers and cyclin-dependent kinase inhibitors Future Med. Chem. 2019 11 2395 2414 10.4155/fmc-2019-0050 31544523
86. Kobayashi J.i. Suzuki M. Tsuda M. Konbu’acidin A, a new bromopyrrole alkaloid with cdk4 inhibitory activity from Hymeniacidon sponge Tetrahedron 1997 53 15681 15684 10.1016/S0040-4020(97)10022-9
87. Moussa A.Y. Mostafa N.M. Singab A.N.B. Pulchranin A: First report of isolation from an endophytic fungus and its inhibitory activity on cyclin dependent kinases Nat. Prod. Res. 2020 34 2715 2722 10.1080/14786419.2019.1585846 30887847
88. Furusaki A. Hashiba N. Matsumoto T. Hirano A. Iwai Y. Omura S. X-ray structure of staurosporine: A new alkaloid from a Streptomyces strain J. Chem. Soc. Chem. Commun. 1978 18 800 10.1039/C39780000800
89. Sanchez-Martinez C. Shih C. Faul M.M. Zhu G. Paal M. Somoza C. Li T. Kumrich C.A. Winneroski L.L. Xun Z. Aryl[a]pyrrolo[3,4-c]carbazoles as selective cyclin D1-CDK4 inhibitors Bioorg. Med. Chem. Lett. 2003 13 3835 3839 10.1016/S0960-894X(03)00791-1 14552791
90. Horton P.A. Longley R.E. McConnell O.J. Ballas L.M. Staurosporine aglycon (K252-c) and arcyriaflavin A from the marine ascidian, Eudistoma sp. Experientia 1994 50 843 10.1007/BF01956468 7925852
91. Hoessel R. Leclerc S. Endicott J.A. Nobel M.E.M. Lawrie A. Tunnah P. Leost M. Damiens E. Marie D. Marko D. Indirubin, the active constituent of a Chinese antileukemia medicine, inhibits cyclin-dependent kinases Nat. Cell Biol. 1999 1 60 67 10.1038/9035 10559866
92. Chiou C.-T. Lee W.-C. Liao J.-H. Cheng J.-J. Lin L.-C. Chen C.-Y. Song J.-S. Wu M.-H. Shia K.-S. Li W.-T. Synthesis and evaluation of 3-ylideneoxindole acetamides as potent anticancer agents Eur. J. Med. Chem. 2015 98 1 12 10.1016/j.ejmech.2015.04.062 25988923
93. Brauers G. Edrada R.A. Ebel R. Proksch P. Wray V. Berg A. Graefe U. Schaechtele C. Totzke F. Finkenzeller G. Anthraquinones and betaenone derivatives from the sponge-associated fungus Microsphaeropsis species: Novel inhibitors of protein kinases J. Nat. Prod. 2000 63 739 745 10.1021/np9905259 10869191
94. Jeong H.W. Kim M.R. Son K.H. Young Han M. Ha J.H. Garnier M. Meijer L. Kwon B.M. Cinnamaldehydes inhibit cyclin dependent kinase 4/cyclin D1 Bioorg. Med. Chem. Lett. 2000 10 1819 1822 10.1016/S0960-894X(00)00357-7 10969976
95. Soni R. Muller L. Furet P. Schoepfer J. Stephan C. Zumstein-Mecker S. Fretz H. Chaudhuri B. Inhibition of Cyclin-Dependent Kinase 4 (Cdk4) by Fascaplysin, a Marine Natural Product Biochem. Biophys. Res. Commun. 2000 275 877 884 10.1006/bbrc.2000.3349 10973815
96. Zupan A. Mikulic-Petkovsek M. Slatnar A. Stampar F. Veberic R. Individual phenolic response and peroxidase activity in peel of differently sun-exposed apples in the period favorable for sunburn occurrence J. Plant Physiol. 2014 171 1706 1712 10.1016/j.jplph.2014.08.010 25209696
97. Pan M.-H. Chen W.-J. Lin-Shiau S.-Y. Ho C.-T. Lin J.-K. Tangeretin induces cell-cycle G1 arrest through inhibiting cyclin-dependent kinases 2 and 4 activities as well as elevating Cdk inhibitors p21 and p27 in human colorectal carcinoma cells Carcinogenesis 2002 23 1677 1684 10.1093/carcin/23.10.1677 12376477
98. Lu X. Jung J.I. Cho H.J. Lim D.Y. Lee H.S. Chun H.S. Kwon D.Y. Park J.H.Y. Fisetin inhibits the activities of cyclin-dependent kinases leading to cell cycle arrest in HT-29 human colon cancer cells J. Nutr. 2005 135 2884 2890 10.1093/jn/135.12.2884 16317137
99. Varghese L. Agarwal C. Tyagi A. Singh R.P. Agarwal R. Silibinin Efficacy against Human Hepatocellular Carcinoma Clin. Cancer Res. 2005 11 8441 8448 10.1158/1078-0432.CCR-05-1646 16322307
100. Sivashanmugam M. Raghunath C. Vetrivel U. Virtual screening studies reveal linarin as a potential natural inhibitor targeting CDK4 in retinoblastoma J. Pharmacol. Pharmacother. 2013 4 256 264 10.4103/0976-500X.119711 24250202 PMC3826001
101. Gutierrez R.M.P. Hoyo-Vadillo C. Anti-inflammatory Potential of Petiveria alliacea on Activated RAW264.7 Murine Macrophages Pharmacogn. Mag. 2017 13 (Suppl. 2) S174 S178 10.4103/pm.pm_479_16 28808377 PMC5538151
102. Bodiba D.C. Prasad P. Srivastava A. Crampton B. Lall N.S. Antibacterial activity of Azadirachta indica, Pongamia pinnata, Psidium guajava, and Mangifera indica and their mechanism of action against Streptococcus mutans Pharmacogn. Mag. 2018 14 76 80 10.1016/j.sajb.2018.02.021 29576705 PMC5858246
103. Rana S. Bendjennat M. Kour S. King H.M. Kizhake S. Zahid M. Natarajan A. Selective degradation of CDK6 by a palbociclib based PROTAC Bioorg. Med. Chem. Lett. 2019 29 1375 1379 10.1016/j.bmcl.2019.03.035 30935795 PMC6487213
104. Zhao B. Burgess K. PROTACs suppression of CDK4/6, crucial kinases for cell cycle regulation in cancer Chem. Commun. 2019 55 2704 2707 10.1039/C9CC00163H 30758029
105. Anderson N.A. Cryan J. Ahmed A. Dai H. McGonagle G.A. Rozier C. Benowitz A.B. Selective CDK6 degradation mediated by cereblon, VHL, and novel IAP-recruiting PROTACs Bioorg. Med. Chem. Lett. 2020 30 127106 10.1016/j.bmcl.2020.127106 32184044
106. Steinebach C. Ng Y.L.D. Sosic I. Lee C.-S. Chen S. Lindner S. Vu L.P. Bricelj A. Haschemi R. Monschke M. Systematic exploration of different E3 ubiquitin ligases: An approach towards potent and selective CDK6 degraders Chem. Sci. 2020 11 3474 3486 10.1039/D0SC00167H 33133483 PMC7552917
107. Verano A.L. You I. Donovan K.A. Mageed N. Yue H. Nowak R.P. Fischer E.S. Wang E.S. Gray N.S. Redirecting the Neo-Substrate Specificity of Cereblon-Targeting PROTACs to Helios ACS Chem. Biol. 2022 17 2404 2410 10.1021/acschembio.2c00439 36007246 PMC13148786
108. Xiong Y. Zhong Y. Yim H. Yang X. Park K.-S. Xie L. Poulikakos P.I. Han X. Xiong Y. Chen X. Bridged Proteolysis Targeting Chimera (PROTAC) Enables Degradation of Undruggable Targets J. Am. Chem. Soc. 2022 144 22622 22632 10.1021/jacs.2c09255 36448571 PMC9772293
109. Li Z. Ma S. Zhang L. Zhang S. Ma Z. Du L. Li M. Targeted Protein Degradation Induced by HEMTACs Based on HSP90 J. Med. Chem. 2023 66 733 751 10.1021/acs.jmedchem.2c01648 36574496
110. Pu C. Liu Y. Deng R. Xu Q. Wang S. Zhang H. Luo D. Ma X. Tong Y. Li R. Development of PROTAC degrader probe of CDK4/6 based on DCAF16 Bioorg. Chem. 2023 138 106637 10.1016/j.bioorg.2023.106637 37276679
111. Bisi J.E. Sorrentino J.A. Jordan J.L. Darr D.D. Roberts P.J. Tavares F.X. Strum J.C. Preclinical development of G1T38: A novel, potent and selective inhibitor of cyclin dependent kinases 4/6 for use as an oral antineoplastic in patients with CDK4/6 sensitive tumors Oncotarget 2017 8 42343 42358 10.18632/oncotarget.16216 28418845 PMC5522071
112. Freeman-Cook K.D. Hoffman R.L. Behenna D.C. Boras B. Carelli J. Diehl W. Ferre R.A. He Y.-A. Hui A. Huang B. Discovery of PF-06873600, a CDK2/4/6 Inhibitor for the Treatment of Cancer J. Med. Chem. 2021 64 9056 9077 10.1021/acs.jmedchem.1c00159 34110834
113. Bozhenko V.K. Kulinich T.M. Kudinova E.A. Boldyrev A. Solodkij V.A. New targeted anti CDK4/6 peptide MM-D37K J. Clin. Oncol. 2013 31 e13545 10.1200/jco.2013.31.15_suppl.e13545
114. Wang Y. Wang J. Ding L. Preparation of (Heterocyclyl)azinylaminopyrimidinyl Tetrahydrobenzimidazopyridines and Dihydrobenzimidazooxazines as Inhibitors of CDK4 and CDK6 for Use as Anticancer Agents WO2018113771 28 June 2018
115. Available online: https://www.medicilon.com.cn//index.php?keywords=RGT-419B&c=list&pagelen=20&tempid=59&catid=48 (accessed on 5 December 2023)
116. O’Rourke K.A.T.E. Highlights from the 2022 Annual ASCO Meeting Cancer 2022 128 3010 3011 10.1002/cncr.34379 35860919
117. Available online: http://www.tykmedicines.com/ty-302/ (accessed on 5 December 2023)
118. Available online: https://db.yaozh.com/linchuangshiyan/bJaRbmNnZ2hqlWRilJaWlg==.html (accessed on 5 December 2023)
119. Available online: http://www.bebettermed.cn/goods-25-view.html (accessed on 5 December 2023)
120. Carlino L. Christodoulou M.S. Restelli V. Caporuscio F. Foschi F. Semrau M.S. Costanzi E. Tinivella A. Pinzi L. Lo Presti L. Structure-activity relationships of hexahydrocyclopenta[c]quinoline derivatives as allosteric inhibitors of CDK2 and EGFR ChemMedChem 2018 13 2627 2634 10.1002/cmdc.201800687 30457710
121. Jain S.K. Bharate S.B. Vishwakarma R.A. Cyclin-dependent kinase inhibition by flavoalkaloids Mini-Rev. Med. Chem. 2012 12 632 649 10.2174/138955712800626683 22512551
122. Available online: https://www.sphchina.com/main_business/pharmaceutical_R&D.html (accessed on 5 December 2023)
123. Liu C. Liu B. Xu C. Zhang P. Li B. Ji B. Zhang B. Liu Q. Zhang J. Yu C. ETH-155008, a Novel Selective Dual Inhibitor of FLT3 and CDK4/6 in Preclinical Treatment of Acute Myeloid Leukemia Blood 2019 134 5114 10.1182/blood-2019-123589
124. Profitós-Pelejà N. Ribeiro M.L. Parra R.J. Fernández-Serrano M. Marín-Escudero P. Makovski-Silverstein A. Cosenza S. Esteller M. Roué G. Prolonged cell cycle arrest by the CDK4/6 antagonist narazaciclib restores ibrutinib response in preclinical models of BTKi-resistant mantle cell lymphoma Hematol. Oncol. 2023 553 554 10.1002/hon.3164_414
125. Available online: http://www.sipac.gov.cn/szgyyq/jsdt/202212/fb4e3e65573e454d8487929ee03b74be.shtml (accessed on 5 December 2023)
126. Bessudo A. Cohen E.E.W. Gutierrez R. Johnson D.H. Rosenberg A. Sukari A. Stemmer S.M. Weinberg B.A. Reuveni H. Schickler M. A phase 1/2 study with open-label, dose escalation phase followed by single-arm expansion at the maximum tolerated dose to assess the safety, tolerability, pharmacokinetics, pharmacodynamics, and efficacy of NT219 injection alone and in combination with cetuximab in adults with advanced solid tumors and head and neck cancer J. Clin. Oncol. 2021 39 TPS3156
127. Available online: http://www.jrfbio.com/list-12-1.html (accessed on 5 December 2023)
128. Available online: http://www.changbaicao.cn/trialsdetail-id-1493.html (accessed on 5 December 2023)
129. Available online: https://www.medicilon.com.cn/tindex.html/QHRD110 (accessed on 5 December 2023)
130. Available online: https://investingnews.com/nuvation-bio-announces-discontinuation-of-nuv-422-clinical-development-program/ (accessed on 5 December 2023)
131. Kong T. Xue Y. Cencic R. Zhu X. Monast A. Fu Z. Pilon V. Sangwan V. Guiot M.-C. Foulkes W.D. eIF4A inhibitors suppress cell-cycle feedback response and acquired resistance to CDK4/6 inhibition in cancer Mol. Cancer Ther. 2019 18 2158 2170 10.1158/1535-7163.MCT-19-0162 31395685 PMC7132330
132. Xu X.-Q. Pan X.-H. Wang T.-T. Wang J. Yang B. He Q.-J. Ding L. Intrinsic and acquired resistance to CDK4/6 inhibitors and potential overcoming strategies Acta Pharmacol. Sin. 2021 42 171 178 10.1038/s41401-020-0416-4 32504067 PMC8027849
133. Finn R.S. Martin M. Rugo H.S. Jones S. Im S.-A. Gelmon K. Harbeck N. Lipatov O.N. Walshe J.M. Moulder S. Palbociclib and letrozole in advanced breast cancer N. Engl. J. Med. 2016 375 1925 1936 10.1056/NEJMoa1607303 27959613