Seliciclib (CYC202, R-roscovitine) is a second-generation orally available cyclin-dependent kinases (CDKs) inhibitor that competes for ATP binding sites on these kinases. It is a direct inhibitor of cyclin CDK2/E, CDK2/A and it has inhibitory effects on cyclin H/CDK7, CDK5, and CDK9. CDKs are enzymes that are central to the process of cell division and cell cycle control and play pivotal roles in cancer cell growth and DNA damage repair. Seliciclib exerts an anti-proliferative effect via several key mechanisms: selective downregulation of proliferative and survival proteins and upregulation of p53, leading to growth arrest or apoptosis. The second one is decreasing phosphorylation of Rb and modulating E2F transcriptional activity leading to growth arrest or apoptosis. Seliciclib is currently in phase II clinical trial as a drug candidate for the treatment of Cushing's disease and as a potential therapeutic agent for the treatment of cystic fibrosis (CF). In addition, it is in Phase II trials for non-small cell lung cancer and nasopharyngeal carcinoma.

A diaminopyrimidine compound R547 is a small molecule selective ATP-competitive inhibitor of cyclin-dependent kinases CDK1, CDK2 and CDK4 and has excellent in vitro cellular potency, inhibiting the growth of various human tumor cell lines. In vivo, R547 showed antitumor activity in all of the models tested to date, including six human tumor xenografts and an orthotopic syngeneic rat model. R547 was being developed by Roche for the treatment of solid tumours. The compound was undergoing clinical development in the US. However, no recent development has been reported and it is assumed to have been discontinued.

AT7519M or AT7519, a small molecule inhibitor of cyclin-dependent kinases 1, 2, 4, 5, and 9, participated in phase II clinical trials in patients with relapsed or refractory chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL). As a result, in CLL, some patients had tumor reductions, but the objective response rate (ORR) was low. In MCL, activity was noted with ORR of 27%. In addition, AT7519M was studied in patients with previously treated multiple myeloma, to understand whether the drug alone or in combination with bortezomib were effective treatments. Recent experiments also have shown that AT7519 is a promising drug for the treatment of high-risk neuroblastoma patients with MYCN amplification. It is known, that MYCN-dependent neuroblastomas have low cure rates with current multimodal treatment.

P276-00 (also known as riviciclib) is a novel, potent, small-molecule, flavone-derived inhibitor of cyclin-dependent kinases (Cdk), Cdk 4 D1, Cdk1 B, and Cdk9 T, with potent cytotoxic effects against chemosensitive and chemoresistant cancer cell lines. P276-00 was in phase II clinical trial for the treatment mantle cell lymphoma, but that study was terminated based on interim results and all subjects were off study at that time. Although, there were not the major safety or tolerability concerns. However, this drug successfully passed phase II clinical trial for the treatment Melanoma, squamous cell carcinoma of head and neck, malignant melanoma and in combination with Gemcitabine in the treatment of advanced pancreatic cancer.

RGB-286638 is a multi-targeted protein kinase inhibitor currently in Phase 1 clinical testing. In vitro cell-free kinase assays indicated that RGB-286638 inhibits CDK1, 2, 3, 4, 5, and 9 and is less active against CDK6 and 7. The RGB-286638 compound has been shown to inhibit the processes controlling cell division in cancer cells by targeting multiple cyclin-dependent kinase proteins involved in regulating the cell cycle. RGB-286638 has also been shown to induce apoptosis (programmed cell death) and to inhibit other important protein kinases involved in the proliferation of cancer cells. RGB-286638 treatment results in tumor regression and increased survival in a number of pre-clinical models of solid and hematological tumors.

Dinaciclib (SCH 727965) is a small molecule inhibitor of cyclin-dependent kinases. Dinaciclib demostrates potent and selective inhibition of CDK2, CDK5, CDK1, and CDK9 activity. Dinaciclib inhibits cell cycle progression and proliferation in various tumor cell lines in vitro. Dinaciclib is a product of a drug discovery collaboration between Pharmacopeia (later Ligand Pharmaceuticals) and Schering-Plough (later Merck & Co.) that began in 1998. Dinaciclib showed promising effect in treating haematological malignancies and solid tumors.

R1530 is the multikinase inhibitor with potential antiangiogenesis and antineoplastic activities. R1530 is also a mitosis-angiogenesis inhibitor (MAI) that inhibits multiple receptor tyrosine kinases involved in angiogenesis, such as vascular endothelial growth factor receptor (VEGFR)-1, -2, -3, platelet-derived growth factor receptor (PDGFR) beta, FMS-like tyrosine kinase (Flt)-3, and fibroblast growth factor receptor (FGFR) -1, -2. In addition, this agent exhibits anti-proliferative activity by initiating mitotic arrest and inducing apoptosis. In the presence of R1530, polyploid cancer cells underwent apoptosis or became senescent which translated into potent in vitro and in vivo efficacy. Normal proliferating cells were resistant to R1530-induced polyploidy thus supporting the rationale for cancer therapy by induced polyploidy. Mitotic checkpoint kinase BubR1 was found downregulated during R1530-induced exit from mitosis, a likely consequence of PLK4 inhibition. R1530 strongly inhibited human tumor cell proliferation and growth factor-driven proliferation of endothelial and fibroblast cells. Significant tumor growth inhibition was demonstrated in a lung cancer xenograft model with a range of once daily, weekly and twice-weekly doses of R1530. Daily doses were most effective in the lung cancer model and also had significant growth inhibitory effects in models of colorectal, prostate, and breast tumors. Tumor regression occurred in all models treated with the maximum tolerated daily dose. The doses of 25 and 50 mg/kg QD resulted in biologically significant increased survival in all tested models. After oral administration in nude mice, R1530 showed good tissue penetration. R1530 had been in phase I clinical trials by Hoffmann-La Roche, Inc. for the treatment of advanced solid tumors. However, the clinical development of R1530 was discontinued.

C16 (PKRi) is a selective inhibitor of the enzyme double-stranded RNA-dependent protein kinase (PKR). PKRi prevents striatal neurodegeneration and improves behavioral outcomes in a chemically induced mouse model of Huntington's disease, the efffect mediated by off-target inhibition of CDKs. PKRi restored memory deficit in a mouse model of Alzheimer's disease. In a mouse model of colon-26 adenocarcinoma, treatment with C16 lead to a significantly higher level of blood glucose and lower level of serum triglyceride compared with placebo group, which indicates potential use for alleviation of cachexia.

JNJ-7706621, developed by Johnson & Johnson Pharmaceutical, is pan-CDK inhibitor with the highest potency on cyclin-dependent kinases: CDK1/2. Inhibition of CDK1 kinase activity, altered CDK1 phosphorylation status, and interference with downstream substrates such as retinoblastoma was also shown in human tumor cells following drug treatment. It also potently inhibits Aurora A/B and has no activity on Plk1 and Wee1. In human cancer cells, treatment with JNJ-7706621 inhibited cell growth independent of p53, retinoblastoma, or P-glycoprotein status; activated apoptosis; and reduced colony formation. At low concentrations, JNJ-7706621 slowed the growth of cells and at higher concentrations induced cytotoxicity. JNJ-7706621 is a unique inhibitor regulating cell cycle progression at multiple points, suggesting that it could be useful for cell cycle analysis and therapy of various cancers, including Ewing's sarcoma.