In a routine screening of our small-molecule compound collection we recently identified 4-arylazo-3,5-diamino-1H-pyrazoles as a novel group of ATP antagonists with moderate potency against CDK2-cyclin E. A preliminary SAR study based on 35 analogues suggests ways in which the pharmacophore could be further optimized, for example, via substitutions in the 4-aryl ring. Enzyme kinetics studies with the lead compound and X-ray crystallography of an inhibitor-CDK2 complex demonstrated that its mode of inhibition is competitive. Functional kinase assays confirmed the selectivity toward CDKs, with a preference for CDK9-cyclin T1. The most potent inhibitor, 4-[(3,5-diamino-1H-pyrazol-4-yl)diazenyl]phenol 31b (CAN508), reduced the frequency of S-phase cells of the cancer cell line HT-29 in antiproliferation assays. Further observed cellular effects included decreased phosphorylation of the retinoblastoma protein and the C-terminal domain of RNA polymerase II, inhibition of mRNA synthesis, and induction of the tumor suppressor protein p53, all of which are consistent with inhibition of CDK9.

Poor therapeutic outcomes and serious side effects, together with acquired resistance to multiple drugs, are common problems of current cancer therapies. Therefore, there is an urgent need for new cancer-targeted drugs, which has led (inter alia) to the development of molecules that can specifically inhibit cyclin-dependent kinases (CDKs). In addition to their cell cycle regulatory functions, CDKs, especially CDK7 and CDK9, play important roles in the regulation of RNA polymerase II-mediated transcription. Here, we report on progress in the preclinical development of CDK inhibitors and their anticancer activities. Special attention is paid to the action mechanisms of the pan-specific CDK inhibitors flavopiridol and roscovitine, which have already entered phase II clinical trials as treatments for various tumours. The links between their ability to inhibit transcription and sensitisation of some types of cancer to apoptosis, mechanisms leading to p53 activation, and their synergistic cooperation with common DNA damaging drugs are also discussed. It has been demonstrated that drug-resistant cancer cells can arise during therapeutic application of small molecule protein kinase inhibitors. Clinical resistance to CDK inhibitors has not yet been described, but by comparing CDKs to other kinases, and CDK inhibitors to other clinically used protein kinase inhibitors, we also discuss possible mechanisms that could lead to resistance to CDK inhibitors.

Deregulation of cyclin-dependent kinases (CDKs) has been associated with many cancer types and has evoked an interest in chemical inhibitors with possible therapeutic benefit. While most known inhibitors display broad selectivity towards multiple CDKs, recent work highlights CDK9 as the critical target responsible for the anticancer activity of clinically evaluated drugs. In this review, we discuss recent findings provided by structural biologists that may allow further development of highly specific inhibitors of CDK9 towards applications in cancer therapy. We also highlight the role of CDK9 in inflammatory processes and diseases.

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Embryonic stem cells (ESCs) proliferate rapidly and have a unique cell-cycle structure with a very short G1 phase. Previous reports suggested that the rapid G1 phase progression of ESCs might be underpinned by high and precocious Cdk2 activity and that Cdk2 activity might be crucial for both cell-cycle regulation and cell-fate decisions in human ESCs. However, the actual role of Cdk2 in cell-cycle progression of mouse ESCs (mESCs) has not been elucidated. In this study, we investigated the effects of down-regulation of Cdk2 activity by olomoucine II in 2 mESC lines. Olomoucine II treatment significantly increased the G1 phase cell numbers, decreased the S phase cell numbers, and inhibited DNA replication in mESCs. In nocodazole-synchronized mESCs, we show that specific down-regulation of Cdk2 activity prolongs G1 phase progression. In addition, down-regulation of Cdk2 activity in mESCs established a somatic cell-like cell cycle and induced expression of differentiation markers. Our results suggest that high Cdk2 activity is essential for rapid G1 phase progression and establishment of ESC-specific cell-cycle structure in mESCs and support the hypothesis of a link between cell-cycle regulation and pluripotency maintenance in ESCs. This study reveals olomoucine II to be an effective tool for manipulation of the cell cycle and pluripotency in ESCs and very likely also for the manipulation of other stem cell types, including cancer stem cells.

A novel chemical tool for the rapid preparation of PROTAC conjugates by persons without extensive synthetic experience or special laboratory equipment.

Roscovitine is a synthetic inhibitor of cyclin-dependent kinases that is currently undergoing clinical trials as a candidate drug for some oncological indications. Its discovery prompted many research teams to further optimize its structure or to initiate their own related but independent studies. This article reviews known roscovitine bioisosteres that have been prepared as CDK inhibitors using different core heterocycles. The individual bioisostere types have been described and explored to a different extent, which complicates direct comparisons of their biochemical activity - only six direct analogs containing different purine bioisosteres have been prepared and evaluated side by side with roscovitine. Only four types of bioisosteres have demonstrated improved biological properties, namely pyrazolo[ 1,5-a]-1,3,5-triazines, pyrazolo[1,5-a]pyrimidines, pyrazolo[1,5-a]pyridines and pyrazolo[4,3-d]pyrimidines.