Current Medicinal Chemistry - Anti-Cancer Agents - Volume 3, Issue 1, 2003

The importance of Cdks in cell cycle regulation, their interaction with oncogenes and tumor suppressors, and their frequent deregulation in human tumors, has encouraged an active search for agents capable of perturbing the function of Cdks. In our laboratories, a variety of selective and potent low molecular weight inhibitors directed against the ATP binding sites of the Cdk1, Cdk2 have been developed. Extensive biological profiling of two distinct classes of Cdk inhibitors - the phenylamino pyrimidines (PAPs) and trisubstituted purines has revealed distinct differences in their cellular effects in normal cells compared to tumor cells. Due to their intact G1 / S checkpoints, normal cells are shown to be reversibly blocked by these Cdk inhibitors in either the G1 / S-phase or at the G2 / M boarder. In transformed cells these control points are either absent or defective and treatment with the compounds resulted in pronounced proliferation block at the G2 / M transition. Furthermore, there is strong evidence that this G2 / M arrest is less well tolerated by the cells and consequently, they undergo apoptotic cell death. Finally, these dual Cdk1 / Cdk2 inhibitors are also found to be significantly more active on proliferating cells compared to quiescent cells reflecting their specific activity. Despite these encouraging results demonstrating a distinct outcome after treatment with such dual Cdk inhibitors in normal compared to de-regulated tumor cells, it remains to be determined whether a comparable therapeutic window might be observed in vivo experiments. Furthermore the intracellular kinase selectivity of inhibitors which are putatively selective in vitro remains a complicating feature that is only recently begun to be addressed by affinity chromatography and phosphoproteomics techniques. Once efficacy can be demonstrated in animal models at well-tolerated doses, there will be strong evidence for the development of cell cycle antagonists for cancer therapy.

Inhibition of cyclin-dependent kinases is a theme of major interest in current anti-cancer agents research. Different classes of chemical inhibitors of these enzymes have been identified during the past decade and the structural basis of inhibition has been elucidated by X-ray crystallography studies of one member of the family, CDK2. In this article, we review the structural biology work that has led to a precise knowledge of the interactions between CDK2 and small organic molecules binding to its ATP pocket that are determinant for inhibitory activity. The use of this information to design or optimize CDK inhibitors by molecular modeling is also reviewed.

Aurora kinases are a family of mitotic serine-threonine kinases (S / T kinases), that functions as a class of novel oncogenes and are over-expressed in several solid tumors including breast, ovary, prostate, pancreas and colorectal cancer. To validate human ARK1 (Aurora2, STK15, HsAIRK1) as a drugable target in pancreatic cancer, we undertook a structure-based approach to design specific inhibitors utilizing homology modeling, affinity docking and an in vitro kinase assay in an iterative process. In this review, we discuss the biology, rationale for targeting and approaches taken to inhibit this family of protein kinases, implicated in dysregulated chromosome segregation and cytokinesis.

Cell cycle checkpoints are activated in response to DNA-damage to ensure that accurate copies of the cellular genome are passed on to the next generation and to avoid replication and segregation of damaged DNA. These cellular control systems can be overcome by combining conventional DNA-damaging agents with compounds that target the cell cycle regulatory pathways, to enhance cytotoxicity. Tumor cells often comprise a corrupted G1 cell cycle checkpoint while the G2 cell cycle checkpoint is still intact. This review describes the concept of G2 checkpoint abrogation with recognized (methylxanthines, UCN-01) and novel G2 checkpoint abrogators to potentiate the cytotoxicity of DNA-damaging drugs and ionizing radiation. It illustrates the potential of G2 checkpoint abrogators to preferentially sensitize p53-mutated, treatment resistant tumor cells for genotoxic treatment. Identification of the targets of caffeine and UCN-01 to be key-players of the G2 checkpoint (ATM / ATR and Chk1, respectively) promoted the search for novel inhibitors of this checkpoint. Even though a direct causal link between G2 checkpoint abrogation and chemo- / radiosensitization is difficult to prove the multitude of these novel compounds validate that inhibition of critical elements of the G2 checkpoint (ATM / ATR-Chk1 / Chk2- CDC25C-cascade) potentiates the cytotoxicity of DNA-damaging agents.

The cyclin dependent kinases (CDKs) are key regulators of cell cycle progression. Lead compounds (from empirical anti-proliferative screening approaches) have been defined which modulate CDK function and have evidence of anti-proliferative activity in tissue culture systems and in some cases anti-tumor activity in vivo in conventional xenogaft models. Two of these, flavopiridol and UCN-01, have entered initial clinical testing. Flavopiridol is a “pan-CDK” inhibitor, with essentially equal potency in inhibiting all CDKs tested. The recent elucidation that in addition to cell cycle regulatory functions, CDK family members have been defined which regulate transcription, neuronal, and secretory function has increased the need for definition of CDK antagonists with greater selectivity. Novel purine, pyrimidine, and benzazepinone derivatives have been characterized in part through the National Cancer Institute's drug screening systems. UCN-01, in contrast to flavopiridol, modulates CDK activity participating in the DNA damage response, possibly through potent inhibition of the chk1 checkpoint kinase, as well as affecting CDK function indirectly through activity on other kinase targets. An unexpected feature in its development has been avid binding to α1 acid glycoprotein. Further progress in CDK modulator development will require the definition of additional lead structures that address issues raised by these early molecules entering into clinical development.

The recognition of cyclin-dependent kinase (CDK) / cyclin complexes by various cell-cycle regulatory proteins, including certain tumour suppressors and transcription factors, occurs at least in part through a protein-protein interaction with a binding groove on the cyclin subunit. Since CDK function is generally deregulated in tumour cells, blocking of this recruitment site prevents recognition and subsequent phosphorylation of CDK substrates and offers a therapeutic approach towards restoration of checkpoint control in transformed cells. Here we discuss the finding that peptides derived from such cyclin-interacting proteins, and rendered permeable through conjugation to cellular delivery vectors, can apparently induce tumour cells to undergo apoptosis selectively. We review the current status of 3D-structural information available on cyclin-peptide interactions and we summarise our extensive peptide structure-activity relationship studies in light of this information. We also show how a combination of molecular modelling and introduction into synthetic peptides of peptidomimetic elements, such as non-natural amino acid residues and conformational constraints, is being used hopefully to arrive at drug candidates capable of modulating CDK function in a selective mechanism-based approach rather than through ATP antagonism.