SNS-032 is a potent and selective inhibitor of CDK2, 7 and 9 and induces cell death by inhibiting cell cycle progression and the expression of antiapoptotic proteins

2079 The cell cycle-regulated CDKs, CDK1, 2, and 4 have been extensively studied as potential therapeutic targets in cancer. Recent research has additionally underscored the potential role of several constitutively active CDKs including CDK7 and 9 as cancer targets. Phosphorylation of the c-terminal domain (CTD) of RNA Polymerase II by CDK7 and 9 are critical steps in transcriptional regulation. Inhibition of these kinases is predicted to have the greatest effect on the expression of proteins with short T \(\frac{1}{2}\) and short-lived mRNA, including proteins involved in apoptotic regulation. CDK7 also activates cell-cycle CDKs 1, 2, 4 and 6. SNS-032 (formerly BMS-387032) has previously been described as a selective inhibitor of CDK2 with potent antitumor activity in animal models. Here we show that in addition to inhibition of CDK2, SNS-032 also inhibits CDK7/cyclinH and CDK9/cyclinT at low nanomolar concentrations in biochemical assays. The compound is highly selective for CDK inhibition; in a panel of 208 kinases, only four non-CDK proteins were inhibited by >50% at 1 μM SNS-032. The cellular pharmacology of SNS-032 mirrors the biochemical data. Cells treated with SNS-032 show a rapid cell cycle arrest and onset of cell death that corresponds with inhibition of multiple substrates of CDK2, 7, and 9. For instance, inhibition of pRb phosphorylation, accumulation of cyclin E protein, and cell-cycle arrest at G1 and G2 are observed in multiple cell lines in a time- and dose-dependent manner, consistent with inhibition of CDK2 and CAK. Furthermore, SNS-032 inhibits CDK9-mediated phosphorylation of Ser2 in the CTD with an IC50= 20 nM. Corresponding with inhibition of RNA polymerase II, the short half-life, anti-apoptotic protein Mcl-1 is rapidly depleted from cells, coincident with the phosphorylation of p53. Expression of Mcl-1 is a candidate predictor of aggressive disease and resistance to chemotherapy in CLL and is essential for survival of B-cell lymphoma and multiple myelomas, supporting the use of SNS-032 as a treatment for these diseases. SNS-032, a selective inhibitor of multiple CDKs involved in apoptosis and cell cycle regulation, has potential for antitumor activity in both solid and hematological cancers. SNS-032 is currently in phase 1 clinical studies.