CeLL CyCLe NeWS & VieWS

Cyclin-dependent kinases (CDKs) drive cell cycle progression and control transcriptional pro-cesses. The dysregulation of multiple CDK family members occurs commonly in human cancer; in particular, the cyclin D-CDK4/6-retinoblastoma protein (RB)-iNK4 axis is universally disrupted, facilitating cancer cell proliferation and prompt-ing long-standing interest in targeting CDK4/6 as an anticancer strategy. Most agents that have been tested inhibit multiple cell cycle and transcriptional CDKs and have carried toxicity. However, several selective and potent inhibitors of CDK4/6 have recently entered clinical trial. PD0332991, the first to be developed, resulted from the introduction of a 2-aminopyridyl sub-stituent at the C2-position of a pyrido(2,3-d) pyrimidin-7-one backbone, affording exqui-site selectivity toward CDK4/6.1 PD0332991 arrests cells in G1 phase by blocking RB phos-phorylation at CDK4/6-specfic sites and does not inhibit the growth of RB-deficient cells.2 Phase i studies conducted in patients with advanced RB-expressing cancers demonstrated mild side effects and dose-limiting toxicities of neutropenia and thrombocytopenia, with prolonged stable disease in 25 % of patients.3,4 in cyclin D1-translocated mantle cell lymphoma, PD0332991 extinguished CDK4/6 activity in patients ’ tumors, resulting in markedly reduced proliferation, and translating to more than 1 year of stability or response in 5 of 17 cases.5 Two recent papers from the Knudsen laboratory make several important obser-vations that will help guide the continued clinical development of CDK4/6 inhibitors. in the study by Dean et al., surgically resected patient breast tumors were grown on a tissue culture matrix in the presence or absence of PD0332991. Crucially, these cultures retained associated stromal components known to play important roles in cancer pathogenesis and therapeutic sensitivities, as well as key histological and molecular features of the pri-mary tumor, including expression of eR, HeR2 and Ki-67. Similar to results in breast cancer cell lines,6 the authors demonstrate that onl