Cell cycle modulators for the treatment of lung malignancies

It has become clear in the past decade that most human malignancies, including lung neoplasms, have aberrations in cell cycle control. The tumor suppressor gene retinoblastoma is an important player in the G1/S transition and its function is abnormal in most human neoplasms. Retinoblastoma function is lost as a result of phosphorylation by the cyclin-dependent kinases (CDKs). Thus, modulation of CDKs may have an important use for the therapy and prevention of human neoplasms. Direct CDK modulators are small molecules that target specifically the adenosine triphosphate binding site of CDKs. In contrast, indirect CDK modulators affect CDK function by modulation of upstream pathways required for CDK activation. The first example of a direct small-molecule CDK modulator tested in the clinic, flavopiridol, is a pan-CDK inhibitor that not only promotes cell cycle arrest but also halts transcriptional elongation, promotes apoptosis, induces differentiation, and has antiangiogenic properties. The second example of direct small-molecule CDK modulators tested in clinical trials is UCN-01 (7-hydroxystaurosporine). UCN-01 has interesting preclinical features: it inhibits Ca2+-dependent protein kinase C, promotes apoptosis, arrests cell cycle progression at G1/S, and abrogates checkpoints upon DNA damage. In summary, novel small-molecule CDK modulators are being tested in the clinic with interesting results. Although these small molecules are directed toward a very prevalent cause of carcinogenesis, their role in the clinical armamentarium is still uncertain.