Cancer Prevention Research Chemoprevention of Mouse Intestinal Tumorigenesis by the Cyclin-Dependent Kinase Inhibitor SNS-032

Despite advances in screening and treatment, colorectal cancer remains the second leading cause of cancer-related death in the United States. Cyclin-dependent kinases (Cdk) are deregulated in colorectal cancer by silencing of the Cdk inhibitor p16 and other mechanisms. We tested whether the small molecule Cdk inhibitor SNS-032 (formerly BMS-387032), which targets Cdk2, Cdk7, and Cdk9, can prevent intestinal tumorigenesis in mouse models. We generated mice with high intestinal tumor loads by combining the multiple intestinal neoplasia (Min) mutation with Ink4a/Arf mutations and inducing colitis with dextran sulfate sodium. p16-null Min mice (n = 17) began dextran sulfate sodium treatment at week 5 and i.p. injection of carrier or SNS-032 at week 6. Mice were sacrificed at week 12. SNS-032 was well tolerated and reduced colon tumor burden to 36% of that in carrier-treated mice (P < 0.001). We then extended the study to Ink4/Arf-null Min mice (n = 14) and increased the drug dose frequency. SNS-032 treatment reduced the intestinal tumor number to 25% and intestinal tumor burden to 16% of carrier-treated mice (P < 0.0001). DNA synthesis in non-neoplastic and tumor epithelial cells, detected by bromodeoxyuridine incorporation, was modestly reduced by acute SNS-032 treatment. The mitotic index, detected by histone H3 phosphorylation, was distinctly decreased (P < 0.03), and apoptosis, detected by caspase 3 activation, was increased (P < 0.005). These results show the chemoprevention of intestinal tumorigenesis by SNS-032. Our findings support further study of Cdk inhibitors for chemoprevention and therapy of colon cancer. Despite advances in screening and treatment, colorectal cancer remains the second leading cause of cancer-related deaths in the United States. Patients with inherited predispositions, chronic ulcerative colitis, or a personal history of colon tumors are at particular high risk. Chemoprevention constitutes an appealing alternative method to combat the disease in such patients. Most chemoprevention has targeted inflammatory mediators (1–3). Among other molecular pathways that are deregulated in colorectal cancer are the cyclin-dependent kinases (Cdk), enzymes that promote cell cycle progression and transcription of genes involved in cell replication and survival. The Cdk inhibitor p16 is frequently inactivated in sporadic and ulcerative colitis–associated neoplasia (4–7). Several small molecule Cdk inhibitors have been developed for cancer therapy and are undergoing clinical investigation (8, 9). Although Cdk inhibitors have been studied for activity against human colorectal cancer cell lines in mouse xenograft studies (10), to our knowledge, no studies have tested their efficacy in treating intestinal tumors arising in situ. Xenograft studies have the advantage of assessing drug efficacy against human colorectal cancer cells but carry the drawbacks of using immunocompromised mice and tumor growth in an artificial setting, typically a pocket of subcutaneous tissue formed by needle injection. We focused our studies on the potential of a newer inhibitor, SNS-032 (11, 12), to suppress intestinal tumorigenesis in a preclinical model. Thus, these studies have the advantages of using immunocompetent hosts, avoiding idiosyncrasies of established cell lines, examining tumor growth in native contexts, and allowing drug access via the native vasculature. Furthermore, specific premalignant states and genotypes can be assessed that mimic those found in human populations. Thus, studies of drug effect on tumorigenesis in situ can have valuable implications for both therapy and chemoprevention. Materials and Methods