Abstract 693: Inhibitory effect of the CDK4/6 inhibitor, PD 0332991, is enhanced by mTOR inhibition in Non-Small Cell Lung Cancer (NSCLC).

Abstract The p16 protein plays a critical role in the regulation of the retinoblastoma (RB) pathway, which inhibits cell growth through cell cycle regulation. The p16 gene, CDKN2a, is inactivated in 70% of NSCLC tumors, resulting in unregulated activation of CDK 4/6, and consequently increased cell cycling. The drug PD 0332991 (PD) is a highly specific inhibitor of CDK4/6. Western blots of p16-deficient NSCLC cell lines have confirmed a decrease in phosphorylated RB with PD-treated cells compared to untreated cells. We have observed a 34% decrease in viability by an MTS assay, and a 30% increase in cell senescence by a β-galactosidase assay in PD-treated p16-deficient cell lines compared to untreated cells. Based on these data, we are currently conducting a single-arm phase II clinical trial of single-agent PD 0332991 in patients with advanced NSCLC who have failed at least one prior chemotherapy regimen. We hypothesized that a combination of PD with other targeted therapies, acting on the same and intersecting pathways, would result in increased tumor cell death. Using MTS viability assays, we tested PD with and without other inhibitors in p16-deficient NSCLC cell lines. These inhibitors were: 1) the demethylating agent, decitabine, since the p16 gene, CDKN2a, is most often inactivated by hypermethylation (PD 66.1%, decitabine 49.7%, combination 80.2% viability); 2) the MEK inhibitor, AZD 6244, based on a report of a potential synthetic lethal interaction between K-Ras and CDK4 (PD 66.1%, AZD 6244 90.1%, combination 72.0% viability); 3) the CDK1/2/9 inhibitor, AZD 5438, since CDK 1 and 2 are thought to play a role in resistance to CDK 4/6 inhibition (PD 66.1%, AZD 5438 76.3%, combination 96.6% viability), and 4) the mTOR inhibitor, everolimus, since the mTOR pathway can upregulate cyclin D1, which complexes with CDK4/6 in the RB pathway (PD 67.3%, everolimus 65.1%, combination 46.4% viability, p<0.02 for PD compared to combination). Based on our studies, we saw that the combination of PD and everolimus significantly decreased cell viability. Western blots of p16-deficient NSCLC cell lines showed a further decrease in phosphorylated RB with the combination compared with PD alone. These data are promising, and we will be further determining the mechanism of inhibition of this combination. This study provides the groundwork for a future clinical trial of everolimus and PD 0332991 in combination in NSCLC patients with p16-deficient tumors. Citation Format: Priya K. Gopalan, Andres Gordillo-Villegas, Maria Zajac-Kaye, Frederic J. Kaye. Inhibitory effect of the CDK4/6 inhibitor, PD 0332991, is enhanced by mTOR inhibition in Non-Small Cell Lung Cancer (NSCLC). [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 693. doi:10.1158/1538-7445.AM2013-693