Winterhoff, Boris

Abstract Background: Cell cycle dysregulation is a common molecular finding in ovarian cancer and the cyclin-D kinases (CDK) represent an attractive target in this pathway. PD 0332991 (Pfizer Inc.) is an orally active potent and highly selective inhibitor of CDK 4 and CDK6 kinases with the ability to block pRb phosphorylation at serine 780 and 795 in the low nanomolar range. To identify predictors of response to PD 0332991, we determined the in vitro sensitivity to PD 0332991 in a large panel of molecularly characterized ovarian cancer cell lines, and then used baseline expression profiles and Western blot analyses to identify biomarkers associated with response to PD 0332991. These biomarkers were then validated in a large cohort of 324 ovarian cancer patients by immunohistochemistry. Methods: 40 human ovarian cancer cell lines were treated with PD 0332991 using two-fold dilutions over 12 concentrations. Dose response curves were generated using a cell count assay to calculate the IC50 across the panel. These data were then analyzed against baseline gene expression data to identify genes associated with sensitivity to PD 0332991. Western blot analysis was performed to validate these markers and to study the effects of PD 0332991 on pRb phosphorylation. Cell cycle analysis was performed using flow-cytometry. Combination studies were performed to analyze the interaction between PD 0332991 and paclitaxel or carboplatin chemotherapy. Tissue microarrays were constructed from 324 ovarian cancer patients and IHC was performed for p16 and Rb expression. Long-term clinical follow up was available for all patients. Results: Concentration-dependent anti-proliferative effects of PD 0332991were seen in all ovarian cancer cell lines tested, but varied significantly between individual cell lines with up to a 3 log-fold difference in the IC50s (IC50 range: 0.02 to >20 μmol/L). Cell lines with high Rb expression and low p16 expression by gene expression array and Western blot analyses were most sensitive to inhibition by PD 0332991. Cell lines treated with PD 0332991 showed clear G0/G1 arrest and a decrease in S-phase fraction in sensitive but not in resistant cell lines. In addition, Western blot showed that pRb phosphorylation is blocked in a concentration and time dependent manner. IHC for Rb and p16 allows the identification of ovarian cancer patients most likely to benefit from CDK4/6 inhibition. Outcome analyses are ongoing. Conclusion: PD 0332991 shows promising biologic activity in ovarian cancer cells. Assessment of Rb and p16 expression may help select patients who benefit from therapeutic strategies targeting CDK4/6 in ovarian cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 25.

Abstract Purpose: PD-0332991 is a selective inhibitor of the CDK4/6 kinases with the ability to block retinoblastoma (Rb) phosphorylation in the low nanomolar range. Here we investigate the role of CDK4/6 inhibition in human ovarian cancer. Experimental Design: We examined the effects of PD-0332991 on proliferation, cell-cycle, apoptosis, and Rb phosphorylation using a panel of 40 established human ovarian cancer cell lines. Molecular markers for response prediction, including p16 and Rb, were studied using gene expression profiling, Western blot, and array CGH. Multiple drug effect analysis was used to study interactions with chemotherapeutic drugs. Expression of p16 and Rb was studied using immunohistochemistry in a large clinical cohort of ovarian cancer patients. Results: Concentration-dependent antiproliferative effects of PD-0332991 were seen in all ovarian cancer cell lines, but varied significantly between individual lines. Rb-proficient cell lines with low p16 expression were most responsive to CDK4/6 inhibition. Copy number variations of CDKN2A, RB, CCNE1, and CCND1 were associated with response to PD-0332991. CDK4/6 inhibition induced G0/G1 cell cycle arrest, blocked Rb phosphorylation in a concentration-and time-dependent manner, and enhanced the effects of chemotherapy. Rb-proficiency with low p16 expression was seen in 97/262 (37%) of ovarian cancer patients and was independently associated with poor progression-free survival (adjusted relative risk 1.49, 95% CI 1.00–2.24, P = 0.052). Conclusions: PD-0332991 shows promising biologic activity in ovarian cancer cell lines. Assessment of Rb and p16 expression may help select patients most likely to benefit from CDK4/6 inhibition in ovarian cancer. Clin Cancer Res; 17(6); 1591–602. ©2011 AACR.