Purpose: Overexpression of cyclin-dependent kinase (CDK) 4 has been shown in a variety of cancers and contributes to tumor cell growth and proliferation. However, the effect of inhibition of CDK4 in ovarian cancer is unknown. Here we investigated the therapeutic effect of CDK4 inhibitor, palbociclib, in combination with paclitaxel in ovarian cancer cells. Experimental Design: Cell viability was determined by MTT assay after incubating with different dosages of palbociclib and/or paclitaxel. Western blot, immunofluorescence, and Calcein AM assays were conducted to determine the potential effects and mechanisms of palbociclib in combination with paclitaxel. CDK4 siRNA was used to validate the outcome of targeting CDK4 in ovarian cancer cells. Immunohistochemistry was performed to evaluate the clinical relevance of CDK4 expression in an ovarian cancer tissue microarray. Results: The combination of palbociclib and paclitaxel significantly enhanced drug sensitivity in both Rb positive (Rb+) and Rb negative (Rb-) ovarian cancer cells. Palbociclib suppressed the phosphorylation of pRb (Ser 807-811, Ser 795, and Ser 780) in multidrug resistant SKOV3TR (Rb+) cells. When combined with paclitaxel, palbociclib induced significant apoptosis in both SKOV3TR (Rb+) and OVCAR8TR (Rb-) cells. Palbociclib also inhibited the activity of P-glycoprotein (Pgp). Additionally, transfection of CDK4 siRNA showed effective knockdown of CDK4 expression and sensitized multidrug resistant SKOV3TR and OVCAR8TR cells to paclitaxel. Furthermore, primary ovarian cancer patients with high expression of CDK4 tended to have unfavorable clinical outcomes. Conclusions: Inhibition of CDK4 either by palbociclib or by specific siRNA can increase the in vitro paclitaxel sensitivity in either Rb+ or Rb- multidrug resistant ovarian cancer cells through increased apoptosis. Therefore, CDK4 may be a promising target in the treatment of ovarian cancer.

Citation Format: YAN GAO, Jacson Shen, Rosemary Foster, Francis J. Hornicek, Zhenfeng Duan. Inhibition of CDK4 sensitizes multidrug resistant ovarian cancer cells to paclitaxel by increasing apoptosis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4678.