Abstract 5047: Synergistic anti-cancer activity of the CDK4/6 inhibitor PD-0332991 in combination with 5-fluorouracil-based chemotherapy in human colon cancer cells

Abstract

Abstract Background: The cell cycle activator cyclin-dependent kinase 4 (CDK4) is overexpressed in many cancers and is a promising target for novel anti-cancer therapies. PD-0332991 (Pfizer, Inc) is a highly specific CDK4/6 inhibitor that has proven anti-cancer efficacy in multiple pre-clinical models, and is currently undergoing Phase II clinical testing in humans. PD-0332991 inhibits tumor cell growth by inducing cell cycle arrest. Sequential therapy with PD-0332991 followed by DNA damaging cytotoxic chemotherapy could produce additive or synergistic anti-cancer effects. Our goal was to assess the anti-tumor efficacy of a combination of PD-0332991 and cytotoxic chemotherapy in vitro, and in vivo. We also sought to determine the mechanisms of anti-cancer activity seen with combination therapy. Methods: First, using human colorectal cancer (CRC) cell lines, we performed in vitro cell growth assays to determine the effects of a combination of PD-0332991 and 5-fluorouracil (5-FU) or oxaliplatin. Next, we tested the activity of PD-0332991, oral capecitabine (an oral 5-FU prodrug), or both in mice bearing human CRC xenografts. Finally, we assessed the mechanisms of cell death in vitro and in resected xenograft tumor samples. Results: The combination of PD-0332991 and 5-FU or oxaliplatin exhibited additive/synergistic anti-cancer activity, in vitro. Furthermore, synergistic anti-cancer activity was observed in mice bearing xenografts of HCT 116 human CRC cells upon treatment with a combination of PD-0332991 and capecitabine. Virtually no anti-tumor activity was observed in mice treated with PD-0332991 or capecitabine alone. In assessing the mechanism of anti-tumor activity, in vitro and in resected xenograft samples, no increase in apoptosis was observed, as determined by annexin V and TUNEL staining. Even when cells were synchronized in G1 with PD-0332991 and then released and treated with 5-FU or oxaliplatin, there was no increase in apoptosis. Thus, assessment of alternative mechanisms of cell death, including cell senescence and autophagy are ongoing. Conclusions: Our results demonstrate an additive/synergistic anti-tumor activity with the combination of PD-0332991 and traditional cytotoxic chemotherapy. The mechanisms of anti-tumor activity have yet to be determined, but are not dependent upon apoptosis. Given the synergistic anti-tumor activity, these results suggest that a clinical trial of PD-0332991 and 5-FU-based chemotherapy is warranted. 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 5047.