Abstract 6369: SHetA2 increases the activity of palbociclib in cervical cancer in vitro and in vivo

Abstract

Abstract Introduction: Standard of care therapies for cervical cancer are harmful to normal cells and cause many side effects. New, less toxic treatments are needed in order to avoid these toxic effects. We are currently studying a promising low-toxicity drug, SHetA2. SHetA2 is a flexible heteroarotinoid that induces apoptosis and G1 cell cycle arrest in cancer cells, but only induces G1 cell cycle arrest in healthy cells. Part of the mechanism of SHetA2 is its degradation of cyclin D1. Cyclin D1 forms a complex with CDK 4/6 and is important for G1 progression. Therefore, we combined SHetA2 with palbociclib. Palbociclib is an FDA approved CDK 4/6 inhibitor that is currently used in the treatment of hormone positive breast cancers and is currently in clinical trials for the treatment of ovarian cancer. Therefore, we hypothesized that SHetA2 and palbociclib would synergistically inhibit growth of cervical cancer in vitro and in cervical cancer cell- derived xenograft tumors. Methods: The effects of SHetA2 and palbociclib alone and in combination were tested on cervical cancer cell lines using the MTT assay. Synergy was evaluated in cervical cancer cell lines by treating the cells with the drugs alone and in combination, then plotting their effects in an isobologram. Protein expression of phosphorylated Rb, and cleaved PARP were measured using western blot. An athymic nude mouse model was utilized to test the SHetA2/palbociclib combination in vivo. Mice were treated every day for 24 days with control solvent, 60mg/kg SHetA2, 100mg/kg palbociclib, or in combination. Differences in tumor growth inhibition between the groups were analyzed using repeated measure one-way ANOVA with multiple comparosions post-hoc analysis. Tumors were evaluated by immunohistochemistry and stained with CD31 and cyclin D1 antibodies. Results: The drug combination consistently demonstrated synergistic activity in three cervical cell lines (CI < 0.5), with strong synergistic (CI < 0.1) activity in two cell lines. There was a consistent increase in cleaved PARP expression and reduction in Rb phosphorylation in the combination group in comparison to all other groups. When compared to control, all treatment groups significantly reduced SiHa xenograft tumor growth (p< 0.05). In post-hoc analysis, the growth inhibition in the combination group was significantly greater compared to the palbociclib only group (p=0.0127). In comparison to tumors in the control group, tumors in the combination treatment group exhibited dramatic reduction of cyclin D1 and angiogenesis. Conclusions: The drug combination was synergistic in vitro and significantly effective in vivo. The mechanism of synergy appears to involve induction of apoptosis and reduction of cyclin D1, phosphorylated Rb, and angiogenesis. Citation Format: Amy Bosley, Rajani Rai, Doris M. Benbrook. SHetA2 increases the activity of palbociclib in cervical cancer in vitro and in vivo [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6369.