BI 6727 and gemcitabine combination therapy in pancreatic cancer

Abstract

Introduction: Though adjuvant chemotherapy improves survival following surgery, pancreatic cancer carries a poor prognosis. Alternatives are required to the current drug regimens consisting of S-phase dependent drugs such as gemcitabine. PLK1 is a passenger protein involved in G2/M phases which presents a novel target to inhibit in combination with current therapies, which may help overcome inate and acquired resistance. Aim: To evaluate the potential role of a novel PLK1 inhibitor, BI 6727 in pancreatic cancer – both as monotherapy and in combination with gemcitabine in vitro. Methods: The IC50 concentrations of both drugs were established in Suit-2, BxPC-3, Panc-1 and MiaPaCa-2 pancreatic cancer cell lines and isobolar analyses undertaken with a variety of combination therapies. Cell cycle analyses were performed with Flow Activated Cell Sorting, with apoptosis and necrosis quantified on the basis of phosphatidylserine cell surface exposure, propidium iodide staining and cleavage of caspase-3. Results: IC50 ranges for BI 6727 and gemcitabine were 53-77nM and 11-34nM respectively across four pancreatic cell lines. Flow cytometry of MiaPaCa-2 cells demonstrated G2 arrest with BI 6727 and S-phase accumulation with gemcitabine monotherapy. Isobolar analyses showed that when added together the combination of drugs was additive, but BI 6727 pretreatment followed by combination was synergistic. Western blotting for cleaved caspase-3 showed evidence of apoptosis with gemcitabine monotherapy but none with BI 6727 treatment. Although membrane inversion was seen with synergistic drug combinations there was no evidence of cleaved-caspase-3, suggesting a modified form of apoptosis. Conclusion: BI 6727 is effective against a variety of pancreatic cancer cells in vitro. Synergy is demonstrated in MiaPaCa-2 cells when BI 6727 is administered prior to combination therapy with gemcitabine, though mode of cell death does not appear to be caspase-dependent. This supports the concept that PLK1 inhibition can overcome gemcitabine resistance in some cells by allowing resistant cells to initiate gemcitabine induced apoptosis, although this is dependent on drug phasing and the full apoptotic pathway may not be achieved.