Abstract A251: MK‐1775, a potent Wee1 inhibitor, synergizes with gemcitabine to achieve tumor regressions in pancreatic tumor xenografts

Background and Aim: Treatment efficacy of DNA damaging agents is determined not only by the amount of therapy‐induced DNA damage but also by the capacity of tumor cells to repair the damaged DNA. When cells are treated with DNA damaging agents, both G 1 and G 2 /M cell cycle checkpoints are activated, leading to cell cycle arrest, thus providing time for the cell to repair the damage and to evade apoptosis before resuming cell cycle. Tumor cells can exploit these repair mechanisms in response to DNA damaging chemotherapeutics, rendering tumors refractory to therapy. Since G 1 checkpoint is frequently compromised due to loss‐of‐function mutation in p53 gene in approximately half of cancers, G 2 /M checkpoint play a pivotal role in preventing the programmed cell death in p53 null tumors. Wee1 kinase, which acts as a critical driver of G 2 ‐M cell cycle progression, plays a critical role in maintaining G 2 arrest through its inhibitory phosphorylation of cdc2. G 2 checkpoint abrogation by Wee1 inhibition may therefore sensitize p53 deficient tumor cells to anti‐cancer agents. MK‐1775, a potent inhibitor of Wee1 tyrosine kinase, is currently being evaluated in several phase I clinical trials. Here, we investigated the efficacy and pharmacodynamic effects of MK‐1775 in both monotherapy and in combination with gemcitabine in pancreatic cancer xenografts with p53 null/intact status. Methods: Fresh pancreatic tumor pieces obtained from patients at the time of surgery in Johns Hopkins Hospital were implanted (s.c) in female athymic mice and grafted tumors were propagated as a live PancXenoBank . Ten patient xenografts (7 with p53 mutated and 3 with p53 wild type) were allowed to grow separately on both flanks of mouse. When tumor reached a volume of ∼200 mm 3 , mice were individually identified and randomly assigned to treatment groups, with 5–6 mice (8–10 evaluable tumors/group) in 1) control; 2) MK‐1775 (30 mg/kg, p.o., twice a week) for 4 weeks; 3) gemcitabine (100 mg/kg, i.p., twice a week) for 4 weeks; 4) gemcitabine followed 24 h later by MK‐1775 in the above dose and frequency. Tumor growth rate/regressions were calculated on day 28. Target modulation by MK‐1775 was assessed by western blot and IHC. Results: MK‐1775 treatment leads to the inhibition of Wee1 kinase and its substrate phospho‐cdc2. Combination of gemcitabine with MK‐1775 abrogated the G 2 arrest to promote the mitotic entry and facilitate tumor cell death. Although, MK‐1775 monotherapy resulted in >50% inhibition of tumor growth in three of ten xenografts compared to control tumors, it did not result in tumor regressions. However, combination of gemcitabine with MK‐1775 produced robust anti‐tumor activity and remarkable increase in tumor regression (3.39 fold) in p53 mutated tumors compared to gemcitabine treatment. Tumor re‐growth curves after the treatment suggest that the effect of the combination therapy is longer‐lasting than that of gemcitabine. None of the agents or combination therapy produced tumor regressions in p53 wild type cases. Conclusions: Our results provide compelling evidence that MK‐1775 synergizes with gemcitabine to achieve tumor regressions, selectively in p53 mutated pancreatic tumor xenografts and this combination merits clinical testing. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A251.