Pancreatic cancer remains among the most lethal cancers with an overall 5-year survival of less than 5%. Chk1 inhibition has emerged as a promising approach to sensitize pancreatic cancer to gemcitabine (Gem), the current standard therapy for both local control and metastatic disease. Here, we characterized the ability of AZD7762, a small molecule Chk1/2 inhibitor currently in Phase I trials, to sensitize pancreatic cancer cells to Gem both in vitro and in vivo.

We first evaluated the in vitro schedule-dependence of AZD7762-mediated Gem sensitization by clonogenic assay. We found that AZD7762 effectively sensitized pancreatic cancer cell lines to Gem-induced loss of clonogenicity, with shifts in IC50 from 3- to 38-fold, compared to cells treated with Gem alone. At low concentrations of Gem (≤IC50), AZD7762 was most effective when present during Gem treatment. At higher concentrations of Gem, however, giving AZD7762 24 h after Gem resulted in the greatest sensitization. We next assessed the molecular mechanisms of AZD7762-mediated chemosensitization in one of these cell lines, MiaPaCa2, by immunoblot, flow cytometry and immunofluorescent staining. Gemcitabine sensitization was associated with inhibition of Chk1, rather than Chk2 and abrogation of the S-M and G2-M checkpoints. Despite checkpoint abrogation, we found that Gem/AZD7762-treated cells were unable to exit S-phase and, over time, AZD7762 inhibited DNA synthesis, as measured by BrdUrd incorporation. This inhibition was associated with pan γ-H2AX staining in Gem-treated cells, in addition to disruption of Rad51 foci and inhibition of homologous recombination repair activity.

We next evaluated the effects of AZD7762 in two different human pancreatic cancer xenograft models; MiaPaCa2, and a patient-derived early passage pancreatic cancer xenograft, EPX-J. AZD7762 had anti-tumor activity both as a single agent, and in combination with Gem in both MiaPaCa2 and EPX-J, with average times to tumor doubling of 37 and 49 days, respectively, in tumors treated with both Gem/AZD7762, compared to 18 and 29 days for tumors treated with Gem alone (*p≤0.05). Chk1 inhibition, as determined by S296- and S345-Chk1 phosphorylation status, correlated with in vivo response. Finally, we found increased S345-Chk1 phosphorylation in normal tissues from mice treated with Gem/AZD7762, suggesting this may be a useful surrogate marker for AZD7762 activity in vivo.

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 1765.