Combination of chloroquine and GX15-070 (obatoclax) results in synergistic cytotoxicity against pancreatic cancer cells

Pancreatic cancer is an aggressive disease with a poor prognosis. Therefore, new treatment is urgently required. GX15-070 is a pan-Bcl-2 inhibitor which has shown promising antitumor activity in different malignancies. We previously demonstrated that clinically achievable concentrations of GX15-070 caused growth arrest in pancreatic cancer cell lines. However, they only induced minimal levels of apoptosis. We hypothesized that GX15-070 induced autophagy in pancreatic cancer cells which blocked apoptosis. In this study, we investigated the effects of GX15-070 on autophagy and the antitumor activities of the combination of GX15-070 and chloroquine (CQ), an autophagy inhibitor, in six pancreatic cancer cell lines. We found that GX15-070 treatment indeed induced autophagy in 5 of the 6 pancreatic cancer cell lines, reflected by the conversion of LC3B-I to LC3B-II and detection of autophagosomes by transmission electron microscopy. Furthermore, we found additive to synergistic antitumor interactions in all six cell lines by MTT assays. CQ significantly enhanced GX15-070-induced apoptosis in the cell line models, possibly due to downregulation of Bcl-2, Bcl-xL and Mcl-1 in the cells by the two agents. These results provide compelling evidence for the further development of the combination of GX15-070 and CQ in pancreatic cancer.