Killing of resistant cancer cells with low Bak by a combination of an antimesothelin immunotoxin and a TRAIL Receptor 2 agonist antibody.

PURPOSE

Many solid tumors express cell surface mesothelin making them attractive targets for antibody-based therapies of cancer. SS1P [antimesothelin(Fv)PE38] is a recombinant immunotoxin (RIT) that has potent cytotoxic activity on several cancer cell lines and clinical activity in mesothelioma patients. Pancreatic cancers express mesothelin and are known to be resistant to most chemotherapeutic agents. The goal of this study is to treat pancreatic cancer with RIT by targeting mesothelin.

EXPERIMENTAL DESIGN

We measured the cytotoxic activity of an antimesothelin immunotoxin on pancreatic cancer cells. We also measured the levels of several pro- and antiapoptotic proteins, as well as the ability of TNF-related apoptosis-inducing ligand (TRAIL) or the anti-TRAIL receptor 2 agonist antibody (HGS-ETR2) to kill pancreatic cells, and the cytotoxic activity of the two agents together in cell culture and against tumors in mice.

RESULTS

In two pancreatic cancer cell lines, immunotoxin treatment inhibited protein synthesis but did not produce significant cell death. The resistant lines had low levels of the proapoptotic protein Bak. Increasing Bak expression enhanced the sensitivity to immunotoxins, whereas Bak knockdown diminished it. We also found that combining immunotoxin with TRAIL or HGS-ETR2 caused synergistic cell death, and together triggered caspase-8 recruitment and activation, Bid cleavage and Bax activation. Combining SS1P with HGS-ETR2 also acted synergistically to decrease tumor burden in a mouse model.

CONCLUSION

Our data show that low Bak can cause cancer cells to be resistant to immunotoxin treatment and that combining immunotoxin with TRAIL or a TRAIL agonist antibody can overcome resistance.