Abstract B37: Mesothelin-targeted immunotoxin RG7787 (LMB-100) preferentially depletes secreted proteins and short-lived intracellular proteins to augment tumor cell killing by taxanes

Mesothelin (MSLN) is a cell surface antigen expressed by a number of solid tumors including most pancreatic adenocarcinomas. RG7787 is a MSLN-targeted immunotoxin that is entering clinical testing for patients with advanced pancreatic cancer. RG7787 binds to MSLN on the cancer cell surface through a high affinity Fab and delivers a modified Pseudomonas exotoxin A (PE) payload into the cell cytosol. PE induces apoptosis by catalyzing the irreversible modification and inactivation of elongation factor-2 (EF-2), halting protein synthesis. This is a novel mechanism of action which produces synergistic tumor killing in combination with nab-paclitaxel in preclinical mouse models and in cell culture. Our goal is to understand the mechanism of this interaction. We have previously demonstrated that taxane does not increase delivery of RG7787 to tumors grown in mice and that taxane does not augment protein synthesis inhibition caused by RG7787. We hypothesized that RG7787 should deplete levels of secreted proteins and short-lived intracellular proteins and that insufficiencies of these factors may enhance taxane-mediated killing. To examine the effect of RG7787 on the pancreatic cancer secretome, we treated pancreatic cancer cells grown in culture or mice bearing tumor xenografts with RG7787 and used ELISA to examine levels of secreted proteins in conditioned medium and interstitial tumor fluid. We found that RG7787 treatment causes dose-dependent decreases in secreted proteins even at non-lethal doses. To demonstrate proof of principle that short-lived proteins are preferentially depleted, we created a reporter cell line co-expressing both GFP and a destabilized fluorescent Cherry protein (Cherry-PEST). We found that treatment with RG7787 reduced the levels of Cherry-PEST but not GFP in a dose-dependent fashion confirming our hypothesis. Consistent with this, we found that levels of the short-lived Bcl-2 family survival factor Mcl-1 are reduced in cells treated with the combination of RG7787 and taxane compared to taxane treatment alone. We conclude that RG7787 depletion of Mcl-1 primes cells for killing by taxane. We are now testing the combination of RG7787 (LMB-100) with nab-paclitaxel in a Phase IB/II trial for patients with pancreatic cancer. Citation Format: Christine Alewine, Rebekah Landsman, Michael Rudloff, Emily Kolyvas, Jinqiu Chen, Ira Pastan.{Authors}. Mesothelin-targeted immunotoxin RG7787 (LMB-100) preferentially depletes secreted proteins and short-lived intracellular proteins to augment tumor cell killing by taxanes. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr B37.