Abstract 4505: Anti-tumor activity in pancreatic cancer of a low immunogenic and clinically optimized anti-mesothelin immunotoxin RG7787
Authors: Pastan Ira , Hollevoet Kevin , Mason-Osann Emily , Alewine Christine , Liu Xiu-Fen 
Published: May 2015

Abstract
Background
Pancreatic ductal adenocarcinoma (PDAC) has a poor prognosis and new therapies are needed. SS1P is a recombinant immunotoxin (RIT) containing an anti-mesothelin dsFv fused to a cytotoxic fragment of Pseudomonas Exotoxin A (PE). In clinical trials, patients developed dose-limiting neutralizing anti-drug antibodies within the first cycle of treatment. RG7787 is a next generation, clinically optimized RIT with decreased immunogenicity and increased resistance to lysosomal proteases. In this study, we evaluated the in vitro and in vivo activity of RG7787 in PDAC.
Method
Five established PDAC cell lines (KLM-1, AsPC-1, BxPC-1, BxpC3, Panc 3.014, and PK-1) and one primary PDAC line directly obtained from a patient tumor (GUMC108) were used. Cell viability and proliferation were calculated by measuring ATP levels with Cell Titer-Glo assays. In vivo activity was evaluated in a subcutaneous KLM-1 mouse tumor model. Delivery of fluor-labeled RG7787 to mouse tumors was assessed by flow cytometry.
Results
RG7787 had significant in vitro activity in the PDAC cell lines tested with most IC50 values in the subnanomolar range. It was significantly more active than SS1P in KLM-1, Panc 3.014 and GUMC108, with similar activity in the other cell lines. GUMC108 (IC50 = 18.9 pM) was the most sensitive cell line. In KLM-1 cells, SS1P and RG7787 had similar internalization rates. RG7787 could be safely administered to mice at a dose of 2.5 mg/ kg IV QOD x3, which is more than five times the maximum tolerated murine dose of SS1P. In vivo uptake studies in a KLM-1 subcutaneous mouse xenograft model demonstrated intracellular delivery of RG7787-Alexa647 to 50% of tumor cells after a single 2.5 mg/kg IV dose. Tumor volume was reduced by 13% after a single cycle of RG7787 treatment (n=6). Two cycles of combination treatment with paclitaxel (50 mg/kg IP) and RG7787 shrank tumors to &lt;10 mm3 in 100% of mice by 30 days (n = 11 over 2 experiments). Two single doses of paclitaxel alone (n=6) induced stable disease without significant tumor regression. Complete remission persisted through day 100 in 2/11 combination treated animals.
Conclusions
RG7787 has superior or similar activity compared to SS1P in established PDAC cell lines and a primary PDAC cell line in vitro. In a KLM-1 mouse xenograft model, the combination of paclitaxel with RG7787 cured 18.1% of mice and reduced tumor size by more than 90% in all other mice treated. RG7787 merits further development for the treatment of pancreatic cancer.
Citation Format: Ira Pastan, Kevin Hollevoet, Emily Mason-Osann, Christine Alewine, Xiu-Fen Liu. Anti-tumor activity in pancreatic cancer of a low immunogenic and clinically optimized anti-mesothelin immunotoxin RG7787. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4505. doi:10.1158/1538-7445.AM2014-4505