Abstract 667: IMGN853, a folate receptor (FR) α-targeting antibody-drug conjugate (ADC), is highly effective against xenograft models with clinically relevant levels of receptor expression
Authors: Ab Olga , Bartle Laura M. , Sun Xiuxia , Wu Rui , Johnson Holly A. , Whiteman Kathleen R. , LaBelle Alyssa , Goldmacher Victor S. 
Published: May 2015

Abstract
IMGN853 comprises the anti-FR-α antibody M9346A conjugated to the cytotoxic maytansinoid molecule DM4 via a stable disulfide-containing linker, sulfo-SPDB. IMGN853 is currently in phase I clinical testing; promising clinical results were reported at ASCO 2013.
To analyze determinants of the sensitivity of FR-α positive cells to IMGN853, we examined antigen density on the cell surface, internalization and processing of IMGN853, and sensitivity of the cells to IMGN853 and to non-conjugated maytansinoid, S-methyl-DM4, in vitro. Antigen density, internalization and processing were analyzed with 3[H] M9346A. Preliminary experiments demonstrated that 3[H] M9346A is an appropriate surrogate reagent to analyze binding and processing of IMGN853. Sensitivity of the cells to IMGN853 and to the unconjugated maytansinoid was analyzed by a cytotoxicity assay.
A panel of seven FR-α positive cell lines was used for the study. Three of the cell lines were sensitive to IMGN853 in an antigen-dependent manner (the cytotoxic effect was blocked in the presence of a saturating excess of the non-conjugated antibody), while four lines were insensitive (the same low cytotoxic activity of IMGN853 was observed with and without the non-conjugated antibody). The proportion of cell surface-bound IMGN853 that was internalized and processed by the sensitive and insensitive cell lines was comparable. The cell lines differed in their FR-α expression, and there was a strong positive correlation between the sensitivity of cells towards IMGN853 and antigen expression on their surface. The cell lines were similar in their sensitivity to S-methyl-DM4. Taken together, the data indicate that the FR-α density on the surface of a cell is a key determinant of its sensitivity to IMGN853.
Having observed a correlation between antigen density and sensitivity of the cells to IMGN853, we set out to test IMGN853 activity in xenograft models expressing FR-α at levels comparable to those on patient tumor cells. First, we developed an IHC assay with the use of anti-FR-α antibody BN3.2 (Novocastra Laboratories, UK) that detected various levels of receptor expression in tumor samples. Next, we compared FR-α expression in ovarian carcinoma (OvCa), non-small cell lung carcinoma (NSCLC) samples and in human xenograft models, and examined five models that represented antigen densities of the majority of FR-α positive OvCa and NSCLC patients. We evaluated the anti-tumor activity of IMGN853 in SCID mice bearing these xenograft models after single intravenous injections of 5, 2.5 or 1.2 mg/kg. IMGN853 was highly active with a minimally effective dose from 1.2 to 2.5 mg/kg depending on the model. There was no activity of the conjugate against a FR-α negative xenograft model. Thus, IMGN853 is highly effective against xenograft models with clinically relevant levels of FR-α expression.
Citation Format: Olga Ab, Laura M. Bartle, Xiuxia Sun, Rui Wu, Holly A. Johnson, Kathleen R. Whiteman, Alyssa LaBelle, Victor S. Goldmacher. IMGN853, a folate receptor (FR) α-targeting antibody-drug conjugate (ADC), is highly effective against xenograft models with clinically relevant levels of receptor expression. [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 667. doi:10.1158/1538-7445.AM2014-667