Ravi J. Chari

Abstract 5677: The anti-EGFR ADC, IMGN289 displays favorable pharmacokinetic properties.

IMGN289 is an antibody-drug conjugate (ADC) in preclinical development for the treatment of EGFR-positive cancers. IMGN289 utilizes the same SMCC thioether linker and DM1 cytotoxic agent used in trastuzumab emtansine (T-DM1), and both conjugates contain approximately 3.5 DM1 molecules per antibody. The stability of the SMCC linker is exemplified by several reports that have shown maytansinoid release only after hydrolysis of the antibody backbone within cells and tissue to yield lysine-SMCC-DM1. Despite these findings, pharmacokinetic (PK) data for T-DM1 point to a faster clearance for conjugated antibody than for total antibody. Additionally, reports of payload release from non-maytansinoid, cysteine-linked ADCs via thiol-maleimide exchange in plasma have led to speculation of DM1 release from T-DM1, and of this occurring via a similar mechanism. Consequently, we conducted a series of studies in mice to examine the PK behavior of IMGN289, its unmodified antibody component (J2898A) and a version of IMGN289 (J2898A-(CH2)3-DM1) in which the SMCC linker is replaced with an all-carbon linker incapable of cleavage via thiol-maleimide exchange. PK was assessed in CD-1 mice administered a single bolus iv dose of the test article. Total antibody and ADC concentrations (species with at least one linked DM1) were measured at periodic intervals over 35 days using standard sandwich ELISA assays. We also employed radioactive assays to directly measure the antibody and ADC components in plasma. For total antibody detection, the antibody or ADC was labeled with the radioactive tracer, N-succinimidyl-2,3-[3H]propionate. For ADC detection, tritium was incorporated into the C20-methoxy group of its maytansinoid moiety. The PK profiles for the ADCs and their unmodified antibody were found to be similar using the ELISA and radioactive assays for total antibody, having only slight changes in the PK parameters for the ADC. Using the ELISA assays, we found the total antibody and ADC PK profiles were indistinguishable for IMGN289, in contrast to the reported PK for T-DM1 in mice. The profiles developed using the radioactive assays were in good agreement with these findings, supporting that the ELISA assays were reliable. To confirm our results were not unique to IMGN289, we repeated the experiments with two additional antibody-SMCC-DM1 conjugates and found similar results. Additionally, the PK profile for the all- carbon linked conjugate, J2898A-C3-[3H]DM1, was similar to that of J2898A-SMCC-[3H]DM1 conjugate, suggesting little if any thiol-maleimide exchange in the latter, consistent with the finding of similar total antibody and ADC PK profiles for Ab-SMCC-DM1 conjugates. In summary, IMGN289 appears to retain the PK properties of its antibody component and avoids the payload release via thiol-maleimide exchange described for cysteine-maleimide linked ADCs. Citation Format: Jose F. Ponte, Xiuxia Sun, Nicholas C. Yoder, Nathan Fishkin, Sharon Wilhelm, Susan J. Hawes, Wayne Widdison, Robert A. Mastico, Jan Pinkas, Ravi J. Chari, Robert J. Lutz, Hans K. Erickson. The anti-EGFR ADC, IMGN289 displays favorable pharmacokinetic properties. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5677. doi:10.1158/1538-7445.AM2013-5677