Abstract 2668: Detoxification of metabolites from antibody-maytansinoid conjugates by human liver microsomes
Authors: Widdison Wayne C. , Wilhelm Sharon , Veale Karen , Kovtun Yelena , Erickson Hans , Audette Charlene , Leeca Barbara , Jones Gregory , Chari Ravi 
Published: February 2011

Abstract
Six antibody-maytansinoid conjugates (AMCs) consisting of the cytotoxic maytansinoid, DM1 or DM4, linked to a tumor-targeting monoclonal antibody are undergoing clinical evaluation, with the most advanced in Phase III testing. In these AMCs, the maytansinoid is attached through its C3 side chain to the antibody via a thioether or disulfide bond. The antibody moiety of the conjugate binds specifically to a tumor cell after which the conjugate is internalized and processed to release one or more maytansinoid metabolites (MMs) which kill the cell. Some of these MMs are hydrophobic and highly potent against both the forming cell and neighboring dividing cells, while others are charged or polar compounds that are highly toxic to the cell that intracellularly released them, but have a relatively low toxicity to adjacent cells due to poor membrane permeability. Similar MMs are formed from AMCs inside cells of the reticulo-endothelial system (RES), the primary route of elimination of proteinaceous therapeutics such as AMCs. We have previously reported, from studies with radiolabeled MMs in mice, that the primary elimination route of RES-formed MMs is via the liver, and entails biliary excretion of relatively low potency charged or polar maytansinoids. It would be desirable to know if MMs generated from AMCs in the RES systems of patients can be inactivated by the oxidative processes of the human liver. To model this, MMs and other maytansinoids were synthesized and then exposed in vitro to human liver microsomes. The resulting microsomal metabolites were identified by HPLC/MS and their cytotoxicities were determined. In general, the major metabolites formed from microsome exposure occurred via oxidation on the C3 side chain, with only minimal microsome-dependent alterations noted in the macrocycle. Non-charged hydrophobic maytansinoids containing at least one sulfur atom were oxidized on the sulfur atom(s) to yield one or more products which were significantly less cytotoxic than the substrate. For example, the highly cytotoxic tumor cell metabolite, S-methyl DM4, was converted to its sulfoxide and sulfone analogues, which were more than 20-fold less potent than S-methyl DM4. Charged maytansinoids containing an amino acid or a carboxylic acid were poorly metabolized by the human liver microsomes even when these maytansinoids contained one or more sulfur atoms. These data suggest that the highly potent, non-charged MMs formed in the tumor cells or the RES systems of patients would be rendered less toxic through oxidation in the liver prior to biliary elimination. Furthermore, the less potent, charged MMs likely would not be further processed by liver in a way that would increase their systemic toxicity.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2668.