Abstract 4398: Maytansinoid release from thioether linked antibody maytansine conjugates (AMCs) under oxidative conditions: Implication for formulation and for ex vivo sample analysis in pharmacokinetic studies

Abstract

Abstract Several antibody—maytansinoid conjugates (AMCs) are in various stages of clinical testing, with the most advanced being trastuzumab-DM1 (T-DM1) in Phase III testing. In T-DM1, the maytansinoid, DM1, is linked to the trastuzumab antibody via a non-reducible (“non-cleavable”) thioether bond using the SMCC (N-succinimidyl-4-(N-maleimidomethyl)-cyclohexane-1-carboxylate) linker. It has been reported that T-DM1 shows evidence of maytansinoid loss in circulation over time based on ex vivo analysis (Liepold, AACR 2009 abstract #2914), and it has been assumed that a slow retro-Michael reaction is the cause of some linker cleavage. In the present study, however, we demonstrate that a more likely source of observed maytansinoid release is a reaction generated ex vivo. In our study, we used a model synthetic compound, DM1-MCC, which has the DM1 maytansinoid linked via a thioether bond like that in T-DM1, and also a thioether-linked AMC. When incubated in human plasma ex vivo, these were found to release maytansinoid sulfonate (DM1-SO3-) under physiological conditions (37°C, pH 7.4) at a slow rate. The formation of an initial sulphenic acid intermediate (RSOH) during β-elimination was confirmed with trapping experiments using dimedone (5,5-dimethyl-1,3-cyclohexanedione). Oxidatively-stressed AMCs and a model substrate with a more hindered thioether made with the maytansinoid DM4 showed a 3-fold increase in rate of sulfoxide formation, but an 8-fold lower rate of β-elimination compared to unhindered thioethers. DM1-MCC and thioether-linked AMCs were incubated at 37°C with thiol-trapping agents, such as 2,2′-dipyridyldisulfide. No release of DM1-thiol was detected under these conditions. In contrast, greater than 20% of conjugated maytansinoid was cleaved from a thioether-linked AMC under oxidative stress conditions (10−6 M H2O2 1 hr, 4°C dialysis, then incubation 18 h at pH 7.4, 37 °C). The present observations suggest that ex vivo thioether oxidation may lead to maytansinoid release from conjugates. This sulfoxide elimination is noted at 37 °C under aqueous conditions and is likely facilitated by the mildly acidic β-hydrogen on the maleimide (pKa ∼ 25). Oxidation of thioethers (such as methionine) is common during protein extraction and concentration, and the present studies underscore the importance of reducing autooxidation of thioether-linked AMCs with regard to sample handling during pharmacokinetic analyses and conjugate formulation. 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 4398.