Abstract A149: The effect of linker on target cell catabolism and PK/PD of trastuzumab maytansinoid conjugates
Authors: Erickson Hans , Lewis Phillips Gail D. , Provenzano Carmela , Leipold Douglas D. , Johnson Holly , Mai Elaine , Audette Charlene , Messick Kirsten , Pinkas Jan , Gupta Manish , Tibbitts Jay 
Published: June 2012

Abstract
Trastuzumab-MCC-DM1 (T-DM1) is an antibody-drug conjugate (ADC) consisting of a maytansinoid cytotoxic agent (DM1) conjugated via a non-reducible thioether-based linker (SMCC) to the humanized HER2 antibody trastuzumab (T). T-DM1 is currently in clinical trials for the treatment of patients with HER2+ metastatic breast cancer. Nonclinical studies of T-DM1 showed slightly greater efficacy in mouse models of HER2+ breast cancer and greater tolerability in rodents than a T-maytansinoid conjugate with a reducible disulfide-linker, T-SPP-DM1. The efficacy data were unexpected because for other targets disulfide-linked conjugates have shown greater activity in mouse xenograft models than non-reducible thioether versions. To explore how the linker component influences the behavior of T-maytansinoid conjugates, in vitro and in vivo efficacy, pharmacokinetic, and target cell metabolism studies evaluating the thioether-linked T-DM1 and the disulfide-linked T-SPP-DM1 were conducted. The two conjugates displayed similar in vitro cytotoxic potency towards the HER2-overexpressing, T-insensitive mammary carcinoma cell line, BT-474EEI. However, T-DM1 was slightly more efficacious than T-SPP-DM1 in mice bearing BT-474EEI tumors. In vitro studies in BT-474EEI cells using [3H]DM1 conjugates were conducted and the maytansinoid metabolites formed within the cells were quantified by HPLC radiochromatography and identified by LC/MS. The sole metabolite of T-DM1 formed was lysine-N -MCC-DM1. HER2-mediated processing of T-SPP-DM1 produced the analogous lysine-N -SPP-DM1 metabolite; however, free DM1 was also found, suggesting subsequent reduction of lysine-SPP-DM1. The metabolites identified for these two T conjugates were similar to those reported previously with maytansinoid conjugates using other antibodies. The catabolism kinetics for the two conjugates were similar, consistent with their similar in vitro cytotoxic activities. The in vivo tumor metabolites of the two conjugates following administration to mice bearing BT-474EEI tumors were identical to those found in vitro. Surprisingly, the concentrations for the tumor-metabolites of T-SPP-DM1 and T-DM1 were nearly the same despite higher plasma concentrations for T-DM1. In contrast to these findings, a similar study with conjugates targeting the CanAg antigen found that concentrations of tumor metabolites of the thioether-linked conjugate were significantly greater than those from disulfide-linked conjugates, in this case paralleling the difference in their relative clearance from plasma. Our results suggest that T-DM1 conjugates are activated by HER2+ cells by mechanisms similar to those reported previously with other maytansinoid conjugates, with the exception that, in vivo, the linker appears to have little influence on the relative amount of tumor metabolites of the two conjugates despite having a large impact on their relative pharmacokinetics. A mechanism for this unexpected observation is being investigated.
Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A149.