Preclinical Development The Effect of Different Linkers on Target Cell Catabolism and Pharmacokinetics/Pharmacodynamics of Trastuzumab Maytansinoid Conjugates

Trastuzumab emtansine (T-DM1) is an antibody–drug conjugate consisting of the anti-HER2 antibody trastuzumab linked via a nonreducible thioether linker to themaytansinoid antitubulin agentDM1. T-DM1has shown favorable safety and efficacy in patients with HER2-positive metastatic breast cancer. In previous animal studies, T-DM1 exhibited better pharmacokinetics (PK) and slightly more efficacy than several disulfide-linked versions. The efficacyfindings are unique, as other disulfide-linked antibody–drug conjugates (ADC) have showngreater efficacy than thioether-linkeddesigns. To explore this further, the in vitro and in vivo activity, PK, and target cell activation of T-DM1 and the disulfide-linked T-SPP-DM1 were examined. Both ADCs showed high in vitro potency, with T-DM1 displaying greater potency in two of four breast cancer cell lines. In vitro target cell processing of T-DM1 and T-SPP-DM1 produced lysine-N-MCC-DM1, and lysineN-SPP-DM1 and DM1, respectively; in vivo studies confirmed these results. The in vitro processing rates for the two conjugate to their respective catabolites were similar. In vivo, the potencies of the conjugates were similar, and T-SPP-DM1 had a faster plasma clearance than T-DM1. Slower T-DM1 clearance translated to higher overall tumor concentrations (conjugate plus catabolites), but unexpectedly, similar levels of tumor catabolite. These results indicate that, although the ADC linker can have clear impact on the PK and the chemical nature of the catabolites formed, both linkers seem to offer the same payload delivery to the tumor. Mol Cancer Ther; 11(5); 1133–42. 2012 AACR.