Tissue distribution, metabolism, and excretion of the antibody-drug conjugate Herceptin-monomethyl auristatin E in rats

5063 Antibody-drug conjugates (ADC) are a relatively new and evolving approach to improve the potency and selectivity of anti-tumor agents. Key questions exist regarding the in vivo disposition of the drug conjugate (i.e. the cytotoxic moiety) as it relates to optimizing dosing and understanding safety. In this study, Herceptin, a humanized monoclonal antibody directed against the Her2 receptor, was conjugated to the cytotoxic drug monomethyl auristatin E (MMAE) through a highly stable protease-cleavable dipeptide linker. The linker is selectively cleaved by lysosomal enzymes after internalization and free MMAE is capable of binding tubulin and blocking cell proliferation. Initial studies in mouse xenograft tumor models have demonstrated enhanced tumor-specific cell killing relative to unconjugated Herceptin. The objective of this study was to characterize the tissue distribution, metabolism and route of elimination of Herceptin-Val-Cit-[14C]MMAE following an IV bolus dose of (11 mg/kg, 100uCi/kg) of the radiolabeled ADC in Sprague-Dawley rats. Tissue distribution was assessed by quantitative whole body autoradiography (QWBA) 1, 4, 7, and 14 days following dosing. The results from QWBA indicated the presence of radioactivity in highly perfused organs (lungs>liver>heart>kidney) on day 1 post-dose, with evidence for hepatic elimination of the drug via the bile/gastro-intestinal tract (GI) on day 4. Radioactivity decreased in most organs with a similar pharmacokinetic profile as blood radioactivity. Persistence of radioactivity was observed in tissues with rapidly dividing cells such as the GI epithelia, bone marrow, spleen and thymus on days 4 and 7. These data are consistent with target organs of toxicity identified in previous toxicology studies. Excretion studies indicated that MMAE was primarily eliminated intact in feces (∼80%) and urine (∼ 6%) within 7 days following dosing. Characterization of radioactivity in rat plasma showed evidence for a time-dependent biotransformation of Herceptin-Val-Cit-[14C]MMAE including the liberation of low levels of free drug. In conclusion, Herceptin-Val-Cit-[14C]MMAE distributes to highly perfused organs. The majority of blood radioactivity represented radiolabeled ADC, however, low levels of free drug were detected. The cytotoxic moiety, MMAE, was primarily eliminated intact in the feces with greater than 86% of drug being excreted 7 days post dose.