Enhancement of ADC selectivity by inverse targeting: Mechanistic studies and optimization - Cancer is a major cause of morbidity and mortality in the US, with 1.8 million cases and 600 thousand cancer deaths projected for 2020. Substantial progress in cancer treatment has been made in the past two decades, largely through the development of highly targeted therapies, including development of antibody-drug conjugates (ADCs). ADCs employ monoclonal antibodies with specificity for tumor-associated antigens to increase the efficiency and selectivity of the delivery of anti-cancer toxins (i.e., payloads) to cancer cells. Although this approach has proven to be successful, with 9 anti-cancer ADCs approved for use in the US (brentuximab vedotin, trastuzumab emtansine, gemtuzumab ozogamicin, inotuzumab ozogamicin, polatuzumab vedotin, enfortumab vedotin, belantamab mafodotin, trastuzumab deruxtecan, and sacituzumab govitecan), ADC therapies are often associated with substantial off-target toxicity, narrow therapeutic windows, and high failure rates in clinical testing. This project introduces a new pharmacokinetic strategy to increase the tumor-selectivity of antibody- directed delivery of anti-cancer drugs. In our approach, payload-binding antibody fragments, termed payload- binding selectivity enhancers (PBSE), are co-administered with ADCs to decrease the exposure of healthy tissues to payload agents, thereby reducing the development of off-target toxicity, increasing the tolerable dose of ADCs, and increasing ADC efficacy. The strategy is based on the recognition that off-site ADC toxicity is primarily attributed to the released (“free”) payload molecule, and also on the hypothesis that PBSE may be employed to prevent cellular entry of free payload molecules in non-targeted cells (by preventing diffusion across plasma membranes) without altering entry of ADCs into targeted cells (which proceeds via receptor mediated endocytosis). Work in this project will focus on the development and evaluation of a novel series of PBSE that have been shown to decrease the cytotoxicity of free SN38 and Dxd. These agents are camptothecin derivatives that are employed as payloads for sacituzumab govitecan and trastuzumab deruxtecan, two recently approved ADC molecules that have shown some efficacy, but substantial toxicity, in clinical investigations. Mechanistic studies proposed in Aim #1 and Aim #2 examine relationships between PBSE molecular attributes (e.g., affinity, molecular modality [i.e., IgG, Fab, scFv, sdAb], selectivity for unconjugated payload, molecular charge, etc.) and PBSE utility in enhancing the pharmacokinetic and pharmacodynamic selectivity of ADC therapy. These findings will be integrated through the use of mechanistic mathematical modeling to assist in the selection of an optimal agent and dosing regimen for evaluation of efficacy and toxicity in Aim #3. The novel agents developed in this work may be suitable for immediate translation toward optimization of sacituzumab govitecan and trastuzumab deruxtecan therapy of refractory and resistant breast cancer.
