Abstract 1781: HPN536, a T cell-engaging, mesothelin/CD3-specific TriTAC for the treatment of solid tumors

Mesothelin (MSLN) is a GPI-linked tumor antigen overexpressed in a variety of solid tumors, including ovarian, pancreatic, lung and triple-negative breast cancer. Normal tissue expression is restricted to single-cell, mesothelial layers lining the pleural, pericardial, and peritoneal cavities. Overexpression of MSLN is associated with poor prognosis in lung adenocarcinoma and triple-negative breast cancer. MSLN has been used as cancer target antigen for numerous modalities, including immunotoxins, vaccines, antibody drug conjugates and CAR-T cells. Early signs of clinical efficacy have validated MSLN as target, but therapies with improved efficacy are still needed to address the significant, unmet medical need posed by MSLN-expressing cancers. HPN536 is a ~50-kDa antibody derivative called TriTAC (Tri-specific T cell Activating Construct) designed to simultaneously bind to MSLN on tumor cells and to CD3ϵ on T cells with an affinity of 1 nM and 14 nM, respectively. Transient bispecific binding leads to the formation of an immunological cytolytic synapse, T cell activation and redirected tumor cell killing. A third domain of HPN536 binds non-covalently to serum albumin with an affinity of 8 nM to extend serum half-life life. Because TriTACs are built using single domain antibodies, TriTACs are much smaller than full size antibodies and are anticipated to demonstrate improved penetration of human tumors compared to full sized antibodies. HPN536 is produced by eukaryotic cell culture and secreted as a highly stable, single polypeptide. It binds with similar affinity to human and cynomolgus MSLN, albumin and CD3. When incubated in co-cultures with resting, human or cynomolgus T cells and human tumor cells, T cells are induced to release cytokines, to proliferate, and to specifically lyse MSLN-positive target cells with EC 50 values at single-digit picomolar concentrations. In an exploratory toxicological study in non-human primates, HPN536 was well tolerated and showed pharmacokinetics in support of weekly dosing in humans. Preclinical characterization suggests that HPN536 is an efficacious and safe novel therapeutic candidate for the convenient treatment of patients with MSLN-expressing malignancies. Citation Format: Richard Austin, Wade Aaron, Patrick Baeuerle, Manasi Barath, Adrie Jones, Susan D. Jones, Che-Leung Law, Kathryn Kwant, Bryan Lemon, Anna Muchnik, Kenneth Sexton, Laurie Tatalick, Holger Wesche, Timothy Yu. HPN536, a T cell-engaging, mesothelin/CD3-specific TriTAC for the treatment of solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1781.