Abstract 5529: Preclinical characterization of LY4336619, a novel ALPPL2xCD3 bispecific antibody, for the treatment of ovarian and endometrial cancer

Abstract Development of CD3-bispecific antibodies has been challenging in solid tumors due to the difficulty identifying tumor antigens with high tumoral expression but with minimal or no expression in normal tissues. Placental-like alkaline phosphatase 2 (ALPPL2) is overexpressed in various solid tumors, including ovarian and endometrial carcinomas, while its expression in normal tissues is largely restricted to the placenta. This expression profile makes ALPPL2 a promising target for CD3-bispecific antibody therapies. We have developed a fully human common light chain-based ALPPL2×CD3 bispecific antibody, LY4336619, on an Fc-silenced IgG1 backbone designed to engage ALPPL2 on tumor cells and CD3 on T cells, facilitating targeted T cell-mediated cytotoxicity against ALPPL2-expressing tumor cells. LY4336619 was generated by pairing a novel anti-ALPPL2 monoclonal antibody, identified through a murine immunization strategy, with an anti-CD3 antibody. The binding, target selectivity, and cytotoxic activity of LY4336619 were evaluated using multiple ALPPL2-expressing cell lines. In vivo efficacy was evaluated using immunocompromised murine models engrafted with human tumor xenografts and human peripheral blood mononuclear cells (PBMCs). Additionally, a comprehensive proteomics analysis was performed to characterize ALPPL2 expression across tumor tissue specimens. LY4336619 specifically bound to ALPPL2-expressing cell lines but spared ALPI and ALPL, the other more widely expressed alkaline phosphatase family members; it also bound to the highly homologous ALPP family member, a protein with similar limited normal expression and association with similar tumor types. LY4336619 engaged with both CD3 and ALPPL2, and induced concentration-dependent T-cell activation and T cell-mediated cytotoxicity in co-culture assays with T cells and ALPPL2-expressing tumor cells. Furthermore, it demonstrated antitumor activity and relatively low cytokine release as a single agent in human PBMC-engrafted mice with human tumor xenografts representing a range of ALPPL2 expression levels. Additional preliminary studies revealed that the combination treatment with LY4170156, a clinical-stage anti-FRα antibody-drug conjugate (ADC), resulted in more sustained and pronounced tumor growth inhibition compared to either monotherapy alone. Our ALPPL2×CD3 bispecific antibody exhibited IgG-like pharmacokinetics and low immunogenic potential. These findings demonstrate that LY4336619 has target specificity, favorable functional activity, and a promising developability profile. LY4336619 has the potential as a first-in-class medicine to treat ALPPL2-positive solid tumors, including ovarian and endometrial cancers. An IND submission is anticipated for 2026. Citation Format: Colleen Burns, Julie Dobkin, Kevin Lindquist, Hee Rae Shin, Cindy Nguyen, Wei Yang, Xiaodong Huang, Marie Mutryn, Whitney S. Helms, Nicholas Arce, Yvonne Mak, Alexandra Antonoplis, Amelie Forest, Abraham Abouzeid, Marta Witek, Claire F. Friedman, Zhao Hai Lu, Mohan Srinivasan, Kyla Driscoll, Andrew Hass, Omar Duramad, Rikke Holmgaard, Kristin Bedard. Preclinical characterization of LY4336619, a novel ALPPL2xCD3 bispecific antibody, for the treatment of ovarian and endometrial cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5529.