Abstract 5070: Anti-tumor activity of HPN328, a DLL3-targeting tri-specific, half-life extended T cell engager, is enhanced by combining with an anti-PD-L1 antibody in an immunocompetent mouse model
Authors: Valenzuela Laura B. , Wu Chi-Heng , Aaron Wade , Banzon Raphalea R. , Bush Mabel , Lin S. Jack , Rocha Sony S. , Thothathri Subramanian , Wesche Holger , Molloy Mary Ellen , Anand Banmeet S. 
Published: April 2023

Abstract
TriTAC® is a T cell engager platform comprised of three binding domains: a target binder for tumor cell engagement, an albumin binder for half-life extension, and a CD3 binder for T cell engagement. HPN328 is a Delta-like ligand 3 (DLL3)-targeting TriTAC currently being evaluated in a Phase 1/2 clinical trial enrolling patients with advanced cancers associated with DLL3 expression, including small cell lung cancer (SCLC) and other neuroendocrine malignancies (NCT04471727). While HPN328 has demonstrated single agent clinical activity in relapsed/refractory SCLC patients, combinations with other immunotherapeutic agents could further potentiate HPN328 anti-tumor activity and enable the testing of HPN328 in earlier lines of therapy. To test this hypothesis, we developed a human CD3ε (hCD3ε) immunocompetent mouse model that has the epitope of human CD3ε, as recognized by TriTAC molecules, specifically knocked-in (KI) to the mouse CD3ε gene. hCD3ε KI mice have comparable immune cell repertoire as wild-type mice. However, unlike wild-type mice, TriTAC molecules bind to the T cells in hCD3ε KI mice. In a MC38 tumor model, an EGFR-targeting TriTAC was able to exert the expected anti-tumor activity and EGFR-dependent toxicity, thereby confirming TriTACs can engage and activate endogenous T cells in this immunocompetent hCD3ε KI mouse model. Next, we tested HPN328 in hCD3ε KI mice with MC38 tumors that have been engineered to express human DLL3. As expected, HPN328 inhibited tumor growth in a dose-dependent manner. HPN328 also resulted in a notable upregulation of PD-L1 in these tumors, suggesting that HPN328 in combination with an anti-PD-L1 antibody could potentially drive deeper anti-tumor responses. Indeed, sub-therapeutic doses of HPN328 in combination with an anti-PD-L1 antibody enhanced the anti-tumor activity of HPN328 when compared to either treatment alone. Atezolizumab and durvalumab, anti-PD-L1 antibodies, are approved in combination with platinum-doublet chemotherapy as part of front-line therapy for extensive-stage SCLC. These results demonstrate the utility of combining anti-PD-L1 antibodies to enhance the anti-tumor activity of HPN328 and further supports investigation of this combinatorial approach in patients. Clinical studies of HPN328 in combination with atezolizumab are planned.
Citation Format: Laura B. Valenzuela, Chi-Heng Wu, Wade Aaron, Raphalea R. Banzon, Mabel Bush, S. Jack Lin, Sony S. Rocha, Subramanian Thothathri, Holger Wesche, Mary Ellen Molloy, Banmeet S. Anand. Anti-tumor activity of HPN328, a DLL3-targeting tri-specific, half-life extended T cell engager, is enhanced by combining with an anti-PD-L1 antibody in an immunocompetent mouse model. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5070.