Background: Neuroendocrine Prostate Cancer (NEPC) is an aggressive variant of prostate cancer that can arise de novo or emerge during prostate cancer progression as a mechanism of therapy resistance. The development of effective therapies for patients with NEPC is a clinical unmet need. Delta-Like Ligand 3 (DLL3) is a negative regulator of Notch signaling and aberrantly expressed on the cell surface in the majority of NEPC and other small cell carcinomas. HPN328 is a DLL3-targeted Tri-specific T cell Activating Construct (TriTAC) that binds to DLL3 on tumor cells, CD3 on T cells, and human serum albumin. Here, we evaluated the specificity and anti-tumor activity of HPN328 in NEPC preclinical models.

Methods: Immunohistochemistry (IHC) was used to evaluate DLL3 expression in prostate cancer preclinical models. We co-cultured DLL3-positive and DLL3-negative tumor cells in vitro with either human PBMCs or T cells and used CellTiter-Glo and live-cell imaging to examine tumor cell viability after the exposure to HPN328. We conducted flow cytometry to examine T cell activation. Patient-derived organoid xenografts were used to determine the anti-tumor activity of HPN328 in vivo and we applied digital spatial profiling (Nanostring) to examine effects on immune cell infiltration.

Results: HPN328 suppressed cell growth and induced cell death in DLL3-positive prostate cancer organoid cell line models at a range of 0.7nM - 10nM with the presence of human T cells. Flow cytometry analysis illustrated that T cell activation markers, such as CD25, were upregulated in a dose- and time-dependent manner. In vivo studies demonstrated robust anti-tumor activity of HPN328 in causing tumor regression and stable disease after stopping the treatment in DLL3-positive models, but not in DLL3-negative models. Sustained anti-tumor effects were observed without adverse events in mice.

Conclusions: The DLL3-targeted TriTAC HPN328 demonstrates robust and specific anti-tumor activity in DLL3-positive NEPC preclinical models in vitro and in vivo. T cell engagers targeting DLL3 represent a promising therapeutic strategy for NEPC.

Acknowledgements: This research is funded by a Helen Trailblazers Award from the Helen Gurley Brown Foundation/DFCI Presidential Initiative (H.B.).

Citation Format: Sheng-Yu Ku, Yasutaka Yamada, Patrick Ng, Laura Sun, Himisha Beltran. DLL3-targeted T cell engager therapy (HPN328) for neuroendocrine prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2896.