Abstract 1773: HPN424, a half-life extended, PSMA/CD3-specific TriTAC for the treatment of metastatic prostate cancer

Metastatic, castrate-resistant prostate cancer (mCRPC) is diagnosed in up to 50,000 patients each year in the US alone, and roughly 27,000 patients will succumb to it every year. Once metastasized beyond regional lymph nodes, the 5-year survival rate is 30%. While novel therapeutics like abiraterone and enzalutamide have improved the treatment options for mCRPC, no curative treatment is available, and new therapies are urgently needed. HPN424 is a ~50-kDa antibody derivative called TriTAC (Tri-specific T cell Activating Construct) under development for the treatment of mCRPC. It is designed to simultaneously bind to CD3ϵ on T cells and to prostate specific membrane antigen (PSMA, FOLH1) on prostate cancer cells. A third domain of HPN424 binds non-covalently to serum albumin for extension of serum half-life. PSMA is expressed in >90% of malignant lesions of patients, and outside the central nervous system, its expression on normal tissue is largely restricted to the prostate. HPN424 binds human PSMA with sub-nanomolar affinity. When incubated in co-cultures with resting, human T cells and prostate cancer cells, it activates T cells and induces cytokine production, proliferation and redirected target cell killing with EC 50 values in the single digit picomolar range. When administered to mice bearing human prostate cancer xenografts and human T cells, HPN424 eradicates subcutaneous tumors. The affinities of HPN424 for human and cynomolgus monkey CD3 and albumin are comparable, while HPN424 binds only marginally to cynomolgus PSMA. HPN424 is very well tolerated in non-human primates, even at high doses, indicating that CD3-binding has little if any pharmacological effect in the absence of target binding. Pharmacokinetic analysis supports weekly administration in humans. Our preclinical data suggest that HPN424 will be highly efficacious, safe and convenient for the treatment of patients with mCRPC. Citation Format: Bryan Lemon, Wade Aaron, Richard Austin, Patrick Baeuerle, Manasi Barath, Adrie Jones, Susan D. Jones, Kathryn Kwant, Che-Leung Law, Anna Muchnik, Kenneth Sexton, Laurie Tatalick, Holger Wesche, Timothy Yu. HPN424, a half-life extended, PSMA/CD3-specific TriTAC for the treatment of metastatic prostate cancer [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 1773.