HPN328, a Trispecific T Cell-activating Protein Construct Targeting DLL3-Expressing Solid Tumors.

Delta-like ligand 3 (DLL3) is expressed in more than 70% of small cell lung cancers (SCLCs) and other neuroendocrine-derived tumor types. SCLC is highly aggressive and limited therapeutic options lead to poor prognosis for patients. HPN328 is a tri-specific T cell activating construct (TriTAC) consisting of three binding domains: a CD3 binder for T cell engagement, an albumin binder for half-life extension, and a DLL3 binder for tumor cell engagement. In vitro assays, rodent models and non-human primates were used to assess the activity of HPN328. HPN328 induces potent dose-dependent killing of DLL3-expressing SCLC cell lines in vitro concomitant with T cell activation and cytokine release. In an NCI-H82 xenograft model with established tumors, HPN328 treatment led to T cell recruitment and anti-tumor activity. In an immunocompetent mouse model expressing a human CD3ε epitope, mice previously treated with HPN328 withstood tumor rechallenge, demonstrating long-term anti-tumor immunity. When repeat doses were administered to cynomolgus monkeys, HPN328 was well tolerated up to 10 mg/kg. Pharmacodynamic changes, such as transient cytokine elevation, were observed, consistent with the expected mechanism of action of T cell engagers. HPN328 exhibited linear pharmacokinetic in the given dose range with a serum half-life of 78 to 187 hours, supporting weekly or less frequent administration of HPN328 in humans. Preclinical and nonclinical characterization suggests that HPN328 is a highly efficacious, safe, and novel therapeutic candidate. A phase 1/2 clinical trial is currently underway testing safety and efficacy in patients with DLL3 expressing malignancies.

TriTACs, a Novel Class of T-Cell-Engaging Protein Constructs Designed for the Treatment of Solid Tumors.

T cells have a unique capability to eliminate cancer cells and fight malignancies. Cancer cells have adopted multiple immune evasion mechanisms aimed at inhibiting T cells. Dramatically improved patient outcomes have been achieved with therapies genetically reprogramming T cells, blocking T-cell inhibition by cancer cells, or transiently connecting T cells with cancer cells for redirected lysis. This last modality is based on antibody constructs that bind a surface antigen on cancer cells and an invariant component of the T-cell receptor. Although high response rates were observed with T-cell engagers specific for CD19, CD20, or BCMA in patients with hematologic cancers, the treatment of solid tumors has been less successful. Here, we developed and characterized a novel T-cell engager format, called TriTAC (for Trispecific T-cell Activating Construct). TriTACs are engineered with features to improve patient safety and solid tumor activity, including high stability, small size, flexible linkers, long serum half-life, and highly specific and potent redirected lysis. The present study establishes the structure/activity relationship of TriTACs and describes the development of HPN424, a PSMA- (FOLH1-) targeting TriTAC in clinical development for patients with metastatic castration-resistant prostate cancer.

Preclinical Characterization of HPN536, a Trispecific, T-Cell-Activating Protein Construct for the Treatment of Mesothelin-Expressing Solid Tumors.

PURPOSE: Mesothelin (MSLN) is a glycophosphatidylinositol-linked tumor antigen overexpressed in a variety of malignancies, including ovarian, pancreatic, lung, and triple-negative breast cancer. Early signs of clinical efficacy with MSLN-targeting agents have validated MSLN as a promising target for therapeutic intervention, but therapies with improved efficacy are still needed to address the significant unmet medical need posed by MSLN-expressing cancers. EXPERIMENTAL DESIGN: We designed HPN536, a 53-kDa, trispecific, T-cell-activating protein-based construct, which binds to MSLN-expressing tumor cells, CD3ε on T cells, and to serum albumin. Experiments were conducted to assess the potency, activity, and half-life of HPN536 in in vitro assays, rodent models, and in nonhuman primates (NHP). RESULTS: HPN536 binds to MSLN-expressing tumor cells and to CD3ε on T cells, leading to T-cell activation and potent redirected target cell lysis. A third domain of HPN536 binds to serum albumin for extension of plasma half-life. In cynomolgus monkeys, HPN536 at doses ranging from 0.1 to 10 mg/kg demonstrated MSLN-dependent pharmacologic activity, was well tolerated, and showed pharmacokinetics in support of weekly dosing in humans. CONCLUSIONS: HPN536 is potent, is well tolerated, and exhibits extended half-life in NHPs. It is currently in phase I clinical testing in patients with MSLN-expressing malignancies (NCT03872206).