Harpoon to Discontinue Once Promising Treatment for Prostate Cancer

Harpoon Therapeutics, Inc. has unfortunately announced that it will move to discontinue one of its promising T cell engager biologics, HPN424. The biologic previously showed potential for success in patients diagnosed with metastatic castration-resistant prostate cancer (mCRPC).

This decision is a result of “a careful and thorough analysis of our HPN424 data, including our clinical results," stated Harpoon President and CEO Julie Eastland. Rather than focusing on the setback, Harpoon chooses to look at future advancements for “portfolio programs that show promising activity.”

Interim data for the study of Harpoon’s HPN424 molecule was reported in June of 2021, with four out of 19 enrolled patients experiencing a decline of prostate-specific antigen (PSA). Participants in the study were following a modified step dose regimen, evaluating the effects of dose escalation. Promise gleamed from positive results of tumor regression, dose tolerability, safety and overall patient stability.

Formerly dubbed the company’s “lead product” to be approved by the Food and Drug Administration (FDA) for patient use, it is unclear what specific factors lead the company to discontinue it.

This news comes after a productive 2021 year for Harpoon, which profited both fiscally and in reputation following its collaboration with AbbVie called Harpoon’s Discovery Collaboration. Harpoon Therapeutics has increasingly invested its funds into research and development, jumping from a $15.1 million-dollar allocation in 2020 to $72.1 million in 2021.

This investment can be seen at work with three other T cell engagement immunotherapies currently undergoing clinical trials, each of which uses Harpoon’s novel method of Tri-specific T cell Activating Construct (TriTAC). This drug delivery system is what attracted AbbVie to form Harpoon’s Discovery Collaboration.

The $50 million collaboration, which focuses on research and development, grants AbbVie rights to HPN217, as well as two other TriTac molecules. The agreement allows for an extension of rights of future molecules, as well, for a potential grand total of six therapeutics. For the rights of each additional molecule, Harpoon stands to gain upwards of $310 million, not including royalties from international marketing and sales.

Several of these TriTAC therapies have been granted special designations by the U.S. Food and Drug Administration (FDA). These therapies include HPN536, which targets pancreatic and ovarian cancers, HPN217, a Fast Track designated therapy for relapsed, refractory multiple myeloma and HPN328, which has Orphan Drug designation to treat small cell lung cancer. Each of these therapies engages a patient’s T cells to attack antigens and proteins. The target antigens vary depending on the disease being treated.

Additionally, Harpoon intends to complete IND filing with the FDA in 2022 for HNP601, an epithelial adhesion molecule (EpCAM) biotherapeutic that will influence the expression of diseases associated with the EpCAM gene. HNP601 utilizes Harpoon’s proprietary ProTriTAC delivery mechanism, which is designed to ensure that the T cell engager is active only once reaching the targeted tumor.

Other upcoming 2022 pipeline milestones for Harpoon include the continuation of a dose-escalation study for HPN328, completion of the HPN536 and HPN424 dose escalation studies and initiation of a dose-expansion study for HPN217.