Harpoon files $86M IPO to help fund its T-cell engager platform

Fresh off raising $70 million through a series C round just over a month ago, Harpoon Therapeutics announced it will go public with an $86.25 million IPO to further support its portfolio of early-stage T-cell engagers aimed at solid tumors.

The South San Francisco-based company filed confidentially with the SEC in late October, before it closed its mid-November fundraising round, which had brought on new investors including OrbiMed, Cormorant, Ridgeback Capital Investments, Lilly Asia Ventures and NS Investment. It plans to trade on the Nasdaq using the symbol HARP, though stock pricing terms have not been disclosed.

Harpoon believes its TriTAC platform can overcome the obstacles in tissue penetration faced by T-cell engager therapies, which have previously seen success against blood cancers, such as with Amgen’s 2014 approval of Blincyto in acute lymphoblastic leukemia.

Its TriTACs are single polypeptides with three binding sites, but are a third the size of monoclonal antibodies. While two sites bind to cancer antigens and T cells, respectively, the third binds with albumin to extend its serum half-life to three or four days, which the company expects will allow once- or twice-weekly dosing.

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Harpoon’s lead candidate, HPN424, is currently in a phase 1 trial of 40 patients with metastatic castration-resistant prostate cancer, with preliminary data expected in 2019, according to the company’s prospectus.

The company hopes to bring its second candidate into clinical development in the first half of 2019, with HPN536 as a treatment for solid tumors expressing mesothelin, an antigen found in mesothelioma and pancreatic, ovarian and lung cancers. Overall, Harpoon plans to have all four of its TriTAC candidates in the clinic by the end of 2020, including in multiple myeloma and small cell lung cancer.

In addition, the company’s second platform, known as ProTriTAC, aims to develop prodrug versions that are inactive until they meet tumor-associated proteases, to avoid damage to healthy tissues. Harpoon plans to launch IND-enabling ProTriTAC studies in 2019.

Harpoon previously signed a discovery and licensing deal with AbbVie in October 2017, granting the Big Pharma worldwide rights to develop and commercialize its TriTAC technology together with soluble T cell receptors. The deal included $17 million upfront, plus up to $600 million in potential milestone payments.