A Cancer Antigen Long Thought Untouchable Is Suddenly the Hottest Target in Oncology

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Issued on behalf of GT Biopharma, Inc.

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From bispecific ADCs at IDEAYA to a GSK partnership at Summit, B7-H3 has become one of the most actively pursued antigens in solid tumor oncology — and a new natural killer cell engager just entered the clinic

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SAN FRANCISCO, May 14, 2026 (GLOBE NEWSWIRE) — USA News Group News Commentary — For more than two decades, B7-H3 sat on the shortlist of theoretically perfect cancer drug targets that nobody could quite figure out how to hit. The protein is broadly overexpressed across some of the most common — and most lethal — solid tumors, including prostate, lung, breast, ovarian, head and neck, and pancreatic cancers. It is largely absent from healthy tissue. It correlates with poor prognosis. On paper, it has every quality a drug developer wants. In practice, three B7-H3-targeting antibody-drug conjugates have entered the clinic, and none have yet been approved [1].

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That is starting to change.

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In the first half of 2026, a wave of new B7-H3-directed programs from across the U.S. oncology landscape has reached early clinical milestones, ranging from bispecific antibody-drug conjugates to systemic radiopharmaceuticals to natural killer cell engagers. The mechanisms vary widely. The target does not.

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GT Biopharma, Inc. (NASDAQ: GTBP) added itself to that list this week, announcing that the first patient has been dosed in a Phase 1 dose-escalation basket trial of GTB-5550, a B7-H3-targeted natural killer (NK) cell engager for solid tumors expressing B7-H3 [2]. GTB-5550 is the third TriKE® (Tri-specific Killer Engager) molecule from GT Biopharma to enter the clinic and the first to be tested with subcutaneous dosing — a notable design choice in a category where most engager therapies have historically required continuous infusion. The dose-escalation phase will focus primarily on prostate cancer, where, according to Dr. Nicholas Zorko of the University of Minnesota, B7-H3 is expressed in over 90% of metastatic castration-resistant tumors and PSA can serve as an early biomarker of therapeutic activity [2].

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“Dosing the first patient in our GTB-5550 Phase 1 trial is a pivotal milestone for GT Biopharma and represents the natural evolution of our TriKE® platform into the broader opportunity of treating patients with a variety of solid tumors,” said Michael Breen, Executive Chairman and Chief Executive Officer of GT Biopharma, in the company’s announcement [2]. After dose escalation, the Phase 1b expansion will enroll patients across up to seven distinct tumor types: castration-resistant prostate, ovarian, breast, head and neck, non-small cell lung, pancreatic, and bladder cancer.

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Step back from the molecule, though, and the more striking story is the company GT Biopharma now finds itself in.

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Big Pharma Buys In: Summit Therapeutics × GSK

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Just four months earlier, Summit Therapeutics Inc. (NASDAQ: SMMT) — currently one of the largest publicly traded oncology biotechs by market capitalization, with a market value around $14 billion as of early 2026 [3] — announced a clinical trial collaboration with GSK plc to evaluate Summit’s lead bispecific antibody ivonescimab in combination with GSK’s novel investigational B7-H3-targeting antibody-drug conjugate, risvutatug rezetecan (also known as GSK’227), across multiple solid tumor settings, including small cell lung cancer [4].