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

Issued on behalf of GT Biopharma, Inc.

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

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].

That is starting to change.

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.

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].

"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.

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

Big Pharma Buys In: Summit Therapeutics × GSK

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].

Risvutatug rezetecan is itself a high-profile asset: GSK acquired exclusive worldwide rights (excluding mainland China, Hong Kong, Macau, and Taiwan) from Hansoh Pharma, and GSK's global Phase 3 trial for the drug in relapsed extensive-stage small cell lung cancer began in August 2025 [4]. The combination study with Summit is expected to begin dosing patients in mid-2026.

The takeaway for investors watching the antigen is that B7-H3 is no longer an academic curiosity. It is at the center of a deal between one of the most-watched names in U.S. oncology biotech and one of the world's largest pharmaceutical companies.

Precision Oncology Gets Bispecific

IDEAYA Biosciences, Inc. (NASDAQ: IDYA), a precision medicine oncology company, has taken a different swing at the same antigen. In February 2026, IDEAYA announced that the first patient had been enrolled in its Phase 1 dose-escalation/expansion trial evaluating IDE034, a potential first-in-class PTK7/B7-H3 bispecific TOP1 antibody-drug conjugate [5]. The design rationale is unusually specific: IDEAYA estimates that B7-H3 and PTK7 are co-expressed in approximately 30–40% of certain large solid tumor types — including lung, breast, ovarian, and colorectal cancers — while exhibiting minimal dual-antigen expression in normal tissue [5]. The drug is designed to be internalized only when both antigens are co-expressed on the same tumor cell, an architecture intended to enhance selectivity and tolerability compared to monovalent antibody formats.

In its first-quarter 2026 update, IDEAYA reaffirmed plans to provide a clinical data update for IDE034 by year-end 2026 and confirmed that patient dosing in the trial had triggered a $5 million milestone payment to its collaboration partner Biocytogen [6].

Beyond Antibodies: A Radiopharmaceutical Approach

A third U.S.-listed program is testing whether B7-H3 can be hit with radiation rather than a payload-conjugated antibody. Radiopharm Theranostics Ltd. (NASDAQ: RADX), through its Radiopharm Ventures joint venture with The University of Texas MD Anderson Cancer Center, is advancing 177Lu-BetaBart (RV-01), a Lutetium-177-tagged engineered monoclonal antibody designed with strong affinity for the 4Ig isoform of B7-H3 — the first clinical trial globally to target B7-H3 with a systemic radiopharmaceutical [7]. Radiopharm dosed the first patient in the First-In-Human Phase 1/2a clinical trial of 177Lu-BetaBart on February 24, 2026 [8]. The trial follows FDA IND clearance received in July 2025, and in January 2026 Radiopharm announced it had increased its ownership stake in Radiopharm Ventures to 87.5%, deepening its exposure to the BetaBart program [7].

Taken together — antibody-drug conjugates, bispecific ADCs, systemic radiopharmaceuticals, natural killer cell engagers — B7-H3 is now being attacked from four distinct therapeutic angles across at least four publicly traded U.S. companies in 2026 alone.

Why Subcutaneous Matters in This Wave

Among that group, GT Biopharma's GTB-5550 stands out for two structural reasons. First, the engager is the only one of the named programs that relies on natural killer cells rather than T cells, antibody payloads, or radiation. NK cells are part of the innate immune system, act faster than T cells, do not require MHC presentation, and have historically been associated with lower rates of cytokine release syndrome — features that have made them a focus of an entire emerging therapeutic category. Second, GTB-5550 is being administered subcutaneously, in the abdominal area, for five consecutive days during Week 1 and Week 2 of each four-week cycle [2]. Subsequent cycles are dosed three times weekly for two weeks followed by two weeks of no treatment.

The implication for patients — and for the eventual commercial logistics of any approved therapy — is that treatment may not require an infusion center.

A Pipeline-Defining Year

GT Biopharma has indicated that it expects to provide updates on the GTB-5550 trial throughout the second half of 2026 as enrollment progresses through the dose-escalation cohorts [2]. Patients are followed for 12 months to determine progression-free survival and overall survival. The trial is registered under clinicaltrials.gov identifier NCT07541573 [2].

In a category that took two decades to get its first programs into the clinic, the pace of activity in 2026 alone is striking. From a small-cap NK cell engager developer to a mega-cap bispecific antibody company partnering with GSK, B7-H3 is no longer the target that pharma circled and walked away from. It is the target everyone is racing to crack — and the patient just dosed in San Francisco this week is one more data point in that race.

Continuing coverage of GT Biopharma, Inc. and the broader B7-H3 oncology landscape is available at gtbiopharma.com.

Contact

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Article Sources

1. ITC-6102RO research — B7-H3 ADC clinical development overview, National Center for Biotechnology Information — https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10439577/

2. GT Biopharma, Inc. — GT Biopharma Announces First Patient Dosed in Phase 1 Trial of GTB-5550 (May 14, 2026) — https://www.globenewswire.com/NewsRoom/ReleaseNg/7602764 

3. Summit Therapeutics (SMMT) Market Capitalization — Public.com / StockAnalysis — https://stockanalysis.com/stocks/smmt/market-cap/

4. Summit Therapeutics Inc. — Summit Therapeutics Announces Clinical Trial Collaboration with GSK to Evaluate Ivonescimab in Combination with GSK's B7-H3 Antibody Drug Conjugate (ADC) (January 12, 2026) — https://smmttx.com/news/press-releases/news-details/2026/Summit-Therapeutics-Announces-Clinical-Trial-Collaboration-with-GSK-to-Evaluate-Ivonescimab-in-Combination-with-GSKs-B7-H3-Antibody-Drug-Conjugate-ADC/default.aspx

5. IDEAYA Biosciences, Inc. — IDEAYA Biosciences Announces First-Patient-In for Phase 1 Trial of IDE034 (February 25, 2026) — https://ir.ideayabio.com/2026-02-25-IDEAYA-Biosciences-Announces-First-Patient-In-for-Phase-1-Trial-of-IDE034,-a-Potential-First-In-Class-B7H3-PTK7-Bispecific-TOP1-ADC

6. IDEAYA Biosciences, Inc. — Q1 2026 Financial Results and Business Update — https://www.sec.gov/Archives/edgar/data/0001676725/000119312526204944/idya-ex99_1.htm

7. Radiopharm Theranostics Ltd. — RAD Increases Ownership in Radiopharm Ventures to 87.5% (January 12, 2026) — https://www.sec.gov/Archives/edgar/data/0001949257/000121390026003148/ea027252001ex99-1_radiopharm.htm

8. Radiopharm Theranostics Ltd. — Radiopharm Theranostics Doses First Patient in Phase 1/2a Clinical Study of BetaBart (RV-01) (February 24, 2026) — https://www.globenewswire.com/news-release/2026/02/24/3243439/0/en/Radiopharm-Theranostics-Doses-First-Patient-in-Phase-1-2a-Clinical-Study-of-BetaBart-RV-01.html

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