Development

GT Biopharma's B7-H3-targeted NK cell engager enters Phase I testing across solid tumors

Edited by Robert McTiernan··3 min read

GT Biopharma (Nasdaq: GTBP), based in San Francisco, has dosed the first patient in a Phase I basket trial evaluating GTB-5550, a B7-H3-targeted natural killer cell engager, in patients with B7-H3-expressing solid tumors. The announcement marks the third molecule from the company's proprietary TriKE platform to enter human testing.

GTB-5550 is a tri-specific fusion protein that simultaneously engages CD16 on NK cells, delivers a wildtype IL-15 proliferative signal, and anchors the complex to B7-H3 on tumor cells. It is also the first nanobody-based TriKE to be administered subcutaneously, a departure from intravenous dosing used in earlier platform molecules.

The Phase Ia dose escalation will evaluate up to six dose levels in prostate cancer patients to establish the maximum tolerated dose. Phase Ib will then expand across seven tumor types: castration-resistant prostate cancer, ovarian, breast, head and neck, non-small cell lung, pancreatic, and bladder cancers. Patients receive subcutaneous injections for five consecutive days in each of the first two weeks of a four-week cycle, with subsequent cycles moving to three-times-weekly dosing. Follow-up extends to 12 months, with progression-free survival and overall survival as endpoints. GT Biopharma anticipates reporting dose escalation updates in the second half of 2026.

Research context

B7-H3, also designated CD276, is a type I transmembrane protein in the B7 immunoregulatory family. Its expression is largely restricted in normal tissue but broadly upregulated across many carcinomas, making it an attractive target for tumor-directed immunotherapy. In metastatic castration-resistant prostate cancer specifically, B7-H3 is expressed in over 90% of tumors, according to the company, and PSA can function as an early pharmacodynamic readout of therapeutic activity — a practical advantage in early-phase dose finding.

The TriKE architecture addresses a known limitation of conventional NK cell therapies: endogenous NK cells are often metabolically exhausted and numerically insufficient in the tumor microenvironment. By incorporating wildtype IL-15 as a structural linker rather than a separate cytokine infusion, GTB-5550 is designed to prime and expand NK cells in situ while simultaneously directing their cytotoxicity toward B7-H3-positive tumor cells via CD16 crosslinking. This tri-functional design distinguishes TriKE molecules from bispecific NK cell engagers that rely solely on receptor bridging without a co-stimulatory signal.

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The subcutaneous delivery format is a material development for the platform. Earlier TriKE molecules required intravenous administration, which constrains outpatient use. If subcutaneous dosing proves pharmacologically equivalent and clinically manageable, it could broaden the practical deployment of NK cell engager therapy beyond academic infusion centers.

GTB-5550's mechanism is biomarker-agnostic in the sense that B7-H3 expression, unlike PSMA or HRR mutation status, is present across the large majority of mCRPC tumors. If the molecule demonstrates activity, it could theoretically be applicable without the molecular pre-selection required for PARP inhibitors or Pluvicto, though formal biomarker stratification strategies have not yet been disclosed for this trial.

Competitive landscape

B7-H3 (CD276) has emerged as a major solid tumor immunotherapy target because of its broad expression across prostate, lung, ovarian, head and neck, and other carcinomas alongside relatively limited expression in normal tissue. The field is currently led by antibody-drug conjugates, including Merck and Daiichi Sankyo’s ifinatamab deruxtecan and GSK/Hansoh Pharma’s risvutatug rezetecan, both of which have shown clinical activity in small cell lung cancer and other solid tumors. Other groups are pursuing radiopharmaceutical and T-cell engager approaches. GTB-5550 instead uses a natural killer cell engager strategy, combining CD16-mediated NK-cell activation with an integrated IL-15 proliferative signal intended to expand and sustain endogenous NK-cell activity within the tumor microenvironment. The subcutaneous formulation further differentiates the program from earlier intravenous NK-cell engager approaches, though whether that translates into improved outpatient usability or comparable pharmacokinetics remains to be established clinically. Cross-trial comparisons are limited by differences in modality, tumor selection, and development stage.

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