GT Biopharma Inc. (GT Biopharma (GTBP)) just took a meaningful step forward in its quest to treat solid tumors. On Thursday, the company announced it had dosed the first patient in a Phase 1 basket trial for GTB-5550, its B7-H3-targeted NK cell engager designed to attack solid tumors that express the B7-H3 protein.
This is a big deal for GT Biopharma because it marks the company's first foray into solid tumors with its TriKE platform. Up until now, the platform has been focused on hematologic malignancies — blood cancers. The company's other lead candidate, GTB-3650, is already in Phase 1 trials for blood cancers, and the progress there gave management the confidence to push into solid tumors.
"The ongoing Phase 1 progress with GTB-3650 in hematologic malignancies now gives us the confidence to advance the platform with GTB-5550 to target B7-H3, which is broadly expressed across many of the most common and difficult-to-treat solid tumor cancers," said Michael Breen, Executive Chairman and CEO of GT Biopharma. "We look forward to providing updates on the trial's progress throughout the second half of 2026."
B7-H3 is a protein that's found on the surface of many solid tumors, making it a promising target for cancer therapies. The idea is that GTB-5550 will act like a bridge, bringing natural killer (NK) cells close to cancer cells so they can destroy them.
Phase 1 Trial: Starting with Prostate Cancer, Then Expanding
The Phase 1 trial is designed in two parts. First comes the dose-escalation phase, which will primarily enroll patients with prostate cancer. The goal here is to find the maximum tolerated dose (MTD) by testing up to six dose levels. This is the first time a nanobody TriKE will be tested using subcutaneous dosing — meaning it's injected just under the skin, which is more convenient for patients than intravenous infusion.
Once the right dose is identified, the trial will move into an expansion phase. That Phase 1b part will enroll patients across up to seven tumor types: castration-resistant prostate cancer, ovarian cancer, breast cancer, head and neck cancer, non-small cell lung cancer, pancreatic cancer, and bladder cancer. The expansion phase will continue to evaluate safety and tolerability while also looking for early signs of anti-tumor activity.
A Unique Dosing Schedule
GTB-5550 has a pretty specific dosing schedule. During the first two weeks of each four-week cycle, patients receive a subcutaneous injection in the abdominal area for five consecutive days in Week 1 and again in Week 2. Then they get two weeks off. In subsequent cycles, the dosing frequency drops to three times per week for two weeks, followed by two weeks off.
Patients are expected to complete at least two cycles, with disease reassessment happening after the second cycle and then every 8 to 12 weeks after that. Treatment can continue as long as it's working and the patient can tolerate it — stopping only if the disease progresses, side effects become unacceptable, the patient decides to stop, or the doctor thinks it's no longer in the patient's best interest.
The trial will also follow patients for 12 months to track progression-free survival (PFS) and overall survival (OS) — standard measures in cancer trials.
Cash Runway Through Q4 2026
As of December 31, 2025, GT Biopharma had about $7 million in cash and cash equivalents. That number improved to an unaudited pro-forma cash balance of roughly $9 million as of January 31, 2026. The company believes this will be enough to fund operations through the fourth quarter of 2026.
Investors seemed pleased with the news. GT Biopharma shares were up 13.94% in premarket trading on Thursday, hitting $0.35. The stock is trading near its 52-week low of $0.26, so there's a long way to go, but the trial progress is a positive signal for a company that's been grinding away in the biotech space.
