Shares of GT Biopharma had their best trading day in more than nine months after the company shared encouraging progress from its ongoing Phase 1 trial of GTB-3650, an experimental treatment for patients with relapsed or refractory CD33-positive blood cancers.

The stock jumped 43.7% to $1.08 at Wednesday's close, before slipping 10.5% in after-hours trading.

The clinical-stage biotech company said enrollment in the study's dose-escalation groups is progressing smoothly and remains“well on track.” Both patients in Cohort 3 have now begun treatment, with no signs of dose-limiting toxicities or tolerability issues so far. 

The company added that the first patient in Cohort 3 has shown“promising evidence of immune activation,” matching the levels of activity seen in patients from the two earlier, lower-dose groups.

GT Biopharma's trial will evaluate the safety, pharmacokinetics/pharmacodynamics of GTB-3650, and evidence of immune activation. GTB-3650 will use the company's TriKE platform to recruit natural killer (NK) cells to fight cancer.

The Phase 1 study can enroll up to 14 patients, with two people in each of seven planned dosing groups. The lowest dose starts at 1.25 micrograms per kilogram per day and could go up to 100 micrograms per kilogram per day in the highest group. Each patient will take the drug in two-week treatment cycles, with two weeks on therapy followed by two weeks off, for as long as four months if they continue to show signs of clinical benefit.

If Cohort 3 is dosed without any problems, GT Biopharma anticipates dosing the first patient in Cohort 4 by the end of 2025. The dose level can be increased further if warranted. The company expects to provide an update on additional data by the end of 2025 and in the first quarter of 2026 as it moves to the higher dose levels in the trial that may yield more robust therapeutic effects.

On Stocktwits, retail sentiment for GT Biopharma was 'extremely bullish' amid a 2,933% surge in 24-hour message volume.

One user said that GT Biopharma shares appeared to be gaining strong upward momentum and could move toward double-digit levels. 

Another user commented that the company's cancer therapy showed significant promise, with the potential for substantial long-term returns.

GT Biopharma's stock has declined 65% so far in 2025.

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