Aprea Therapeutics Reports Early Success For WEE1 Inhibitor APR-1051 With Confirmed Partial Response In Phase 1 ACESOT-1051 Trial

Aprea Therapeutics, Inc. announced a confirmed partial response (PR) in its ongoing ACESOT-1051 clinical trial evaluating APR-1051, a selective WEE1 kinase inhibitor being developed for patients with biomarker-defined solid tumors. This confirmation marks a meaningful clinical development for the company’s precision oncology program.

The confirmed partial response was observed in a patient with PPP2R1A-mutated endometrial cancer who is currently receiving the 220 mg once-daily dose. Aprea previously reported on February 18, 2026, that the patient showed a 50% reduction in target lesion size at their first imaging assessment, meeting RECIST criteria for a partial response, along with a drop in CA-125 levels. The second imaging assessment confirmed the response, showing an additional 9.5% decrease in lesion size and a reduction in CA-125 to 40.2 U/mL, down from a baseline of 362 U/mL.

ACESOT-1051 is a Phase 1 trial designed to evaluate the safety, tolerability, pharmacokinetics, and early anti-tumor activity of APR-1051 in patients with advanced solid tumors that carry specific cancer-associated genetic alterations. So far, 24 patients have been treated at dose levels ranging from 10 mg to 220 mg once daily. Two patients—both with endometrial cancers harboring PPP2R1A mutations—have achieved partial responses, with one of these responses now confirmed. Both individuals remain on treatment.

Additionally, five patients have achieved stable disease as their best overall response. These include individuals with HPV-positive head and neck squamous cell carcinoma, as well as patients with colorectal and endometrial cancers containing relevant genomic alterations. Overall, APR-1051 has shown a favorable safety profile, with most side effects being mild to moderate (Grade 1 or 2), commonly involving nausea and fatigue.

Eugene Kennedy, Chief Medical Advisor at Aprea, stated that the data emerging from the study continue to reinforce the potential of APR-1051. He noted that the confirmed partial response in the 220 mg cohort demonstrates sustained anti-tumor activity, adding that the treatment appears generally well tolerated with an encouraging therapeutic window. According to Kennedy, these findings strengthen confidence in APR-1051’s ability to effectively target WEE1 in genetically driven cancers, where patients often have limited treatment options.

Dose escalation in the trial is ongoing, with plans to advance to Dose Level 9 (300 mg once daily) in the second quarter of 2026. At the same time, Aprea intends to enroll additional patients whose tumor types or specific genetic mutations suggest a higher likelihood of responding to WEE1-targeted therapy. This includes patients with uterine serous carcinoma, colorectal cancers, and HPV-positive tumors. Further details about the ACESOT-1051 trial can be found under ClinicalTrials.gov identifier NCT06260514.