Aprea Therapeutics' APR-1051 achieves second partial response in phase I solid tumor trial

# Aprea Therapeutics Reports Second Partial Response for APR-1051 WEE1 Inhibitor in Phase I Solid Tumor Trial Pennsylvania-based Aprea Therapeutics (Nasdaq: APRE) [announced](https://www.globenewswire.com/news-release/2026/02/18/3240156/0/en/Aprea-Therapeutics-Announces-Additional-Positive-Clinical-Activity-for-WEE1-Inhibitor-APR-1051-Including-Second-Partial-Response-in-Ongoing-ACESOT-1051-Trial.html) preliminary data from the ongoing Phase I ACESOT-1051 trial showing a second unconfirmed partial response for its oral WEE1 kinase inhibitor APR-1051 in a patient with advanced endometrial cancer. The response, observed at the 220 mg dose level, included a 50% reduction in target lesion measurements and an 87% decline in the tumor biomarker CA-125, with only Grade 1 adverse events reported. For a company that pivoted its entire pipeline after the failure of its TP53-reactivating compound eprenetapopt in Phase III, the emerging clinical activity of APR-1051 in genomically selected cancers provides early validation of a new strategic direction — and enters a WEE1 inhibitor field where no drug has yet reached regulatory approval. ## Trial Specifics The [ACESOT-1051 trial](https://clinicaltrials.gov/ct2/show/NCT06260514) is a first-in-human, open-label Phase I dose-escalation study evaluating the safety, pharmacokinetics, pharmacodynamics, and preliminary anti-tumor activity of single-agent APR-1051 in patients with advanced solid tumors harboring cancer-associated genetic alterations. The drug is administered orally once daily in continuous 28-day cycles. Twenty-two patients have been treated to date across dose cohorts ranging from 10 mg to 220 mg, with the 220 mg cohort currently enrolling. The dose-escalation portion is designed to enroll up to 50 patients. The second partial response, reported at the first on-treatment imaging assessment in a patient at the 220 mg dose level (Cohort 8), met RECIST v1.1 criteria. The patient's CA-125 levels fell from 362 U/mL at baseline to 47 U/mL. Her tumor harbored a PPP2R1A mutation — the same alteration found in the first responding patient, who achieved an unconfirmed partial response at the 150 mg dose level. Both patients remain on treatment. The company also reported stable disease in five additional patients across multiple tumor types, including HPV-positive head and neck squamous cell carcinoma (HNSCC), FBXW7-mutated colorectal cancer, and CCNE1/TP53-mutated uterine cancer, at doses ranging from 70 mg to 150 mg. Both partial responses require confirmation at subsequent imaging assessments per standard criteria. APR-1051 has been generally well tolerated across all dose levels evaluated. The company stated that no class-limiting toxicities have been observed to date, a point it emphasized given the historical difficulty of developing WEE1 inhibitors with adequate therapeutic indices. Aprea plans to expand enrollment of PPP2R1A-mutated endometrial cancer and HPV-positive HNSCC patients within the study, signaling a shift toward genomically guided cohort enrichment. A further trial update is expected in Q2 2026. "We believe the emergence of a second unconfirmed partial response strengthens the clinical trend we are observing as dose escalation progresses," said Oren Gilad, President and CEO of Aprea. Eugene Kennedy, Chief Medical Advisor, noted that the second responding patient "was refractory to her most recent two prior therapies," adding that "the response in a tumor with a PPP2R1A alteration underscores the potential of genomically guided patient selection in our DDR program." ## APR-1051 WEE1 Inhibitor: Mechanism and Research Context WEE1 is a cell-cycle checkpoint kinase that restrains cyclin-dependent kinase (CDK) activity to enforce the intra-S and G2/M checkpoints, providing cells time to repair DNA damage before entering mitosis. Inhibiting WEE1 forces cells with pre-existing replication stress or unrepaired DNA damage into premature mitosis, resulting in mitotic catastrophe and cell death. This mechanism is [particularly relevant](https://app.allsci.com/article/W4213192564) in cancers that have lost other DNA damage response safeguards — for instance, tumors with TP53 mutations that lack a functional G1 checkpoint become heavily dependent on the G2/M checkpoint that WEE1 controls. Preclinical work has also shown that WEE1 inhibition can [activate innate immune signaling pathways](https://app.allsci.com/article/W4389340197), including endogenous retroviral element and double-stranded RNA sensing, which may have implications for combination strategies with immune checkpoint inhibitors. The focus on PPP2R1A mutations as a biomarker for WEE1 sensitivity is a relatively novel element of Aprea's development strategy. PPP2R1A encodes a subunit of the protein phosphatase 2A (PP2A) complex, and mutations in this gene are recurrent in endometrial cancers, particularly serous subtypes. The mechanistic link between PP2A dysfunction and WEE1 dependence is an area of active investigation, but the clinical observation of two partial responses in two PPP2R1A-mutated tumors provides an early, hypothesis-generating signal. APR-1051 does not currently hold any US FDA designations such as Fast Track, Orphan Drug, or Breakthrough Therapy, and it remains unapproved in all jurisdictions globally. The molecule was not originated at Aprea Therapeutics. It was initially developed as ZN-c3 by Zentalis Pharmaceuticals, passed through Acrotech Biopharma (a subsidiary of Aurobindo Pharma), and was acquired by Aprea in 2024 as the company sought to rebuild its pipeline following the Phase III failure of eprenetapopt (APR-246) in myelodysplastic syndromes. Aprea now holds exclusive global rights and is the sole developer. ### WEE1 Kinase Inhibitor Clinical Trial Landscape and Competing Programs No WEE1 inhibitor has received regulatory approval for any indication. The field's trajectory has been shaped by the experience of adavosertib (AZD1775), the first WEE1 inhibitor to enter clinical trials, which demonstrated anti-tumor activity across multiple solid tumor types but was ultimately limited by myelosuppression and gastrointestinal toxicity. AstraZeneca largely discontinued its internal development of adavosertib, though some investigator-sponsored studies continue. The tolerability challenges that constrained adavosertib have made therapeutic index the central differentiating question for every subsequent entrant. Key competitors include: * **Azenosertib (ZN-c3) — Pfizer (via acquisition of Zentalis Pharmaceuticals, 2024).** This is the most advanced next-generation WEE1 inhibitor and the most direct competitor to APR-1051. It is in [Phase II development](https://clinicaltrials.gov/ct2/show/NCT04158336), with dedicated expansion cohorts in [uterine serous carcinoma](https://clinicaltrials.gov/ct2/show/NCT05368506) and other CCNE1-amplified or TP53-mutated solid tumors. Preliminary data presented at [ASCO 2023](https://meetings.asco.org/abstracts-presentations/226553) showed clinical activity with a tolerability profile that appeared more manageable than adavosertib's. Pfizer's acquisition provides substantial development and commercialization resources. Notably, azenosertib shares an origin with APR-1051 — both trace back to Zentalis's WEE1 program, though they are distinct molecules. * **Debio 0123 — Debiopharm Group (Switzerland-based).** This oral WEE1 inhibitor is in [Phase I/II development](https://clinicaltrials.gov/ct2/show/NCT05235035) in advanced solid tumors and is differentiated by its brain-penetrant properties, which could open applications in CNS metastases or primary brain tumors that other WEE1 inhibitors cannot readily address. * **ART0380 — Novartis (via acquisition of Artios Pharma, 2024; UK-based).** Now part of Novartis's DNA damage response portfolio, ART0380 is an oral WEE1 inhibitor in [Phase I/II](https://clinicaltrials.gov/ct2/show/NCT04386902) in biomarker-selected advanced solid tumors. * **IMP7068 (formerly SY-4835) — Impact Therapeutics (China-based).** This selective oral WEE1 inhibitor is in [Phase I/II](https://clinicaltrials.gov/ct2/show/NCT05224856) in advanced solid tumors, primarily enrolling in China. * **Adavosertib (AZD1775) — AstraZeneca.** While AstraZeneca has largely stepped back from active development, adavosertib generated the most extensive [clinical dataset](https://pubmed.ncbi.nlm.nih.gov/32174170/) for WEE1 inhibition, including [Phase II data in uterine serous carcinoma](https://ascopubs.org/doi/10.1200/JCO.20.02923) that helped establish the rationale now being pursued by multiple competitors. Aprea's differentiation rests on two claims: first, that APR-1051 can deliver WEE1 inhibition with a wider therapeutic window than predecessors, as suggested by the absence of class-limiting toxicities in the 22 patients treated to date; and second, that its biomarker strategy — particularly the focus on PPP2R1A mutations — can identify patients most likely to respond. Both propositions remain early-stage. The partial responses are unconfirmed, the patient numbers are small, and the maximum tolerated dose has not yet been established. Whether the tolerability profile holds at higher doses and with longer treatment durations will be a defining question as the ACESOT-1051 trial progresses through Q2 2026 and beyond. In a competitive field where multiple well-resourced companies are pursuing the same target, the data that emerge from the next several dose cohorts will determine whether APR-1051 can carve out a distinct clinical niche or whether the WEE1 inhibitor space consolidates around the programs with the most capital behind them. --- Spot something wrong? [Report an issue with this article](https://newsgen-prod.reframedata.com/feedback/apr-1051-partial-response-aprea-therapeutics)