Stemline's MEN2312 breast cancer trial timeline slips 16 months
The Phase 1 dose-finding study now targets an October 2027 primary completion, extending a delay that has already stretched more than two years from the original estimate.
Executive Summary
- Stemline Therapeutics pushed back the primary completion date for its Phase 1 trial of MEN2312 in advanced breast cancer, the second such delay on this trial.
- The dose-finding study is measuring dose-limiting toxicity and a recommended Phase 2 dose, not efficacy, so the near-term readout will speak to tolerability rather than clinical benefit.
- A Pfizer KAT6 inhibitor already in Phase 2 for the same target in breast cancer sets the pace MEN2312 must eventually match on a target with only three industry programs testing it directly.
- The completion date has slipped twice while enrollment has grown, a pattern consistent with a still-recruiting first-in-human study rather than a program in distress.
The delay
The trial's primary completion date, originally estimated for October 1, 2025, moved first to June 1, 2026 in January 2026, then to October 24, 2027 in the latest registry update, a 16-month extension from the prior estimate and a cumulative 753-day slip from the original projection. The study, first posted in October 2024 and still recruiting, is a first-in-human evaluation of MEN2312 in adults with advanced breast cancer whose tumors carry PIK3CA, AKT1, or PTEN alterations and who have progressed on a CDK4/6 inhibitor combined with fulvestrant or an aromatase inhibitor. The trial spans 44 sites in the United States and Spain. NCT06638307+1A First-in-Human Study of MEN2312 in Adults With Advanced Breast CancerNCT06638307Stemline Therapeutics Timeline Delayed for Phase 1 MEN2312 Trial in Advanced Breast CancerJul 17, 2026
Probability of SuccessBased on the AppliedXL Probability of Success model. For more information about the methodology, read the research here.
What it will test
The primary endpoints are the number of participants with dose-limiting toxicity and the recommended Phase 2 dose of MEN2312, evaluated through Day 28 and Month 6, respectively. Secondary measures include overall response rate, progression-free survival, duration of response, and pharmacokinetic parameters for both monotherapy and combination dosing. Because the design is open-label with no comparator arm, the trial can establish tolerability and an initial dose to carry forward; it cannot, on its own, establish whether MEN2312 produces a clinical benefit in this heavily pretreated population. NCT06638307A First-in-Human Study of MEN2312 in Adults With Advanced Breast CancerNCT06638307
Enrollment context
The anticipated enrollment target rose from 124 to 240 patients in September 2025, a 93.5% increase. That increase in a Phase 1 study reads as expanded dose-exploration scope rather than a distress signal, consistent with how enrollment growth is typically read at this stage. The eligibility criteria were also refined twice, most recently in June 2026, to sharpen the biomarker and prior-therapy requirements for enrolled patients. NCT06638307A First-in-Human Study of MEN2312 in Adults With Advanced Breast CancerNCT06638307
Competitive position
MEN2312 targets KAT6, a histone acetyltransferase, and its closest comparator is Pfizer's PF-07248144, a KAT6 inhibitor already in a Phase 2 trial for the same indication. Two other KAT6 programs, IDEAYA Biosciences' IDE574 and Beijing Konruns Pharmaceutical's KC1086, remain in Phase 1 testing in tumors of varied primary sites. Only four industry trials study KAT6 in breast cancer across four sponsors, and none has yet completed or been terminated, so there is no resolved outcome to benchmark against. With Pfizer's program a phase ahead, MEN2312's dose-finding data will need to clear a comparable tolerability bar to keep pace with the more advanced KAT6 entrant.
Sponsor track record
Stemline Therapeutics has completed 9 of 12 prior trials globally, with 3 terminated, and maintains five other trials currently recruiting or active. The sponsor has experienced delays exceeding 90 days on two of its trials, including this one.
This analysis was produced using AI-assisted reporting systems, AppliedXL data, and official public records. These systems undergo editorial review, quality checks, and regular audits by human experts. Errors may still occur, as with any automated system. Always consult the linked primary sources. Read our AI Editorial Policy.