Strong Response Rates Mark a Turning Point for Advanced Systemic Mastocytosis Treatment
Cogent Biosciences has released positive top-line results from its pivotal APEX Part 2 trial, revealing that bezuclastinib achieved a 57% objective response rate (ORR) by mIWG criteria in patients with advanced systemic mastocytosis (AdvSM). With 80% ORR measured by pure pathological response criteria and deep biomarker reductions in a heavily affected patient population, the data signal potential shifts in AdvSM care as regulatory milestones approach.
Bezuclastinib Yields Rapid, Deep Responses and Consistent Disease Control
Out of 81 patients enrolled in APEX Part 2, 68 were evaluable for the primary endpoint. The majority saw clinical improvements rapidly—median time to response was just 2.0 months. Remarkably, 89% of patients experienced at least a 50% reduction in both serum tryptase and bone marrow mast cells, indicating bezuclastinib’s powerful impact on disease drivers. A significant 91% reduction in the KIT D816V mutation burden (a key disease mutation) was also observed in a subset of patients, reflecting the therapy's targeted approach.
| Clinical Outcome | Result |
|---|---|
| Objective Response Rate (mIWG criteria) | 57% |
| Objective Response Rate (PPR criteria) | 80% |
| ≥50% Reduction in Serum Tryptase | 89% |
| ≥50% Reduction in Bone Marrow Mast Cells/Clearance of Aggregates | 89% |
| ≥50% Reduction in KIT D816V Variant Allele Frequency | 91% |
Safety Profile: Minimal Dose Reductions and No Discontinuations Signal Patient-Friendly Option
The therapy's tolerability stands out: just 14.8% of patients needed a dose reduction, and there were no treatment discontinuations due to adverse events. Most side effects were low-grade, such as hair color change (30.9%), neutropenia (29.6%), altered taste (28.4%), and mild, reversible elevations in liver enzymes. Importantly, even a single grade 3 transaminase event resolved with dose modification, and the patient continued therapy.
Strategic Outlook: Regulatory Submissions and Commercialization on the Horizon
Following these results, Cogent anticipates submitting a new drug application (NDA) for bezuclastinib in AdvSM to the FDA in the first half of 2026. These results mark the third pivotal positive outcome for bezuclastinib this year—preceded by favorable data from the SUMMIT (NonAdvSM) and PEAK (GIST) trials—setting the stage for potential commercial launches later in 2026.
What’s Next for Investors and Patients?
Detailed trial data will be shared at a major medical meeting in 2026. Cogent is also rolling out inducement equity awards as it ramps up its commercial strategy. For patients and investors alike, these results are a notable milestone: they highlight not only an effective and well-tolerated therapy, but also an expanding portfolio poised for regulatory progress and potential market entry.
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