On December 6, 2025 Cogent Biosciences, Inc. (NASDAQ: COGT) reported complete results from the registration-directed Part 2 of the SUMMIT clinical trial of bezuclastinib in patients with nonadvanced systemic mastocytosis (NonAdvSM). As previously reported, bezuclastinib demonstrated clinically meaningful and highly statistically significant improvements across the primary and all key secondary endpoints. New results further highlight the benefit of bezuclastinib on patient-reported symptoms and objective measures of mast cell burden and demonstrate significant correlation between improvement in disease pathology and patient-reported symptom severity.

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"We are excited to present additional data from the SUMMIT trial that support our conviction that bezuclastinib will be the best-in-class treatment option for patients with nonadvanced systemic mastocytosis," said Andrew Robbins, Cogent’s President and Chief Executive Officer. "We remain on track to submit our first New Drug Application for bezuclastinib in NonAdvSM with the FDA this month and are encouraged by the increased interest in our Expanded Access Program."

"Nonadvanced systemic mastocytosis patients currently have very limited treatment options, and the benefit bezuclastinib demonstrated in the SUMMIT trial across measures of disease pathology and symptomatic improvement is very exciting for this patient population," said Lindsay Rein, MD, Associate Professor of Medicine in the Division of Hematologic Malignancies and Cellular Therapy, Duke University. "The SUMMIT trial results match my clinical experience using bezuclastinib with NonAdvSM patients, delivering rapid and deep improvement in symptom control and objective measures of disease without tolerability challenges."

SUMMIT Trial Data

In the registration-directed Part 2 of the SUMMIT clinical trial, 118 patients received bezuclastinib once daily plus best supportive care (BSC) and 60 patients received placebo plus BSC. The study included adults with a NonAdvSM diagnosis confirmed by central pathology review, and moderate-to-severe symptom burden despite an optimized regimen of BSC.

Following completion of the 24-week treatment period, patients had the option to receive bezuclastinib in an open-label extension study. Baseline patient demographics were balanced between treatment arms and reflected significant disease burden. Disease symptoms were assessed using the Mastocytosis Symptom Severity Daily Diary (MS2D2).

Bezuclastinib delivered clinically meaningful and statistically significant symptomatic improvement

Outcome measure Bezuclastinib Placebo p-value
At 24 weeks of treatment (primary endpoint and key secondary endpoints)
Mean change TSS (%) -24.3 (-43%) -15.4 (-29%) p<0.001
Proportion of patients with ≥50% reduction in TSS 34.3% 18.1% p=0.01
Proportion of patients with ≥30% reduction in TSS 65.4% 38.6% p<0.001
For patients treated through 48 weeks (follow-up data cut off Nov 2025)
Mean change TSS (%) -32.0 (-54%) n/a n/a
Proportion of patients with ≥50% reduction in TSS 56.4% n/a n/a
Proportion of patients with ≥30% reduction in TSS 86.2% n/a n/a

Across several additional key secondary endpoints, bezuclastinib demonstrated rapid, deep and sustained improvement on objective disease markers of mast cell burden. At week 24, 87.4% of patients achieved ≥50% reduction in serum tryptase levels, 75.6% of patients demonstrated ≥50% reduction in bone marrow mast cells or clearance of aggregates and 85.7% of patients achieved ≥50% reduction in KIT D816V variant allele frequency or undetectable, each of which was statistically significant when compared to placebo. Additional pathobiology data from SUMMIT patients will be shared in an oral presentation on Monday, December 8th at ASH (Free ASH Whitepaper).

SUMMIT Subgroups

As part of the SUMMIT study, patients with Smoldering Systemic Mastocytosis (n=8 bezuclastinib arm, n=4 placebo arm) and patients who had previously been treated with avapritinib (n=11 bezuclastinib arm, n=3 placebo arm) were enrolled. Patients treated with bezuclastinib in these subgroups showed a mean change in TSS of -35.6 and -21.6, respectively. The response in objective measures of disease burden in these patients was consistent with results from the broader SUMMIT population, as were their related adverse events and overall tolerability.

Safety Data

As previously reported on July 7, 2025, the majority of treatment emergent adverse events (TEAEs) (98.3% in bezuclastinib arm vs. 88.3% in placebo arm) were of low grade. The most frequent TEAEs reported on bezuclastinib treatment were hair color change (69.5% bezuclastinib vs. 5.0% placebo), altered taste (23.7% bezuclastinib vs. 0% placebo), nausea (22.0% bezuclastinib vs. 13.3% placebo) and ALT/AST elevations (22.0% bezuclastinib vs. 6.6% placebo; ≥Gr 3, 5.9% vs. 0%). Serious AEs occurred in 4.2% of patients treated with bezuclastinib, compared to 5.0% of patients treated with placebo. Discontinuations due to treatment-related AEs occurred in 5.9% of patients treated with bezuclastinib, all due to ALT/AST elevations and all patients fully resolved. There were no hepatic AEs reported in any patient other than transient and manageable lab abnormalities.

SUMMIT Long Term Follow-up

Data from longer term follow-up in patients participating in the SUMMIT trial are expected to be presented at an upcoming scientific meeting in Q1 2026. Preliminary 48-week data will be shared during the investor call scheduled for Monday, December 8th.

(Press release, Cogent Biosciences, DEC 6, 2025, View Source [SID1234661197])