Celldex barzolvolimab Phase 3 trials meet all endpoints in CSU

Celldex Therapeutics has reported positive topline results from two concurrent Phase 3 trials of barzolvolimab in chronic spontaneous urticaria (CSU), with both studies meeting their primary endpoint and all key secondary endpoints. The readout, covering 1,939 randomised patients across EMBARQ-CSU1 and EMBARQ-CSU2, is among the largest datasets generated in antihistamine-refractory CSU to date.

Barzolvolimab is a humanised monoclonal antibody that targets the KIT receptor, which is critical for mast cell function and survival. By inhibiting KIT, the drug suppresses the mast cell activity that underpins CSU, a condition characterised by spontaneous hives and angioedema. The NASDAQ-listed company intends to submit a Biologics License Application (BLA) to the FDA in 2027.

Trial data

The primary endpoint in both studies was mean change from baseline in the weekly urticaria activity score (UAS7) at week 12. Against a placebo arm that showed reductions of roughly 10 to 11 points from a baseline UAS7 of around 30, both barzolvolimab dose regimens (150 mg every four weeks and 300 mg every eight weeks) produced reductions of approximately 20 points, each reaching p values below 0.00001.

Complete response, defined as a UAS7 of zero (the complete absence of itch and hives), was achieved in roughly 42 to 46 percent of patients on active treatment at week 12, rising to 45 to 54 percent by week 24, compared with 9 to 18 percent on placebo. In the subgroup of patients whose disease is refractory to omalizumab, the approved first-line biologic in CSU, complete response rates of 41 to 55 percent were observed at week 12, representing a clinically meaningful signal in a population with limited options.

Diane Young, Chief Medical Officer at Celldex, said the data showed "unprecedented complete response rates" and "strong differentiation in patient populations underserved by existing therapies, including those with severe disease or angioedema." The company characterises barzolvolimab as a potential first-line option for severe or angioedema-predominant CSU and a second-line candidate for patients who have failed omalizumab. Safety through the 24-week placebo-controlled period was consistent with prior Phase 2 experience.

Market context and regulatory path

CSU affects an estimated 0.5 to 1 percent of the global population, and a meaningful proportion of patients remain inadequately controlled despite antihistamines and omalizumab. That refractory segment represents a commercially attractive and scientifically underserved opportunity, and several programmes are competing for it. AstraZeneca and Sanofi are among larger companies with assets in adjacent mast cell and IgE pathways, while Novartis's omalizumab (Xolair) remains the only approved biologic for CSU.

Barzolvolimab's KIT inhibition mechanism is mechanistically distinct from anti-IgE or anti-IL-4 receptor approaches, and the omalizumab-refractory complete response data will be particularly scrutinised by regulators and payers, as demonstrating benefit in a prior-biologic-failure population often strengthens a label's commercial positioning. The company said full results will be presented at an upcoming medical meeting before the BLA submission, which will give the field more granular safety and durability data.

Treatment in both EMBARQ trials continues to week 52, and Celldex has established a Phase 3b long-term extension study for patients completing the main trials. Barzolvolimab is also being evaluated in Phase 3 studies in cold urticaria and symptomatic dermographism, and in a Phase 2 study in atopic dermatitis, suggesting the company is building a broad mast cell franchise around the asset.