Barzolvolimab (CDX-0159): A First-in-Class Mast Cell-Depleting Antibody Heading Toward Its Phase 3 Readout in Chronic Hives (September 17, 2026)
Table of Contents
- An antibody that removes the cell, not just the signal
- How it works: blocking KIT (CD117)
- The lead disease: antihistamine-refractory chronic hives
- What the Phase 2 data showed
- Beyond spontaneous hives: cold urticaria and dermographism
- The Phase 3 test: EMBARQ-CSU1 and EMBARQ-CSU2
- Safety: side effects that trace back to the target
- Why it matters and what to watch
An antibody that removes the cell, not just the signal
Barzolvolimab, known by its development code CDX-0159, is an investigational monoclonal antibody from Celldex Therapeutics built around an idea that is unusual in allergy and dermatology: rather than blocking one of the chemicals a mast cell releases, it aims to get rid of the mast cells themselves. Mast cells are the immune cells that sit behind hives, angioedema and much of allergic inflammation - when they degranulate they flood the skin with histamine and other mediators. Most treatments work around them: antihistamines block histamine after it is released, and the anti-IgE antibody omalizumab lowers one route of activation. Barzolvolimab instead targets the mast cell's lifeline. It is often described as a first-in-class 'mast cell-depleting' antibody.
How it works: blocking KIT (CD117)
The target is KIT, also called CD117 or the stem cell factor receptor - a receptor tyrosine kinase on the surface of mast cells. Mast cells depend on KIT signaling, switched on when the growth factor stem cell factor (SCF) binds the receptor, for their development, survival in tissues and activation. Barzolvolimab binds a specific part of KIT and potently blocks its activity, cutting off the SCF signal. Deprived of KIT signaling, mast cells decline in number. Researchers can see this happening: barzolvolimab produces a dose-dependent fall in plasma tryptase, a blood marker of how many mast cells are present, and reduces mast cells seen on skin biopsy. In other words, the drug's effect on its target is measurable in patients, not just inferred.
The lead disease: antihistamine-refractory chronic hives
Barzolvolimab's most advanced program is chronic spontaneous urticaria (CSU) - recurring hives and swelling that appear without an obvious external trigger and last six weeks or more. Guidelines escalate from second-generation antihistamines to higher doses and then to omalizumab, but a substantial group of patients keep breaking out despite everything, and their disease can be miserable and unpredictable. That refractory population is exactly where barzolvolimab is aimed. By depleting the effector cells upstream of the many mediators they release, the antibody is designed to control the disease more completely than blocking any single downstream signal.
What the Phase 2 data showed
In Phase 2 dose-finding studies in antihistamine-refractory CSU, barzolvolimab produced rapid, deep and durable reductions in the standard measure of hives activity, the 7-day Urticaria Activity Score (UAS7). Large proportions of patients reached well-controlled or complete responses, benefit was seen across subgroups (including patients with different baseline IgE levels), and the effect held up over long-term follow-up reported out to 52 and even 76 weeks - an unusually long window for this kind of study. Those results are what justified moving into a large Phase 3 program.
Beyond spontaneous hives: cold urticaria and dermographism
Barzolvolimab has also been tested in the chronic inducible urticarias, where hives are triggered by a physical stimulus. In a Phase 2 study over a 20-week placebo-controlled period, up to roughly 66% of patients with cold urticaria (hives triggered by cold) and 49% with symptomatic dermographism (hives triggered by skin stroking or pressure) achieved a complete response - meaning the provoked hive reaction essentially resolved. On the strength of those data, Celldex has initiated a global Phase 3 program in cold urticaria and symptomatic dermographism. The company is additionally studying barzolvolimab in prurigo nodularis, atopic dermatitis and eosinophilic esophagitis, probing whether mast-cell depletion can help other diseases in which those cells play a role.
The Phase 3 test: EMBARQ-CSU1 and EMBARQ-CSU2
The pivotal evidence will come from two replicate global Phase 3 trials in antihistamine-refractory CSU: EMBARQ-CSU1 (NCT06445023) and EMBARQ-CSU2 (NCT06455202). Together they enrolled 1,939 patients - described as the largest Phase 3 program ever conducted in antihistamine-refractory CSU, and one that deliberately includes patients who are experienced with or refractory to advanced therapy. Enrollment finished ahead of Celldex's own guidance, and topline results are expected in the fourth quarter of 2026. If those data are positive, Celldex has said it plans to file a Biologics License Application (BLA) in 2027. As of this writing the Phase 3 results have not been released, so the durable, deep responses seen in Phase 2 still have to be confirmed at scale.
Safety: side effects that trace back to the target
Barzolvolimab has generally been well tolerated in trials, and its most characteristic side effects are instructive because they flow directly from its mechanism. KIT is important not only to mast cells but also to pigment-producing cells and to parts of blood-cell development, so blocking it can cause reversible changes in hair or skin color and transient drops in neutrophils (a type of white blood cell), reported in a minority of patients and increasing somewhat with longer treatment. Reassuringly, the pigment changes and neutropenia observed on study have been mild and reversible - neutropenia tended to resolve even while treatment continued and was not linked to more infections - and no unexpected safety signals emerged over long-term follow-up. Still, because barzolvolimab is investigational, its full benefit-risk profile in everyday use will only be clear once Phase 3 and any regulatory review are complete.
Why it matters and what to watch
Barzolvolimab is worth following on two levels. Practically, it could become a new option for people with chronic hives that antihistamines and omalizumab cannot control, and potentially for cold urticaria, dermographism and other mast cell-driven conditions. Conceptually, it is a real-world test of whether depleting an entire effector-cell lineage - rather than blocking one mediator at a time - can be done safely enough to enter mainstream care. The late-2026 EMBARQ readout is the milestone that will answer the first question and inform the second. Barzolvolimab is an investigational biologic given by injection in clinical settings; it is not approved, and nothing here is medical advice - anyone dealing with severe or persistent urticaria should work with an allergist, immunologist or dermatologist.
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Citations
- [1] A Phase 3 Study of Barzolvolimab in Chronic Spontaneous Urticaria (EMBARQ-CSU1; NCT06445023) - ClinicalTrials.gov Source
- [2] A Phase 3 Study of Barzolvolimab in Chronic Spontaneous Urticaria (EMBARQ-CSU2; NCT06455202) - ClinicalTrials.gov Source
- [3] Celldex Completes Enrollment in Global Phase 3 Studies (EMBARQ-CSU1 and EMBARQ-CSU2) of Barzolvolimab - Celldex Therapeutics Source
- [4] Celldex Advances Barzolvolimab into Phase 3 for Cold Urticaria and Symptomatic Dermographism - Dermatology Times Source
- [5] Anti-KIT Barzolvolimab for Chronic Spontaneous Urticaria - PMC (peer-reviewed) Source
- [6] Randomized dose-finding study of anti-KIT barzolvolimab in chronic spontaneous urticaria - Journal of Allergy and Clinical Immunology Source
- [7] Anti-KIT antibody, barzolvolimab, reduces skin mast cells and disease activity in chronic inducible urticaria - PubMed Source
- [8] Celldex Presents Phase 2 Cold Urticaria and Symptomatic Dermographism Results (up to 66% ColdU and 49% SD complete response at Week 20) - Celldex Therapeutics Source
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