Celldex's hives drug won two Phase 3 trials decisively, and the stock fell 18% on two anaphylaxis cases

On September 22, 2026 Celldex Therapeutics reported topline results from EMBARQ-CSU1 (963 patients) and EMBARQ-CSU2 (976 patients), two Phase 3 trials of barzolvolimab, an antibody against the KIT receptor that depletes mast cells, in chronic spontaneous urticaria in patients not controlled by antihistamines. Patients received 150 mg every four weeks or 300 mg every eight weeks, after a loading dose, or placebo for 24 weeks.

Both trials met the primary endpoint and all key secondary endpoints. The weekly urticaria activity score fell by 19.7–20.5 points on drug against 10.7–11.4 on placebo at week 12 (p<0.00001 in every arm). The share of patients completely free of hives and itch was 42.1–45.7% on drug against 9.3–12.6% on placebo at week 12, and 45.1–54.0% against 15.4–17.6% at week 24. In patients who had previously failed omalizumab (Xolair), complete-response rates at week 12 were 41.7–55.3% against 9.3–15.1%.

BioPharma Dive and Endpoints News reported two life-threatening anaphylaxis cases in one barzolvolimab dose group, and one in the placebo arm; the chief executive called the incidence “extremely low” among about 2,400 patients treated. Other reported side effects included changes in neutrophil counts and in hair and skin colour. The press release gave no safety tables. The trials continue to 52 weeks, and Celldex plans a BLA submission in 2027.

Celldex shares closed at $37.89 on September 21, 2026 and at $31.12 on September 23, a two-day fall of 17.9%, and 30.6% below the August 12, 2026 high of $44.87. Cantor Fitzgerald called the results the best Phase 3 data set it had seen across pivotal studies and models $4.8 billion in peak sales; Stifel noted that Dupixent also carries anaphylaxis warnings.

Opportunity

The obvious reading is that a mast-cell-depleting drug has shown the risk everyone feared, and that a boxed warning will confine it to a small refractory niche behind Sanofi and Regeneron’s Dupixent, approved for the condition in 2025, and Novartis’s oral BTK inhibitor Rhapsido, approved later that year.

What that reading may miss is the size of the efficacy gap. Complete clearance in roughly half of patients, including those who failed Xolair, is a level no approved therapy has shown in pivotal trials of this size, and in chronic urticaria complete clearance is what patients and dermatologists pay for. Two cases in roughly 2,400 treated patients is a rate that labels and first-dose monitoring routinely manage in allergy and immunology.

The information the market needs to judge the signal — when the reactions occurred, whether they followed the loading dose, how they were treated, whether both patients were in the same dose regimen — has not been published.

Hypothesis: the 18% fall prices the anaphylaxis cases as a commercial ceiling when the likelier outcome, if the reactions cluster around the first injections, is a label warning plus in-office first dosing that a specialist-prescribed biologic can absorb; if so, the disclosure of the full safety data re-prices the stock toward the efficacy.

A second inference: at a sharply lower price, a best-in-class efficacy asset in a large chronic immunology indication is the kind of single-product company a large pharma buyer looks at once the safety profile is fully known.

How it could play out

Celldex presents full safety data at a medical meeting or in the 52-week readout → the anaphylaxis cases are shown to be early, loading-dose related and resolved → analysts model a warning and first-dose observation rather than a restrictive REMS → the stock recovers toward its pre-readout level as peak-sales models return to the multi-billion range → other indications (cold urticaria, symptomatic dermographism) follow on the same mechanism. The alternative: more cases emerge through week 52 or the FDA requires a REMS, and the drug becomes a third-line option behind an oral pill and Dupixent.

Questions worth asking

• When did the two anaphylaxis cases occur — after the loading dose, early in treatment, or late — and in which dosing regimen? Early, loading-dose events point to a manageable label; late or random events point to a commercial ceiling.
• How does a 45–54% complete-response rate at 24 weeks compare with Rhapsido’s and Dupixent’s pivotal complete-response rates, and how much of the prescribing decision in chronic urticaria turns on that number?
• Do the neutropenia and depigmentation effects, both expected from KIT inhibition, stay stable through 52 weeks or accumulate?
• Does the anaphylaxis signal read across to other mast-cell-directed programmes, or is it specific to barzolvolimab?
• Which large immunology companies lack a chronic-urticaria asset and could see Celldex as an acquisition after the safety data are public?

Where to look

• Celldex Therapeutics (CLDX) — the single-asset company holding barzolvolimab, down 17.9% in two sessions and 30.6% from its August high
• Novartis (NOVN.SW) — owns Rhapsido, the oral competitor; the clearest loser if barzolvolimab’s label is clean
• Sanofi (SAN.PA) and Regeneron (REGN) — Dupixent, the established biologic in the indication
• Roche (ROG.SW) — Xolair, the incumbent whose failures are the refractory population barzolvolimab cleared
• Jasper Therapeutics (JSPR) — a smaller KIT-targeted antibody developer exposed to any class read-across on safety
• Evommune (EVMN) — a mast-cell-directed chronic urticaria developer at a one-year low, exposed to the competitive bar barzolvolimab has set

Thesis check

The efficacy is primary-sourced from the company’s release, large, replicated across two trials and consistent in patients who failed the incumbent drug, which is as strong as Phase 3 evidence gets.

The weak link is that the safety data that caused the sell-off have not been published: without knowing when the reactions happened and in which regimen, the market’s discount cannot be judged too large or too small, and a single-asset company with a 2027 filing is exposed to anything the remaining 52-week follow-up turns up. A restrictive REMS against an oral competitor would justify the fall.

Sources

Celldex press release (GlobeNewswire), Sep 22 2026 · BioPharma Dive, Sep 22 2026 · Endpoints News, Sep 22 2026 · RTTNews, Sep 2026 · Novartis (Rhapsido approval), 2025