What happened

Shares of Celldex Therapeutics (CLDX -4.68%) were tanking 34.1% as of 11:56 a.m. EDT on Monday. The big decline came after Celldex announced interim results from a phase 1b study of CDX-0159 in treating patients with chronic inducible urticaria (CIndU), severe hives that affect around 0.5% of the population.

So what

Celldex reported that eight of the 10 patients who were assessed for at least 15 days after treatment with CDX-0159 experienced a complete response. One of the patients experienced a partial response. That sounds like great news, so why did the biotech stock sink?

It could be that investors are concerned about the safety profile for CDX-0159. Celldex stated that the drug "was generally well-tolerated." However, the company also said that one patient experienced a severe reaction with a "brief loss of consciousness, followed by shaking and sweating." This patient was treated with antihistamines and steroids, and recovered quickly.

Two scientists in a lab with one looking through a microscope

Image source: Getty Images.

Marcus Maurer, professor of dermatology and allergy at Charite-Universitatsmedizin in Berlin and lead investigator for the study, didn't seem to share such concerns. He stated, "As a physician, it is exciting to see such high complete response rates, which are made even more noteworthy by the fact that the responses are developing rapidly after treatment and are coupled with a well-tolerated safety profile to date."

Celldex Therapeutics CEO Anthony Marucci also was positive about the interim data, stating, "The interim results reported today further reinforce our enthusiasm for CDX-0159."

Now what

The company expects to complete testing of additional cohorts in the phase 1b study over the next few weeks. Celldex plans to report updated results later in the near future, probably at a medical conference this summer. It also should announce results from a phase 1b study in chronic spontaneous urticaria by the end of 2021.