The Food and Drug Administration has placed a partial clinical hold on Biohaven's late-stage trials for opakalim, its experimental epilepsy drug, halting new patient enrollment and sending the company's shares tumbling as much as 17% in Thursday trading.

The hold, effective September 4, does not affect the more than 600 patients already enrolled and dosing in Biohaven's pivotal BHV7000-303 study, which the company says remains fully enrolled and on track to report topline results in the second half of 2026, according to a filing with the Securities and Exchange Commission. Only enrollment of new patients into the ongoing trials is paused.

Regulators cited insufficient data to assess the human relevance of a metabolite identified in rodent toxicology studies of opakalim, formally known as BHV-7000, according to BioPharma Dive. Biohaven has said it believes the finding may be specific to rodents and is preparing additional nonclinical studies to address the FDA's concerns.

Shares of Biohaven (NYSE: BHVN) fell more than 13% in premarket trading and extended losses once markets opened, according to BioSpace, which described the hold as "another hit" for a drug candidate already under scrutiny from investors.

The timing stings: the hold lands barely two weeks after Biohaven struck a deal on August 26 granting South Korea's SK Biopharmaceuticals worldwide rights to opakalim outside a handful of markets, a licensing agreement worth up to $795 million in potential payments. That deal had been viewed as validation of the drug's commercial prospects heading into its pivotal trial readout.

Opakalim is a potassium channel opener Biohaven has been developing as a treatment for focal epilepsy, one of several pipeline bets the company has made as it works to diversify beyond its blockbuster migraine drug Nurtec.

What's next: investors are now watching for the additional nonclinical data Biohaven plans to submit to the FDA to address the metabolite question, as well as the BHV7000-303 topline readout expected later this year — a result that will land regardless of whether the enrollment hold has been lifted by then.