The US Food and Drug Administration has imposed a partial clinical hold on Biohaven’s epilepsy candidate opakalim. The hold took effect on 4 September 2026. Biohaven disclosed it in a Form 8-K filed on 10 September 2026. The measure stops new enrolment across the programme. Patients already randomised continue to receive treatment.
It lands nine days after Biohaven licensed the asset worldwide to SK Biopharmaceuticals. Reuters reported that Biohaven shares fell more than 13 per cent in premarket trading on 10 September. The commercial stakes are unusually clear. SK agreed to pay 350 million US dollars at closing, and that closing has not yet happened.
What the FDA asked Biohaven to resolve
Opakalim, also known as BHV-7000, is a selective Kv7.2/7.3 potassium channel activator. It is in Phase 2/3 development for refractory focal onset epilepsy. That population has seizures which persist despite at least two antiseizure medicines.
The filing says it was determined that there was insufficient information about a specific metabolite. That gap concerns the risks of the metabolite to human subjects at this time. The metabolites were observed during rodent testing. Biohaven says it had already updated global regulatory authorities on the ongoing nonclinical work. The agency then instructed the company to pause new enrolment until further studies are complete.
Biohaven told investors the significance to humans was uncertain. The company said the finding may be specific to rodents. It also said opakalim has generally been safe and well tolerated across more than 1,200 participants dosed to date. Biohaven expects to supply the additional data within weeks.
What has not been disclosed
The public record behind this partial clinical hold is thin. Neither party has named the metabolite. Neither has described the nature of the rodent finding.
LSDN could not establish which organ system was affected. Nor could it establish the doses tested, the exposure multiples, or whether any effect was dose related. The agency’s own wording has not been published, because clinical hold letters are not routinely released.
Biohaven announced the SK licence by press release on 26 August 2026. It disclosed the partial clinical hold through a Form 8-K on 10 September. Both are standard routes, and the filing sets out the position in the company’s own words.
Which studies the partial clinical hold actually stops
Three registered studies sit inside the opakalim programme. SK Biopharmaceuticals named all three in its own announcement of the licence.
RISE 2 is registered as NCT06132893. RISE 3 is registered as NCT06309966. A long-term open-label safety study is registered as NCT06443463. Biohaven names all three identifiers in its own programme disclosures. LSDN could not independently confirm current registry status for the three studies as at 10 September 2026.
Biohaven completed enrolment in RISE 3 in June 2026. Topline results from that study are still guided to the second half of 2026. The enrolment pause therefore bites on RISE 2 and on the open-label extension.
More than 600 participants already assigned to treatment may continue. Biohaven also extended the enrolment pause beyond US sites on a voluntary basis. That distinction matters for sponsors running global programmes. An FDA hold binds the US investigational new drug application, not trial sites in other jurisdictions.
The action also differs from a sponsor stopping a study outright. Novo Nordisk halted two late-stage heart failure trials on futility grounds this month. Here the studies continue, and only the intake of new participants has stopped.
What the UK research records show
The Health Research Authority holds records for two of the three studies. RISE 3 received a favourable opinion from Wales REC 3 on 18 June 2024. Its references are 24/WA/0136 and IRAS 1009549. The sponsor is listed as Biohaven Therapeutics Ltd.
The long-term safety study received a favourable opinion from East Midlands Derby Research Ethics Committee on 21 August 2024. Its references are 24/EM/0197 and IRAS 1010242. That record gives approximately 660 eligible subjects.
LSDN found no HRA research summary for RISE 2 as at 10 September 2026. The registry record for NCT06132893 does not list the United Kingdom among its countries. UK exposure therefore runs through RISE 3, where enrolment is already complete, and through the open-label extension.
A deal signed but not yet closed
Biohaven announced the worldwide licence on 26 August 2026. Its Form 8-K sets out the terms. Biohaven Bioscience Ireland Limited granted SK an exclusive, royalty-bearing, worldwide licence to the Kv7 platform.
The upfront fee is 400 million US dollars. Of that, 350 million is payable at closing and 50 million one year later. Development and regulatory milestones run to a further 150 million. SK also assumes about 245 million US dollars of Knopp Biosciences obligations and future Kv7 milestones.
Royalties are tiered from the mid-teens to the low twenties on US net sales. Ex-US royalties are mid-single digit. Both companies put the headline value of the transaction at up to 795 million US dollars.
The 8-K describes closing as subject to required antitrust clearances and other customary closing conditions. It does not itself identify any clinical or regulatory closing condition. Biohaven said on 10 September that both companies were still completing the Hart-Scott-Rodino submission required for closing. Biohaven continues to run RISE 2 and RISE 3 under the agreement.
Analyst sentiment had already cooled before the partial clinical hold became public. RBC Capital Markets downgraded Biohaven to Sector Perform on 8 September 2026. Analyst Leonid Timashev cut the price target to 19 dollars from 23 dollars. He cited valuation after a rally of about 100 per cent from the year’s lows. He also pointed to limited patient data and fewer near-term catalysts.
That sequencing is the commercial heart of the story. A partial clinical hold has landed on the asset while the buyer’s largest payment remains contingent on closing. How pharma chooses between licensing and acquisition shapes who absorbs that risk. The published summary of the agreement does not settle the point.
Biohaven’s filing addresses that point directly. It states that all clinical and nonclinical data was fully disclosed to SK before signing. That disclosure expressly included the metabolite material submitted to global regulatory authorities. SK separately told Korean media it had identified the finding during due diligence. SK said it expects limited impact on development timelines, and LSDN has not verified that.
The Kv7 class carries a safety history
Kv7 activation is not a new idea in epilepsy. Retigabine, sold in the United States as ezogabine under the brand Potiga, reached the market first.
In 2013 the FDA added a boxed warning covering retinal abnormalities, possible vision loss and blue skin discolouration. In June 2015 the agency reviewed further safety reports. It concluded those risks could be managed adequately under the existing labelling. It required an additional study, and the boxed warning stayed in place.
GlaxoSmithKline withdrew the product by the end of June 2017. It cited commercial reasons and declining use, and said safety was not the driver. The class therefore meets any new preclinical safety question with history attached. That history explains why a metabolite query draws more attention here than it might elsewhere.
Biohaven has also promoted the asset’s tolerability. On 26 May 2026 it reported new clinical data ahead of its research and development day. An open-label extension in focal epilepsy followed more than 100 six-month completers. All were taking opakalim 75 mg once daily. Of those, 54 per cent achieved at least a 50 per cent seizure reduction against pre-randomisation baseline. Dizziness was reported in 5.0 per cent of participants treated.
A separate proof of concept study in idiopathic generalised epilepsy reported no cases of somnolence, dizziness, fatigue or memory impairment. That study enrolled 27 subjects in total, 15 of them on opakalim. It closed before reaching its prespecified sample size, and no formal statistical testing was provided. Jason Lerner is Medical Director, Research and Development and Epilepsy Development Lead at Biohaven. He framed the wider dataset as seizure control without burdensome side effects. That framing now sits alongside a partial clinical hold and an unresolved preclinical question.
Opakalim has also missed endpoints outside epilepsy. A Phase 2/3 bipolar mania study failed on the Young Mania Rating Scale, announced on 3 March 2025. A Phase 2 proof-of-concept study in major depressive disorder missed its primary endpoint, announced on 24 December 2025. Neither result concerned safety.
The rival that plans to file this month
Xenon Pharmaceuticals is developing azetukalner, another Kv7 opener, in the same indication. Its Phase 3 X-TOLE2 study randomised 380 adults with treatment-resistant focal onset seizures.
At 25 mg, median monthly seizure frequency fell 53.2 per cent against 10.4 per cent on placebo. The responder rate at that dose was 54.8 per cent against 20.8 per cent. Discontinuations for adverse events reached 14.5 per cent at 25 mg, against 3.2 per cent on placebo.
Those figures come from a randomised, placebo-controlled trial. The opakalim responder rate cited earlier comes from an open-label extension with no placebo arm. The two sets of numbers are not directly comparable.
Xenon says that if approved, azetukalner would be the only Kv7 opener available for treating epilepsy. Chief Financial Officer Tucker Kelly spoke at a Wells Fargo healthcare conference on 9 September 2026. He said Xenon planned to file its new drug application that month.
Xenon has not commented on its rival’s position. LSDN found no statement from the company on the matter.
What UK prescribers would be waiting for
SK already sells cenobamate, marketed as XCOPRI in the United States. NICE recommends cenobamate as an add-on for focal onset seizures in adults with drug-resistant epilepsy.
Technology appraisal TA753 sets the conditions. It applies where epilepsy has not been adequately controlled with at least two antiseizure medicines. It positions cenobamate as a second-line add-on, used after at least one first-line add-on has failed. Treatment must be started by a clinician with epilepsy expertise.
Any Kv7 asset entering the UK would be arguing for space in that same crowded tier. SK would also be positioning a second epilepsy product against its own first.
What to watch next
Three things will settle how serious this partial clinical hold proves to be. The first is the nonclinical package Biohaven has promised within weeks. Its content, and the agency’s response, are not on a published timetable.
The second is antitrust clearance, and whether the licence closes on its original terms. The third is the RISE 3 topline, still guided to the second half of 2026. A clean readout would change the tone of this episode considerably.
SK has guided to a US launch in 2029. A partial clinical hold resolved within weeks would leave that timeline intact. One that runs longer would not. Trade readers tracking licensing risk should watch the gap between signing and closing on this transaction.
