The U.K. medicines agency declined to clear a human challenge study, a design that intentionally infects healthy volunteers with flu, forcing antiviral maker Traws Pharma to search for backup compounds.
Traws Pharma has deferred a planned mid-stage flu trial after the U.K.'s medicines regulator issued a negative review, the second time in less than a year that a Western regulator has pushed back on the company's lead antiviral over safety concerns the company has not fully disclosed.
The Phase 2a study of tivoxavir marboxil, or TXM, was designed to test the drug by intentionally infecting roughly 150 healthy volunteers with seasonal flu under controlled conditions, a so-called human challenge trial. The U.K. Medicines and Healthcare products Regulatory Agency declined to clear the protocol, prompting Traws to delay a study that had been scheduled to begin in June, according to Fierce Biotech reporting.
The decision leaves Traws without a clear path forward for a drug meant to anchor its flu business. Roughly $20 million in funding is tied to the deferred trial — $10 million linked to MHRA approval for the challenge study and a further $10 million tied to the announcement of trial data — and the company is now actively advancing backup antiviral candidates with TXM-like long-duration pharmacokinetics and, in Traws's own framing, "devoid of any mutagenic potential," per the same Fierce Biotech report.
The mutagenicity language is the thread that ties the two regulatory setbacks together. In 2025 the U.S. Food and Drug Administration refused to let Traws pursue TXM for bird flu, citing the drug's animal-data profile. Traws responded by pivoting to a human-challenge design in healthy volunteers, a study format that lets a small, controlled trial substitute for the larger field trials regulators usually require. Challenge trials in healthy volunteers come with a higher regulatory bar, especially for a drug that regulators have already asked questions about.
The U.K. agency's specific objections are not stated in Traws's announcement or in the available reporting. The strongest public clue is the same one the FDA flagged: the drug's potential to damage DNA. Traws's statement says only that the company is taking "next steps" and that the negative review will not delay its other programs.
What a human challenge trial actually looks like helps explain why regulators treat them with extra caution. A sponsor recruits healthy adults, screens them carefully, and then deliberately exposes them to the pathogen under quarantine, in this case seasonal flu, so that researchers can measure whether a drug blunts infection or symptoms in a tight, controlled setting. Because volunteers are being made sick on purpose, regulators want strong evidence that the drug itself does not introduce a separate risk. Concerns about a drug's potential to damage DNA sit squarely in that category.
Traws is now redesigning around that bar. The company is searching its portfolio for compounds with the same long-acting profile as TXM, which can be given as a single dose, but without the mutagenicity question. The framing matters: Traws is pitching this as a search for compounds that will "exclude potential regulatory concerns," not a retreat.
The pressure is real. Robert Redfield, who ended a stint as director of the U.S. Centers for Disease Control and Prevention in 2023 after beginning in 2018, serves as Traws's chief medical officer. Iain Dukes is the company's CEO. The $20 million tied to the deferred trial is part of the company's runway math — Traws's cash extends into the first quarter of next year — and the silence on what the U.K. agency flagged leaves investors and clinicians reading the same tea leaves: a regulator, asked to clear a drug for prophylactic use in healthy people, said no, and the company is not yet saying why.
The open question is what, exactly, the U.K. agency flagged. Traws's announcement does not say, and the most useful next document would be a fuller account of the review itself, paired with a primary Traws statement and an hVIVO confirmation of the study's status.