The FDA issued a complete response letter rejecting Replimune's melanoma drug candidate RP1, an oncolytic virus therapy combined with nivolumab, citing inadequate and uncontrolled clinical trial data, patient heterogeneity, and problems with confirmatory trial design. The company's CEO expressed surprise at the FDA's decision, given prior alignment on study design. The rejection caused a steep decline in Replimune’s stock price. This action reflects the FDA’s increasingly stringent approach to gene and cell therapy approvals.