FDA has twice declined to approve Replimune Group’s oncolytic immunotherapy RP1, despite the program receiving breakthrough therapy designation and fast-track attention after early trial results. The drug is being developed as a tumor-injected engineered herpesvirus intended to lyse cancer cells and stimulate local immune activation. Reporting emphasizes that RP1’s development has become harder than expected for Replimune, with CEO Sushil Patel characterizing the FDA’s feedback as atypically stringent. The rejection has left patients and researchers focused on whether additional evidence will be sufficient for an eventual review. The situation underscores the high bar for approval in late-stage melanoma settings, where there are few established second-line options for patients who do not respond to standard approaches.