A United States Food and Drug Administration (FDA) advisory committee has voted 10 to 3 that efficacy findings from Replimune Group’s IGNYTE study of RP1 plus nivolumab are evaluable and clinically meaningful, handing the experimental melanoma treatment a significant regulatory reprieve after two earlier rejection letters.
The committee considered RP1, also known as vusolimogene oderparepvec, in combination with Bristol Myers Squibb’s nivolumab for adults with unresectable advanced cutaneous melanoma whose cancer progressed during a programmed death receptor-1-blocking treatment. Replimune Group is seeking accelerated approval under the proposed brand name Tudriqev.
The favorable vote does not constitute FDA approval. Advisory committee recommendations are not binding, and the agency must decide whether the unresolved weaknesses in the single-arm IGNYTE study can be accepted in light of the durability of reported responses, the treatment’s safety profile and the limited options available after anti-PD-1 therapy fails.
That distinction is unusually important for RP1. FDA reviewers maintained before the meeting that the study could not reliably establish how much benefit came from the injected virus, whether the reported response rate was calculated consistently or whether local treatment of measured tumors distorted the efficacy assessment.
Replimune Group, meanwhile, argued that the 33.6% objective response rate, 16.4% complete response rate and activity in noninjected tumors supported a real systemic antitumor effect. The resulting 10 to 3 vote moved the programme closer to a possible approval, but it did not erase the methodological dispute that has followed the application since 2025.
What exactly did FDA advisers support in the 10 to 3 vote on Replimune’s RP1?
The committee was not asked simply whether RP1 should be approved. Its voting question focused on whether the efficacy findings from IGNYTE could be evaluated and considered clinically meaningful.
That wording reflects the central problem facing the application. The debate was not primarily about an alarming safety signal or a manufacturing failure. It concerned whether a single-arm study involving intratumoral treatment could provide sufficiently reliable evidence that RP1 contributes meaningful benefit when administered with nivolumab.
Ten members concluded that the findings were interpretable and clinically meaningful despite the study’s weaknesses. Three voted against that conclusion, demonstrating that the committee did not regard the evidence as straightforward or methodologically clean.
The majority appeared persuaded by the depth and durability of certain responses, regression in tumors that were not injected, the need for additional post-checkpoint-inhibitor treatments and the relative tolerability of the regimen. Patients and melanoma specialists also described a treatment setting in which conventional comparisons can fail to capture the importance of durable responses among heavily pretreated individuals.
The FDA can now approve the application, issue another complete response letter, narrow the proposed indication, request additional analyses or attach post-approval requirements. A favorable committee vote strengthens Replimune Group’s case, but the agency’s clinical reviewers retain substantial reasons for caution.
How did the 140-patient IGNYTE study support Replimune’s melanoma application?
The pivotal evidence comes from a 140-patient cohort within the Phase 1 and Phase 2 IGNYTE study. Participants had unresectable stage IIIB through stage IV cutaneous melanoma and confirmed progression following anti-PD-1-based treatment.
All participants received RP1 through direct injection into selected tumors alongside intravenous nivolumab. The primary efficacy endpoint was objective response rate assessed by an independent review committee using Response Evaluation Criteria in Solid Tumors version 1.1.
Replimune Group reported confirmed responses in 47 of the 140 patients, producing an objective response rate of 33.6%. Twenty-three patients achieved a complete response and 24 achieved a partial response, corresponding to complete and partial response rates of 16.4% and 17.1%, respectively.
The company reported a median response duration of 24.8 months. Updated follow-up indicated that an estimated 72.3% of responders remained in response at one year, 51.4% at two years and 44.8% at three years.
Those durability figures make the dataset more compelling than a short-lived tumor-shrinkage signal. A response lasting approximately two years can be important in advanced melanoma, particularly after a checkpoint inhibitor has already failed.
The study also produced evidence of activity beyond injected tumors. Replimune Group reported reductions in 50 of 52 measured noninjected visceral lesions among confirmed responders, with at least a 30% reduction in approximately 65% of those lesions.
That observation supports the company’s proposed systemic mechanism. It remains an exploratory lesion-level analysis from a selected group of responding patients, however, and it cannot replace a randomized comparison across the entire study population.
Why did the FDA reject RP1 twice despite the reported 33.6% response rate?
Replimune Group initially submitted its biologics license application in November 2024. The FDA issued the first complete response letter in July 2025 after determining that IGNYTE was not an adequate and well-controlled investigation capable of providing substantial evidence of effectiveness.
The agency questioned the heterogeneity of the enrolled population, including differences in prior treatments, the setting and duration of anti-PD-1 exposure, baseline disease burden and earlier use of combination checkpoint inhibitors. These differences complicated comparisons with historical studies.
The FDA also concluded that IGNYTE was not designed to isolate the contribution of RP1. Every patient received both RP1 and nivolumab, leaving no concurrent arm receiving nivolumab alone or another treatment that could show how much of the observed response was specifically attributable to the oncolytic virus.
Replimune Group resubmitted the application in October 2025 with limited early data from the randomized IGNYTE-3 trial. The FDA considered those results too immature and issued a second complete response letter in April 2026.
Only 40 patients were represented in that early, unplanned Phase 3 analysis. Three investigator-assessed responses occurred among 22 patients receiving RP1 plus nivolumab, compared with no responses among 18 patients receiving physician-selected treatment.
The agency found that the analysis lacked sufficient follow-up, independent radiology review and statistical controls. It therefore did not resolve the deficiencies associated with the original single-arm dataset.
Replimune Group resubmitted the application again in June after further discussions with the FDA. This submission added longer-term survival information but continued to rely principally on the same 140-patient IGNYTE cohort.
Why does injecting RP1 directly into tumors complicate response assessment?
RP1 is administered directly into tumors every two weeks for up to eight initial doses. The injection schedule begins at a lower viral concentration before increasing from the second treatment visit.
Direct tumor injection creates a response-assessment problem because standard solid-tumor criteria were developed largely for systemic medicines. A lesion subjected to a local intervention may shrink because of the injected treatment, physical disruption, inflammation, altered imaging characteristics or a combination of these effects.
The FDA argued that measuring injected lesions as though they were unaffected by a local procedure could inflate the apparent systemic response. It had previously asked Replimune Group to track injected and noninjected lesions separately so reviewers could determine whether tumors distant from the injection site also responded.
The company presented reductions in noninjected tumors, including visceral lesions, as evidence that RP1 activated a broader immune response. FDA reviewers responded that the classification of lesions as injected or noninjected was not consistently tied to each assessment time point.
Investigators could choose which lesions to inject and could later inject lesions that had enlarged or newly appeared. Lesions never selected for injection were consequently not a random sample. They might have been inaccessible, small or already improving, creating selection bias.
FDA reviewers also identified 14 patients classified as responders who received RP1 or nivolumab after an initial progression event. In some cases, new or enlarging lesions were subsequently injected before a later response was recorded.
Continuing immunotherapy beyond apparent progression can be appropriate because immune treatments sometimes cause pseudoprogression. However, inconsistent treatment and response rules can change who is counted as a responder and how long the response appears to last.
The agency ultimately said it could not conduct a dependable sensitivity analysis to calculate a corrected objective response rate or response duration. The committee majority nevertheless judged that the totality of the evidence retained clinical meaning.
How is RP1 designed to kill tumors and reactivate immune responses after PD-1 failure?
RP1 is derived from a genetically modified herpes simplex virus type 1. It is engineered to replicate selectively within tumors, break cancer cells apart and expose tumor-associated antigens to the immune system.
The virus contains a fusogenic protein intended to increase fusion and destruction of tumor cells. It is also engineered to express granulocyte-macrophage colony-stimulating factor, an immune-signalling protein intended to promote recruitment and activation of antigen-presenting cells.
The therapeutic concept extends beyond destroying the injected lesion. Tumor-cell rupture is intended to release antigens that help generate a wider T-cell response against cancer cells elsewhere in the body.
Nivolumab blocks the programmed death receptor-1 pathway that tumors can use to suppress T-cell activity. Combining RP1 with nivolumab is intended to create new immune recognition inside the tumor while removing one of the inhibitory signals limiting the resulting response.
This creates a biologically credible strategy for anti-PD-1-resistant melanoma. A tumor that initially lacked sufficient immune-cell infiltration might become more visible to the immune system after viral infection, inflammation and antigen release.
Biological plausibility cannot independently establish clinical effectiveness. The decisive question is whether adding RP1 produces better patient outcomes than nivolumab rechallenge or other available treatments, and that comparison was absent from the pivotal IGNYTE cohort.
Could RP1 provide a less intensive alternative to Amtagvi cell therapy?
Treatment options after anti-PD-1 therapy include checkpoint-inhibitor combinations, targeted treatment for eligible BRAF-mutated cancers, chemotherapy, clinical trials and Iovance Biotherapeutics’ Amtagvi.
Amtagvi, also known as lifileucel, received accelerated FDA approval for selected adults with previously treated unresectable or metastatic melanoma. Its authorization was supported by a 31.5% objective response rate among 73 patients treated within the recommended dose range.
The numerical response rates for Amtagvi and RP1 appear similar, but direct comparison is inappropriate. The studies enrolled different populations, applied different response procedures and did not randomize the treatments against each other.
The practical differences are substantial. Amtagvi requires surgical removal of tumor tissue, individualized expansion of tumor-infiltrating lymphocytes, lymphodepleting chemotherapy, cell infusion and interleukin-2 administration at a specialized treatment centre.
The Amtagvi label includes a boxed warning covering treatment-related mortality, prolonged severe blood-cell depletion, severe infection and cardiopulmonary and renal complications. Its potential benefit must be weighed against an intensive treatment process that some patients cannot tolerate or access.
RP1 would also require suitable injectable tumors and repeated treatment visits. Deep or visceral lesions may require image-guided procedures and specialist involvement, making administration more complicated than an ordinary infusion.
Even so, RP1 plus nivolumab could represent a less intensive and more widely deployable approach than personalized cell therapy if its efficacy is confirmed. Its commercial position would depend on which patients have accessible lesions, how often interventional radiology is required and whether durable systemic responses occur beyond the injected sites.
Does the RP1 safety profile justify regulatory flexibility around uncertain efficacy?
Safety was not the main reason for the two FDA rejection letters. In the 140-patient melanoma cohort, the most frequent treatment-emergent adverse events included fatigue, fever, chills, nausea and diarrhoea.
Most common events were classified as mild or moderate. Replimune Group reported no treatment-related grade 5 adverse event in the pivotal cohort, and the overall safety pattern was broadly consistent with nivolumab plus additional low-grade effects associated with intratumoral viral treatment.
That profile influenced the benefit-risk discussion. Regulators may be more willing to tolerate uncertainty when a treatment addresses a serious condition, produces potentially durable responses and does not add severe toxicity for a large proportion of patients.
Approval would still require controls for administering a live, genetically modified virus. Clinics would need procedures covering preparation, storage, injection-site care, accidental exposure, viral shedding and management of patients or close contacts who may be particularly vulnerable to infection.
Repeated intratumoral injections can also produce discomfort and procedural complications. Accessibility may depend on tumor location, with superficial skin or nodal lesions easier to treat than tumors requiring ultrasound or computed-tomography-guided injection.
A manageable safety profile improves RP1’s clinical proposition. It cannot, however, establish that the treatment works or resolve uncertainty over whether nivolumab contributed independently to the responses.
Why will the randomized IGNYTE-3 trial remain crucial even after a possible approval?
IGNYTE-3 is a randomized, open-label Phase 3 study designed to enroll approximately 400 patients with unresectable or metastatic melanoma following anti-PD-1 and anti-CTLA-4 treatment, or patients considered ineligible for anti-CTLA-4 therapy.
Participants receive RP1 plus nivolumab or a physician-selected treatment. Control options can reflect the patient’s clinical circumstances, creating a more relevant comparison than relying on historical response rates assembled from separate studies.
The primary endpoint is overall survival. Progression-free survival and objective response rate are important secondary measures.
A survival endpoint should clarify whether tumor responses translate into longer life. It will also provide a treatment-level comparison unaffected by some of the interpretive problems surrounding injected-lesion measurements.
If the FDA grants accelerated approval, continued authorization would likely depend on Replimune Group completing IGNYTE-3 and confirming clinical benefit. A failure to verify benefit could lead to label changes or withdrawal of the indication.
The confirmatory study must enroll rapidly enough to produce interpretable results without being undermined by crossover, changing treatment standards or uneven use of control therapies. It must also demonstrate that the combination’s effect is not limited to patients with easily injected superficial disease.
Strong overall-survival, progression-free-survival and response findings would convert the current debate into conventional comparative evidence. Weak or inconclusive results would validate the concerns that made the single-arm IGNYTE application so controversial.
What would FDA approval establish, and which questions would remain unanswered?
Accelerated approval would establish that the FDA considers RP1 plus nivolumab’s response data reasonably likely to predict clinical benefit in the proposed population. It would allow commercial use under an approved label while confirmatory evidence continues to mature.
It would not prove that the combination improves overall survival, nor would it demonstrate superiority over Amtagvi, nivolumab plus ipilimumab, nivolumab plus relatlimab or physician-selected therapy. Those questions require randomized evidence.
The final label could influence how broadly the treatment is used. Eligibility may be shaped by prior treatment, melanoma subtype, the presence of injectable lesions and precautions associated with an oncolytic virus.
The agency may also require detailed response verification, post-marketing safety monitoring and completion of IGNYTE-3 within an agreed timeline. Manufacturing consistency will remain important because an engineered viral treatment must maintain identity, potency and purity across commercial batches.
The advisory committee’s vote has transformed the regulatory outlook for RP1, but it has not made the evidentiary controversy disappear. The majority concluded that clinically meaningful activity could still be recognized inside an imperfect study, while FDA reviewers argued that the design prevents reliable attribution of benefit.
That tension now defines the decision facing the agency. Approval would give patients another option after checkpoint-inhibitor failure while shifting the burden of definitive proof to IGNYTE-3. Another rejection would preserve a stricter evidentiary standard but delay access to a treatment that produced prolonged complete responses in a subset of patients with limited alternatives.
For Replimune Group, the panel vote is a major recovery from two complete response letters. For melanoma care, the more important question is whether a tumor-injected virus can generate reproducible systemic benefit. Only the randomized trial can provide an answer sturdy enough to settle that debate.
