Merck and Moderna Report First Positive Phase 3 for a Personalized mRNA Cancer Vaccine in High-Risk Melanoma

Merck and Moderna on August 19 announced the first positive Phase 3 readout for a personalized mRNA cancer vaccine. The combination of Moderna's mRNA-4157 (V940) with Merck's Keytruda reduced the risk of recurrence or death by 44 percent compared with Keytruda alone in patients with resected high-risk melanoma, with a hazard ratio of 0.56 over a median 34 months of follow-up. The readout, which is the first pivotal trial to demonstrate a clinically meaningful and statistically significant benefit for a personalized neoantigen mRNA cancer vaccine, was highlighted by the Neuron AI daily digest on August 19 and confirmed by Merck and Moderna in their joint press release.

How the Vaccine Is Made for Each Patient

mRNA-4157 is a neoantigen vaccine engineered for each patient individually. After surgical resection of the tumor, the patient's tumor and normal tissue samples are sequenced to identify up to 34 neoantigens — mutations unique to that patient's tumor. A custom mRNA construct encoding those neoantigens is manufactured in roughly six to eight weeks and shipped back to the treating site, where it is administered alongside Keytruda. The treatment is designed to teach the patient's immune system to recognize and attack any residual cancer cells expressing those mutations.

What the Data Show

The Phase 3 trial, named INTerpath-001, enrolled 1,089 patients with resected stage IIB-IV melanoma. Patients were randomized to receive mRNA-4157 plus Keytruda or Keytruda alone after surgery. At a median 34-month follow-up, the combination arm showed a 44 percent reduction in the risk of recurrence or death compared with Keytruda monotherapy. The benefit was consistent across major prespecified subgroups, including PD-L1 status and stage at resection. Grade 3 or higher adverse events occurred in 25 percent of patients in the combination arm versus 18 percent in the Keytruda-only arm, driven primarily by immune-related events that resolved with standard management.

Why This Matters Beyond Melanoma

The Phase 3 readout validates a class of medicines that has been in clinical development for nearly a decade and that previously struggled to demonstrate consistent benefit in randomized trials. Personalized neoantigen mRNA vaccines are now being tested in non-small cell lung cancer, renal cell carcinoma, pancreatic cancer, and bladder cancer. Merck and Moderna have signaled that INTerpath-001 data will be filed to the FDA and EMA in the fourth quarter of 2026, with potential approval and launch in the first half of 2027 if review proceeds on a standard timeline. Pricing has not been disclosed but is expected to follow the value-based frameworks being negotiated with payers for cell and gene therapies.

The Manufacturing Question

The bottleneck is now manufacturing scale, not clinical efficacy. Each patient's vaccine requires a dedicated mRNA construct made from their tumor's sequencing data, with a six-to-eight-week turnaround. Moderna has been investing in a dedicated manufacturing facility in Norwood, Massachusetts, and signed a $250 million manufacturing partnership with Lonza in 2025. The companies have indicated that the first commercial doses will be limited to the patients who can be sequenced and treated within the production window, with capacity expanding through 2027 and 2028 as the Norwood facility reaches full output.