Full Phase 3 Trial Confirms Moderna and Merck mRNA Vaccine Halves Melanoma Recurrence Risk

Phase 3 INTerpath-001 trial results for Moderna and Merck's personalized mRNA cancer vaccine, intismeran, show it cuts the risk of melanoma recurrence or death by 42 percent when paired with the immunotherapy Keytruda. The data, which leaked online on October 22 ahead of a medical conference in Madrid, confirms earlier mid-stage findings across more than 1,130 patients.[1][2][4]

The findings represent the first successful late-stage validation of a personalized cancer vaccine, a concept that has challenged oncology researchers for decades. By adapting the mRNA technology that drove the global response to the COVID-19 pandemic, the treatment offers a blueprint for preventing aggressive skin cancers from returning after surgery.[1][5]

Patients enrolled in the trial had completely resected stage 2B to 4 melanoma, meaning their visible tumors had been surgically removed but they remained at high risk for microscopic recurrence. They were randomly assigned to receive either the combination therapy or Keytruda alone for approximately one year.[1][4]

Engineering a Custom Immune Response

The treatment relies on the genetic sequencing of a patient's surgically removed tumor to identify up to 34 specific mutated proteins, known as neoantigens. The custom-built vaccine then delivers mRNA instructions to the patient's cells, training the immune system to recognize and attack those exact targets.[1]

Creating the vaccine requires a complex logistical chain, starting with the surgical resection and genetic sequencing of the tumor tissue. The genetic data must then be translated into a custom mRNA sequence, manufactured in a specialized facility, and shipped back to the clinic within weeks to begin the immunization schedule.[1]

Intismeran is administered once every three weeks for up to nine doses, while Keytruda is given every six weeks for up to nine cycles. Keytruda, a widely used checkpoint inhibitor, complements the vaccine by removing the natural brakes that cancer cells exploit to hide from immune cells.[1]

"With this ability to create new T cells that are specific to your tumor... the clinical impact is very profound," Moderna CEO Stephane Bancel said following the data release.[5]

He noted that the Phase 3 results proved the mechanism could deliver lasting protection at a large scale, marking a new chapter in oncology.[5]

Consistent Long-Term Protection

The combination therapy also reduced the risk of distant metastasis—cancer spreading to other organs—by 38 percent compared to Keytruda alone. Preventing metastasis is a critical benchmark in melanoma care, as the disease becomes significantly more lethal once it migrates to the lungs, liver, or brain.[3][4]

After a median follow-up of roughly two years, 82.2 percent of patients receiving the dual treatment remained cancer-free. In contrast, 70 percent of those on the standard immunotherapy regimen survived without a recurrence over the same 24-month period, highlighting the added benefit of the personalized vaccine.[3]

The trial data showed that the clinical benefit became particularly pronounced after patients completed the initial six-month immunization phase. During the first six months, the recurrence rates were statistically similar between the two study arms as the immune system learned to mount its defense.[4]

A prespecified sensitivity analysis focusing exclusively on the period after the vaccine regimen finished revealed a 58 percent lower risk of recurrence or death. It also demonstrated a 50 percent reduction in the risk of distant metastasis or death once the full immunization sequence was complete.[4]

Safety Profile and Market Reaction

The Phase 3 trial reported that no new safety signals emerged from the combination therapy. The adverse events were consistent with the known side effects of Keytruda and the reactogenicity typically associated with mRNA vaccines, such as fatigue and injection site pain.[4]

The results closely mirrored the efficacy seen in the Phase 2b trial, which had previously reported a 49 percent reduction in recurrence risk.[1]

"I was surprised by the fact that the results replicated the Phase 2 study so closely," Dr. David Berman noted regarding the consistency of the data.[1]

Despite the clinical success, Moderna's stock price dropped by more than 17 percent after the results leaked, while Merck shares fell nearly 2 percent. Analysts attributed the decline to sky-high investor expectations that had already driven Moderna's valuation up significantly since August.[5]

The companies had previously announced in August that the Phase 3 trial met its primary endpoint, causing Moderna shares to nearly quadruple by late October. The subsequent market reaction occurred because the detailed 42 percent efficacy figure matched, rather than exceeded, the baseline established by earlier studies.[5]

Expanding the Oncology Pipeline

The trial results were officially scheduled for presentation on October 24 at the European Society for Medical Oncology congress in Madrid. The premature release of the data on an oncology website forced the companies to issue a formal statement confirming the statistics two days early.[4][5]

Moderna and Merck are now preparing to file for regulatory approval in the United States, with a potential commercial launch targeted for 2027. The approval would mark a regulatory milestone for individualized neoantigen therapies in the adjuvant setting.[3][5]

The intismeran and Keytruda combination is not limited to melanoma and is currently being evaluated in clinical trials for other malignancies. Ongoing studies are testing the dual regimen in patients with lung, bladder, and kidney cancers to determine if the mechanism translates across tumor types.[5]

If successful in broader applications, the mRNA platform could fundamentally alter standard oncology protocols. The ability to rapidly manufacture a bespoke vaccine for a patient's specific tumor mutations represents a shift away from broad-spectrum treatments toward highly targeted cellular defense.[1][3]