Drugmakers Moderna and Merck announced a major advance in testing a personalized cancer treatment for melanoma, a dangerous form of skin cancer.

The companies did not disclose full results from a Phase 3 trial but said they plan to share the data at upcoming medical meetings and with regulators. The trial involved melanoma patients who had their cancer removed.

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The treatment combines KEYTRUDA, an existing cancer immunotherapy drug, with a custom-made mRNA therapy designed specifically for each patient’s tumor.

To create the treatment, doctors analyze a patient’s tumor after surgery and identify the unique mutations, or “fingerprint,” of the cancer. The companies then develop a personalized mRNA therapy designed to help the patient’s immune system recognize and attack any remaining cancer cells.

According to Merck and Moderna, patients who received the treatment were less likely to have their cancer return.

“By intervening earlier in the course of disease, when many cancers are considered most treatable, the goal of adjuvant therapy given after surgery is to increase the possibility of cure for more patients,” said Dr. Dean Y. Li, president of Merck Research Laboratories. “These first Phase 3 findings for intismeran in combination with KEYTRUDA as adjuvant therapy reinforce the promise of a more personalized approach to cancer treatment.”

The drugmakers said the personalized mRNA approach could eventually be used in treatments for other types of cancer.

“For many years, the idea of creating an mRNA treatment designed specifically for an individual patient’s cancer was aspirational. We are now helping turn that vision into a reality,” said Stéphane Bancel, Moderna’s CEO. “Together with Merck, we have started to demonstrate the transformative potential of this technology to address critical unmet needs in the adjuvant melanoma setting.”

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If approved by the FDA, the treatment could eventually become available to melanoma patients after surgery to help prevent the cancer from returning.

A successful Phase 3 trial generally means a treatment has shown effectiveness and acceptable safety in a large patient group. Phase 4 studies take place after approval and are used to monitor rare side effects and evaluate how a drug performs in broader populations over time.