Herpes Virus Could Help Treat Advanced Cancer

While it may give billions of people worldwide painful cold sores, a common virus could be harnessed to provide a much-need new cancer treatment.

This is the conclusion of a study by researchers with Keck Medicine at the University of Southern California, who have shown in early clinical trials that a genetically modified version of herpes simplex virus type 1 (HSV-1) can be used against advanced melanoma.

Melanoma is a form of skin cancer that originates in the cells that give our skin its pigment. In its advanced form, however, it spreads to other sites in the body.

"These findings are very encouraging because melanoma is the fifth most common cancer for adults—and about half of all advanced melanoma cases cannot be managed with currently available immunotherapy treatments," said paper author and oncologist Dr. Gino Kim In of Keck Medicine in a statement.

"The survival rate of untreatable advanced melanoma is only a few years, so this new therapy offers hope to patients who may have run out of options to fight the cancer."

In their study, In and colleagues recruited 140 patients with advanced melanoma that either did not respond—or was no longer responding—to immunotherapy.

Each participant had multiple tumors, some of which were "superficial," in that they were located on or just beneath the skin, while others were located deeper in the body, such as in organs like the liver or lungs.

"Unlike other immunotherapy drugs, RP1 is an 'intratumoral' therapy that is injected directly into melanoma tumors," In told Newsweek.

Both deep and superficial tumors were treated with a modified version of HSV-1 in combination with the immunotherapy agent, nivolumab (which helps the immune system's T-cells fight tumors), every two weeks for up to 16 weeks in total.

If the patients appeared to be responding to the treatment, they then continued receiving nivolumab every four weeks for up to two years.

The modified HSV-1 virus used in the study is named "RP1." It is designed to specifically target and destroy cancerous tumors while also stimulating the body's white blood cells to seek and destroy other cancer cells across the entire body. Unlike its natural predecessor, RP1 does not cause herpes.

The U.S. Food and Drug Administration approved RP1 for review in patients with advanced melanoma that was not responding to immunotherapy in January this year.

By the trial's end, the team found that tumors had shrunk by at least 30 percent in one-third of the patients—while nearly one-in-six saw their tumors disappear completely.

"Importantly, RP1 was very well tolerated with approximately 90 percent of patients having only mild side effects, such as fatigue, fever and flu-like symptoms," noted In.

Comparing the outcomes of both tumors that were directly treated and those that were not, the team found that uninjected tumors shrank and/or disappeared just as frequently.

"This result suggests that RP1 is effective in targeting cancer throughout the entire body and not just the injected tumor," said In.

This, she continued, "expands the potential effectiveness of the drug, because some tumors may be more difficult or impossible to reach."

In added that—while it is too early to tell if the positive outcomes stay permanent—she is optimistic about RP1 therapy's potential.

She explained: "I believe that oncolytic viruses will open up an important new approach to fighting cancer in some patients in the near future."

With the first two phases of the clinical trial now complete, the researchers are now moving to confirm their findings in a larger, global population of more than 400 trial patients.

Keck Medicine will once again serve as one of the sites for this next trial.

Patients interested in participating are encouraged to contact Keck Medicine Oncology Clinical Research Program Manager, Sandy Tran, by email.

The clinical trials are being sponsored by the biotechnology company Replimune, which manufactures RP1 as well as other viral-based cancer therapies.

As the mechanism of RP1's action is not specific to melanoma, the researchers say that the treatment may also have potential in treating other cancer types.

Do you have a tip on a health story that Newsweek should be covering? Do you have a question about melanoma treatment? Let us know via [email protected].

Reference

Gino In, Michael Wong, Joseph Sacco, Eva Muñoz Couselo, Dirk Schadendorf, Georgia Beasley, Jiaxin Niu, Bartosz Chmielowski, Trisha Wise-Draper, Mohammed Milhem, Tawnya Bowles, Katy Tsai, Celeste Lebbe, Caroline Gaudy-Marqueste, Adel Samson, Junhong Zhu, Marcus Viana, Jeannie Hou, & Caroline Robert. (2025). Response analysis for injected and non-injected lesions and of the safety and efficacy of superficial and deep/visceral RP1 injection in the registrational cohort of anti–PD-1–failed melanoma patients of the IGNYTE trial. 2025 ASCO Annual Meeting.