Scientists just weaponized something that affects two-thirds of the world’s population. Something that causes painful cold sores and fever blisters. Something most people would never associate with healing.
Researchers took this common virus, modified its genetic code, and transformed it into a precision cancer-killing machine. Early results show that tumors are shrinking dramatically in patients who have exhausted every other treatment option.
One-third of people in a recent clinical trial experienced something their doctors thought impossible: their advanced melanoma responding to treatment after everything else had failed. Some saw their cancer disappear completely.
What makes this approach revolutionary isn’t just that it works where other treatments don’t. Genetically engineered viruses target cancer cells while leaving healthy tissue untouched, then trigger the immune system to hunt down and eliminate remaining cancer cells that may be hiding throughout the body.
The Virus That Gives Cold Sores Just Became a Cancer Killer
Herpes simplex virus type 1 typically causes oral herpes infections that millions of people recognize as cold sores. Scientists saw potential in how this virus naturally invades cells and replicates inside them.
Genetic engineering transformed HSV-1 into RP1, an oncolytic virus designed specifically to target cancer cells. Modifications prevent RP1 from causing herpes infections while enhancing its ability to destroy tumors.
RP1 represents a new class of cancer immunotherapy drugs called oncolytic viruses. These engineered pathogens infect cancer cells, replicate inside them, and destroy them from within while stimulating broader immune responses.
Dr. Gino Kim In, medical oncologist at Keck Medicine of USC and principal investigator at the clinical trial site, explained the urgent need: “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. 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.”
Melanoma spreads aggressively once it reaches advanced stages, moving from skin to lymph nodes, organs, and bones. Standard immunotherapy fails in roughly half of all advanced cases.
When Standard Cancer Treatments Stop Working
Advanced melanoma patients typically receive immunotherapy drugs that activate the immune system to recognize and attack cancer cells. These treatments work initially for many patients but eventually stop being effective.
Primary resistance occurs when cancer never responds to immunotherapy despite adequate treatment time. Secondary resistance develops when tumors initially shrink but later begin growing again.
Treatment-resistant melanoma leaves patients with few options and grim prognoses. Median survival drops to approximately one year after immunotherapy failure, forcing doctors and families to discuss end-of-life planning rather than cure possibilities.
Patients in the IGNYTE clinical trial had already tried multiple treatments without success. Some had received anti-PD-1 therapy combined with anti-CTLA-4, representing the most aggressive immunotherapy combinations available.
Standard care offered these patients little beyond palliative measures. They needed something different, something that could bypass the resistance mechanisms their cancers had developed.
The IGNYTE Trial: 140 Patients, One Bold Experiment
Keck Medicine at the University of Southern California joined other sites worldwide in testing RP1 combined with nivolumab immunotherapy. Researchers enrolled 140 patients whose advanced melanoma had confirmed progression despite anti-PD-1 treatment lasting at least eight weeks.
Patients needed multiple tumors for trial participation because researchers injected some while monitoring others. Both superficial tumors visible on or just beneath the skin and deep tumors in organs like the liver and lungs qualified for treatment.
Every two weeks for up to eight cycles, patients received RP1 injections directly into tumors plus nivolumab infusions. Responders continued nivolumab alone every four weeks for up to two years.
Researchers measured both treated and untreated tumors throughout the trial. Would injecting some tumors affect cancer elsewhere in the body? Could a localized treatment produce system-wide benefits?
Results published in the Journal of Clinical Oncology and presented at the 2025 American Society of Clinical Oncology annual meeting exceeded expectations for this desperate patient population.
One-Third of “Hopeless” Cases Saw Tumors Shrink
Among all 140 patients, 32.9% experienced confirmed objective responses with tumors shrinking by at least 30%. Nearly one in six patients achieved a complete response with tumors disappearing entirely.
Median duration of response reached 33.7 months, meaning half of responding patients maintained their improvements for nearly three years. More than two-thirds of responses remained ongoing at one year after initial tumor shrinkage.
Survival rates dramatically exceeded expectations for treatment-resistant melanoma. At one year, 75.3% of patients remained alive. At two years, 63.3% continued surviving despite having cancers that had defeated all prior treatments.
Responses occurred across different patient subgroups, including those with primary resistance, secondary resistance, and tumors lacking PD-L1 expression. Even patients who had received prior anti-PD-1 plus anti-CTLA-4 combination therapy experienced meaningful response rates.
Most remarkably, uninjected tumors shrank or disappeared just as frequently as injected ones, demonstrating system-wide anticancer effects.
The Virus That Hunts Cancer Throughout Your Body
RP1 replicates inside cancer cells, bursting them open while releasing cancer antigens that alert the immune system. Dying cancer cells become beacons that train white blood cells to recognize and destroy similar cells elsewhere.
Dr. In emphasized this systemic effect: “This result suggests that RP1 is effective in targeting cancer throughout the entire body and not just the injected tumor, which expands the potential effectiveness of the drug because some tumors may be more difficult or impossible to reach.”
Among responding patients, 93.6% of injected lesions and 79% of non-injected lesions showed reductions of 30% or greater. Deep tumors in organs responded similarly to accessible skin lesions.
Visceral organ tumors in the lungs and liver responded particularly well. Among 52 non-injected visceral lesions in responding patients, 96.2% showed some reduction, and 65.4% shrank by at least 30%.
Pseudoprogression occurred frequently, where tumors initially appeared larger before shrinking. This temporary growth reflects immune cell infiltration attacking cancer rather than true progression.
How Scientists Turn a Virus Into a Cancer Weapon
Researchers selected a new clinical strain of HSV-1 specifically for enhanced tumor-killing ability across multiple human cancer cell lines. Genetic modifications added two important proteins to amplify anticancer effects.
GM-CSF protein stimulates the production and activation of immune cells that attack cancer. GALV-GP-R- protein causes infected cancer cells to fuse, dramatically increasing immunogenic cell death that alerts the immune system.
These modifications make RP1 distinctly different from T-VEC, the first FDA-approved oncolytic virus introduced in 2015. RP1 demonstrates superior performance in patients whose cancers progressed despite prior anti-PD-1 therapy.
Engineering focused on selectivity, ensuring RP1 preferentially infects and replicates in cancer cells while sparing healthy tissue. Normal cells resist infection through intact antiviral defenses that cancer cells often lose during malignant transformation.
Combination with nivolumab enhances results beyond either treatment alone. Nivolumab blocks PD-1 receptors that cancer uses to hide from immune surveillance, complementing RP1’s immune activation.
Side Effects: Surprisingly Mild
Safety profile analysis revealed 90% of patients experienced only grade 1 or 2 adverse events classified as mild. Common side effects included fatigue, fever, chills, and flu-like symptoms consistent with immune system activation.
Only 12.9% of patients had grade 3 or 4 adverse events considered moderate to severe. Five patients experienced grade 4 events, but no treatment-related deaths occurred throughout the trial.
Favorable safety compares positively with alternatives for treatment-resistant melanoma. Lifileucel, the only FDA-approved therapy after anti-PD-1 failure, causes grade 3/4 adverse events in nearly all patient,s with treatment-related mortality reaching 7.5%.
Anti-PD-1 plus anti-CTLA-4 combination therapy produces grade 3 or higher adverse events in 57% of patients. RP1 plus nivolumab offers superior tolerability while delivering comparable or better response rates.
Most side effects resolved quickly without intervention. Injection site reactions remained localized and temporary. Systemic symptoms like fatigue and fever typically lasted only days after each treatment.
FDA Fast-Tracks This Treatment
Priority review status granted by the FDA in January 2025 accelerates the approval process for RP1 combined with nivolumab. This designation recognizes urgent unmet medical needs in advanced melanoma patients whose cancers failed immunotherapy.
Priority review shortens FDA evaluation timelines from standard ten months to six months. Expedited assessment helps potentially lifesaving treatments reach desperate patients faster.
Regulatory approval requires confirming phase 2 findings through larger randomized trials. IGNYTE-3 phase 3 trial now enrolls more than 400 participants globally to validate earlier results.
Comparison against the physician’s choice treatment allows direct assessment of RP1 plus nivolumab benefits versus current standard options. Randomized design addresses limitations of single-arm phase 2 study.
Keck Medicine continues participating as a trial site with Dr. In leading patient enrollment. Interested patients can contact the clinical research program to discuss qualification criteria and participation.
RP1 Opens Doors for New Cancer Battles
Dr. In expressed optimism about broader implications: “I believe that oncolytic viruses will open up an important new approach to fighting cancer in some patients in the near future.”
Oncolytic virus mechanisms aren’t specific to melanoma. RP1 could potentially treat breast, lung, liver, and other cancers. Expanding research explores applications across tumor types.
Biomarker analysis revealed broad immune activation in responding patients, including increased CD8+ T-cell infiltration and elevated PD-L1 expression. Gene expression analysis showed 313 genes upregulated in responders versus only five in non-responders.
Treatment-induced immune signatures help predict which patients benefit most from RP1 therapy. Future refinements may enable patient selection based on tumor characteristics and immune profiles.
Combining oncolytic viruses with immunotherapy represents a paradigm shift from directly attacking cancer toward activating natural immune defenses. Success validates the approach and encourages the development of additional viral therapies targeting different cancer types through similar mechanisms.
