After years of research and extensive human trials, only one virus specifically engineered to target cancer has gained approval from US and European regulators. Following promising results in treating melanoma—a notably aggressive skin cancer—approval may soon be granted.

The genetically altered herpes virus, known as RP1, was injected into the tumors of 140 patients with advanced melanoma who did not respond to conventional treatments. All participants also received a medication called nivolumab, designed to enhance the immune response against the tumors.

In 30% of the treated individuals, tumors shrank, including those that were not directly injected. Notably, in half of these cases, the tumors were completely eradicated.

“Half of the patients who responded experienced a complete response, meaning total disappearance of all tumors,” said Gino Kim from the University of Southern California. “I am thrilled with these results,” he added, noting that other treatments for patients at this stage often perform poorly and have harsher side effects.

A larger trial involving 400 participants is currently in progress; however, RP1 may receive approval from the US Food and Drug Administration (FDA) to be used in conjunction with Nivolumab for treating advanced melanoma before the trial concludes. The New Scientist reports that “the FDA is anticipated to make a decision by the end of this month.”

For over a century, it has been recognized that viral infections can aid in cancer treatment, though intentionally infecting someone with a “wild” virus poses significant risks. In the 1990s, scientists attempted to genetically modify viruses to effectively target cancer while leaving healthy cells unharmed.

These engineered viruses function in two main ways: First, they directly invade cancer cells, causing them to rupture and die. Secondly, they stimulate immune responses aimed at all cancer cells present in the body.

For instance, T-VEC, a modified herpes simplex virus, was engineered to release an immune-boosting factor called GM-CSF within infected tumor cells. T-VEC received approval in 2015 in both the US and Europe for treating inoperable melanoma.

Unfortunately, T-VEC’s use is limited as it was only tested and approved for injection into skin tumors. Many patients with advanced melanoma have deeper tumor locations, as noted.

With RP1, the strategy shifted to administering it into deeper tumors. RP1, like T-VEC, is a herpes simplex virus but has undergone various enhancements. It notably aids in fusing tumor cells with adjacent ones, thus boosting viral spread within the tumor and reinforcing the immune response.

Though there have been no direct comparisons between T-VEC and RP1, RP1 demonstrates a greater likelihood of reducing all tumors, rather than just those directly injected. “It indicates a more pronounced systemic effect,” experts state.

Thus, should RP1 gain approval, its application is expected to be far broader than that of T-VEC. Experts believe this could significantly enhance the overall interest in utilizing cancer-targeting viruses. “There seems to be increasing enthusiasm for this approach.”