An already-approved immunotherapy drug can dramatically shrink — or even eliminate — tumors associated with a rare and aggressive form of melanoma, a new clinical trial has found.

About 71% of desmoplastic melanoma patients treated with pembrolizumab (Keytruda) had no detectable cancer remaining when it came time to surgically remove their tumor, researchers reported Jan. 29 in the journal Nature Cancer.

As a result, patients were spared from either disfiguring surgeries or more treatment, researchers said.

“We’re seeing that desmoplastic melanoma, which can be challenging to remove surgically, responds extremely well to immunotherapy,” said senior researcher Dr. Antoni Ribas, director of the UCLA Health Jonsson Comprehensive Cancer Center’s Tumor Immunology Program.

“We found that giving pembrolizumab before surgery is a powerful and safe approach that reduces the need for invasive procedures and improves long-term outcomes,” he said in a news release.

Desmoplastic melanoma accounts for only about 4% of all skin melanomas, according to Memorial Sloan Kettering Cancer Center.

But because the melanoma grows deep into tissues, sometimes along nerves, it can make surgery challenging and potentially disfiguring, researchers said in background notes.

Typical treatment involves a wide surgical excision followed by radiation therapy. Advanced cases tend to be largely resistant to chemotherapy or immunotherapy, researchers said.

However, earlier findings had shown that pembrolizumab shrank tumors in nearly 90% of patients with advanced, inoperable desmoplastic melanoma, researchers noted.

This prompted the new study, which looked at whether pembrolizumab could help patients with surgically removable melanomas.

Pembrolizumab works by blocking a mechanism that cancer cells use to evade detection by the immune system, according to Drugs.com. The U.S. Food and Drug Administration (FDA) first approved it in 2014 as a treatment for inoperable melanoma, and it has since been indicated to treat a wide variety of cancers.

For this study, 28 patients with surgically removable desmoplastic melanoma received three infusions of pembrolizumab over nine weeks prior to scheduled surgery.

By the time of surgery, 71% of patients had no detectable cancer, researchers found.

At a three-year follow-up, 95% of patients had not died from melanoma and 74% remained cancer-free, the study said.

“Giving pembrolizumab before surgery led to very high rates of tumor clearance, few serious side effects, and excellent three-year survival for patients with this rare and difficult-to-treat cancer,” Ribas said.

“Taken together with our earlier findings in patients with advanced disease, these results represent a true shift in how desmoplastic melanoma is treated, moving away from repeated surgeries and radiation toward a single therapy that offers durable control, improved survival and better quality of life,” he said.