The findings could represent a new standard of treatment for this extremely rare tumor.
The study was inspired by a previous retrospective analysis which found an overall response rate of 77% and a complete response of 32% to anti–PD-1 monotherapy.
The ORR is about double what is seen in melanoma more generally, according to Kari Kendra, MD, PhD, who presented the study at the annual meeting of the American Association for Cancer Research.
“Our study was a positive study. Of note, in the retrospective study, they saw a complete response rate of 32%, which was amazingly similar to what we found. [The findings support] the use of single agent anti–PD-1 immunotherapy as first line treatment for most patients with unresectable desmoplastic melanoma. [There was 89% overall response and we saw] dramatic responses across the board,” said Dr. Kendra, who is a medical oncologist at Ohio State University Wexner Medical Center, Columbus.
The findings drew a strong reaction. “In a rare tumor session, to see response curves like that, it’s just outstanding,” said the session’s cochair Brian Van Tine, MD, PhD, who is a professor of medical oncology at Washington University in St. Louis.
“This really is one of the highest tumor response rates to immunotherapy that we are seeing in any cancer. And I think may also highlight the fact that we shouldn’t think of all cutaneous melanomas as one disease, given the heterogeneity in tumor responses based on some of the pathologic and molecular characteristics,” said Zeynep Aroglu, MD, who served as a discussant but was also one of the investigators who enrolled patients for the trial.
Desmoplastic melanoma represents about 4% of all cutaneous melanoma diagnoses, and its unique pathology can make it difficult to diagnose. That often leads to a late diagnosis, according to Dr. Aroglu. They typically occur in elderly patients, in the head and neck area, and are associated with sun exposure. DM also tends to have a high mutation burden, Dr. Aroglu said during the session.
It remains to be seen why there is such a high response rate in this tumor type, even among tumor types with mutation burdens that are nearly as high. DM tumors are often driven by neurofibromatosis type 1, but other tumors driven by NF-1 don’t have as high of a response rate to immunotherapy. The tumor environment could also play a role, she said.
“Is it a combination of all these factors? I think some of the ongoing analysis of tumor samples that Dr. Kendra mentioned may help to answer some of these questions,” Dr. Aroglu continued.
She also noted that the melanoma field is increasingly turning to combination of anti–PD-1 therapy with agents like that target LAG3 or CTLA4. Such combinations can achieve higher response rates, but at a cost of higher rates of grade 3-4 adverse events than anti–PD-1 inhibitors alone. “I wonder if for desmoplastic melanomas in light of this data, do we consider de-escalating therapy, given these very high response rates to PD-1 alone, given also the elderly age of many of these patients, because even the PD-1–LAG3 combo still has a higher rate of toxicity than PD-1 monotherapy. Perhaps the immunotherapy combinations can be reserved for those rare desmoplastic patients who are resistant to PD-1 alone,” said Dr. Aroglu.