Melanoma is one of the rarer types of skin cancer but causes the majority of skin cancer related deaths.
The drug, called interferon alpha (IFNa), the only drug approved for this purpose, is used to clean up microscopic tumour cells that may remain in the body after surgery for the disease.
Researchers from the Ohio State University Comprehensive Cancer Center, who led the study, say that these findings underscore the need to develop ways to make melanoma cells more vulnerable to the drug, or to overcome the block within the cells that prevents them from responding to it.
The study showed that melanoma cells taken directly from patients, as well as those grown in the laboratory, responded poorly to IFNa, even when the drug was given at very high doses, while immune cells responded well to the same substance.
"IFNa is effective in only 10 to 20 percent of patients, but it's the best therapy available for these patients, and no therapies on the horizon have been proven any more effective," said principal investigator William E. Carson, III, professor of surgery and a melanoma specialist at Ohio State's James Cancer Hospital and Solove Research Institute.
"It is critical that we understand exactly how this drug works and learn how to improve its effectiveness," he added.
IFNa is an immune-system hormone made by the body to help other immune cells recognize and destroy developing tumours. As a drug, the substance is used to treat melanoma and other cancers.
"The new findings are significant because they confirm that the immune system, and not the tumour cell, is the primary target of IFNa," Prof. Carson said.
Formerly, it was thought that IFNa acted directly on melanoma-tumour cells to stop their growth. But earlier research by Carson's laboratory and others suggested that the drug has a greater effect on the immune system.
"The present study confirms that earlier work. The new findings are significant because they confirm that the immune system, and not the tumour cell, is the primary target of IFNa," Prof. Carson said.
"We show for the first time that even normal melanocytes are inherently less responsive to IFNa compared to immune cells," said first author Gregory B. Lesinski, a research assistant professor in the department of molecular virology, immunology and medical genetics.
Melanocytes are the normal cells that, when cancerous, cause melanoma.
"Some unknown factor in melanoma cells seems to turn down their response to IFNa. We are now trying to understand what that factor might be," Prof. Lesinski said.
The study appears in the journal Clinical Cancer Research.