A Merck & Co. Inc. drug to treat a slow-growing cancer that affects infection-fighting white blood cells received federal approval Friday in the U.S.

The U.S. Food and Drug Administration approved the drug Zolinza to treat advanced cases of cutaneous T-cell lymphoma (CTCL), a rare cancer that's a form of non-Hodgkin's lymphoma. Roughly 20,000 Americans have the cancer, Merck said.

The drug is to be used when the disease persists, gets worse or comes back during or after treatment with other medicines, the FDA said.

Zolinza is the first new cancer drug from Merck, based in Whitehouse Station, N.J., in roughly 20 years.

Neither Zolinza nor its generic name, vorinostat, appears under Health Canada's online drug and health products listings.

While the market as a treatment for CTCL is limited, Merck is studying the drug as a possible treatment for many other cancers, including breast and lung. Additional approvals from the FDA could vastly expand the market for the drug.

CTCL affects white blood cells called T-lymphocytes. The malignant cells are deposited in the skin, forming tumours and causing dry skin, rash and itchiness.

In two clinical trials, the cancer-related skin lesions in 30 per cent of 107 patients tested showed improvement, the FDA said. The patients previously had received other treatment.

Zolinza is the first in a new class of cancer drugs to win FDA approval. The drugs, called histone deacetylase inhibitors, are thought to silence some genes that, when left unchecked, allow cancerous cells to proliferate.

"We see CTCL as the tip of the iceberg," said Dr. Stanley Frankel, senior director of clinical research in oncology for Merck. "It proves that this pathway can be attacked effectively and therefore makes for an entirely new way of treating cancers."

Merck acquired the drug in its 2004 purchase of Aton Pharma Inc. of Tarrytown, N.Y.

The FDA granted Zolinza "orphan drug" status, which is meant to spur development of treatments for rare diseases. The status gives Merck seven years of legal protection against introduction of an identical competing product, along with the ability to write off some of the costs of developing the drug.

The most serious side-effects of the drug include blood clots in the lungs, dehydration and anemia, the FDA said.

Zolinza has not been tested in pregnant women; tests done in animals suggest it may be harmful to fetuses.