WASHINGTON (AP) – A Merck & Co. Inc. drug received federal approval Friday to treat a slow-growing cancer that affects infection-fighting white blood cells.

The Food and Drug Administration approved the drug, Zolinza, to treat advanced cases of cutaneous T-cell lymphoma, 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.

While the market as a treatment for the cancer, also called CTCL, is limited, Merck is studying the drug as a tumor-fighting treatment in many other cancers, including of the breast and lung. Additional approvals from the FDA could vastly expand the market for the drug, also known as vorinostat.

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

In two clinical trials, the cancer-related skin lesions in 30 percent of the 107 patients tested showed improvement, the FDA said. The patients were given Zolinza after the disease had recurred following other treatments.

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 make for an entirely new way of treating cancers.”

The cancer.gov Web site lists 36 clinical trials involving Zolinza, alone or in combination with other drugs.

While some early trial results seem promising, Zolinza’s effect on the larger cancer question remains to be seen, said Dr. Len Lichtenfeld, the American Cancer Society’s deputy chief medical officer. The pathway targeted by the drug is a relative newcomer in the cancer field, he added.

“There is a lot of excitement about targeted therapies. This is not the same type of mechanism as those other agents; nonetheless, any new approach to treating cancer – any novel approach that uses a new pathway, that can be taken by mouth, has limited side effects – merits more attention and hopefully will be successful over time,” Lichtenfeld said.

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.

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Food and Drug Administration: http://www.fda.gov/