WASHINGTON (AP) - The first in a new class of therapy for rheumatoid arthritis won federal approval, although studies show Kineret promises just modest effectiveness.

The Food and Drug Administration said the new drug should be reserved for arthritis patients who have failed today's best treatments.

Still, many doctors eagerly were awaiting Amgen Inc.'s drug because there are few options for the more than 2 million sufferers of rheumatoid arthritis.

This is not the common arthritis associated with the wear-and-tear of aging. With rheumatoid arthritis, the immune system goes awry and attacks patients' joints, causing swelling and stiffness as rogue immune cells eat away cartilage and eventually erode bone. It mostly strikes women, usually between ages 25 and 50, and can disable them within 10 years.

Kineret, known chemically as anakinra, works differently from other treatments, blocking a protein called interleukin-1 that is one cause of the joint swelling.

Studies show that after six months of treatment, about 38 percent of Kineret patients suffered less swelling and pain, compared with 22 percent who got a dummy shot.

In contrast, about two-thirds of patients improve with the competing drugs Enbrel and Remicade, which block a different inflammatory immune protein, called tumor necrosis factor or TNF, said the FDA's Dr. Karen Weiss.

Prescription-only Kineret will be on pharmacy shelves within two weeks, and will cost about $11,088 a year, said Thousand Oaks, Calif.-based Amgen. That is slightly lower than its anti-TNF competitors, which cost around $12,000 a year.

Kineret's main side effect is irritation at the injection site. Because it suppresses the immune system, it also causes a small risk of serious infection - 2 percent of Kineret patients versus less than 1 percent of placebo patients.

Patients should stop taking Kineret if they develop an infection, but can resume the treatment once the infection clears, Amgen said.

The anti-TNF drugs also cause the infection risk, so patients should not use Kineret together with either Enbrel or Remicade, Weiss said.

One small study suggested that such combination therapy left 7 percent of patients with a serious infection, and occasionally caused a severe drop in infection-fighting white blood cells.

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