Biotechnology firm Immunex Corp won permission on Tuesday to market its flagship drug, Enbrel, for treating early rheumatoid arthritis, which the company said boosted the fast-selling drug's potential market to about one million patients.

The move by the US Food and Drug Administration adds to Enbrel's prior approval for relieving symptoms of moderate to severe forms of the disease and juvenile rheumatoid arthritis. The drug's sales were $367 million last year.

``We estimate that under the previous label we had roughly 300,000 patients, and with this new indication we have an additional 700,000 patients who qualify for Enbrel,'' Immunex spokesman Tim Warner said.

In addition, the FDA gave Seattle-based Immunex permission to claim that Enbrel can delay the progress of rheumatoid arthritis, Warner said.

Between 60,000 and 70,000 patients have used Enbrel to date, Warner added.

Immunex officials have billed Enbrel as the driver of the company's growth in 2000. Earlier on Tuesday the company said Enbrel sales were $50 million in May, putting it on pace to bring in $600 million a year.

American Home Products Corp owns a majority stake in Immunex, and AHP unit Wyeth-Ayerst Laboratories co-markets Enbrel in North America.

One industry analyst said the FDA action was good news for Immunex even though she was unsure how quickly doctors would begin using Enbrel in early-stage patients. Enbrel is more expensive than the standard treatment, the pill methotrexate, and must be given by injection.

``It's very encouraging, but clearly we remain cautious only because it's expensive and has a less convenient mode of administration,'' said PaineWebber analyst Elise Wang, who predicted Enbrel sales of $650 million this year, $864 million next year and $975 million in 2002.

Rheumatoid arthritis causes painful, swollen joints. Enbrel, known generically as etanercept, works by inhibiting tumor necrosis factor, a protein linked to inflammation.

The FDA approval follows an advisory panel recommendation in April to include early-stage patients. But members also called for studies of the Enbrel's long-term effects. The drug has been on the market only since November 1998, and the FDA has raised questions about its possible link to serious infections.

Less serious side effects seen with Enbrel include nausea, rashes and mouth ulcers.

Enbrel competes against Remicade, a rheumatoid arthritis drug made by Johnson & Johnson unit Centocor. The FDA also is reviewing whether to expand Remicade's approved uses.