Drug manufacturer Amgen Inc. generated strong sales last year, but its share of the lucrative market for rheumatology and dermatology medications declined, according to the company’s 2006 annual report.

The report, released Friday, showed a continued rise in sales of the drug Enbrel, manufactured by Amgen in its West Greenwich plant, one of the world’s largest biotechnology manufacturing facilities. The success of this medication has driven rapid, high-income job growth in a sector that the state’s economic-development officials are promoting.

Sales of the medication, used to treat immune and inflammatory disorders such as rheumatoid arthritis and psoriasis, grew 12 percent last year to $2.9 billion, according to the report.

The increased revenue from Enbrel resulted from growing demand for the medication and from a 4.9-percent price increase imposed last May, the report said. That helped boost the company’s total product sales to $13.9 billion, a 15-percent increase over the prior year.

But the report also acknowledged heightened “competitive activities” that have slowed the growth in sales of the drug.

In 2005, Enbrel sales increased 35 percent, and total product sales jumped 20 percent over the previous year, according to the company’s previous annual report. The company also manufactures several other medications, including drugs to treat anemia and a drug that treats a side effect of chronic kidney disease.

The new annual report did not include precise data on Enbrel’s share of the nearly $6-billion market for these medications. Amgen spokesmen in Rhode Island and at company headquarters in Thousand Oaks, Calif., were unavailable for comment yesterday.

The company’s annual meeting will be held this year in Rhode Island, at the Westin Providence hotel, May 9.

The report released Friday said Enbrel sales will continue to “drive year-over-year sales growth in 2007.”

The company has spent more than $1 billion on its facility in West Greenwich. In September 2005, the U.S. Food and Drug Administration approved Amgen’s second drug manufacturing facility in Rhode Island, giving the company the capacity to double its U.S. production of Enbrel.

Since 1999, Amgen has competed with Johnson & Johnson to sell medication to patients suffering from rheumatoid arthritis.

Although only Enbrel can be administered at home, the two companies split the market, with Enbrel generating about $868 million in sales in 2002 and Johnson & Johnson bringing in $901 million from the drug Remicade, according to Decision Resources Inc., a research firm for the pharmaceutical and biotech industries.

The following year, the pharmaceutical company Abbott began selling the drug Humira to treat rheumatoid arthritis. It can be injected without visiting a doctor, and a patient only needs two doses a month — half the frequency required with Enbrel.

An aggressive advertising campaign followed the launch of Humira, as the company promoted the convenience of the new drug.

By 2005, Abbott was earning more than $942 million from sales of the drug. That revenue could grow if Japan approves the sale of the drug. In 2005, Japanese spent $95 million on Enbrel and Remicade, according to Decision Resources.

“Humira has risen quickly and made a big impact on the market very quickly, especially for a third-to-market drug,” Melissa Stolper, a director at Decision Resources, said yesterday.

But the overall market for all three drugs is expected to continue to grow, and analysts say Enbrel will remain a highly profitable medication for Amgen.

“It’s always great to grow your market share, but the important thing for a company is to grow its sales,” said Eric Schmidt, biotechnology analyst at Cowen and Co.

Cowen projects average sales of Enbrel to grow by 10 percent annually over the next five years, Schmidt said. “This is a very important product to them, and it’s only going to increase its importance,” he said.

Although patients taking Humira require fewer injections, Stolper said, many rheumatologists and dermatologists are more comfortable prescribing Enbrel, which has been used by some chronically ill patients for the past nine years.

Despite the increased competition, Stolper said, the price of rheumatoid arthritis drugs is projected to steadily rise.

“They only tend to switch a patient if a patient feels strongly about the convenience issue or has diminishing efficacy over time,” Stolper said.

bgedan@projo.com