Merrimack Pharmaceuticals, a biotech aiming to develop autoimmune disease and cancer drugs, closed its fourth round of financing Wednesday. The company secured $37.3 million, bringing its total raised to $87 million.

Return investors Sorenson Development, Unilever Technology Ventures Fund, and Wharton Biotechnology Partners led the round. The money will take the company through Phase II Food and Drug Administration trials on its rheumatoid arthritis drug, says COO Vincent Simmon.

Enbrel, a drug made by Amgen and Wyeth, hogs 75 percent of the arthritis market, with 2004 sales of $1.9 billion. Humira, made by Abbott, takes second place. Both drugs block the production of a protein called tumor necrosis factor, or TNF.

Despite Enbrel’s iron grip on the market, there is still an unmet medical need for better drugs and a chance for startups to make money.

Anti-TNF drugs only work in about 50 to 60 percent of patients. For an unlucky few, the drugs can cause lymphoma and reactivate tuberculosis. The need for something better has attracted startups. The PhRMA, a pharmaceutical industry organization, lists 43 arthritis products in some stage of clinical trials.

Four-year-old Merrimack has tackled the problem in a unique way. Its innovation consists of a way of genetically engineering the production of a protein called alpha-fetoprotein. Human fetuses make this protein in huge amounts. About 75 to 90 percent of moms-to-be experience arthritis remission during the last trimester, when the compound gets into the mother’s circulation in significant quantities. “It’s probably pretty safe,” says Mr. Simmon.

The link to pregnancy and mothers means the drug might work. Big players such as Bristol-Myers Squibb already have new rheumatoid arthritis drugs further along in the FDA approval process, however.