Federal regulators have approved an alternative version of Remicade, an expensive injected drug widely used for rheumatoid arthritis and other immune system disorders.

The Food and Drug Administration said Friday that it approved Renflexis, developed by Samsung Bioepis Co. of South Korea. It will be marketed by its U.S. partner, Merck & Co. of Kenilworth, New Jersey.

The companies refused to disclose the list price. Without insurance, Remicade generally costs more than $30,000 annually for adults; dosage varies with the patient's weight.

Merck said Renflexis should be available in six months. The delay is required under U.S. regulations for a relatively new category called biosimilars. That's the industry term for generic but not quite identical versions of biotech drugs.