Two local researchers show why Eli Lilly and Co. and its peers are interested in developing medicines to treat autoimmune diseases: The costs of treating them are growing twice as fast as the prescription drug market.

Dr. Vivianne Beyer of St. Vincent Health and Dr. Stephen E. Wolverton of the Indiana University School of Medicine examined the trends for treating psoriasis in the January issue of the Archives of Dermatology, according to a story by BusinessWeek.

Psoriasis, a chronic autoimmune skin disease that affects 4.5 million to 7.5 million people, costs Americans more than $3 billion each year in medicines, doctors visits, lab tests and monitoring.

The average annual costs for brand-name psoriasis treatments rose by 66 percent from 2000 to 2008, Beyer and Wolverton found. Costs for all prescription drugs rose 31 percent over the same time period.

One driver of the increase has been new biotech drugs, which range in cost from $18,000 to $28,000 each year.

Autoimmune disease is one of three key focuses at Lilly’s biotech development hub in San Diego. Also, in December, Lilly paid $90 million to acquire the rights to an oral treatment for autoimmune diseases developed by Delaware-based Incyte Corp.

Other autoimmune diseases include rheumatoid arthritis, lupus and inflammatory bowel syndrome.