Pharmacology of auranofin: overview and update.

Auranofin, the only approved oral gold complex of value in suppressing rheumatoid arthritis, differs from injectable gold compounds molecularly and pharmacologically. Although comparably efficacious, the side-effect profiles of oral and intramuscular gold differ, and the withdrawal rate for adverse reactions is several-fold lower with auranofin. Considerably less elemental gold is available to the internal milieu with auranofin than with gold sodium thiomalate (3 mg/week vs. 25 mg/week), a difference reflected in lower blood, synovial fluid and tissue gold levels. Approximately 25% of the administered auranofin dose is absorbed orally, and 85% is recovered in feces. Serum gold levels are 300-400 micrograms/dl one week after injectable gold and 60-70 micrograms/dl daily with auranofin (6 mg/d). But, surprisingly, the fraction of gold associated with the red blood cell fraction is higher with auranofin. Blood gold levels do not correlate with clinical response to treatment or frequency or type of adverse reaction, regardless of the gold preparation used. Similar results obtain with penicillamine. Methotrexate blood levels are not related to the development of hepatic fibrosis. The mechanisms of gold action in rheumatoid arthritis are unknown, despite the laboratory definition of multiple antiinflammatory, immunologic and other effects. Sulfhydryl binding activity, an established property of injectable gold compounds and penicillamine of potential importance pharmacodynamically, is limited with auranofin.