Comparison of the kinetics of parenteral and oral gold.

The intramuscular (gold sodium thiomalate and aurothioglucose) and the orally (auranofin) administered gold compounds exhibit contrasting patterns of absorption, excretion and body tissue and fluid levels. The parenteral compounds are fully absorbed after injection but negligibly absorbed orally. Approximately 25% of the gold in auranofin is orally absorbed. Serum gold levels peak several hours after injection during conventional weekly treatment, attaining concentrations of 600-800 micrograms/dl, and then decline gradually, reaching 300-350 micrograms/dl before the next injection. Whole blood gold levels with auranofin vary from 10 to 90 micrograms/dl with doses of 1-9 mg/day. Blood gold levels plateau after 6-8 weeks with the injectable compounds and after 12 weeks with oral gold, reflecting the shorter blood half-life of gold sodium thiomalate (5.5 days) than of auranofin (17-26 days). A larger fraction of gold is within or attached to circulating blood cells, especially erythrocytes, with auranofin than with injectable gold. Fourty percent of the administered dose is excreted during injectable chrysotherapy, and 75-100% is recovered in excreta with auranofin. Parenteral gold is excreted primarily in urine (70%) while auranofin gold is recovered primarily in faeces (95%). Approximately 43% of intravenous radiolabelled gold sodium thiomalate is retained in the body at 60 days and 30% at 180 days; only 15% of radiolabelled auranofin remains at 10 days and less than 1% at 180 days. During injectable therapy, the total body burden of gold rises steadily; preliminary studies suggest minimal tissue accumulation with auranofin.