Etretinate. Persistent serum levels after long-term therapy

In 47 patients who received long-term etretinate therapy, we measured serum etretinate concentrations from one to 244 weeks after the discontinuation of therapy. The earliest posttreatment, nondetectable serum concentration of etretinate was observed at five weeks after treatment. Detectable serum concentrations (0.05 to 1.2 micrograms/dL) were observed more than two years (108, 111, 131, 136, and 150 weeks) following the discontinuation of therapy. Sequential serum concentrations obtained on eight individual patients were used to determine half-lives for this late-phase elimination. The median half-life for the 12 curves obtained was 12.5 weeks (range, 5.3 to 24.8 weeks). Since etretinate is stored in fat, we compared each patient's deviation from ideal body weight as a measure of excess body fat with various pharmacokinetic factors of etretinate elimination. Overweight patients tended to have slower elimination, maintain higher serum concentrations, and clear etretinate later.