Improvement and assessment of enzyme-linked immunosorbent assay to detect low FK506 concentrations in plasma or whole blood within 6 hours.

FK506, a promising new immunosuppressant, is currently under clinical investigation. Because dose-dependent toxicity is possible, blood concentrations of FK506 should be monitored. We improved the original ELISA of FK506 by shortening the incubation time. With some modification of materials, results are obtained within 6 h instead of 2 days, with similar or even better precision. Internal and external quality-control programs showed that our results correlated satisfactorily both with values determined by the original method and the theoretical expected values. Either plasma (detection limit 0.1 microgram/L) or whole-blood (detection limit 1 microgram/L) samples can be used. The sensitivity of the method makes it particularly useful for accurate pharmacokinetic studies or measurement of low blood concentrations. Twenty-four drugs and nine biological variables showed no significant interference on the assay. Study of the concentration- and temperature-dependent distribution of FK506 shows that the drug is largely bound to erythrocytes (ratio of blood to plasma concentrations is 10-40); as the erythrocytes become saturated, more of the drug is found in the plasma. Plasma concentrations may vary according to the blood temperature. We conclude that whole blood should be used for FK506 monitoring, as it is for monitoring cyclosporine.