Increased sensitivity in a radioimmunoassay for measuring cyclosporine in lipoprotein fractions.

We describe a modification of the commercially available cyclosporine (CAS) 3H-RIA kit, allowing detection of drug in lipoprotein fractions obtained by ultracentrifugation. Sodium bromide solutions containing the lipoprotein fractions were dried and reconstituted with drug-free plasma. Methanolic extractions were centrifuged, and 250 microL of the supernate was dried and reconstituted with 50 microL of methanol. Buffer, radioactive tracer, and monoclonal antibody were then added and incubated for 2 h at 4 degrees C. Samples were subjected to charcoal de-activation and then centrifuged, and the radioactivity in the supernate was counted. The limit of detection of CSA was 2.5 micrograms/L; analytical recovery was between 97.5% and 101.3%. Within- and between-day CVs were less than 9%. We conclude that the present method may be beneficial for therapeutic drug monitoring of CSA in lipoprotein fractions.