EVALUATION OF AN IMMUNOASSAY (EMIT) FOR MYCOPHENOLIC-ACID IN PLASMA FROM RENAL-TRANSPLANT RECIPIENTS COMPARED WITH A HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY ASSAY

Mycophenolic acid is an immunosuppressant administered as a bioavailab le ester, mycophenolate mofetil. The pharmacokinetics of mycophenolic acid have been reported to be variable. Accurate measurement of concen trations of this drug could be important to adjust doses. The aim of t his study was to compare the enzyme-multiplied immunoassay technique ( EMIT [Dade Behring; San Jose, CA, U.S.A.]) for mycophenolic acid with a high-performance liquid chromatographic (HPLC) assay using samples c ollected from renal transplant recipients. The HPLC assay used solid p hase extraction and a C18 stationary phase with ultraviolet (UV) detec tion (254 nm). The immunoassay required no manual sample preparation. Plasma samples (n = 102) from seven patients, collected at various tim es after a dose, were analyzed using both methods. Both assays fulfill ed quality-control criteria. Higher concentrations were consistently m easured in patient samples when using EMIT. The mean (+/- standard dev iation [SD]) bias (EMIT-HPLC) was 1.88 +/- 0.86 mg/L. The differences in concentrations were higher in the middle of a dosage interval, sugg esting that a metabolite might have been responsible for overestimatio n. Measurement of glucuronide concentrations by HPLC demonstrated only a weak correlation between assay differences and glucuronide concentr ations. If the crossreacting substance is active, EMIT could provide a superior measure of immunosuppression; if inactive, further work is n eeded to improve antibody specificity. In conclusion, it was found tha t EMIT overestimates the concentration of mycophenolic acid in plasma samples from renal transplant recipients compared with HPLC analysis.