Comparison of a new enzymatic assay with a high-performance liquid chromatography/ ultraviolet detection method for therapeutic drug monitoring of mycophenolic acid in adult liver transplant recipients

Mycophenolic acid (MPA) is used to prevent graft rejection. The methods used for determining the plasma MPA concentration in liver transplant recipients are the enzyme-multiplied immunoassay technique (EMIT), high-performance liquid chromatography with ultraviolet detection (HPLC-UV), and most recently mass spectrometry. EMIT has been reported to overestimate the MPA concentration by 30% to 35% in comparison with HPLC-UV. Recently, a new automated enzymatic assay based on inosine monophosphate dehydrogenase inhibition has been designed. The aim of the present investigation was to compare this technique with validated HPLC-UV in adult liver transplant recipients treated with tacrolimus or cyclosporine. One hundred seventy-six samples from 50 adult liver transplant recipients were analyzed with both techniques. Patients received mycophenolate mofetil (2 or 3 times daily) coadministered with cyclosporine microemulsion (n = 18) or tacrolimus (n = 32). Samples were drawn over an interdose interval during the early or late posttransplantation period. The Passing-Bablok regression and Bland-Altman plot were used to compare the 2 techniques. The Passing-Bablock regression, calculated from 166 samples, showed very good agreement between the enzymatic assay and the HPLC-UV method: enzymatic assay = 1.0204 (95% confidence interval, 0.9942, 1.0478) x HPLC-UV + 0.0201 (-0.0442, 0.0882). No significant bias was found between the techniques (Bland-Altman plot), and the median relative difference was 2.7% (95% confidence interval, -0.4, 6.6). In conclusion, the enzymatic assay showed an excellent correlation with HPLC-UV. Therefore, this method was proved valid and reliable for the monitoring of the plasma MPA concentration in adult liver transplant recipients treated with cyclosporine microemulsion or tacrolimus.