Analytical validation of a homogeneous immunoassay for determination of mycophenolic acid in human plasma.

Mycophenolic acid (MPA) is an immunosuppression agent for the prophylaxis of organ rejection in patients receiving allogeneic transplants. The drug is administered based in 2 formulations, mycophenolate mofetil (MMF) and enteric-coated mycophenolate sodium (EC-MPS). MPA acts by specific, reversible, uncompetitive inhibition of inosine monophosphate dehydrogenase (IMPDH) and thus blocks the proliferation of both T- and B-activated lymphocytes. Therapeutic drug monitoring (TDM) constitutes an important part of immunosuppressive treatment because of the demonstrated significant intraindividual and interindividual variability of its pharmacokinetic behavior. TDM is required to optimize immunosuppressive efficacy. We present the analytical validation of a homogeneous particle-enhanced turbidimetric inhibition immunoassay (PETINIA) technique for determination of MPA in human plasma, and compare with a homogeneous enzyme immunoassay technique (EMIT; reference method), both methods adapted on a Dimension analyzer (Siemens). We examined 50 human plasma samples from kidney transplant recipients treated with MMF or EC-MPA, which were analyzed simultaneously by both methods. The interassay precision was 5.95% at a concentration of 1.0 μg/mL, 3.47% at 7.5 μg/mL, and 3.75% at 12.0 μg/mL. The bias of PETINIA-MPA for each of the 3 quality control sample was <3.0%. Least squares linear regression yielded an r-value of 0.994 with the following linear regression equation: PETINIA = 0.939 * EMIT - 0.063. Bland-Altman comparison presented a mean negative difference of -0.312 μg/mL (standard deviation [SD], 0.441), namely, -7.6% for PETINIA-MPA. The PETINIA assay for monitoring MPA concentrations is an acceptable method for routine clinical use, with interassay imprecision (% coefficient of variation) ranging from 5.9% to 3.7% below and above the therapeutic concentration range, respectively. In conclusion, MPA-EMIT and PETINIA-MPA methods on Dimension analyzer have a good correlation (r = 0.994), but PETINIA-MPA method demonstrates a negative average difference of -7.6% in comparison with EMIT-MPA method.