A limited sampling model for estimation of total and unbound mycophenolic acid (MPA) area under the curve (AUC) in hematopoietic cell transplantation (HCT)

Objectives: Renal transplant patients with suboptimal mycophenolic acid (MPA) areas under the curves (AUCs) are at greater risk of acute rejection. In hematopoietic cell transplantation, a low MPA AUC is also associated with a higher incidence of acute graft versus host disease. Therefore, a limited sampling model was developed and validated to simultaneously estimate total and unbound MPA AUC(0-12) in hematopoietic cell transplantation patients. Methods: Intensive pharmacokinetic sampling was performed at steady state between days 3 to 7 posttransplant in 73 adult subjects while receiving prophylactic mycophenolate mofetil I g per 12 hours orally or intravenously plus cyclosporine. Total and unbound MPA plasma concentrations were measured, and total and unbound AUC(0-12) was determined using noncompartmental analysis. Regression analysis was then performed to build IV and PO, total and unbound AUC(0-12) models from the first 34 subjects. The predictive performance of these models was tested in the next 39 subjects. Results: Trough concentrations poorly estimate observed total and unbound AUC(0-12) (r(2) < 0.48). A model with 3 concentrations (2-, 4-, and 6-hour post start of infusion) best estimated observed total and unbound AUC(0-12) after IV dosing (r(2) > 0.99). Oral total and unbound AUC(0-12) was more difficult to estimate and required at least 4 concentrations (0-, 1-, 2-, and 6-hour post dose) in the model (r(2) > 0.85). The predictive performance of the final models was good. Eighty-three percent of IV and 70% of PO AUC(0-12) predictions fell within 20% of the observed values without significant bias. Conclusion: Trough MPA concentrations do not accurately describe MPA AUC(0-12). Three intravenous (2-, 4-, 6-hour post start of infusion) or 4 oral (0-, 1-, 2-, and 6-hour post dose) MPA plasma concentrations measured over a 12-hour dosing interval will estimate the total and unbound AUC(0-12) nearly as well as intensive pharmacokinetic sampling with good precision and low bias. This approach simplifies AUC(0-12) targeting of MPA post hematopoietic cell transplantation.