Pharmacokinetics and Pharmacodynamics of Mycophenolic Acid and Their Relation to Response to Therapy of Childhood-Onset Systemic Lupus Erythematosus

ObjectivesMycophenolic acid (MPA) is the active form of mycophenolate mofetil (MMF), which is currently used off-label as immunosuppressive therapy in childhood-onset SLE (cSLE). The objectives of this study were to (1) characterize the pharmacokinetics (MPA-PK) and pharmacodynamics (MPA-PD) of MPA and (2) explore the relationship between MPA-PK and cSLE disease activity.MethodsMPA-PK [area under the curve from 0-12 hours (AUC0-12)] and MPA-PD [inosine-monophosphate dehydrogenase (IMPDH) activity] were evaluated in cSLE patients on stable MMF dosing. Change in SLE disease activity while on MMF therapy was measured using the British Isles Lupus Assessment Group (BILAG) index.ResultsA total of 19 AUC0-12 and 10 IMPDH activity profiles were included in the analysis. Large interpatient variability in MPA exposure (AUC0-12) was observed (mean ± SE: 32 ± 4.2 mg h/L; coefficient of variation: 57%). Maximum MPA serum concentrations coincided with maximum IMPDH inhibition. AUC0-12 and weight-adjusted MMF dosing were only moderately correlated (r = 0.56, P = 0.01). An AUC0-12 of ≥30 mg h/L was associated with decreased BILAG scores while on MMF therapy (P = 0.002).ConclusionWeight-adjusted MMF dosing alone does not reliably allow for the prediction of exposure to biologically active MPA in cSLE. Individualized dosing considering MPA-PK appears warranted as this allows for better estimation of immunologic suppression (IMPDH activity). Additional controlled studies are necessary to confirm that an MPA AUC0-12 of at least 30 mg h/L is required for cSLE improvement.