Free mycophenolic acid exposure for therapeutic drug monitoring in paediatric lupus nephritis: associations with treatment response and haematologic toxicity.

OBJECTIVE: This study aimed to evaluate the associations of free mycophenolic acid (f-MPA) concentration and total MPA (t-MPA) concentration with treatment response and haematologic toxicity and to determine whether f-MPA provides additional safety-related value for therapeutic drug monitoring (TDM) in patients with paediatric lupus nephritis (LN) treated with mycophenolate mofetil (MMF). METHODS: Sixty-five patients with paediatric LN receiving MMF were prospectively enrolled. The t-MPA and f-MPA were measured simultaneously using validated ultrafiltration and LC-MS/MS. Full pharmacokinetic profiles were obtained over 12 hours, and area under the concentration-time curve (AUC) was calculated for t-MPA (t-MPA-AUC) and f-MPA (f-MPA-AUC). Univariate analysis identified factors influencing efficacy (relapse-free survival) and adverse drug reaction (ADR) over 12 months. Receiver operating characteristic (ROC) curves established predictive thresholds and Kaplan-Meier methods analysed time-to-event. RESULTS: The 12-month relapse-free rate was 78.7% (95% CI 66.6% to 90.9%). Both t-MPA-AUC and f-MPA-AUC significantly correlated with relapse-free survival (ROC AUC 0.70 each; optimal thresholds: t-MPA-AUC=31.63 µg·h/mL, f-MPA-AUC=354.45 ng·h/mL). The average remission duration above these thresholds was 10.88 and 11.00 months. The overall MMF-related ADR incidence was 25.2%, including 17 haematological events (26.2%). The t-MPA-AUC, f-MPA-AUC, glucose, β2-microglobulin and C3 were haematological ADR risk factors. The f-MPA-AUC independently predicted haematological ADRs (optimal threshold: 492.96 ng·h/mL; specificity 86.2%). CONCLUSION: Both t-MPA-AUC and f-MPA-AUC effectively predict MMF efficacy in paediatric LN. However, f-MPA-AUC demonstrates superior predictive value for safety outcomes, specifically haematological ADRs. This supports f-MPA as a potentially better TDM metric for optimising MMF therapy safety in this population.