Volumetric Absorptive Microsampling of Saliva for Pharmacokinetic Evaluation of Mycophenolic Acid and Its Glucuronide Metabolite in Pediatric Renal Transplant Recipients: Bioanalytical Method Validation and Clinical Feasibility Evaluation

Background: Mycophenolic acid (MPA) is frequently used in pediatric renal transplantation as part of immunosuppressive therapy, yet therapeutic drug monitoring (TDM) remains challenging. Accurate monitoring is essential due to MPA’s narrow therapeutic window, variable pharmacokinetics, and high protein binding. This study examined whether saliva could serve as a non-invasive alternative to plasma for measuring MPA exposure. Methods and Results: Concentrations of MPA and its primary glucuronide metabolite (MPAG) were determined in plasma, capillary blood, plasma ultrafiltrate, wet saliva, and dried saliva collected using volumetric absorptive microsampling (VAMS). A novel LC–MS/MS method for quantifying MPA and MPAG in dried saliva collected with the Mitra™ device was developed and validated within a 1–700 μg/L calibration range, demonstrating robust analytical performance. Dried and wet saliva showed high correlation (r = 0.99 and 0.98 for MPA and MPAG, respectively). However, both salivary matrices—dried saliva collected with Mitra™ (vsMPA, vsMPAG) and wet saliva (sMPA, sMPAG)—exhibited poor correlation with unbound (fMPA, fMPAG) and total plasma concentrations (tMPA, tMPAG). A modest, yet positive, correlation was observed between the measured concentrations for the following pairs: sMPA versus fMPA (r = 0.376, p = 0.1036), sMPA versus tMPA (r = 0.305, p = 0.1904), sMPAG versus fMPAG (r = 0.205, p = 0.3851), and sMPAG versus tMPAG (r = 0.472, p = 0.0012). Pharmacokinetic parameters supported these findings, highlighting discrepancies between saliva and plasma. Conclusions: From a clinical perspective, saliva sampling—although minimally invasive and patient-friendly—does not offer a reliable substitute for plasma in routine TDM of MPA and MPAG. Capillary blood collected through VAMS remains a promising alternative for long-term monitoring of pediatric patients; however, several considerations still need to be addressed.