Flatter Tacrolimus Pharmacokinetic Profiles in Pediatric Kidney Transplant Recipients Receiving Liquid Formulations: A Quality Improvement Study

BACKGROUND: Compounded tacrolimus suspensions are frequently used in pediatric kidney transplant recipients who cannot reliably swallow capsules or who receive medication via a gastrostomy tube. Clinicians at our center observed unusually flat mini-pharmacokinetic (mini-PK) profiles in patients receiving liquid tacrolimus. METHODS: We retrospectively analyzed 13 tacrolimus mini-PK profiles (C0, C1, C2, C4) from 7 pediatric kidney transplant recipients receiving compounded liquid tacrolimus, measured by LC-MS/MS in routine clinical care. As a contemporaneous comparator measured by the same assay, we assembled mini-PK profiles from pediatric recipients receiving capsule tacrolimus at our center (December 2024 onward). A historical capsule cohort from Berlin measured with Abbott Tacrolimus II immunoassay was used only as external context. RESULTS: ratios, consistent with a trough-dependent change in apparent PK shape. CONCLUSIONS: In this Quality Improvement (QI) analysis, pediatric patients receiving compounded liquid tacrolimus demonstrated flatter mini-PK profiles than contemporaneous capsule recipients when evaluated within the same analytical platform. These findings support closer attention to PK shape (not trough alone) in clinically complex children requiring liquid formulations and motivate prospective studies that control for gastrointestinal comorbidity, co-medications, and administration route.