Mycophenolic Acid Exposure Optimization Based on Vitamin D Status in Children with Systemic Lupus Erythematosus: A Single-Center Retrospective Study

Introduction Systemic lupus erythematosus (SLE) can affect bone metabolism and homeostasis of serum electrolytes that are associated with abnormal levels of vitamin D. Mycophenolate mofetil (MMF) is a commonly used immunosuppressant with the active metabolite mycophenolic acid (MPA). The area under the plasma concentration–time curve (AUC) of MPA is often monitored during the treatment to assess the exposure levels. This study aims to explore the association between exposure levels of MPA and 25-hydroxyvitamin D [25(OH)D] levels in children with SLE. Methods Repeated measured data of children with SLE who were treated with MMF and under therapeutic drug monitoring (TDM) were retrospectively collected from the electronic medical records. MPA exposure levels were reflected by the area under the concentration–time curve over 24 h (AUC 0–24h ). Univariate and multivariate linear regression models were employed to analyze factors associated with 25(OH)D levels. Hierarchical linear models were developed to analyze the intra- and inter-individual effects of AUC 0–24h on the variance of 25(OH)D levels. Results Data from 184 children with SLE (142 female and 42 male) with 518 follow-ups were collected. The median age was 14 years (range 3–18 years) at TDM. Children with normal 25(OH)D levels had significantly higher AUC 0–24h than children with low 25(OH)D levels (98.71 vs. 84.05 mg·h/L, P = 0.004). Intra- and inter-individual effects of AUC 0–24h on 25(OH)D levels were similar ( γ_10 = 0.034 vs. γ_01 = 0.037) but only the intra-individual effect was significant ( P = 0.001) in hierarchical models. Other associated factors include age, sex, season at measurement, glucocorticoid daily dose, and external vitamin D 3 supplements. Conclusion 25(OH)D levels are associated with MPA exposure levels, and may serve as a potential indicator to optimize the exposure level of MPA during treatment. AUC 0–24h of 98.71 mg·h/L or AUC 0–12h of 49.36 mg·h/L could be the targeted exposure level for children with SLE.