The Use of Serum Methadone/Metabolite Ratios to Monitor Changing Perinatal Pharmacokinetics

Objectives: Pregnancy profoundly alters drug metabolism, accelerating clearance and confounding medication management, primarily through induction of CYP450 enzymes. Methadone is a CYP450 substrate with altered pharmacokinetics during pregnancy. We report on the use of serum methadone/metabolite ratios (MMRs) to monitor changes in methadone metabolism through the perinatal period and to objectively guide methadone dosing. Previous research found average MMRs in nonpregnant populations of between 11.3 and 12.7. Methods: Serum methadone and its major metabolite 2-ethylidene1,5-dimethyl-3,3-diphenylpyrrolidine concentrations were analyzed in 67 samples from 23 pregnant patients treated for opioid use disorder, and their calculated ratio was used to document changes in methadone clearance across trimesters and postpartum. Lower ratios indicate increased clearance. Results: The average MMR during pregnancy was 6.1. Ratios declined significantly from trimester 1 to trimester 3 (P = 0.007), and then rose significantly from trimester 3 to postpartum (P = 0.001). The per cent of ratios that were 4 or less, indicating ultrarapid metabolism, increased from 8% to 30% to 38% across trimesters, and decreased to 5% postpartum. Forty-four per cent of individual patients had at least 1 prepartum ratio of 4 or less. Conclusions: This study documents significant metabolic changes occurring perinatally, which indicate the need for both changes in methadone dose and dose frequency to maintain maternal/fetal stability, and also dose reductions as hypermetabolism reverses postpartum. MMRs provide an objective tool to more efficiently improve the safety and efficacy of methadone dosing perinatally.