PROJECT ABSTRACT The current US opioid epidemic includes a rising number of pregnant and postpartum women struggling with opioid use disorders (OUD). The standard of care for pregnant and postpartum women with OUD is treatment with medication for OUD (MOUD), specifically methadone or sublingual buprenorphine (SL BUP). However, in recent years, newer options for the treatment of OUD have emerged that have not been studied in pregnant and postpartum women. Specifically, the use of naltrexone (NTX), an opioid antagonist, could be advantageous in pregnant women with OUD as it avoids fetal exposure to opioids and eliminates the risk for neonatal opioid withdrawal syndrome (NOWS). Preliminary studies demonstrate minimal impact on pregnancy and fetal outcomes with NTX exposure, however there is an overall lack of prospective human data fully evaluating safety, efficacy, and the full range of outcomes. In addition, the pharmacokinetics, pharmacogenomics, breast milk transfer and safety of NTX when used during pregnancy and the postpartum period are unknown. Our parent study examined 50 pregnant women on NTX compared with 50 on SL BUP in a multi-centered prospective comparative cohort study with the following 4 specific aims: 1) Safety and efficacy of NTX versus SL BUP: Mother-infant dyads are followed throughout the pregnancy until 12 months postpartum to examine maternal outcomes, fetal outcomes, and infant outcomes. 2) Pharmacokinetics of NTX: The pharmacokinetics of NTX in pregnant and postpartum women is examined. 3) Pharmacogenomics and epigenetic modification: We examine the impact of genetic polymorphisms on treatment response, and the impact of maternal treatment on DNA methylation. 4) NTX breastfeeding safety. We will measure breast milk concentrations from lactating women on NTX along with simultaneous maternal and infant plasma concentrations. Recently, extended-release formulations of subcutaneous buprenorphine (SC BUP) have been developed and are commonly used in non- pregnant individuals. These long-acting formulations of BUP allow for more individualization of treatment and could improve compliance and treatment retention. In clinical practice, more providers have moved towards using SC BUP in the postpartum period, however no studies of SC BUP in pregnant, postpartum, or lactating women have been conducted. We have added the following two aims as part of an administrative supplement: 1) Pharmacokinetics of SC BUP in lactating women and their infants: We will enroll 10 postpartum women who are on SC BUP who are breastfeeding to determine pharmacokinetics and breastmilk levels. 2) Safety and efficacy of SC BUP in postpartum breastfeeding women and their infants: We will follow the dyads to 12 months postpartum to examine maternal and infant outcomes. The results of this study hold the promise to guide planned future studies examining the use of both NTX and other agonist medications in the context of best practice treatment for OUD in pregn...