Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.07.25.221192v1?rss=1 Authors: Safa, A., Lau, A. R., Aten, S., Schilling, K., Bales, K. L., Miller, V. A., Fitzgerald, J., Chen, M., Hill, K., Dzwigalski, K., Obrietan, K., Phelps, M. A., Sadee, W., Oberdick, J. Abstract: Neonatal opioid dependence is an enormous and growing medical challenge. Newborns exposed to prenatal opioids often experience intense postnatal withdrawal after cessation of the opioid, called neonatal opioid withdrawal syndrome (NOWS). They may also show delays in developmental milestones. Thus, a pharmacotherapy that could be delivered in conjunction with a prenatal opioid, and that could prevent fetal opioid dependence without interfering with pain or opioid use management of the mother, would be highly desirable. In a prior study in mice we demonstrated that the peripherally selective neutral opioid antagonist, 6{beta}-naltrexol (6BN), has properties that make it a promising candidate for such an approach. In the current work we extend our studies of 6BN to pregnant guinea pigs, which are a more suitable model than mice and rats, and allows for the detection of robust spontaneous neonatal withdrawal. We find that prenatal methadone (MTD) significantly aggravates two classic maternal separation stress behaviors in newborn guinea pigs: calling (vocalizing) and searching (locomotion) - natural attachment behaviors thought to be controlled by the endogenous opioid system. In addition, prenatal MTD significantly increases the levels of plasma cortisol in newborns exposed to maternal separation stress, showing that cessation of MTD at birth engages the hypothalamic-pituitary-adrenal (HPA) axis. Most importantly, our work shows that 6BN can prevent all these effects of prenatal methadone with high potency (50% inhibitory dose, ID50, of ~0.02 mg/kg), at doses unlikely to induce maternal withdrawal or to interfere with opioid analgesia based on many prior published studies. This potency is surprising in the face of both the long half-life of methadone and our pharmacokinetic (PK) studies showing slow placental transit of 6BN in guinea pigs. Furthermore, we demonstrate an unexpectedly high potency of 6BN in preventing opioid withdrawal in adult guinea pigs (ID50 = 0.01 mg/kg), in spite of the fact that our PK data in both mice and guinea pigs show the relative exclusion of 6BN from the adult CNS, presumably by virtue of the blood brain barrier and associated extrusion pumps. Thus, novel opioid receptor mechanisms may need to be hypothesized to account for the high potency of 6BN for preventing opioid withdrawal. In conclusion, 6BN is an attractive compound for development of a preventive therapy for NOWS, and guinea pigs are an ideal model to further test it. Copy rights belong to original authors. Visit the link for more info