title: Metabolic and toxicological considerations of the opioid replacement therapy and analgesic drugs: methadone and buprenorphine.
abstract: BACKGROUND
  Methadone and buprenorphine are maintenance replacement therapies for opioid dependence; they are also used for pain management. Methadone and buprenorphine (to a lesser extent) have seen sharp increases in mortality associated with their use. They have distinct routes of metabolism (mostly cytochrome P450 dependent), and distinct pharmacologic activity of metabolites. As such, metabolism may play a role in differences in their toxicity.
  
  
  METHODS
  This article reviews peer-reviewed literature obtained from PubMed searches and literature referenced within. The review considers first an overview of drug use and mortality over the past decade. It then provides extensive detail on the in vitro and in vivo human metabolism of methadone and buprenorphine. Using both human and experimental animal studies it then presents the pharmacodynamic activity of parent drug and metabolites at the mu-opioid receptor, as P-glycoprotein substrates and plasma/brain concentration ratios, and activity at the hERG K(+) channel. Lessons learned from drug interaction studies in humans are then examined in an attempt to bring together the combined information.
  
  
  CONCLUSIONS
  The use and misuse of these drugs contributes to the epidemic in opioid-associated mortalities. A better understanding of metabolism-, transport- and co-medication-induced changes will contribute to their safer use.
authors.affiliation: University of Utah College of Pharmacy, Department of Pharmacology and Toxicology, Salt Lake City, UT 84108, USA. david.moody@utah.edu
authors.forename: David E
authors.initials: DE
authors.lastname: Moody
mesh_terms: D000701:Analgesics, Opioid / Q000008:administration & dosage* / Q000009:adverse effects / Q000378:metabolism; D000818:Animals; D002047:Buprenorphine / Q000008:administration & dosage* / Q000009:adverse effects / Q000378:metabolism; D004347:Drug Interactions; D006801:Humans; D008691:Methadone / Q000008:administration & dosage* / Q000009:adverse effects / Q000378:metabolism; D058850:Opiate Substitution Treatment / Q000379:methods; D009293:Opioid-Related Disorders / Q000188:drug therapy; D010146:Pain / Q000188:drug therapy
chemical_list: D000701:Analgesics, Opioid; D002047:Buprenorphine; D008691:Methadone