Central Nervous System Distribution of an Opioid Agonist Combination with Synergistic Activity

Jessica I Griffith, Minjee Kim, Daniel Bruce, Cristina D Peterson, Kelley F. Kitto, Afroz Shareef Mohammad, Sneha Rathi, Carolyn A. Fairbanks, George L. Wilcox, William F. Elmquist

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


Novel combinations of specific opioid agonists like loperamide and oxymorphindole targeting the µ- and δ-opioid receptors, respectively, have shown increased potency with minimized opioid-associated risks. However, whether their interaction is pharmacokinetic or pharmacodynamic in nature has not been determined. This study quantitatively determined whether these drugs have a pharmacokinetic interaction that alters systemic disposition or central nervous system (CNS) distribution. We performed intravenous and oral in vivo pharmacokinetic assessments of both drugs after discrete dosing and administration in combination to determine whether the combination had any effect on systemic pharmacokinetic parameters or CNS exposure. Drugs were administered at 5 or 10 mg/kg i.v. or 30 mg/kg orally to institute for cancer research (ICR) mice and 5 mg/kg i.v. to Friend leukemia virus strain B mice of the following genotypes: wild-type, breast cancer resistance protein ( Bcrp -/- ) (Bcrp knockout), Mdr1a/b -/- [P-glycoprotein (P-gp) knockout], and Bcrp -/- Mdr1a/b -/- (triple knockout). In the combination, clearance of oxymorphindole (OMI) was reduced by approximately half, and the plasma area under the concentration-time curve (AUC) increased. Consequently, brain and spinal cord AUCs for OMI in the combination also increased proportionately. Both loperamide and OMI are P-gp substrates, but administration of the two drugs in combination does not alter efflux transport at the CNS barriers. Because OMI alone shows appreciable brain penetration but little therapeutic efficacy on its own, and because loperamide's CNS distribution is unchanged in the combination, the mechanism of action for the increased potency of the combination is most likely pharmacodynamic and most likely occurs at receptors in the peripheral nervous system. This combination has favorable characteristics for future development. SIGNIFICANCE STATEMENT: Opioids have yet to be replaced as the most effective treatments for moderate-to-severe pain and chronic pain, but their side effects are dangerous. Combinations of opioids with peripheral activity, such as loperamide and oxymorphindole, would be valuable in that they are effective at much lower doses and have reduced risks for dangerous side effects because the µ-opioid receptor agonist is largely excluded from the CNS.

Original languageEnglish (US)
Pages (from-to)34-46
Number of pages13
JournalJournal of Pharmacology and Experimental Therapeutics
Issue number1
StatePublished - Jan 1 2022

Bibliographical note

Funding Information:
This work was supported by The University of Minnesota PharmacoNeuro Immunology Training Grant, T32 DA007097–32, supported D.J.B. NIDA R01 DA 015438–10 to G.L.W. supported initial studies. We also acknowledge preclinical development grants from the University of Minnesota Committee on Pharmaceutical Development (CPD) and MN-REACH Program (to G.L.W.). G.L.W. holds the R.W. Goltz Professorship in Dermatology, which also provided support.

Publisher Copyright:
Copyright © 2021 by The American Society for Pharmacology and Experimental Therapeutics.

PubMed: MeSH publication types

  • Journal Article
  • Research Support, Non-U.S. Gov't


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