Norbuprenorphine (NBP)

Norbuprenorphine is the primary active metabolite of buprenorphine. Norbuprenorphine acts as a μ-opioid, δ-opioid, and nociceptin receptor full agonist, as well as a κ-opioid receptor partial agonist. Norbuprenorphine crosses the blood-brain-barrier similarly to buprenorphine and likely contributes to its effects. Norbuprenorphine is a major metabolite of buprenorphine and potent agonist of μ, δ, and κ opioid receptors. Compared with buprenorphine, norbuprenorphine causes minimal antinociception but greater respiratory depression. It is unknown whether the limited antinociception is caused by low efficacy or limited brain exposure. Norbuprenorphine is an in vitro substrate of the efflux transporter P-glycoprotein (Mdr1), but the role of P-glycoprotein in norbuprenorphine transport in vivo is unknown.