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| Routes of administration | By mouth, parenteral |
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| Formula | C18H24Br2N2O |
| Molar mass | 444.211 g·mol−1 |
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U-77891 is an opioid analgesic drug that was first synthesized in 1983 by the Upjohn company.[1] It was originally synthesized to prove that the removal of a single methylene spacer of the benzamide would alter a κ-opioid receptor agonist such as U-50488 into an μ-opioid receptor agonist, as well as producing a semi-rigid derivative of U-47700. This would help elucidate the relative positions of the hydrogen-bond acceptors and substituted aromatic system to find the compound with the lowest Ki value in a series of benzamide opioids dating back to the 1970s.[2] The original work found a mixture of agonists and antagonists.[3]
U-77891 acts as an agonist of the μ-opioid, δ-opioid and κ-opioid receptors with Ki values of 2, 105 and 2300 nM, respectively.[2] The compound has ED50 values of 0.02 mg/kg and 0.21 mg/kg in mouse phenylquinone writhing and tail-flick assays.[2] One reason for the high potency is the LogP of 4.57,[4] allowing it to accumulate in fatty tissue such as the brain.