Synthesis and biological evaluation of an orally active glycosylated endomorphin-1

The endogenous opioid peptide endomorphin-1 (1) was modified by attachment of lactose to the N-terminus via a succinamic acid spacer to produce compound 2. The carbohydrate modification significantly improved the metabolic stability and membrane permeability of 2 while retaining μ-opioid receptor binding affinity and agonist activity. Analogue 2 produced dose-dependent antinociceptive activity following intravenous administration in a chronic constriction injury (CCI) rat model of neuropathic pain with an ED(50) of 8.3 (± 0.8) μmol/kg. The corresponding ED(50) for morphine was 2.6 (± 1.4) μmol/kg. Importantly, compound 2 produced dose-dependent pain relief after oral administration in CCI rats (ED(50) = 19.6 (± 1.2) μmol/kg), which was comparable with that of morphine (ED(50) = 20.7 (±3.6) μmol/kg). Antineuropathic effects of analogue 2 were significantly attenuated by pretreatment of Animals with the opioid antagonist naloxone, confirming opioid receptor-mediated analgesia. In contrast to morphine, no significant constipation was produced by compound 2 after oral administration.