| In Vitro | In vitro activity: Alvimopan (also known as ADL 8-2698; LY 246736; HSDB-7704) is a novel peripheral micro opioid antagonist in clinical development for the management of post-operative ileus and opioid-induced bowel dysfunction. The long duration of action of alvimopan might be related to a slower dissociation rate from the micro opioid receptor compared to other shorter acting antagonists. The dissociation rate of alvimopan from the micro opioid receptor (t(1/2)=30--44 min) was comparable to that of the long acting partial agonist buprenorphine (t(1/2)=44 min), but was slower than those of the antagonists naloxone (t(1/2)=0.82 min) and N-methylnaltrexone (t(1/2)=0.46 min). Also, increases in the apparent affinities and potencies of buprenorphine and alvimopan, but not of naloxone and methylnaltrexone, were observed upon preincubation with the micro opioid receptor. Consistent with its long duration of action, alvimopan has a slow dissociation rate from the micro opioid receptor compared to other shorter acting antagonists and may be more potent if administered prior to dosing with exogenous opioids.
Kinase Assay: Alvimopan is highly selective (by ≥227-fold) for the human μ receptor over the κ subtype, but has a more modest (≥6-fold) μ/δ receptor selectivity. In the guinea pig isolated ileum, alvimopan is a potent antagonist of morphine, DAMGO or endomorphin-1-induced, and μ opioid receptor-mediated, inhibition of electrically-evoked contractions (pA2 values of 9.6 or 9.7). The δ and κ antagonist potencies of alvimopan are lower in the guinea pig ileum (pA2 values of 8.7 and 7.8, respectively). Alvimopan (1 or 10 μM) has no significant affinity for a broad range of non-opioid receptors, ion channels and enzymes at which it has been tested.
Cell Assay: |
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| In Vivo | In animals, alvimopan antagonizes centrally mediated, morphine-induced analgesia only at relatively high doses, with very high plasma concentrations needed to cross the blood-brain barrier. After intravenous administration, alvimopan is approximately 200-times more potent at blocking peripheral verses central μ-receptors. After oral administration, alvimopan is also highly active. In dogs, intravenous administration of alvimopan provided dose-dependent increases in peak plasma concentrations and plasma area under the concentration-time curve. However, as a result of poor systemic absorption, oral doses up to 100 mg/kg produced low plasma concentrations (mean Cmax =92.9 ng/ml), which resulted in an oral bioavailability of approximately 0.03%. The half-life of alvimopan is estimated to be approximately 10 min after intravenous administration in dogs and rabbits. |
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