Alvimopan

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^[2].

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^[1].