Abstract

The irreversible μ-opioid antagonists β-funaltrexamine (β-FNA) and β-chlornaltrexamine (β-CNA) are important pharmacological tools but have a κ-agonist activity and, in the latter case, low selectivity. This work examines whether clocinnamox (C-CAM) and the newer analog, methocinnamox (M-CAM), represent improved long-lasting antagonists for examining μ-opioid-mediated effects in vivo. β-FNA, β-CNA, C-CAM, and M-CAM were compared after systemic administration in mice and in vitro. β-FNA and β-CNA were effective agonists in the writhing assay, reversible by the κ-antagonist norbinaltorphimine. Neither C-CAM nor M-CAM had agonist activity in vivo. M-CAM was devoid of agonist action at cloned opioid receptors. All four compounds depressed the dose-effect curve for the μ-agonist morphine in the warm-water tail-withdrawal test 1 h after administration; at 48 h, recovery was evident. In the writhing assay, the dose-effect curve for morphine was shifted in a parallel fashion in the order M-CAM ≫ C-CAM > β-CNA ≥ β-FNA. In comparison with their ability to shift the dose-effect curve for bremazocine (κ) and BW373U86 (δ), β-CNA was the least μ-selective, followed by C-CAM < β-FNA < M-CAM. M-CAM (1.8 mg/kg) produced a 74-fold increase in the ED₅₀ of morphine while showing no effect on bremazocine or BW373U86 dose-response curves. In binding assays, C-CAM and M-CAM were 8-fold selective for μ- over κ-receptors, whereas β-FNA and β-CNA were μ/δ-, but not μ/κ, selective. However, ex vivo binding assays confirmed the μ-receptor selectivity of M-CAM. M-CAM is thus a potent, long-lasting, and specific antagonist at μ-receptors in vivo that lacks confounding agonist actions.