Synthesis and pharmacological evaluation of novel selective MOR agonist 6β-pyridinyl amidomorphines exhibiting long-lasting antinociception

Abstract

It was previously reported that 6β-aminomorphinan derivatives show high affinity for opiate receptors. Novel 6β-heteroarylamidomorphinanes were designed based on the MOR selective antagonist NAP. The 6β-aminomorphinanes were prepared with stereoselective Mitsunobu reaction and subsequently acylated with nicotinic acid and isonicotinic acid chloride hydrochlorides. The receptor binding and efficacy were determined in vitro and the analgesic activity was studied in vivo. The in vitro studies revealed moderate selectivity for MOR. At least two compounds in this series exhibited long-acting analgesic response when administered subcutaneously and intracerebroventricularly. When the substances were given intracerebroventricularly to mice, they showed analgesic potency comparable to morphine.

Fig. 1. Structures of 6β-amidomorphinanes.

Fig. 2. Synthesis of 6β-amidomorphinans 9a–g. a) Acetic anhydride, NaHCO₃ water r.t. 2 h; b) benzene/Ph₃P, phthalimide, DIAD, r.t. 1 h; c) ethanol/hydrazine monohydrate, heating 3 h; d) hydrazine monohydrate/RANEY® nickel, ethanol, r.t. 2 h; e) dichloromethane/acyl chloride, triethylamine; f) K₂CO₃/methanol, heating 4 h.

Fig. 3. General structure of the studied compounds.

Fig. 4. Two independent determinations of the cumulative dose–response curves were performed on groups of mice (n = 5) for antinociception in the tail flick assay with the compounds given intracerebroventricularly. The animals were tested 15 min later at the peak effect to generate the antinociceptive dose–response curve. Each point represents mean ± SEM for 10 mice. ED₅₀ values (and 95% CI) were: 9c: 2.0 μg (1.1, 3.6); 9d: 0.51 μg (0.3, 0.9); 9e: 0.7 μg (0.4, 1.2); 9f: 1.8 μg (1.0, 3.2); morphine: 0.53 μg (0.27, 1.0).

Fig. 5. A) Antinociceptive effect of compounds 9c–f. CD1 mice (n = 10) were given the compounds sc at 30 mg kg^–1 and the antinociceptive effects were assessed 30 min post-injection using the radiant heat tail-flick method. Results (mean ± SEM): 9c: 38% ± 13%; 9d: 35% ± 10%; 9e: 78% ± 11%; 9f: 83% ± 11%. B) Time course of the antinociceptive effect of morphine (4.5 mg kg^–1), compound 9e (30 mg kg^–1), and 9f (30 mg kg^–1) following sc administration to CD1 mice. Two independent determinations were performed on groups of mice (n = 10) for antinociception in the tail flick assay. C) Time course of the antinociceptive effect of morphine (1 μg), compound 9e (7.5 μg), and 9f (7.5 μg) following icv administration to CD1 mice. Two independent determinations were performed on groups of mice (n = 10) for antinociception in the tail flick assay. D) Reversal of antinociception by selective antagonists. Groups of CD1 mice (n = 10) received 9e (30 mg kg^–1, sc), or 9f (30 mg kg^–1) with or without β-funaltrexamine (β-FNA; 40 mg kg^–1, sc). β-FNA was administered 24 hours before agonist testing. All antinociception testing was performed 30 min after the administration of 9e and 9f. Similar results were observed in two independent replications. Antinociception was antagonized by β-FNA (two-way ANOVA followed by the Bonferroni post hoc comparison test, p < 0.05). The means in each determination were determined as the percentage maximal possible effect (%MPE) [(observed latency – baseline latency)/(maximal latency – baseline latency)] × 100. The baseline latencies were between 2–3 s and the maximal latency was 10 s to avoid tissue damage.