Synthesis, Opioid Receptor Binding, and Biological Activities of Naltrexone-Derived Pyrido- and Pyrimidomorphinans

Abstract

A series of pyrido- and pyrimidomorphinans (6a-h and 7a-g) were synthesized from naltrexone and evaluated for binding and biological activity at the opioid receptors. The unsubstituted pyridine 6a displayed high affinities at opioid delta, mu, and kappa receptors with K(i) values of 0.78, 1.5, and 8.8 nM, respectively. Compound 6a was devoid of agonist activity in the mouse vas deferens (MVD) and guinea pig ileum (GPI) preparations but was found to display moderate to weak antagonist activity in the MVD and GPI with K(e) values of 37 and 164 nM, respectively. The pyrimidomorphinans in general displayed lower binding potencies and delta receptor binding selectivities than their pyridine counterparts. Incorporation of aryl groups as putative delta address mimics on the pyrido- and pyrimidomorphinan framework gave ligands with significant differences in binding affinity and intrinsic activity. Attachment of a phenyl group at the 4'-position of 6a or the equivalent 6'-position of 7a led to dramatic reduction in binding potencies at all the three opioid receptors, indicating the existence of a somewhat similar steric constraint at the ligand binding sites of delta, mu, and kappa receptors. In contrast, the introduction of a phenyl group at the 5'-position of 6a did not cause any reduction in the binding affinity at the delta receptor. In comparison to the unsubstituted pyridine 6a, the 5'-phenylpyridine 6c showed improvements in mu/delta and kappa/delta binding selectivity ratios as well as in the delta antagonist potency in the MVD. Interestingly, introduction of a chlorine atom at the para position of the pendant 5'-phenyl group of 6c not only provided further improvements in delta antagonist potency in the MVD but also shifted the intrinsic activity profile of 6c from an antagonist to that of a mu agonist in the GPI. Compound 6d thus possesses the characteristics of a nonpeptide mu agonist/delta antagonist ligand with high affinity at the delta receptor (K(i) = 2.2 nM), high antagonist potency in the MVD (K(e) = 0.66 nM), and moderate agonist potency in the GPI (IC(50) = 163 nM). Antinociceptive evaluations in mice showed that intracerebroventricular (icv) injections of 6d produced a partial agonist effect in the 55 degrees C tail-flick assay and a full agonist effect in the acetic acid writhing assay (A(50) = 7.5 nmol). No signs of overt toxicity were observed with this compound in the dose ranges tested. Moreover, repeated icv injections of an A(90) dose did not induce any significant development of antinociceptive tolerance in the acetic acid writhing assay. The potent delta antagonist component of this mixed mu agonist/delta antagonist may be responsible for the diminished propensity to produce tolerance that this compound displays.

Building a Better Analgesic: Multifunctional Compounds that Address Injury-Induced Pathology to Enhance Analgesic Efficacy while Eliminating Unwanted Side Effects

“…1A), where TFL in mice treated with 1 bolus of the highest doses remained above baseline levels for 180 minutes. The calculated intracerebroventricular A 50 was 0.93 mg [95% confidence interval (CI) 5 0.62-1.39 mg] in mice, similar to the published data for morphine (A 50 5 0.68 mg, 95% CI 5 0.51-0.92 mg; Ananthan et al, 1999).…”

Potentiation of Low Dose Ketamine Effects by Naltrexone: Potential Implications for the Pharmacotherapy of Alcoholism

“…However, clinical trials of naltrexone treatment for alcohol dependence study doses up to 100 mg. Naltrexone, at 25 mg, should have produced a nearly complete blockade of m-opiate receptors (Lee et al, 1988). But, it is possible that higher naltrexone doses would have produced greater effects via d-or k-opiate receptors, where it has nearly 16.7-fold and 5.6-fold lower affinity, respectively (Ananthan et al, 1999). Second, this study was conducted in healthy human subjects without a family history of alcohol dependence.…”

Salvinorin A: A potent naturally occurring nonnitrogenous κ opioid selective agonist

“…Detailed on-line protocols are available for all assays at the NIMH-PDSP web site (http:͞͞pdsp.cwru.edu). opioid radioligand-binding assays in situ in guinea pig brain and opioid receptor (MOR)-and ␦ opioid receptor (DOR)-binding assays in rat brain were performed as previously detailed (11). Initial screening assays were performed by using 10 M Salvinorin A or 10 M LSD by using quadruplicate determinations and the percent inhibition of specific binding determined.…”