Probes for narcotic receptor-mediated phenomena. 27. Synthesis and pharmacological evaluation of selective delta-opioid receptor agonists from 4-[(alphaR)-alpha-(2S,5R)-4-substituted-2, 5-dimethyl-1-piperazinyl-3-methoxybenzyl]-N,N-diethylbenzamides and their enantiomers

Evaluation of N-substitution in 6,7-benzomorphan compounds

6,7-benzomorphan derivatives, exhibiting different mu, delta, and kappa receptor selectivity profiles depending on the N-substituent, represent a useful skeleton for the synthesis of new and better analgesic agents. In this work, an aromatic ring and/or alkyl residues have been used with an N-propanamide or N-acetamide spacer for the synthesis of a new series of 5,9-dimethyl-2'-hydroxy-6,7-benzomorphan derivatives (12-22). Data obtained by competition binding assays showed that the mu opioid receptor seems to prefer an interaction with the 6,7-benzomorphan ligands having an N-substituent with a propanamide spacer and less hindered amide. Highly stringent features are required for delta receptor interaction, while an N-acetamide spacer and/or bulkier amide could preferentially lead to kappa receptor selectivity. In the propanamide series, compound 12 (named LP1) displayed high mu affinity (Ki=0.83 nM), good delta affinity (Ki=29 nM) and low affinity for the kappa receptor (Ki=110 nM), with a selectivity ratio delta/mu and kappa/mu of 35.1 and 132.5, respectively. Further, in the adenylyl cyclase assay, LP1 displayed a mu/delta agonist profile, with IC50 values of 4.8 and 12 nM at the mu and delta receptors, respectively. The antinociceptive potency of LP1 in the tail-flick test after sc administration in rat was comparable with the potency of morphine (ED50=2.03 and 2.7 mg/kg, respectively), and was totally reversed by naloxone. LP1, possessing a mu/delta agonist profile, could represent a lead in further developing benzomorphan-based ligands with potent in vivo analgesic activity and a reduced tendency to induce side effects.

Role of delta opioid efficacy as a determinant of mu/delta opioid interactions in rhesus monkeys

Delta opioid agonists can selectively enhance the antinociceptive effects of mu opioid agonists without enhancing some other, potentially undesirable mu agonist effects. However, the degree of delta receptor efficacy required to produce this profile of interactions is unknown. To address this issue, the present study examined interactions produced by the mu agonist fentanyl and the intermediate-efficacy delta opioid MSF61 in rhesus monkeys. For comparison, interactions were also examined between fentanyl and the relatively high-efficacy delta agonist SNC243A and the delta antagonist naltrindole, which has negligible efficacy at delta receptors. Two different behavioral procedures were used: (a) a warm-water tail-withdrawal assay of thermal nociception, and (b) an assay of schedule-controlled responding for food reinforcement. Drug interactions within each procedure were evaluated using dose-addition analysis to compare experimental results with expected additivity. Drug interactions across procedures were evaluated using dose-ratio analysis to assess relative potencies to produce antinociception vs. response-rate suppression. As expected, dose-addition analysis found that fentanyl/SNC243A interactions were superadditive in the assay of antinociception but additive in the assay of schedule-controlled responding. Conversely, fentanyl/MSF61 interactions were generally additive in both procedures, and fentanyl/naltrindole interactions were additive or subadditive in both procedures. Dose-ratio analysis found that fentanyl alone produced antinociception and rate suppression with similar potencies. Some fentanyl/SNC243A mixtures produced antinociception with up to 4-fold greater potency than rate-suppression. However, fentanyl/MSF61 and fentanyl/naltrindole mixtures produced antinociception with lower potency than rate suppression. These results suggest that relatively high delta receptor efficacy is required for mu/delta antinociceptive synergy.

Potent, orally bioavailable delta opioid receptor agonists for the treatment of pain: discovery of N,N-diethyl-4-(5-hydroxyspiro[chromene-2,4'-piperidine]-4-yl)benzamide (ADL5859)

Selective delta opioid receptor agonists are promising potential therapeutic agents for the treatment of various types of pain conditions. A spirocyclic derivative was identified as a promising hit through screening. Subsequent lead optimization identified compound 20 (ADL5859) as a potent, selective, and orally bioavailable delta agonist. Compound 20 was selected as a clinical candidate for the treatment of pain.

3D-QSAR comparative molecular field analysis on delta opioid receptor agonist SNC80 and its analogs

Three-dimensional quantitative structure-activity relationship (3D-QSAR) models were constructed using comparative molecular field analysis (CoMFA) for a series of delta opioid receptor agonists: SNC80 analogs. Quantum chemical calculations on SNC80 show that protonation is preferred at the basic N4 atom over the alternative N1 atom, accordingly N4 protonation may contribute significantly to ligand-receptor interactions under physiologically relevant conditions. Statistically significant and predictive CoMFA models were achieved by pooling biological data from independent published sources, including compounds with both alphaR and alphaS benzylic configurations. Improved CoMFA models were obtained when the compounds were considered as N4-protonated species rather than neutral compounds. The influence of various atomic partial-charge formalisms, alignment schemes and additional molecular descriptors was evaluated in order to produce the highest quality models. In addition, separate CoMFA models were generated for compounds with only the alphaR benzylic configuration. These CoMFA models showed excellent internal predictability and consistency, and external validation using test-set compounds yielded predicted pIC50 values within 1log unit of the corresponding experimentally measured values. Key insights into the structure-activity relationship derived from the CoMFA analysis concur with experimentally observed data, thus the CoMFA models presented here find utility for predicting the binding affinity, and guiding the design, of novel SNC80 analogs and related delta opioid receptor agonists.

Molecular modeling directed synthesis of a bicyclic analogue of the δ opioid receptor agonist SNC 80

In order to find novel delta opioid receptor agonists, the pharmacophoric benzhydryl moiety of the lead compound SNC 80 (1) was dissected and the phenyl residues were attached to different positions of the 6,8-diazabicyclo[3.2.2]nonane core system (4). The position of the carboxamido group, the stereochemistry, the C3/C4 bond order and the kind and length of the spacer X were considered. The resulting compounds were compared with the four energetically most favourable conformations of SNC 80 by a multifit analysis. These calculations led to the structures 5-10, which fit best to SNC 80. Herein the synthesis of one of these compounds (9) is described. Starting from (S)-glutamate two alternative routes are detailed to obtain the key intermediate 14. A variation of the Dieckmann cyclization, which uses trapping of the first cyclization product with ClSiMe(3) provided the mixed acetal 20, which was carefully hydrolyzed to yield the bicyclic ketone 17. Stereoselective addition of phenylmagnesium bromide, dehydration, LiAlH(4) reduction and exchange of the N-6 residue afforded the designed compound 9. The affinities of 9 towards delta, mu, kappa and ORL1 receptors were determined in receptor binding studies with radioligands. Only moderate receptor affinity was found.

Factors influencing agonist potency and selectivity for the opioid delta receptor are revealed in structure-activity relationship studies of the 4-[(N-substituted-4-piperidinyl)arylamino]-N,N-diethylbenzamides

A study of the effect of transposition of the internal nitrogen atom for the adjacent benzylic carbon atom in delta-selective agonists such as BW373U86 (1) and SNC-80 (2) has been undertaken. It was shown that high-affinity, fully efficacious, and delta opioid receptor-selective compounds can be obtained from this transposition. In addition to the N,N-diethylamido group needed as the delta address, the structural features identified to promote delta receptor affinity in the set of compounds studied included a cis relative stereochemistry between the 3- and 4-substituents in the piperidine ring, a trans-crotyl or allyl substituent on the basic nitrogen, the lack of a 2-methyl group in the piperidine ring, and either no substitution or hydroxyl substitution in the aryl ring not substituted with the N,N-diethylamido group. Structural features found to be important for mu affinity include hydroxyl substitution in the aryl ring, the presence of a 2-methyl group in a cis relative relationship to the 4-amino group as well as N-substituents such as cyclopropylmethyl. It was also determined that mu receptor affinity could be increased while maintaining delta receptor affinity, especially when hydroxyl-substituted compounds are considered. Additionally, it was discovered that the somewhat lower mu/delta selectivities observed for the piperidine compounds relative to the piperazine-based ligands appear to arise as a consequence of the carbon-nitrogen transposition which imparts an overall lower delta and higher mu affinity to the piperidine-based ligands. This higher affinity for the mu receptor, apparently intrinsic to the piperidine-based compounds, suggests that ligands of this class will more easily be converted to mu/delta combination agonists compared to the piperazine ligands such as 1. This is particularly important since analogues of 1, which show both mu- and delta-type activity, are now recognized as important for their strong analgesia and cross-canceling of many of the side effects found in agonists operating exclusively from either the delta or mu opioid receptor.

New diarylmethylpiperazines as potent and selective nonpeptidic delta opioid receptor agonists with increased In vitro metabolic stability

Nonpeptide delta opioid agonists are analgesics with a potentially improved side-effect and abuse liability profile, compared to classical opioids. Andrews analysis of the NIH nonpeptide lead SNC-80 suggested the removal of substituents not predicted to contribute to binding. This approach led to a simplified lead, N, N-diethyl-4-[phenyl(1-piperazinyl)methyl]benzamide (1), which retained potent binding affinity and selectivity to the human delta receptor (IC(50) = 11 nM, mu/delta = 740, kappa/delta > 900) and potency as a full agonist (EC(50) = 36 nM) but had a markedly reduced molecular weight, only one chiral center, and increased in vitro metabolic stability. From this lead, the key pharmacophore groups for delta receptor affinity and activation were more clearly defined by SAR and mutagenesis studies. Further structural modifications on the basis of 1 confirmed the importance of the N, N-diethylbenzamide group and the piperazine lower basic nitrogen for delta binding, in agreement with mutagenesis data. A number of piperazine N-alkyl substituents were tolerated. In contrast, modifications of the phenyl group led to the discovery of a series of diarylmethylpiperazines exemplified by N, N-diethyl-4-[1-piperazinyl(8-quinolinyl)methyl]benzamide (56) which had an improved in vitro binding profile (IC(50) = 0.5 nM, mu/delta = 1239, EC(50) = 3.6 nM) and increased in vitro metabolic stability compared to SNC-80.