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. Author manuscript; available in PMC: 2009 Jun 8.
Published in final edited form as: J Med Chem. 2006 Jul 13;49(14):4044–4047. doi: 10.1021/jm0601250

Discovery of Novel Triazole-Based Opioid Receptor Antagonists

Qiang Zhang 1,‡,†, Susan M Keenan 1,‡, Youyi Peng 1,‡, Anil C Nair 1,∥,#, Seong Jae Yu 1,∥,§, Richard D Howells 1,⊥, William J Welsh 1,*,‡
PMCID: PMC2693423  NIHMSID: NIHMS109984  PMID: 16821764

Abstract

We report the computer-aided design, chemical synthesis, and biological evaluation of a novel family of δ opioid receptor (DOR) antagonists containing a 1,2,4-triazole core structure that are structurally distinct from other known opioid receptor active ligands. Among those δ antagonists sharing this core structure, 8 exhibited strong binding affinity (Ki = 50 nM) for the DOR and appreciable selectivity for δ over μ and opioid receptors (δ/μ = 80; δ/κ > 200).

Opioid analgesics are the mainstay for treatment of moderate to severe pain. Research on opioids and their receptors has remained active over the past decade.1 Three opioid receptor subtypes, designated as δ, κ, and μ, have been identified in the central nervous system (CNS) and periphery2,3 and are products of three distinct and extensively studied genes. Recent evidence suggests that subtype-selective opioid receptor agonists and antagonists offer great potential as therapeutic agents devoid of the numerous adverse side effects (e.g., respiratory depression, physical dependence, and gastrointestinal effects) associated with morphine.4 In particular, δ-selective antagonists have been shown to modulate the development of tolerance5,6 and dependence on μ agonists such as morphine,7 to offset the behavioral effects of drugs of abuse such as cocaine,8 and to elicit favorable immunomodulatory9 and emotional effects.10 On the other hand, δ-selective agonists have been shown to elicit the prototypical analgesic effects of clinically available opioids.4 They may also provide unique benefits as cardioprotective and neuroprotective agents11 and as treatments for depression and anxiety.12,13

In view of their broad range of pharmacological applications, the δ-selective opioids have attracted interest in our laboratory and elsewhere. Given the paucity of high-quality X-ray crystal structure data for GPCRs such as the opioid receptor, our drug design strategy has relied on ligand-based molecular modeling approaches. An additional component of our drug discovery paradigm is the proprietary Shape Signatures computational tool that provides unique capabilities for scaffold hopping in the search for new lead compounds.14,15

A three-point pharmacophore was extracted by overlaying a series of high-affinity opioid receptor ligands including the δ-antagonist naltrindole.16 This pharmacophore model (Figure 1, gray) comprised the basic nitrogen atom, the centroid of the phenol ring (A), and the centroid of the hydrophobic ring (B). Virtual screening of an in-house database of ∼1.2 million commercially available small-molecule chemicals was conducted to identify structures matching this three-point pharmacophore. Additional molecular models were developed for a distinct series of DOR-selective agonists17 and antagonists18 to demonstrate the structural requirement for δ selectivity. Promising chemical entities were then subjected to filters using an expanded Lipinski rule of five19 hierarchical scheme. The substituted 1,2,4-triazoles (Figure 1) emerged from this scheme as an interesting core structural framework for our DOR active agents. In selecting appropriate substitution patterns for the 1,2,4-triazole ring to confer δ binding affinity and selectivity, we exploited the “message–address” concept20,21 associated with classical morphine-like opioids. For instance, a sterically bulky group (e.g., tert-butyl) was attached to the B aryl group to mimic the δ “address” in our 1,2,4-triazoles. Several di- and trisubstituted 1,2,4-triazoles (Table 1) were selected for chemical synthesis and biological evaluation. Structural alignment of naltrindole and 8 in the conformation adopted in its X-ray crystal structure reveals good overlap between the tert-butyl group of 8 and the δ “address” of naltrindole (Figure 2, Supporting Information).

Figure 1.

Figure 1

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Comparison of the structures of naltrindole and the present 1,2,4-triazoles.

Table 1.

Structures and Opioid Receptor Binding Affinities for Substituted 1,2,4-Triazoles

% inhibitiona
 Ki (nM)b
 selectivity ratio
compd R1 R2 R3 δ μ κ δ μ κ δ/μ δ/κ
1 3-OCH3 3-tert-butyl H 31 11 5 >10000 >10000 >10000 na
2 3-OH 3-tert-butyl H 91 78 66 230 850 1500 3.70 6.52
3 3-OCH3 4-tert-butyl H 8 9 8 >10000 >10000 >10000 na
4 3-OH 4-tert-butyl H 54 16 16 ∼10000 >10000 >10000 na
5 3-OH 3-phenyl H 84 84 42 140 1000 >10000 7.14 >71.4
6 3-OH 4-phenyl H 75 60 46 1500 >10000 >10000 >6.66 >6.66
7 3-OH 3,4-(CH=CH)2 H 68 48 27 2100 >10000 >10000 >4.76 >4.76
8 3-OH 4-tert-butyl N(CH3)2 94 32 6 50 4000 >10000 80 >200
9 3-OCH3 4-tert-butyl N(CH3)2 28 50 3 >10000 >10000 >10000 na
10 3-OH 3-tert-butyl N(CH3)2 76 19 6 1050 >10000 >10000 >9.5 >9.5
11 3-OH 3-phenyl N(CH3)2 86 23 8 150 >10000 >10000 >66.6 >66.6
12 3-OH 4-phenyl N(CH3)2 82 15 11 130 >10000 >10000 >76.9 >76.9
13 3-OH 3,4-(CH=CH)2 N(CH3)2 68 48 27 480 >10000 >10000 >20.8 >20.8
14 4-OH 4-tert-butyl N(CH3)2 27 8 21 >10000 >10000 >10000 na
15 3-OH 4-tert-butyl N(CH2CH2)2NCH3 18 5 12 >10000 >10000 >10000 na
16 3-OH 3-tert-butyl CH2N(CH3)2 65 27 10 2600 >10000 >10000 >3.86 >3.86
17 3-OH 4-tert-butyl CH2N(CH3)2 88 20 9 460 >10000 >10000 >21.6 >21.6
18 3-OH 4-tert-butyl (CH2)2N(CH3)2 90 23 19 900 >10000 >10000 >11.1 >11.1
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a

Compounds, initially screened at 10 μM, are expressed as percentage inhibition of the reference compound which is normalized to 100%. (−)-[9−3H]bremazocine was used as the radiolabeled ligand.

b

Inhibitory effect to (−)-[9−3H]bremazocine on membranes isolated from HEK 293 cells stably transfected with mouse (δ and μ) or rat (κ) opioid receptors. Ki values are the average of two to three independent experiments.

Figure 2.

Figure 2

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Up-regulation results of compounds 2 and 8.

Three separate reaction schemes were developed for the synthesis of the 1,2,4-triazoles (Scheme 1) with thioamides as key intermediates. In most cases, thioamides were synthesized from the corresponding amides.22,23 For 1, 2, 10, and 16, a coupling reaction of arylmagnesium reagents with isothiocyanates was conducted to synthesize the thioamides.24,25 Amidrazones could be efficiently prepared by reaction of thioamides with excess hydrazine at room temperature. Cyclization of amidrazones with different reagents led to products with methoxyl groups at the R2 positions. 1−7 were obtained using trimethyl orthoformate as the cyclization reagent,26 while 8−15 involved cyclization of amidrazones with phosgeninium salts (Viehe's salts), which were easily synthesized from the corresponding amines.27 Compounds 16 and 17 were synthesized from 1 and 3, respectively, through reaction with Eschenmoser's salt.28,29 Compound 18 was prepared directly by condensation of amidrazone with 3-N,N-dimethylaminopropionic acid hydrochloride in the presence of dicyclohexyldiimide (DCC) (Scheme 1). For most of the products, the final step of cleaving methoxyl groups at R2 was completed easily by reaction with BBr3 in dichloromethane. Where hydrolysis of the methoxyl group was incomplete using the above BBr3 procedure, excess NaSH was added to achieve ether cleavage in acceptable yields.

Scheme 1.

Scheme 1

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General Synthesis of Substituted 1,2,4-Triazolesa

Initially, 1−4 were synthesized to evaluate the feasibility of our approach (Table 1). Radioligand binding assays revealed that 2 binds to all three opioid receptors with Ki values of 230 (δ), 850 (μ), and 1500 (κ) nM, respectively. As anticipated, it exhibited some subtype selectivity for the δ over μ and κ opioid receptors. Structural analogues (5−18) were synthesized in order to increase the δ binding affinity and selectivity (Table 1). Several of the subject compounds (e.g., 5, 8, 11, 12) exhibited selectivity for the δ over μ and κ opioid receptors, which concurs with our initial design strategy to confer δ selectivity. The inhibitory activity was much greater at all three opioid receptors for compounds with R1 = OH (2 and 4) compared with R1 = OCH3 (1 and 3). In fact, the latter compounds showed very limited inhibitory activity for any of the opioid receptors even at 10 μM. Comparison of 8 and 14 indicates that the binding affinity for all three opioid receptor subtypes was virtually abolished when the hydroxyl substituent at R1 is moved from the meta to para position on the aromatic ring. Although this single example precludes making generalizations, the strong preference for the meta over para phenolic moiety is consistent with the familiar SAR of morphine-like opioids.30–33

For 1−7, R2 substitutions were preferred at the meta position over the para position (e.g., Ki(δ) = 230 nM for 2 vs ∼10 000 nM for 4). For compounds with R3 substitutions, namely 8−17, the opposite trend was observed in cases exhibiting an appreciable affinity difference (see 8 vs 10). Compound 8 (Ki(δ) = 50 nM), with R2 = p-tert-butyl and R3 = N(CH3)2, yielded the best results overall among this first generation of triazole-based opioid receptor active agents in terms of δ binding affinity and subtype selectivity. It is worth noting that introduction of groups more highly constrained than tert-butyl at R2 failed to increase binding affinity for the δ receptor. For example, the δ binding affinity was poorer for 11, 12 and 13 (Ki = 150, 130, and 480 nM, respectively) than for 8 (Ki = 50 nM).

The functional activity of our substituted 1,2,4-triazoles on the opioid receptors was determined by receptor up-regulation assays. Incubation of the δ opioid receptor with 2 and 8 produced a sharp increase in receptor expression, suggesting that the subject compounds are δ opioid antagonists (Figure 2). Interestingly, 8 exhibited >3-fold up-regulation of the δ opioid receptor in this assay. The pharmacological significance of this observation is currently under investigation in our laboratory.

In fact, a N,N-dimethylamino group at R3 did produce a sharp increase in binding affinity to the δ receptor. Compare, for example, the Ki(δ) of 8 (50 nM, R3 = N(CH3)2) with its simple homologue 4 (∼10 000 nM, R3 = H). One might reasonably attribute the greater activity of 8 over 4 to the strong basicity of the N atom at R3. Nevertheless, 8 is only slightly more basic than 4 (pKa(pred) = 3.36 vs 2.18).17 Ab initio quantum mechanical calculations on 8 at the HF/6−31G** level of theory, in vacuum and aqueous (implicit solvation) conditions, indicated that the most basic atom is not the N in R3 = N(CH3)2 (i.e., Nsub) but rather N1 or N2 in the triazole ring (Table 2). Among the four N atoms in 8, the rank of basicity is N1 ∼ N2 > Nsub > N4. These results suggest that Nsub is less basic than the triazole-ring atoms N1 and N2, although it should be restated that all of the N atoms in 8 are weakly basic. It is evident that the basicity of the N(CH3)2 group is mitigated by its strong conjugation with the triazole ring. One might suspect that disrupting this conjugation by extension of the substituent group would afford a basic N atom and thereby enhance binding affinity. However, 17 (R3 = CH2N(CH3)2) and 18 (R3 = (CH2)2N(CH3)2) showed >6-fold decrease in binding affinity to the DOR compared with 8.

Table 2.

Relative Energies of Protonationa Obtained from HF/6−31G** ab Initio Calculations on Compound 8 Assuming Vacuum and Aqueous Conditions

graphic file with name nihms-109984-f0004.jpg
protonation site vacuuma aqueousa
N1 1.07 0.00
N2 0.00 0.25
N4 56.85 53.28
Nsub 25.29 13.30
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a

In units of kcal/mol.

In conclusion, we report here a novel family of δ-selective opioid receptor antagonists containing the 1,2,4-triazole core structure. The subject compounds are chemically and structurally distinct from the classical opioids such as morphine and other known small-molecule opioids (e.g., (+)-4-[(α)R)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80)). Moreover, these compounds are synthetically accessible as pure compounds in high yield and, uncommon among opioids, lack chiral centers. Compound 8, the most active among this first generation of substituted 1,2,4-triazoles, exhibited strong binding affinity (Ki = 50 nM) and appreciable selectivity (selectivity ratio: δ/μ = 80; δ/κ > 200) for the δ opioid receptor. The weak basicity of 8 (pKa(pred) = 3.36) favors the neutral (unprotonated) form under physiological conditions (pH 7.4). Virtually all known opioids, whether agonists or antagonists, contain at least one basic N atom. The only exception to our knowledge is the agonist salvinorin A, a natural compound extracted from S. divinorum,34 and a series of cyclic peptides reported by Schiller et al.35 that act as δ and μ receptor antagonists. The present compounds thus represent the first nonpeptidic δ-selective opioid antagonists lacking a basic N atom.

Acknowledgment

Funding for this research was provided to W.J.W. by the Biotechnology Research & Development Corporation (Peoria, IL) and to R.D.H. by the National Institute on Drug Abuse (Grant DA09113). The authors also thank Dr. John Duchek of Tyco-Mallinckrodt, Inc. (St. Louis, MO) for fruitful discussions. Access to the computational facilities at the UMDNJ Informatics Institute, supported in part by the National Library of Medicine (Grant G08 LM6230−07), is also gratefully acknowledged.

Footnotes

Supporting Information Available: Experimental procedures for the synthesis of all new compounds, details on the molecular modeling and in vitro assays, and the X-ray crystal structure of 8 together with the crystallographic structural data. This material is available free of charge via the Internet at http://pubs.acs.org.

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