Novel C-terminus modifications of the Dmt-Tic motif: a new class of dipeptide analogues showing altered pharmacological profiles toward the opioid receptors

The design, synthesis and pharmacological evaluation of a novel class of Dmt-Tic dipeptide analogues are described. These resulting analogues bearing different C-terminal functionalities were found to bind to the human delta receptor with high affinity. One specific class of dipeptides bearing urea/thiourea functionalities showed partial to full activation of the delta receptor. Several dipeptides also showed good binding affinities with full activation of the human kappa receptor, a novel property for those ligands.

N,N-Diethyl-4-(phenylpiperidin-4-ylidenemethyl)benzamide: a novel, exceptionally selective, potent delta opioid receptor agonist with oral bioavailability and its analogues

The design, synthesis, and pharmacological evaluation of a novel class of delta opioid receptor agonists, N, N-diethyl-4-(phenylpiperidin-4-ylidenemethyl)benzamide (6a) and its analogues, are described. These compounds, formally derived from SNC-80 (2) by replacing the piperazine ring with a piperidine ring containing an exocyclic carbon carbon double bond, were found to bind with high affinity and exhibit excellent selectivity for the delta opioid receptor as full agonists. 6a, the simplest structure in the class, exhibited an IC(50) = 0.87 nM for the delta opioid receptors and extremely high selectivity over the mu receptors (mu/delta = 4370) and the kappa receptors (kappa/delta = 8590). Rat liver microsome studies on a selected number of compounds show these olefinic piperidine compounds (6) to be considerably more stable than SNC-80. This novel series of compounds appear to interact with delta opioid receptors in a similar way to SNC-80 since they demonstrate similar SAR. Two general approaches have been established for the synthesis of these compounds, based on dehydration of benzhydryl alcohols (7) and Suzuki coupling reactions of vinyl bromide (8), and are herewith reported.

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.

Inverse agonism by Dmt-Tic analogues and HS 378, a naltrindole analogue

The potent delta-opioid receptor antagonist H-2',6-L-tyrosine(Dmt)-1, 2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic-OH) exhibited partial inverse agonism (EC(50)=6.35 nM, E(max)=-18.87%) for [35S]GTPgammaS binding and H-Dmt-Tic-NH(2) was a neutral antagonist (no effect up to 30 microM). In contrast N,N(CH(3))(2)-Dmt-Tic-NH(2) was a full inverse agonist (EC(50)=2.66 nM, E(max)=-35.95%) similar to ICI 174864 ([N,N-diallyl-Tyr(1),Aib(2,3),Leu(5)]enkephaline) but with a 3.5-fold higher EC(50). In comparison, naltrindole was a neutral antagonist while its analogue HS 378 was a partial inverse agonist (E(max)=-12.99%).

Novel Dmt-Tic dipeptide analogues as selective delta-opioid receptor antagonists

A series of Dmt-Tic analogues with substitution on the Tic aromatic ring has been synthesized and evaluated for opioid receptor affinity and activation. Incorporation of large hydrophobic groups at position 7 of Tic did not greatly alter the delta opioid receptor binding affinities of the dipeptides whereas substitution at position 6 substantially diminished their affinity. These modified Dmt-Tic peptides showed binding affinities as low as 2.5 nM with up to 500-fold selectivity for the delta versus mu opioid receptor and proved to be delta receptor antagonists.

Characterization of [125I]AR-M100613, a high-affinity radioligand for delta opioid receptors

AR-M100613 ([I]-Dmt-c[-D-Orn-2-Nal-D-Pro-D-Ala-]) is the iodinated analog of a cyclic casomorphin previously shown to be a potent antagonist at the delta opioid receptor. Specific [125I]AR-M100613 binding to rat whole brain membranes was saturable, reversible, and best fit to a one-site model (Kd = 0.080 +/- 0.008 nM, Bmax = 45.2 +/- 4.4 fmol/mg protein). [125I]AR-M100613 binding was displaced with high affinity by the delta opioid receptor ligands SNC-80, Deltorphin II and DPDPE but not the mu or kappa-selective receptor ligands DAMGO and U69593. Residual non-selective binding of [125I]AR-M 100613 to mu opioid receptors is blocked by the addition of CTOP to the assay buffer. [35S]GTPgammaS binding assays indicate that AR-M100613 is a potent, selective, and reversible antagonist for delta opioid receptors in rat brain membranes. The high-affinity, high specific activity, low nonspecific binding and antagonist profile of [125I]AR-M100613 favor its use as a radiochemical probe for delta opioid receptors.

Design, synthesis, and evaluation of Phe-Gly mimetics: heterocyclic building blocks for pseudopeptides

Enantiopure heterocyclic Boc-protected Phe-Gly dipeptidomimetics containing 1,3,4-oxadiazole, 1,2,4-oxadiazole, and 1,2,4-triazole ring systems have been synthesized as building blocks in the synthesis of pseudopeptides. Three derivatives (1-3) have the carboxylic acid function directly bound to the heterocyclic ring, and three derivatives (4-6) have an extra methylene group between the heterocyclic ring and the acid function to allow for an increased conformational flexibility. The mimetics were used as Phe-Gly replacements in the biologically active peptides dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH(2)) and substance P (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-MetNH(2), SP). The pseudopeptide synthesis was performed using solid-phase methodology on a MBHA-resin using Boc-chemistry. The biological evaluation was performed by testing the micro- and delta-opioid receptor affinities of the dermorphin pseudopeptides and the NK(1) receptor affinities of the SP pseudopeptides. The results showed that all mimetics except 3 were excellent replacements of Phe-Gly in dermorphin since they displayed affinities for the micro-receptor (IC(50) = 12-31 nM) in the same range as dermorphin itself (IC(50) = 6.2 nM). The agonist activity of three pseudopeptides at human micro-receptors was also evaluated. It was shown that the tested compounds retained their agonist activity. The SP pseudopeptides showed considerably lower affinities (IC(50) > 1 microM) for the NK(1) receptor than SP itself (IC(50) = 1.5 nM) indicating that the Phe-Gly replacements prevent the pseudopeptides from adopting bioactive conformations.