Enkephalin analogues with 2',6'-dimethylphenylalanine replacing phenylalanine in position 4

Stapling of leu-enkephalin analogs with bifunctional reagents for prolonged analgesic activity

The design and synthesis of leu-enkephalin analogs by replacing the glycine residues with N-(2-thioethyl)glycines and opening the cyclisation potential is presented. The cyclization (stapling) was achieved using bifunctional reagents (hexafluorobenzene and trithiocyanuric acid derivatives). The CD conformational studies of the stapled analogs suggest that the peptides adopt the type I β-turn conformation, which is in agreement with the theoretical analysis. The analog containing a trithiocyanuric acid derivative with a benzyl substituent shows potent analgesic activity.

(L)-Monomethyl Tyrosine (Mmt): New Synthetic Strategy via Bulky 'Forced-Traceless' Regioselective Pd-Catalyzed C(sp2)-H Activation

The enormous influence in terms of bioactivity, affinity, and selectivity represented by the replacement of (L)-2,6-dimethyl tyrosine (Dmt) instead of Phenylalanine (Phe) into Nociceptin/orphanin (N/OFQ) neuropeptide analogues has been well documented in the literature. More recently, the non-natural amino acid (L)-2-methyl tyrosine (Mmt), with steric hindrance included between Tyr and Dmt, has been studied because of the modulation of steric effects in opioid peptide chains. Here, we report a new synthetic strategy to obtain Mmt based on the well-known Pd-catalyzed ortho-C(sp2)-H activation approach, because there is a paucity of other synthetic routes in the literature to achieve it. The aim of this work was to force only the mono-ortho-methylation process over the double ortho-methylation one. In this regard, we are pleased to report that the introduction of the dibenzylamine moiety on a Tyr aromatic nucleus is a convenient and traceless solution to achieve such a goal. Interestingly, our method provided the aimed Mmt either as N-Boc or N-Fmoc derivatives ready to be inserted into peptide chains through solid-phase peptide synthesis (SPPS). Importantly, the introduction of Mmt in place of Phe1 in the sequence of N/OFQ(1-13)-NH2 was very well tolerated in terms of pharmacological profile and bioactivity.

An investigation of the polymorphism of a potent nonsteroidal anti-inflammatory drug flunixin

Flunixin, a potent nonsteroidal anti-inflammatory drug widely used in veterinary medicine, was found to exist in at least two crystal forms, in contrast to clonixin which exists in four solvent-free forms and multiple solvates.

Developmental potential for endomorphin opioidmimetic drugs

Morphine, which is agonist for μ-opioid receptors, has been used as an anti-pain drug for millennia. The opiate antagonists, naloxone and naltrexone, derived from morphine, were employed for drug addiction and alcohol abuse. However, these exogenous agonists and antagonists exhibit numerous and unacceptable side effects. Of the endogenous opioid peptides, endomorphin(EM)-1 and endomorphin(EM)-2 with their high μ-receptor affinity and exceptionally high selectivity relative to δ- and κ-receptors in vitro and in vivo provided a sufficiently sequence-flexible entity in order to prepare opioid-based drugs. We took advantage of this unique feature of the endomorphins by exchanging the N-terminal residue Tyr¹ with 2′,6′-dimethyl-l-tyrosine (Dmt) to increase their stability and the spectrum of bioactivity. We systematically altered specific residues of [Dmt¹]EM-1 and [Dmt¹]EM-2 to produce various analogues. Of these analogues, [N-allyl-Dmt¹]EM-1 (47) and [N-allyl-Dmt¹]EM-2 (48) exhibited potent and selective antagonism to μ-receptors: they completely inhibited naloxone- and naltrexone-induced withdrawal from following acute morphine dependency in mice and reversed the alcohol-induced changes observed in sIPSC in hippocampal slices. Overall, we developed novel and efficacious opioid drugs without deleterious side effects that were able to resist enzymatic degradation and were readily transported intact through epithelial membranes in the gastrointestinal tract and the blood-brain-barrier.

2',6'-dimethylphenylalanine: a useful aromatic amino Acid surrogate for tyr or phe residue in opioid peptides

Two aromatic amino acids, Tyr(1) and Phe(3) or Phe(4), are important structural elements in opioid peptides because they interact with opioid receptors. The usefulness of an artificial amino acid residue 2',6'-dimethylphenylalanine (Dmp) was investigated as an aromatic amino acid surrogate for several opioid peptides, including enkephalin, dermorphin, deltorphin, endomorphin, dynorphin A, and nociceptin peptides. In most peptides, substitutions of Phe(3) by a Dmp residue produced analogs with improved receptor-binding affinity and selectivity, while the same substitution of Phe(4) induced markedly reduced receptor affinity and selectivity. Interestingly, replacement of Tyr(1) by Dmp produced analogs with unexpectedly high affinity or produced only a slight drop in receptor affinity and bioactivity for most peptides. Thus, Dmp is also a useful surrogate for the N-terminal Tyr residue in opioid peptides despite the lack of a phenolic hydroxyl group, which is considered necessary for opioid activity. The Dmp(1)-substituted analogs are superior to 2',6'-dimethyltyrosine (Dmt)(1)-substituted analogs for high receptor selectivity since the latter generally have poor receptor selectivity. Thus, Dmp is very useful as an aromatic amino acid surrogate in opioid peptides and may be useful for developing other novel peptide mimetics with high receptor specificity.

Bifunctional [2',6'-dimethyl-L-tyrosine1]endomorphin-2 analogues substituted at position 3 with alkylated phenylalanine derivatives yield potent mixed mu-agonist/delta-antagonist and dual mu-agonist/delta-agonist opioid ligands

Endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2) and [Dmt1]EM-2 (Dmt = 2',6'-dimethyl-l-tyrosine) analogues, containing alkylated Phe3 derivatives, 2'-monomethyl (2, 2'), 3',5'- and 2',6'-dimethyl (3, 3', and 4', respectively), 2',4',6'-trimethyl (6, 6'), 2'-ethyl-6'-methyl (7, 7'), and 2'-isopropyl-6'-methyl (8, 8') groups or Dmt (5, 5'), had the following characteristics: (i) [Xaa3]EM-2 analogues exhibited improved mu- and delta-opioid receptor affinities. The latter, however, were inconsequential (Kidelta = 491-3451 nM). (ii) [Dmt1,Xaa3]EM-2 analogues enhanced mu- and delta-opioid receptor affinities (Kimu = 0.069-0.32 nM; Kidelta = 1.83-99.8 nM) without kappa-opioid receptor interaction. (iii) There were elevated mu-bioactivity (IC50 = 0.12-14.4 nM) and abolished delta-agonism (IC50 > 10 muM in 2', 3', 4', 5', 6'), although 4' and 6' demonstrated a potent mixed mu-agonism/delta-antagonism (for 4', IC50mu = 0.12 and pA2 = 8.15; for 6', IC50mu = 0.21 nM and pA2 = 9.05) and 7' was a dual mu-agonist/delta-agonist (IC50mu = 0.17 nM; IC50delta = 0.51 nM).

ORL1 and opioid receptor preferences of nociceptin and dynorphin A analogues with Dmp substituted for N-terminal aromatic residues

Nociceptin (NOC) and dynorphin A (DYN) analogues containing 2',6'-dimethylphenylalanine (Dmp) in place of Phe or Tyr in position 1 and/or 4 were synthesized and their metabolic stability and receptor-binding properties were investigated. [Dmp1]NOC(1-13)-NH2 (1) possessed high ORL1 receptor affinity comparable to that of the parent peptide with substantially improved affinities for kappa-, mu-, and delta-opioid receptors. However, Dmp4 substitution of NOC peptide (2) reduced ORL1 receptor affinity. [Dmp1]DYN(1-13)-NH2 (4) and its Dmp4 analogue (5) possessed a 3-fold greater kappa-opioid receptor affinity and improved kappa-receptor selectivity compared to the parent peptide. Analogue 4 however exhibited an unexpectedly low in vitro bioactivity (GPI assay), suggesting, the phenolic hydroxyl group at the N-terminal residue in DYN peptide is extremely important for activation of the kappa-opioid receptor. Analogue 5 possessed an improved kappa-opioid receptor selectivity with an IC50 ratio of 1(kappa)/509(mu)/211598(delta); thus, this peptide may serve as a highly selective kappa-receptor agonist for pharmacological study. Dmp1 substitution in both the NOC and DYN peptides improved metabolic stability toward these peptides, while Dmp4 substitution provided no additional metabolic stability.

Enantioselective Synthesis of a Phenylalanine Library Containing Alkyl Groups on the Aromatic Moiety: Confirmation of Stereostructure by X-Ray Analysis

Six phenylalanine analogues containing 2'-methyl-, 2',6'-dimethyl-, 2'-ethyl-6'-methyl-, 2'-isopropyl-6'-methyl-, 2',4',6'-trimethyl-, and 3',5'-dimethyl-L-phenylalanine were synthesized enantioselectively through asymmetric hydrogenation of acetamidoacrylate derivatives. Enzymatic digestion and X-ray analysis supported the L-configuration of the phenylalanine derivatives obtained.

Dmt and opioid peptides: a potent alliance

The introduction of the Dmt (2',6'-dimethyl-L-tyrosine)-Tic pharmacophore into the design of opioid ligands produced an extraordinary family of potent delta-opioid receptor antagonists and heralded a new phase in opioid research. First reviewed extensively in 1998, the incorporation of Dmt into a diverse group of opioid molecules stimulated the opioid field leading to the development of unique analogues with remarkable properties. This overview will document the crucial role played by this residue in the proliferation of opioid peptides with high receptor affinity (K(i) equal to or less than 1 nM) and potent bioactivity. The discussion will include the metamorphosis between delta-opioid receptor antagonists to delta-agonists based solely on subtle structural changes at the C-terminal region of the Dmt-Tic pharmacophore as well as their behavior in vivo. Dmt may be considered promiscuous due to the acquisition of potent mu-agonism by dermorphin and endomorphin derivatives as well as by a unique class of opioidmimetics containing two Dmt residues separated by alkyl or pyrazinone linkers. Structural studies on the Dmt-Tic compounds were enhanced tremendously by x-ray diffraction data for three potent and biologically diverse Dmt-Tic opioidmimetics that led to the development of pharmacophores for both delta-opioid receptor agonists and antagonists. Molecular modeling studies of other unique Dmt opioid analogues illuminated structural differences between delta- and mu-receptor ligand interactions. The future of these compounds as therapeutic applications for various medical syndromes including the control of cancer-associated pain is only a matter of time and perseverance.

2',6'-Dimethylphenylalanine (Dmp) Can Mimic the N-Terminal Tyr in Opioid Peptides

Substitution of 2',6'-dimethyltyrosine (Dmt) for the N-terminal Tyr in opioid peptides has recently been shown to be a promising tool for improving opioid receptor affinity and biological activity. We have also demonstrated that another unnatural amino acid, 2',6'-dimethylphenylalanine (Dmp), is not only an excellent substitute for Phe at position 3 but also can mimic the aromatic N-terminal Tyr residue in a micro opioid receptor-selective dermorphin analogue (YRFB: Tyr-D-Arg-Phe-betaAla-NH(2)). To further evaluate the value of Dmp in opioid peptides, we investigated Dmp(1)-substituted analogues of the delta receptor ligands, deltorphin II (DLT: Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH(2)) and enkephalin (ENK: Tyr-Gly-Gly-Phe-Leu). In the receptor binding assay, both [Dmp(1)]DLT and [Dmp(1)]ENK bound to the delta-receptor with high affinity and selectivity, and were nearly as effective as the parent peptides. The potency of the Dmp(1)-peptides on the MVD and GPI assays correlated well with the receptor binding affinity data. These results are in contrast to the tendency of corresponding Dmt(1)-analogues to have poor receptor selectivity. Taken together with the results with YRFB, we conclude that the Dmp(1)-peptide is superior to the corresponding Dmt(1)-peptide in its receptor selectivity. [Dmp(1)]DLT and [Dmp(1)]YRFB may serve as pharmacological tools for the studies of ligand recognition and opioid receptor signal transduction.

Dermorphin tetrapeptide analogues with 2',6'-dimethylphenylalanine (Dmp) substituted for aromatic amino acids have high mu opioid receptor binding and biological activities

To investigate the value of the 2',6'-dimethylphenylalanine (Dmp) residue as an aromatic amino acid substitution, we prepared analogues of the mu opioid receptor-selective dermorphin tetrapeptide Tyr-D-Arg-Phe-betaAla-NH(2) (YRFB) in which Dmp or its D-isomer replaced Tyr(1) or Phe(3). Replacing Phe(3) with Dmp essentially tripled mu receptor affinity and the receptor's in vitro biological activities as determined with the guinea pig ileum (GPI) assay but did not change delta receptor affinity. Despite an inversion of the D configuration at this position, mu receptor affinity and selectivity remained comparable with those of the L-isomer. Replacing the N-terminal Tyr residue with Dmp produced a slightly improved mu receptor affinity and a potent GPI activity, even though the substituted compound lacks the side chain phenolic hydroxyl group at the N-terminal residue. Dual substitution of Dmp for Tyr(1) and Phe(3) produced significantly improved mu receptor affinity and selectivity compared with the singly substituted analogues. Subcutaneous injection of the two analogues, [Dmp(3)]YRFB and [Dmp(1)]YRFB, in mice produced potent analgesic activities that were greater than morphine in the formalin test. These lines of evidence suggest that the Dmp residue would be an effective aromatic amino acid surrogate for both Tyr and Phe in the design and development of novel opioid mimetics.

Endomorphin 2 analogues containing Dmp residue as an aromatic amino acid surrogate with high mu-opioid receptor affinity and selectivity

To investigate the effectiveness of a 2',6'-dimethylphenylalanine (Dmp) residue as an aromatic amino acid surrogate, endomorphin 2 (EM(2): Tyr-Pro-Phe-Phe-NH(2)) analogues were prepared, in which the constitutive aromatic amino acids (Tyr(1), Phe(3), or Phe(4)) were replaced by Dmp or its isomer, D-Dmp. Replacement of Phe(3) by Dmp increased the affinity over 10-fold for both mu- and delta-opioid receptors, without affecting receptor selectivity. In contrast, replacement of Phe(4) considerably reduced the mu-receptor affinity and selectivity. These data indicated that the Dmp-substitution of Phe(3), but not Phe(4), in EM(2) is favorable for improving mu-receptor specificity. Inversion of the chirality of the substituted Dmp residue resulted in marked decrease in the mu-receptor affinity. Replacement of Tyr(1) by Dmp yielded an analogue that exhibited only a limited decrease in mu-receptor affinity and GPI potency, despite the lack of a phenolic hydroxyl group at the N-terminal residue. In contrast, D-Dmp(1)- or Phe(1)-substitution of Tyr(1) resulted in a significant decrease in mu-receptor affinity and GPI potency. These results suggested that the Dmp residue can mimic Tyr(1), which is one of the critical structural elements of opioid peptides.

Dermorphin and deltorphin heptapeptide analogues: replacement of Phe residue by Dmp greatly improves opioid receptor affinity and selectivity

The usefulness of 2,6-dimethylphenylalanine (Dmp) as a Phe surrogate in two opioid peptides, dermorphin (DM) and deltorphin II (DT), was investigated. Compared to DM, [L-Dmp(3)]DM (1) showed a 170-fold increase in mu affinity and only a 4-fold increase in delta affinity, resulting in a 40-fold improvement in mu receptor selectivity. Compared to DT, [L-Dmp(3)]DT (3) showed a 22-fold increase in delta affinity and somewhat of a loss in mu affinity, and consequently a marked (75-fold) improvement in delta receptor selectivity. The D-Dmp replacement, however, resulted in a great loss in receptor selectivity in each of the peptides. The specific receptor interactions of 1 and 3 were confirmed by in vitro bioassays. Analogues 1 and 3 seem to be useful as pharmacological tools for the study of opioid systems.