Replacement of the N-terminal Tyrosine Residue in Opioid Peptides with 3-(2,6-Dimethyl-4-carbamoylphenyl)propanoic Acid (Dcp) Results in Novel Opioid Antagonists

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.

Cyclic Opioid Peptides

For decades the opioid receptors have been an attractive therapeutic target for the treatment of pain. Since the first discovery of enkephalin, approximately a dozen endogenous opioid peptides have been known to produce opioid activity and analgesia, but their therapeutics have been limited mainly due to low blood brain barrier penetration and poor resistance to proteolytic degradation. One versatile approach to overcome these drawbacks is the cyclization of linear peptides to cyclic peptides with constrained topographical structure. Compared to their linear parents, cyclic analogs exhibit better metabolic stability, lower offtarget toxicity, and improved bioavailability. Extensive structure-activity relationship studies have uncovered promising compounds for the treatment of pain as well as further elucidate structural elements required for selective opioid receptor activity. The benefits that come with employing cyclization can be further enhanced through the generation of polycyclic derivatives. Opioid ligands generally have a short peptide chain and thus the realm of polycyclic peptides has yet to be explored. In this review, a brief history of designing ligands for the opioid receptors, including classic linear and cyclic ligands, is discussed along with recent approaches and successes of cyclic peptide ligands for the receptors. Various scaffolds and approaches to improve bioavailability are elaborated and concluded with a discourse towards polycyclic peptides.

Evaluation of the Antihyperuricemic Activity of Phytochemicals from Davallia formosana by Enzyme Assay and Hyperuricemic Mice Model

Abnormal serum urate levels are recognized as a critical factor in the progression of several chronic diseases. To evaluate the antihyperuricemic effect of Davallia formosana, the inhibitory activities of 15 isolated phytochemicals, including five novel compounds of 6,8-dihydroxychromone-7-C-β-d-glucopyranoside (1), 6,8,3′,4′-tetrahydroxyflavanone-7-C-β-d-glucopyranoside (2), 6,8,4′-trihydroxyflavanone-7-C-β-d-glucopyranoside (3), 8-(2-pyrrolidinone-5-yl)-catechin-3-O-β-d-allopyranoside (4), and epiphyllocoumarin-3-O-β-d-allopyranoside (5), were examined against xanthine oxidase (XOD) and in a potassium oxonate-(PTO-) induced acute hyperuricemic mice model. The results indicated that compounds 3 and 5 significantly inhibited XOD activity in vitro and reduced serum uric acid levels in vivo. This is the first report providing new insights into the antihyperuricemic activities of flavonoid glycosides which can possibly be developed into potential hypouricemic agents.

Novel opioid peptide derived antagonists containing (2S)-2-methyl-3-(2,6-dimethyl-4-carbamoylphenyl)propanoic acid [(2S)-Mdcp]

A synthesis of the novel tyrosine analogue (2 S)-2-methyl-3-(2,6-dimethyl-4-carbamoylphenyl)propanoic acid [(2 S)-Mdcp] (15) was developed. In (2 S)-Mdcp, the amino and hydroxyl groups of 2',6'-dimethyltyrosine are replaced by a methyl and a carbamoyl group, respectively, and its substitution for Tyr (1) in opioid agonist peptides resulted in compounds showing antagonism at all three opioid receptors. The cyclic peptide (2 S)-Mdcp-c[D-Cys-Gly-Phe(pNO 2)-D-Cys]NH 2 (1) was a potent and selective mu antagonist, whereas (2 S)-Mdcp-c[D-Pen-Gly-Phe(pF)-Pen]-Phe-OH (3) showed subnanomolar delta antagonist activity and extraordinary delta selectivity.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Further studies of tyrosine surrogates in opioid receptor peptide ligands

A series of opioid peptide ligands containing modified N-terminal tyrosine (Tyr) residues was prepared and evaluated against cloned human mu, delta, and kappa opioid receptors. This work extends the recent discovery that (S)-4-carboxamidophenylalanine (Cpa) is an effective tyrosine bioisostere. Amino acids containing negatively charged functional groups in place of tyrosine's phenolic hydroxyl lacked receptor affinity, while exchange of Tyr for (S)-4-aminophenylalanine was modestly successful. Peptides containing the new amino acids, (S)-4-carboxamido-2,6-dimethylphenylalanine (Cdp) and (S)-beta-(2-aminobenzo[d]thiazol-6-yl)alanine (Aba), displayed binding (K(i)) and functional (EC(50)) profiles comparable to the parent ligands at the three receptors. Cdp represents the best performing Tyr surrogate in terms of overall activity, while Cpa and Aba show a subtle proclivity toward the delta receptor.