Synthesis and opioid activity of [D-Pro10]dynorphin A-(1-11) analogues with N-terminal alkyl substitution

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.

Addiction and the cerebellum with a focus on actions of opioid receptors

Increasing evidence suggests that the cerebellum could play a role in the higher cognitive processes involved in addiction as the cerebellum contains anatomical and functional pathways to circuitry controlling motivation and saliency. In addition, the cerebellum exhibits a widespread presence of receptors, including opioid receptors which are known to play a prominent role in synaptic and circuit mechanisms of plasticity associated with drug use and development of addiction to opioids and other drugs of abuse. Further, the presence of perineural nets (PNNs) in the cerebellum which contain proteins known to alter synaptic plasticity could contribute to addiction. The role the cerebellum plays in processes of addiction is likely complex, and could depend on the particular drug of abuse, the pattern of use, and the stage of the user within the addiction cycle. In this review, we discuss functional and structural modifications shown to be produced in the cerebellum by opioids that exhibit dependency-inducing properties which provide support for the conclusion that the cerebellum plays a role in addiction.

Dynorphin A analogs for the treatment of chronic neuropathic pain

Chronic pain is one of the most ubiquitous diseases in the world, but treatment is difficult with conventional methods, due to undesirable side effects of treatments and unknown mechanisms of pathological pain states. The endogenous peptide, dynorphin A has long been established as a target for the treatment of pain. Interestingly, this unique peptide has both inhibitory (opioid in nature) and excitatory activities (nonopioid) in the CNS. Both of these effects have been found to play a role in pain and much work has been done to develop therapeutics to enhance the inhibitory effects. Here we will review the dynorphin A compounds that have been designed for the modulation of pain and will discuss where the field stands today.

Structure-Activity Relationships of the Peptide Kappa Opioid Receptor Antagonist Zyklophin

The dynorphin (Dyn) A analogue zyklophin ([N-benzyl-Tyr(1)-cyclo(d-Asp(5),Dap(8))]dynorphin A(1-11)NH2) is a kappa opioid receptor (KOR)-selective antagonist in vitro, is active in vivo, and antagonizes KOR in the CNS after systemic administration. Hence, we synthesized zyklophin analogues to explore the structure-activity relationships of this peptide. The synthesis of selected analogues required modification to introduce the N-terminal amino acid due to poor solubility and/or to avoid epimerization of this residue. Among the N-terminal modifications, the N-phenethyl and N-cyclopropylmethyl substitutions resulted in analogues with the highest KOR affinities. Pharmacological results for the alanine-substituted analogues indicated that Phe(4) and Arg(6), but interestingly not the Tyr(1) phenol, are important for zyklophin's KOR affinity and that Arg(7) was important for KOR antagonist activity. In the GTPγS assay, while all of the cyclic analogues exhibited negligible KOR efficacy, the N-cyclopropylmethyl-Tyr(1) and N-benzyl-Phe(1) analogues were 28- and 11-fold more potent KOR antagonists, respectively, than zyklophin.

The effects of C-terminal modifications on the opioid activity of [N-benzylTyr(1)]dynorphin A-(1-11) analogues

Structural modifications affecting the efficacy of analogues of the endogenous opioid peptide dynorphin (Dyn) A have focused on the N-terminal "message" sequence based on the "message-address" concept. To test the hypothesis that changes in the C-terminal "address" domain could affect efficacy, modified amino acids and cyclic constraints were incorporated into this region of the partial agonist [N-benzylTyr(1)]Dyn A-(1-11). Modifications in the C-terminal domain of [N-benzylTyr(1)]Dyn A-(1-11)NH(2) resulted in increased kappa opioid receptor (KOR) affinity for all of the linear analogues but did not affect the efficacy of these peptides at KOR. Cyclization between positions 5 and 8 yielded [N-benzylTyr(1),cyclo(d-Asp(5),Dap(8))]Dyn A-(1-11)NH(2) (zyklophin, 13) ( J. Med. Chem. 2005 , 48 , 4500 - 4503 ) with high selectivity for KOR. In contrast to the linear peptides, this peptide exhibits negligible efficacy in the adenylyl cyclase (AC) assay and is a KOR antagonist. These data are consistent with our hypothesis that appropriate modifications in the "address" domain of Dyn A analogues may affect efficacy.

Peptide kappa opioid receptor ligands: potential for drug development

While narcotic analgesics such as morphine, which act preferentially through mu opioid receptors, remain the gold standard in the treatment of severe pain, their use is limited by detrimental liabilities such as respiratory depression and drug dependence. Thus, there has been considerable interest in developing ligands for kappa opioid receptors (KOR) as potential analgesics and for the treatment of a variety of other disorders. These include effects mediated both by central receptors, such as antidepressant activity and a reduction in cocaine-seeking behavior, and activity resulting from the activation of peripheral receptors, such as analgesic and anti-inflammatory effects. While the vast majority of opioid receptor ligands that have progressed in preclinical development have been small molecules, significant advances have been made in recent years in identifying opioid peptide analogs that exhibit promising in vivo activity. This review will focus on possible therapeutic applications of ligands for KOR and specifically on the potential development of peptide ligands for these receptors.

[Nα-BenzylTyr,cyclo(d-Asp5,Dap8)]- dynorphin A-(1−11)NH2 Cyclized in the “Address” Domain Is a Novel κ-Opioid Receptor Antagonist

The cyclic dynorphin A analogue [N(alpha)-benzylTyr(1),cyclo(D-Asp(5),Dap(8))]dynorphin A-(1-11)NH(2) (Dap = 2,3-diaminopropionic acid) exhibits nanomolar affinity (30 nM) and high selectivity (K(i) ratio (kappa/mu/delta) = 1/194/330) for kappa-opioid receptors. This analogue antagonizes dynorphin A-(1-13)NH(2) at kappa-opioid receptors in the adenylyl cyclase assay (K(B) = 84 nM). This is the first dynorphin A-based antagonist with modifications in the C-terminal "address" domain that alter efficacy and thus represents a novel selective kappa-opioid receptor antagonist.

Synthesis of novel basic head-to-side-chain cyclic dynorphin A analogs: Strategies and side reactions

Novel N-terminus-to-side-chain cyclic analogs of the opioid peptide dynorphin (Dyn) A-(1-11)NH(2) were prepared that retain the basicity of the N-terminal amine and restrict the backbone conformation around the important Tyr(1) residue. Cyclic peptides were synthesized in which the N-terminal amine and the N(epsilon)-amine of a Lys at position 3 or 5 were attached to the alpha-carbon and carbonyl of an acetyl group, respectively. Several synthetic strategies were explored with detailed analysis of the side reactions in order to obtain the desired cyclic peptides. One of the side reactions observed involved premature loss of the N-terminal 9-fluorenylmethoxycarbonyl (Fmoc) group during the neutralization step following deprotection of the Mtt (4-methyltrityl) protecting group from the side chain of Lys. The successful strategy involved the synthesis of the linear peptide up through Gly(2) and functionalization through the N(epsilon)-amine of Lys. A linear N-terminal alkylated analog was prepared by alkylation of the peptide on the resin with an equimolar amount of bromoacetamide, followed by treatment of the peptide with Fmoc-OSu prior to cleavage from the resin to facilitate separation by reversed phase high performance liquid chromatography of unreacted peptide from the desired alkylated product. The novel N-terminal cyclic Dyn A analogs and the linear analog were evaluated for their opioid receptor affinities. These peptides exhibited large losses in affinity for opioid receptors; the low affinity of the linear N-terminal alkylated peptide suggested that the alpha-acetamide group on the N-terminal amine resulted in unfavorable interactions with opioid receptors.

Structure-activity relationships of dynorphin analogs substituted in positions 2 and 3

Following up on the observation that the dynorphin analog [Pro(3)]Dyn A(1-11)-NH(2) 2 possesses high affinity and selectivity for the kappa opioid receptor, a number of related peptides were prepared and characterized by radioligand binding and [(35)S]GTPgammaS assays. While incorporation of 2-azetidine carboxylic acid in position 3 led to the equally potent analog 3, the corresponding analog containing piperidine-2-carboxylic acid showed a nearly 90-fold reduction in kappa affinity. Differential preferred bond angles phi in the three building blocks might account for these observations. Compounds 2 and 3 were kappa antagonists with IC(50) values of 380 and 350 nM, respectively. The Sar(3) analog 7 and the Sar(2) analog 8 were kappa agonists, with greater selectivity than Dyn A(1-11)-NH(2) 1. In view of their high kappa affinities (8: K(i) = 1.5 nM; 2: K(i) = 2.4 nM), the new analogs were surprisingly weak kappa agonists or antagonists, e.g., the EC(50) value for the agonist 8 was 280 nM. Different kappa receptor subtypes in binding vs functional assays can not account for these results, since both assays were performed using the same membrane preparation.

Structure-activity relationships of dynorphin a analogues modified in the address sequence

The peptide [Pro3]Dyn A(1-11)-NH2 2 exhibits high affinity (K(i) = 2.4 nM) and over 2000-fold selectivity for the opioid receptor. Stepwise removal of the C-terminal residues from this ligand demonstrated that its positively charged Arg residues, particularly Arg6 and Arg7, were crucial for binding to the kappa receptor. Analogues shorter than seven amino acids lacked significant affinity for opioid receptors. Comparison with a series of truncated analogues of Dyn A showed that the relative losses in binding potency differed only slightly between the two series. The neutral residues Ile8 and Pro10 could be removed without significant loss in affinity for the kappa receptor. Their replacement, in the Pro3 analogue, with additional Arg residues led to analogues with improved kappa affinity (e.g., [Pro3,Arg8]Dyn A(1-11)-NH2 20: K(i)(kappa) = 0.44 nM). This type of modification did not compromise the high kappa selectivity of the Pro3 analogues. These findings support the view that a negatively charged domain in the putative second extracellular loop of the kappa receptor selectively recognizes residues 6-11 of dynorphin through electrostatic interactions. As with parent compound 2, analogue 20 and related compounds displayed kappa antagonist properties.

A novel N-terminal cyclic dynorphin A analogue cyclo(N,5)[Trp(3),Trp(4),Glu(5)] dynorphin A-(1-11)NH(2) that lacks the basic N-terminus

A novel N-terminal-to-side chain cyclic dynorphin A analogue lacking the basic N-terminus was designed based on Ac[Lys(2),Trp(3),Trp(4),d-Ala(8)]dynorphin A-(1-11)NH(2) (Wan et al. J. Med. Chem. 1999, 42, 3011-3013). cyclo(N,5)[Trp(3),Trp(4),Glu(5)]dynorphin A-(1-11)NH(2) showed similar kappa opioid receptor affinity (K(i) = 27 nM) and selectivity (K(i) ratio (kappa/mu/delta) = 1/12/330) to the linear peptide and antagonized dynorphin A-(1-13)NH(2) at kappa opioid receptors. This is the first opioid peptide cyclized through the N-terminus that retains high opioid receptor affinity.

Recent advances in selective opioid receptor agonists and antagonists

Opioid analgesics provide outstanding benefits for relief of severe pain. The mechanisms of the analgesia accompanied with some side effects have been investigated by many scientists to shed light on the complex biological processes at the molecular level. New opioid drugs and therapies with more desirable properties can be developed on the bases of accurate insight of the opioid ligand-receptor interaction and clear knowledge of the pharmacological behavior of opioid receptors and the associated proteins. Toward this goal, recent advances in selective opioid receptor agonists and antagonists including opioid ligand-receptor interactions are summarized in this review article.

[2',6'-Dimethyltyrosine]dynorphin A(1-11)-NH2 analogues lacking an N-terminal amino group: potent and selective kappa opioid antagonists

Recent studies showed that dermorphin and enkephalin analogues containing two methyl groups at the 2',6'-positions of the Tyr(1) aromatic ring and lacking an N-terminal amino group were moderately potent delta and mu opioid antagonists. These results indicate that a positively charged N-terminal amino group may be essential for signal transduction but not for receptor binding and suggested that its deletion in agonist opioid peptides containing an N-terminal 2',6'-dimethyltyrosine (Dmt) residue may represent a general way to convert them into antagonists. In an attempt to develop dynorphin A (Dyn A)-derived kappa opioid antagonists, we prepared analogues of [Dmt(1)]Dyn A(1-11)-NH2 (1), in which the N-terminal amino group was either omitted or replaced with a methyl group. This was achieved by replacement of Tyr(1) with 3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid (Dhp) or (2S)-2-methyl-3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid [(2S)-Mdp]. Compounds were tested in the guinea pig ileum and mouse vas deferens bioassays and in rat and guinea pig brain membrane receptor binding assays. All analogues turned out to be potent kappa antagonists against Dyn A(1-13) and the non-peptide agonist U50,488 and showed only weak mu and delta antagonist activity. The most potent and most selective kappa antagonist of the series was [(2S)-Mdp(1)]Dyn A(1-11)-NH2 (5, dynantin), which showed subnanomolar kappa antagonist potency against Dyn A(1-13) and very high kappa selectivity both in terms of its K(e) values determined against kappa, mu, and delta agonists and in terms of its ratios of kappa, mu, and delta receptor binding affinity constants. Dynantin is the first potent and selective Dyn A-derived kappa antagonist known and may complement the non-peptide kappa antagonists norbinaltorphimine and GNTI as a pharmacological tool in opioid research.

[Pro(3)]Dyn A(1-11)-NH(2): a dynorphin analogue with high selectivity for the kappa opioid receptor

A proline scan at positions 2 and 3 of the opioid peptide dynorphin A(1-11)-NH(2) led to the discovery of the analogue [Pro(3)]Dyn A(1-11)-NH(2). This analogue possesses high affinity and selectivity for the kappa opioid receptor (K(i)(kappa) = 2.7 nM, K(i) ratio kappa/micro/delta = 1/2110/3260). The gain in selectivity is achieved through an overall reduction of opioid receptor affinity which is most pronounced at micro and delta receptors. The Pro(3) analogue exhibits antagonist properties. Despite its high kappa affinity, [Pro(3)]Dyn A(1-11)-NH(2) is a relatively weak antagonist in both the [(35)S]GTPgammaS assay (IC(50) = 380 nM) and the guinea pig ileum assay (K(e) = 244 nM). Discrepancies between GPI and binding assay have often been ascribed to differential kappa receptor subtypes prevailing in central vs peripheral neurons. Since the [(35)S]GTPgammaS assay uses the same membrane preparations as the binding assay, differential kappa subtypes can be ruled out as an explanation in this case, and the observed behavior rather seems to reflect an intrinsic property of the ligand.

Conformation-activity relationships of opioid peptides with selective activities at opioid receptors

The discovery of endogenous opioid peptides 25 years ago opened up a new chapter in efforts to understand the origins and control of pain, its relationships to other biological functions, including inflammatory and other immune responses, and the relationships of opioid peptides and their receptors to a variety of undesirable or toxic side effects often associated with the nonpeptide opiates such as morphine including addiction, constipation, a variety of neural toxicities, tolerance, and respiratory depression. For these investigations the need for potent and highly receptor selective agonists and antagonists has been crucial since they in principle allow one to distinguish unequivocally the roles of the different opioid receptors (mu, delta, and kappa) in the various biological and pathological roles of the opioid peptides and their receptors. Conformational and topographical constraint of the linear natural endogenous opioid peptides has played a major role in developing peptide ligands with high selectivity for mu, delta, and kappa receptors, and in understanding the conformational, topographical, and stereoelectronic structural requirements of the opioid peptides for their interactions with opioid receptors. In turn, this had led to insights into the three-dimensional pharmacophore for opioid receptors. In this article we review and discuss some of the developments that have led to potent, selective, and stable peptide and peptidomimetic ligands that are highly potent and selective, and that have delta agonist, mu antagonist, and kappa agonist biological activities (other authors in this issue will discuss the development of other types of activities and selectivities). These have led to ligands that provide unique insight into opioid pharmacophores and the critical roles opioid ligands and receptor scan play in pain, addiction, and other human maladies.