N-Guanidyl and C-Tetrazole Leu-Enkephalin Derivatives: Efficient Mu and Delta Opioid Receptor Agonists with Improved Pharmacological Properties

Analgesic Peptides: From Natural Diversity to Rational Design

Pain affects one-third of the global population and is a significant public health issue. The use of opioid drugs, which are the strongest painkillers, is associated with several side effects, such as tolerance, addiction, overdose, and even death. An increasing demand for novel, safer analgesic agents is a driving force for exploring natural sources of bioactive peptides with antinociceptive activity. Since the G protein-coupled receptors (GPCRs) play a crucial role in pain modulation, the discovery of new peptide ligands for GPCRs is a significant challenge for novel drug development. The aim of this review is to present peptides of human and animal origin with antinociceptive potential and to show the possibilities of their modification, as well as the design of novel structures. The study presents the current knowledge on structure-activity relationship in the design of peptide-based biomimetic compounds, the modification strategies directed at increasing the antinociceptive activity, and improvement of metabolic stability and pharmacodynamic profile. The procedures employed in prolonged drug delivery of emerging compounds are also discussed. The work summarizes the conditions leading to the development of potential morphine replacements.

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.

Structure-activity study of oncocin: On-resin guanidinylation and incorporation of homoarginine, 4-hydroxyproline or 4,4-difluoroproline residues

Antimicrobial peptides (AMPs) often display guanidinium functionalities, and hence robust synthetic procedures are needed to facilitate access to analogues with unnatural homologues of arginine (Arg = R). Initially, a resin-bound Arg/Pro-rich fluoren-9-yl-methyloxycarbonyl-protected fragment (Fmoc-RPRPPR) of the AMP oncocin (i.e., VDKPPYLPRPRPPRRIYNR-NH2) was employed in a comparative on-resin assessment of commercial guanidinylation reagents head-to-head with the recently studied bis-Boc-protected triazole-based reagent, 1H-triazole-1-[N,N'-bis(tert-butoxycarbonyl)]-carboxamidine, which was synthesized by a chromatography-free procedure. This reagent was found to enable quantitative conversion in solid-phase peptide synthesis (SPPS) of peptides displaying homoarginine (Har) residues and/or an N-terminal guanidinium group. SPPS was used to obtain analogues of the 18-mer oncocin with single as well as multiple Arg → Har modifications. In addition, the effect of replacement of proline (Pro) residues in oncocin was explored by incorporating single or multiple trans-4-hydroxy-l-proline (Hyp) or 4,4-difluoro-l-proline (Dfp) residues, which both affected hydrophobicity. The resulting peptide library was tested against both Gram-negative and Gram-positive bacteria. Analysis of the minimal inhibitory concentrations (MICs) showed that analogues, displaying modifications at positions 4, 5 and 12 (originally Pro residues), had retained or slightly improved antimicrobial activity. Next, an oncocin analogue with two stabilizing l-Arg → d-Arg replacements in the C-terminal part was further modified by triple-replacement of Pro by either Dfp or Hyp in positions 4, 5, and 12. The resulting analogue displaying three Pro → Dfp modifications proved to possess the best activity profile: MICs of 1-2 µg/mL against E. coli and Klebsiella pneumoniae, less than 1% hemolysis at 800 µg/mL, and an IC50 above 1280 µg/mL in HepG2 cells. Thus, incorporation of bis-fluorinated Pro residues appears to constitute a novel tool in structure-activity studies aimed at optimization of Pro-rich AMPs.

Tetrazole Diversification of Amino Acids and Peptides via Silver-Catalyzed Intermolecular Cycloaddition with Aryldiazonium Salts

Selective structural modification of amino acids and peptides is a central strategy in organic chemistry, chemical biology but also in pharmacology and material science. In this context, the formation of tetrazole rings, known to possess significant therapeutic properties, would expand the chemical space of unnatural amino acids but has received less attention. In this study, we demonstrated that the classic unimolecular Wolff rearrangement of α-amino acid-derived diazoketones could be replaced by a faster intermolecular cycloaddition reaction with aryldiazonium salts under identical practical conditions. This strategy provides an efficient synthetic platform that could transform proteinogenic α-amino acids into a plethora of unprecedented tetrazole-decorated amino acid derivatives with preservation of the stereocenters. Density functional theory studies shed some light on the reaction mechanism and provided information regarding the origins of the chemo- and regioselectivity. Furthermore, this diazo-cycloaddition protocol was applied to construct tetrazole-modified peptidomimetics and drug-like amino acid derivatives.

AgeMTPT, a Catalyst for Peptide N-Terminal Modification

A key goal of synthetic biology is to enable designed modification of peptides and proteins, both in vivo and in vitro. N- and C-Terminal modification enzymes are crucial in this regard, but there are a few enzymatic options to protect peptide termini. AgeMTPT protects the N-terminus of short peptides with isoprene and the C-terminus as a methyl ester, but its substrate scope is unknown, limiting its application. Here, we investigate the substrate selectivity of the prenyltransferase domain, revealing a requirement for N-terminal aromatic amino acids, but with tolerance for diverse uncharged amino acids in the remaining positions. To demonstrate the potential of the enzyme, substrate selectivity data were used in the enzymatic modification of leu-enkephalin at the critical N-terminal residue. AgeMTPT active site mutagenesis led to an enzyme with expanded substrate scope, including the reverse geranylation of the N-termini of peptides. These data reveal potential applications of enzymatic peptide protection in synthetic biology.

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.

Tetrazole conjoined organosilane and organosilatrane via the ‘click approach’: a potent Mycobacterium tuberculosis enoyl ACP reductase inhibitor and a dual sensor for Fe(iii) and Cu(ii) ions

This article includes the synthesis and characterization of tetrazole-allied organosilane and organosilatrane. The tetrazole-allied silatrane was explored for molecular docking and optical aspects.

Evaluation of 1H-Triazole-1-[N,N'-Bis(tert-butoxycarbonyl)]carboxamidine in Solution-Phase and On-Resin Guanidinylation

Several guanidines and guanidinylated peptides have substantial potential as therapeutics, but efficient guanidinylation reagents are vital for easy access to these compounds. Presently, pyrazole-1-carboxamidine type reagents are commonly used in the transformations of amines into corresponding guanidines. Here, we report a comparative study of the utility of 1H-triazole-1-[N,N'-bis(tert-butoxycarbonyl)]carboxamidine, which was synthesized in two steps and readily upscaled to gram amounts. It exhibited excellent performance in solution-phase reactions, rapidly converting a set of representative aliphatic primary and unhindered secondary amines as well as aniline into the corresponding bis(tert-butoxycarbonyl)-protected guanidines. To enable a direct assessment of the reactivity of guanidinylation reagents, conversions were performed in deuterated solvents (d₇-DMF or d₈-THF), allowing for continuous analysis of the reaction mixtures by ¹H and ¹³C NMR. Likewise, 1H-triazole-1-[N,N'-bis(tert-butoxycarbonyl)]carboxamidine proved to be a versatile reagent in solid-phase conversions, for example, a resin-bound test peptide (KFFKFFK) was fully guanidinylated in only 2 h by using 2 equivalents of the reagent per free amino group. Also, 1H-triazole-1-[N,N'-bis(tert-butoxycarbonyl)]carboxamidine proved capable of completely guanidinylating more sterically hindered N-terminal residues (e.g., N-methyl amino acids or a peptoid) in resin-bound peptides. Its superior reactivity and stability demonstrated under heating conditions make 1H-triazole-1-[N,N'-bis(tert-butoxycarbonyl)]carboxamidine a valuable guanidinylation reagent both in solution- and solid-phase synthesis.

An Effective and Safe Enkephalin Analog for Antinociception

Opioids account for 69,000 overdose deaths per annum worldwide and cause serious side effects. Safer analgesics are urgently needed. The endogenous opioid peptide Leu-Enkephalin (Leu-ENK) is ineffective when introduced peripherally due to poor stability and limited membrane permeability. We developed a focused library of Leu-ENK analogs containing small hydrophobic modifications. N-pivaloyl analog KK-103 showed the highest binding affinity to the delta opioid receptor (68% relative to Leu-ENK) and an extended plasma half-life of 37 h. In the murine hot-plate model, subcutaneous KK-103 showed 10-fold improved anticonception (142%MPE·h) compared to Leu-ENK (14%MPE·h). In the formalin model, KK-103 reduced the licking and biting time to ~50% relative to the vehicle group. KK-103 was shown to act through the opioid receptors in the central nervous system. In contrast to morphine, KK-103 was longer-lasting and did not induce breathing depression, physical dependence, and tolerance, showing potential as a safe and effective analgesic.

The Meta-Position of Phe4 in Leu-Enkephalin Regulates Potency, Selectivity, Functional Activity, and Signaling Bias at the Delta and Mu Opioid Receptors

As tool compounds to study cardiac ischemia, the endogenous δ-opioid receptors (δOR) agonist Leu⁵-enkephalin and the more metabolically stable synthetic peptide (d-Ala², d-Leu⁵)-enkephalin are frequently employed. However, both peptides have similar pharmacological profiles that restrict detailed investigation of the cellular mechanism of the δOR’s protective role during ischemic events. Thus, a need remains for δOR peptides with improved selectivity and unique signaling properties for investigating the specific roles for δOR signaling in cardiac ischemia. To this end, we explored substitution at the Phe⁴ position of Leu⁵-enkephalin for its ability to modulate receptor function and selectivity. Peptides were assessed for their affinity to bind to δORs and µ-opioid receptors (µORs) and potency to inhibit cAMP signaling and to recruit β-arrestin 2. Additionally, peptide stability was measured in rat plasma. Substitution of the meta-position of Phe⁴ of Leu⁵-enkephalin provided high-affinity ligands with varying levels of selectivity and bias at both the δOR and µOR and improved peptide stability, while substitution with picoline derivatives produced lower-affinity ligands with G protein biases at both receptors. Overall, these favorable substitutions at the meta-position of Phe⁴ may be combined with other modifications to Leu⁵-enkephalin to deliver improved agonists with finely tuned potency, selectivity, bias and drug-like properties.