Synthesis and In vitro Biological Activity of Cyclic Lipophilic χ-Conotoxin MrIA Analogues

The chemical synthesis of α-conotoxins and structurally modified analogs with enhanced biological stability

α-Conotoxins are peptide neurotoxins isolated from the venom ducts of carnivorous marine cone snails that exhibit exquisite pharmacological potency and selectivity for various nicotinic acetylcholine receptor subtypes. As such, they are important research tools and drug leads for treating various diseases of the central nervous system, including pain and tobacco addiction. Despite their therapeutic potential, the chemical synthesis of α-conotoxins for use in structure-activity relationship studies is complicated by the possibility of three disulfide bond isomers, where inefficient folding methods can lead to a poor recovery of the pharmacologically active isomer. In order to achieve higher yields of the native isomer, especially in high-throughput syntheses it is necessary to select appropriate oxidative folding conditions. Moreover, the poor biochemical stability exhibited by α-conotoxins limits their general therapeutic applicability in vivo. Numerous strategies to enhance their stability including the substitution of disulfide bond with diselenide bond and N-to-C cyclization via an oligopeptide spacer have successfully overcome these limitations. This chapter describes methods for performing both selective and nonselective disulfide bond oxidation strategies for controlling the yields and formation of α-conotoxin disulfide bond isomers, as well as methods for the production of highly stable diselenide-containing and N-to-C cyclized conotoxin analogs.

Drugs from slugs. Part II--conopeptide bioengineering

The biological transformation of toxins as research probes, or as pharmaceutical drug leads, is an onerous and drawn out process. Issues regarding changes to pharmacological specificity, desired potency, and bioavailability are compounded naturally by their inherent toxicity. These often scuttle their progress as they move up the narrowing drug development pipeline. Yet one class of peptide toxins, from the genus Conus, has in many ways spearheaded the expansion of new peptide bioengineering techniques to aid peptide toxin pharmaceutical development. What has now emerged is the sequential bioengineering of new research probes and drug leads that owe their lineage to these highly potent and isoform specific peptides. Here we discuss the progressive bioengineering steps that many conopeptides have transitioned through, and specifically illustrate some of the biochemical approaches that have been established to maximize their biological research potential and pharmaceutical worth.

Conotoxin engineering: dual pharmacophoric noradrenaline transport inhibitor/integrin binding peptide with improved stability

A dual-pharmacophoric peptide was engineered by grafting the integrin binding RGD motif between the C- and N-termini of a disulfide-rich noradrenaline transporter inhibiting χ-conotoxin resulting in a stable backbone cyclized peptide. The construct maintained two independent biological activities and showed increased plasma stability with no adverse effects observed following administration to rats, highlighting the potential value of pharmacophore grafting into constrained peptide scaffolds.

Improving the stability of α-conotoxin AuIB through N-to-C cyclization: the effect of linker length on stability and activity at nicotinic acetylcholine receptors

Modification of α-conotoxin frameworks through cyclization via an oligopeptide linker has previously been shown as an effective strategy for improving in vivo stability. We have extended this strategy by investigating cyclic analogs of α-conotoxin AuIB, a selective α(3)β(4) nicotinic acetylcholine receptor (nAChR) antagonist, to examine a range of oligopeptide linker lengths on the oxidative formation of disulfide bonds, activity at nAChRs, and stability to degradation by chymotrypsin. Upon nondirected random oxidation, the ribbon isomer formed preferentially with the globular isomer occurring as a minor by-product. Therefore, a regioselective disulfide bond forming strategy was used to prepare the cAuIB-2 globular isomer in high yield and purity. The cAuIB-2 globular isomer exhibited a threefold decrease in activity for the α(3)β(4) nAChR compared to wild-type-AuIB, although it was selective for α(3)β(4) over α(7) and α(4)β(2) subtypes. On the other hand, the cAuIB-2 ribbon isomer was shown to be inactive at all three nAChR subtypes. Nonetheless, all of the cyclic analogs were found to be significantly more stable to degradation by chymotrypsin than wild-type AuIB. As such, the cAuIB-2 globular isomer could constitute a useful probe for studying the role of the α(3)β(4) nAChR in a range of in vivo experimental paradigms.

Synthetic α-conotoxin mutants as probes for studying nicotinic acetylcholine receptors and in the development of novel drug leads

α-Conotoxins are peptide neurotoxins isolated from venomous marine cone snails that are potent and selective antagonists for different subtypes of nicotinic acetylcholine receptors (nAChRs). As such, they are valuable probes for dissecting the role that nAChRs play in nervous system function. In recent years, extensive insight into the binding mechanisms of α-conotoxins with nAChRs at the molecular level has aided in the design of synthetic analogs with improved pharmacological properties. This review examines the structure-activity relationship studies involving α-conotoxins as research tools for studying nAChRs in the central and peripheral nervous systems and their use towards the development of novel therapeutics.