Recent progress towards pharmaceutical applications of disulfide-rich cyclic peptides

Cyclotides and conotoxins are two classes of disulfide-rich peptides that occur in plants and animals respectively and are the major focus of study in our laboratory. In the last three years there has been significant progress in studies of these two classes of compounds and in this article we provide an overview of the findings from our laboratory in this period. Highlights include the discovery of cyclotides in the Fabaceae and Solanaceae plant families, members of which are widely used in human nutrition, and the discovery of new classes of cyclotide precursors. These discoveries confirm the widespread distribution of cyclotides in the plant kingdom and the diversity of precursor proteins involved in their biosynthesis. Other studies have delineated the mode of action of naturally occurring cyclotides and have demonstrated the versatility of synthetic cyclotides as stable protein engineering frameworks, with applications in drug design. Conotoxins continue to be a rich source of inspiration for drug design programs, and we summarize here a range of recent studies from our laboratory focusing on the development of novel synthetic strategies and the delineation of structure-activity relationships. A major highlight was the development of an orally active cyclized conotoxin derivative that is highly efficacious in a rat model of neuropathic pain. Overall the studies described herein provide much encouragement for continuing efforts to develop peptides as drugs.