Synthesis and structural properties of cyclic peptides incorporating novel non-reducible cystine mimics

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The aim of this project is to develop a range of bioactive cyclic peptides incorporating lanthionine and cystathione as metabolically stable, non-reducible replacements for disulfide linkages. By studying the conformational and biological properties of these disulfide replacements, we aim to understand how they could best be used as tools to study and alter biological processes, and to evaluate them as possible leads in drug discovery programmes.The resurgence of interest in cyclic peptides has been driven by their potency and selectivity when binding to therapeutically relevant targets, such as receptors, protein-protein interactions (PPI) and transcription factors, which are difficult to address with small molecule therapeutics or with biologics. The conventional approach to applying a conformational constraint to a peptide lead is to cyclise by introducing a disulfide bridge between two Cys residues. However, such linkages are reduced in vivo and are not metabolically stable; thus there is considerable interest in making cyclic peptides which have a thioether linkage (from incorporation of lanthionine or cystathionine) instead. Indeed, CRB are frequently asked by their clients to provide cyclic peptide analogues containing these unnatural linkages for early stage drug discovery programmes.The Tabor group have pioneered a solid-phase peptide synthesis (SPPS) approach to lanthionine- containing peptides.1,2,3 This involves the stereoselective synthesis of orthogonally protected lanthonine, the incorporation of this residue in a linear peptide, selective removal of the protecting groups on the w-amino acid moiety, cyclisation on-resin and chain extension. This is now the method used by groups worldwide for the chemical synthesis of lantibiotics, an emerging class of antimicrobial peptides. However, this method has not yet been widely used to prepare non-reducible, conformationally constrained analogues of other biologically active peptides,3,4,5 and the effects of introducing this bridge on the conformation and biological properties of these peptides is also not well understood. This project will therefore focus on applying the SPPS approach to synthesising thioether bridged conformationally constrained analogues of key peptides involved in receptor binding. In order to understand the effect that these disulphide bond replacements have on the peptide conformation, and in particular to determine whether the peptides can adopt the biologically active conformation, the structural properties of these peptides will be analysed by NMR, using NAMFIS- analysis,6 in collaboration with Prof Mate Erdelyi (University of Uppsala). The student will also spend 3 - 4 months receiving training in this technique at Uppsala. Workplan:Year 1 Months 1 - 6 LIDo training courses (SysMIC, Bio-Industry). Small-scale synthesis of orthogonally protected lanthionine and cystathionine amino acid building blocks.Months 7 - 12 Synthesis of two analogues of vasopressin,7 one with with lanthionine and one with cystathionine replacements for the native disulfide bondYear 2 Months 13 - 18 NMR studies of cyclic peptides: training in NAMFIS technique (University of Uppsala, Sweden: Prof Mate Erdelyi)Months 19 - 24 CRB Placement 1 (Section 5)Year 3 Months 25 - 32 Synthesis of all variants thioether bridge lengths and a-stereochemistry of vasopressin,7 somatostatin5 and of the client peptide synthesised in Placement 1: NMR analysis, biological evaluation (calcium flux FLIPR assay).Months 33 - 38 CRB Placement 2 (Section 5)Year 4 Months 39 - 42 NMR analysis of peptides synthesised in Placement 2 Months 43 - 48 Thesis writing