Synthesis And Applications Of Novel Cyclic Peptide By Lysine Tethering

Natural peptides are favored by researchers because of their unique functions and characteristics such as antibacterial,anti-tumor,signal transduction,immune regulation and so on.Although peptides own excellent potential as therapeutics,natural peptides have low biostability and are hydrolyzed easily by proteases,resulting in short half-life in the body and low bioavailability,which limits their clinical development and widespread applications of peptide drugs.Many researchers have tried to overcome these shortcomings by chemically modifying natural peptides.There are a large number of literature reports which provided many chemical methods to modify peptides,such as replacing L-amino acids with D-amino acids,introducing unnatural amino acids,conjugating to a polymer,and cyclization.However,many results show that modified peptide cannot reproduce the biological activity of the peptide precursor,which makes the synthesis of stable peptides with retained bioactivity still a challenge in pharmaceutical chemical synthesis.Cyclic peptide molecules(especially multicyclic peptides)mainly improve the stability,structural rigidity and binding affinity of peptides precursor by restricting their conformation,which are a class of modulators with great potential to resist protease hydrolysis.In previous literature reports,researchers have developed various cyclization methods,such as lactam method,olefin metathesis method,alkylation method,disulfide bond method,thioether bond method,etc.,to synthesize stable cyclic peptides by head to tail,side chain to side chain,or side chains to end cyclization of a linear peptide.Recently,our group had developed a new peptide cyclization method by connecting the ε-amino groups of two lysine through N-alkylation reaction.This new cyclization protocol can provide cyclic peptide with not only enhanced protease stability,but also increased biological activity.Therefore,this research aims to further synthesize a more stable bicyclic peptide structure based on the N-alkylation modification of the ε-amino group of lysine.In principle,the bicyclic peptide could be synthesized by the alkylation reaction of a linear peptide containing multiple lysines by a centrosymmetric small molecular(1,3,5-tribromomethyl benzene).The selective N-alkylation of the ε-amino groups of lysine at i,i+4 and i+7 positions of the linear peptide cannot avoid the production of multiple by-products,however the conversion rate of the target product can be significantly improved by screening of the N-alkylation reaction conditions.This novel cyclic structure makes the protease cleavage site shielded by the peptide loop,and also increased the structural rigidity due to the restriction of peptide conformation,therefore,the synthesized bicyclic peptide have increased resistance to proteolytic enzymes.Meanwhile,the bicyclic peptide obtained by this cyclization protocol showed not only significantly prolonged half-life in serum,but also improved biological activity,meanwhile with low hemolytic toxicity and cytotoxicity.In addition,in order to further improve the stability of the peptides,this research also tried to modify the above-mentioned synthetic bicyclic peptides by head-to-tail cyclization to get a tricyclic peptides,which may allow the bioactive precursor peptide to exert a long-lasting therapeutic effect in the body.