Studies towards the syntheses and analyses of short peptides as novel antimycobacterial agents

Abstract

Tuberculosis (TB) is a communicable disease caused by M. tuberculosis. One-third population of the world is affected by TB. The emerging drug-resistance against current TB-drugs, necessitates the need for novel antimycobacterial agents which can target mycobacteria, especially the drug-resistant strains. Several antimycobacterial peptides are reported in the literature and are promising as next generation anti-tuberculosis drugs to combat drug resistance. This thesis reports several synthetic peptides (some based on naturally occurring sequences) designed to have potential antimycobacterial activity. A hexapeptide, calpinactam, which is selectively active against mycobacteria, and four new analogues were synthesised by replacing the caprolactam ring with Ac7c, Dpg, lysine hydroxamic acid and ornithine hydroxamic acid residues. Dpg and lysine hydroxamic acid analogues were conjugated with the cell penetrating peptide TAT. The antimycobacterial activity of these analogues were studied against M. marinum, M. abscessus M. smegmatis mc2155, M. smegmatis BSG201 and M. tuberculosis ΔLeuΔPanCD strain. TAT conjugated Dpg analogue (3.7) has shown same level of antimycobacterial activity as calpinactam against M. tuberculosis ΔLeuΔPanCD strain and this analogue has better solubility than calpinactam. Further, it was found that the calpinactam does not inhibit the growth of M. marinum, and M. abscessus.CPP could help the drug to enter the mycobacterial cells which has developed the drug-resistance possibly via efflux pump mechanism. To study this prodrug effect, two CPP-drug conjugates of anti-tuberculosis fluoroquinolones class drug, levofloxacin were synthesised via formation of amide bond (4.1) and ester bond (4.4). These compounds were tested against M. smegmatis BSG201 and the compound 4.1 was found to be equally active as levofloxacin iv against M. smegmatis BSG201 which suggests that the drug levofloxacin is released with its free functional carboxylic acid group, inside the mycobacterial cell. This feature is important for a prodrug. However, further studies with drug-resistant strains are required to confirm this finding. Non-methylated and methylated linear analogues of auxarthrides were synthesised. Crystal structures of analogues 5.5 and 5.14 (Figure i) were studied using X-ray crystallography and deposited in Cambridge Crystallographic Data Centre (CCDC) database. It was found that the linear analogues are in trans (ttt) geometry which is unfavourable for cyclization. Figure i: Crystal structure of analogue 5.14 (N-Me-Phe-Val-Ile-Leu) Various cyclization conditions, reported in the literature, were applied on these linear analogues. The effect of N-methylation on cyclization of tetrapeptides was studied while standardizing the synthetic protocol. Characterization of linear and cyclic analogues were performed by ESI-MS and NMR studies and reported in this thesis.