Chemical Conversion of Natural Polymyxin B and Colistin to Their N-Terminal Derivatives

Abstract

The chemical conversions of natural polymyxin B and colistin, which are fatty-acylated cyclic decapeptides, to polymyxin (2–10) and colistin (2–10) derivatives were examined. The Nα-free and side chain Nγ-protected nonapeptides, i.e., tetrakis(Nγ-trifluoroacetyl)–polymyxin B (2–10) and tetrakis(Nγ-trifluoroacetyl)–colistin (2–10), were prepared by trifluoroacetylation of polymyxin B and colistin, followed by chemical cleavage with 50% methanesulfonic acid to remove Nα-alkanoyl–Nγ-trifluoroacetyl–Dab–OH. The Nγ-protected nonapeptides were useful starting materials for the semi-synthesis of N-terminal derivatives by selective Nα-acylation at Thr2, followed by the removal of the Nγ-trifluoroacetyl protecting group with aqueous piperidine. Further, myristoyl–polymyxin B (2–10) and myristoyl–colistin (2–10) retained their antimicrobial activity with an MIC of 2–4 nmol mL−1 against Escherichia coli, Salmonella Typhimurium, and Pseudomonas aeruginosa. They also retained their high lipopolysaccahride (LPS) binding activity. Acetyl–polymyxin B (2–10) and acetyl–colistin (2–10) exhibited very low biological activities, except for a high bactericidal activity specifically against Pseudomonas aeruginosa with an MIC of 2 nmol mL−1. The distinct sensitivity of three Gram-negative bacteria tested toward acetyl-nonapeptides suggested that the N-terminal hydrophobic character of the fatty-acylated polymyxin peptides was necessary for the bactericidal activity against Escherichia coli and Salmonella Typhimurium, but not against Pseudomonas aeruginosa.

Semi-synthesis of Polymyxin B (2-10) and Colistin (2-10) Analogs Employing the Trichloroethoxycarbonyl (Troc) Group for Side Chain Protection of .ALPHA.,.GAMMA.-Diaminobutyric Acid Residues

“…Semi-synthesis As reported previously, 2) the preparation of N a -free and N g -protected nonapeptides from N g -protected cyclic decapeptides was the key step for the semi-synthesis of N-terminal nonapeptide analogs of polymyxin B and colistin. In the present study, Troc was examined as an acid-resistant protecting group for the Dab side chains of polymyxin B and colistin.…”

“…After cleavage, the product was treated with diluted ammonia to reverse the N-O migration products, as reported previously. 2) Following purification by RP-HPLC, the new key compounds, tetrakis(N g -Troc)-polymyxin B (2-10) (3) and tetrakis(N g -Troc)-colistin (2-10) (4), were obtained in 18.9% and 15.2% yield, respectively (Fig. 1).…”

“…Polymyxin B (2-10) and colistin (2)(3)(4)(5)(6)(7)(8)(9)(10) are cyclic nonapeptides obtained respectively from polymyxin B and colistin by enzymic removal of fatty acyl-Dab-OH with ficin 13) or papain. 14) The toxicity of the nonapeptides is known to be markedly low in comparisons with the parent compounds, polymyxin B and colistin; however, both nonapeptides are very weak bactericidals despite the fact that they retain considerable binding activity to LPS.…”

“…Tetrakis(N g -Tfa)-polymyxin B (2-10) and tetrakis(N gTfa)-colistin (2)(3)(4)(5)(6)(7)(8)(9)(10), which have N a -free and N g -protected amino functions, were useful starting materials for the semisynthesis of their N-terminal derivatives. However, the yields of tetrakis(N g -Tfa)-polymyxin B (2-10) and tetrakis(N g -Tfa)-colistin (2-10) were only 8-11%.…”

Development of new polymyxin derivatives for multi-drug resistant Gram-negative infections

“…The loss of potentiating activity against P. aeruginosa is presumably due to the substitution of D-serine for Dab at amino acid 3 as N-acetyl-PMBN retains some direct activity against P. aeruginosa. 63 Interestingly, the renal clearance of NAB741 in rats was 4000-fold higher than that of colistin.…”