Stewart J. Wood

Structural Correlates of Antibacterial and Membrane-Permeabilizing Activities in Acylpolyamines

ABSTRACT A homologous series of mono- and bis-acyl polyamines with varying acyl chain lengths originally synthesized for the purpose of sequestering lipopolysaccharide were evaluated for antimicrobial activity to test the hypothesis that these bis-cationic amphipathic compounds may also bind to and permeabilize intact gram-negative bacterial membranes. Some compounds were found to possess significant antimicrobial activity, mediated via permeabilization of bacterial membranes. Structure-activity relationship studies revealed a strong dependence of the acyl chain length on antimicrobial potency and permeabilization activity. Homologated spermine, bis-acylated with C8 or C9 chains, was found to profoundly sensitize Escherichia coli to hydrophobic antibiotics such as rifampin. Nonspecific cytotoxicity is a potential drawback of these membranophilic compounds. However, the surface activity of these cationic amphipaths is strongly attenuated under physiological conditions via binding to serum albumin. Significant antibacterial activity is still retained in the presence of physiological concentrations of human serum albumin, suggesting that these compounds may serve as leads in the development of novel adjuncts to conventional antimicrobial chemotherapy.

Controlling plasma protein binding: structural correlates of interactions of hydrophobic polyamine endotoxin sequestrants with human serum albumin.

Hydrophobically substituted polyamine compounds, particularly N-acyl or N-alkyl derivatives of homospermine, are potent endotoxin (lipopolysaccharide) sequestrants. Despite their polycationic nature, the aqueous solubilites are limited owing to the considerable overall hydrophobicity contributed by the long-chain aliphatic substituent, but solubilization is readily achieved in the presence of human serum albumin (HSA). We desired first to delineate the structural basis of lipopolyamine-albumin interactions and, second, to explore possible structure-activity correlates in a well-defined, congeneric series of N-alkyl and -acyl homospermine lead compounds. Fluorescence spectroscopic and isothermal titration calorimetry (ITC) results indicate that these compounds appear to bind to HSA via occupancy of the fatty-acid binding sites on the protein. The acyl and carbamate compounds bind HSA the strongest; the ureido and N-alkyl analogues are significantly weaker, and the branched alkyl compound is weaker still. ITC-derived dissociation constants are weighted almost in their entirety by enthalpic deltaH terms, which is suggestive that the polarizability of the carbonyl groups facilitate, at least in large part, their interactions with HSA. The relative affinities of these lipopolyamines toward HSA is reflected in discernible differences in apparent potencies of LPS-sequestering activity under experimental conditions requiring physiological concentrations of HSA, and also of in vivo pharmacodynamic behavior. These results are likely to be useful in designing analogues with varying pharmacokinetic profiles.