Bioresponsive dextrin-colistin conjugates as antimicrobial agents for the treatment of Gram-negative infection

Abstract

Multidrug-resistant Gram-negative infection is an important cause of mortality and morbidity. Management of these infections is often dependent upon “treatment of last resort” "small molecule" antibiotics which suffer from significant toxicity and an indiscriminate volume of distribution. The aim of this study was to develop a prototype polymer-antibiotic conjugate that may be customised by polymer modification and binding chemistry to afford selective, controlled release at an infected site. These studies employed the biodegradable, naturally-occurring polymer, dextrin, and a polymyxin antibiotic, colistin, as the first model combination. Physicochemical characterisation of a library of succinoylated dextrins and dextrin-colistin conjugates demonstrated that conjugation of dextrin to colistin was feasible and reproducible, resulting in masking of colistin's amino groups through incorporation in peptide bonds. Exposure to physiological -amylase activity resulted in controlled degradation of the dextrin component, leading to sustained colistin release. Following exposure of the conjugates to physiological concentrations of -amylase, minimally-modified, low molecular weight dextrin, conjugated to colistin, demonstrated significantly earlier, maximal release of colistin and subsequent reinstatement of antimicrobial activity. At maximum unmasking, the lead conjugate reported equivalent antimicrobial activity to the current clinical formulation of colistin (Colimycin®) against a range of MDR organisms including: A. baumannii, K. pneumoniae and E. coli. A static two-compartment dialysis bag model was developed under infinite sink conditions, which demonstrated that the conjugates were able to suppress bacterial growth over a significantly greater duration than colistin sulfate. Ex vivo studies of infected human wound fluid samples confirmed that colistin could be readily liberated from conjugate in infected sites. Significantly higher amylase activity in these wound fluid samples supported the notion of locally-triggered, enzymatically-mediated unmasking. An in vivo intravenous, pharmacokinetic model in rats demonstrated the increased half-life associated with conjugation and succinoylation. Moreover, the dextrin-colistin conjugates were better tolerated than colistin sulfate at higher concentrations. These studies have demonstrated the feasibility of developing this new class of “nanoantibiotics” and highlighted their potential usefulness as bioresponsive nanomedicines for the treatment of MDR Gram-negative infection.