Nutrient-Mediated Delivery of Antimicrobials

Background
The problem of antimicrobial resistance is now so severe that it has been described as critical by the World Health Organisation and some authorities believe that mankind is now entering a post-antibiotic era. Fluoroquinolones (FQ) such as ciprofloxacin are widely used for the treatment of a range of infections, however, with their increased use the incidence of resistance in almost all bacteria that fluoroquinolones are used against has emerged.
The challenge is to chemically modify FQs to increase intracellular concentrations and overcome permeability and efflux mediated resistance.
We have explored the use of bacterial nutrient transport to deliver antibiotics into the bacterial cell (Trojan horse approach). To date we have investigated iron carriers (siderophores-citrate, Staphyloferrin A) and carbohydrates attached to the piperazinyl nitrogen of ciprofloxacin via a non-biolabile linker. Screens against our established panel of bacterial strains confirmed that ciprofloxacin, when conjugated, retains antibacterial activity, however, its potency is reduced. Subsequent DNA gyrase inhibition assays revealed that the decrease in activity is mainly due to an impaired ability of the ciprofloxacin conjugate to inhibit gyrase, the target of the drug, rather than lack of bacterial uptake. We believe that the presence of the conjugated moiety limit binding to DNA gyrase5-7. This suggests that a 'prodrug approach', were the nutrient is cleaved, with intracellular release of the fluoroquinolone will improve

Project Aims: The primary aim of this research is to conjugate FQs to key nutrients via an intracellular, reductively activated, biolabile linker in order to evade FQ permeability resistance and increase intracellular accumulation. The research workflow is:
(1) To synthesise a ciprofloxacin conjugates with a self immolative linker. The design of the linker is such that, after active transport, intracellular reduction (via intracellular sulfhydryl-containing species such as glutathione) will release free ciprofloxacin in the bacterial cytoplasm. Initially the conjugate will use a citrate unit (our model siderophore) attached to FQ COOH. The conjugate will be screened against wild-type E. coli. The minimum inhibitory concentrations/minimum bactericidal concentrations obtained will be compared to those of the parent drug ciprofloxacin and a corresponding analog with non-labile linker.
(2) The siderophore components that we propose to explore are inspired by the structure of the salmochelins, a new class of siderophores that has only recently discovered. The main advantage of these glycosylated siderophores is that they are able to evade the immune response of the mammalian host. In addition, they are less susceptible to serum albumin binding and membrane partitioning, both factors that reduce the concentration of conventional siderophores in the serum.