Synchrotron microanalysis of gallium as a potential novel therapy for urinary tract infections

Most urinary tract infections in humans and dogs are caused by uropathogenic strains of , and increasing antimicrobial resistance among these pathogens has created a need for a novel approach to therapy. Bacterial iron uptake and metabolism are potential targets for novel antimicrobial therapy, as iron is a limiting factor in . growth during infection. As a trivalent metal of similar atomic size to iron (III), gallium can interact with a wide variety of biomolecules that normally contain or interact with iron. Gallium compounds disrupt bacterial iron metabolism, are known to accumulate at sites of infection and inflammation in mammals, exert antimicrobial activity against multiple bacterial pathogens in vitro, and may be good candidates as novel antimicrobial drugs. We assessed the suitability of orally administered gallium maltolate as a potential new antimicrobial therapy for urinary tract infections by evaluating its distribution into the bladder, its activity against uropathogenic . in vitro, and its pharmacokinetics and efficacy in a mouse cystitis model. Using a novel application of synchrotron-based analytical methods, we confirmed the distribution of gallium to the bladder mucosa and characterized the relationship between iron and gallium distribution in the bladder. In vitro experiments with human and canine uropathogenic . isolates demonstrated that gallium maltolate exerts antimicrobial effects in a time-dependent, bacteriostatic manner. Minimum inhibitory concentrations ranged from 0.144 µmol/mL to >9.20 µmol/mL with a median of 1.15 µmol/mL. Isolates resistant to ampicillin, ciprofloxacin, or with decreased susceptibility to cephalothin were susceptible to the antimicrobial activity of gallium maltolate, suggesting that resistance to conventional antimicrobials does not predict resistance to gallium maltolate. Pharmacokinetic studies in healthy mice and in a mouse model of urinary tract infection confirmed that gallium is absorbed into systemic circulation after oral administration of gallium maltolate. Gallium is slowly eliminated from the body, with a trend toward longer terminal half-lives in blood and bladder for infected mice relative to healthy mice. This study did not reveal any statistically significant effect of infection status on maximum blood gallium concentrations (4.46 nmol/mL, 95% confidence interval 2.75 nmol/mL – 6.18 nmol/mL and 4.80 nmol/mL, 95% confidence interval 2.53 nmol/mL – 7.06 nmol/mL in healthy and infected mice, respectively) or total gallium exposure in blood and kidney as represented by area under the concentration vs. time curves. Gallium exposure in the bladder was significantly greater for mice with urinary tract infections than for healthy mice. The investigation of gallium distribution within tissues represented a novel application of synchrotron-based analytical techniques to antimicrobial pharmacokinetics. Prior to analysing tissue samples, a library of x-ray absorption spectra was assembled for gallium compounds in both the hard and soft x-ray ranges. The suitability of hard x-ray fluorescence imaging and scanning and transmission x-ray microscopy for localizing and speciating trace elements in tissues was subsequently assessed. Of these methods, only hard x-ray microprobe analysis was well-suited to the analysis and was successfully used for this application. This approach confirmed that gallium arrives at the bladder mucosa after oral administration of gallium maltolate. Furthermore, comparison of iron and gallium distribution within the bladder mucosa indicated that these elements are similarly but not identically distributed and that they do not significantly inhibit one another’s distribution. This finding suggests that gallium may be distributed in part via pathways that do not involve iron. Despite the favorable distribution characteristics of gallium and the persistence of gallium in target tissues following the oral administration of gallium maltolate, its efficacy in a mouse model of urinary tract infection was disappointing. In this study, no statistically significant difference was detected between gallium maltolate, ciprofloxacin and sham treatments in their ability to eliminate bacteria in the urinary tracts. The failure of ciprofloxacin treatment to render bladder tissue culture-negative for an organism that is classified as ciprofloxacin-susceptible in vitro is consistent with observations from other research groups. The similar lack of efficacy observed for gallium maltolate may be related to the large gap between minimum inhibitory concentrations observed in vitro and gallium concentrations observed in tissues from treated mice, but may also be related to the small study size if the effect size of gallium maltolate treatment is small. Given the magnitude of the difference between tissue concentrations and minimum inhibitory concentrations, it may not be possible to increase the dose sufficiently to achieve therapeutic concentrations without causing toxicity. While the results of these experiments suggest that orally administered gallium maltolate may not be a reasonable antimicrobial drug candidate for treating urinary tract infections caused by uropathogenic . , it may be useful for other applications. Other bacterial pathogens may be more susceptible to the antimicrobial effects of gallium maltolate, and local or topical administration could produce much higher concentrations than we observed following oral administration. Continued development of the synchrotron-based analytical techniques used in these experiments could provide new and important opportunities to investigate antimicrobial distribution and metabolism within cells and tissues, particularly for metal-based drugs