Diseases due to M. tuberculosis (M.tb) and M. avium/intracellular complex (MAC) cause significant worldwide morbidity and mortality especially in AIDS patients. MAC are uniformly multidrug-resistant (MDR) and MDR strains of M. tb are becoming more prevalent. Treatment requires multiple antibiotics with significant toxicity administered over months to years. Thus there is the need to evaluate novel, effective therapeutic agents with the promise of reducing the duration of therapy. M.tb and MAC are representative of a group of intracellular pathogens that enter and multiply within mononuclear phagocytes. Iron (Fe) availability is critical for mycobacterial growth, making disruption of this aspect of mycobacterial metabolism an attractive target for antimicrobial therapy. Gallium (Ga), a group IIIA transition metal, particularly in the form of Ga nitrate [Ga(NO3)3], has been used clinically to localize neoplasms and inflammatory sites due to its concentration in tumor cells and macrophages and also to treat malignant neoplasms and associated hypercalcemia. The effects of Ga relate to its ability to substitute for Fe in many biomolecular processes, thereby disrupting them. The investigators' preliminary studies indicate that Ga inhibits the growth of M.tb, including an MDR strain, and MAC extracellularly and within human macrophages. Ga treatment is cidal against M.tb growing in macrophages. The Ga mediated growth inhibition is additive with other antimycobacterial drugs. The effect of Ga is reversed with excess Fe and Ga interrupts the ability of intracellular M. tb to acquire exogenous Fe. Finally, their studies indicate that Ga reduces the enzymatic activity of purified recombinant RR from M.tb. Thus, they hypothesize that Ga compounds may represent a new class of agents for treating mycobacterial infections through disruption of bacterial Fe-dependent metabolic pathways. The investigators propose the following two specific aims: 1) examine mechanism(s) whereby Ga is acquired by and exhibits its microbicidal effects on M.tb and MAC, and 2) determine the mechanism(s) whereby Ga trafficks from the extracellular environment to the mycobacterial phagosome in macrophages and its effect on mycobacterial viability in this location. Methods proposed include cell culture, radiolabeled Ga binding studies and assays of enzyme activity. The goal is to characterize completely the mechanism of action of Ga against pathogenic mycobacteria with the eventual goal to determine the feasibility of the use of Ga compounds as therapeutic agents for this important group of pathogens.
