Characterization of Alkyltriphenylphosphonium Cations and Their Interaction with Bacterial Cells

Abstract

Tuberculosis (TB) is a difficult to treat disease caused by the bacterium Mycobacterium tuberculosis. M. tuberculosis is able to shut down its metabolism in response to diverse environmental cues and enter a stage of non-replicating persistence that makes it resistant to many frontline TB drugs. This is further compounded by the “walling off” of M. tuberculosis in granulomatous lesions during infection. New drugs and strategies are in desperate need to combat TB, which currently kills two million people a year. The goal of this thesis was to explore the chemotherapeutic potential of alkyltriphenylphosphonium (alkylTPP) cations; lipophilic positively charged molecules known to accumulate at biological membranes in response to the membrane potential. To address this goal, a structure-function analysis of alkylTPP cations was carried out against several clinically important microorganisms: Mycobacterium tuberculosis, Staphylococcus aureus, Enterococcus faecalis and Escherichia coli; and the non-pathogenic Mycobacterium smegmatis. In addition, we determined if these compounds were toxic to murine RAW macrophages. A series of alkylTPP cations ranging in lipophilicity were characterized, where their toxicity against each cell type was used as a measure of effective accumulation. AlkylTPP cations were shown to be highly toxic to bacteria and mammalian macrophages at concentrations of as low as 1 – 2 μg/mL, where this toxicity increased with respect to lipophilicity. This was deemed an important structure- function relationship for their efficacy. The alkylTPP cation Aa10 was shown to be an effective inhibitor of all bacterial strains used in this study, where it elicited bactericidal killing in M. smegmatis and collapsed the membrane potential. On the basis of these data it is proposed that Aa10 inhibits bacterial growth in a bactericidal manner by dissipating the membrane potential. At toxic concentrations this is due to the accumulation of positively charged alkylTPP cations in the cytoplasmic membrane, where the specifics of this mechanism are yet to be defined. This is validated by the ability of Aa10 to effectively inhibit the anaerobic growth of E. faecalis JH2-2, implying that the action of Aa10 is not dependent on an electron transport chain. Future work will focus on investigating other structure- function relationships that attribute to effective alkylTPP cation toxicity. This includes the addition of different substituents around the central phosphonium ion and variations of the central cation (such as ammonium). Defining these relationships is key in developing alkylTPP cations for a therapeutic application.