valinomycin and Tuberculosis

We report the discovery of a series of new drug leads that have potent activity against Mycobacterium tuberculosis as well as against other bacteria, fungi, and a malaria parasite. The compounds are analogues of the new tuberculosis (TB) drug SQ109 (1), which has been reported to act by inhibiting a transporter called MmpL3, involved in cell wall biosynthesis. We show that 1 and the new compounds also target enzymes involved in menaquinone biosynthesis and electron transport, inhibiting respiration and ATP biosynthesis, and are uncouplers, collapsing the pH gradient and membrane potential used to power transporters. The result of such multitarget inhibition is potent inhibition of TB cell growth, as well as very low rates of spontaneous drug resistance. Several targets are absent in humans but are present in other bacteria, as well as in malaria parasites, whose growth is also inhibited.

Topics: Anti-Infective Agents; Antineoplastic Agents; Antitubercular Agents; Bacteria; Breast Neoplasms; Cell Proliferation; Drug Design; Drug Discovery; Female; Fungi; Humans; Malaria, Falciparum; MCF-7 Cells; Membrane Transport Proteins; Models, Molecular; Molecular Structure; Mycobacterium tuberculosis; Plasmodium falciparum; Structure-Activity Relationship; Tuberculosis; Tumor Cells, Cultured

Pyrazinamide (PZA) is an important front-line anti-tuberculosis drug that is active only at acid pH. However, acid pH causes significant difficulty for PZA susceptibility testing. A common problem in PZA testing is false resistance caused by large bacterial inocula. This study investigated the relationship of false resistance to numbers of bacilli, pH and other factors that potentially affect susceptibility to PZA. Large inocula (10(7-8) bacilli/ml) of M. tuberculosis H37Ra caused significant increase in medium pH from 5.5 towards neutrality, and thus produced false resistance results. The increase in medium pH was determined to be a function of live bacilli; heat-killed bacilli had little or no effect. Susceptibility to PZA and its active derivative pyrazinoic acid (POA) was comparable on 7H11 agar medium, but POA was less active than PZA in liquid medium containing bovine serum albumin (BSA), suggesting that susceptibility to PZA or POA was reduced in the presence of BSA, because of its neutralising effect on medium pH and significant POA binding. A 3-month-old H37Ra culture was shown to be more susceptible to PZA exposure than a 4-day log-phase culture, suggesting that PZA is more active for non-growing bacilli. Finally, reserpine, an inhibitor of POA efflux pump, increased susceptibility to PZA even near neutral pH 6.8, with an MIC of 400 mg/L compared with 1,000 mg/L without reserpine. These findings should have implications for understanding the mode of action of PZA and for PZA susceptibility testing.

Topics: Antitubercular Agents; Cell Survival; Colony Count, Microbial; Culture Media; Drug Interactions; Drug Resistance, Bacterial; Humans; Hydrogen-Ion Concentration; Ionophores; Microbial Sensitivity Tests; Mycobacterium tuberculosis; Pyrazinamide; Reserpine; Serum Albumin, Bovine; Time Factors; Tuberculosis; Valinomycin