Drug-resistant tuberculosis: what is to be done?

2-Mercapto-Quinazolinones as Inhibitors of Type II NADH Dehydrogenase and Mycobacterium tuberculosis: Structure-Activity Relationships, Mechanism of Action and Absorption, Distribution, Metabolism, and Excretion Characterization

Mycobacterium tuberculosis (MTb) possesses two nonproton pumping type II NADH dehydrogenase (NDH-2) enzymes which are predicted to be jointly essential for respiratory metabolism. Furthermore, the structure of a closely related bacterial NDH-2 has been reported recently, allowing for the structure-based design of small-molecule inhibitors. Herein, we disclose MTb whole-cell structure–activity relationships (SARs) for a series of 2-mercapto-quinazolinones which target the ndh encoded NDH-2 with nanomolar potencies. The compounds were inactivated by glutathione-dependent adduct formation as well as quinazolinone oxidation in microsomes. Pharmacokinetic studies demonstrated modest bioavailability and compound exposures. Resistance to the compounds in MTb was conferred by promoter mutations in the alternative nonessential NDH-2 encoded by ndhA in MTb. Bioenergetic analyses revealed a decrease in oxygen consumption rates in response to inhibitor in cells in which membrane potential was uncoupled from ATP production, while inverted membrane vesicles showed mercapto-quinazolinone-dependent inhibition of ATP production when NADH was the electron donor to the respiratory chain. Enzyme kinetic studies further demonstrated noncompetitive inhibition, suggesting binding of this scaffold to an allosteric site. In summary, while the initial MTb SAR showed limited improvement in potency, these results, combined with structural information on the bacterial protein, will aid in the future discovery of new and improved NDH-2 inhibitors.

Tuberculosis chemotherapy in the 21 century: Back to the basics

The key to successful elimination of tuberculosis (TB) is treatment of cases with optimum chemotherapy. Poor chemotherapy over time has led to drug-resistant disease. Drug resistance of Mycobacterium tuberculosis develops by the selective growth of resistant mutants. The incidence of drug-resistant cases depends on the number of bacilli and the drug-resistant mutants in the lesion. The latter is low for individual drugs and even lower for two and three drugs. Therefore, use of combination chemotherapy with three or more drugs results in cure. However, irregular treatment, inadequate drugs, inadequate drug doses or addition of a single drug to a failing regimen allows selective growth of resistant mutants and acquired drug-resistant TB. Contacts of these resistant cases develop primary drug resistant TB. Thus, drug resistance in tuberculosis is a "man-made problem". Anti-TB chemotherapy must be given optimally by (i) ensuring adequate absorption of drugs, (ii) timely diagnosis and management of drug toxicities and (iii) treatment adherence. New classes of anti-TB drugs are needed; but are unlikely to become available soon. It is vital that the 21(st) century physicians understand the basic principles of TB chemotherapy to ensure efficient use of available drugs to postpone or even reverse epidemics drug-resistant TB.