Mycobacterium tuberculosis Folate Metabolism and the Mechanistic Basis for para -Aminosalicylic Acid Susceptibility and Resistance

Abstract

b para-Aminosalicylic acid (PAS) entered clinical use in 1946 as the second exclusive drug for the treatment of tuberculosis (TB). While PAS was initially a first-line TB drug, the introduction of more potent antitubercular agents relegated PAS to the secondline tier of agents used for the treatment of drug-resistant Mycobacterium tuberculosis infections. Despite the long history of PAS usage, an understanding of the molecular and biochemical mechanisms governing the susceptibility and resistance of M. tuberculosis to this drug has lagged behind that of most other TB drugs. Herein, we discuss previous studies that demonstrate PAS-mediated disruption of iron acquisition, as well as recent genetic, biochemical, and metabolomic studies that have revealed that PAS is a prodrug that ultimately corrupts one-carbon metabolism through inhibition of the formation of reduced folate species. We also discuss findings from laboratory and clinical isolates that link alterations in folate metabolism to PAS resistance. These advancements in our understanding of the basis of the susceptibility and resistance of M. tuberculosis to PAS will enable the development of novel strategies to revitalize this and other antimicrobial agents for use in the global effort to eradicate TB. Mycobacterium tuberculosis is responsible for approximately 8.6 million new cases of active tuberculosis (TB) infection and 1.3 million deaths annually despite the existence of TB therapy (1). While this therapy has a high success rate in curing drugsusceptible TB infections, it is challenging, in part because it requires a minimum of 6 months of treatment with drugs that are associated with adverse reactions (2, 3). These factors contribute to treatment errors and noncompliance, which have been implicated in the emergence of drug-resistant strains of M. tuberculosis (4, 5). Further, subsequent relapse of the disease can occur and is associated with a high incidence of drug resistance (6). Together, these complications have enabled the emergent spread of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains of M. tuberculosis that require greater than 2 years of therapy with second-line drugs and threaten the efficacy of existing TB therapy (1, 7). Elucidating the mechanisms that govern the susceptibility and resistance of M. tuberculosis to existing antitubercular agents will facilitate the discovery of new therapeutic approaches to shorten treatment times and counter drug-resistant TB.para-Aminosalicylic acid (PAS) entered clinical use as a bacteriostatic antitubercular agent in 1946 (8). Shortly before the introduction of PAS, the discovery of streptomycin as a therapeutic tool had dramatically improved TB survival rates (9). At that time, it was apparent that the rapid emergence of streptomycin-resistant M. tuberculosis strains posed a threat to this monotherapy strategy for TB infection (9). As PAS was effective against streptomycinresistant strains of M. tuberculosis (10), it was soon recognized that combination therapy could reduce th...

Update of Antitubercular Prodrugs from a Molecular Perspective: Mechanisms of Action, Bioactivation Pathways, and Associated Resistance

“…Because TAC, ISO, PTH andE TH are activated by EthA, mutationso nethA/ethR genes lead to cross-resistance between ETH, PTH, TACa nd ISO. [111] Nowadays,P AS is an important second-line antitubercular drug in multidrug therapy to treat MDR and XDR tuberculosis. [109,110] In the past, PASw as used as af irst-line antitubercular agent in association with streptomycin and isoniazid buti tw as later replaced by ethambutol, ab etter-tolerated agent.…”

“…[111] The main PASm echanism of action is the inhibition, once activated, of the dihydrofolate reductase DfrA, encoded by the dfrA gene, and involved in folate metabolism for the conversion of dihydrofolate( H 2 PteGlu) to tetrahydrofolate (H 4 PteGlu)( Scheme 10). [111] The main PASm echanism of action is the inhibition, once activated, of the dihydrofolate reductase DfrA, encoded by the dfrA gene, and involved in folate metabolism for the conversion of dihydrofolate( H 2 PteGlu) to tetrahydrofolate (H 4 PteGlu)( Scheme 10).…”

“…Resistance to PASi sa ssociated with multiple mechanisms including:1 )reductiono fb ioactivation by FolC, 2) attenuation of DfrA inhibition by the RibD pathway,3 )attenuation of DfrA inhibition by ThyX pathway,and 4) active efflux pump Ta p. [111] Mutations in folC gene, encoding PASa ctivating enzyme FolC, are identified as predominant mutationsi nP AS-resistant strains. [116] More precisely,m utationsi nt he coding sequence of the H 2 Pte binding pocket of dihydrofolate synthase FolC result in decreasing dihydrofolate synthase activity and suppress the bioactivation of the PASm etabolite hydroxy-H 2 Pte.…”

“…[109,110] In the past, PASw as used as af irst-line antitubercular agent in association with streptomycin and isoniazid buti tw as later replaced by ethambutol, ab etter-tolerated agent. [111] Nowadays,P AS is an important second-line antitubercular drug in multidrug therapy to treat MDR and XDR tuberculosis.…”

“…As other prodrugs, PASe xhibits severale ffects on M. tuberculosis and its complete mechanism of action is still unknown, even if some mechanistic proposals are made in the literature. [111] The main PASm echanism of action is the inhibition, once activated, of the dihydrofolate reductase DfrA, encoded by the dfrA gene, and involved in folate metabolism for the conversion of dihydrofolate( H 2 PteGlu) to tetrahydrofolate (H 4 PteGlu)( Scheme 10). [109,111] Reduced folate species, as H 4 PteGlu, serve as essential cofactors in the transfer of onecarbon units in the synthesiso fs everale ssential bacterial components such as methionine, glycine, serine, pantothenate, purines and thymidine.…”