Auranofin Activity Exposes Thioredoxin Reductase as a Viable Drug Target in Mycobacterium abscessus

Abstract

Nontuberculous mycobacteria (NTM) are highly drug-resistant, opportunistic pathogens that can cause pulmonary disease. The outcomes of the currently recommended treatment regimens are poor, especially for Mycobacterium abscessus. New or repurposed drugs are direly needed. Auranofin, a gold-based antirheumatic agent, was investigated for Mycobacterium tuberculosis. Here, we test auranofin against NTM in vitro and ex vivo. We tested the susceptibility of 63 NTM isolates to auranofin using broth microdilution. Next, we assessed synergy between auranofin and antimycobacterial drugs using the checkerboard method and calculated the fractional inhibition concentration index (FICI). Using time-kill kinetics assays (TK), we assessed pharmacodynamics of auranofin alone and in combination with drug combinations showing the lowest FICIs for M. abscessus CIP 104536. A response surface analysis was used to assess synergistic interactions over time in TKs. Primary isolated macrophages were infected with M. abscessus and treated with auranofin. Finally, using KEGG Orthology, we looked for orthologues to auranofins drug target in M. tuberculosis. M. abscessus had the lowest auranofin MIC50 (2 μg/ml) among the tested NTM. The lowest average FICIs were observed between auranofin and amikacin (0.45) and linezolid (0.50). Auranofin exhibited concentration-dependent killing of M. abscessus, with >1-log killing at concentrations of >2× MIC. Only amikacin was synergistic with auranofin according to Bliss independence. Auranofin could not lower the intracellular bacterial load in macrophages. Auranofin itself may not be feasible for M. abscessus treatment, but these data point toward a promising, unutilized drug target.

AC2P20 selectively kills Mycobacterium tuberculosis at acidic pH by depleting free thiols

“…50 Indeed, chloroquine has recently been shown to kill Mtb in vivo by targeting redox homeostasis 50 and auranon also shows promising antimycobacterial activity. 23,51 Furthermore, agents targeting respiration may similarly have activity by promoting redox imbalance. Thus, targeting redox-homeostasis represents an important new approach to treating TB.…”

AC2P20 selectively killsM. tuberculosisat acidic pH by depleting free thiols

“…Furthermore, direct perturbations to either redox-homeostasis or pH-homeostasis results in decreased drug tolerance and enhanced Mtb killing [50]. Indeed, chloroquine has recently been shown to kill Mtb in vivo by targeting redox homeostasis [50] and auranofin also shows promising antimycobacterial activity [23,51]. Furthermore, agents targeting respiration may similarly have activity by promoting redox imbalance.…”

Pipeline of anti‐Mycobacterium abscessus small molecules: Repurposable drugs and promising novel chemical entities

“…Other proposed targets of potential therapeutic interest in M. abscessus , based on the bactericidal activity of novel compounds whose mechanisms of action were partially defined against M. tuberculosis or other microorganisms, include the thioredoxin reductase (MAB_4940), an enzyme required for the survival of M. tuberculosis under oxidative stress and a target of auranofin (MIC 50 of auranofin against M. abscessus = 2 µg/ml) 95 ; the NaMN adenylyltransferase (NadD) involved in NAD biosynthesis (MIC of M. tuberculosis NadD inhibitors in the range of 25–62 µM against M. abscessus ) 96 ; the ClpC1/P1/P2 proteolytic complex responsible for the degradation of intracellular proteins and the target of the cyclic peptide, rufomycin I (MIC of rufomycin I against M. abscessus = 0.4 µM) 97 ; and the cell division protein FtsZ (MIC of C109, an inhibitor of FtsZ in Burkholderia cenocepacia , against M. abscessus = 4‐8 µg/ml) 98 …”

“…A few other approved drugs—auranofin (a gold‐containing drug for the treatment of rheumatoid arthritis), flomoxef (an antibiotic for the treatment of upper urinary tract infections), metronidazole (an antiprotozoal agent), delamanid (an antitubercular drug)—failed to show potential for repositioning in the treatment of M. abscessus infections 95,123,152‐154 …”