A systematic review of the literature on mechanisms of 5-nitroimidazole resistance in Trichomonas vaginalis

Abstract

Background: Trichomonas vaginalis is the most common non-viral sexually transmitted infection. 5-Nitroimidazoles [metronidazole (MTZ) and tinidazole (TDZ)] are FDA-approved treatments. To better understand treatment failure, we conducted a systematic review on mechanisms of 5-nitroimidazole resistance.

Methods: PubMed, ScienceDirect and EMBASE databases were searched using keywords Trichomonas vaginalis, trichomoniasis, 5-nitroimidazole, metronidazole, tinidazole and drug resistance. Non-English language articles and articles on other treatments were excluded.

Results: The search yielded 606 articles, of which 550 were excluded, leaving 58 articles. Trichomonas vaginalis resistance varies and is higher with MTZ (2.2-9.6%) than TDZ (0-2%). Resistance can be aerobic or anaerobic and is relative rather than absolute. Differential expression of enzymes involved in trichomonad energy production and antioxidant defenses affects 5-nitroimidazole drug activation; reduced expression of pyruvate:ferredoxin oxidoreductase, ferredoxin, nitroreductase, hydrogenase, thioredoxin reductase and flavin reductase are implicated in drug resistance. Trichomonas vaginalis infection with Mycoplasma hominis or T. vaginalis virus has also been associated with resistance. Trichomonas vaginalis has two genotypes, with greater resistance seen in type 2 (vs type 1) populations.

Discussion: 5-Nitroimidazole resistance results from differential expression of enzymes involved in energy production or antioxidant defenses, along with genetic mutations in the T. vaginalis genome. Alternative treatments outside of the 5-nitroimidazole class are needed.

Keywords: 5-Nitroimidazole; Trichomonas vaginalis; drug resistance; metronidazole; tinidazole.

Fig. 1.

Flow chart. Flow chart of the databases used to perform the systematic review of the literature. Included are the number of articles reviewed for study inclusion and those that were excluded for various reasons.

Fig. 2.

MTZ activation through the energy production pathway in T. vaginalis. (a) Overview of T. vaginalis and the MTZ drug activation pathway. (1) MTZ enters the cell by passive diffusion and is subsequently metabolized in the hydrogenosome. (2) Pyruvate is produced from glucose through glycolysis in the cytosol. (3) Inside the hydrogenosome, PFOR facilitates oxidative decarboxylation of pyruvate by reducing and transferring electrons to Fdx. Fdx then reduces the nitro group of MTZ, which creates a cytotoxic nitro radical anion. (4) Activated MTZ then interacts with T. vaginalis DNA causing damage and death of the organism. (b) Detailed energy production and MTZ drug activation pathway in the hydrogenosome. Solid line: Major energy production pathway: Pyruvate is the major intermediate product produced from glucose through the glycolytic pathway (pathway 1) as well as through an alternative malate-dependent pathway (pathway 2). PFOR then transfers electrons from pyruvate to the electron acceptor Fdx, producing acetyl-CoA, which is through ASCT/SCS cycle reduced to acetate, producing ATP (pathway 3). The reduced Fdx is used to produce H₂ by the HYD enzyme. Dashed line: Drug activation pathway – Once MTZ enters the hydrogenosome, it competes with HYD for the electron carrying Fdx. MTZ is reduced to a nitro radical anion that interacts with T. vaginalis DNA causing damage and death of the organism. PFOR, pyruvate:ferredoxin oxidoreductase; Fdx, ferredoxin; HYD, hydrogenase; H₂, hydrogen; CO₂, carbon dioxide; MTZ, metronidazole; MDH, malate dehydrogenase; NADPH, nicotinamide adenine dinucleotide phosphate; NADH:FOR, nicotinamide adenine dinucleotide:ferredoxin oxidoreductase; PEPCK, phosphoenolpyruvate carboxykinase; GTP/GDP, guanosine triphosphate/guanosine diphosphate; PDC, pyruvate decarboxylase; LDH, lactate dehydrogenase; ADH, alcohol dehydrogenase; ASCT, acetate:succinate CoA-transferase; SCS, succinyl CoA synthetase; OX, oxidized; RED, reduced.

Fig. 3.

Aerobic resistance to MTZ arises from deficient O₂ scavenging pathways. (a) Normal T. vaginalis antioxidant defense pathways – (1) ROS, such as O₂⁻, can be harmful to trichomonads, and multiple pathways can control and ‘neutralize’ ROS. SOD reduces O₂⁻ to molecular O₂ while also producing water and H₂O₂. (2) Intracellular O₂ can be reduced to H₂O₂ by FR1 or by (3) NADH oxidase into H₂O. The cytotoxic H₂O₂ produced by these pathways further metabolized by the trichomonad. (4) H₂O₂ can be indirectly reduced to H₂O, through the activation of the flavin-containing enzyme TrxR by NADPH. Electrons from NADPH are transferred from TrxR to Trx, which activates thioredoxin-dependent peroxidases such as TrxP to reduce H₂O₂ to H₂O. (b) Aerobic resistance pathway – (1) TrxR, like Fdx and NTR, can reduce MTZ. Once reduced, MTZ can form covalent adducts with TrxR and Trx. These adducts inhibit the activities of the enzymes, leading to increased levels of cytotoxic H₂O₂. (2) FR1 and NADH oxidase activities are significantly decreased or absent in MTZ resistant strains. (3) Intracellular O₂ levels increase in the absence of normally functioning O₂-scavenging pathways, resulting in the inactivation of MTZ from its nitro radical anion in a process known as futile cycling. ROS, reactive oxygen species; O₂⁻, superoxide; SOD, superoxide dismutase; O₂, oxygen; H₂O₂, hydrogen peroxide; FR1, flavin reductase 1; NADH, nicotinamide adenine dinucleotide; TrxR, thioredoxin reductase; NTR, nitroreductase; NADPH, nicotinamide adenine dinucleotide phosphate; Trx, thioredoxin; TrxP, thioredoxin peroxidase; MTZ, metronidazole; OX, oxidized; RED, reduced.