These data add to the mounting evidence against the dogma that PFOR/Fd is the only couple with a low enough redox potential to reduce metronidazole in anaerobes and point to the multi-factorial nature of metronidazole resistance.

Trichomoniasis is the most common, sexually transmitted infection. It is caused by the flagellated protozoan parasite Trichomonas vaginalis. Symptoms include vaginitis and infections have been associated with preterm delivery, low birth weight and increased infant mortality, as well as predisposing to HIV/AIDS and cervical cancer. Trichomoniasis has the highest prevalence and incidence of any sexually transmitted infection. The 5-nitroimidazole drugs, of which metronidazole is the most prescribed, are the only approved, effective drugs to treat trichomoniasis. Resistance against metronidazole is frequently reported and cross-resistance among the family of 5-nitroimidazole drugs is common, leaving no alternative for treatment, with some cases remaining unresolved. The mechanism of metronidazole resistance in T. vaginalis from treatment failures is not well understood, unlike resistance which is developed in the laboratory under increasing metronidazole pressure. In the latter situation, hydrogenosomal function which is involved in activation of the prodrug, metronidazole, is down-regulated. Reversion to sensitivity is incomplete after removal of drug pressure in the highly resistant parasites while clinically resistant strains, so far analysed, maintain their resistance levels in the absence of drug pressure. Although anaerobic resistance has been regarded as a laboratory induced phenomenon, it clearly has been demonstrated in clinical isolates. Pursuit of both approaches will allow dissection of the underlying mechanisms. Many alternative drugs and treatments have been tested in vivo in cases of refractory trichomoniasis, as well as in vitro with some successes including the broad spectrum anti-parasitic drug nitazoxanide. Drug resistance incidence in T. vaginalis appears to be on the increase and improved surveillance of treatment failures is urged.

No abstract

Metronidazole (Mz)-resistant Giardia and Trichomonas were inhibited by 1 of 30 new 5-nitroimidazole drugs.Another five drugs were effective against some but not all of the Mz-resistant parasites. This study provides the incentive for the continued design of 5-nitroimidazole drugs to bypass cross-resistance among established 5-nitromidazole antiparasitic drugs.Metronidazole (Mz) and a related 5-nitroimidazole, tinidazole, are the only drugs recommended for the treatment of trichomoniasis and are the most-prescribed drugs for the treatment of giardiasis. However, clinical resistance to these drugs has been well documented; and in the event of overt clinical resistance to Mz in trichomonads, there is no alternative for treatment, when one keeps in mind the documented crossresistance between the currently used 5-nitroimidazole drugs and their worldwide availability (7,8,18,23). Some success has been obtained with quinacrine and albendazole in combination with Mz in cases of giardiasis treatment failures (23). On the positive side, a great deal of flexibility is offered by the side chains attached to the imidazole ring structure that bear the all important nitro group (17).The mechanisms of Mz resistance in Giardia and Trichomonas have been well studied in laboratory-induced resistance (18). It occurs by down-regulation of pathways, especially the enzyme pyruvate:ferredoxin oxidoreductase (PFOR) and ferredoxin (Fd) pathway, that activate Mz to its toxic radical state. The PFOR-Fd couple has an electron potential sufficiently low to activate Mz, while no such electron couple is present in the mammalian host (9). In the laboratory we see a threefold down-regulation of PFOR activity in Mz-resistant (Mz r ) Giardia duodenalis (14), and in highly Mz r Trichomonas vaginalis the activity of the hydrogenosome organelle is downregulated such that there is no detectable PFOR or Fd expression (4,11,18). Thus, Mz is not activated to its toxic radical state in these cells. On the other hand, it is well documented that clinically Mz r T. vaginalis strains do not have down-regulated hydrogenosomes, and the mechanism of Mz resistance in these cells is not understood (8).Previously, we showed that some 5-nitroimidazole derivatives were significantly more effective antiprotozoal agents (based on in vitro molar drug concentrations) than Mz against Mz-susceptible (Mz s ) parasites but were not as effective against Mz r parasites (17). Given the impetus for the development of 5-nitroimidazole drugs that vary markedly in their efficacies (both positively and negatively), we tested 30 new 5-nitroimidazoles in our anaerobic drug susceptibility screening assay (16) for their efficacies against T. vaginalis and G. duodenalis, with the focus on laboratory-derived Mz r (Mz rl ) lines and clinical isolates derived from patients with treatment failures.Parasites were cultured axenically in anaerobic TYI-S-33 (6), which was modified as described previously (16 All new 5-nitroimidazole compounds (Fig. 1) were identified by spectral data, purified b...

Infections with the diarrheagenic protozoan pathogen Giardia lamblia are most commonly treated with metronidazole (Mz). Treatment failures with Mz occur in 10 to 20% of cases and Mz resistance develops in the laboratory, yet clinically, Mz-resistant (Mz r ) G. lamblia has rarely been isolated from patients. To understand why clinical Mz r isolates are rare, we questioned whether Mz resistance entails fitness costs to the parasite. Our studies employed several newly generated and established isogenic Mz r cell lines with stable, high-level resistance to Mz and significant cross-resistance to tinidazole, nitazoxanide, and furazolidone. Oral infection of suckling mice revealed that three of five Mz r cell lines could not establish infection, while two Mz r cell lines infected pups, albeit with reduced efficiencies. Failure to colonize resulted from a diminished capacity of the parasite to attach to the intestinal mucosa in vivo and to epithelial cells and plastic surfaces in vitro. The attachment defect was related to impaired glucose metabolism, since the noninfectious Mz r lines consumed less glucose, and glucose promoted ATP-independent parasite attachment in the parental lines. Thus, resistance of Giardia to Mz is accompanied by a glucose metabolism-related attachment defect that can interfere with colonization of the host. Because glucose-metabolizing pathways are important for activation of the prodrug Mz, it follows that a fitness trade-off exists between diminished Mz activation and reduced infectivity, which may explain the observed paucity of clinical Mz r isolates of Giardia. However, the data also caution that some forms of Mz resistance do not markedly interfere with in vivo infectivity.