The metabolism of metronidazole (1-2'-hydroxyethyl-2-methyl-5-nitroimidazole)

Metronidazole: an update on metabolism, structure-cytotoxicity and resistance mechanisms

Metronidazole, a nitroimidazole, remains a front-line choice for treatment of infections related to inflammatory disorders of the gastrointestinal tract including colitis linked to Clostridium difficile. Despite >60 years of research, the metabolism of metronidazole and associated cytotoxicity is not definitively characterized. Nitroimidazoles are prodrugs that are reductively activated (the nitro group is reduced) under low oxygen tension, leading to imidazole fragmentation and cytotoxicity. It remains unclear if nitroimidazole reduction (activation) contributes to the cytotoxicity profile, or whether subsequent fragmentation of the imidazole ring and formed metabolites alone mediate cytotoxicity. A molecular mechanism underpinning high level (>256 mg/L) bacterial resistance to metronidazole also remains elusive. Considering the widespread use of metronidazole and other nitroimidazoles, this review was undertaken to emphasize the structure-cytotoxicity profile of the numerous metabolites of metronidazole in human and murine models and to examine conflicting reports regarding metabolite-DNA interactions. An alternative hypothesis, that DNA synthesis and repair of existing DNA is indirectly inhibited by metronidazole is proposed. Prokaryotic metabolism of metronidazole is detailed to discuss new resistance mechanisms. Additionally, the review contextualizes the history and current use of metronidazole, rates of metronidazole resistance including metronidazole MDR as well as the biosynthesis of azomycin, the natural precursor of metronidazole. Changes in the gastrointestinal microbiome and the host after metronidazole administration are also reviewed. Finally, novel nitroimidazoles and new antibiotic strategies are discussed.

PEG-metronidazole conjugates: synthesis, in vitro and in vivo properties

Metronidazole (MTZ), a drug used for the treatment of protozoal infections caused by protozoa and anaerobic microorganisms, was conjugated to linear or branched poly(ethylene glycol) of 5,000, 10,000 and 20,000 Da. An ester linkage between polymer and drug was used in the coupling to yield a polymeric prodrug. The modification allowed overcoming the known MTZ solubility problem leading us to obtain a bioconjugate more suitable for parental administration. The conjugates of various molecular weight polymers have been tested in vitro toward chemical degradation and digestive enzymes. It was found that molecular weight and shape of PEG is critical for the prodrugs stability. Good resistance in the stomach acidic media was found and a slow release of the drug in the large intestinal fluid may take place. In vivo studies carried out following i.v. or s.c. administration to mice revealed improved pharmacokinetics properties upon conjugation.

The mechanisms of action of antiprotozoal and anthelmintic drugs in man

The mechanisms of action of antiprotozoal and anthelmintic drugs are reviewed according to: (1) drugs interfering with metabolic processes; (2) drugs interfering with reproduction and larval physiology; and (3) drugs interfering with neuromuscular physiology of parasites.

Changes in membrane-bound hydrolases by metronidazole in rat renal brush border

The antiprotozoal drug metronidazole, when administered orally at a dose level of 100 mg/kg body wt. daily for 7 days to rats, brought about significant elevation of renal brush-border-membrane-bound hydrolytic enzymes, such as alkaline phosphatase, maltase, sucrase, and leucine aminopeptidase (LAP). Kinetic analysis of the enzymes (substrate saturation) indicated that the drug produced an increase in the maximum of apparent initial enzyme velocity (Vmax), while the substrate affinity constant (Km) remained unaltered. These changes were not recovered to the normal level even after the drug regimen was stopped and the animals were allowed to recover for a period of 7 days. Lipid analysis of brush border membrane (BBM) revealed a significant elevation in the cholesterol, phospholipid, and ganglioside levels, while no marked change was recorded in triglyceride, free fatty acid and plasmalogen. Study of the temperature-dependent parameters of the enzymes showed that metronidazole induced a shift in the transition temperature (To) in LAP with nearly total reversibility in the recovery group. No such change was seen in the other enzymes. However, there also was a lowering in the energy of activation (Ea) below To, which returned to normal after the treatment was withdrawn.

Review article: metronidazole and the anaerobic gut flora

Metronidazole is a nitro-imidazole drug which was discovered nearly 30 years ago. Metronidazole has remained the mainstay of anti-anaerobic therapy following a change observation that its activity included anaerobic organisms. The predominant human reservoir of these anaerobic organisms is the gastrointestinal tract. In this review, the anaerobic flora and their pathogenic potential are described. The main characteristics of metronidazole and the role of selective decontamination of the gastrointestinal tract are also discussed.

Synthesis and biodistribution of 18F-labeled fluoronitroimidazoles: potential in vivo markers of hypoxic tissue

Three 18F labeled fluoronitroimidazoles have been prepared as potential in vivo markers of hypoxic cells in tumors, and ischemic areas of the heart and brain. 1-(2-Nitroimidazolyl)-3-[18F]fluoro-2-hydroxypropanol (18F]fluoro-normethoxymisonidazole) 4, 1-(2-[18F]fluoroethyl)-2-nitroimidazole 7, and 1-(2-[18F]-fluoroethyl)-2-methyl-5-nitroimidazole ([18F]fluoro-norhydroxymetronidazole) 10 were prepared in average radiochemical yields of less than 1%, 23% and 15-43% (8% at the no carrier-added level) respectively at end-of-synthesis. The in vivo biodistribution in rats was determined for each of the 18F labeled fluoronitroimidazoles. At 1 and 3 h after administration, the tissue distribution of each of the 18F labeled nitroimidzaoles was quite uniform and consistent with that of nitroimidazoles previously studied. These results suggest the need for a suitable animal model to evaluate their potential as in vivo markers of hypoxic tissue in the brain.

Observation of drug metabolites and endogenous compounds in human urine by 1H nuclear magnetic resonance spectroscopy

High-resolution 1H nuclear magnetic resonance (1H-n.m.r.) spectroscopy at 300 MHz has been used to monitor the excretion in human urine of the drug metronidazole and its metabolites. One or more resonances from all of the major metabolites are resolved in spectra of whole urine, and can be used to quantify excretion of the parent drug and its metabolites. The spectra also contain resonances from a large number of endogenous metabolites, many of which are readily identified. 1H n.m.r. should be useful in monitoring, simultaneously and non-destructively, the urinary excretion of drugs and drug metabolites, as well as endogenous compounds in either normal or disease states.

1H nuclear magnetic resonance study of metronidazole metabolism by perfused rat liver

High-resolution 1H nuclear magnetic resonance (NMR) spectroscopy at 300 MHz has been used to monitor the metabolism of metronidazole by the perfused rat liver. Samples of perfusate removed at various times during the perfusion show resonances from metronidazole and its major metabolites, as well as those from organic metabolites released from the liver. The major metabolite of metronidazole is shown to be its glucuronide conjugate. Metronidazole metabolism and glucuronide production are both stimulated in livers from rats pretreated with phenobarbitone.

The in vitro effect of some nitroimidazoles on microtubule formation

The in vitro effects of some nitroimidazoles (metronidazole, ornidazole) and their metabolites on microtubule formation have been tested. Cyclic metabolites are without effect. Metabolites proceeding from cleavage of the imidazole ring inhibit microtubule formation and reduce the polymerization rate of tubulin. This inhibitory effect might be correlated to some of the side-effects of these drugs. Isaxonine phosphate corrects this effect.

Disposition of oral metronidazole in hepatic cirrhosis and in hepatosplenic schistosomiasis

The pharmacokinetics of metronidazole 500 mg orally were determined in patients with hepatosplenic schistosomiasis and normal controls in the Sudan, and in cirrhotics and normal controls in Bristol. Plasma metronidazole levels were above the minimum inhibitory concentration of most susceptible anaerobic bacteria for four to six hours post-dose in all groups. Liver disease did not markedly influence the disposition of single oral doses of metronidazole. Cirrhotics showed some prolongation of metronidazole half-life, and somewhat greater metronidazole concentrations 24 hours after the dose. Concentrations of the oxidative metabolite of metronidazole were lower in Sudanese patients and normal controls than in normal British subjects. In chronic liver disease adjustment of metronidazole dosage is probably not required provided renal function is unimpaired.

Response of murine tumours to combinations of CCNU with misonidazole and other radiation sensitizers

The effect of combinations of the conventional chemotherapeutic agent 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) and nitroimidazole radiation sensitizers was evaluated in female C3H mice. Tumour response to single-agent or combination therapy was assessed in a tumour growth-delay assay. In the KHT sarcoma the simultaneous addition of misonidazole (MISO) was found to increase significantly the tumour growth delay resulting from CCNU treatment. The observed enhancement ratios (ER) increased with MISO dose, and ranged from 1.3 to 1.9 for sensitizer doses of 0.25-1.0 mg/g. The combination of CCNU and 1.0 or 0.5 mg/g MISO in the RIF-1 tumour or the MT-1 tumour produced ERs of approximately 2.0 and approximately 1.5 respectively. In the KHT sarcoma a series of other nitroimidazole sensitizers, including Ro-05-9963, SR-2555, SR-2508 and metronidazole (METRO), were also evaluated at equimolar doses (5 mmol/kg) in combination with a 20mg/kg dose of CCNU. Unlike MISO, these compounds in general failed to enhance the CCNU cytotoxicity in this tumour model. However, SR-2508 did enhance the response of the RIF-1 tumour to large single doses of CCNU, though not as much as MISO. Normal-tissue toxicity was determined using peripheral white blood cell (WBC) counts 3 days after treatment. CCNU doses of 10-50 mg/kg given either alone or in simultaneous combination with 0.5 or 1.0 mg/g MISO were studied. WBC toxicity increased with CCNU dose, but the addition of MISO at either dose did not significantly enhance this normal-tissue toxicity.

Metabolic studies of ornidazole in the rat, in the dog and in man

1. Ornidazole, labelled with 14C in the imidazole ring, administered orally to rats, dogs and men was largely excreted in the urine, predominantly as metabolites, with less than 4% of the drug being excreted unchanged. Free and conjugated metabolites were found in the ratio of approx. 1 : 2. 2. The pattern of free ornidazole and metabolites was different in the three species: while ornidazole predominated in man, ornidazole and metabolite M1 in the dog, the most extensive metabolic pattern was found in the rat. 3. The following metabolites were identified: M1, 1-chlorlo-3-(2-hydroxymethyl-5-nitro-1-imidazolyl)-2-propanol; M2, 2-methyl-5-nitroimidazole; M3, N-(3-chloro-2-hydroxypropyl)acetamide: M4, 3-(-2-methyl-5-nitro-1-imidazolyl)-1, 2-propanediol; M5, acetamide. 4. The formation of metabolite M3 and M5 indicated cleavage of the imidazole ring between N-1/C-5 and C-2/C-3. Other ring scissions were not observed. Metabolites carrying a free amino group were not detected. On the basis of the structures identified, a scheme is suggested for the metabolism of ornidazole.

Species differences in metabolism of tiaramide hydrochloride, a new anti-inflammatory agent

1. Urinary excretion of the radioactivity in 24 h after oral administration of [14C]tiaramide hydrochloride was 67% of the dose in mice, 59% in rats, 41% in dogs and 74% in monkeys. 2. The serum half-lives of tiaramide after intravenous administration were approximately 0-2 h in mice, 0-8 h in rats and 0-5 h in dogs. 3. Marked species variations were noted in the composition of metabolites in the serum and urinary radioactivity. The major metabolites found were 1-[(5-chloro-2-oxo-3(2H)-benzothiazolyl)acetyl]-piperazine (DETR) and 4-[(5-chloro-2-oxo-3(2H)-benzothiazolyl)acetyl]-1-piperazineacetic acid (TRAA) in mice, TRAA and 4-[(5-chloro-2-oxo-3(2H)-benzothiazolyl)acetyl]-1-piperazineethanol 1-oxide (TRNO) in rats, TRNO and tiaramide-O-glucuronide (TR-O-Glu) in dogs, and TRAA and TR-O-Glu in monkeys. 4. The binding of tiaramide to plasma protein of the various species of animals and human was about 24-34% and the extent of the binding of tiaramide to human plasma protein was independent of drug concentration within the range of 1-100 micron.

Uptake of metronidazole and its effect on viability in trichomonads and Entamoeba invadens under anaerobic and aerobic conditions

[(14)C]metronidazole used at the chemotherapeutic concentration of 10 mug/ml is taken up rapidly by the anaerobic protozoa Tritrichomonas foetus, Trichomonas vaginalis, and Entamoeba invadens kept under anaerobic conditions. It can be calculated that within 30 to 60 min the intracellular concentration of the label is 50 to 100 times higher than in the medium. The presence of air markedly suppresses the uptake in the trichomonads and abolishes it in E. invadens. The suppression disappears after anaerobic conditions are established. The rate of uptake in T. foetus is dependent on the concentration of the drug in the range studied (1 to 200 mug/ml). Analysis of double reciprocal plots suggests that the drug enters the cells predominantly or exclusively by diffusion. The major factor driving the uptake is most likely the intracellular biotransformation of the compound. If less than 3 mug of drug per mg of protein is taken up by T. foetus no decrease in viability is observed. Above this level the cytotoxic activity corresponds roughly to the amount accumulated in the cell, irrespective of whether the conditions are anaerobic or aerobic.

GLC analysis of metronidazole in human plasma

A sensitive procedure was developed for metronidazole in human plasma. The metronidazole is extracted from the plasma with chloroform and determined as the trimethylsilyl derivatives by GLC using a flame-ionization detector. Myristyl alcohol is used as the internal standard for quantitation by relative peak height.

The fate of metronidazole and tis implications in chemotherapy

1. [14C]Metronidazole was rapidly absorbed from the gastro-intestinal tract of rats giving maximum blood levels of radioactivity, equivalent to 6-4 and 6-7 mug metronidazole per ml blood, 1 h after oral dosing. 2. There was rapid equilibration between blood and most tissues, although radioactivity was concentrated in the liver, kidney, gastro-intestinal tract and vaginal secretions. 3. The half-life of clearance of radioactivity from the majority of tissues was between 3 and 4 h, although it was longer in the skin (8 h) and gastro-intestinal tract (14 h). 4. Fourteen radioactive excretion products were detected in rat urine and all the major products were identified. These all contained a nitro group and represented 97 degrees of the total radioactivity excreted in urine. 5. Unchanged metronidazole was secreted throughout the entire length of the gastro-intestinal tract and into the vagina of rats. 6. A hypothesis has been proposed to explain the high clinical efficacy of metronidazole in treating trichomonal and amoebic infections.