Serum and tissue tinidazole concentrations after intravenous infusion

Trichomoniasis and its treatment

Trichomonas vaginalis has long been recognized as a cause of infectious vaginitis in women. More recently, studies have demonstrated a significant burden of disease in men with urethritis or men at high risk for sexually transmitted diseases. There is increasing interest in this pathogen as more data accumulates linking it to HIV transmission and perinatal morbidity. New diagnostic methods have emerged that may increase sensitivity of diagnosis or improve point-of-care access to testing. Nitroimidazoles remain the mainstay of therapy. Metronidazole and tinidazole are highly effective as single-dose therapy. Unfortunately, despite the link between T. vaginalis infection and perinatal morbidity, nitroimidazole therapy during pregnancy remains controversial. Although metronidazole resistance is currently uncommon, pharmacological features and nitroimidazole resistance patterns suggest that tinidazole may be more effective in treating patients with metronidazole treatment failure. Alternatives to nitroimidazole therapy are few, and most have limited efficacy and significant toxicity.

Pharmacokinetics and pharmacodynamics of the nitroimidazole antimicrobials

Metronidazole, the prototype nitroimidazole antimicrobial, was originally introduced to treat Trichomonas vaginalis, but is now used for the treatment of anaerobic and protozoal infections. The nitroimidazoles are bactericidal through toxic metabolites which cause DNA strand breakage. Resistance, both clinical and microbiological, has been described only rarely. Metronidazole given orally is absorbed almost completely, with bioavailability > 90% for tablets; absorption is unaffected by infection. Rectal and intravaginal absorption are 67 to 82%, and 20 to 56%, of the dose, respectively. Metronidazole is distributed widely and has low protein binding (< 20%). The volume of distribution at steady state in adults is 0.51 to 1.1 L/kg. Metronidazole reaches 60 to 100% of plasma concentrations in most tissues studied, including the central nervous system, but does not reach high concentrations in placental tissue. Metronidazole is extensively metabolised by the liver to 5 metabolites. The hydroxy metabolite has biological activity of 30 to 65% and a longer elimination half-life than the parent compound. The majority of metronidazole and its metabolites are excreted in urine and faeces, with less than 12% excreted unchanged in urine. The pharmacokinetics of metronidazole are unaffected by acute or chronic renal failure, haemodialysis, continuous ambulatory peritoneal dialysis, age, pregnancy or enteric disease. Renal dysfunction reduces the elimination of metronidazole metabolites; however, no toxicity has been documented and dosage alterations are unnecessary. Liver disease leads to a decreased clearance of metronidazole and dosage reduction is recommended. Recent pharmacodynamic studies of metronidazole have demonstrated activity for 12 to 24 hours after administration of metronidazole 1 g. The post-antibiotic effect of metronidazole extends beyond 3 hours after the concentration falls below the minimum inhibitory concentration (MIC). The concentration-dependent bactericidal activity, prolonged half-life and sustained activity in plasma support the clinical evaluation of higher doses of metronidazole given less frequently. Metronidazole-containing regimens for Helicobacter pylori in combination with proton pump inhibitors demonstrate higher success rates than antimicrobial regimens alone. The pharmacokinetics of metronidazole in gastric fluid appear contradictory to these results, since omeprazole reduces peak drug concentration and area under the concentration-time curve for metronidazole and its hydroxy metabolite; however, concentrations remain above the MIC. Other members of this class include tinidazole, ornidazole and secnidazole. They are also well absorbed and distributed after oral administration. Their only distinguishing features are prolonged half-lives compared with metronidazole. The choice of nitroimidazole may be influenced by the longer administration intervals possible with other members of this class; however, metronidazole remains the predominant antimicrobial for anaerobic and protozoal infections.

Concentrations of metronidazole and tinidazole in male genital tissues

The steady-state concentrations of metronidazole and tinidazole in male genital tissues were analyzed in patients subjected to elective gonadal surgery. The nitroimidazoles were administered orally at 500 mg every 8 h, beginning 5 days before the operation. Eight hours after the last dose, the concentrations of tinidazole were 24.1 +/- 2.5 micrograms (mean +/- standard error of the mean)/g of prostatic tissue, 29.1 +/- 2.9 micrograms/g of vas deferens, 22.1 +/- 2.1 micrograms/g of epididymis, and 18.6 +/- 2.3 micrograms/g of testis. The corresponding values of metronidazole were 14.3 +/- 1.8 micrograms/g, 15.9 +/- 1.2 micrograms/g, 14.0 +/- 1.2 micrograms/g, and 12.5 +/- 1.7 micrograms/g, respectively.

Concentrations of metronidazole and tinidazole in female reproductive organs after a single intravenous infusion and after repeated oral administration

Concentrations of metronidazole and tinidazole in serum and gynecological organs were analyzed after a single 500 mg intravenous infusion and after three days of treatment with 400 mg t.i.d. of metronidazole or 500 mg b.i.d. of tinidazole. The studies were performed in 67 patients subjected to hysterectomy and/or oophorectomy because of myomatosis uteri, carcinoma uteri or endometriosis. At the time of organ removal (about 30 min after infusion), metronidazole and tinidazole levels in serum were 14.5 +/- 0.45 mg/l and 12.3 +/- 0.38 mg/l, respectively. Concentrations of both drugs in the uterus and Fallopian tube were about the same as the simultaneous serum levels and concentrations in the ovaries about 55% thereof. At steady-state, the concentrations of tinidazole in serum (23.5 +/- 1.0 mg/l) were remarkably higher than those of metronidazole (13.5 +/- 0.84 mg/l) about three hours after the last oral dose. Drug concentrations in organs of the female reproductive tract were 70 to 100% those of the simultaneous serum levels.

Comparative pharmacokinetics of metronidazole and tinidazole as influenced by administration route

Serum kinetics of metronidazole and tinidazole were compared in four separate randomized crossover studies. Single doses of each drug were given to healthy volunteers through intravenous infusion (500 mg over 20 min, six persons), by mouth (500 mg, nine persons), by rectum (1,000 mg, six persons), or intravaginally (500 mg, six persons). Concentrations of the unchanged drugs in serum, measured by high-pressure liquid chromatography, were similar after oral and intravenous administration, with mean peaks of 9.0 and 9.4 micrograms/ml for metronidazole and 7.5 and 10.1 micrograms/ml for tinidazole. Concentrations of tinidazole were significantly higher than those of metronidazole from 4 h onwards after intravenous infusion, and from 3 h onwards after administration by mouth. After rectal administrations, a significant difference was seen only at 48 h. After vaginal dosing, however, concentrations of metronidazole were significantly higher than those of tinidazole between 1.5 and 12 h. Bioavailability of either drug, calculated according to the formula (area under the curve for oral administration)/(area under the curve for infusion), was practically complete after oral administration and was poorer after rectal and especially vaginal administration. Whenever the parameters were calculable, the elimination half-life of tinidazole (range of means, 14.0 to 14.7 h) was significantly longer and total clearance (40.3 to 47.6 ml/min) was lower than the corresponding values of metronidazole (7.9 to 8.8 h and 71.8 to 80.1 ml/min, respectively).