Bioequivalence of allopurinol and its metabolite oxipurinol in two tablet formulations.

OBJECTIVE: To test for the bioequivalence of two allopurinol 300 mg tablet formulations (generic allopurinol (Normon) and Zyloric tablets). METHOD: A single dose study was carried out in 24 healthy volunteers with a two-sequence, crossover block-randomized design. Blood samples were taken prior to each administration and at 19 points within 72 h after the dose. Plasma concentrations of allopurinol and oxypurinol were determined by HPLC. The pharmacokinetic parameters Cmax and Tmax were obtained directly from plasma allopurinol and oxypurinol concentrations. ke was estimated by log-linear regression and AUC was calculated by the linear trapezoidal rule for both allopurinol and oxypurinol. The pharmacokinetic parameters AUC and Cmax were tested for equivalence after log-transformation of data. Differences of Tmax were evaluated by a non-parametric test. The 90% standard confidence intervals of the mean values for the test/reference ratios were for AUC and for Cmax, within the acceptable bioequivalence limits of 0.80-1.25 for both allopurinol and oxypurinol. CONCLUSION: The two formulations are bioequivalent and therefore interchangeable.

Gender differences in the disposition of metronidazole.

OBJECTIVE: Gender is usually considered to be one of the factors influencing disposition of drugs, but the evidence available is sometimes conflicting and information for a large number of frequently used drugs is lacking. An evaluation of sex differences in the disposition ofmetronidazole was carried out during a bioequivalence study. SUBJECTS AND METHODS: Twenty-four volunteers (12 males and 12 females) were included in an open, single-dose, two-sequence, crossover randomized trial with a one-week washout interval. All volunteers received in each period, a single 250 mg dose of one of the two study formulations of metronidazole. Venous blood samples were collected immediately before and at 15 time points in an 48-hour interval after drug administration; metronidazole concentrations were determined by HPLC. Non-compartmental pharmacokinetic analysis was performed and log-transformed AUC(0-infinity) and Cmax were tested for bioequivalence. Sex differences were evaluated by means of a 4-factor (sex, sequence, treatment and period) ANOVA. RESULTS: The studied formulations were found bioequivalent according to international standards: average 90% confidence interval for AUC(0-infinity) was 98 to 104 and for Cmax 93 to 115. After correction for the administered dose/kg, AUC was about 12% lower in females than in males (p = 0.0388) and, therefore, a higher calculated oral Cl/kg was found in females. Apparent distribution volume, after correction for weight, was significantly higher in males (p = 0.0019). Metronidazole half-life and MRT were shorter in females than in males (p - 0.0014 and p = 0.0002, respectively). CONCLUSIONS: Data obtained in this study suggest that metronidazole clearance in females is about 12% higher than in males although these differences are probably of no clinical relevance.

Evaluation of sex differences in the pharmacokinetics of ranitidine in humans.

A bioequivalence study of two oral formulations of 300 mg ranitidine was carried out in 16 healthy volunteers (8 men and 8 women), and the pharmacokinetics in both sexes were compared. There was bioequivalence of both formulations. The terminal half-life of ranitidine was 7% shorter and the oral apparent clearance 10.5% higher in women (1.44 L/h/kg) than in men (1.29 L/h/kg), although this difference did not reach statistical significance. No differences were observed in maximum concentration (Cmax) or the time of its occurrence (tmax). Sex, age, and weight did not correlate significantly with oral clearance. These results suggest that there are no sex differences in the pharmacokinetics of ranitidine, or that any differences would not be of clinical relevance. It also should be emphasized that bioequivalence trials also can be used to study other pharmacokinetic or pharmacodynamic characteristics of drugs without damaging the main endpoint of the study.

Vasodilator effect and tolerance induced by the nitrocompound SIN-1 in rabbit femoral artery.

The aim of this study was to investigate vascular actions of SIN-1 in comparison with those of other nitrovasodilators in rabbit femoral arteries. SIN-1 induced relaxations that were unaltered by endothelium removal, inhibition of nitric oxide (NO) synthase and inhibition of the formation of oxygen reactive species such as anion superoxide, hydrogen peroxide and hydroxyl radical. Oxyhemoglobin reduced the relaxation caused by SIN-1 and exogenous NO. Exogenous NO and nitroglycerin relaxations were endothelium-independent. However, those produced by sodium nitroprusside were increased in endothelium denuded segments. Preincubation of segments with nitroglycerin (100 mM) produced a marked tolerance, whereas this effect did not occur when preincubation was done with SIN-1 (100 mM). To analyze the in vivo tolerance, rabbits were chronically treated with SIN-1 or nitroglycerin (15 mg/kg every 8 h, s.c.). Chronic treatment with SIN-1 for 5 days did not reduce relaxation response. However, 3 days of treatment with nitroglycerin induced a marked reduction of relaxations. Chronic treatment with nitroglycerin did not modify the relaxation caused by SIN-1, i.e., there was no cross-tolerance between the two drugs. These results suggest that the endothelium-independent relaxation elicited by SIN-1 is not modulated by endothelial NO or free radicals, and does not produce tolerance after its prolonged administration as occurs with nitroglycerin.

Effect of angiotensin converting enzyme inhibitors on quality of life in hypertensive patients. Pharmacodynamic basis.

The aim of this review is to comment the results described in the literature concerning the possible pharmacodynamic mechanisms involved in the improvement of quality of life of angiotensin converting enzyme inhibitors that is just a working hypothesis. These drugs, widely used in the treatment of hypertension, prevent the formation of angiotensin II and the generation of free radicals, as well as the hydrolysis of bradykinin, enkephalins and endorphins. Different mechanisms have been implicated on quality of life: 1) increase of bradykinin levels in the central nervous system that would trigger the release of nitric oxide (NO), noradrenaline, acetylcholine, excitatory amino acids and vasopressin which are involved in memory and cognition; 2) increase of brain blood supply by enhanced NO synthesis; 3) interference with cholinergic mechanisms in the central nervous system by angiotensin II inhibition of acetylcholine release; 4) decrease of endorphin metabolism; and 5) interaction with hypothalamic-pituitary-adrenal axis that releases ACTH and vasopressin.