Inter- and intra-individual variation in the metabolism of methaqualone in man after a single oral dose

Influence of menstrual cycle on the pharmacokinetics of paracetamol through salivary compartment in healthy subjects

Hormonal changes during the different phases of menstrual cycle may influence drug disposition. The objective of this study was to investigate the influence of the menstrual cycle on the pharmacokinetics of paracetamol through salivary compartment in young healthy Indian women (n = 12) with regular menstrual cycles. The subjects received an oral dose of 1 g paracetamol on the 3rd, 13th, and 23rd days of their menstrual cycle in a 3 x 3 randomized crossover design. Saliva samples were collected at predetermined time intervals and the paracetamol content in them was estimated using an established HPLC method. The pharmacokinetics of paracetamol was worked out using a model-independent method employing WinNonlin 3.1. The mean Cmax of paracetamol was significantly (p < 0.05) lower (31.5%) in the ovulatory phase than in the follicular phase. The mean AUC0-t and AUC0-infinity values were significantly (p < 0.05) lower in the ovulatory phase than those in the luteal phase. These changes could be due to increased first-pass metabolism and decreased bioavailability of paracetamol during the ovulatory phase.

Sex specificity in methadone analgesia in the rat: a population pharmacokinetic and pharmacodynamic approach

PURPOSE

To quantify the extent to which a sex-specific dichotomy in the temporal evolution of the analgesic effect, after intravenous (i.v.) methadone injection in the rat, relates to the pharmacokinetics (PK) and pharmacodynamics (PD) that mediate the dose-to-effect pathway.

METHODS

Tail-flick analgesia was measured after i.v. methadone injection (0.35 mg/kg) in female (n = 16) and male (n = 16) Sprague-Dawley rats. The PK were evaluated in separate female (n = 56) and male (n = 56) rats after they had received the same dose of methadone i.v. (0.35 mg/kg). A bicompartmental model described the kinetics and a sigmoid Emax model-related drug effect vs. simulated concentrations (pharmacodynamics) at the times of effect measurement. All model parameters as well as interanimal and assay variabilities were estimated with a mixed-effects population method using the program NONMEM.

RESULTS

The area under the effect-time curve (AUCE0-120) was (mean +/- interanimal SD) 1859+/-346 min in the females, which was significantly lower than the 4871+/-393 min in the males (P < 0.0001). On the contrary, the profiles of concentration vs. time were higher in females and, therefore, corresponded inversely to the effect vs. time-relative magnitudes. The central volume of distribution, V1, was 1.94+/-0.37 l/kg for female rats and 3.01+/-0.33 l/kg for male rats. Also, the central clearance was 0.077+/-0.006 l/min/kg and 0.102+/-0.005 l/min/ kg, respectively, for female and male rats. Both parameters differed significantly between sexes (P < 0.0001). The pharmacodynamic maximum observed effect parameter (Emax) was 37%+/-29% in female rats and 85%+/-16% in male rats, and these values were significantly different (P < 0.0001). The parameter for the concentration eliciting half of Emax (EC50) was 24.1+/-7.5 microg/l in female rats and 20.3+/-2.9 microg/l in male rats, and the Hill-related exponent, gamma, was 6.3+/-3.9 in female rats and 5.5+/-4.1 in male rats. These parameters did not differ significantly (at the P < 0.05 level).

CONCLUSIONS

A sex-specific dichotomy in the methadone antinociceptive effect, in the rat, was not proportionally related to plasma concentrations. Each sex corresponded to a distinct subpopulation of the PK parameters and one of the pharmacodynamic parameters (Emax). When the course of a drug involves PK or PD subpopulations, PK/PD modeling can afford the safest prediction of the effect-time evolution for a particular dose.

Enantiomeric analysis of the five major monohydroxylated metabolites of methaqualone in human urine by chiral capillary electrophoresis

Methaqualone (MQ; 2-methyl-3-o-tolylquinazolin-4(3H)-one) is a hypnotic and anticonvulsive drug in which the rotation about the nitrogen-to-aryl bond between the planar 2-methyl-quinazolin-4(3H)-one structure and the o-tolyl moiety is sterically hindered at body temperature. MQ and its five major monohydroxylated metabolites found in urine, 4'-hydroxymethaqualone (4'OH-MQ), 2'-hydroxymethaqualone (2'-OH-MQ), 3'-hydroxymethaqualone (3'OH-MQ), 2-hydroxymethaqualone (2OH-MQ) and 6-hydroxymethaqualone (6OH-MQ), are thus chiral substances whose enantiomers are shown to be separable by chiral capillary electrophoresis at pH 2.1 in the presence of 50 mM (2-hydroxypropyl)-beta-cyclodextrin (OHP-beta-CD). Other neutral derivatives of beta-CD, namely (2-hydroxypropyl)-gamma-CD, (2,3,6-trimethyl)-beta-CD, and (2,6-di-O-methyl)-beta-CD were found to be able to resolve the enantiomers of some but not all of these six components. With OHP-beta-CD, simultaneous analysis of the enantiomers of MQ and its five metabolites is hampered by the difficulty in separating MQ and 4'OH-MQ, the major urinary metabolite. A two-step solid phase extraction process is shown to permit discrimination between these two compounds and thus analysis of MQ enantiomers in unhydrolyzed urines that were collected overnight after administration of 250 mg of racemic MQ. Furthermore, analysis of liquid/liquid or solid-phase extracts of enzymatically hydrolyzed urines reveals the distribution of the enantiomers of the five hydroxymetabolites of MQ and, for the first time, insight into the stereoselectivity of the MQ metabolism. The major metabolite, 4'OH-MQ, is shown to be excreted almost exclusively as single enantiomer. The two urinary enantiomers of 6OH-MQ are present at about equal amounts, whereas unequal amounts are noted for the enantiomers of 3'OH-MQ, 2OH-MQ, and 2'OH-MQ.

Pharmacokinetic changes of theophylline and amikacin through the menstrual cycle in healthy women

The objective of this open-label, single-dose study was to clarify the influence of the menstrual cycle on the pharmacokinetics of theophylline (n = 10) and amikacin (n = 8) in young healthy Japanese women with regular menstrual cycles. Each subject received an intravenous infusion of theophylline or amikacin sulfate at four different phases--mid-follicular (phase I), peri-ovulatory (phase II), mid-luteal (phase III), and premenstrual days (phase IV). In the theophylline study, there were no significant differences in the pharmacokinetic parameters among the four phases studied. In the amikacin study, CLtot was 15% higher in phase III than in phase I (p < 0.01). Vd beta was 35% higher in phase III than in phase I (p < 0.05). The other pharmacokinetic parameters of amikacin were not significantly altered during the menstrual cycle. Evidence suggests that the phase of the menstrual cycle may be a factor in determining the pharmacokinetics of amikacin.

Comparison of methaqualone excretion patterns using Abuscreen ONLINE and EMIT II immunoassays and GC/MS

A study was performed to compare the ONLINE and EMIT II immunoassays with gas chromatographic/mass spectrometric (GC/MS) analysis of methaqualone metabolites on urine using samples obtained from a clinical study. Urine was collected over a 72 h period from six healthy adults (4 male, 2 female) after oral dosing with 200 mg methaqualone (MTQ). Each urine sample was analyzed by ONLINE and EMIT II. The samples were then analyzed by GC/MS, hydrolyzed with beta-glucuronidase and again analyzed by GC/MS. Both immunoassays showed greater than 600 ng/ml concentrations of drug in each sample by the second void and remained highly positive for the rest of the 72 h. Unhydrolyzed samples analyzed by GC/MS showed both low concentrations of MTQ as well as its five major hydroxylated metabolites. The hydrolyzed samples analyzed by GC/MS showed high concentrations of the hydroxylated metabolites with the 2'-hydroxy and 3'-hydroxy metabolites being present at the highest concentrations, the 4'-hydroxy metabolite at a lower amount and the 6-hydroxy and 2-hydroxy metabolites at the lowest concentrations. The GC/MS data coupled with the antibody cross-reactivity data indicate that the major species in clinical samples that cross-react in both immunoassays are the conjugated forms of the hydroxylated metabolites of MTQ. Therefore when confirming by GC/MS after an immunoassay screen it would be prudent to confirm for the major hydroxylated metabolites as glucuronides of MTQ instead of the parent drug.

Interindividual and intraindividual variability in labetalol pharmacokinetics

The between-subject and within-subject variability in the pharmacokinetics of labetalol at steady state were determined. Sixteen nonobese normal volunteers (mean age, 27 years) received five different formulations of labetalol orally on five different occasions every 12 hours for five doses. A 7-day washout separated each administration phase. Plasma concentration-time data for labetalol were obtained over the 24-hour period after the fifth dose in each phase. Labetalol concentrations in plasma were measured using high-performance liquid chromatography (HPLC). Pharmacokinetic parameters for each subject after each study phase were estimated. The mean V beta/F, Vdss/F, TBC/F, t1/2 beta, and AUC tau 0 for each subject ranged between 18.1 and 161.9 L/kg, 7.1 and 53.9 L/kg, 1.3 and 5.72 L/hr/kg, 6.9 and 11.0 hours, and 154 and 520 micrograms.hr/L, respectively, indicating large interindividual variability. Considerable intraindividual variability in each of the pharmacokinetic parameters was also observed. These data indicate that a larger number of subjects will be required to detect "significant" differences in the disposition of labetalol.

The effect of age on the competitive C- and N-oxidative pathways of methaqualone in women

The urinary excretion of the N-oxide and the glucuronides of five C-monohydroxy metabolizes of methaqualone has been studied in a group of young women aged 24-34 years and in a group of elderly women aged 85-87 years. The rate of appearance of the metabolites in the urine was slower in the elderly group but the relative importance of the six metabolites was the same in the two groups. There were differences in some metabolite ratios between the two groups, but only one difference approached statistical significance. The metabolite excretion in a 80 year old woman who had received methaqualone daily for over ten years was consistent with that of the elderly group. The results indicate that the ageing process per se and chronic weak hepatic enzyme induction in the elderly are not accompanied by changes in the relative importance of competitive metabolic pathways.

Sex-related differences in drug disposition in man

Sex-related differences in the disposition of some analgesics, anxiolytics and hypnotics have recently been reported. With certain benzodiazepines, sex has been shown to be a more important determinant of variability in drug disposition than age, while with other benzodiazepines an age-related decline in clearance was more pronounced in men than women. In young healthy adults these sex-related differences in drug disposition were related to the phase of the menstrual cycle, oral contraceptive steroid administration, and variations in plasma concentrations of albumin, alpha 1-acid glycoprotein, free fatty acids and sex hormones. While none of the sex-related differences so far reported necessitates the modification of a therapeutic dosage regimen, it is prudent that future protocols for pharmacokinetic studies should regard age, sex, the menstrual cycle and oral contraceptive steroids as potential sources of variability.

Drug prescribing in renal failure: dosing guidelines for adults

The data base for rational guidelines to safe, efficacious drug prescribing in adults with renal insufficiency are presented in tabular form. Current medical literature was extensively surveyed to provide as much specific information as possible. When information is lacking, however, recommendations are based on pharmacokinetic variables in normal subjects. Nephrotoxicity, important adverse effects, and special considerations in renal patients are noted. Adjustments are suggested for hemodialysis and peritoneal dialysis when appropriate.

The influence of oral contraceptives on the metabolism of methaqualone in man

1 Oral contraceptives were shown to suppress the mid-cycle increase in methaqualone metabolism observed in premenopausal women not receiving oral contraceptive therapy. No women were identified in whom this suppression was not observed. 2 Combined oestrogen-progestogen contraceptives produced the effect in all nine women studied. The effect was also observed in one woman who was receiving progestogen-only contraceptives. 3 The effect of the combined contraceptives was observed within one month of the commencement of the contraceptive therapy. 4 The results emphasise the need to monitor the effect of the menstrual cycle in women not receiving oral contraceptive therapy when the effects of such therapy is studied. 5 These effects are more likely to be the consequence of an inhibition of hormonal control of hepatic metabolic activity by the synthetic steroids than they are to simple inhibition of hepatic metabolism.

The influence of the menstrual cycle on the metabolism and clearance of methaqualone

1 The rate of methaqualone metabolism in women was shown to be significantly increased at the time of ovulation. 2 The apparent first order rate constants for the formation of five C-monohydroxy metabolites of methaqualone on day 15 of the menstrual cycle were approximately double that on day 1. 3 The N-oxidation of methaqualone showed considerable inter-individual variation in its sensitivity to the menstrual cycle, and in a group of ten women the difference in N-oxide excretion between days 1 and 15 was not statistically significant. 4 The serum clearance of methaqualone on day 15 was higher (mean value 94.6 ml min-1 on day 1, 176.0 ml min-1 on day 15), serum half-life shorter (mean t1/2 beta 16.3 h on day 1, 11.6 h on day 15) and the AUC alpha smaller (mean value 44.0 micrograms ml-1 h on day 1, 24.4 micrograms ml-1 h on day 15) than on day 1. 5 The relative importance of the five hydroxy metabolites was unchanged during the menstrual cycle but the C/N oxidation ratio was greater on day 15 than on day 1. 6 The data for methaqualone metabolism in a control group of men was similar to than in women on day 1 of a menstrual cycle.

Metabolic oxidation of methaqualone in extensive and poor metabolisers of debrisoquine

The metabolism of methaqualone to the glucuronides of 5 C-monohydroxy metabolites and to the N-oxide has been studied in 2 groups of healthy young adults phenotyped as extensive and poor metabolisers of debrisoquine. No significant interphenotype differences were observed with respect to the excretion of any of the 6 metabolites. It is probable that the genetic regulation of the pathways leading to these metabolites is at a locus other than that which is responsible for the regulation of the oxidation of debrisoquine, guanoxan, phenacetin, phenytoin and sparteine.

The kinetics of the urinary excretion of the N-oxide and glucuronides of methaqualone in man

The urinary excretion of the N-oxide and the glucuronides of five C-monohydroxy metabolites of methaqualone has been studied following the oral administration of a single dose of the drug. The apparent first order rate constants for the excretion of each metabolite (kme) were shown to be numerically smaller than the overall elimination rate constant for methaqualone (k10). The Kme values tended to be greater than or equal to the corresponding apparent first order rate constants for the formation of the metabolite (km) but corresponding kme and km values were always of the same order magnitude. The kme values for the glucuronides were much smaller than the literature kme value for paracetemol glucuronide. The rate of renal elimination of the metabolites was variably sensitive to urine flow but over a period of time of 8 hours or greater the total amount of metabolite recovered in the urine was was independent of the total urine volume.

The metabolism of methaqualone in patients with biliary cirrhosis or secondary carcinoma of the liver

The metabolism of methaqualone has been studied in three patients with secondary carcinoma of the liver and two with biliary cirrhosis. The urinary excretion of five C-monohydroxy metabolites and the N-oxide was studies in the 24 h period immediately after oral dosing with 250 mg methaqualone (Melsed). In both patients with biliary cirrhosis and one with primary carcinoma of the bile duct or pancreas with secondaries in the liver the pattern of metabolites was normal. In a patient with oat cell carcinoma with secondaries in the liver some metabolite patterns were disturbed and increased metabolite excretion occurred. A patient with primary carcinoma of the breast with secondaries in the liver gave a completely abnormal metabolite pattern.

Metabolism of methaqualone in geriatric patients

The urinary excretion of five C-monohydroxy metabolites and the N-oxide of methaqualone has been measured in a group of eleven geriatric patients aged 71--90 years. The total excretion of the six metabolites in 24 h after the oral administration of a single dose was approximately one-half of that in a group of young healthy adults. The relative importance of the six metabolites was 4'-hydroxy greater than N-oxide greater than 2'-hydroxymethyl = 3'-hydroxy greater than 6-hydroxy = 2-hydroxymethyl which was the same order as that in young adults. The ratio of C-to N-oxidation was also the same in the two groups. There was no impairment of conjugation of the C-hydroxy metabolites with glucuronic acid in the geriatric group but there was greater interindividual variation in metabolite excretion. There was also evidence for delayed metabolism in the geriatric patients.