A simple HPLC assay for urinary paracetamol metabolites and its use to characterize the C3H mouse as a model for paracetamol metabolism studies

Bioavailability of paracetamol with/without caffeine in Egyptian patients with hepatitis C virus

PURPOSE

This study investigates the involvement of liver dysfunction in the modulation of paracetamol pharmacokinetic profile in genotype-4 HCV patients treated with either paracetamol alone (Para) or in combination with caffeine (Para-Caf).

METHODS

Twenty healthy volunteers and 20 Child-Pugh B HCV patients, each divided into two equal subgroups, were examined, whose liver/kidney functions were correlated with their main clinical manifestation. After an overnight fasting, healthy and hepatic subjects received either a single dose of Para (1000 mg paracetamol) or Para-Caf (1000 mg paracetamol/130 mg caffeine). Two milliliters of saliva samples were collected prior to and at different time-intervals after drug administration and analyzed using HPLC.

RESULTS

There was a noticeable increase in the mean concentration time profile of salivary paracetamol concentrations in hepatic patients, with concomitant decrease in paracetamol clearance (CLT), along with induction in the primary pharmacokinetic (PK) parameters, C max, AUC(0-8 h) and AUC(0-∞) (by about 95, 82, and 64 %, respectively, after treatment with Para, and 98, 96, and 101 %, respectively, after treatment with Para-Caf), when compared with the corresponding parameters in healthy subjects. Additionally, the healthy subjects treated with Para-Caf exhibited bioinequivalent increase in C max, K a, and t 1/2 with decrease in T max when compared with the healthy individuals treated with Para alone. A similar pattern was recorded in hepatic patients after addition of caffeine to paracetamol, with even augmented significant increase in K a and t 1/2 (by 100 and 32 %, respectively).

CONCLUSIONS

Liver dysfunction modified the PK of paracetamol expressed as earlier effective paracetamol concentration, with obvious decrease in its clearance. Caffeine induced faster absorption (evidenced by shorter T max and higher K a) and prolonged t 1/2 of paracetamol, the effects that were more profound in hepatic patients. Further studies are needed to evaluate the influence of liver damage on paracetamol pharmacokinetics whenever repeated dosing is applied, to avoid possible drug accumulation.

Uncertainty factors for chemical risk assessment: interspecies differences in glucuronidation

For the risk assessment of effects other than cancer, a safe daily intake in humans is generally derived from a surrogate threshold dose (e.g. NOAEL) in an animal species to which an uncertainty factor of 100 is usually applied. This 100-fold is to allow for possible interspecies (10-fold) and interindividual (10-fold) differences in response to a toxicant, and incorporates toxicodynamic and toxicokinetic aspects of variability. The current study determined the magnitude of the interspecies differences in the internal dose of compounds for which glucuronidation is the major pathway of metabolism in either humans or in the test species. The results showed that there are major interspecies differences in the nature of the biological processes which influence the internal dose, including the route of metabolism, the extent of presystemic metabolism and enterohepatic recirculation. The work presented does not support the refinement of the interspecies toxicokinetic default to species- and pathway-specific values, but demonstrates the necessity for risk assessments to be carried out using quantitative chemical-specific data which define the fundamental processes which will influence the internal dose of a chemical (toxicokinetics), or the interaction of toxicant with its target site (toxicodynamics).

Correlation of in vitro and in vivo acetaminophen availability from albumin microaggregates oral modified release formulations

The aim of this study was to develop albumin microaggregated oral formulations for controlled drug release, and to reveal the possible influence of the release site on drug absorption. Acetaminophen was chosen as the model drug, which is included in the Class 1 group of the Biopharmaceutics Classification System (BCS). Albumin micro aggregates were formulated into tablets to obtain different drug release rates: Immediate Release (IR) tablets, multiparticulate systems with an intermediate release rate, and matrix systems showing slow release rate. The properties of the products were initially tested via dissolution studies, and then via bioavailability studies in healthy volunteers. Controlled release albumin microaggregated acetaminophen formulations for oral administration were obtained. The extent of drug absorption was comparable for all formulations, suggesting that the differences found in saliva concentration and urine cumulative profiles could be attributed merely to differences in drug release kinetics, as confirmed by the in vitro-in vivo correlation study. Therefore, it can be concluded that extended release of acetaminophen does not influence its absorption via intestinal heterogeneity.

Assay of paracetamol and its metabolites in urine, plasma and saliva of children with chronic liver disease

A high-performance liquid chromatographic (HPLC) assay of paracetamol and its metabolites is described. The method for quantifying the metabolism of paracetamol in children with chronic liver disease and the good correlation between plasma and salivary concentrations of paracetamol is demonstrated. Despite an increasing bias between the two methods with increasing concentration of paracetamol, it is concluded that salivary assay is satisfactory for characterising paracetamol pharmacokinetics in the group of patients studied.

Selective effects of a bacterial infection (Actinobacillus pleuropneumoniae) on the hepatic clearances of caffeine, antipyrine, paracetamol, and indocyanine green in the pig

1. In order to investigate the effect of a bacterial acute phase response model on drug disposition in vivo, plasma clearances of antipyrine, caffeine, paracetamol and indocyanine green were investigated in the healthy and Actinobacillus pleuropneumoniae-infected pig. 2. Indocyanine green plasma and endogenous creatinine clearance were not changed during the infection, which indicates that hepatic blood flow and renal function were not significantly affected. 3. In the A. pleuropneumoniae-infected pig, plasma clearances of antipyrine and caffeine, both marker substrates for hepatic oxidative biotransformation, were decreased by 72 and 68% respectively. The clearance of paracetamol, a drug mainly glucuronidated in the pig, was reduced by 39%. 4. It is concluded that the most important change in drug elimination during an acute phase response induced by A. pleuropneumoniae is a suppression of oxidative hepatic biotransformation.

Dose-dependent pharmacokinetic interaction between antipyrine and paracetamol in vivo and in vitro when administered as a cocktail in pig

1. The pharmacokinetic interactions between paracetamol (PA) and antipyrine (AP) were studied in pigs in order to investigate the usefulness of this combination for the simultaneous assessment of oxidative and conjugative metabolism. 2. When both drugs were given at a dose of 5 mg/kg, AP plasma clearance was reduced from 2.22 to 0.96 lh-1 kg-1. PA clearance was not changed in comparison with control values. 3. At a dose of 2 mg/kg no pharmacokinetic interaction between the two drugs was observed. 4. The only oxidative AP metabolite found in urine was 4-hydroxyantipyrine (4-OHA). It accounted for 80% of the dose and, like PA, it was completely glucuronidated. 5. The glucuronidation of PA has been studied in vitro in pig liver microsomes. The apparent Km value for PA glucuronidation was 40 mM with a Vmax = 54 nmol min-1 mg protein-1. To determine if 4-OHA and PA competed for the same UDP-glucuronosyl-transferase, the effect of 4-OHA and AP on PA glucuronidation was studied. It appeared that 4-OHA was a competitive inhibitor with a Ki app = 0.07 microM, whereas AP had no effect. 5. Results suggest a dose-dependent interaction between AP and PA, which may be due to competition at the level of glucuronidation. Therefore, the usefulness of AP and PA in vivo in a cocktail for metabolism studies is limited.

Perturbation of paracetamol urinary metabolic ratios by urine flow rate

The effects of high and low urine flow rates on the urinary metabolic ratios for paracetamol glucuronidation, sulphation and oxidation were determined at steady-state in seven healthy young adult volunteers. Metabolic partial clearances were unaffected by urine flow rate, but individual paracetamol metabolic ratios varied 2.5- to 3.2-fold over a 7.4-fold range of urine flow rates (0.81-6.00 ml min-1). The change in metabolic ratios was due entirely to a 2.5-fold change in renal clearance of unchanged paracetamol. These data emphasise the limitations of the metabolic ratio as a measure of intrinsic clearance for compounds which undergo some degree of tubular reabsorption.

Egg albumin microspheres containing paracetamol for oral administration. II. In vivo investigation

Egg albumin microspheres containing paracetamol for oral administration were prepared and their in vivo characteristics evaluated. The egg albumin microspheres were able to improve the organoleptic characteristics of paracetamol formulations. The pharmacokinetic characteristics of three different formulations of paracetamol were evaluated in six volunteers. The formulations administered orally were: (1) Paracetamol granulated with lactose (reference), (2) Egg albumin microspheres and (3) Egg albumin microspheres coated with polymethacrylate. The pharmacokinetic characteristics for formulations 1 and 2 were similar but formulation 3 gave significant differences (p less than 0.05) in Ka, Cmax and tmax. No significant differences in relative bioavailability were observed.

Characterization of paracetamol UDP-glucuronosyltransferase activity in human liver microsomes

A specific high performance liquid chromatographic assay has been developed for the measurement of paracetamol glucuronide formation by the microsomal fraction of human liver. The procedure has been used to characterize paracetamol glucuronidation kinetics in human livers microsomes and to assess the substrate specificity of the paracetamol UDP-glucuronosyltransferase (UDPGT) activity. Paracetamol glucuronidation followed Michaelis-Menten kinetics, suggesting the involvement of a single form of UDPGT, or possibly two or more forms of UDPGT with similar affinities for paracetamol, in this reaction. Mean apparent Km and Vmax values were 7.37 +/- 0.99 mM and 4.76 +/- 1.35 nmol/min/mg, respectively. Addition of the non-ionic detergent Brij 58 to microsomal incubations resulted in approximately 50% activation of microsomal paracetamol UDPGT-activity. This contrasts to the approximately three-fold activation of 4-methylumbelliferone, morphine and 4-nitrophenol glucuronidation observed following Brij 58 treatment of human liver microsomes. The glucuronidated xenobiotics chloramphenicol, digitoxigenin monodigitoxoside, 4-hydroxybiphenyl, 4-methylumbelliferone, morphine, 1-naphthol and 4-nitrophenol were screened for inhibitory effects on paracetamol glucuronidation. Of these compounds, only digitoxigenin monodigitoxoside and 1-naphthol were found to cause significant inhibition of paracetamol UDPGT activity. Along with the results of previous studies of the kinetics and inhibitor profile of human liver glucuronidation reactions (Miners et al., Biochem Pharmacol 37: 665-671, 1988 and 37: 2839-2845, 1988), these data indicate that the model glucuronidated substrates paracetamol, morphine and 4-methyllumbelliferone may be used to differentiate at least four human liver UDPGT isozyme activities.

Effect of subacute oral intake of the food antioxidant butylated hydroxyanisole on clinical parameters and phase-I and -II biotransformation capacity in man

A study is presented in which eight healthy male non-smoking volunteers ingested a daily amount of 0.5 mg/kg butylated hydroxyanisole (BHA) for 10 consecutive days. Blood samples were taken on days -6 and 0 before and on days 4 and 7 after the first BHA administration for the assessment of standard clinical plasma parameters (L-aspartate aminotransferase, L-alanine-aminotransferase, L-gamma-glutamyltranspeptidase, creatine phosphokinase, lactate dehydrogenase, total protein, albumin, urea, creatinine, Na+, and Cl-). Antipyrine (500 mg p.o.) and paracetamol (500 mg p.o) were administered before and during BHA administration as test substances to measure phase-I and phase-II biotransformation capacity. Saliva samples and urine were subsequently collected for the assessment of kinetic parameters (e.g. saliva elimination half-life, saliva clearance, apparent volume of distribution) and urinary excretion of metabolites. Kinetic plasma parameters of BHA itself were determined in plasma samples obtained via a catheter in an arm vein after oral BHA intake on days 0 and 7. Levels of antipyrine, paracetamol, BHA and metabolites in plasma, saliva or urine were quantified by standard or newly developed reversed-phase high-performance liquid chromatography methods. Urinary excretion of Na+, K+, and Cl-, as well as osmolality of urine were measured on three days before and six days during BHA administration. Generally, no significant differences were detected in the parameters measured, indicating that oral administration of BHA to men for 10 days remains without effects on clinical biochemical parameters and phase-I and phase-II biotransformation capacity. In contrast, urinary excretion of metabolites of BHA was significantly increased on days 3 and 7 vs. the first day of BHA administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete separation of urinary metabolites of paracetamol and substituted paracetamols by reversed-phase ion-pair high-performance liquid chromatography

A reversed-phase high-performance liquid chromatographic procedure has been developed for the separation of thirteen urinary metabolites of the analgesic drug paracetamol. The method involved the use of radially compressed columns packed with octadecylsilica with a particle diameter of 5 micron. Metabolites were chromatographed by linear gradient elution using an ion-pair solvent system composed of tetrabutylammonium hydroxide and Tris buffered to pH 5.0 with phosphoric acid, and acetonitrile as the organic solvent. Analyses can be performed at the rate of three per hour. This method enables the direct identification of sulphate and glucuronide conjugates of 3-thiomethylparacetamol and 3-thiomethylparacetamol sulphoxide which have only previously been detected following enzyme hydrolysis of urine samples. The application of this fully optimised separation to the study of the metabolism of substituted paracetamols is also discussed.

Comparison of paracetamol metabolism in young adult and elderly males

Paracetamol metabolism was compared in groups of young adult (mean age 20.8 years) and elderly (mean 79.3 years) males. Apparent oral clearance and half-life and partial metabolic clearances to the glucuronide and glutathione-derived conjugates of paracetamol were not significantly different between the two groups. However, the partial metabolic clearance to paracetamol sulphate was 23.1% lower and renal clearance of unchanged drug was 42.9% lower in the elderly compared to the young adults. Despite the decreased clearance by these pathways, reduction in paracetamol dosage should not be necessary in the elderly.

Automated liquid chromatographic method for the determination of paracetamol and six metabolites in human urine

An automated liquid chromatographic method, with a coefficient of variation for total imprecision of less than 4%, has been developed for the quantitative determination of paracetamol, paracetamol-4-glucuronide, paracetamol-4-sulphate, paracetamol-3-cysteine, paracetamol-3-mercapturate, 3-hydroxyparacetamol-3-sulphate and 3-methoxyparacetamol-4-sulphate in urine samples. The gradient elution system was based on 0.067 M phosphate buffer (pH 2.0) and acetonitrile on an octadecylsilica column. The on-column detection limit using an ultraviolet detector at 254 nm for each of the compounds using 3-hydroxyacetanilide as internal standard was of the order of 10-50 ng from urine and 2-10 ng from water. Application of the method to 24-h urine samples from subjects who had received a therapeutic dose of the drug confirmed the findings of previous studies for the importance of the glucuronide, sulphate, mercapturate and cysteine conjugates. 3-Hydroxyparacetamol-3-sulphate was shown to be present in the urine of all volunteers and to account for up to 5% of the dose. 3-Methoxyparacetamol-4-sulphate was not detected in any urine samples and if present as a metabolite must account for less than 0.1% of the dose.

Pharmacokinetic consequences and toxicologic implications of metyrapone-induced alterations of acetaminophen elimination in man

This study examined the effect of metyrapone on the elimination rate of acetaminophen and on the apparent formation rate of acetaminophen metabolites in man. Metyrapone treatment, 1.5 g, increased the half-life of acetaminophen, decreased the fraction of the dose recovered in the urine as the glucuronide and increased the fraction of the dose recovered in urine as the sulfate and mercapturate conjugates. The apparent rate constant for the formation of acetaminophen glucuronide was significantly decreased by metyrapone while the apparent rate constants for the formation of the sulfate and mercapturic acid metabolites were unchanged or slightly increased, respectively. These data indicate that metyrapone inhibits acetaminophen glucuronidation and possibly enhances the oxidation of acetaminophen to its quantitatively minor yet highly toxic reactive metabolite. The extent to which the parallel pathways of acetaminophen elimination are also affected by inhibitors of cytochrome P-450-mediated oxidation will limit the efficacy of these types of potential antidotes for the treatment of acetaminophen overdose.

Paracetamol metabolism in pregnancy

Paracetamol disposition was studied in groups of pregnant and non-pregnant women of comparable age. Paracetamol apparent oral clearance was 58% higher and elimination half-life was 28% lower in the pregnant women compared to the control group. The higher clearance in the pregnant women was due to increased activity of the glucuronidation (75% higher) and oxidative (88% higher) pathways of paracetamol metabolism. There was no difference between the two groups in paracetamol sulphation or renal clearance of unchanged drug.

Dissociation of increased sulfation from sulfate replenishment and hepatoprotection in acetaminophen-poisoned mice by N-acetylcysteine stereoisomers

N-Acetylcysteine stereoisomers were compared for their ability to alter the sulfation and hepatotoxicity of acetaminophen. The clinically used L-isomer increased urinary excretion of inorganic sulfate 2-3 fold and prevented liver injury, but failed to increase acetaminophen sulfation in mice. Conversely, the nonphysiologic D-isomer failed to increase urinary excretion of inorganic sulfate or prevent hepatotoxicity, but increased acetaminophen sulfation appreciably (by 39%). The basis of the incongruence between changes in the availability of inorganic sulfate and the sulfation of acetaminophen is not known. These data indicate that a modest increase in acetaminophen sulfation, occurring alone following N-acetylcysteine treatment, is insufficient to explain the profound efficacy of the antidote in mice, and further suggest that this holds true for other species, such as humans, that are comparatively poor in the sulfoconjugation of acetaminophen.

Formation of reactive metabolites of phenacetin in humans and rats

The metabolism of phenacetin to reactive intermediates in humans was estimated from the excretion of thio adducts in urine. N-Hydroxyphenacetin, a precursor of reactive metabolites, was also quantified. Following an oral dose of phenacetin (10 mg/kg) to humans, these metabolites in 24 h urine were: paracetamol-3-cysteine, 4.4% dose; paracetamol-3-mercapturate, 3.9%; 3-thiomethylparacetamol, 0.4%; N-hydroxyphenacetin, 0.5%. Rats showed a considerable increase in N-hydroxyphenacetin excretion after chronic dosing with phenacetin at high dosage (500 mg/kg) for one month. chronic dosing with a low dose (50 mg/kg) did not increase N-hydroxyphenacetin excretion, but a marked increase occurred on concomitant administration of aspirin and caffeine.

Mechanism of action of paracetamol protective agents in mice in vivo

The mechanism of action of cysteine, methionine, N-acetylcysteine (NAC) and cysteamine in protecting against paracetamol (APAP) induced hepatotoxicity in male C3H mice in vivo has been investigated by, characterising the effect of the individual protective agents on the metabolism of an hepatotoxic dose of APAP, and determining the efficacy of the protective agents in animals treated with buthionine sulphoximine (BSO), a specific inhibitor of glutathione (GSH) synthesis. Co-administration of cysteine, methionine or NAC increased, while co-administration of cysteamine decreased, the proportion of GSH-derived conjugates of APAP excreted in the urine of mice administered APAP, 300 mg/kg. Pretreatment of animals with BSO abolished the protective effect of cysteine, methionine and NAC, whereas cysteamine still afforded protection against APAP after BSO treatment. In conjunction with other data, these results suggest the most likely mechanism for the protective effect of cysteine, methionine and NAC is by facilitating GSH synthesis, while the most likely mechanism for the protective effect of cysteamine is inhibition of cytochrome P-450 mediated formation of the reactive metabolite of APAP.

The effects of buthionine sulphoximine (BSO) on glutathione depletion and xenobiotic biotransformation

Buthionine sulphoximine (BSO) is an inhibitor of gamma-glutamylcysteine synthetase (gamma-GCS) and, consequently lowers tissue glutathione (GSH) concentrations. In fed male C3H mice, liver and kidney GSH levels were depleted by BSO in a dose dependent manner with maximum effect (35% of initial levels) occurring with doses between 0.8 and 1.6 g/kg, i.p. At these doses maximum effects on gamma-GCS and GSH were observed 2-4 hr after BSO administration; initial gamma-GCS activity and GSH content were restored approximately 16 hr post BSO. BSO, either in vivo or in vitro, had no effect on hepatic microsomal cytochrome P-450 levels, a range of cytochrome P-450 dependent enzyme activities or p-nitrophenol glucuronyl transferase activity. Similarly, BSO had no effect on phenol sulphotransferase and two GSH-transferase activities in the 105,000 g supernatant fraction. BSO had no effect on the duration of hexobarbitone induced narcosis in mice. Consistent with specific inhibition of GSH synthesis, BSO pretreatment of mice decreased the proportion of a 50 mg/kg dose of paracetamol excreted in the urine as GSH-derived conjugates but did not affect paracetamol clearance through the glucuronidation or sulphation pathways. Since BSO does not affect cytochrome P-450 or conjugating enzyme activity, its use as a specific depletor of tissue GSH in the investigation of mechanisms of xenobiotic-induced toxicities is preferable to the standard GSH-depleting agents as these have other enzymic effects.

Influence of sex and oral contraceptive steroids on paracetamol metabolism

Paracetamol metabolism was investigated in eight healthy males, eight healthy females and eight healthy females receiving oral contraceptive steroids (OCS). Paracetamol clearance was 22% greater in males compared to the control female group. This difference was entirely due to increased activity of the glucuronidation pathway in males, there being no sex-related differences in the sulphation or oxidative metabolism of paracetamol. Paracetamol clearance in females using OCS was 49% greater than in the control females. Glucuronidation and oxidative metabolism were both induced in OCS users (by 78% and 36% respectively) but sulphation was not altered. Although sex-related differences in paracetamol metabolism are unlikely to be of clinical importance, induction of paracetamol metabolism by OCS may have clinical and toxicological consequences.