Paracetamol (acetaminophen) pharmacodynamics: interpreting the plasma concentration

Interpretation of analgesic and antipyretic responses documented after paracetamol administration is confused because response is not directly related to concentration in the blood, but rather to an effect compartment. The effect compartment does not have real measurable concentrations, but concentrations equate approximately to those observed in the cerebrospinal fluid. A time delay exists before drug reaches the effect compartment, and the equilibration half-time between the central and effect compartment is described by a single first-order parameter (Teq or T(1/2)keo), reported to be approximately 1 h for paracetamol. Paediatric analgesic studies are limited because they have only explored postoperative pain after tonsillectomy or day-stay surgery. Other pain types and pain confounders have not been investigated. Adult studies are also similarly limited. Studies investigating antipyresis have not explored the maximum response, limiting the precision of any EC(50) estimate. The influence of the cyclical nature of fever or initial temperature is seldom accounted for in antipyretic studies. Target effect compartment concentrations of 5 mg/l for fever and 10 mg/l for pain do not seem unreasonable on the basis of current literature. Speed of onset may be shortened by giving a larger initial dose or improving absorption characteristics. Consequent plasma concentration achieved, differences in effect compartment equilibration times, and the shape of the effect compartment concentration-response curve help to explain differences between common analgesic/antipyretic drugs.

Evaluation of intramuscular lateral distribution profile of topically administered acetaminophen in rats

To clarify to what extent topically administered drug molecules horizontally permeate into tissues surrounding the administration site, the intramuscular lateral concentration profile of acetaminophen was investigated in vivo using the microdialysis method in rats. When acetaminophen was intramuscularly administered for 6 h in a pinpoint manner at a constant rate of 3 microg/min, it was clearly detected in the muscle surrounding the administration site, being 17.5 microg/ml when measured at a 2 mm distance from the administration site. The concentration in the muscle was decreased as the distance increased, and those measured at 5 mm and 40 mm were 0.35 microg/ml and 0.09 microg/ml, respectively. In addition, it was shown that the concentration in the muscle at 40 mm reflected the compound's concentration in plasma, but not the compound's horizontal permeation from the administration site. With these observations, the intramuscular distribution profile of acetaminophen was numerically characterized according to Fick's law. As a result, it was revealed that horizontal permeation is the primary process accountable for the increased intramuscular concentration only in the area adjacent to the administration site, and the radius of the adjacent area was calculated to be 5.80 mm for acetaminophen.

Safety and efficacy of intravenous N-acetylcysteine for acetaminophen overdose: analysis of the Hunter Area Toxicology Service (HATS) database

BACKGROUND

Acetaminophen (N-acetyl-p-aminophenyl; APAP) is the leading drug used in self-poisoning and frequently causes hepatotoxicity, including acute liver failure.

OBJECTIVE

To provide descriptive data on the safety and efficacy of intravenous N-acetylcysteine (IV-NAC) in the treatment of APAP toxicity, based on information in the Hunter Area Toxicology Service (HATS) database involving residents of the Greater Newcastle Area of New South Wales, Australia.

METHODS

This was a retrospective analysis of all APAP overdoses from January 1987 to January 2003. Data were collected prospectively according to a published protocol and included patient characteristics, exposures to APAP and other potential toxins, treatments, and outcomes. Primary safety/tolerability endpoints included the mortality rate and incidence of adverse drug reactions, while efficacy endpoints included alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels.

RESULTS

Of 1749 patients, 399 (22.8%) were treated with IV-NAC. Of these, 37 (9.3%) had an adverse drug reaction to IV-NAC, of which seven (1.8% of total) were anaphylactoid. There were five deaths in hospital (mortality rate = 0.3%), including two attributed to APAP (0.1%) and none to IV-NAC. Of 64 patients who were treated with IV-NAC within 8 hours after APAP ingestion and had available ALT/AST data, two (3.1%) developed hepatotoxicity (AST/ALT > 1000 IU/L) compared with 32 (25%) of 128 patients receiving IV-NAC > 8 hours after APAP ingestion (p = 0.0002). A total of 26 patients (15.6%) receiving IV-NAC treatment within 8 hours after APAP ingestion had hospitalization stays > 48 hours compared with 70 (33.3%) receiving IV-NAC > 8 hours after ingestion (p < 0.0001).

CONCLUSIONS

For patients with APAP overdose seen in the HATS database of New South Wales, Australia, in-hospital death was infrequent (< 1%) and hepatotoxicity was significantly less likely when IV-NAC was administered within 8 hours after APAP ingestion compared with longer intervals (p < 0.01). As a descriptive retrospective database analysis, this study could not exclude certain sources of bias, including temporal changes over the 16-year course of data collection in the use of IV-NAC and low ascertainment of mild, self-limiting reactions to IV-NAC.

Enteral Exsorption of Acetaminophen after Intravenous Injection in Rats: Influence of Activated Charcoal on This Clearance Path

The fate of acetaminophen after intravenous injection in whole bowel-irrigated rats (n = 40) and the influence of activated charcoal on the kinetics were investigated. After randomization to four groups (n = 10, each group), plasma concentration and the quantities of acetaminophen and metabolites excreted into bile, urine and intestine were determined using an in vivo model with or without orally administered activated charcoal and with or without bile duct cannulation. The cumulative amount of acetaminiphen and metabolites exsorbed into the small intestine within 3.5 hr after intravenous injection was about 20% of dose in the animals with bile duct cannulation and about 7% of dose in the animals without. Correspondingly, about 13% of dose was detected in the externalized bile. Activated charcoal did not influence the amount exsorbed into the small intestine. Terminal half-life in plasma ranged from 35 to 51 min. within the four treatment groups without statistically significant difference (P = 0.152). Correspondingly, the area under the curve did not vary much and ranged between 2.6 and 3.3 g/min./l (P = 0.392). Deposition of acetaminophen and metabolites in liver and kidney after 3.5 hr was marginal and ranged between 0.02% and 0.6% of the dose within all groups. The excretion of acetaminophen and metabolites into urine varied strikingly between 31% and 56% of the dose within all groups and correlated with diuresis. The lack of effect of activated charcoal on the elimination of acetaminophen and metabolites may be due to the small amount of the drug being exsorbed into the intestine or the reduced adsorbent capacity of activated charcoal to acetaminophen and metabolites, which also could be influenced by inadequate luminal stirring.

Gastric Emptying of Water in Obese Pregnant Women at Term

BACKGROUND

Healthy nonpregnant and pregnant patients may ingest clear liquids until 2 h before induction of anesthesia without adversely affecting gastric volume. In this study, we compared gastric emptying in obese, term, nonlaboring pregnant women (prepregnancy body mass index >35 kg/m2) after the ingestion of 50 and 300 mL of water.

METHODS

Gastric emptying was assessed in 10 obese, term pregnant volunteers using both serial gastric ultrasound examinations and acetaminophen absorption in a crossover study design. After an overnight fast, volunteers ingested 1.5 g acetaminophen and 50 or 300 mL water (randomly assigned) on two occasions separated by at least 2 days. Serial gastric antrum cross-sectional areas were determined using gastric ultrasound imaging and the half-time to gastric emptying (T([1/2])) was calculated. Areas under the plasma acetaminophen concentration versus time curve (AUC), peak concentrations (C(max)), and time to peak concentration (t(max)) for 50 mL and 300 mL ingestions were compared.

RESULTS

Mean prepregnancy body mass index was 41 +/- 9 kg/m(2). Gastric emptying T([1/2]) was not different after ingestion of 300 mL water compared with 50 mL (23 +/- 11 min vs 32 +/- 15 min). There were no differences between acetaminophen AUCs at 60, 90, or 120 min, C(max) or t(max) after ingestion of 300 mL compared with 50 mL of water.

CONCLUSIONS

Gastric emptying in obese, nonlaboring term pregnant women is not delayed after ingestion of 300 mL compared with 50 mL of water. Gastric antral volume after ingestion of 300 mL of water is similar to the baseline fasting level at 60 min.

Efflux transporter expression and acetaminophen metabolite excretion are altered in rodent models of nonalcoholic fatty liver disease

Efflux transporters are responsible for the excretion of numerous xenobiotics and endobiotics and thus play an essential role in proper liver and kidney function. Nonalcoholic fatty liver diseases (NAFLDs) comprise a spectrum of disorders that range from simple fatty liver (SFL) to nonalcoholic steatohepatitis (NASH). Although the precise events leading to NAFLD are unclear, even less is known about the effects on efflux transporter expression and drug disposition. The purpose of this study was to determine the effect of NAFLD on efflux transporter expression in rat liver as well as on acetaminophen (APAP) metabolite excretion. To simulate SFL and NASH, rats were fed either a high-fat (HF) or a methionine- and choline-deficient (MCD) diet for 8 weeks. In the livers of MCD rats, there were striking increases in both mRNA and protein levels of multidrug resistance-associated protein (Mrp) 3, Mrp4, and breast cancer resistance protein, as well as increased Mrp2 protein. After administration of a nontoxic dose of APAP, biliary concentrations of APAP-sulfate, APAP-glucuronide (APAP-GLUC), and APAP-glutathione were reduced in MCD rats. The effects of the HF diet on both transporter expression and APAP disposition were by comparison far less dramatic than the MCD diet-induced alterations. Whereas APAP-sulfate levels were also decreased in MCD rat plasma, the levels of the Mrp3 substrate APAP-GLUC were elevated. Urinary elimination of APAP metabolites was identical between groups, except for APAP-GLUC, the concentration of which was 80% higher in MCD rats. These studies correlate increased hepatic Mrp3 protein in the MCD model of NASH with increased urinary elimination of APAP-GLUC. Furthermore, the proportional shift in elimination of APAP metabolites from bile to urine indicates that MCD-induced alterations in efflux transporter expression can affect the route of drug elimination.

Effect of acute paracetamol overdose on changes in serum and urine electrolytes

AIMS

To investigate the effects of acute paracetamol overdose on renal function, serum and urine electrolyte excretion in man.

METHODS

Two studies were performed in patients admitted with paracetamol overdose: a retrospective study examining changes in serum electrolytes, and a prospective study evaluating changes in serum and urine electrolytes. A control group with SSRI overdose was included in the prospective study.

RESULTS

There was a significant dose-dependent relationship between admission (4 h) paracetamol concentration and fall in serum potassium in the retrospective study (P < 0.01) and a significant positive relationship between serum paracetamol at 4 h and fractional excretion of potassium at 12 h postingestion (P < 0.01) in the prospective study. No changes were seen in the control group. No cases developed renal failure.

CONCLUSIONS

Paracetamol overdose is associated with dose-related hypokalaemia, and kaliuresis of short duration (<24 h), suggesting a specific renal effect of paracetamol in overdose perhaps via cyclo-oxygenase inhibition. This effect seems distinct from any nephrotoxic effect of paracetamol.

Determination of bergenin in rat plasma by high-performance liquid chromatography

A simple, sensitive, selective and reproducible reversed-phase high-performance liquid chromatography (HPLC) method was developed for the determination of bergenin in rat plasma after intravenous administration. Acetaminophen was successfully used, as internal standard (IS) for calibration. The chromatographic separation was accomplished on a reversed-phase C18 column using a mobile phase consisting of methanol-water (20:80, v/v, pH 2.50) and a detection wavelength of 275 nm. Retention times of bergenin and acetaminophen were approximately 9.9 and 6.1 min and no interfering peak of the blank plasma chromatograms was observed. Good linearity was achieved in the range of 0.3 - 100 microg/ml (r2 = 0.9998). The extraction recoveries of bergenin from plasma was 70.82%, 69.44%, 70.98% at concentrations of 5, 50, 100 microg/ml. Intra-assay and inter-assay variabilities were 0.92 - 2.60% and 2.31 - 2.95%, respectively. The accuracy was validated by relative error (RE%), which was in the range of -0.05 - 1.74%. The capability of the assay to pharmacokinetic studies was demonstrated by the determination of bergenin in plasma after intravenous administration to rats in doses of 7.5 mg/kg, 15.0 mg/kg, and 30.0 mg/kg, using water as the solvent. The half-lives for distribution and elimination are not related to administered doses. A biphasic phenomenon with a rapid distribution followed by a slower elimination phase was observed from the plasma concentration-time curve and the pharmacokinetics was based on first order kinetics.

Pharmacokinetics of an oral drug (acetaminophen) administered at various times relative to subcutaneous injection of pramlintide in subjects with type 2 diabetes

Pramlintide, an adjunct treatment to mealtime insulin for patients with type 2 and type 1 diabetes, aids glycemic control by suppressing postprandial glucagon secretion, slowing gastric emptying, and enhancing satiety. Because gastric emptying affects oral medication absorption, this placebo-controlled, single-blind, crossover study examined the absorption of 1000 mg of acetaminophen elixir administered -2, -1, 0, +1, and +2 hours relative to pramlintide (120 microg) or 0 hours relative to placebo in 24 patients with type 2 diabetes. When acetaminophen administration occurred 0, +1, or +2 hours relative to pramlintide, the maximum observed plasma concentration of acetaminophen decreased 14% to 29%, and time to maximum observed plasma concentration increased by 0.8 to 1.2 hours compared with administration 0 hours relative to placebo. Pramlintide treatment slowed but did not alter the extent of acetaminophen absorption (area under the concentration-time curve). No serious adverse events or withdrawals were reported. Oral agents should be administered at least 1 hour before or 2 hours after pramlintide injection if rapid onset of action is required for efficacy.

Effect of aspirin, paracetamol and their nitric oxide donating derivatives on exudate cytokine and PGE2 production in zymosan-induced air pouch inflammation in rats

Effects of different doses of aspirin, compared to equimolar doses of nitric oxide (NO)-donating aspirin (NCX 4016), and of a single dose of paracetamol, compared to an equimolar dose of NO-donating paracetamol (NCX 701) were investigated in acute zymosan-induced air pouch inflammation in rats. Treatments were administered by orogastric route, and interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha) and prostaglandin E(2) (PGE(2)) levels in the exudates were analysed 4 h after zymosan injection by enzyme immunoassay (EIA). Aspirin, at 10, 30 and 100 mg/kg doses, increased IL-1beta levels in exudates, however, only the highest dose lead to a significant increase when compared to control, whereas a significant increase in TNF-alpha level was observed at all doses tested. NCX 4016, at equimolar doses for aspirin, i.e., 18.6, 55.8 and 186 mg/kg, respectively, did not cause any changes in exudate IL-1beta or TNF-alpha levels. These effects were significantly different, when aspirin was compared with the corresponding NCX 4016 group. Nevertheless, the ability of aspirin and NCX 4016 to inhibit PGE(2) synthesis in the exudate where comparable. Although paracetamol significantly increased exudate TNF-alpha level compared to the control group and NCX 701 group, neither paracetamol, nor NCX701 treatments changed the levels of exudate IL-1beta significantly. As expected, paracetamol and NCX 701 showed poor PGE(2) inhibition. At high doses, aspirin and NCX 4016 decreased the number of polymorphonuclear leukocytes in the exudate. However, this inhibition was not significantly different from the control group. Paracetamol and NO-paracetamol did not cause any change in the number of polymorphonuclear leukocytes in exudate. These results indicated that aspirin and NCX 4016 possessed different effects on cytokine production or release, despite the fact that both drugs inhibited the synthesis of PGE(2) in a similar way. Unlike paracetamol, which increased exudate TNF-alpha level, NCX 701 had no effect on TNF-alpha level in the exudates.

Comparison of Urine and Serum Testing for Early Detection of Acetaminophen Ingestion

Acetaminophen is a common ingestant that is routinely screened for, with serum testing, after overdoses. We compared a modified, qualitative, colorimetric, o-cresol urine test for acetaminophen with the standard laboratory serum immunoassay. Twenty-nine adult volunteers were given 975 mg of acetaminophen, and then serum and urine samples were tested at predefined intervals for up to 4 hours. The presence of acetaminophen in the urine samples was determined with a modified o-cresol assay by two independent reviewers and was verified by using high-performance liquid chromatography. Compared with the acetaminophen serum immunoassay, the acetaminophen urine test had specificity of 97% (95% confidence interval, 80-100%) and sensitivity of 100% (95% confidence interval, 81-100%) at 45 minutes. Further studies to improve and to accelerate processing of the qualitative colorimetric o-cresol urine test could allow the test to be used in specific military and civilian clinical scenarios.

High-amylose carboxymethyl starch matrices for oral sustained drug-release: In vitro and in vivo evaluation

High-amylose corn starch, that contains 70% of amylose chains and 30% of amylopectin, has been used to obtain substituted amylose (SA) polymers. Tablets have been prepared by direct compression, i.e. dry mixing of drug and SA, followed by compression, which is the easiest way to manufacture an oral dosage form. Until now, their controlled-release properties have been assessed only by an in vitro dissolution test. Amylose-based polymers are normally subject to biodegradation by alpha-amylase enzymes present in the gastrointestinal tract, but matrix systems show no significant degradation of tablets by alpha-amylase in vitro. High-amylose sodium carboxymethyl starch (HASCA) is an interesting excipient for sustained drug-release in solid oral dosage forms. In addition to the easy manufacture of tablets by direct compression, the results show that in vitro drug-release from an optimized HASCA formulation is not affected by either acidic pH value or acidic medium residence time. In addition, a compressed blend of HASCA with an optimized quantity of sodium chloride provides a pharmaceutical sustained-release tablet with improved integrity for oral administration. In vivo studies demonstrate extended drug absorption, showing that the matrix tablets do not disintegrate immediately. Nevertheless, acetaminophen does not seem to be the most appropriate drug for this type of formulation.

Interaction between traditional Chinese medicine and Western medicine in rats--In-Chen-How and acetaminophen

The purpose of this study is to evaluate the interaction effects of In-Chen-How (Artemisia capillaries Thunb.) on the pharmacokinetics of acetaminophen and on liver microsomal cytochrome P450 enzyme activity in rats. The rats were divided into control group (n = 8) without In-Chen-How and the pretreated group (n = 8) administered with In-Chen-How (approximately 1.0 mL x kg(-1), according to weight) for 5 consecutive days. Rats in the control group received water simultaneously. Each rat was then given acetaminophen. The pharmacokinetic parameters of acetaminophen of the two groups were significantly different. In the In-Chen-How pretreated group, the maximum concentration of acetaminophen and the area under the plasma concentration-time curve were reduced about 58.4%, 56.7% and 55.4%. To further explain the results, liver microsomal suspensions were obtained from rats that were randomly divided into control and In-Chen-How pretreated group. The levels of CYP1A2 and CYP2E1 in hepatic microsomal protein from pretreated group were increased as compared to that from the control group. It indicated that In-Chen-How can stimulate the activity of CYP isozymes. The changes in the pharmacokinetics of acetaminophen resulting from the administration of In-Chen-How are related to an increase in metabolic activity of CYP1A2 and CYP2E1.

Rectal acetaminophen does not reduce morphine consumption after major surgery in young infants

BACKGROUND

The safety and value of acetaminophen (paracetamol) in addition to continuous morphine infusion has never been studied in newborns and young infants. We investigated the addition of acetaminophen to evaluate whether it decreased morphine consumption in this age group after major thoracic (non-cardiac) or abdominal surgery.

METHODS

A randomized controlled trial was performed in 71 patients given either acetaminophen 90-100 mg kg(-1) day(-1)or placebo rectally, in addition to a morphine loading dose of 100 microg kg(-1) and 5-10 microg kg(-1) h(-1) continuous infusion. Analgesic efficacy was assessed using Visual Analogue Scale (VAS) and COMFORT scores. Extra morphine was administered if VAS was > or = 4.

RESULTS

We analysed data of 54 patients, of whom 29 received acetaminophen and 25 received placebo. Median (25-75th percentile) age was 0 (0-2) months. Additional morphine bolus requirements and increases in continuous morphine infusion were similar in both groups (P = 0.366 and P = 0.06, respectively). There was no significant difference in total morphine consumption, respectively, 7.91 (6.59-14.02) and 7.19 (5.45-12.06) mug kg(-1) h(-1) for the acetaminophen and placebo group (P = 0.60). COMFORT [median (25-75th percentile) acetaminophen 10 (9-12) and placebo 11 (9-13)] and VAS [median (25-75th percentile) acetaminophen 0.0 (0.0-0.2) and placebo 0.0 (0.0-0.3)] scores did not differ between acetaminophen and placebo group (P = 0.06 and P = 0.73, respectively).

CONCLUSIONS

Acetaminophen, as an adjuvant to continuous morphine infusion, does not have an additional analgesic effect and should not be considered as standard of care in young infants, 0-2 months of age, after major thoracic (non-cardiac) or abdominal surgery.

Isobolographic analysis of the antinociceptive interactions between ketoprofen and paracetamol

The present study was undertaken to evaluate the antinociceptive interaction between paracetamol and ketoprofen. The antinociceptive effect of oral administration of the drugs alone or in combination was evaluated using the mouse abdominal constriction test. The data were interpreted by isobolographic analysis to establish the nature of the interaction. The effective dose that produced 50% antinociception (ED(50,mix)) was calculated from the log dose-response curve of fixed-ratio combinations of paracetamol with ketoprofen. This ED(50,mix) was compared to the theoretical additive ED(50,add) by isobolographic analysis. The experimental ED(50,mix) was found to be significantly smaller than the theoretically calculated ED(50,add), indicating a synergistic antinociceptive interaction between ketoprofen and paracetamol. Pharmacokinetic studies were carried out with mice treated with combined ketoprofen (12 mg/kg) and paracetamol (36 mg/kg). Plasma levels of ketoprofen were not changed by concurrent paracetamol treatment, and similarly no statistically significant difference was observed between paracetamol alone and the combination with ketoprofen. The pharmacokinetic analysis revealed that the combination of ketoprofen with paracetamol exerted a synergistic (supra-additive) interaction that was not associated with a pharmacokinetic interaction. The results of this study demonstrate significant synergism between ketoprofen and paracetamol.

Acetaminophen-induced hepatic failure with encephalopathy in a newborn

We report a case of oral acetaminophen toxicity in a term newborn infant successfully treated with a 20 h intravenous N-acetylcysteine infusion protocol without any adverse effects. This case report supports the use of N-acetylcysteine to treat neonatal acetaminophen toxicity and highlights the need for better education of parents regarding the appropriate use and dosage of acetaminophen in newborns.

[Quantitative analysis of heart scintigraphy (99mTc-MIBI GSPECT) with regional myocardial wall motion and thickening assessment for detecting myocardial injury in patients after acute paracetamol poisoning]

The aim of the study was to evaluate the myocardium function in acute paracetamol poisoning using 19mTc-MIBI GSPECT.

MATERIAL AND METHODS

Under examination there were 25 acutely paracetamol poisoned patients (age: 24.5 +/- 6 years) treated at the Ward of Toxicology and Environmental Diseases. The control group necessary to perform quantitative analysis of myocardial scintigraphy consisted of 20 people examined in Nuclear Medicine Unit with normal results of 99mTc-MIBI GSPECT examination.

RESULTS

The quantitative analysis of the heart scintigraphy (GSPECT) revealed that, the average value of left ventricle ejection fraction (EF) in examined women and men after paracetamol poisoning was lower than in control group, while end diastolic (EDV) and end systolic volumes (ESV) were higher; these differences were not statistically significant. Indicators of regional wall motion of the left ventricle were lower for the most of the heart segments. According to the control group, they were significantly lower for anterior and inferior LV segments. Wall thickening analysis revealed the impaired systolic thickening of majority of heart segments in examined group of patients; the wall thickening was the lowest for infero-lateral and septal segments in women and for inferior, infero-lateral and anterior segments in men.

CONCLUSIONS

The quantitative analysis of myocardial perfusion scintigraphy (99mTc-MIBI GSPECT) with wall motion and wall thickening assessment, showed that the left ventricle function was usually diffusely depressed in paracetamol poisoned patients. The disturbances of regional wall motion of anterior and inferior wall (LV) were mainly visible.

Effect of acetaminophen administration to rats chronically exposed to depleted uranium

The extensive use of depleted uranium (DU) in both civilian and military applications results in the increase of the number of human beings exposed to this compound. We previously found that DU chronic exposure induces the expression of CYP enzymes involved in the metabolism of xenobiotics (drugs). In order to evaluate the consequences of these changes on the metabolism of a drug, rats chronically exposed to DU (40mg/l) were treated by acetaminophen (APAP, 400mg/kg) at the end of the 9-month contamination. Acetaminophen is considered as a safe drug within the therapeutic range but in the case of overdose or in sensitive animals, hepatotoxicity and nephrotoxicity could occur. In the present work, plasma concentration of APAP was higher in the DU group compared to the non-contaminated group. In addition, administration of APAP to the DU-exposed rats increased plasma ALT (p<0.01) and AST (p<0.05) more rapidly than in the control group. Nevertheless, no histological alteration of the liver was observed but renal injury characterized by incomplete proximal tubular cell necrosis was higher for the DU-exposed rats. Moreover, in the kidney, CYP2E1 gene expression, an important CYP responsible for APAP bioactivation and toxicity, is increased (p<0.01) in the DU-exposed group compared to the control group. In the liver, CYP's activities were decreased between control and DU-exposed rats. These results could explain the worse elimination of APAP in the plasma and confirm our hypothesis of a modification of the drug metabolism following a DU chronic contamination.

Pharmacokinetics/Pharmacodynamics of Acetaminophen Analgesia in Japanese Patients with Chronic Pain

Acetaminophen (APAP) is a popular analgesic. In the present study, we characterized the pharmacokinetics and pharmacodynamics of APAP in the Japanese. Five healthy volunteers were administered 1000 mg of APAP orally. Five patients with chronic pain were administered the optimal oral dose of APAP ranging from 600 to 1000 mg to allow for an adequate analgesic effect. Plasma APAP and APAP metabolite concentrations were measured in the volunteers, plasma APAP concentrations and pain scores using a visual analog scale were measured in the patients with chronic pain. Patient data were fitted to a first-order absorption one-compartment model with delayed effects accounted for by an effect compartment. A sigmoid Emax model was used as the pharmacodynamic model. Acetaminophen-cysteine metabolites, which are conjugates of the toxic metabolite N-acetyl-p-benzoquinone-imine, were detected in the plasma at levels lower than 0.2 microg/ml, but no side effects were observed. The pharmacokinetic and pharmacodynamic parameter (mean+/-S.D.) estimates were as follows: clearance, 18.7+/-4.7 l/h; distribution volume, 30.9+/-6.8 l; absorption rate constant, 2.4+/-1.3 h(-1); rate constant for the elimination of APAP from the effect compartment, 1.3+/-0.5 h(-1); maximum pain relief score, 4.6+/-2.2 units; effect compartment concentration at 50% maximum, 2.0+/-1.2 microg/ml; and sigmoid factor, 1.3+/-0.7. These results suggest that these parameters can be used to determine an effective APAP dosage regimen for Japanese patients with chronic pain.

Interindividual variability in plasma concentrations after systemic exposure of swine to dietary doxycycline supplied with and without paracetamol: a population pharmacokinetic approach

Anorexigenic substances released during infection may hinder the therapeutic efficacy of in-feed antibiotics. Paracetamol (acetaminophen; PARA) inhibits the anorexia of infection and seems to improve the clinical efficacy of doxycycline (DOX) against bacterial respiratory disease in swine herds. In order to verify whether PARA selectively stimulates intake of DOX-medicated feed in diseased pigs, we documented the pharmacokinetics (PK) of DOX when coadministered with PARA and examined the effect of in-feed PARA on the interindividual variability in plasma concentrations after systemic exposure to in-feed DOX in swine herds with respiratory disease. Systemic exposure to DOX was measured with the area under the curve (AUC) of its plasma concentrations over time. First, a rich-sampling PK study of in-feed and i.v. DOX (10 mg/kg of BW) and PARA (30 and 10 mg/kg of BW, respectively) was performed on 5 pigs. The PK profiles of in-feed DOX were used in mathematical simulations to determine 5 optimal sampling times for the farm-based population PK study. A randomized, blind, parallel PK study was performed in 2 herds with bacterial respiratory disease, where liquid feed was fortified with DOX alone (5 mg x kg of BW(-1) x meal(-1)) or combined with PARA (15 mg x kg of BW(-1) x meal(-1)). Medicated meals were given twice, 12 h apart, to group-housed growing pigs (n > 50 pigs x treatment(-1) x herd(-1), totaling 215 pigs). Plasma concentrations of DOX and PARA were measured with HPLC. At variance with our expectations, PARA decreased (P = 0.069) mean AUC of in-feed DOX and did not decrease its variability (P > 0.34). Mean AUC of DOX increased with feed intake and with initial exposure to DOX, and was greater in sick animals. Therefore, symptomatic PARA-induced improvement in bacterial respiratory disease control with DOX is more likely caused by its analgesic/antipyretic effects than by its orexigenic effect. Interindividual variation in the AUC of DOX was large in pigs given group medication, even when sufficient feeding space was allowed and the amount of feed offered was greater than their requirements. Therefore, future studies to improve the efficacy of group antibiotic therapy should focus on feeding behavior characteristics as well as biopharmaceutical properties of medicated feeds.

Effects of Combined UDP-Glucuronosyltransferase (UGT) 1A1*28 and 1A6*2 on Paracetamol Pharmacokinetics in β-Thalassemia/HbE

In addition to pathophysiological changes, genetic variations can alter drug pharmacokinetics in patients with thalassemia. Numerous drugs are metabolized by UDP-glucuronosyltransferases (UGT) including paracetamol (PCM), a widely used analgesic. Co-occurrence of the UGT1A1 polymorphism (UGT1A1*28) and the UGT1A6 polymorphism (UGT1A6*2) may affect PCM glucuronidation. To elucidate the effect of these combined polymorphisms on the PCM metabolism in thalassemic patients, 15 beta-thalassemia/hemoglobin E subjects with three different UGT1A genotypes received a single oral dose of 1,000 mg PCM. Drug disposition was determined by HPLC. Patients who have UGT1A6*2 without UGT1A1*28 showed a significant, lower area under concentration-time curve (AUC(0)-->infinity) of PCM, PCM-glucuronide and PCM-sulfate than those of the patients with wild-type UGT1A1 and UGT1A6 (p < 0.05). In addition, a high elimination rate constant and clearance of PCM and its metabolites were also found in these patients (p < 0.05). Ourstudy suggests that a subtherapeutic level of PCM may occur in patients who have UGT1A6*2 without UGT1A1*28.

Interaction between paracetamol and warfarin in patients: a double-blind, placebo-controlled, randomized study

BACKGROUND AND OBJECTIVES

Paracetamol (acetaminophen) has occasionally been reported to interact with warfarin. The primary end-point of this study was to investigate whether paracetamol initiation potentiates the anticoagulant effect of warfarin and the mechanism of the interaction.

DESIGN AND METHODS

In a double-blind placebo-controlled, randomized, cross-over study, 20 patients on stable oral anticoagulant therapy with warfarin for at least 1 month were randomized to receive placebo or paracetamol 1g four times daily for 14 days. International Normalized Ratio (INR) and clotting factors activities were measured before the first administration and then on days 2, 4, 7, 9, 11,14.

RESULTS

Mean INR rose rapidly after the start of paracetamol and was significantly increased within one week of paracetamol intake compared to placebo, p=0.0002. The INR values reached a mean maximum of 3.45+/-0.78 with paracetamol versus 2.66+/-0.73 with placebo (p=0.03), corresponding to a maximum increase from baseline of 1.20+/-0.62 with paracetamol versus 0.37+/-0.48 with placebo (p<0.001). Together with the rise in INR on paracetamol treatment there were significant reductions in the vitamin K-dependent clotting factors II, VII, IX and X.

INTERPRETATION AND CONCLUSIONS

The most plausible hypothesis to explain the in vivo interaction is that paracetamol (or its metabolites) interfere with enzymes involved in vitamin K-dependent coagulation factor synthesis. Paracetamol at 4 g daily (a dose higher than that used in clinical practice) potentiates the anticoagulant response produced by warfarin. Clinicians should be aware of this clinically significant and underestimated interaction.

The effect of anesthetic technique on early postoperative gastric emptying: comparison of propofol-remifentanil and opioid-free sevoflurane anesthesia

PURPOSE

A postoperative decrease in the gastric emptying (GE) rate may delay the early start of oral feeding and alter the bioavailability of orally administered drugs. The aim of this study was to compare the effect on early gastric emptying between two anesthetic techniques.

METHODS

Fifty patients (age, 19-69 years) undergoing day-case laparoscopic cholecystectomy were randomly assigned to received either total intravenous anesthesia with propofol/remifentanil/rocuronium (TIVA; n = 25) or inhalational opioid-free anesthesia with sevoflurane/rocuronium (mask induction; GAS; n = 25). Postoperative gastric emptying was evaluated by the acetaminophen method. After arrival in the recovery unit, acetaminophen (paracetamol) 1.5 g was given through a nasogastric tube, and blood samples were drawn during a 2-h period. The area under the serum-acetaminophen concentration curve from 0-60 min (AUC60), the maximal concentration (Cmax), and the time to reach C-max (Tmax) were calculated.

RESULTS

Twelve patients were excluded due to surgical complications (e.g., conversion to open surgery) and difficulty in drawing blood samples (TIVA, n = 7; GAS, n = 5). Gastric emptying parameters were (mean +/- SD): TIVA, AUC60, 2458 +/- 2775 min.micromol.l(-1); Cmax, 71 +/- 61 micromol.l(-1); and Tmax, 81 +/- 37 min; and GAS, AUC60, 2059 +/- 2633 min.micromol.l(-1); Cmax, 53 +/- 53 micromol.l(-1); and Tmax, 83 +/- 41 min. There were no significant differences between groups.

CONCLUSION

There was no major difference in early postoperative gastric emptying between inhalation anesthesia with sevoflurane versus total intravenous anesthesia with propofol-remifentanil. Both groups showed a pattern of delayed gastric emptying, and the variability in gastric emptying was high. Perioperative factors other than anesthetic technique may have more influence on gastric emptying.