Effect of cimetidine and other antihistaminics on the elimination of aminopyrine, phenacetin and caffeine

Modeling caffeine concentrations with the Stanford Caffeine Questionnaire: preliminary evidence for an interaction of chronotype with the effects of caffeine on sleep

OBJECTIVE

To examine the validity of a novel caffeine intake questionnaire and to examine the effects of caffeine on sleep in college students.

METHODS

One-week, ad libitum behavior of 50 university students (28 female, 22 male; aged 20.9 ± 1.78 years) was examined with sleep logs, wrist actigraphy, and a novel daily questionnaire assessing caffeine intake at different times of day. Saliva samples were collected for caffeine assessment (questionnaire validation) and DNA extraction, and for analysis of a single nucleotide polymorphism in the adenosine receptor 2A (ADORA2A) gene.

RESULTS

The caffeine questionnaire was able to accurately predict salivary concentrations of caffeine (R(2) = 0.41, P<0.001). Estimations of integrated salivary caffeine concentration during sleep were correlated with wake after sleep onset (WASO) most strongly in morning-type individuals (R(2) = 0.49; P<0.001, ANOVA), less so in intermediate chronotypes (R(2) = 0.16; P<0.001, ANOVA), and not significantly in evening-types (R(2) = 0.00098; P = 0.13, ANOVA). Using multivariate modeling methods we found that the ADORA2A genotype did not moderate the effects of caffeine on WASO, but did independently alter WASO such that those with the CC genotype had nearly three-times as much WASO as those with CT or TT.

CONCLUSIONS

Our questionnaire was able to accurately predict salivary caffeine concentrations and helped to describe a novel relationship between the effects of caffeine on sleep and genotype and chronotype.

Biodistribution, binding specificity and metabolism of [18F]fluoroethylflumazenil in rodents

Pre-clinical studies were carried out in order to characterize in rodents the biodistribution, the binding specificity and the metabolism of [18F]Fluoroethylflumazenil ([18F]FEF), a potential candidate for in vivo imaging of the benzodiazepine receptors. In vivo competition with flumazenil indicates that [18F]FEF binds specifically to the benzodiazepine receptor in the brain. The accumulation of [18F]FEF was significantly lower than using [3H]Flumazenil. The rather low accumulation in the brain is due to a rapid metabolism of [18F]FEF in hydrophylic metabolites which cannot cross the blood brain barrier, and are rapidly eliminated in the urine. Inhibition of the metabolism by acetaminophen (chemically induced hepatitis) led to a significant increase of the radioactivity found in the circulating blood and in the brain, while these results were not observed using classical inhibitors of the cytochrome CYP450, cimetidine and ketoconazole.

Colchicine: 1998 update

OBJECTIVE

To present an update of the use of colchicine in patients with familial Mediterranean fever (FMF) and other rheumatic and nonrheumatic diseases.

DATA SOURCES

Published studies on colchicine retrieved from MEDLINE searches from 1987 to 1997 and reports presented at national and international meetings. STUDIES SELECTION AND EXTRACTION: All studies were reviewed by the authors. Reports addressing the topics of colchicine pharmacokinetics, biological effects, indications for use, and side effects were selected.

DATA SYNTHESIS

Colchicine is an alkaloid that may interfere with microtubule formation, thereby affecting mitosis and other microtubule-dependent functions. It has a bioavailability of 25% to 50% when administered orally. Colchicine and its metabolites are excreted through the urinary and biliary tracts. It may be used while breast feeding; however, amniocentesis should be performed when used in pregnancy. The drug may be given to children with FMF. The efficacy of colchicine has been proved in FMF, gout, Behcet's disease, and cirrhosis. Its place in the treatment of scleroderma, sarcoidosis, and skin disorders remains to be determined. Gastrointestinal side effects occur early and are most common manifestations of colchicine toxicity. Severe colchicine toxicity results in multiple organ failure, convulsions, coma, and death. Potentially, effective treatment with Fab anti-colchicine antibodies unfortunately is unavailable; therefore, treatment is supportive.

CONCLUSIONS

Colchicine is a relatively safe and effective medication for several disorders when used in appropriate dosage in patients with normal kidney and liver function.

Aminopyrine infusion breath test for the determination of changes in P450 metabolism in vivo

1. An osmotic mini-pump was used to maintain a constant infusion of radiolabelled [N-dimethyl-14C] aminopyrine into a rat. After implanting the mini-pump, 14CO2 expiration rate was constant within 12 h, and this rate was maintained for 192 h. 2. Treatment with 2-diethylaminoethyl-2,2-diphenylvalerate HCl (SKF 525-A) or cimetidine, inhibitors of P450-dependent metabolism, resulted in both dose- and time-dependent inhibition of the expiration of 14CO2.

Characterization of the independent and combined effects of two inhibitors on oxidative drug metabolism in rat liver microsomes

To evaluate how two inhibitors influence oxidative drug metabolism, this study investigated the inhibitory effects of mexiletine with cimetidine and mexiletine with lidocaine, both individually and in combination, on the oxidative metabolism of two probe substrates, aminopyrine and aniline in rat liver microsomes. Mexiletine was a competitive inhibitor of aminopyrine N-demethylation, whereas cimetidine was a mixed type of inhibitor (Ki = 2.00 +/- 0.04 and 0.20 +/- 0.02 mM, respectively). For aniline hydroxylation, mexiletine exhibited a mixed type of inhibition, whereas lidocaine was a noncompetitive inhibitor (Ki = 0.60 +/- 0.07 and 8.50 +/- 0.12 mM, respectively). The combined inhibition of either mexiletine with cimetidine or mexiletine with lidocaine on aminopyrine and aniline metabolism was close to the fully additive effects of the individual compounds when their individual concentrations were below a 2-fold Ki concentration, regardless of the apparent kinetic inhibition type. The combined inhibition was less than fully additive when the individual concentrations were twice the Ki or above. These results demonstrate that, when two inhibitors of oxidative drug metabolism are combined, both the Ki values and the concentrations of inhibitors play important roles in determining the extent of additive inhibition of enzyme activity.

Pharmacological attempts to improve the bioavailability of oral etoposide

Etoposide demonstrates incomplete and variable bioavailability after oral dosing, which may be due to its concentration and pH-dependent stability in artificial gastric and intestinal fluids. The use of agents that may influence etoposide stability and, thereby, bioavailability, was investigated in a number of clinical studies. Drugs that influence the rate of gastric emptying, while modulating the time of drug absorption, did not significantly alter the etoposide area under the concentration-time curve (AUC) or bioavailability. Specifically, metoclopramide had little effect on the etoposide absorption profile and did not significantly alter the AUC (AUC with etoposide alone, 68.4 +/- 20.3 micrograms ml-1 h, versus 74.3 +/- 25.9 micrograms ml-1 h with metoclopramide), suggesting that in most patients the drug is already emptied rapidly from the stomach. In contrast, propantheline produced a dramatic effect on etoposide absorption, delaying the time of maximal concentration tmax from 1.1 to 3.5 h (P < 0.01), but again without a significant improvement in drug AUC or bioavailability across the 24-h study period (AUC with etoposide alone 78.3 +/- 19.1 micrograms ml-1 h, versus 88.1 +/- 23.6 micrograms ml-1 h with propantheline). The effect of these drugs on the absorption of oral paracetamol, a drug included in the study as a marker of gastric emptying, was exactly the same as that found for etoposide, with no change in AUC being observed after metoclopramide or propantheline administration but a significant delay in tmax being seen on co-administration with etoposide and propantheline. The co-administration of ethanol or bile salts (agents that significantly improved the stability of etoposide in artificial intestinal fluid) with oral etoposide similarly had no effect on improving the etoposide AUC or reducing the variability in AUC, suggesting that drug stability in vivo was not affected by these agents. In the third study the co-administration of cimetidine had no effect on the pharmacokinetics of oral or i.v. etoposide, despite the previous observation that etoposide stability was markedly improved at pH 3-5 as compared with pH 1 in artificial gastric fluid. This series of studies, designed to investigate factors that improved etoposide stability in laboratory studies, failed to demonstrate any potentially useful improvement in AUC or bioavailability in the clinical setting.

Cimetidine prevents and partially reverses CCl4-induced liver cirrhosis

Liver injury produced by CCl4 depends on its metabolism by the liver cytochrome P450 enzyme system to a highly reactive intermediate (CCl3.). Cimetidine impairs cytochrome P450 and stimulates regenerative processes acting on DNA synthesis. This work was performed to investigate whether cimetidine may prevent CCl4-induced liver cirrhosis. Male Wistar rats were used: animals in group 1 received CCl4 (0.04 g per 100 g, i.p.) three times a week for 8 weeks; group 2 was treated with CCl4 plus cimetidine (120 mg kg-1, p.o.) three times a week for 8 weeks; group 3 received CCl4 for 8 weeks and then cimetidine for 4 weeks. Alkaline phosphatase, gamma-glutamyl transpeptidase (gamma-GTP) and alanine aminotransferase (ALT) activities, as well as protein and bilirubin, were measured in serum; collagen and lipoperoxidation were quantified in liver. Intoxication with CCl4 increased (P < 0.05) serum activities of alkaline phosphatase, gamma-GTP and ALT, and bilirubin concentration; liver collagen and lipoperoxidation were also increased. Cimetidine treatment prevented or reverted the increases in the three enzyme activities and in bilirubin content and the fall in proteins. It is worth noting that cimetidine co-treatment completely prevented both the increase in collagen content and the lipid peroxidation. The protective effect of cimetidine can be attributed to a reduction in cytochrome P450. However, it could also stimulate regenerative processes.

Inhibition of acetaminophen oxidation by cimetidine and the effects on glutathione and activated sulphate synthesis rates

The aim of the present study was to examine the effects of the hepatotoxic drug, acetaminophen, on the synthesis rates of glutathione, activated sulphate (PAPS, adenosine 3'-phosphate 5'-phosphosulphate) and the acetaminophen metabolites, acetaminophen-glutathione and acetaminophen-sulphate after inhibition of cytochrome P-450 drug oxidation by cimetidine in isolated rat hepatocytes. The synthesis rates of glutathione and PAPS were determined simultaneously by an established method based on trapping of radioactivity (35S) in the prelabelled glutathione and PAPS pools. Preincubation of the hepatocytes with 60 micrograms/ml cimetidine for 30 min. did not affect PAPS (1.71 versus 1.78 nmol/10(6) cells) nor glutathione concentration (16.0 versus 16.4 nmol/10(6) cells). The subsequent incubation with 5 mM acetaminophen resulted in decreased PAPS synthesis in the cimetidine treated cells [0.79 x 10(3) versus 0.92 x 10(3) nmol/(10(6) cells.hr)] (P < 0.05). There was no difference in PAPS concentration or acetaminophen-sulphate synthesis [1.73 versus 1.79 nmol/10(6) cells and 13.0 versus 12.9 nmol/(10(6) cells.hr), respectively]. Decreased PAPS synthesis may be related to decreased ATP supply or may be the result of a feed-back regulation due to diversion of sulphur from glutathione synthesis to sulfoxidation. The glutathione synthesis was not significantly affected by cimetidine treatment [57 x 10(3) versus 27 x 10(3) nmol/(10(6) cells.hr)]. As expected acetaminophen-glutathione synthesis decreased by 38% [1.66 versus 2.68 nmol/(10(6) cells.hr)] (P < 0.01). Also the glutathione concentration was lower in cimetidine treated cells [15.2 versus 15.9 nmol/10(6) cells] (P < 0.05). We have previously shown that glutathione synthesis was reduced if substrate availability decreased (acetaminophen concentration lowered).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a subacute treatment in rats by a fresh cola extract on EEG and pharmacokinetics

The compared effects of an acute and subacute treatment by fresh cola extract and caffeine on the caffeine pharmacokinetics and on cortical activities by spectral analysis of the electroencephalogram (EEG) are studied in rats. After acute cola administration, we observed an increase in half-life elimination of caffeine and a stabilization of its plasma/erythrocyte ratio. Chronic administration revealed differences in cola-caffeine penetration in erythrocytes and a reduction of the area under the curve (AUC) and plasma/erythrocyte ratio. We also noted a significant difference in the binding of the caffeine on plasma proteins after subacute administration of cola seed extract. Cola seed treatment induces an increase in the cortical activity with a widening of the dominant frequency spectrum 7- to 10-Hz band of EEG, whereas caffeine alone induces a shift of the dominant frequency band toward higher frequencies. The observed delay to obtain the greatest EEG effect related to the caffeine contained in cola seeds can be partially explained by the pharmacokinetic data.

Theophylline pharmacokinetics: effect of continuous versus intermittent cimetidine i.v. infusion

The comparative effects of continuous versus intermittent cimetidine infusion on theophylline pharmacokinetics were evaluated in 12 nonsmoking healthy male volunteers. Each subject received aminophylline 0.9 mg/kg/hr over 6 hours alone (control) and in random order at 1 week intervals, in combination with intermittent cimetidine (300 mg IV over 15 minutes every 6 hours) and continuous cimetidine (50 mg/hr IV) infusions. Both cimetidine regimens were administered for a total of 50 hours. Serial plasma samples were obtained and assayed for theophylline by HPLC. No significant differences existed in mean theophylline clearance and mean volume of distribution among control, intermittent or continuous cimetidine regimens; the power was greater than 80% to detect a 30% change in clearance. Only a minor difference in theophylline half-life between control and continuous cimetidine infusion (7.59 +/- 2.52 vs. 9.05 +/- 3.17 hr; P less than .05) was observed. These findings do not support a clinically significant interaction between IV aminophylline and cimetidine administered IV either as a low dose continuous infusion or as an intermittent infusion.

Selective in-vitro inhibition of hepatic oxidative metabolism by quinolones: 7-ethoxyresorufin and caffeine as model substrates

The in-vitro inhibition of several metabolic pathways has been studied in 3-methylcholanthrene-treated rats. The specificity of the 7-ethoxyresorufin O-de-ethylase reaction has been determined in the presence and absence of ciprofloxacin, enoxacin, norfloxacin, ofloxacin, nalidixic acid, oxolinic acid and pipemidic acid. For the caffeine N3-demethylation reaction, enoxacin and pipemidic acid were used. Enoxacin (IC50 = 105 microM, Ki = 65 microM) and pipemidic acid (IC50 = 115 microM, Ki = 160 microM) significantly inhibited 7-ethoxyresorufin O-de-ethylase reaction and caffeine N3-demethylation (IC50 = 60 microM for enoxacin and IC50 = 185 microM for pipemidic acid) by a competitive mechanism. Other quinolones had lower or no (ofloxacin) inhibitory capacity. The order of inhibitory activity observed is in agreement with results obtained previously from in-vivo studies in man. No activity was detected towards ethylmorphine N-demethylation.

The effect of liver dysfunction on colchicine pharmacokinetics in the rat

Recent work has shown that colchicine may benefit patients with primary biliary or alcoholic cirrhosis. However, very little is known about its pharmacokinetics in the presence of impaired liver function. To study this we examined the effects of three models of experimental liver dysfunction and one of cytochrome P-450 inhibition on colchicine elimination in the rat. The models of experimental liver dysfunction included bile duct ligation (with sham-operated controls), alpha-naphthylisothiocyanate-induced intrahepatic cholestasis and galactosamine-induced diffuse hepatocellular necrosis. The control group had a colchicine clearance of 77.33 ml/min.kg +/- 8.27 ml/min.kg, a half-life of 16.68 min +/- 0.97 min and a volume of distribution of 1.84 L/kg +/- 0.15 L/kg. Cimetidine administration, 120 mg/kg intraperitoneally 15 min before colchicine administration, caused clearance to decrease by 32% (p less than 0.05) and half-life to increase by 38% (p less than 0.05). Volume of distribution did not change. At 48 hr after bile duct ligation, colchicine clearance decreased by 84% (p less than 0.05), terminal half-life increased to 513.7 min +/- 106.6 min (p less than 0.05) and volume of distribution increased by 175% (p less than 0.05). Colchicine pharmacokinetics in sham-operated rats were not statistically different from the above mentioned controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cimetidine on hepatic biochemical changes, liver toxicity and major urinary metabolite excretion of trichloroethylene in rats

The effects of cimetidine (CIM) (an inhibitor of the hepatic microsomal monooxygenase system) on the metabolism and hepatotoxicity of trichloroethylene (TRI) were studied in male Sprague-Dawley rats. Rats were given three doses of 120 mg/kg i.p. (low-dose regimen) of CIM at 0, 6 and 11 h for 1 day, or ten doses of 200 mg/kg (high-dose regimen) at 8, 11, 14 and 17 h for 2 days and 8 and 11 h on 3rd day. Trichloroethylene (0.5 or 0.65 ml/kg) was administered i.p. 1 h after 2nd dose (low-dose regimen) or 9th dose (high-dose regimen) of CIM. In the low-dose regimen study, the activity of hepatic microsomal aminopyrine N-demethylase was decreased 1 and 5 h after the second dose and 7 h after the third dose of CIM, but became normal 20 h after the last dose. The cytochrome P-450 content and the activities of aniline hydroxylase and epoxide hydratase remained unchanged. Trichloroethylene at both dose levels produced liver toxicity, as verified by increase in activities of SDH and SGPT as well as by liver histology. Cimetidine alone had no such effect. An apparent reduction in TRI toxicity by CIM (at both dose regimens) could be observed histologically. The biochemical tests (SDH and SGPT) corroborated the histological changes only when TRI was given at a dose of 0.5 ml/kg combined with a high-dose regimen of CIM. Cimetidine at both dose regimens had a tendency to decrease the in vivo metabolism of TRI.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of rifampicin-induced theophylline metabolism by diltiazem/rifampicin coadministration in healthy volunteers

On the basis of reports that diltiazem binds to the hepatic microsomal enzymes and inhibits the metabolism of coadministered drugs, we investigated the effect of a 240 mg daily dose of the calcium channel blocker for a week on theophylline metabolism in 8 healthy male volunteers aged 20-24 years and weighing 56-68 kg. None of theophylline half-life (9.7 h), volume of distribution (0.514 l.kg-1) and total clearance (0.63 ml.min-1.kg-1) after pretreatment differed from the respective control value of 9.5 h, 0.519 l.kg-1 and 0.65 ml.min-1.kg-1. In 6 of the volunteers, 600-mg daily dose of rifampicin for a week induced theophylline metabolism. The control half-life of the bronchodilator (9.6 h) was reduced to 5.5 h and its total clearance (0.64 ml.min-1.kg-1) increased to 1.22 ml.min-1.kg-1. There was no change in volume of distribution. In these 6 volunteers, intake of diltiazem (240 mg daily), concurrently with rifampicin for a week, significantly elevated theophylline half-life to 6.2 h as well as reduced its clearance to 1.03 ml.min-1.kg-1. The volume of distribution did not change. Furthermore, diltiazem-induced absolute fall in theophylline clearance correlated significantly with its post-rifampicin value (r = 0.895). It is suggested that diltiazem has no effect on theophylline metabolism in non-induced subjects. However, rifampicin-induced metabolism of the bronchodilator could be attenuated by diltiazem when the two drugs are given concurrently.

Effect of aging on hepatic elimination of cimetidine and subsequent interaction of aging and cimetidine on aminopyrine metabolism

Aging and cimetidine may each impair hepatic microsomal drug metabolism. To test if and by what mechanisms advanced age may increase sensitivity to the inhibitory effects of cimetidine, the interaction of these two factors with aminopyrine metabolism in the rat was studied using a correlative approach. Initial studies using the aminopyrine breath test indicated that a 40 mg/kg dose of cimetidine, i.p., impaired the 14CO2 exhaled by up to 76% more in aged (26-month) than in young (3- to 4-month-old) rats. Using an isolated liver perfusion to dissect out hepatic components of this phenomenon, it was found that various doses of cimetidine impaired aminopyrine clearance to a greater degree (P less than 0.05) in aged than in young livers. However, cimetidine metabolism in this system ranged from 36 to 78% less in aged versus young livers (P less than 0.05). Subsequent in vitro studies indicated that microsomes isolated from aged livers also averaged a 76% lower rate of cimetidine metabolism (P less than 0.05). A fixed cimetidine concentration, however, inhibited aminopyrine demethylation to the same degree in aged versus young rats (P less than 0.05). In vivo pharmacokinetics showed an age-related decrease in both aminopyrine and cimetidine systemic clearance. In the young rat the liver contributed about 30% to total systemic clearance of cimetidine. In the aged rat, all clearance was renal. Despite a decrease in glomerular filtration rate, net tubular cimetidine secretion was well-maintained. Despite this, absence of the hepatic component resulted in decreased overall systemic clearance of the drug in aged rats. It is concluded that (1) the aged rat liver exhibits impaired cimetidine metabolism, resulting in decreased overall systemic clearance of the drug despite normal net renal tubular secretion, (2) there is no age-related enhanced sensitivity to cimetidine of the hepatic microsomal oxidizing system using aminopyrine as the probe drug, and (3) the larger inhibition of aminopyrine metabolism in aged rats following various doses of cimetidine is due to decreased overall cimetidine clearance, resulting in higher concentrations of the inhibitor in the liver of aged rats.

The effects of ageing on aminopyrine and caffeine breath tests in the rat

The influence of ageing on demethylation of 14C-aminopyrine (AP) and 14C-caffeine (Caf) in Wistar littermate rats was studied serially using the 14CO2-breath test. In both sexes, the elimination half-life (t1/2) of 14C-AP fell from a mean of approximately 75 min when weaned at 17 days to 45 min at maturity (49 days); thereafter t1/2 became prolonged, being most prolonged (82 min) in elderly male rats (210 days). Similarly, t1/2 of 14C-Caf was also prolonged with ageing. These studies using in vivo and longitudinal techniques indicate the importance of ageing on hepatic demethylation.

Effects of chronic treatment with amiodarone on hepatic demethylation and cytochrome P450

The effect of chronic treatment with amiodarone on hepatic oxidative metabolism using an in-vivo [14C]aminopyrine breath test and on hepatic cytochrome P450 was examined in Wistar rats. Aminopyrine demethylation was significantly impaired but returned to pretreatment values following amiodarone for 4 weeks. In contrast the levels of cytochrome P450 were significantly depressed during treatment and at 4 weeks following treatment. While an inhibitory effect on oxidative metabolism may explain the reported drug interactions with amiodarone, the discrepancy between its in-vivo effects and cytochrome P450 levels may suggest the development of 'compensatory' extra-hepatic site of drug metabolism.

The aminopyrine breath test does not correlate with histologic disease severity in patients with cholestasis

To determine whether the aminopyrine breath test can be used to document the presence of cirrhosis in patients with cholestatic liver disease, 19 patients (13 primary biliary cirrhosis, 4 sclerosing cholangitis and 2 chronic extrahepatic bile duct obstruction) underwent clinical and biochemical evaluations, liver biopsies and an aminopyrine breath test. Results were compared with those in 10 patients with biopsy-proven chronic active hepatitis with bridging and/or cirrhosis and in 22 healthy subjects. The aminopyrine breath test results in the 10 cholestatic patients with cirrhosis were not significantly different from the results in precirrhotic cholestatic patients (mean +/- S.D., 11.2 +/- 5.0 vs. 11.6 +/- 2.8% dose per 2 hr, p greater than 0.05) or healthy subjects (11.5 +/- 2.9% dose per 2 hr). In contrast, the results in the patients with chronic hepatitis were markedly depressed (3.2 +/- 1.9% dose per 2 hr, p less than 0.05). The aminopyrine breath test results did not correlate with results of conventional liver function tests in the cholestatic patients. These results demonstrate that the aminopyrine breath test is not clinically useful in identifying the presence of cirrhosis in patients with cholestatic liver disease, and provide further evidence that decreased microsomal enzyme function is a late feature of cholestatic liver disease.

Accumulation of caffeine in healthy volunteers treated with furafylline

The pharmacokinetics and tolerance of repeated oral doses of furafylline were investigated in normal volunteers. In accord with predictions from single dose studies, steady state was achieved on the first day following the administration of 90 mg and maintained by subsequent daily doses of 30 mg. When corrected for body weight there were no significant differences in minimum and maximum plateau levels of furafylline between males (1.2-2.0 micrograms ml-1; mean body weight 67.2 kg) and females (1.6-2.6 micrograms ml-1; mean body weight 54.9 kg). The half-life of elimination was less when the plasma concentration was lower than 600 ng ml-1 than during the stationary phase of treatment. Despite constant plasma levels the repeated administration of furafylline appeared to be associated with the onset of adverse xanthine-like side effects, a finding which was subsequently traced to the presence of, and possible synergism with, accumulating serum levels of caffeine in those volunteers drinking caffeine containing beverages. Subsequent studies showed that a single dose (90 mg) of furafylline results in a rapid accumulation of caffeine given orally (100 mg twice daily) and that this is accompanied by an elimination half-life of some 50 h and an abrupt decrease in metabolite levels. The furafylline-induced accumulation of caffeine was not influenced by the smoking habits of the subjects, implying that the metabolite pathway blocked by furafylline is the demethylation of caffeine in position 3, an implication confirmed by the reduced formation of paraxanthine. This demonstration of an unacceptable level of adverse side effects resulting from a potent inhibiting effect of furafylline on the metabolism of a normal dietary constituent has obvious implications in the interpretation of drug-induced toxicity.

Cimetidine inhibits cerebral and hepatic mitochondrial respiration in rat

The presence of cimetidine in the incubation medium of rat brain mitochondria caused decreased oxygen uptake, especially during oxidative phosphorylation (state 3). This inhibition of the respiratory control and of ATP synthesis was dose-dependent. The same observations were made for hepatic mitochondria. The significance of these results is discussed in terms of both the neurological side-effects of cimetidine and its effect on regulatory mechanisms of cerebral or hepatic blood flow.

Cimetidine enhances and phenobarbital decreases parathion toxicity

Parathion toxicity has been attributed to its metabolic product paraoxon which is formed in the mammal liver through the multiple oxidase enzymes. These are induced by barbiturates and inhibited by SKF 525 A and cimetidine. We assessed the effects of these drugs on the acute toxicity of parathion in rats by measuring the rate of survival at 24 h after the administration of the oral LD50 of parathion to four groups of rats: control and pretreated with the aforementioned drugs. Additional rats of these groups were used to measure the capability of liver isolated microsomes to transform p-nitroanisole to p-nitrophenol. In the control and cimetidine groups we also measured the transformation of parathion to paraoxon and p-nitrophenol by the liver microsomes. Phenobarbital increased the survival 100% whereas cimetidine and SKF 525 A dramatically potentiated parathion toxicity. Phenobarbital increased the formation of p-nitrophenol but cimetidine and SKF-525 A produced the opposite effect. Paraoxon and p-nitrophenol from parathion were decreased by cimetidine. Our results strongly suggest that parathion itself is largely responsible of its toxicity and the inhibition of its metabolism is harmful rather than beneficial.

Cimetidine induces hepatic heme oxygenase activity without altering hepatic heme catabolism

Cimetidine inhibits oxidative drug metabolism; it is not known whether this drug alters the catabolic fate of hepatic heme. We therefore investigated hepatic heme turnover both by a 14CO breath test and directly by labeling the heme pool. Neither acute (150 mg/kg i.p.) nor chronic (150 mg/kg i.p. bid for 3 days) cimetidine administration significantly affected hepatic heme turnover. Chronic, but not acute, cimetidine significantly (p less than 0.025) increased heme oxygenase activity. Cimetidine inhibited heme oxygenase activity in vitro at concentrations achieved in vivo.

Evidence for induction of hepatic microsomal cytochrome P-450 by cimetidine: binding and kinetic studies

The interaction of cimetidine with liver microsomes has been examined by spectral and equilibrium partition studies. First, difference spectroscopy has been used to evaluate the proportion of cytochrome P-450 in rat liver microsomes that exhibits an affinity for cimetidine in the pharmacologically relevant, low micromolar range of drug concentration. The value of 0.45 so obtained has confirmed that a substantial proportion of rat liver cytochrome P-450 has a high binding affinity for this drug. Second, a study of the binding of cimetidine to human liver microsomes by difference spectroscopy and partition equilibrium has detected a similar interaction, thus providing direct support for the postulate that the clinically observed impairment of oxidative drug metabolism may be due in part to inhibition of cytochrome P-450 monooxygenase by cimetidine. Hepatic microsomes from cimetidine-pretreated rats have been shown to exhibit elevated cytochrome P-450 specific content but a decreased proportion of sites with high affinity for the drug; this finding has been shown not to be the consequence of cimetidine-mediated, time-dependent, irreversible monooxygenase inhibition. Although cimetidine pretreatment caused enhanced specific activity of 7-ethoxyresorufin O-dealkylation, the specific activities for O-dealkylation of 7-ethoxycoumarin and 4-nitroanisole were decreased, as were those for the N-dealkylation of morphine, ethylmorphine, aminopyrine, and dimethylnitrosamine. Since cimetidine pretreatment was shown to cause no change in the Michaelis constants for oxidation of morphine or 7-ethoxyresorufin, it is argued that these results provide strong presumptive evidence for changes in the relative abundance of isoenzymes catalyzing these various oxidations. Thus, a dual role of cimetidine, acting both as inhibitor and inducer of the cytochrome P-450 system, is proposed to account for the impaired oxidative metabolism of some drugs that occurs during coadministration with this H2-receptor antagonist.

The hepatic and renal mechanisms of drug interactions with cimetidine

Cimetidine is one of the most frequently prescribed drugs with a known potential to interfere with the metabolic disposition of numerous other medications. Some of these drug interactions may be hazardous to patients. The hepatic and renal mechanisms of drug interactions with cimetidine are discussed in this article. The interactive processes include inhibition of hepatic microsomal enzyme activity, reduction of liver blood flow, and competition for renal tubular secretory sites. The clinical significance and proposed mechanisms of interaction for individual drugs are also presented.

The effect of cimetidine on serum concentrations of piroxicam

To evaluate the effect of cimetidine on serum concentrations of piroxicam, we administered a single 20-mg oral dose of piroxicam to 10 healthy male volunteers on 2 occasions. The first was given on day 1 of the study and the second on day 15, 7 days after starting cimetidine 300 mg orally 4 times a day. Nineteen blood samples were drawn for 7 days after each piroxicam dose to characterize its pharmacokinetics. Piroxicam was analyzed by high-performance liquid chromatography. The mean piroxicam elimination rate constants (Kel), elimination half-lives, and area under the serum concentration-time curves (AUC) were as follows (mean +/- standard deviation): (formula; see text) Data were analyzed with a Wilcoxon matched-pairs, signed-ranks, two-tailed statistical test. Although the increase in AUC was statistically significant, it was of low amplitude (mean 15%) and is probably not clinically significant. The results of this study suggest that cimetidine does not significantly alter the elimination kinetics of a single dose of piroxicam in young healthy males. Additional investigation is needed to confirm these findings in other patient populations.

Omeprazole: effects on oxidative drug metabolism in the rat

Omeprazole, a substituted benzimidazole and a potent gastric antisecretory drug has been tested for inhibition of microsomal drug oxidative function in the rat. A single dose of 40 mg/kg prolonged pentobarbitone sleeping times from 118 (range 73-168) min to 195 (159-222) min (P less than 0.01), pentobarbitone half-lives from 89 (63-114) to 112 (54-146) min (P less than 0.05) and aminopyrine breath 14CO2 half-lives from 43 (37-51) to 56 (49-79) min (P less than 0.05). Omeprazole in doses of 20 mg/kg or less had no significant effect. In prolonging pentobarbitone sleeping times omeprazole 40 mg/kg and an equimolar (30 mg/kg) dose of cimetidine were approximately equipotent. These results contrast with studies in man in which much smaller doses of omeprazole have been shown to produce clinically significant inhibition of drug metabolism.

Evidence for lipid involvement in the high-affinity interaction between cimetidine and rat liver cytochrome P-450

The spectral interaction between cimetidine and a cytochrome P-450 fraction isolated from liver of untreated rats has been shown to be markedly affected by dilauroylphosphatidylcholine. In the absence of the lipid the pigment preparation yielded a binding curve characteristic of a single isoenzyme species with low affinity for the drug, whereas its inclusion led to the observation of a much stronger interaction with a dissociation constant close to that obtained for the high-affinity component(s) of the parent microsomes; material with lower affinity was also observed. Gel chromatography and partition equilibrium studies yielded results which precluded interpretation of this finding either in terms of incomplete incorporation of the pigment into the phospholipid or of disproportionate solvation of cimetidine into the lipid phase of the reconstituted phospholipid complex. In contrast, phospholipid caused only a minor change in the strength of cimetidine binding by the predominant liver cytochrome P-450 from phenobarbitone-pretreated rats. Pronounced lipid sensitivity of cimetidine-binding affinity is thus not a general feature of the microsomal cytochrome P-450 system but rather a specific characteristic of individual isoenzyme species.

Caffeine disposition in obesity

Caffeine pharmacokinetics were studied in 16 obese (mean +/- s.e. mean body weight; 110 +/- 8 kg; % ideal body weight (IBW); 186 +/- 14%) and 23 normal body weight (64 +/- 3 kg; 103 +/- 3% IBW) subjects. Eight obese and four control subjects were cigarette smokers. After abstaining from caffeine for 48 h and an overnight fast, each subject ingested 162 mg caffeine orally. Concentrations of caffeine were measured in plasma samples obtained during the 24 h following the dose and pharmacokinetic variables were determined. The apparent volume of distribution was increased markedly in obese subjects (69.9 +/- 5.9 vs 43.6 +/- 2.8 l; P less than 0.001) in the absence of any change in oral clearance (135 +/- 14-obese vs 112 +/- 12 ml/min; NS), resulting in a trend toward increased elimination half-life (7.05 +/- 1.08-obese vs 5.40 +/- 0.40 h; NS). Apparent volume of distribution correlated well with percent IBW (r = 0.65; P less than 0.001). Caffeine clearance, suggested as a measure of in vivo cytochrome P-448 activity in humans, was not altered in obesity. In contrast, the extent of caffeine distribution increased in direct relation to body weight. If caffeine is used therapeutically, the loading dose should be calculated as a function of total body weight. Since clearance of caffeine is not related to body weight, these data indicate that a chronic dosing regimen to maintain a given plasma caffeine concentration should not be altered due to obesity.

The effect of glutathione depletion on 14CO2 evolution from [14C]methyl-labeled aminopyrine in intact rats

The effect of hepatic glutathione depletion on 14CO2 evolution from [14C]methyl-labeled aminopyrine was assessed in fed male Sprague-Dawley rats. Within 30 min of i.p. administration of either diethylmaleate or phorone, hepatic glutathione fell approximately 75 to 80% and remained depressed for the ensuing 120 min. [14C]Aminopyrine was i.p. administered 30 min after the glutathione-lowering agents (zero time) and exhaled 14CO2 was collected at 15-min intervals for the next 120 min. Parameters of 14CO2 exhalation including peak exhalation rate, cumulative exhalation from 0 to 120 min and the elimination rate constant were all impaired in glutathione-depleted rats. Metabolism of the [14C]methyl groups involves N-demethylation with formation of formaldehyde, oxidation to formate and conversion to 14CO2. Glutathione depletion did not affect CO2 evolution from i.p. administered formate or bicarbonate. The glutathione-dependent step presumably involves either or both generation of formaldehyde or its subsequent oxidation to formate.

Analgesic effects of antihistaminics

The literature provides considerable evidence indicating that several, but not all antihistaminics, are indeed analgesic agents and some are analgesic adjuvants as well. Those for which effectiveness is reported includes diphenhydramine, hydroxyzine, orphenadrine, pyrilamine, phenyltoloxamine, promethazine, methdilazine, and tripelennamine. The proposed mechanisms of analgesic action of antihistaminics are reviewed and discussed. The literature suggests that more than one mechanism of action exists for them. There is considerable evidence suggesting that histaminergic and serotoninergic central pathways are involved in nociception and that antihistaminic drugs can modulate their responses (1). The evidence for a role for norepinephrine and dopamine and the effects of antihistaminics on them are less well established. Still other pathways have been proposed. A greater understanding of pain mechanisms will aid in elucidating the role of antihistaminics in analgesia.

Nizatidine, a new histamine H2-receptor antagonist, and hepatic oxidative drug metabolism in the rat: a comparison with structurally related compounds

The effects of nizatidine (a new H2-receptor antagonist) and of related compounds were studied on oxidative drug metabolism in the rat both in vivo and in vitro. Nizatidine is a structural analog of the H2-receptor antagonists ICI 125,211 (Tiotidine) and ranitidine (Zantac). Nizatidine (120 mg/kg, ip) had no effect on the [14C]aminopyrine (ABT) or [14C]caffeine breath (CBT) tests, nor on the clearance from plasma of aminopyrine despite high tissue and plasma concentrations of nizatidine. Binding of nizatidine (1 mM) to rat hepatic microsomal P-450 determined by spectral analysis was not observed. In vitro aminopyrine demethylation was inhibited by nizatidine only at high concentrations (Ki = 92 mM). Cimetidine, ICI 125,211, and imidazole bind avidly to rat hepatic microsomal cytochrome P-450 and are potent inhibitors of aminopyrine demethylation in vitro. Imidazole inhibited the aminopyrine breath test, while imidazole, ranitidine, and ICI 125,211 inhibited the caffeine breath in vivo. These data indicate that nizatidine has no acute inhibitory effect on hepatic oxidative drug metabolism in the rat, both in vitro and in vivo. The composite structural-activity data suggest that inhibition of in vivo oxidative drug metabolism by H2-antagonists may not depend primarily on either the imidazole ring side chain or the thiazole ring per se. Furthermore, the in vivo inhibition may not correlate with in vitro data.

Comparison of inflammatory changes in established type II collagen- and adjuvant-induced arthritis using outbred Wistar rats

Type II collagen- and adjuvant-induced arthritis in outbred Wistar rats were compared using parameters that measured the inflammatory response, cellular and humoral immunity, blood protein changes, drug metabolism and histopathological and bony changes of the inflamed paws. There was a lesser incidence (40-70%) and severity of collagen disease than the adjuvant model (incidence approximately 100%). The use of MDP increased the incidence and severity of collagen arthritis. The acute phase protein response (plasma fibrinogen) was similar in both models during the peak of inflammatory response. Drug metabolism was inhibited in both type II collagen boosted with MDP or M. butyricum sensitized rats with arthritis; however, arthritic rats sensitized with collagen alone produced no inhibition. Only collagen arthritic rats produced type II collagen antibody and exhibited delayed hypersensitivity to type II collagen. Bony changes as assessed by radiographic evaluation were more severe in adjuvant arthritic rats than in the collagen arthritic model; histopathological findings from these animals confirmed this observation. The primary lesions in both models were periosteal reaction of the bone and ankylosis. Several classes of antiarthritic drugs were compared in both models using paw edema measurements and bony changes by radiographic evaluation. Drugs with inhibitory activity in both models were indomethacin, methylprednisolone, D-penicillamine and gold sodium thiomalate. Levamisole, chloroquine and auranofin were inactive in both models.

In vivo interactions between H2-receptor antagonists and ethanol metabolism in man and in rats

The influence of a 7-day medication of either cimetidine (1,000 mg per day) or ranitidine (300 mg per day) on serum ethanol concentrations after a single oral dose of ethanol (0.8 gm per kg body weight) was investigated in a randomized placebo-controlled study in eight male volunteers. Compared with the placebo, cimetidine but not ranitidine produced a significant increase in both the peak serum ethanol concentration (85.9 +/- 3.5 vs. 73.0 +/- 3.2 mg dl-1, p less than 0.02) and in the area under the serum ethanol concentration time curve (350 +/- 19 vs. 304 +/- 25 mg dl-1 hr-1, p less than 0.05). However, the ethanol elimination rate was not affected by cimetidine. When ethanol (1.0 gm per kg body weight) was administered intravenously, cimetidine failed to induce a change in ethanol metabolism. Furthermore, the effect of H2-receptor antagonists was studied in animal experiments. Female Sprague-Dawley rats received a single dose of ethanol (7 or 3 gm per kg body weight) together with an intraperitoneal injection of either cimetidine (120 mg per kg body weight), ranitidine (120 mg per kg body weight) or isotonic saline. After alcohol absorption, ethanol elimination was significantly inhibited by both cimetidine (3.99 +/- 0.39 vs. 5.68 +/- 0.23 mmoles kg-1 hr-1, p less than 0.02) and ranitidine (4.21 +/- 0.14 vs. 5.68 +/- 0.23 mmoles kg-1 hr-1, p less than 0.02) at high ethanol concentrations (60 to 20 mM) but not at blood ethanol concentrations below 20 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Bioluminescence assays for bile acids in the detection and follow-up of experimental liver injury

We evaluated the usefulness of recently developed bioluminescence assays for serum bile acids (BA) in the detection and follow-up of experimental liver injury. Liver damage was induced in rats by either D-galactosamine or CCl4, and BA were compared to SGPT and aminopyrine breath test (ABT). In severe liver injury, following D-galactosamine administration, all three methods revealed a significant difference from control values. The degree of abnormality was, however, far greater with SGPT and BA than with ABT. In moderate liver injury, induced by CCl4, the increase in BA was not significant. Values of SGPT and BA showed a very good correlation (3 alpha-OH: r = 0.88; 7 alpha-OH: r = 0.90; 12 alpha-OH: r = 0.83; p less than 0.001 for all correlations). Application of different assays for 3 alpha-OH, 7 alpha-OH and 12 alpha-OH BA allowed us to assess changes in individual BA. A 96-hr follow-up study in D-galactosamine-treated animals showed an increase in BA up to 48 hr and a decrease thereafter. The bioluminescence assays for BA are simple, rapid and require only 10 microliter of serum. Thus, these assays may be the method of choice in detecting and monitoring liver injury in small laboratory animals.

Selective inhibition of acetaminophen oxidation and toxicity by cimetidine and other histamine H2-receptor antagonists in vivo and in vitro in the rat and in man

Acetaminophen-induced hepatotoxicity results from hepatic enzymatic oxidation of acetaminophen to a toxic, electrophilic intermediate. Acetaminophen is ordinarily eliminated after conjugation with glucuronic acid and sulfate to nontoxic derivatives. Cimetidine has been shown to inhibit the hepatic oxidation of a number of drugs and to protect rats from acetaminophen-induced hepatic necrosis. The aim of this study was to define the mechanism by which cimetidine reduced acetaminophen-induced hepatic necrosis and to determine whether inhibition of formation of the reactive metabolite(s) of acetaminophen occurred also in man. In vivo cimetidine pretreatment decreased covalent binding of [3H]acetaminophen to the liver from 552 +/- 23.8 to 170 +/- 31.6 nmol/g protein 2 h after a toxic dose of acetaminophen in 3-methylcholanthrene pretreated rats (P less than 0.05). Cimetidine pretreatment also significantly reduced the rate of hepatic glutathione depletion. Both cimetidine and metiamide produced dose-dependent inhibition of acetaminophen oxidation in vitro, whereas inhibition by ranitidine and cimetidine sulfoxide was quantitatively less. Inhibition of acetaminophen oxidation by cimetidine and metiamide was primarily competitive with an inhibition constant (Ki) of 130 +/- 16 and 200 +/- 50 microM, respectively. By contrast, cimetidine inhibited acetaminophen glucuronidation minimally with a Ki of 1.39 +/- 0.23 mM. Similar results were obtained using human liver microsomes as a source of enzymes. In a dose-related fashion, cimetidine also reduced acetaminophen-induced toxicity to human lymphocytes when incubated with microsomes and NADPH. Pharmacokinetics of acetaminophen elimination were studied in normal volunteers with and without co-administration of cimetidine 300 mg every 6 h. In normal volunteers, cimetidine decreased the fractional clearance of the oxidized (potentially toxic) metabolites of acetaminophen more than the conjugated metabolites. This finding confirmed the hypothesis that cimetidine is a relatively selective inhibitor of the oxidation of acetaminophen to reactive metabolites in man as well as in animals. When considered together with the results of previous studies showing improved survival and decreased hepatoxicity in acetaminophen-poisoned animals, the present results provide a rational basis for assessing possible benefits of cimetidine treatment of acetaminophen overdoses in man.

Cimetidine-drug interactions

The use of cimetidine, the histamine H2 receptor antagonist, is associated with a relatively low incidence of adverse reactions. However, its liberal use has led to the identification of several clinically significant cimetidine-drug interactions that can lead to drug accumulation, toxicity, and life-threatening sequelae. A review of the literature and the clinical significance and physiologic basis of these interactions are presented. Recommended management of cimetidine-drug interactions is discussed.

Factors affecting the response to inhibition of drug metabolism by cimetidine--dose response and sensitivity of elderly and induced subjects

The effect of cimetidine on oxidative drug metabolism was characterised using antipyrine clearance in a group of healthy volunteers. In six subjects cimetidine produced a dose dependent reduction of antipyrine clearance: 400 mg/day (16.8 +/- 2.2%, mean +/- s.e. mean), 800 mg/day (26.3 +/- 1.5%) and 1600 mg/day (33.5 +/- 2.4%). The effect of cimetidine (800 mg/day) was of similar magnitude (approximately 25%) in two groups of six young (21-26 years) and six elderly (65-78 years) subjects. The effect of pretreatment begun just 1 h before administration of antipyrine was similar to that of 24 h pretreatment and that reported for chronic cimetidine pretreatment. The percentage reduction in antipyrine clearance produced by cimetidine 800 mg/day was greater (44 +/- 5 vs 24 +/- 3%; P less than 0.05) in six subjects who had been pretreated with the hepatic enzyme inducer rifampicin (600 mg/day for 21 days) than in the control uninduced state. Although cimetidine was capable of rapidly reversing the effect of rifampicin on antipyrine clearance, following withdrawal of both rifampicin and cimetidine there was still evidence of enzyme induction. These results suggest that the effect of cimetidine on oxidative metabolism is dose dependent, is more marked in enzyme induced subjects, is independent of the duration of pretreatment and is of similar magnitude in young and elderly subjects.

Effect of cimetidine on the metabolism of cholephilic dyes in Gilbert's syndrome

The metabolism of indocyanine green, bromsulphophthalein and bilirubin was studied before and after a treatment for 2 weeks with cimetidine 1.0 g/day in 12 subjects with Gilbert's syndrome. There was no effect on the plasma clearance of indocyanine green and bromsulphophthalein, on the maximal biliary excretion capacity of bromsulphophthalein, on serum bilirubin concentrations or on the biliary metabolites of bromsulphophthalein and bilirubin. It is concluded that in Gilbert's syndrome two weeks of treatment with cimetidine had no effect on liver blood flow or on conjugation mechanisms which were independent of cytochrome P450.

Effect of cimetidine on the pharmacokinetics of quinidine and lidocaine in the rat

Because of previously reported drug interactions involving cimetidine and liver-metabolized drugs, the intravenous pharmacokinetics of quinidine (25 mg/kg) and lidocaine (15 mg/kg) were investigated in anesthetized rats pretreated with a single intraperitoneal dose of cimetidine (60 mg/kg) and compared with saline pretreated controls. Significant reductions of 35 and 23% in the respective total clearances of quinidine and lidocaine were observed in the presence of cimetidine. The quinidine volume of distribution was significantly decreased in the cimetidine-treated rats, while the lidocaine volume of distribution was not altered significantly. There was no significant change in the elimination half-life for either drug in the presence of cimetidine. These results suggest cautious use of quinidine or lidocaine when cimetidine is prescribed concurrently.