Interaction of cimetidine with liver microsomes

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, "general chemicals," natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10-15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Drug interactions through binding to cytochrome p 450: the experience with h2-receptor blocking agents

H2-receptor blocking agents, such as cimetidine, ranitidine or oxmetidine, are consumed in large amounts often together with a variety of other drugs. There is increasing evidence that cimetidine interferes with the hepatic elimination of several drugs, thereby aggravating the effects of the comedication. Microsomal studies in vitro revealed that cimetidine binds in therapeutic concentrations to cytochrome P450, which may represent the primary mechanism for its ability to inhibit drug metabolism and thereby interact with other drugs. The structurally different ranitidine (replacement of the imidazole in cimetidine by a furan ring) is about five times as potent as a H2-receptor blocker and displays low affinity for binding sites on cytochrome P 450. Therefore, therapeutic doses of ranitidine do not impair the metabolism of other drugs. Preliminary data with oxmetidine suggest that it too does not interfere at the level of hepatic elimination. Thus, it is concluded that new therapeutic agents should be tested for their ability to bind to cytochrome P 450 to determine possible risks of drug interactions.

Prediction of Cytochrome P450 Profiles of Environmental Chemicals with QSAR Models Built from Drug-like Molecules

The human cytochrome P450 (CYP) enzyme family is involved in the biotransformation of many xenobiotics. As part of the U.S. Tox21 Phase I effort, we profiled the CYP activity of approximately three thousand compounds, primarily those of environmental concern, against human CYP1A2, CYP2C19, CYP2C9, CYP2D6, and CYP3A4 isoforms in a quantitative high throughput screening (qHTS) format. In order to evaluate the extent to which computational models built from a drug-like library screened in these five CYP assays under the same conditions can accurately predict the outcome of an environmental compound library, five support vector machines (SVM) models built from over 17,000 drug-like compounds were challenged to predict the CYP activities of the Tox21 compound collection. Although a large fraction of the test compounds fall outside of the applicability domain (AD) of the models, as measured by k-nearest neighbor (k-NN) similarities, the predictions were largely accurate for CYP1A2, CYP2C9, and CYP3A4 ioszymes with area under the receiver operator characteristic curves (AUC-ROC) ranging between 0.82 and 0.84. The lower predictive power of the CYP2C19 model (AUC-ROC = 0.76) is caused by experimental errors and that of the CYP2D6 model (AUC-ROC = 0.76) can be rescued by rebalancing the training data. Our results demonstrate that decomposing molecules into atom types enhanced the coverage of the AD and that computational models built from drug-like molecules can be used to predict the ability of non-drug like compounds to interact with these CYPs.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Metabolism of territrem B and C in liver microsomes from 14-wk-old Wistar rats is catalyzed by cytochrome P-450 3A

The metabolism of territrem B (TRB) and territrem C (TRC) in liver microsomes of 14-wk-old male and female Wistar rats was investigated. Metabolism of TRB to 4beta-hydroxylmethyl-4beta-demethylterritrem B (MB2), O-demethylation of the methoxy group of the aromatic moiety of TRB to form MB4 (same structure as TRC), and metabolism of TRC to 4beta-hydroxylmethyl-4beta-demethylterritrem C (MC) were observed in both genders. However, the amounts of MB2, MB4, and MC formed in females were much lower than in males. To investigate which cytochrome P-450 (CYP450) isoforms were involved in each step, four CYP450 isotype-specific inhibitors (furafylline, orphenadrine, cimetidine, and troleandomycin) and antibodies against CYP1A1, CYP2B1, CYP2C11, or CYP3A2 were used. Formation of MB2, MB4, and MC was markedly inhibited by cimetidine and troleandomycin, but less by furafylline and orphenadrine. Anti-CYP3A2 antibody completely inhibited MB, MB, and MC formation, while antibodies against CYP1A1, CYP2B1, or CYP2C11 produced no marked effect. Of the seven tested supersomes from baculovirus-transformed insect cells expressing rat CYP450 isoforms (1Al, 1A2, 2B1, 2C11, 2C12, 3A1, and 3A2), only those expressing CYP3A1 and CYP3A2 metabolized TRB and TRC. The amounts of MB2, MB4, and MC formed by male and female rat liver microsome preparations were related to the testosterone 6beta-hydroxylase activity and CYP3A1/2 protein content of the preparation. Immunoblotting showed that CYP3A1 was expressed in both genders, but at different levels, while CYP3A2 was only expressed in males. These results suggest that the formation of MB2, MB4, and MC in liver microsomes from 14-wk-old rats of either gender is mediated by CYP3A1 and CYP3A2.

Metabolism of territrem a in liver microsomes from male wistar rats: 3. Cytochrome p-450 isoforms catalyzing tra metabolism

The cytochrome P-450 isoforms involved in territrem A (TRA) metabolism in liver microsomes of male Wistar rats have been characterized. Pretreatment with phenobarbital (PB) or dexamethasone (DEX) resulted in a similar significant increase in TRA metabolic activity. Although PB treatment resulted in a significant elevation in CYP2B, CYP2C11, and CYP3A levels, only CYP3A levels were significantly increased by DEX treatment. Cimetidine markedly reduced the formation of the TRA metabolites 4beta-hydroxymethyl-4beta-demethylterritrem A (MA(1)), 4beta-oxo-4beta-demethylterritrem A (MAX) and 2-dihydro-4beta-demethylterritrem A (MA(2)) in liver microsomes from 2-wk-old rats (mainly containing CYP3A2) and 7-wk-old rats (containing CYP2B, CYP2C11, and CYP3A2). SKF 525A, which inhibits CYP2B, CYP2C11, and CYP3A2, and orphenadrine, which inhibits CYP2B, also decreased MA(2) formation in liver microsomes from 7-wk-old phenobarbital-pretreated rats. The formation of MA(1) and MAX was not affected. Furthermore, an immunoinhibition study demonstrated that anti-CYP3A2 antibody reduced MA(1), MAX, and MA(2) formation to nondetectable levels in liver microsomes from 2- and 7-wk-old rats, whereas anti-CYP2C11 or anti-CYP2B antibody, respectively, had no marked effect on MA(1), MAX, and MA(2) formation in liver microsomes from 7-wk-old untreated or PB-treated rats. These results suggest that the CYP3A isoform is mainly responsible for MA(1), MAX, and MA(2) formation in liver microsomes in male Wistar rats.

Effects of pantoprazole, a novel H+/K+-ATPase inhibitor, on duodenal ulcerogenic and healing responses in rats: a comparative study with omeprazole and lansoprazole

BACKGROUND

Pantoprazole, 2-[(2-pyridylmethyl) sulphinyl] benzimidazole, is a new substituted benzimidazole that inhibits the parietal cell H+/K+-ATPase.

METHODS

In the present study, the anti-secretory and anti-ulcer activities of pantoprazole were compared with those of omeprazole and lansoprazole in rats.

RESULTS

Pantoprazole (0.3-3 mg/kg, p.o.) as well as omeprazole (1-10 mg/kg, p.o.) and lansoprazole (1-10 mg/kg, p.o.) dose-dependently decreased both basal acid secretion in pylorus-ligated rats and the stimulated acid secretion induced by mepirizole in acute fistula rats, and the effects of pantoprazole were more potent than those of omeprazole and lansoprazole, the ED50 values for the stimulated acid secretion being 0.8, 2.0 and 1.2 mg/kg, respectively. Neither of these drugs had any effect on duodenal HCO3- secretion. These pump inhibitors prevented the development of duodenal lesions in response to mepirizole in a dose-related manner, the ED50 values for pantoprazole, omeprazole and lansoprazole being 0.4, 2.0 and 1.3 mg/kg, respectively. Likewise, pantoprazole showed the healing promoting action on chronic duodenal ulcers induced by acetic acid, and this effect was also more potent when compared to omeprazole or lansoprazole.

CONCLUSIONS

We conclude that pantoprazole exhibited both anti-ulcer and healing promoting effects on duodenal ulcers in rats, and the effects may be attributable to its potent anti-secretory action. Other pump inhibitors such as omeprazole and lansoprazole were almost equally effective as pantoprazole, yet this drug was most potent on the basis of ED50 values.

Toxicokinetics of organic solvents: a review of modifying factors

This article reviews, with an emphasis on human experimental data, factors known or suspected to cause changes in the toxicokinetics of organic solvents. Such changes in the toxicokinetic pattern alters the relation between external exposure and target dose and thus may explain some of the observed individual variability in susceptibility to toxic effects. Factors shown to modify the uptake, distribution, biotransformation, or excretion of solvent include physical activity (work load), body composition, age, sex, genetic polymorphism of the biotransformation, ethnicity, diet, smoking, drug treatment, and coexposure to ethanol and other solvents. A better understanding of modifying factors is needed for several reasons. First, it may help in identifying important potential confounders and eliminating negligible ones. Second, the risk assessment process may be improved if different sources of variability between external exposures and target doses can be quantitatively assessed. Third, biological exposure monitoring may be also improved for the same reason.

The actions of the H2-blocker cimetidine on the toxicity and biotransformation of the phosphorothioate insecticide parathion

Parathion, like most organophosphorus insecticides currently in use, must undergo cytochrome P450(P450)-dependent activation in order to exert its acute mammalian toxicity (cholinergic crisis). Since P450 isoforms play such an important role in mediating the toxicity of parathion and related insecticides, factors which significantly alter P450 activities, such as exposure to certain xenobiotics, can also be expected to affect the toxicity of these potentially hazardous insecticides. Cimetidine is a H2-histamine antagonist that has been shown to inhibit several P450-isoforms. In addition, administration of cimetidine has been reported to result in clinically significant pharmacokinetic interactions with a wide variety of drugs. In the present study coexposure to cimetidine and parathion resulted in a moderate increase in the toxicity of this pesticide. However, coexposure to cimetidine and paraoxon did not alter the toxicity of the organophosphate, indicating that cimetidine likely affected P450-dependent formation of paraoxon from parathion. In vitro incubations of mouse hepatic microsomes demonstrated that, in addition to reducing the velocity of P450-dependent metabolism of parathion, cimetidine increased the proportion of paraoxon formed (activation). and decreased the proportion of p-nitrophenol formed (detoxification). Since parathion is not eliminated significantly by other routes in the mouse, the bulk of parathion in vivo was metabolized by P450 (although more slowly) in the presence of cimetidine, leading to a greater amount of paraoxon produced, and therefore greater toxicity. Incubations with individual P450 isoforms suggested that cimetidine could act by inhibition of P450 isoforms that detoxify parathion to a greater degree than cimetidine-resistant isoforms, and/or cimetidine could alter the proportions of detoxification versus activation of certain individual isoforms.

The effect of three H2 receptor antagonists on the disposition of cyclosporin A in the in situ perfused rat liver model

The in situ perfused rat liver model was used to investigate the effect of three H2 receptor antagonists on the disposition of cyclosporin A (CyA) and the major human metabolite, AM1. Perfusion experiments, using standard techniques, were carried out on four groups (one control and three H2-receptor antagonist-treated groups) of male Sprague-Dawley rats (300-350 g). All animals received CyA, 2.5 mg; the three treated groups received cimetidine (8 mg), ranitidine (3 mg), or famotidine (0.4 mg). Perfusate and bile samples were collected and assayed for CyA, AM1, and the H2 receptor antagonists by HPLC. Results indicated that CyA perfusate concentrations in the controls and cimetidine and ranitidine-treated groups were not significantly different, although levels in the famotidine group were significantly higher at all times (p < 0.05), except 30 min, compared to the controls. However, examination of the AM1 perfusate and bile data and the apparent metabolic clearance data indicated that CyA metabolism was still occurring, despite the presence of the H2 receptor antagonist. It is suggested that the absence of a interaction may be attributed to a lack of specificity of the H2 receptor antagonists for CYP3A, the isoenzyme responsible for CyA metabolism.

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.

Cimetidine as adjunctive treatment for acetaminophen overdose

The aim of this study was to determine if cimetidine in addition to N-acetylcysteine and standard supportive care provide additional hepatoprotection following acute acetaminophen poisoning. It was designed as a prospective study with alternate month treatment protocol, and the work was carried out at a regional certified poison information centre. For a 2-year period, consultations received by the Rocky Mountain Poison Center involving acute acetaminophen overdose patients with a serum level above the nomogram line, but who would not receive N-acetylcystine therapy until at least 8 h postingestion, were prospectively evaluated for adjunctive treatment with cimetidine. All patients received standard supportive therapy and N-acetylcysteine treatment. During odd numbered months, cimetidine 300 mg was administered intravenously every 6 h for the duration of N-acetylcysteine therapy. Forty-one cimetidine treated patients were compared to 66 patients in the control group. The peak measured AST levels (+/- s.e.) were 1259+/-330 and 1301+/-451 for the control and cimetidine treatment groups, respectively (P = 0.94). Fourteen of 64 patients (21%) in the control group and 8/41 patients (20%) in the cimetidine group developed an AST > 1000 IUL-1. There were no statistical differences between the cimetidine-treated and control groups when classified by AST < 100 IUL-1, 100-1000 IUL-1, or > 1000 IUL-1. The addition of cimetidine therapy to standard N-acetylcysteine treatment did not provide additional hepatoprotection in acutely acetaminophen poisoned patients when treatment was started later than 8 h post overdose.(ABSTRACT TRUNCATED AT 250 WORDS)

Variables affecting nicotine metabolism

Nicotine metabolism is exceedingly sensitive to perturbation by numerous host factors. To reduce the large variations and discrepancies in the literature pertaining to nicotine metabolism, investigators in future studies need to recognize and better control these host factors. Recent advances in the understanding of nicotine metabolism have suggested new approaches to elucidating underlying mechanisms of certain toxic effects associated with cigarette smoking.

Individual variation in first-pass metabolism

Individual variation in pharmacokinetics has long been recognised. This variability is extremely pronounced in drugs that undergo extensive first-pass metabolism. Drug concentrations obtained from individuals given the same dose could range several-fold, even in young healthy volunteers. In addition to the liver, which is the major organ for drug and xenobiotic metabolism, the gut and the lung can contribute significantly to variability in first-pass metabolism. Unfortunately, the contributions of the latter 2 organs are difficult to quantify because conventional in vivo methods for quantifying first-pass metabolism are not sufficiently specific. Drugs that are mainly eliminated by phase II metabolism (e.g. estrogens and progestogens, morphine, etc.) undergo significant first-pass gut metabolism. This is because the gut is rich in conjugating enzymes. The role of the lung in first-pass metabolism is not clear, although it is quite avid in binding basic drugs such as lidocaine (lignocaine), propranolol, etc. Factors such as age, gender, disease states, enzyme induction and inhibition, genetic polymorphism and food effects have been implicated in causing variability in pharmacokinetics of drugs that undergo extensive first-pass metabolism. Of various factors considered, age and gender make the least evident contributions, whereas genetic polymorphism, enzymatic changes due to induction or inhibition, and the effects of food are major contributors to the variability in first-pass metabolism. These factors can easily cause several-fold variations. Polymorphic disposition of imipramine and propafenone, an increase in verapamil first-pass metabolism by rifampicin (rifampin), and the effects of food on propranolol, metoprolol and propafenone, are typical examples. Unfortunately, the contributions of these factors towards variability are unpredictable and tend to be drug-dependent. A change in steady-state clearance of a drug can sometimes be exacerbated when first-pass metabolism and systemic clearance of a drug are simultaneously altered. Therefore, an understanding of the source of variability is the key to the optimisation of therapy.

Interaction of cimetidine with cytochrome P450 and effect on mixed-function oxidase activities of liver microsomes

The histamine H2-receptor antagonist drug, cimetidine (CM), was investigated to determine its effect on the metabolism of 'model' alkoxyphenoxazone substrates ethoxyresorufin (ER) and pentoxyresorufin (PR). The investigation was carried out under different conditions in rat liver microsomes from rodents pretreated with various classical cytochrome P450 inducers. CM exerted a dual effect on uninduced and PB-induced liver microsomal ethoxyresorufin-O-de-ethylase (EROD) activities; it was initially stimulatory but became inhibitory. However, when 3-MC-induced preparations were used. CM only exerted an inhibitory effect on EROD activity over the whole concentration range (0.01 microM-20 mM). Pre-incubating PB-induced and uninduced liver microsomes with NADPH before the addition of ER and CM decreased the stimulatory effect of CM and increased the inhibitory effect in the concentration range (5-20 mM). With 3-MC-induced preparations, the inhibition of EROD was only marginally potentiated. Overall conclusions for the diphasic effects of CM on biological activities were due to CM binding with differing affinities to different P450s. Subsequent decreases and increases in stimulation and inhibition, respectively, on pre-incubation with NADPH were thought to be due to an increased affinity of CM for reduced cytochrome P450.

Chemistry and biology of heme. Effect of metal salts, organometals, and metalloporphyrins on heme synthesis and catabolism, with special reference to clinical implications and interactions with cytochrome P-450

Although free porphyrins occur in nature in small quantities, no known function has been assigned to them. In contrast, heme and cobalamin, which are Fe and Co chelates of porphyrins or porphyrin derivatives, respectively, carry out crucial biological functions. Heme is the prosthetic group for a number of hemoproteins. These include myoglobin and hemoglobin, which carry out oxygen binding or transport; mitochondrial cytochromes aa3, b, c, and c3, which are important in transferring electrons; microsomal cytochrome P-450, which catalyzes mixed-function oxidations; catalase, which decomposes H2O2; peroxidase, which activates H2O2; and tryptophan pyrrolase, which catalyzes the oxidation of tryptophan. Recently, heme has also been shown to be the prosthetic group of prostaglandin and peroxide synthetase and indoleamine dioxygenase. The elegant studies of the biochemical pathway for the formation of heme demonstrated the arrangement in the porphyrin macrocycle of the carbon and nitrogen atoms originating from the eight glycine and the succinic acid molecule that are the precursors of porphyrins. There are eight enzymes involved in the synthesis of heme. The first and last three of these enzymes are localized in mitochondria, while the intermediate enzymes are localized in cytosol. The catalytic site of HMOX recognizes metalloporphyrins with central metal atoms other than iron; it favors some of these metalloporphyrins over heme as a potential substrate, sometimes by a large factor, permitting the synthetic heme analogue to serve as a potent competitive inhibitor of HMOX reaction. Since these synthetic metalloporphyrins do not bind molecular oxygen, they are not metabolically degraded by ring rupture and do not add to the body pool of bile pigment. One possible consequence of this competitive inhibition of heme degradation is suppression of bile pigment formation to such a degree that excessive plasma levels of bilirubin may be diminished. The studies of Drummond and Kappas (1981) and later studies in rats, mice, monkeys, and man, and also our studies have proved the latter phenomenon. The compound does not appear to affect the metabolic disposition of preformed bilirubin but inhibits biliary bilirubin excretion derived from the metabolism of endogenous or exogenous heme. Whether some of the effect of Sn-PP on naturally occurring or experimentally induced jaundice in animals reflects diversion of heme to nonheme to oxygenase-dependent pathways of heme metabolism, or whether a pathway which is normally latent becomes activated concurrent with HMOX inhibition is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of inhibition and induction of cytochrome P-450 isozymes on hyperoxic lung injury in rats

Pulmonary oxygen toxicity most likely results from excessive production of reactive oxygen species. The role of the cytochromes P-450 in this process is controversial because these enzymes have been reported both to enhance hyperoxic lung injury and to protect from the damaging effects of 100% oxygen. We sought to further determine the role of the cytochromes P-450 in hyperoxic lung injury by inhibiting and inducing pulmonary cytochrome P-450 isozymes in rats. Treatment with the cytochrome P-450 inhibitor cimetidine or 8-methoxypsoralen did not improve survival or reduce lung edema in rats exposed to 100% oxygen. The activity of cytochrome P-450IIB1, the major pulmonary cytochrome P-450 isozyme in rats, was clearly inhibited by 8-methoxypsoralen. beta-Naphthoflavone (beta NF), a selective inducer of cytochrome P-450IA1, was administered in two-dose and five-dose regimens. The two-dose regimen produced significant and sustained induction of cytochrome P-450IA1 activity, but survival in these rats was not improved when exposed to 100% oxygen. In rats treated with five doses of beta NF, a small increase in survival time was found from 71.1 +/- 8.7 to 88.0 +/- 20.2 h; however, there was no difference in the induction of cytochrome P-450IA1 activity between this five-dose regimen and the two-dose regimen. The small improvement in survival after five doses of beta NF is thus unrelated to cytochrome P-450IA1 induction. We conclude that neither inhibition of cytochrome P-450IIB1 activity nor induction of cytochrome P-450IA1 activity protects adult rats against hyperoxic lung injury.

Ranitidine treatment and cortisol metabolism in man

Experimental evidence suggested that H2-receptor antagonists may inhibit not only hepatic but also adrenal cytochrome P-450 dependent monooxygenases. Therefore, the effects of ranitidine (150 mg b.i.d. over 14 days) on cortisol metabolism and antipyrine clearance have been investigated in nine healthy volunteers. Urinary excretion of 6 beta-hydroxycortisol (6 beta-OHC) and 17-hydroxycorticosteroids (17-OHCS) remained unaffected by ranitidine pretreatment, as did the calculated 6 beta-OHC/17-OHCS ratio. Only marginal effects were observed on antipyrine kinetics and metabolite formation. We conclude that neither adrenal production of corticosteroids nor their hepatic metabolism is affected by ranitidine administration.

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.

H1 and H2 receptor antagonists and hepatic oxygen supply-demand relationship in pigs

The hypothesis that histamine receptor (H1 and H2) blockade beneficially affects the hepatic oxygen supply-demand relationship was tested during experiments performed on 13 miniature pigs. Hepatic arterial and portal blood flows were measured with electromagnetic flowmeters. Cardiac output was determined by thermodilution. H1 and H2 receptor blockade was achieved with promethazine, 5 mg.kg-1 and cimetidine 30 mg.kg-1 IV, respectively. The study demonstrated no significant effect of H1 and H2 receptor blockade on hepatic oxygen uptake and no noticeable effects of cimetidine on hepatic circulation. However, promethazine decreased total hepatic blood flow, primarily by decreasing portal blood flow; this resulted in an increase in oxygen extraction as reflected in a decreased oxygen content in hepatic venous blood. The results reject the posed hypothesis: H1 receptor antagonist promethazine decreased, while H2 receptor antagonist cimetidine did not affect hepatic blood flow and oxygen supply; hepatic oxygen demand remained unaffected during H1 and H2 receptor blockade.

Effect of H2-receptor antagonists on the pharmacokinetics of 5-fluorouracil in the rat and monkey

The effect of short term (7 days) and long term (28 days) pretreatment with the imidazole H2-receptor antagonist, cimetidine (CMT), on the pharmacokinetics of 5-fluorouracil (5-FUra) has been studied in the rat and cynomolgus monkey. Short-term pretreatment of rats with CMT significantly increased t1/2.z by 29 per cent and AUC by 40 per cent: total body clearance was decreased by 30 per cent. Long-term pretreatment exaggerated these effects. By contrast, short- and long-term pretreatment with the furan H2-receptor antagonist, ranitidine, caused no significant effect on 5-FUra kinetics. In the monkey, 7 days pretreatment with CMT increased t1/2.z by 32 per cent leaving other 5-FUra kinetic parameters unchanged; 28 days pretreatment increased both t1/2.z by 41 per cent and AUC by 100 per cent with a decrease in total body clearance of 5-FUra of 48 per cent. CMT, but not RNT, inhibited cytosolic dihydropyrimidine dehydrogenase (DPD), the enzyme responsible for 5-FUra catabolism. It is proposed that the observed effects of CMT on 5-FUra kinetics are the result of inhibition of DPD. This interaction has the potential for increasing systemic drug toxicity and it is therefore advisable that where H2-receptor blockade is administered concurrently with 5-FUra that a non-imidazole based H2-receptor antagonist such as RNT should be substituted for CMT.

Cimetidine suppresses chemically induced experimental hepatic porphyria

The ability of cimetidine to reduce the activity of hepatic aminolevulinic acid synthase (ALA-S) was examined in allylisopropyl acetamide (AIA) treated porphyric adult rats. A dose of 20 mg cimetidine/100 gm body weight resulted in a 50% decrease in the AIA-induced hepatic ALA-S activity compared to rats treated with AIA alone. Heme oxygenase activity was decreased 25% compared to rats treated with AIA alone. The effects of AIA and cimetidine on cytochrome P-450 were not additive, suggesting competition for a common site of interaction. The results suggest that cimetidine may prove to be useful in treating porphyria in humans.

Comparison of inhibitory effects of new quinolones on drug metabolizing activity in the liver

The effects of three new quinolones (enoxacin (ENX), norfloxacin (NFLX) and ofloxacin (OFLX] on acetaminophen-induced liver injury in rats were examined and compared with their effects on the elimination half-life (T1/2) of theophylline (in vivo) and on the 7-ethoxycoumarin (7-EC) O-deethylase activity in liver microsomes (in vitro). ENX, NFLX and OFLX (75 or 300 mg/kg) were administered orally to rats 1 hr before, simultaneously with, and 1 hr after the acetaminophen injection (800 mg/kg). Biochemical liver function tests, drug metabolizing activity in liver microsomes, the total glutathione content of the liver and histological changes were examined 5 hr after the acetaminophen injection. ENX markedly reduced acetaminophen-induced liver injury and NFLX slightly but significantly did so, but no protective effect was observed with OFLX treatment. ENX markedly and NFLX slightly prolonged the T1/2 of theophylline, but OFLX did not affect it. In addition, ENX markedly and NFLX slightly inhibited the 7-EC O-deethylase activity in liver microsomes, but OFLX again had no effect. These findings indicated that ENX markedly inhibited the activity of cytochrome P-450 in liver microsomes and NFLX did so slightly, while OFLX had no such effect. Slight variations in the structures of these quinolones might explain the differences in their effects on cytochrome P-450 activity.

Effect of cimetidine and ranitidine on the hepatic and renal elimination of nicotine in humans

In a randomized, double-blind, cross-over experiment, 6 healthy consenting male subjects were administered cimetidine 600 mg or ranitidine 300 mg or placebo p.o. q12h x 2 days. Nicotine bitartrate was administered i.v. on day 2 (1 microgram/kg/min) x 30 min. After cimetidine mean nicotine total and metabolic clearances were decreased by 30% and 27% while after ranitidine the clearances were decreased by 10% and 7% respectively. Since smokers regulate their smoke intake based in large part on their nicotine blood levels these results suggest that the diminished nicotine total clearance in the presence of cimetidine could be important in assisting smoking reduction or cessation.

Biotransformation of lovastatin--III. Effect of cimetidine and famotidine on in vitro metabolism of lovastatin by rat and human liver microsomes

The effects of the H2-receptor antagonists, cimetidine and famotidine, on the microsomal metabolism of [14C]lovastatin were investigated. Liver microsomes were prepared from control, phenobarbital- and 3-methylcholanthrene-pretreated rats and humans (male and female). Concentration-dependent inhibition of the metabolism of lovastatin (0.1 mM) was observed with cimetidine (0.1 to 1.0 mM). In contrast, famotidine at a similar concentration was a very weak inhibitor. The formation of 6'beta-hydroxy-lovastatin, the major microsomal metabolite of lovastatin, was similarly inhibited. The results suggest that in vivo metabolic interaction with concomitantly administered lovastatin is less likely with famotidine than with cimetidine. Phenobarbital pretreatment produced 58% stimulation in overall metabolism, whereas 3-methylcholanthrene pretreatment had no effect relative to control rats (5.4 nmol/mg protein/min). Liver microsomes from phenobarbital-pretreated rats produced 67% more of the 6'beta-hydroxy-lovastatin but 63-66% less of the 3''-hydroxy and 6'-exomethylene metabolites. Liver microsomes from 3-methylcholanthrene-treated rats also produced less 3"-hydroxy-lovastatin (49%) but similar quantities of the other two metabolites. 6'beta-Hydroxy-lovastatin was a major metabolite with human liver microsomes. Interestingly with these microsomes, hydroxylation at the 3''-position of the molecule was a negligible pathway and hydrolysis to the hydroxy acid form was not observed. The formation of 6'-exomethylene-lovastatin was also catalyzed by human liver microsomes (0.5 to 0.8 nmol/mg protein/min).

Do all histamine2-antagonists cause a warfarin drug interaction?

Cimetidine, the first marketed histamine2-receptor antagonist, has been shown to decrease the clearance of warfarin consistently through inhibition of cytochrome P-450 metabolism. The clinical significance of this drug-drug interaction has been questioned due to: (1) the lowering of the warfarin therapeutic range, (2) the lowering of the total daily therapeutic cimetidine dosage, (3) the advent of once-daily cimetidine dosing, and (4) the demonstration that the clearance of the less active warfarin R-enantiomer is decreased to a greater extent than the more active S-enantiomer. Ranitidine has been implicated in both increasing and decreasing warfarin's hypoprothrombinemic-effect (noted in the warfarin package insert), despite the majority of investigations demonstrating no warfarin clearance changes. Careful examination of the implicating data indicates that the majority of the warfarin pharmacodynamic and pharmacokinetic variance that occurs with combined ranitidine-warfarin therapy cannot be attributed to a drug-drug interaction. No data are available to implicate the newer histamine2-antagonists, famotidine and nizatidine, in causing a decrease in warfarin metabolism.

Kinetics of oral and intravenous mexiletine: lack of effect of cimetidine and ranitidine

The pharmacokinetics of mexiletine, a Class I antiarrhythmic drug, was investigated in 6 healthy volunteers after single oral doses and 15 min intravenous infusions of 3 mg/kg. Cimetidine and ranitidine are commonly used H2-receptor antagonists, which interact adversely with many drugs, e.g. inhibition of the metabolism of Class I antiarrhythmics such as lidocaine and quinidine by cimetidine. To investigate the effects of the two drugs on the kinetics of mexiletine, cimetidine 800 mg.day-1 or ranitidine 600 mg.day-1 were administered orally for one week. Neither H2-receptor antagonist altered the distribution and elimination of mexiletine, nor did they affect its overall kinetics, or excretion of the metabolites para- and 4-OH-methylmexiletine after oral and intravenous administration of mexiletine.

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)

Decreased estrogen hydroxylation in male rat liver following cimetidine treatment

Administration of cimetidine (600 mumol/kg x 5) to adult male rats resulted in 55 and 25% decreases, respectively, in estradiol 2- and 16 alpha-hydroxylation. The same treatment also decreased the activities of ethylmorphine demethylase, aryl hydrocarbon hydroxylase, aniline hydroxylase and heme oxygenase but did not inhibit the activities of 7-ethoxycoumarin de-ethylase and delta-aminolevulinic acid synthase or decrease cytochrome P-450 content. In vitro addition of cimetidine (10-300 microM) also inhibited estradiol hydroxylations, and the effect was additive in rats pretreated with cimetidine in vivo; the other enzymic activities studied were completely unaffected by in vitro addition of cimetidine. In contrast, there was no effect of cimetidine either in vivo or in vitro on any of these activities in female rats. The results point to a wide variation in the susceptibilities of different isozymes of cytochrome P-450 to inhibition by cimetidine and suggest that such differential susceptibilities are also highly dependent on the sex of the animal.

A comparison of the influence of famotidine and cimetidine on phenytoin elimination and hepatic blood flow

The H2-receptor antagonist cimetidine has been reported to decrease the hepatic clearance of numerous drugs by inhibiting cytochrome P-450 metabolism, decreasing liver blood flow or both. In this open-label, randomized crossover study we determined whether therapeutic doses of famotidine, a newer H2-receptor antagonist, has similar effects. Ten healthy subjects received single doses of both phenytoin 100 mg orally and indocyanine green intravenously without other treatment, and then again during treatment with famotidine or cimetidine. After a drug-free period, this sequence was repeated with the alternate H2-receptor antagonist. Cimetidine decreased the plasma clearance of phenytoin by 16% +/- 14% (mean +/- s.d.), but was not found to have a significant influence on phenytoin volume of distribution or terminal elimination rate constant nor on blood clearance of indocyanine green. Famotidine was not found to alter either phenytoin or indocyanine green kinetics.

Haematological adverse effects of histamine H2-receptor antagonists

Histamine H2-receptor antagonists are widely used in the treatment of gastrointestinal diseases related to gastric acid hypersecretion. Cimetidine was introduced into medical practice in 1976 and ranitidine, famotidine and nizatidine in 1981, 1985 and 1987, respectively. Haematological adverse effects are relatively uncommon and most have been reported in cases of cimetidine administration. These adverse effects are reviewed under 4 main headings: (a) blood cytopenias and leucocytosis; (b) coagulation disorders related to drug interactions with oral anticoagulants; (c) reduction of dietary iron absorption; and (d) reduction of dietary cobalamin absorption. 85 reported cases of blood cytopenias attributed to these drugs are reviewed, of which 75 (88%) were associated with cimetidine therapy. In postmarketing surveillance studies, the incidence of cimetidine-associated blood cytopenia has been evaluated at about 2.3 per 100,000 patients. Neutropenia and agranulocytosis are by far the most frequently encountered. Whatever the drug or the type of cytopenia, this adverse effect is almost always rapidly reversible when treatment is stopped. Moreover, in several cases other factors such as underlying diseases or additional drugs could have been responsible, at least partly, for the cytopenia. The pathophysiological basis of these adverse effects remains poorly explained. Various mechanisms have been proposed, which in some cases are probably associated: (a) direct toxicity for haemopoietic stem cells; (b) drug-induced immune reactions leading to blood or bone marrow cell damage, and (c) drug interactions, with increased and prolonged action of potentially haematotoxic drugs. Mechanisms (a) and (c) appear to be of particular clinical importance in cases of impaired renal elimination of H2-receptor antagonists. Cimetidine and probably to a lesser extent ranitidine potentiate the action of oral anticoagulants of both coumarin and indanedione structure. This may result in haemorrhagic complications. Such action is a consequence of the reduced hepatic metabolism of oral anticoagulants through a dose-dependent, reversible inhibition of cytochrome P450. Malabsorption of dietary iron and cobalamin appears to result from inhibition of gastric secretion by the H2-receptor antagonists. This is of no clinical importance in short term treatment, but long term use of H2-receptor antagonists may theoretically contribute to the occurrence of iron or cobalamin deficiency anaemia.

Nizatidine. A preliminary review of its pharmacodynamic and pharmacokinetic properties, and its therapeutic use in peptic ulcer disease

Nizatidine is an H2-receptor antagonist which in animal studies was more active on a weight-for-weight basis than cimetidine in inhibiting basal and stimulated gastric acid secretion. Similarly, studies in humans have confirmed that nizatidine is a potent inhibitor of basal, nocturnal and stimulated gastric acid secretion. As might be expected at this stage of its development, published therapeutic experience with nizatidine is limited. Nevertheless, multicentre therapeutic trials have shown that nizatidine 300mg at bedtime or 150mg twice daily is significantly more effective than placebo for healing active duodenal ulcer, and is apparently as effective as standard doses of ranitidine in increasing the rate of healing of both duodenal and gastric ulcers, and as effective as a standard dose of cimetidine in active duodenal ulcer. When used prophylactically a single 150mg dose of nizatidine at night produces a decrease in the incidence of ulcer recurrence compared with placebo, and a similar rate of decrease to that achieved with ranitidine 150mg. Nizatidine is well tolerated. Unlike cimetidine it does not have any antiandrogenic effects or alter the hepatic metabolism of drugs. However, only wider clinical experience with nizatidine can accurately determine its relative efficacy and tolerability compared with other antiulcer therapy. Thus, early clinical experience suggests that nizatidine is a useful alternative to the histamine H2-receptor antagonists presently in clinical use.

Identification of structural characteristics of some potential H2-receptor antagonists that determine the interaction with rat hepatic P-450

Several potential H2-receptor antagonists have been tested in vitro, using liver microsomal preparations from untreated rats, in order to study their interaction with P-450. The aim of this investigation was to establish structure-activity relationships for the P-450-inhibition developed by cimetidine and related drugs. Most of the compounds tested demonstrate an inhibitory activity and a binding ability to P-450, via type II (ligand type) binding. Our results strongly indicate that the cyano-guanidine moiety is an essential structural feature for both the inhibition of a ferrocytochrome P-450-metabolic intermediate complex formation occurring during the metabolism of tofenacine, and the binding of the compounds to the heme iron of P-450. The presence of an imidazole group is not necessary for these activities. Furthermore, it is pointed out that the lipophilic character of the cyano-guanidine side chain contributes to the interaction of the test compounds with P-450, since a trend for a parabolic relationship between lipophilicity and inhibitory activity or binding ability is observed. Finally, under the experimental conditions used, no increase of the inhibitory activity of cimetidine on the metabolism of tofenacine and 7-ethylresorufin is observed after preincubation of rat liver microsomes with cimetidine, confirming earlier results in similar studies.

Lack of efficacy of cimetidine and ranitidine as inhibitors of tolbutamide metabolism

We investigated the effect of 4 days pretreatment with cimetidine (1.2 g daily) and ranitidine (0.3 g daily) on tolbutamide disposition in an open randomised, cross-over study design involving 8 healthy adult male volunteers. Control half-life of tolbutamide 6.29 h was not significantly altered by cimetidine (6.93 h) or ranitidine (6.97 h). The corresponding apparent oral clearance (ml.min-1.kg-1) were not significantly different: 0.26 (control), 0.24 (cimetidine) and 0.25 (ranitidine). Apparent volume of distribution was also unaltered 0.140 l.kg-1 (control), 0.141 l.kg-1 (cimetidine) and 0.146 l.kg-1 (ranitidine). It is suggested that the hepatic monooxygenase isozyme that catalyses the rate-limiting conversion of tolbutamide to its hydroxy derivative is not susceptible to the inhibitory effect of cimetidine or ranitidine.

Clinical characteristics of roxatidine acetate: a review

Pharmacodynamic studies revealed that 150 mg of roxatidine acetate were optimal in suppressing gastric acid secretion, and that a single bedtime dose of 150 mg was more effective than a dose of 75 mg twice daily in terms of inhibiting nocturnal acid secretion. When administered orally as a capsule containing a granule formulation, the drug displayed modified-release properties, which led to a sustained suppression of gastric acid secretion. Clinical trials revealed that roxatidine acetate, 75 mg twice daily and 150 mg at night, was highly effective in healing duodenal and gastric ulcers and in reducing ulcer pain, over 4, 6, and 8 weeks of therapy. A steady reduction in diameter was observed in those ulcers not completely healed during therapy. The single bedtime dose regimen, while producing the same degree of healing as the divided daily dose during controlled clinical trials, may be of greater value in therapeutic use owing to improved patient compliance. In all efficacy criteria (cure, reduction in ulcer size, and pain relief) there was no significant difference between roxatidine acetate in a total daily dose of 150 mg, ranitidine in a total daily dose of 300 mg, and cimetidine in a total daily dose of 800 mg. Prevention of gastric and duodenal ulcer relapse was achieved by roxatidine acetate, 75 mg at night for 6 months, in about 70% of patients, as determined in open, pilot studies--a rate comparable to those reported for cimetidine and ranitidine. Roxatidine acetate shares with ranitidine an improved safety profile when compared with cimetidine. Human pharmacology studies and short-term and long-term clinical trials have all shown that roxatidine acetate is an exceptionally well tolerated compound, without the antiandrogenic activity and interference with hepatic drug metabolism which have characterized cimetidine treatment. A reason for the improved safety profile of roxatidine acetate may be its greater potency than cimetidine (six times less potent) and ranitidine (half as potent), so that lower doses of roxatidine acetate, representing a lower chemical load, are therapeutically effective. The novel structure of roxatidine acetate probably also underlies the improved safety of the compound.

Interaction of roxatidine acetate with antacids, food and other drugs

The inhibition of hepatic mixed-function oxidase microsomal enzymes by cimetidine can lead to clinically important drug interactions. The metabolism of antipyrine is used as an index of hepatic enzymatic activity. The pharmacokinetic profiles of salivary antipyrine obtained following treatment with roxatidine acetate 75 mg or placebo twice a day for 7 days showed similar characteristics with no difference in the areas under the plasma concentration-time curves. In addition, roxatidine acetate 75 mg daily did not modify the clearance of propranolol, diazepam, desmethyldiazepam or controlled release theophylline preparations. Furthermore, there was no interference in the bioavailability of roxatidine acetate 150 mg daily when administered alone or in combination with a meal or antacids.

Therapeutic approach in patients with concomitant disease/drug--drug interactions (roxatidine acetate)

Possible mechanisms of drug interactions with H2-antagonists are outlined. The mode of action of roxatidine acetate on hepatic microsomal enzymes is contrasted with those of cimetidine and ranitidine, and their differing structure-activity relationships are discussed. In the light of the mechanisms of drug interactions with H2-antagonists, clinical studies with roxatidine acetate are contrasted with published interaction data of cimetidine and ranitidine. The therapeutic consequences of these data are considered.

Interactions of the histamine H2-receptor antagonist etintidine with rat liver cytochrome P-450: a comparison with cimetidine

The two imidazole histamine H2-receptor antagonists etintidine and cimetidine interact with the rat liver microsomal cytochrome P-450. From type II spectral changes follows that the affinity of rat liver microsomal preparations for etintidine is about 5 times as high as for cimetidine when comparing both high and low affinity binding sites. After pretreatment with phenobarbital etintidine inhibited benzphetamine N-demethylation competitively (app. Ki: 4.0 mmol/l). Cimetidine inhibited benzphetamine N-demethylation in the same range. After pretreatment with phenobarbital both drugs inhibited the oxidation of benzo(a)pyrene for which etintidine showed a higher inhibitory potency than cimetidine. However, this oxidation could not be inhibited when microsomes of 5,6-benzoflavone pretreated rats were used. After pretreatment with 5,6-benzoflavone only etintidine but not cimetidine inhibited the O-deethylation of ethoxyresorufin competitively (app. Ki: 0.2 mmol/l). Etintidine and cimetidine were metabolized by rat liver microsomes to their corresponding sulphoxides. In conclusion, etintidine may cause mainly the same drug interactions as cimetidine but seems to be a more potent inhibitor.

New synthetic quinolone antibacterial agents and serum concentration of theophylline

The effect of pipemidic acid and five new synthetic antibacterial agents--norfloxacin, enoxacin, ofloxacin, ciprofloxacin, and pefloxacin--on the serum level of theophylline was studied in healthy male adult volunteers after concomitant oral administration of these agents with a slow release preparation of theophylline. The results indicated that enoxacin, ciprofloxacin, and pipemidic acid might decrease the clearance of theophylline in the liver, and the attention should be paid in clinical use when enoxacin or pipemidic acid is coadministered with theophylline.

Chloroquine elimination in humans: effect of low-dose cimetidine

A controlled study was carried out in ten healthy, male volunteers (randomly distributed into control and test groups of five subjects each) to determine the effect of low-dose cimetidine on chloroquine elimination. The control group subjects received two tablets of chloroquine sulfate (300-mg base) only, while the test group subjects took 400-mg cimetidine at bedtime for four days prior to chloroquine (two tablets of chloroquine sulfate) administration and throughout the duration of the study. Blood samples, 5 mL, were collected periodically after chloroquine administration. The samples were assayed for chloroquine and monodesethylchloroquine using a combination of thin-layer chromatography and spectrophotometry. Wilcoxon's test for unpaired data at P less than .05 was used to determine if there was any significant difference in the elimination of chloroquine in the test group when compared with the control group. The apparent oral clearance rate of chloroquine was reduced from 0.49 +/- 0.04 L/d/kg in the control group to 0.23 +/- 0.02 L/d/kg in the test group, and the elimination half-life was prolonged from 3.11 days in the control group to 4.62 days in the test group. There was a 47.04% reduction in the AUC0-7d of monodesethylchloroquine, the major metabolite of chloroquine, in the test group when compared with the control group. The apparent volume of distribution at steady state was increased from 0.46 +/- 0.07 L/kg in the control group to 0.72 +/- 0.10 L/kg in the test group. All these changes were statistically significant. The conclusion is that cimetidine impairs the elimination of chloroquine in healthy subjects.

Lack of inhibitory effect of metronidazole on theophylline disposition in healthy subjects

The effect of metronidazole, at a dose of 1.2 g daily, on theophylline disposition was studied in 10 healthy adult volunteers. Neither theophylline half-life, volume of distribution nor total body clearance was altered by the anti-microbial. It is concluded that metronidazole does not impair theophylline metabolism despite the fact that it is a substituted imidazole.

Factors affecting the pharmacokinetics of nifedipine

The plasma pharmacokinetics of nifedipine and the formation of its metabolites have been studied in volunteers under conditions which would affect the activity of the cytochrome P-450 system. The pharmacokinetics of a 10-mg capsule of nifedipine were not significantly different between smokers and non-smokers of similar age. After pretreatment with cimetidine, which inhibits the activity of cytochrome P-450, the peak plasma concentration and area under the plasma-time concentration curve for nifedipine were increased by a mean 84%. In contrast, pre-treatment with ranitidine which has little effect on cytochrome P-450, did not significantly alter nifedipine pharmacokinetics. Smoking does not contribute significantly to the variability in nifedipine pharmacokinetics. However, the interaction between nifedipine and cimetidine, but not ranitidine, may be of clinical importance.

Pharmacokinetic interactions of cimetidine 1987

The number of studies on drug interactions with cimetidine has increased at a rapid rate over the past 5 years, with many of the interactions being solely pharmacokinetic in origin. Very few studies have investigated the clinical relevance of such pharmacokinetic interactions by measuring pharmacodynamic responses or clinical endpoints. Apart from pharmacokinetic studies, invariably conducted in young, healthy subjects, there have been a large number of in vitro and in vivo animal studies, case reports, clinical observations and general reviews on the subject, which is tending to develop an industry of its own accord. Nevertheless, where specific mechanisms have been considered, these have undoubtedly increased our knowledge on the way in which humans eliminate xenobiotics. There is now sufficient information to predict the likelihood of a pharmacokinetic drug-drug interaction with cimetidine and to make specific clinical recommendations. Pharmacokinetic drug interactions with cimetidine occur at the sites of gastrointestinal absorption and elimination including metabolism and excretion. Cimetidine has been found to reduce the plasma concentrations of ketoconazole, indomethacin and chlorpromazine by reducing their absorption. In the case of ketoconazole the interaction was clinically important. Cimetidine does not inhibit conjugation mechanisms including glucuronidation, sulphation and acetylation, or deacetylation or ethanol dehydrogenation. It binds to the haem portion of cytochrome P-450 and is thus an inhibitor of phase I drug metabolism (i.e. hydroxylation, dealkylation). Although generally recognised as a nonspecific inhibitor of this type of metabolism, cimetidine does demonstrate some degree of specificity. To date, theophylline 8-oxidation, tolbutamide hydroxylation, ibuprofen hydroxylation, misonidazole demethylation, carbamazepine epoxidation, mexiletine oxidation and steroid hydroxylation have not been shown to be inhibited by cimetidine in humans but the metabolism of at least 30 other drugs is affected. Recent evidence indicates negligible effects of cimetidine on liver blood flow. Cimetidine reduces the renal clearance of drugs which are organic cations, by competing for active tubular secretion in the proximal tubule of the kidney, reducing the renal clearances of procainamide, ranitidine, triamterene, metformin, flecainide and the active metabolite N-acetylprocainamide. This previously unrecognised form of drug interaction with cimetidine may be clinically important for both parent drug, and metabolites which may be active.(ABSTRACT TRUNCATED AT 400 WORDS)

Cimetidine inhibition of theophylline elimination: the influence of adult age and the time course

The effect of placebo or cimetidine at a dose of 1 g daily on theophylline elimination was studied in a double-blind control manner in twelve healthy adult males aged 19-31 years. In a group of six, placebo had no effect on any of theophylline half-life (t1/2), volume of distribution (Vd), and total body clearance (CL). In another group of six, theophylline t1/2 rose significantly from 5.48 to 9.04 h (p less than 0.001) after 48 h and to 8.98 h (p less than 0.001) after a week pretreatment with cimetidine. Correspondingly, CL (ml min-1kg-1) fell to 0.66 (p less than 0.025) and 0.60 (p less than 0.01). Changes after 48 h pretreatment were not different from those after a week pretreatment. Seven days after the last cimetidine dose, theophylline disposition had reverted to the control level. There was no significant change in Vd. In five elderly subjects aged 56-68 years, a week of dosing with cimetidine caused a rise in t1/2 (7.17 h to 10.39 h; p less than 0.001) and a fall in CL (0.77 ml min-1kg-1 to 0.53 ml min-1kg-1; p less than 0.005) without significantly altering Vd. In two subjects, there was restoration of theophylline disposition to the control level 72 h after cessation of cimetidine intake. Changes in the elderly were not significantly different from those in the younger ones. Data indicate that cimetidine-induced impairment of theophylline disposition is of rapid onset, has rapidly attainable maximum effect, and is rapidly reversible. Furthermore elderly individuals are just as susceptible to it as the younger adults.