Cimetidine, but not oxmetidine, penetrates into the cerebrospinal fluid after a single intravenous dose

Activation of carbonic anhydrase isoforms involved in modulation of emotional memory and cognitive disorders with histamine agonists, antagonists and derivatives

Carbonic anhydrases (CAs, EC 4.2.1.1) activators were shown to be involved in memory enhancement and learning in animal models of cognition. Here we investigated the CA activating effects of a large series of histamine based compounds, including histamine receptors (H1R - H4R) agonists, antagonists and other derivatives of this autacoid. CA activators may be thus useful for improving cognition as well as in diverse therapeutic areas (phobias, obsessive-compulsive disorder, generalised anxiety, post-traumatic stress disorders), for which activation of this enzyme was recently shown to be involved.

Curcuma longa normalized cimetidine-induced pituitary-testicular dysfunction: Relevance in nutraceutical therapy

BACKGROUND

The increasing incidence of chemically induced infertility is both a social threat and a threat to the continuation of life itself. Treatment or management therapy is often expensive. This study investigated the effects of acetone extract of a local plant (Curcuma longa) in a Wistar rat model of cimetidine-induced pituitary-testicular dysfunction.

METHODS

Thirty-five male Wistar rats were divided into 7 groups of 5 rats. After a phytochemical screening of an acetone extract of C. Longa, cimetidine and the extract at three doses, 200, 400 and 600 mg/kg, were orally co-administered to the rats for 28 consecutive days. Comparisons were made (at P < 0.05) against a control (2 mL/kg distilled water), a standard treatment group (cimetidine + 50 mg/kg vitamin C), a toxic group (60 mg/kg cimetidine) and a group receiving extract alone.

RESULTS

Cimetidine administration was associated with deleterious alterations to sperm motility, sperm count and sperm viability, as well as derangements in the plasma levels of FSH, LH and testosterone (P < 0.05). Both brain and testicular GSH and TBARS levels were significantly altered following cimetidine administration, and distortions were seen in the pituitary and testicular histoarchitecture. These changes were significantly normalized by co-administration of graded doses of the extract, with an associated improvement of both pituitary and testicular histology.

CONCLUSION

Acetone extract of C. Longa normalized cimetidine-induced pituitary-testicular dysfunction in Wistar rats. This presents the extract as a potential nutraceutical choice against chemically induced reproductive toxicity.

Pharmacokinetic and pharmacodynamic properties of histamine H2-receptor antagonists. Relationship between intrinsic potency and effective plasma concentrations

Histamine H2-receptor antagonists are a unique class of compounds. Pharmacologically they are characterised as a family by their ability to inhibit the secretion of gastric acid, and kinetically they are classified as a family by their similarity in absorption, distribution and elimination. All the H2-receptor antagonists exhibit classical competitive drug-receptor interactions, with Schild slope parameters not significantly different from unity. Comparison of the values of the negative logarithm of the molar concentration of antagonist in the presence of which the potency of the agonist is reduced 2-fold (PA2) indicates that famotidine is about 20 to 50 times more potent than cimetidine and 6 to 10 times more potent than ranitidine. To date, famotidine is the most potent among marketed H2-receptor antagonists. Oral absorption of all the H2-receptor antagonists under clinical investigation is fairly rapid. Peak plasma concentrations are usually attained within 1 to 3h after the dose, but a second peak after oral administration has been observed with cimetidine, ranitidine, famotidine, ramixotidine and etintidine. The mean oral bioavailability for the H2-antagonists ranges from 50 to 70%. Reports on the plasma profiles after intravenous administration are available only for cimetidine, ranitidine, famotidine and nizatidine: plasma concentrations of all 4 decline in a biexponential manner. All of the H2-antagonists are eliminated quite rapidly, with a terminal half-life of 1 to 3h and a total body clearance of 24 to 48 L/h. Elimination is mainly attributable to renal excretion, with renal clearances ranging from 13.8 to 30 L/h. As the values for renal clearance greatly exceed the glomerular filtration rate (6 to 7.2 L/h), it is apparent that renal tubular secretion plays an important role. There is a simple, direct correlation between plasma concentrations of H2-receptor antagonists and the inhibition of gastric acid secretion. This implies a rapid equilibration between drug concentration in plasma and at the site of action, and a reversible drug-receptor interaction. Success in correlating the plasma concentration of H2-receptor antagonists and their pharmacological effects stems from reliable and precise measurement of both items. Despite the heterogeneous nature of data sources, 50% inhibitory concentration (IC50) values for cimetidine, ranitidine, famotidine, nizatidine, etintidine and roxatidine obtained in vitro appear to be in good agreement with those determined in vivo. These results suggest that at an early stage of development of an H2-receptor antagonist, IC50 determined from in vitro studies may be useful as a first approximation to predict the clinically effective concentration of the new agent.

Drug metabolizing enzymes in the brain and cerebral microvessels

Several families of brain parenchyma and microvessel endothelial cell enzymes can metabolize substrates of exogenous origin. This xenobiotic metabolism includes functionalization and conjugation reactions and results in detoxication, but also possibly in the formation of pharmacologically active or neurotoxic products. The brain is partially protected from chemical insults by the physical barrier formed by the cerebral microvasculature of endothelial cells, which prevents the influx of hydrophilic molecules. These cells provide also, as a result of their drug-metabolizing enzyme activities, a metabolic barrier against penetrating lipophilic substances. The involvement of these enzymatic activities in neurotoxic events, probably responsible for neuronal dysfunctioning and/or death, neurodegenerative diseases and normal aging, is discussed.

Clinical pharmacokinetics of drugs used in the treatment of gastrointestinal diseases (Part II)

Part I of this article, which appeared in the previous issue of the Journal, covered the following agents: histamine H2-receptor antagonists (cimetidine, ranitidine, famotidine, nizatidine); muscarinic-M1-receptor antagonists (pirenzepine); proton pump inhibitors (omeprazole); site-protective agents (colloidal bismuth subcitrate, sucralfate); antacids and prostaglandin analogues; antiemetics and prokinetics (metoclopramide, domperidone, cisapride); and antispasmodics. In Part II, we consider the anti-inflammatory salicylates, nonspecific antidiarrhoeal agents, laxatives and cathartics.

Oxmetidine in the short term treatment of active duodenal ulcer. A review and commentary

Oxmetidine is an H2-antagonist like cimetidine containing an imidazole ring in its molecule, but differing from cimetidine in that it contains in the side-chain a substituted isocytosine moiety in place of the cyanoguanidine group. Nine controlled clinical trials are critically analysed in detail. The overall results show that the antiulcer activity of oxmetidine is not significantly different from that of cimetidine with mean healing rates by week 4 of 74.9% and 75.3%, respectively. Healing rates proved to be lower in smokers than in non-smokers in all trials but one. Satisfactory response as regards symptoms was obtained both with oxmetidine and cimetidine. A certain degree of variability with regard to the untoward effects was found, but in all cases failed to prove significant from the clinical point of view. However, a better definition of oxmetidine safety requires a study on a large number of patients and for a longer period.

Cimetidine elimination from the cerebrospinal fluid of the rat

The major goal of this study was to develop a small animal model that could be used to assess quantitatively the clearance of cimetidine from the cerebrospinal fluid (CSF) under relatively physiologic conditions. In addition, we addressed questions related to the pathways involved in the elimination of cimetidine from the CSF. We administered high and low bolus doses of cimetidine together with inulin, as a marker of bulk flow, into the lateral ventricle of anesthetized rats and sampled CSF from the cisterna magna. Principles of linear pharmacokinetic systems were applied to the data to obtain clearances from the CSF. The clearance of inulin was 2.02 +/- 0.22 microliters/min, which is in excellent agreement with the CSF production rate of 2.2 microliters/min in anesthetized rats. The clearance of cimetidine from the CSF following the administration of a low dose was 11.8 +/- 3.1 microliters/min, which is in good agreement with the cimetidine CSF clearance in the rat obtained previously in studies using the technique of ventriculocisternal perfusion. A 32% decrease in the CSF clearance of cimetidine (P less than 0.05) was observed when the high dose was administered, suggesting that CSF elimination is saturable. The clearance of inulin was unaffected by the high dose of cimetidine. This study demonstrates that the technique of lateral ventricle injection and sampling from the cisterna magna is useful in quantitatively assessing the elimination of compounds from the CSF in the rat under relatively physiologic conditions.

Behavioural effects of histamine and its antagonists: a review

This review focuses on the behavioural effects of histamine and drugs which affect histaminergic function, particularly the H1- and H2-receptors antagonists. Research in this area has assumed considerable importance with increasing interest in the role of brain histamine, the clinical use of both H1 and H2 antagonists and evidence of nonmedical use of H1 antagonists. Results from a number of studies show that H1 and H2 antagonists have clear, but distinct subjective effects and that H1 antagonists have discriminative effects in animals. While H1 antagonists are reinforcers in certain conditions, histamine itself is a punisher. Moderate doses of H1 antagonists affect psychomotor performance in some situations, but the results are variable. The exceptions are terfenadine and astemizole, which do not seem to penetrate the blood-brain barrier readily. In studies of schedule-controlled behaviour, marked changes in response rate have been observed following administration of H1 antagonists, with the magnitude and direction dependent on the dose and the baseline behaviour. Histamine reduces avoidance responding, an effect mediated via H1-receptors. Changes in drinking and aggressive behaviour have also been observed following histamine administration and distinct roles for H1- and H2-receptors have been delineated. Separate H1- and H2-receptor mechanisms have also been suggested to account for changes in activity level. While the H2 antagonists do not always have strong behavioural effects when administered peripherally, there is evidence that cimetidine has a depressant effect on sexual function. These and other findings reveal an important role for histaminergic systems in a wide range of behaviour.

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 penetrates brain and inhibits non-opiate footshock-induced analgesia

The inhibition of hindpaw (non-opiate) footshock-induced analgesia (HP-FSIA) by cimetidine, the histamine H2-receptor antagonist, was characterized in rats, and the drug's presence in brain was demonstrated. Cimetidine (100 mg/kg, IP) inhibited HP-FSIA when administered 30 min before testing, but was inactive when testing began sooner (15 min) or later (1-4 hr) than this time. Lower doses (20 mg/kg) were also ineffective when given 30 min before testing, whereas higher doses (200 mg/kg) effectively inhibited the response. Increasing the footshock current from 4 mA (which elicited cimetidine-sensitive analgesia) to higher currents (5 and 6 mA) yielded cimetidine-insensitive analgesia. Administration of isotopically labeled cimetidine (100 mg/kg, IP, 30 min) yielded whole brain cimetidine levels of 1.95 nmols/g, respectively, with a brain/blood ratio of 0.017. These findings confirm a limited penetration of brain by cimetidine, and show that large peripheral doses of cimetidine are required to block brain H2-receptors. The specific dose and time requirements for cimetidine to inhibit the HP-FSIA are probably attributable to the brain drug levels that can be achieved after peripheral administration.

Adverse reactions and interactions with H2-receptor antagonists

Histamine H2-receptor antagonists have been used in the treatment of gastrointestinal diseases for more than a decade and during this period have become one of the most commonly prescribed groups of drugs in the world. The deserved popularity of the H2-receptor antagonists reflects, in part, their therapeutic efficacy, which has revolutionised the treatment of peptic ulcer disease. An equally, or more, important reason for the widespread use of H2-receptor antagonists is their remarkably low toxicity. We have attempted, in this review, to present a detailed account of the minor and more serious adverse reactions, while emphasising the low incidence of the former and the rarity of the latter. The toxicology of the H2-receptor antagonists is discussed under two main headings: adverse effects; and drug interactions. The latter category is potentially the more significant, since the frequent use of therapy with multiple drugs may give rise to drug interactions, some of which are serious and may even be lethal. These drug interactions occur especially in the gastrointestinal tract, the liver and the kidneys. Thus, the absorption of other drugs may be altered because the H2-receptor antagonists inhibit gastric secretion--an effect illustrated by ketoconazole, the absorption of which is reduced when given in combination with cimetidine. Very important drug interactions are caused by inhibition of the hepatic microsomal enzyme cytochrome P450 by some of the H2-receptor antagonists. This effect appears to be related to the chemical structure of the individual H2-receptor antagonists and is not attributable to histamine H2-receptor blockade. For example, cimetidine is a powerful inhibitor of cytochrome P450, while the interaction of ranitidine with this system is weaker. Consequently, cimetidine reduces the metabolism of many drugs which are normally degraded by phase I reactions, leading to potentially toxic plasma concentrations of therapeutic agents such as some oral anticoagulants, beta-blockers, anticonvulsants, benzodiazepines and xanthines. Some of the H2-receptor antagonists are actively secreted by the renal tubules and may thus compete with other drugs for cationic tubular transport mechanisms, resulting in reduced urinary excretion and hence potentially toxic plasma concentrations. This type of drug interaction has been reported after administration of both cimetidine and ranitidine with procainamide or quinidine.(ABSTRACT TRUNCATED AT 400 WORDS)

Clinical pharmacokinetics of cimetidine

Cimetidine is the first histamine H2-receptor antagonist with wide clinical application. It is a weak base and a highly water-soluble compound which can be measured in biological fluids by a number of high-pressure liquid chromatographic methods. Following intravenous administration, the plasma concentration profile follows multicompartmental characteristics. The total systemic clearance is high (500 to 600 ml/min) and is mainly determined by renal clearance. The volume of distribution (Vd beta or Vdss) is of the order of 1 L/kg and this about equals bodyweight. Elimination half-life is approximately 2 hours. Following oral administration of cimetidine, 2 plasma concentration peaks are frequently observed, probably due to discontinuous absorption in the intestine. The absolute bioavailability in healthy subjects is about 60%. In patients with peptic ulcer disease, bioavailability is around 70%, but the variation is much greater than in healthy subjects. Absorption and clearance of cimetidine are linear after 200 and 800mg doses. Mean steady-state plasma concentrations on a standard 1000mg daily dose are 1.0 microgram/ml (range 0.64-1.64 micrograms/ml) and are reproducible after treatment periods of up to 2 years. When taken with food, the extent of absorption is unaltered, but a delay occurs and only 1 peak in the plasma concentration curve is apparent. Partial gastrectomy (Billroth I, II) causes an increase in systemic availability of cimetidine by an unclear mechanism. Distribution of cimetidine leads to extensive uptake into kidney, lung and muscle tissues. It distributes into the cerebrospinal fluid (CSF) at a ratio of 0.1 to 0.2 compared with plasma. The mean saliva to plasma ratio is 0.2 (range 0.1-0.55). Plasma protein binding is 20%, and there is no relevant effect of changes in binding on the pharmacokinetics of cimetidine. Uptake of cimetidine into red blood cells leads to concentrations equal to those in plasma. Between 50 and 80% of the dose administered intravenously is recovered in urine as unchanged cimetidine. This fraction is less after oral doses, but is independent of the amount of the dose. In ulcer patients, 40% is recovered unchanged in urine after oral administration. Biliary excretion of cimetidine accounts for only 2% of the dose.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative randomized, double-blind study of oxmetidine versus cimetidine for short-term treatment of duodenal and prepyloric ulceration

101 patients with endoscopically verified duodenal or prepyloric ulcerations were treated in a 4- to 8-week double-blind randomized trial with oxmetidine 400 mg twice daily (51 patients) or cimetidine 1 g daily (50 patients). Both groups had free access to antacid tablets for symptomatic relief. There were no significant differences between the two treatment alternatives with regard to ulcer healing, relief of ulcer pain, or antacid consumption. The ulcer healing rate after 4 weeks of treatment was 80% in the oxmetidine group and 74% in the cimetidine group and, after 8 weeks, 92% and 86%, respectively. The differences in healing rate at 4 and 8 weeks and the corresponding 95% confidence limits were 6 +/- 16% and 4 +/- 13%, respectively. Both duodenal and prepyloric ulcerations healed to the same extent in the two regimens.