The drug interaction potential of ranitidine: an update

Effect of ranitidine on the pharmacokinetics and pharmacodynamics of prasugrel and clopidogrel

OBJECTIVE

Clopidogrel is an oral thienopyridine antiplatelet agent indicated for the treatment of atherothrombotic events in patients with acute coronary syndrome (ACS). Prasugrel, a novel oral thienopyridine, is under investigation for the reduction of atherothrombotic events in patients with ACS undergoing percutaneous coronary intervention. Prasugrel's solubility decreases with increasing pH, suggesting that concomitantly-administered medications that increase gastric pH may lower the rate and/or extent of prasugrel absorption. This study evaluated the influence of ranitidine coadministration on the pharmacokinetics and pharmacodynamics of the respective active metabolite of prasugrel and clopidogrel.

RESEARCH DESIGN AND METHODS

In this open-label, two-period, two-treatment, crossover study, 47 healthy male subjects were randomized to one of two study arms, receiving either prasugrel (60-mg loading dose [LD], 10-mg maintenance dose [MD] for 7 days; n = 23) or clopidogrel (600-mg LD, 75-mg MD for 7 days; n = 24). In one treatment period, subjects received prasugrel or clopidogrel alone, and in the alternate period received the same thienopyridine with ranitidine (150 mg twice daily, starting 1 day before the LD). Pharmacokinetic parameter estimates (AUC(0-t last), C(max), and t(max)) and inhibition of platelet aggregation (IPA) by light transmission aggregometry were assessed at multiple time points after the LD and final MD.

RESULTS

Ranitidine had no clinically significant effect on the area under the plasma-concentration-time curve (AUC) and did not affect the time to C(max) (t(max)) for active metabolites of either prasugrel or clopidogrel. It reduced the geometric mean maximum concentrations of active metabolite (C(max)) after a prasugrel and clopidogrel LD by 14% and 10%, respectively, but these differences were not statistically significant. When coadministered with a 60-mg prasugrel LD, ranitidine did not affect the time to, or magnitude of, peak IPA, but did result in a modest reduction at 0.5 h from 67.4 to 55.1% (p < 0.001). Ranitidine did not affect prasugrel IPA during MD. For clopidogrel, IPA was not affected by ranitidine. Prasugrel and clopidogrel were both well-tolerated, with/without ranitidine.

CONCLUSIONS

Results from this study suggest that there is no significant drug-drug interaction between oral ranitidine therapy and concomitantly-administered prasugrel or clopidogrel.

Safety issues relating to long-term treatment with histamine H2-receptor antagonists

H2-receptor antagonist therapy is associated with a low incidence of adverse reactions. Adverse events reported in clinical trials of ranitidine in daily doses of up to 1200 mg include headache, tiredness and mild gastrointestinal disturbances, but the incidence is similar to or less than that for placebo. High doses of cimetidine (> 5 g/day) can cause reversible impotence or gynaecomastia. While ranitidine exhibits no clinically significant drug-drug interactions, cimetidine interacts with many drugs metabolized by cytochrome P450. In contrast to ranitidine and cimetidine, where safety data are available for up to 10 years of continuous therapy, experience with famotidine and nizatidine is limited. The safety of long-term H2-receptor antagonist therapy needs to be considered in relation to the potential consequences of prolonged acid suppression, including the risk of proliferation of gastric flora and the risk of developing enterochromaffin-like cell hyperplasia, which could in turn, theoretically, lead to gastric malignancy. Such problems have not been observed in patients during long-term therapy at low or full doses of H2-receptor antagonists. Standard doses of currently available H2-receptor antagonists permit acid secretion in response to food and other stimuli, and this daily acid tide prevents persistent bacterial colonization.

No association between CYP2D6 polymorphisms and personality trait in Japanese

AIMS

The polymorphic enzyme CYP2D6 is expressed not only in liver but also in brain at low concentrations. CYP2D6 mediates, to some extent, the synthesis of the neurotransmitters, serotonin and dopamine. We investigated a possible association between the genetic polymorphism of CYP2D6 and individual personality trait.

METHODS

Mentally and physically healthy volunteers were recruited (n = 342). Temperament and Character Inventory (TCI) and CYP2D6 genotyping were performed in all subjects. We detected mutated alleles which were identified using the Amplichip CYP450 DNA chip.

RESULTS

The number of phenotypes, assumed by genotype for ultrarapid metabolizers (UM), extensive metabolizers (EM), intermediate metabolizers (IM) and poor metabolizers (PM) were 4 (1.1%), 262 (76.6%), 75 (21.9%) and 1 (0.3%), respectively. There were no differences in scores for novelty seeking, harm avoidance, reward dependence or persistence among the CYP2D6 phenotypes. The number of mutated alleles for CYP2D6 did not differ for scores of novelty seeking, harm avoidance, reward dependence or persistence. In subitem analyses, only RD3 (attachment) had a significant difference both in the CYP2D6 phenotype (P < 0.05) and genotype (P < 0.05).

CONCLUSIONS

This study did not demonstrate a significant association between CYP2D6 activity and personality trait because of the small interindividual variability in CYP2D6 activity within the Japanese population.

Relationship between Type A and B personality and debrisoquine hydroxylation capacity

AIM

A person with Type A personality is an 'aggressor' compared with the rarely harried Type B. Although debrisoquine hydroxylase (CYP2D6) capacity has been associated with personality, no study has specifically investigated its association with personality Type A and B. Therefore the aim of this research was to study the impact of CYP2D6 on Type A and B personality.

METHODS

Type A and B personality questionnaires were administered to 48 healthy patients undergoing elective orthopaedic surgery. After obtaining informed consent, patients were genotyped for the various CYP2D6 alleles by allele-specific polymerase chain reaction. Based on the genotypes, patients were grouped as extensive metabolizer (EM)1 (normal) (CYP2D6*1/*1), EM2 (intermediate) (CYP2D6*1/*4, CYP2D6*1/*5, CYP2D6*1/*9 and CYP2D6*1/*10) and EM3 (slow) (CYP2D6*4/*10, CYP2D6*5/*10, CYP2D6*10/*10 and CYP2D6*10/*17). Chi(2) was used to determine the relationship between the groups and personality types.

RESULTS

The percentages of patients who were of the EM1, EM2 and EM3 groups were 20.8%, 52.1% and 27.1%, respectively. There was a significant difference (P = 0.032) between the three groups in terms of personality type, in which EM1 showed a tendency to be of personality Type A while EM2 and EM3 tended to be of personality Type B.

CONCLUSION

The study suggests that there is a relationship between CYP2D6 activity and Type A and B personality.

Pharmacokinetic considerations in the eradication of Helicobacter pylori

As Helicobacter pylori plays an important role in the aetiopathogenesis of peptic ulcer, therapeutic strategies aimed at maintaining long term remission have shifted from the control of intragastric pH to targeting H. pylori. According to recent international guidelines the clinical goals--rapid ulcer healing and prevention of relapse--can be best accomplished by combination therapy consisting of an antisecretory drug (proton pump inhibitor or ranitidine) and 2 antimicrobial agents (preferable amoxicillin, clarithromycin or metronidazole). When applying such multidrug regimens, possible synergy between the agents suggests that pharmacokinetic considerations might help to improve H. pylori eradication rates, which should be above 85 to 90% on an intention-to-treat basis. The present review summarises the pharmacokinetic properties and interaction potential of all drugs presently used in the various H. pylori eradication regimens, with emphasis on particular patient populations such as the elderly and those with renal impairment. The drugs considered are omeprazole, lansoprazole, pantoprazole, rabeprazole, ranitidine and ranitidine bismutrex, bismuth salts, amoxicillin, clarithromycin, azithromycin, roxithromycin, metronidazole, tinidazole and tetracycline. When addressing the clinically important questions of the efficacy, safety and costs of the recommended regimens, the impact of drug disposition on H. pylori eradication should not be neglected.

Gender-related differences in pharmacokinetics and their clinical significance

In this review I have attempted to summarize gender differences in pharmacokinetics involving the cytochrome P450 (CYP) isozymes of young and mature adults, excluding the effects of the menstrual cycle, use of oral contraceptives and pregnancy. Sex differences in drug metabolism and elimination are mainly related to steroid hormone levels. CYP3A4, responsible for the metabolism of over 50% of therapeutic drugs, exhibits higher activity in women than in men. Nonetheless, the absence of a sex difference has been reported by some workers. The activity of several other CYP (CYP2C19, CYP2D6, CYP2E1) isozymes and the conjugation (glucuronidation) activity involved in drug metabolism may be higher in men than in women. Drug metabolism in women is affected by sex-specific factors (menopause, pregnancy and menstruation) in addition to the cigarette smoking, drug ingestion and alcohol consumption that are more commonly observed factors in men. Furthermore, they are affected by physiological factors such as drug absorption, protein binding and elimination. Thus, careful attention should be paid to the side-effects and toxicity arising from sex differences in drug metabolism in clinical situations. Although there are specific ethical considerations regarding carrying out drug trials in women, the relationship between the side-effects and toxicity that may be influenced by hormones during drug metabolism and drug treatment needs further study.

Update: genetic polymorphism of drug metabolizing enzymes in humans

The cytochrome P450 (P450 or CYP) monooxygenases, CYP2D6, CYP2C19, CYP2E1 and CYP2C9, and non-P450 monooxygenases, N-acetyltransferase, thioprine methyltransferases and dihydropyrimidine dehydrogenase, all display polymorphism. CYP2D6 and CYP2C19 have been studied extensively and, despite their low abundance in the liver, they have been found to catalyse the metabolism of many drugs. CYP2D6 has many allelic variants, whereas CYP2C19 has only two. Most variants are translated into inactive, truncated proteins or fail to express protein. There is, as yet, no clear information about CYP2E1 polymorphism. In addition, genetic differences in certain foreign-compound metabolizing enzymes, such as Phase II enzymes, have been shown to be associated with an increased risk of developing environmentally and occupationally related diseases such as cancer. When two drugs that are substrates of a polymorphic CYP enzyme are administered concomitantly during drug therapy, each will compete for that enzyme and competitively inhibit the metabolism of the other substrate. This can result in toxicity. Patients who are poor metabolizers (PMs), extensive metabolizers (EMs) and ultrarapid metabolizers (URMs) can be identified. Having such information will help in determining the appropriate dosage of certain drugs when treating patients with an inherited abnormality of a drug-metabolizing enzyme. In view of the remarkable progress in this particular field, it is to be expected that more genetic polymorphisms will be discovered in the near future.

Clinically significant pharmacokinetic drug interactions between antiepileptic drugs

Pharmacokinetic interactions between antiepileptics represent a major potential complication of epilepsy treatment because drug combinations are common. This review discusses pharmacokinetic drug interactions of clinical significance involving antiepileptics and cytochrome P450 (CYP). Most commonly used antiepileptics are eliminated through hepatic metabolism, catalysed by the enzymes CYP2C9, CYP2C19 and CYP3A4 and uridine diphosphate glucuronosyltransferase (UDGPT). Antiepileptics are associated with a wide range of drug interactions, including hepatic enzyme induction and inhibition. Phenytoin, phenobarbiral, primidone and carbamazepine induce CYP and UDPGT enzymes while valproic acid inhibits them. Avoidance of unnecessary polypharmacy, selection of alternative agents with lower interaction potential and careful dosage adjustments based on serum drug concentration monitoring and clinical observation are the main methods for reducing the risks associated with these interactions.

Study of the association between cytochromes P450 2D6 and 2E1 genotypes and the risk of drug and chemical induced idiosyncratic aplastic anaemia

A genetic susceptibility to drug or chemical toxicity may provide a basis for an increased risk of idiosyncratic aplastic anaemia (AA). The cytochrome P450 enzymes are responsible for the metabolism of many drugs, some of which have been linked to AA. Mutations in the cytochrome P450 CYP2D6 gene result in absent or impaired enzyme activity in about 7% of Caucasians, whereas a specific mutation in the 5'-regulatory region of the CYP2E1 gene causes overexpression of the gene. We evaluated the frequency of allelic variants of CYP2D6 and CYP2E1 using allele-specific PCR amplification and restriction enzyme analysis of blood mononuclear cell DNA among 54 Caucasian AA patients. CYP2D6 and CYP2E1 were chosen because of the link between AA and the antipsychotic drug remoxipride (CYP2D6 substrate) and benzene (CYP2E1 substrate), respectively. Results were compared with 53 controls matched for age, sex and ethnicity. The percentage of AA patients homozygous for the CYP2D6*3, CYP2D6*4 alleles (poor metabolizer phenotype) and the CYP2E1 mutant allele (overexpression) was 0%, 4% and 0%, respectively, and the percentage of heterozygotes was 2%, 28% and 15%, respectively. For normal controls the corresponding results for homozygous mutants were 0%, 4% and 0% and for heterozygotes 4%, 25% and 6%, respectively. We concluded that there were no major differences in the frequencies of the genetic polymorphisms between this series of AA patients and controls, but due to the low number of cases with the poor metabolizer phenotype and those with a history of drug exposure, the power of the study was too low to disprove an interaction.

Clinically important pharmacokinetic drug-drug interactions: role of cytochrome P450 enzymes

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues and many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In the future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

Clinical importance of non-genetic and genetic cytochrome P450 function tests in liver disease

Liver disease is associated with reduced metabolic capacity for drugs that are metabolized by oxidative biotransformation. Three cytochrome P450 (P450 or CYP) gene families in liver microsomes (CYP 1, CYP2 and CYP3) appear to be responsible for much of the drug metabolism that takes place. The genetic polymorphism of the CYPs responsible for debrisoquine/ sparteine (CYP2D6) metabolism and S-mephenytoin (CYP2C19) metabolism has been well documented, but information on the impairment of each isoform in liver disease is still limited. There are two types of hepatic P450 function tests. One type consists of non-genetic P450 function tests (CYP1A2, 2A6, 2C9/10, 2E1 and 3A3/4), and probe drugs include caffeine, catalysed by CYP1A2, coumarin by CYP2A6, phenytoin by CYP2C6, chlorzoxazone by CYP2E1, and nifedipine, erythromycin and lidocaine by CYP3A3/4. The second type of genetic P450 function tests (CYP2C19 and CYP2D6) involves probe drugs such as S-mephenytoin, catalysed by CYP2C19, and debrisoquine and sparteine, catalysed by CYP2D6. The metabolism of the probe drugs used in non-genetic P450 function tests in patients with liver disease falls into two categories: reduced (CYP1A2, CYP2C, 2E1 and 3A) and unchanged (CYP2C). In genetic P450 function tests there seems to be a lesser degree of inhibition in poor metabolizers (PMs) than extensive metabolizers (EMs) among patients with liver disease. There have been very few reports on changes in metabolism of the probe drugs used in genetic P450 function tests in liver disease. In this paper the subject is reviewed.

The safety of ranitidine in over a decade of use

BACKGROUND

Ranitidine hydrochloride (Zantac) is one of the most extensively studied and widely used drugs of all time. This has provided an excellent opportunity to define its safety profile.

METHODS

Data from 189 controlled clinical trials in which more than 26,000 patients received daily doses of ranitidine for 4 weeks or more were reviewed. More than 80% of patients were treated with up to 300 mg ranitidine daily; the remaining patients received doses of up to 1200 mg daily. Eighty-seven trials were placebo controlled. Analyses of post-marketing surveillance and a database of all spontaneously reported adverse events were also evaluated.

RESULTS

Overall in the clinical trial programme adverse events were reported by 20% of those receiving ranitidine compared with 27% of those receiving placebo. The pattern of events was similar in all treatment groups with no evidence of dose-related toxicity in regimens encompassing an eightfold range of therapeutic doses. Similarly in a programme of studies designed to evaluate a dose of ranitidine of 75 mg for non-prescription (over-the-counter) use in the treatment of heartburn, ranitidine was not associated with an adverse event profile distinct from that of placebo. Analysis of spontaneously reported adverse event data allowed identification of rare idiosyncratic events.

CONCLUSIONS

Review of data from a large population of controlled clinical trials with analyses of postmarketing surveillance studies and spontaneously reported adverse events confirmed the excellent safety profile of ranitidine.

Clinical pharmacokinetics of alfentanil, fentanyl and sufentanil. An update

Alfentanil, fentanyl and sufentanil are synthetic opioid analgesics acting at specific opioid receptors. These opioids are widely used as analgesics to supplement general anaesthesia for various surgical procedures or as primary anaesthetic agents in very high doses during cardiac surgery. Fentanyl and sufentanil especially are administered via infusion for long term analgesia and sedation in intensive care patients. Opioid analgesics are mainly administered using the intravenous route. However, other techniques of administration, including epidural, intrathecal, transdermal and intranasal applications, have been demonstrated. Important pharmacokinetic differences between alfentanil, fentanyl and sufentanil have been shown in many reports. Alfentanil has the most rapid analgesic onset and time to peak effect as well as the shortest distribution and elimination half-lives. The volume of distribution and total body clearance of this agent are smaller when compared with those of fentanyl and sufentanil. The pharmacokinetics of the opioid analgesics can be affected by several factors including patient age, plasma protein content, acid-base status and cardiopulmonary bypass, but not significantly by renal insufficiency or compensated hepatic dysfunction. In addition, pharmacokinetic properties can be influenced by changes in hepatic blood flow and administration of drug combinations which compete for the same plasma protein carrier or metabolising pathway. Although comparing specific pharmacokinetic parameters such as half-lives is deeply entrenched in the literature and clinical practice, simply comparing half-lives is not a rational way to select an opioid for specific requirements. Using pharmacokinetic-pharmacodynamic models, computer simulations based on changes in the effect site opioid concentration or context-sensitive half-times seem to be extremely useful for selecting an opioid on a more rational basis.

Clinical pharmacokinetics and metabolism of chloroquine. Focus on recent advancements

This paper presents the current state of knowledge on chloroquine disposition, with special emphasis on stereoselectivity and microsomal metabolism. In addition, the impact of the patient's physiopathological status and ethnic origin on chloroquine pharmacokinetics is discussed. In humans, chloroquine concentrations decline multiexponentially. The drug is extensively distributed, with a volume of distribution of 200 to 800 L/kg when calculated from plasma concentrations and 200 L/kg when estimated from whole blood data (concentrations being 5 to 10 times higher). Chloroquine is 60% bound to plasma proteins and equally cleared by the kidney and liver. Following administration chloroquine is rapidly dealkylated via cytochrome P450 enzymes (CYP) into the pharmacologically active desethylchloroquine and bisdesethylchloroquine. Desethylchloroquine and bisdesethylchloroquine concentrations reach 40 and 10% of chloroquine concentrations, respectively; both chloroquine and desethylchloroquine concentrations decline slowly, with elimination half-lives of 20 to 60 days. Both parent drug and metabolite can be detected in urine months after a single dose. In vitro and in vivo, chloroquine and desethylchloroquine competitively inhibit CYP2D1/6-mediated reactions. Limited in vitro studies and preliminary data from clinical experiments and observations point to CYP3A and CYP2D6 as the 2 major isoforms affected by or involved in chloroquine metabolism. In vitro efficacy studies did not detect any difference in potency between chloroquine enantiomers but, in vivo in rats, S(+)-chloroquine had a lower dose that elicited 50% of the maximal effect (ED950) than that of R(-)-chloroquine. Stereoselectivity in chloroquine body disposition could be responsible for this discrepancy. Chloroquine binding to plasma proteins is stereoselective, favouring S(+)-chloroquine (67% vs 35% for the R-enantiomer). Hence, unbound plasma concentrations are higher for R(-)-chloroquine. Following separate administration of the individual enantiomers, R(-)-chloroquine reached higher and more sustained blood concentrations. The shorter half-life of S(+)-chloroquine appears secondary to its faster clearance. Blood concentrations of the S(+)-forms of desethylchloroquine always exceeded those of the R(-)-forms, pointing to a preferential metabolism of S(+)-chloroquine.

Safety of acid-suppressing drugs

There is an extensive literature on the adverse effects of drugs that inhibit gastric acid secretion. This study presents a critical examination of interactions between antisecretory drugs and other compounds, the frequency of serious adverse effects relating to various body systems, the safety of antisecretory drugs in pregnancy, and longer-term safety data from postmarketing surveillance studies. While interactions with some other drugs, alcohol, and certain carcinogens are of potential concern, in practice clinically significant reactions appear to be rare if they occur at all. A small number of major side-effects have been documented, but they occur rarely, and postmarketing surveillance has not detected other longer-term sequelae. Safety of these drugs in pregnancy is not established, as data are so few. It is concluded that antisecretory agents, by comparison with most other classes of drugs, are remarkably well tolerated.

Aplastic anaemia following exposure to 3,4-methylenedioxymethamphetamine ('Ecstasy')

We report two cases of aplastic anaemia following exposure to 'Ecstasy' (MDMA, 3,4-methylenedioxymethamphetamine). In both cases the aplastic anaemia resolved spontaneously 7-9 weeks after presentation. Long-term bone marrow culture study of one patient demonstrated complete normalization of haemopoiesis at time of haematological recovery, suggesting either that damage to the haemopoietic stem cell had been only transient, or that a more mature, committed progenitor cell was the target. Because MDMA may have been a factor in the aetiology of the bone marrow suppression in these two cases, we recommend close haematological monitoring of young adults presenting with toxicity from MDMA, and a detailed history of exposure to recreational drugs in all new patients presenting with aplastic anaemia.

The effect of oral omeprazole on the disposition of lignocaine

The effects of the gastric antisecretory drug, omeprazole, on the disposition of lignocaine were studied in 10 healthy male volunteers in a double-blind, randomised, placebo-controlled, two-period crossover trial. Omeprazole 40 mg or placebo was taken daily for one week before administration of lignocaine 1 mg.kg-1 (3.7 mumol.kg-1) given intravenously over 10 min. Venous concentrations of lignocaine and its metabolite monoethylglycinexylidine were measured in plasma with reversed phase liquid chromatography. The mean (95% CI) areas under the curve at infinity for lignocaine after pretreatment with omeprazole or placebo were 6.67 (4.90-8.45) mumol.h.l-1 and 6.14 (5.05-7.23) mumol.h.l-1, respectively (p = 0.44). The respective areas for monoethylglycinexylidine were 1.85 (1.25-2.45) mumol.h.l-1 and 1.79 (1.44-2.14) mumol.h.l-1 (p = 0.78). Similarly, omeprazole had no significant effect on the half-lives of lignocaine or methylglycinexylidine.

In-vitro potencies of histamine H2-receptor antagonists on tetraethylammonium uptake in rat renal brush-border membrane vesicles

The histamine H2 antagonists cimetidine, ranitidine and famotidine are organic bases that are cleared from the body by active renal tubular secretion involving the organic cation transporter in the proximal tubule. To determine the potential for competition for the transporter between these drugs and other drugs, their inhibitory potencies were assessed in-vitro, using rat renal brush-border membrane vesicles and tetraethylammonium as the substrate. The concentration-dependent effect of cimetidine, ranitidine and famotidine on the 15-s proton-stimulated uptake of tetraethylammonium into the membrane vesicles was studied using five different rat kidneys. The order of inhibition potencies was: cimetidine (mean IC50 = 1.07 microM) > famotidine (2.43 microM) < ranitidine (55.4 microM). The results indicate the potential for drug interactions in the kidney, especially for cimetidine and famotidine.

Risk for adverse events among patients receiving intravenous histamine2-receptor antagonists

OBJECTIVE

To identify risk factors for adverse drug reactions (ADRs) in patients receiving intravenous histamine2-receptor antagonists (H2-RAs).

DESIGN

The study hypothesis was evaluated by performing a logistic regression procedure with a backward elimination of the explanatory variables associated with ADRs.

MAIN OUTCOME MEASURES

ADRs temporally associated with the use of intravenous H2-RAs served as the dependent variable. Background information about the patients and drug use evaluation criteria in three general areas were entered into the regression analysis.

SETTING

Hospitals were selected from the southeastern US, based on their willingness to participate and their characteristics. Participating hospitals exhibited a variety of sizes and ownership arrangements.

PATIENTS

1200 adult patients who were receiving intravenous H2-RAs.

RESULTS

Seven percent of patients experienced a presumed ADR (PADR) to intravenous H2-RAs. The only risk factor for ranitidine was for patients who did not have their dosage corrected for renal function ("overdose"); these patients were twice as likely to experience a PADR compared with patients who received the correct dosage as determined by their renal function. Two risk factors for cimetidine were identified: (1) patients taking cimetidine with another medication known to cause a drug interaction; and (2) patient age. No risk factors were identified for famotidine.

CONCLUSIONS

The two risk factors for ADRs identified in this study are preventable. Healthcare providers should strive to prevent ADRs by adjusting patients' dosages based on their renal function and by monitoring patients receiving cimetidine with another medication known to interact with cimetidine.

Renal clearance of pyridostigmine in myasthenic patients and volunteers under the influence of ranitidine and pirenzepine

1. To assess the contribution of tubular secretion to the renal excretion of pyridostigmine, and its modification by other cationic drugs, six volunteers were given single oral doses of 60-mg pyridostigmine bromide with and without co-administration of ranitidine or pirenzepine. Renal clearances were determined by h.p.l.c. analysis of pyridostigmine and enzymic measurement of endogenous creatinine in plasma and urine. 2. In patients with myasthenia receiving continuous pyridostigmine therapy, renal clearance values were obtained in the same manner with and without ranitidine (10 patients) or pirenzepine (nine patients) co-medication. 3. Pyridostigmine was not bound to plasma proteins. Its renal clearance averaged 6.65 ml/min per kg (350% of the creatinine clearance) in all subjects, 74% being due to net tubular secretion. 4. Mean values for pyridostigmine renal clearance and for clearance by secretion were decreased in the presence of pirenzepine, but plasma concentrations were not affected significantly. Ranitidine caused a small non-significant decrease of the pyridostigmine clearance in patients. 5. Pyridostigmine had a higher elimination (2 h-1) than the absorption rate constant (0.23 h-1) when administered orally as a non-retarded preparation. 6. The renal clearance of creatinine was slightly increased by pyridostigmine in volunteers and slightly decreased by pirenzepine in the total group of subjects.

Pharmacology of gastric acid inhibition

Gastric acid secretion is precisely regulated by neural (acetylcholine), hormonal (gastrin), and paracrine (histamine; somatostatin) mechanisms. The stimulatory effect of acetylcholine and gastrin is mediated via increase in cytosolic calcium, whereas that of histamine is mediated via activation of adenylate cyclase and generation of cAMP. Potentiation between histamine and either gastrin or acetylcholine may reflect postreceptor interaction between the distinct pathways and/or the ability of gastrin and acetylcholine to release histamine from mucosal ECL cells. The prime inhibitor of acid secretion is somatostatin. Its inhibitory paracrine effect is mediated predominantly by receptors coupled via guanine nucleotide binding proteins to inhibition of adenylate cyclase activity. All the pathways converge on and modulate the activity of the luminal enzyme, H+,K(+)-ATPase, the proton pump of the parietal cell. Precise information on the mechanisms involved in gastric acid secretion and the identification of specific receptor subtypes has led to the development of potent drugs capable of inhibiting acid secretion. These include competitive antagonists that interact with stimulatory receptors (e.g. muscarinic M1-receptor antagonists and histamine H2-receptor antagonists) as well as non-competitive inhibitors of H+,K(+)-ATPase (e.g. omeprazole). The histamine H2-receptor antagonists (cimetidine, ranitidine, famotidine, nizatidine and roxatidine acetate) continue as first-line therapy for peptic ulcer disease and are effective in preventing relapse. Although they are generally well tolerated, histamine H2-receptor antagonists may cause untoward CNS, cardiac and endocrine effects, as well as interfering with the absorption, metabolism and elimination of various drugs. The dominance of the histamine H2-receptor antagonists is now being challenged by omeprazole. Omeprazole reaches the parietal cell via the bloodstream, diffuses through the cytoplasm and becomes activated and trapped as a sulfenamide in the acidic canaliculus of the parietal cell. Here, it covalently binds to H+,K(+)-ATPase, the hydrogen pump of the parietal cell, thereby irreversibly blocking acid secretion in response to all modes of stimulation. The main potential drawback to its use is its extreme potency which sometimes leads to virtual anacidity, gastrin cell hyperplasia, hypergastrinaemia and, in rats, to the development of carcinoid tumours. The cholinergic receptor on the parietal cell has recently been identified as an M3 subtype and that on postganglionic intramural neurones of the submucosal plexus as an M1 subtype.(ABSTRACT TRUNCATED AT 400 WORDS)