Pharmacokinetic interactions of cimetidine 1987

Pediatric flecainide pharmacogenomics: a roadmap to delivering precision-based care to pediatrics arrhythmias

Flecainide acetate is a Class 1c anti-arrhythmic with a potent sodium voltage gated channel blockade which is utilized for the second-line treatment of tachyarrhythmias in children and adults. Given its narrow therapeutic index, the individualization of drug therapy is of utmost importance for clinicians. Despite efforts to improve anti-arrhythmic drug therapy, there remain knowledge gaps regarding the impact of variation in the genes relevant to flecainide's disposition and response. This variability is compounded in developing children whose drug disposition and response pathways may remain immature. The purpose of this comprehensive review is to outline flecainide's disposition and response pathways while simultaneously highlighting opportunities for prospective investigation in the pediatric population.

An observational study to justify and plan a future phase III randomized controlled trial of metformin in improving overall survival in patients with inoperable pancreatic cancer without liver metastases

PURPOSE

Metformin has plausible direct and indirect anti-cancer properties against pancreatic adenocarcinoma cells. However, metformin may only be efficacious in patients with inoperable pancreatic ductal adenocarcinoma (PDAC) without liver metastases. Absorption may be decreased by gastrointestinal symptoms and proton pump inhibitors (PPIs). We aimed to justify and inform a future phase III trial of metformin versus placebo on survival in inoperable PDAC by documenting prevalence of patients meeting eligibility criteria, gastrointestinal symptoms and PPI use.

METHODS

Patient notes with PDAC were reviewed at a large teaching hospital over 2 years. Study variables were obtained from multiple sources of information.

RESULTS

141 participants were identified (51.8% female), of which 37.6% were not prescribed metformin at diagnosis and had no radiological hepatic metastases. Characteristics were similar between non-metformin and metformin users. In eligible patients, 65.2% reported nausea and vomiting and 46.2% were prescribed PPIs.

CONCLUSION

Approximately, a third of all patients with inoperable PDAC are eligible for a future trial of metformin, allowing an estimate of the number of hospitals required for recruitment. Nausea and vomiting are common and should be managed effectively to prevent trial dropouts. PPI use is frequent and their influence on metformin's pharmacodynamic actions needs to be clarified.

Evaluation and Quantitative Prediction of Renal Transporter-Mediated Drug-Drug Interactions

With numerous drugs cleared renally, inhibition of uptake transporters localized on the basolateral membrane of renal proximal tubule cells, eg, organic anion transporters (OATs) and organic cation transporters (OCTs), may lead to clinically meaningful drug-drug interactions (DDIs). Additionally, clinical evidence for the possible involvement of efflux transporters, such as P-glycoprotein (P-gp) and multidrug and toxin extrusion protein 1/2-K (MATE1/2-K), in the renal DDIs is emerging. Herein, we review recent progress regarding mechanistic understanding of transporter-mediated renal DDIs as well as the quantitative predictability of renal DDIs using static and physiologically based pharmacokinetic (PBPK) models. Generally, clinical DDI data suggest that the magnitude of plasma exposure changes attributable to renal DDIs is less than 2-fold, unlike the DDIs associated with inhibition of cytochrome P-450s and/or hepatic uptake transporters. It is concluded that although there is a need for risk assessment early in drug development, current available data imply that safety concerns related to the renal DDIs are generally low. Nevertheless, consideration must be given to the therapeutic index of the victim drug and potential risk in a specific patient population (eg, renal impairment). Finally, in vitro transporter data and clinical pharmacokinetic parameters obtained from the first-in-human studies have proven useful in support of quantitative prediction of DDIs associated with inhibition of renal secretory transporters, OATs or OCTs.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Pharmacological Approach to the Prevention and Treatment of Stress Ulcers

The mortality associated with bleeding stress ulcers in patients in intensive care units exceeds 50%. Iden tification of patients at risk and use of early and effec tive prophylaxis are necessary in the management of patients in intensive care units. The use of antacids is inconvenient, expensive, and associated with electro lyte disturbances and erratic pH control. H2-receptor antagonists are the preferred agents for stress ulcer pro phylaxis because of their proven efficacy, safety, and ease of administration. Adjunct therapy with cyto protective agents may be useful in patients with com promised mucosal defences.

Drug Interactions with Antiulcer Agents: Considerations in the Treatment of Acid-Peptic Disease

All of the antiulcer agents have been implicated in drug interactions. These agents generally influence the absorption, metabolism, or elimination of other medications. However, these interactions can lead to alterations in pharmacodynamic response. The mechanisms by which antiulcer agents produce drug interactions differ among the agents. It is beyond the scope of this article to review all of the drug interactions that have been reported with antiulcer agents. However, it is the intent to provide the reader with a detailed understanding of the mechanisms by which antiulcer agents may interact with other medications and to provide insight into factors that may influence the potential magnitude or clinical consequences of these interactions. An understanding of antiulcer drug interactions will aid pharmacists in assisting clinicians with drug selection and/or monitoring of drug interactions. Specifically, pharmacists can assist with the identification of potential antiulcer drug interactions and develop strategies designed to minimize adverse consequences of these interactions.

Substrate-Dependent Inhibition of the Human Organic Cation Transporter OCT2: A Comparison of Metformin with Experimental Substrates

The importance of the organic cation transporter OCT2 in the renal excretion of cationic drugs raises the possibility of drug-drug interactions (DDIs) in which an inhibitor (perpetrator) drug decreases OCT2-dependent renal clearance of a victim (substrate) drug. In fact, there are clinically significant interactions for drugs that are known substrates of OCT2 such as metformin. To identify drugs as inhibitors for OCT2, individual drugs or entire drug libraries have been investigated in vitro by using experimental probe substrates such as 1-methyl-4-phenylpyridinium (MPP⁺) or 4–4-dimethylaminostyryl-N-methylpyridinium (ASP⁺). It has been questioned whether the inhibition data obtained with an experimental probe substrate such as MPP⁺ or ASP⁺ might be used to predict the inhibition against other, clinical relevant substrates such as metformin. Here we compared the OCT2 inhibition profile data for the substrates metformin, MPP⁺ and ASP⁺. We used human embryonic kidney (HEK 293) cells stably overexpressing human OCT2 as the test system to screen 125 frequently prescribed drugs as inhibitors of OCT2-mediated metformin and MPP⁺ uptake. Data on inhibition of OCT2-mediated ASP⁺ uptake were obtained from previous literature. A moderate correlation between the inhibition of OCT2-mediated MPP⁺, ASP⁺, and metformin uptake was observed (pairwise r_s between 0.27 and 0.48, all P < 0.05). Of note, the correlation in the inhibition profile between structurally similar substrates such as MPP⁺ and ASP⁺ (Tanimoto similarity T = 0.28) was even lower (r_s = 0.27) than the correlation between structurally distinct substrates, such as ASP⁺ and metformin (T = 0.01; r_s = 0.48) or MPP⁺ and metformin (T = 0.01; r_s = 0.40). We identified selective as well as universal OCT2 inhibitors, which inhibited transport by more than 50% of one substrate only or of all substrates, respectively. Our data suggest that the predictive value for drug-drug interactions using experimental substrates rather than the specific victim drug is limited.

Targeted disruption of organic cation transporter 3 attenuates the pharmacologic response to metformin

Metformin, the most widely prescribed antidiabetic drug, requires transporters to enter tissues involved in its pharmacologic action, including liver, kidney, and peripheral tissues. Organic cation transporter 3 (OCT3, SLC22A3), expressed ubiquitously, transports metformin, but its in vivo role in metformin response is not known. Using Oct3 knockout mice, the role of the transporter in metformin pharmacokinetics and pharmacodynamics was determined. After an intravenous dose of metformin, a 2-fold decrease in the apparent volume of distribution and clearance was observed in knockout compared with wild-type mice (P < 0.001), indicating an important role of OCT3 in tissue distribution and elimination of the drug. After oral doses, a significantly lower bioavailability was observed in knockout compared with wild-type mice (0.27 versus 0.58, P < 0.001). Importantly, metformin's effect on the plasma glucose concentration-time curve was reduced in knockout compared with wild-type mice (12 versus 30% reduction, respectively, P < 0.05) along with its accumulation in skeletal muscle and adipose tissue (P < 0.05). Furthermore, the effect of metformin on phosphorylation of AMP activated protein kinase, and expression of glucose transporter type 4 was absent in the adipose tissue of Oct3(-/-) mice. Additional analysis revealed that an OCT3 3' untranslated region variant was associated with reduced activity in luciferase assays and reduced response to metformin in 57 healthy volunteers. These findings suggest that OCT3 plays an important role in the absorption and elimination of metformin and that the transporter is a critical determinant of metformin bioavailability, clearance, and pharmacologic action.

Membrane transporters in drug development

Membrane transporters can be major determinants of the pharmacokinetic, safety and efficacy profiles of drugs. This presents several key questions for drug development, including which transporters are clinically important in drug absorption and disposition, and which in vitro methods are suitable for studying drug interactions with these transporters. In addition, what criteria should trigger follow-up clinical studies, and which clinical studies should be conducted if needed. In this article, we provide the recommendations of the International Transporter Consortium on these issues, and present decision trees that are intended to help guide clinical studies on the currently recognized most important drug transporter interactions. The recommendations are generally intended to support clinical development and filing of a new drug application. Overall, it is advised that the timing of transporter investigations should be driven by efficacy, safety and clinical trial enrolment questions (for example, exclusion and inclusion criteria), as well as a need for further understanding of the absorption, distribution, metabolism and excretion properties of the drug molecule, and information required for drug labelling.

Role of cytochrome P450 in drug interactions

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. 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 future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

A study of the pharmacokinetic interactions of the direct renin inhibitor aliskiren with allopurinol, celecoxib and cimetidine in healthy subjects

OBJECTIVE

Aliskiren is the first in a new class of orally effective direct renin inhibitors approved for the treatment of hypertension. This multiple-dose study investigated the potential for pharmacokinetic interactions between aliskiren and three drugs, each predominantly eliminated by a different clearance/metabolic pathway: allopurinol (glomerular filtration), celecoxib (cytochrome P450 metabolism) and cimetidine (P-glycoprotein and organic anion/cation transporters).

RESEARCH DESIGN AND METHODS

Three open-label, multiple-dose studies in healthy subjects investigated possible pharmacokinetic interactions between aliskiren 300 mg od and allopurinol 300 mg od (n = 20), celecoxib 200 mg bid (n = 22), or cimetidine 800 mg od (n = 22). Subjects received aliskiren alone or co-administered with allopurinol, celecoxib or cimetidine. Allopurinol and celecoxib were also administered alone and in combination with aliskiren. Plasma drug concentrations were determined by LC/MS/MS.

RESULTS

Co-administration of aliskiren with allopurinol had no effect on allopurinol AUC(tau) (ratio of geometric means 0.93 [90% CI, 0.88, 0.98]) or oxypurinol AUC(tau) (mean ratio 1.12 [90% CI, 1.08, 1.16]) and C(max) (mean ratio 1.08 [90% CI, 1.04, 1.13]), with 90% CI within the bioequivalence range 0.80-1.25, and a minor effect on allopurinol C(max) (mean ratio 0.88 [90% CI, 0.78, 1.00]). Aliskiren co-administration had no effect on AUC(tau) or C(max) of celecoxib (mean ratios and 90% CI within range 0.80-1.25). Neither allopurinol nor celecoxib significantly altered aliskiren AUC(tau) or C(max) (geometric mean ratios 0.88-1.02 with 90% CI including 1.00, but with some 90% CI outside the 0.80-1.25 range due to high variability). Co-administration of aliskiren with cimetidine increased aliskiren AUC(tau) by 20% (mean ratio 1.20 [90% CI, 1.07, 1.34]) and C(max) by 25% (mean ratio 1.25 [90% CI, 0.98, 1.59]).

CONCLUSIONS

In this multiple-dose study, aliskiren showed no clinically relevant pharmacokinetic interactions when co-administered with allopurinol, celecoxib or cimetidine in healthy subjects.

Time-dependent inhibition of human drug metabolizing cytochromes P450 by tricyclic antidepressants

AIMS

To investigate time-dependent inhibition (TDI) of human drug metabolizing CYP enzymes by tricyclic antidepressants (TCAs).

METHODS

CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A/CYP3A4 activities were investigated following co- and preincubation with TCAs using human liver microsomes (HLM) and human recombinant CYP proteins (expressed in Escherichia coli) as the enzyme sources. A two-step incubation method was employed to examine the in vitro mechanism-based inactivation (MBI) criteria. Potential metabolite-intermediate complex (MIC) formation was studied by spectral analysis.

RESULTS

TCAs generally exhibited significant TDI of recombinant CYP1A2, CYP2C19 and CYP2D6 (>10% positive inhibition differences between co- and preincubation conditions). TDI of recombinant CYP2C9 was minor (<10%), and was minor or absent in experiments utilizing recombinant CYP3A4 or HLM as the enzyme sources. Where observed, TDI of recombinant CYP occurred via alkylamine MIC formation, but evidence to support similar behaviour in HLM was limited. Indeed, only secondary amine TCAs reduced the apparent P450 content of HLM (3-6%) consistent with complexation. As a representative TCA, nortriptyline fulfilled the in vitro MBI criteria using recombinant CYP2C19 and CYP3A4 (K(I) and k(inact) values of 4 microm and 0.19 min(-1), and 70 microm and 0.06 min(-1)), but not with the human liver microsomal enzymes.

CONCLUSIONS

TCAs appear to have minimal potential for MBI of human liver microsomal CYP enzymes involved in drug metabolism. HLM and recombinant CYP (expressed in E. coli) are not equivalent enzyme sources for evaluating the TDI associated with some drugs.

Evaluation of 3-O-methylfluorescein as a selective fluorometric substrate for CYP2C19 in human liver microsomes

Cytochrome P450 (P450) fluorometric high-throughput inhibition assays have been widely used for drug-drug interaction screening particularly at the preclinical drug discovery stages. Many fluorometric substrates have been investigated for their selectivity, but most are found to be catalyzed by multiple P450 isozymes, limiting their utility. In this study, 3-O-methylfluorescein (OMF) was examined as a selective fluorescence substrate for CYP2C19 in human liver microsomes (HLMs). The kinetic studies of OMF O-demethylation in HLMs using a liquid chromatography/mass spectrometry method exhibited two-enzyme kinetics with apparent K(m) and V(max) values of 1.14 +/- 0.90 microM and 11.3 +/- 4.6 pmol/mg/min, respectively, for the high affinity component(s) and 57.0 +/- 6.4 microM and 258 +/- 6 pmol/mg/min, respectively, for the low affinity component(s). Studies utilizing cDNA-expressed individual P450 isoforms and P450-selective chemical inhibitors showed that OMF O-demethylation to fluorescein was selective for CYP2C19 at substrate concentrations < or =1 microM. At substrate concentrations > or =10 microM, other P450 isozymes were found to catalyze OMF O-demethylation. In HLMs, analysis of the two-enzyme kinetics in the presence of P450 isozyme-selective chemical inhibitors (ticlopidine for CYP2C19, sulfaphenazole for CYP2C9, and furafylline for CYP1A2) indicated that CYP2C19 was the high affinity component and CYP2C9 was the low affinity component. Based on these findings, a fluorometric assay was developed using 1 microM OMF and 2 microM sulfaphenazole for probing CYP2C19-mediated inhibition in HLMs. The IC(50) data of 13 substrates obtained from the fluorometric assay developed in this study correlated well with that reported in the literature using nonfluorescence assays.

OPC-67683, a nitro-dihydro-imidazooxazole derivative with promising action against tuberculosis in vitro and in mice

BACKGROUND

Tuberculosis (TB) is still a leading cause of death worldwide. Almost a third of the world's population is infected with TB bacilli, and each year approximately 8 million people develop active TB and 2 million die as a result. Today's TB treatment, which dates back to the 1970s, is long and burdensome, requiring at least 6 mo of multidrug chemotherapy. The situation is further compounded by the emergence of multidrug-resistant TB (MDR-TB) and by the infection's lethal synergy with HIV/AIDS. Global health and philanthropic organizations are now pleading for new drug interventions that can address these unmet needs in TB treatment.

METHODS AND FINDINGS

Here we report OPC-67683, a nitro-dihydro-imidazooxazole derivative that was screened to help combat the unmet needs in TB treatment. The compound is a mycolic acid biosynthesis inhibitor found to be free of mutagenicity and to possess highly potent activity against TB, including MDR-TB, as shown by its exceptionally low minimum inhibitory concentration (MIC) range of 0.006-0.024 microg/ml in vitro and highly effective therapeutic activity at low doses in vivo. Additionally, the results of the post-antibiotic effect of OPC-67683 on intracellular Mycobacterium tuberculosis showed the agent to be highly and dose-dependently active also against intracellular M. tuberculosis H37Rv after a 4-h pulsed exposure, and this activity at a concentration of 0.1 microg/ml was similar to that of the first-line drug rifampicin (RFP) at a concentration of 3 microg/ml. The combination of OPC-67683 with RFP and pyrazinamide (PZA) exhibited a remarkably quicker eradication (by at least 2 mo) of viable TB bacilli in the lung in comparison with the standard regimen consisting of RFP, isoniazid (INH), ethambutol (EB), and PZA. Furthermore, OPC-67683 was not affected by nor did it affect the activity of liver microsome enzymes, suggesting the possibility for OPC-67683 to be used in combination with drugs, including anti-retrovirals, that induce or are metabolized by cytochrome P450 enzymes.

CONCLUSIONS

We concluded that based on these properties OPC-67683 has the potential to be used as a TB drug to help combat the unmet needs in TB treatment.

Modulation of antipyrine clearance by polysaccharide peptide (PSP) isolated from Coriolus versicolor in the rat

Polysaccharide peptide (PSP), isolated from Coriolus versicolor COV-1, has been previously shown to have immuno-stimulatory, anti-tumour and analgesic effects in animal models. When used as an adjunct in cancer chemotherapy in clinical trials carried out in China, PSP improved the quality of life in the patients by improving appetite and alleviating symptoms associated with cancer chemotherapy. In this study, the effects of non-toxic doses of PSP on phase I metabolism was investigated in the rat, using the conventional probe antipyrine. Acute PSP (3-5 micromol/kg, i.p.) treatment did not produce significant changes in antipyrine clearance. Sub-chronic treatment with PSP (1-3 micromol/kg/day, i.p., 3 days) decreased the antipyrine clearance (30-35%), with an increase in the plasma half-life (T1/2) by 55% and an increase in the area under concentration-time curve (AUC) by 61%. Total hepatic cytochrome P450 (P450) was dose-dependently decreased (32-54%) after sub-chronic, but not the acute treatment of PSP. Given that PSP can affect phase I metabolism and hepatic cytochrome P450 content, the concomitant use of PSP with other therapeutic agents that undergo phase I metabolism should be carefully monitored.

Drug-drug interactions involving membrane transporters in the human kidney

The kidneys play a critical role in the elimination of xenobiotics. Factors affecting the ability of the kidney to eliminate drugs may result in marked changes in the pharmacokinetics of a given compound. Drug-drug interactions due to competitive inhibition of renal organic anion or cation secretion systems have been noticed clinically for a long time. However, our understanding of the physical sites of interactions, that is, the specific transport proteins that the interacting drugs act on, has just begun very recently. This review summarises the latest progress in molecular identification and functional characterisation of major drug transporters in the human kidney. In particular, the review focuses on relating cloned renal drug transporters to clinically observed drug-drug interactions. The authors' opinion on the current status and future directions of research in these areas is also offered.

Liver enzyme induction and inhibition: implications for anaesthesia

Recent breakthroughs in molecular biology have enabled a reclassification of drug metabolising enzymes based on their amino acid sequence. This has led to a better understanding of drug metabolism and drug interactions. The majority of these drug metabolising enzymes may be either induced or inhibited by drugs or by extraneous substances including foodstuffs, cigarette smoke and environmental pollutants. Virtually all drugs used in anaesthesia are metabolised by either hepatic phase 1 or phase II enzymes. This review considers the classification of drug metabolising enzymes, explains the mechanisms of enzyme induction and inhibition, and also considers how the action of drugs commonly used by anaesthetists, including opioids and neuromuscular blocking drugs, may be altered by this mechanism.

Influence of famotidine on verapamil pharmacokinetics in rats

The effect of famotidine (4 mg/kg, p.o.) on the kinetic profile of co-administered verapamil (20 mg/kg(-1), p.o.) was studied in the rat. Plasma verapamil levels were collected serially for 12 h and measured using sensitive HPLC method. The pharmacokinetic parameters (elimination half-life, area under the plasma concentration-time curve, peak plasma levels and the times to attain these plasma levels) of verapamil were evaluated in the rat. The results indicate that co-administered famotidine did not significantly alter the pharmacokinetic profile of verapamil in the rat. The present finding suggests that famotidine may safely be co-administered with verapamil but clearly further studies in human subjects are needed to reliably rule out the potential interaction of these two drugs.

Metformin transport by renal basolateral organic cation transporter hOCT2

PURPOSE

Metformin, an antihyperglycemic agent, is eliminated by tubular secretion in addition to glomerular filtration in the human kidney. This study was performed to characterize metformin transport by human organic cation transporter 2 (hOCT2), the most abundant organic cation transporter in the basolateral membranes of the human kidney.

METHODS

Accumulation of [14C]metformin was assessed by the tracer experiments in the human embryonic kidney (HEK293) cells expressing hOCT2.

RESULTS

The transport of [14C]metformin was markedly stimulated in hOCT2-expressing cells compared with the vector-transfected cells. The accumulation of [14C]metformin was concentrative and was dependent on the membrane potential, showing consistency with the characteristics of hOCT2. The apparent Km and Vmax values of [14C]metformin transport by hOCT2-expressing HEK293 cells were 1.38+/-0.21 mM and 11.9+/-1.5 nmol mg protein(-1) min(-1), respectively. The order of the potencies of unlabeled biguanides to inhibit [14C]metformin transport by hOCT2 was phenformin > buformin > metformin. Furthermore, [14C]metformin transport was inhibited slightly or moderately by cationic drugs such as procainamide and quinidine at respective therapeutic concentrations.

CONCLUSIONS

Metformin is transported by the basolateral organic cation transporter hOCT2 in the human kidney. hOCT2 could play a role in the drug interactions between metformin and some cationic drugs.

Science review: The use of proton pump inhibitors for gastric acid suppression in critical illness

Prophylaxis is routinely provided for critically ill patients admitted to intensive care units (ICUs) who are at high risk for stress-related mucosal damage (SRMD), an erosive process of the gastroduodenum associated with abnormally high physiological demands. Traditionally, treatment options have included sucralfate, antacids and histamine H2 receptor antagonists (H₂RAs). The H₂RAs are currently the most widely used agents in prophylactic acid suppression; however, proton pump inhibitors (PPIs) have recently replaced H₂RAs in the treatment of many acid-related conditions. PPIs achieve a more rapid and sustained increase in gastric pH and are not associated with the rapid tachyphylaxis seen with H₂RAs. As a result, and after the introduction of intravenous formulations, PPIs are beginning to be used for the prophylaxis of SRMD in critically ill adults. The high prevalence of renal and hepatic impairment among the ICU population, as well as the need for multiple drug therapy in many patients, means that pharmacokinetic characteristics and the potential for drug interactions may be important considerations in the choice of prophylactic agent. This review seeks to present the pharmacological evidence that may inform decision-making about the prescription of drugs for prophylaxis of SRMD.

Impact of cytochrome P-450 inhibition by cimetidine and induction by carbamazepine on the kinetics of hypericin and pseudohypericin in healthy volunteers

This study evaluated the influence of cimetidine and carbamazepine on the pharmacokinetics of the St. John's wort (SJW) ingredients hypericin and pseudohypericin. In a placebo-controlled, double blind study, 33 healthy volunteers were randomized into three treatment groups that received SJW extract (LI160) with different comedications (placebo, cimetidine, and carbamazepine) for 7 days after a run-in period of 11 days with SJW alone. Hypericin and pseudohypericin pharmacokinetics were measured on days 10 and 17. Between-group comparisons showed no statistically significant differences in AUC(0-24), C(max), and t(max) values for hypericin and pseudohypericin. Within-group comparisons, however, revealed a statistically significant increase in hypericin AUC(0-24) from a median of 119 (range 82-163 microg h/l) to 149 microg h/l (61-202 microg h/l) with cimetidine comedication and a decrease in pseudohypericin AUC(0-24) from a median of 51.0 (16.4-102.9 microg h/l) to 36.4 microg h/l (14.0-102.0 microg h/l) with carbamazepine comedication compared to the baseline pharmacokinetics in each group. Hypericin and pseudohypericin pharmacokinetics were only marginally influenced by comedication with the enzyme inhibitors and inducers cimetidine and carbamazepine.

Influence of cimetidine co-administration on the pharmacokinetics of acebutolol enantiomers and its metabolite diacetolol in a rat model: the effect of gastric pH on double-peak phenomena

Acebutolol (AC) is a chiral beta-adrenergic receptor-blocking agent, which has been shown to be clinically effective in hypertension. The plasma concentration-time profiles of AC exhibit two peaks following oral administration of racemate for both R- and S-enantiomers. In the present study, the absorption of AC after a single dose was studied as a function of gastric pH in male Sprague-Dawley rats. Furthermore, the effect of cimetidine (CIM) on pharmacokinetic parameters of AC and its metabolite diacetolol (DC) was evaluated. CIM (50 mg kg(-1)) was administered via jugular vein 30 min prior to AC administration to elevate the intragastric pH. AC (50 mg kg(-1)) was administered orally by gavage and serial blood samples were collected before and for 8h after AC administration. Plasma samples were assayed for AC and DC, pharmacokinetic parameters were estimated and compared with those of control. The concentration-time profiles and the pharmacokinetics of AC were unchanged after co-administration of CIM. The oral absorption of AC, as assessed by the area under the plasma concentration-time curve (AUC) and the amount of unchanged drug recovered in the urine were not affected by CIM. The amount of metabolite recovered in the urine and the rate of absorption, however, were significantly altered. These are unlikely to be of clinically importance as we have found that the extent of absorption was not changed. We, therefore, concluded that intragastric elevation of pH has no effect either on generation of multiple peaking or on pharmacokinetic parameters of AC.

Metabolism of zaleplon by human liver: evidence for involvement of aldehyde oxidase

1. The metabolism of Zaleplon (CL-284,846; ZAL) has been studied in precision-cut human liver slices and liver cytosol preparations. 2. Human liver slices metabolized ZAL to a number of products including 5-oxo-ZAL (M2), N-desethyl-5-oxo-ZAL (M1) and N-desethyl-ZAL (DZAL), the latter metabolite being known to be formed by CYP3A forms. 3. Human liver cytosol preparations catalysed the metabolism of ZAL to M2. Kinetic analysis of three cytosol preparations revealed mean (+/- SEM) K(m) and V(max) of 93 +/- 18 mm and 317 +/- 241 pmol/min/mg protein, respectively. 4. Using 16 individual human liver cytosol preparations a 33-fold variability in the metabolism of 80 micro M ZAL to M2 was observed. Correlations were observed between M2 formation and the metabolism of the aldehyde oxidase substrates phenanthridine (r(2) = 0.774) and phthalazine (r(2) = 0.460). 5. The metabolism of 80 micro M ZAL to M2 in liver cytosol preparations was markedly inhibited by the aldehyde oxidase inhibitors chlorpromazine, promethazine, hydralazine and menadione. Additional kinetic analysis suggested that chlorpromazine and promethazine were non-competitive inhibitors of M2 formation with K(i) of 2.3 and 1.9 micro M, respectively. ZAL metabolism to M2 was also inhibited by cimetidine. 6. Incubations conducted with human liver cytosol and H(2)(18)O demonstrated that the oxygen atom incorporated into ZAL and DZAL to form M2 and M1, respectively, was derived from water and not from molecular oxygen. 7. In summary, by correlation analysis, chemical inhibition and H(2)(18)O incorporation studies, ZAL metabolism to M2 in human liver appears to be catalysed by aldehyde oxidase. With human liver slices, ZAL was metabolized to products dependent on both aldehyde oxidase and CYP3A forms.

Inhibition of zaleplon metabolism by cimetidine in the human liver: in vitro studies with subcellular fractions and precision-cut liver slices

1. The effect of cimetidine on the metabolism of zaleplon (ZAL) in human liver subcellular fractions and precision-cut liver slices was investigated. 2. ZAL was metabolized to a number of products including 5-oxo-ZAL (M2), which is known to be formed by aldehyde oxidase, N-desethyl-ZAL (DZAL), which is known to be formed by CYP3A forms, and N-desethyl-5-oxo-ZAL (M1). 3. Human liver microsomes catalysed the NADPH-dependent metabolism of ZAL to DZAL. Kinetic analysis of three microsomal preparations revealed mean (+/-SEM) S(50) and V(max) of 310 +/- 24 micro M and 920 +/- 274 pmol/min/mg protein, respectively. 4. Human liver cytosol preparations catalysed the metabolism of ZAL to M2. Kinetic analysis of three cytosol preparations revealed mean (+/-SEM), K(m) and V(max) of 124 +/- 14 micro M and 564 +/- 143 pmol/min/mg protein, respectively. 5. Cimetidine inhibited ZAL metabolism to DZAL in liver microsomes and to M2 in the liver cytosol. With a ZAL substrate concentration of 62 micro M, the calculated mean (+/-SEM, n = 3) IC50 were 596 +/- 103 and 231 +/- 23 micro M for DZAL and M2 formation, respectively. Kinetic analysis revealed that cimetidine was a competitive inhibitor of M2 formation in liver cytosol with a mean (+/-SEM, n = 3) K(i) of 155 +/- 16 micro M. 6. Freshly cut human liver slices metabolized ZAL to a number of products including 1, M2 and DZAL. 7. Cimetidine inhibited ZAL metabolism in liver slices to M1 and M2, but not to DZAL. Kinetic analysis revealed that cimetidine was a competitive inhibitor of M2 formation in liver slices with an average (n = 2 preparations) K(i) of 506 micro M. 8. The results demonstrate that cimetidine can inhibit both the CYP3A and aldehyde oxidase pathways of ZAL metabolism in the human liver. Cimetidine appears to be a more potent inhibitor of aldehyde oxidase than of CYP3A forms and hence in vivo is likely to have a more marked effect on ZAL metabolism to M2 than on DZAL formation. 9. The results also demonstrate that precision-cut liver slices may be a useful model system for in vitro drug-interaction studies.

Clinical pharmacokinetics of quetiapine: an atypical antipsychotic

Quetiapine is a dibenzothiazepine derivative that has been evaluated for management of patients with the manifestations of psychotic disorders. In pharmacokinetic studies in humans, quetiapine was rapidly absorbed after oral administration, with median time to reach maximum observed plasma concentration ranging from 1 to 2 hours. The absolute bioavailability is unknown, but the relative bioavailability from orally administered tablets compared with a solution was nearly complete. Food has minimal effects on quetiapine absorption. The drug is approximately 83% bound to serum proteins. Single and multiple dose studies have demonstrated linear pharmacokinetics in the clinical dose range (up to 375mg twice daily). The drug is eliminated with a mean terminal half-life of approximately 7 hours. The primary route of elimination is through hepatic metabolism. In vitro studies show that quetiapine is predominantly metabolised by cytochrome P450 (CYP) 3A4. After administration of [14C]quetiapine, approximately 73% of the radioactivity was excreted in the urine and 21% in faeces. Quetiapine accounted for less than 1% of the excreted radioactivity. 11 metabolites formed through hepatic oxidation have been identified. Two were found to be pharmacologically active, but they circulate in plasma at 2 to 12% of the concentration of quetiapine and are unlikely to contribute substantially to the pharmacological effects of the drug. The pharmacokinetics of quetiapine do not appear to be altered by cigarette smoking. Oral clearance declines with age, and was reduced in 2 of 8 patients with hepatic dysfunction but not in patients with renal impairment. Quetiapine has no effect on the in vitro activity of CYP1A2, 2C9, 2C19, 2D6 and 3A4 at clinically relevant concentrations. The lack of effect of quetiapine on hepatic oxidation was confirmed in vivo by the lack of effect of quetiapine on antipyrine disposition. Quetiapine had no effect on serum lithium concentration. Phenytoin and thioridazine increase the clearance of quetiapine, and ketoconazole decreases clearance. No clinically significant effects of cimetidine, haloperidol, risperidone or imipramine on the pharmacokinetics of quetiapine were noted. Quetiapine dosage adjustment, therefore, may be necessary when coadministered with phenytoin, thioridazine or other potent CYP3A4 inducers or inhibitors. The relationship between the therapeutic effects and the plasma concentrations of quetiapine has been investigated in a multicentre clinical trial. There was no statistically significant association between trough plasma quetiapine concentration and clinical response as measured by traditional assessments of psychotic symptom severity. Subsequent clinical studies of the plasma concentration versus effect relationships for quetiapine may help to further define guidelines for dosage regimen design.

Simultaneous determination of testosterone metabolites in liver microsomes using column-switching semi-microcolumn high-performance liquid chromatography

A sensitive and selective column-switching semi-microcolumn high-performance liquid chromatographic (HPLC) method has been developed for the simultaneous determination of testosterone and eight of its metabolites (6alpha-, 6beta-, 16alpha-, 16beta-, 7alpha-, 2alpha-, and 2beta-hydroxytestosterone, and androstenedione) in liver microsomes. After incubation for 10 min, testosterone and its metabolites were extracted from the microsomes with ethyl acetate, and the extract was evaporated to dryness. The residue was dissolved in the mobile phase and loaded onto the HPLC system. The analytes were first concentrated in a precolumn and subsequently transferred to the analytical column, where they were separated using linear gradient elution. A UV detector set at 254 nm was used to detect the analytes. This newly developed method clearly separated TES and the metabolites with high resolution and was found to be reproducible with intra- and interday variability of <10.7%. This method has been subsequently used to determine the testosterone hydroxylation activities catalyzed by 15 different recombinant CYP isozymes. The results confirmed the formation of stereoselectively hydroxylated metabolites by each CYP isozyme.

Inhibitory effects of the medicinal herb, Thonningia sanguinea, on liver drug metabolizing enzymes of rats

In this study we examined the effect of the aqueous extract of Thonningia sanguinea (T.S.) on 7-ethoxyresorufin O-deethylase (EROD, CYP1A1), 7-pentoxyresorufin O-dealkylase (PROD, CYP2B1/2), 7-methoxyresorufin O-demethylase (MROD, CYP1A2), aniline hydroxylase (aniline, CYP2E1), p-nitrophenol hydroxylase (PNPH, CYP2E1) and erythromycin N-demethylase (ERDM, CYP3A1) in rat liver in vitro and in vivo. Although T.S. extract increased ERDM activity in induced rat liver microsomes, it showed a dose-dependent inhibitory effect in vitro on other P450 monooxygenase activities particularly EROD and PROD, which are mediated primarily by CYP1A1 and CYP2B1/2, respectively. PROD, EROD and MROD activities were also decreased by 18%, 19% and 40%, respectively, in hepatic microsomes prepared from rats treated with T.S. extract for 3 days. Kinetic analysis of CYP activity of 3-methylchloranthrene-induced microsomes demonstrated that T.S. inhibited EROD and MROD activities by a noncompetitive and competitive mechanism, respectively. The analysis of alterations produced by T.S. on PROD kinetic parameters in phenobarbital-induced microsomes suggested that the inhibition is noncompetitive. Pretreatment of rats with T.S. prolonged pentobarbital and phenobarbital sleeping time; however, plasma phenobarbital concentration determined on awakening showed no significant difference between control and T.S.-treated rats. T.S. was also found to be a potent inhibitor of the liver cytosolic glutathione S-transferase. These data suggest that selective modulation of CYP isoenzymes by T.S. might contribute to protection of the liver from xenobiotic-induced intoxication or to alteration of the action of drug(s) concomitantly administered besides its antioxidative properties.

Mefloquine pharmacokinetics in healthy subjects and in peptic ulcer patients after cimetidine administration

The pharmacokinetics of orally administered mefloquine were determined in six healthy male subjects and in six ulcer patients before and after a 3-day course of cimetidine (400 mg morning and evening). Peak plasma concentrations Cmax and AUC0-infinity were similarly and significantly (P < 0.05) increased after cimetidine pretreatement in both healthy subjects and peptic ulcer patients Cmax was increased by 42.4% and 20.5% while AUC0-infinity was increased by 37.5% in healthy and peptic ulcer subjects respectively. The values of t1/2ab absorption and t1/2 beta elimination, total crearance CLT/F and volume of distribution were altered to varying levels after cimetidine treatment but the changes were not statistically significant in both healthy and peptic ulcer subjects. The established long t1/2 beta and this apparent interaction between mefloquine and cimetidine which resulted in increased mefloquine plasma concentration might be of clinical significant in patients with neurological/psychiatric history.

Inhibitory effects of amiodarone and its N-deethylated metabolite on human cytochrome P450 activities: prediction of in vivo drug interactions

AIMS

To predict the drug interactions of amiodarone and other drugs, the inhibitory effects and inactivation potential for human cytochrome P450 (CYP) enzymes by amiodarone and its N-dealkylated metabolite, desethylamiodarone were examined.

METHODS

The inhibition or inactivation potency of amiodarone and desethylamiodarone for human CYP activities were investigated using microsomes from B-lymphoblastoid cell lines expressing CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. The in vivo drug interactions of amiodarone and desethylamiodarone were predicted in vitro using the 1+Iu/Ki values.

RESULTS

Amiodarone weakly inhibited CYP2C9, CYP2D6, and CYP3A4-mediated activities with Ki values of 45.1-271.6 microm. Desethylamiodarone competitively inhibited the catalytic activities of CYP2D6 (Ki=4.5 microm ) and noncompetitively inhibited CYP2A6 (Ki=13.5 microm ), CYP2B6 (Ki=5.4 microm ), and CYP3A4 (Ki=12.1 microm ). The catalytic activities of CYP1A1 (Ki=1.5 microm, alpha=5.7), CYP1A2 (Ki=18.8 microm, alpha=2.6), CYP2C9 (Ki=2.3 microm, alpha=5.9), and CYP2C19 (Ki=15.7 microm, alpha=4.5) were inhibited by desethylamiodarone with mixed type. The 1+Iu/Ki values of desethylamiodarone were higher than those of amiodarone. Amiodarone inactivated CYP3A4, while desethylamiodarone inactivated CYP1A1, CYP1A2, CYP2B6, and CYP2D6.

CONCLUSIONS

The interactions between amiodarone and other drugs might occur via the inhibition of CYP activities by its N-dealkylated metabolite, desethylamiodarone, rather than by amiodarone itself. In addition, the inactivation of CYPs by desethylamiodarone as well as by amiodarone would also contribute to the drug interactions.

Effect of trimethoprim on the renal clearance of lamivudine in rats

Lamivudine undergoes minimal metabolism and renal clearance of the unchanged drug is the predominant mechanism of clearance. The effect of trimethoprim on the renal clearance of lamivudine was investigated in rats in-vivo. Total renal clearance of lamivudine was about three times higher than the glomerular filtration rate in rats receiving an infusion of tritium-labelled lamivudine. Concomitant infusion of trimethoprim reduced the renal clearance of lamivudine to about half, but did not affect the level of radioactivity in the renal cortex. When rats received an infusion of lamivudine with probenecid, cimetidine or quinidine, the renal clearance of lamivudine was only significantly reduced by co-administration of cimetidine. These findings suggest that secretion in the renal proximal tubule takes an active part in the total renal clearance of lamivudine, and that cationic drugs such as trimethoprim and cimetidine may inhibit the secretion of lamivudine without greatly affecting the concentration of lamivudine in the renal cortex.

Effect of cimetidine and ranitidine on pharmacokinetics and pharmacodynamics of a single dose of dofetilide

AIMS

The aim of this open-label, placebo-controlled, randomized, four-period crossover study was to determine the effects of cimetidine and ranitidine on the pharmacokinetics and pharmacodynamics of a single dose of dofetilide.

METHODS

Twenty healthy male subjects received 100 or 400 mg twice daily of cimetidine, 150 mg twice daily of ranitidine, or placebo for 4 days. On the second day, a single oral 500 microg dose of dofetilide was administered immediately after the morning doses of cimetidine, ranitidine, or placebo. Treatment periods were separated by 1-2 weeks. Pharmacokinetic parameters were determined from plasma and urinary dofetilide concentrations; prolongation of the QTc interval was determined from three-lead electrocardiograms.

RESULTS

Ranitidine did not significantly affect the pharmacokinetics or pharmacodynamics of dofetilide; however, a dose-dependent increase in exposure to dofetilide was observed with cimetidine. When dofetilide was administered with 100 and 400 mg of cimetidine, the area under the plasma concentration-time curve of dofetilide increased by 11% and 48% and the maximum plasma dofetilide concentration increased by 11% and 29%, respectively. The respective cimetidine doses reduced renal clearance of dofetilide by 13% and 33% and nonrenal clearance by 5% and 21%. Dofetilide-induced prolongation of the QTc interval was enhanced by cimetidine; the mean maximum change in QTc interval from baseline was increased by 22% and 33% with 100 and 400 mg of cimetidine, respectively. However, the relationship between the prolongation of the QTc interval and plasma dofetilide concentrations was unaffected by cimetidine or ranitidine; a 1 ng ml-1 increase in plasma dofetilide concentration produced a 17-19 ms prolongation of the QTc interval. Dofetilide was well tolerated, with no treatment-related adverse events or laboratory abnormalities.

CONCLUSIONS

These results suggest that cimetidine increased dofetilide exposure by inhibiting renal tubular dofetilide secretion, whereas ranitidine did not. This effect is not an H2-receptor antagonist class effect but is specific to cimetidine. If therapy with an H2-receptor antagonist is required, it is recommended that cimetidine at all doses be avoided; since ranitidine has no effect on dofetilide pharmacokinetics or prolongation of the QTc interval, it can be seen as a suitable alternative.

Regulation of paracellular absorption of cimetidine and 5-aminosalicylate in rat intestine

PURPOSE

Isolating the relative contributions of parallel transcellular and paracellular transport to the intestinal absorption of small hydrophilic molecules has proven experimentally challenging. In this report, lumenal appearance of drug metabolite is utilized as a tool to assess the contribution of paracellular transport to the absorption of cimetidine and 5-aminosalicylate (5ASA) in rat small intestine.

METHODS

Steady-state intestinal absorption and elimination of cimetidine and 5ASA were studied in single-pass intestinal perfusions in rats.

RESULTS

Both drugs were metabolized in intestinal epithelia with subsequent metabolite secretion into the intestinal lumen. Jejunal cimetidine absorption decreased with increasing perfusion concentration while the ratio of lumenal metabolite to lumenal drug loss increased. Cimetidine uptake at perfusion concentrations above 0.4 mM resulted in over 80% drug elimination into the jejunal lumen. Inhibition of intracellular metabolism of cimetidine by methimazole did not alter epithelial uptake but totally abolished transepithelial cimetidine flux indicating an elevation of intracellular cimetidine. Similarly, co-perfusion of 5ASA with cimetidine and methimazole totally abolished 5ASA absorption but increased lumenal levels of N-acetyl 5ASA indicating an increase in intracellular uptake of 5ASA.

CONCLUSIONS

Cimetidine and 5ASA absorption across rat jejunal epithelia are exclusively paracellular. Elevation of intracellular cimetidine, inferred from mass balance considerations, restricts paracellular transport of both drugs.

An in-vitro study on the metabolism of rokitamycin and possible interactions of the drug with rat liver microsomes

This in-vitro study was designed to identify the enzyme(s) involved in the major metabolic pathway of rokitamycin, i.e. the formation of leucomycin A7, and to assess possible interactions of the drug with rat liver microsomes. Formation of leucomycin A7 was NADPH-independent and was not appreciably inhibited by anti-rat NADPH cytochrome P-450 reductase serum or cimetidine, a nonspecific inhibitor of cytochrome P-450 isoforms. Eadie-Hofstee plots for the formation of leucomycin A7 were indicative of apparently monophasic behaviour for six rat liver microsomes tested. The mean (+/- s.d.) kinetic parameters, Km, Vmax and Vmax/Km, for the formation of leucomycin A7 from rokitamycin were 47+/-13 microM, 390+/-56 nmol min(-1) (mg protein)(-1) and 8.6+/-1.6 mL min(-1) (mg protein)(-1), respectively. Three esterase inhibitors (100 microM), bis-nitrophenylphosphate, physostigmine and metrifonate inhibited the formation of leucomycin A7 by more than 60%. Metabolism of rokitamycin was inhibited by terfenadine, but not by mequitazine, whereas chlorpheniramine and theophylline activated the formation of leucomycin A7. Rokitamycin, leucomycin A7, leucomycin V, erythromycin and clarithromycin were weak inhibitors of CYP3A-catalysed 3-hydroxylation of quinine with mean IC50 values ranging from 71 to >100 microM. It is concluded that in rat liver microsomes the formation of leucomycin A7 from rokitamycin is catalysed mainly by an esterase (possibly cholinesterase, EC3.1.1.8), but not by cytochrome P-450 enzyme(s). Although in this in-vitro animal study CYP3A activity was barely inhibited by rokitamycin, the possibility cannot be totally discounted in man when rokitamycin is co-administered with drugs metabolized by CYP3A.

Clinically significant pharmacokinetic drug interactions with benzodiazepines

The reports of interactions between benzodiazepines (BZPs) and other drugs (e.g., antidepressants, selective serotonin reuptake inhibitors, antiulcer drugs, antiepileptic drugs, macrolide antibiotics) during their combined use are reviewed. In general, metabolism of BZPs is delayed when combined with a number of other drugs but some reports have suggested otherwise. In recent years, the cytochrome P450 (P450 or CYP) isoenzyme that catalyses the metabolism of BZPs has also been identified. BZPs are mainly catalysed by CYP3A4. When published reports are studied, it appears necessary to be exceptionally careful about interactions mainly between BZPs and selective serotonin reuptake inhibitors, cimetidine, antiepileptic drugs, macrolide antibiotics and antimycotics. More information is necessary to identify individuals at greatest risk of drug interactions and adverse events.

Effects of cimetidine on the pharmacokinetics of proguanil in healthy subjects and in peptic ulcer patients

The pharmacokinetics of orally administered proguanil and its metabolites were determined in six healthy volunteers and in six peptic ulcer patients, before and after a 3-day course of cimetidine (400 mg given two times daily for 2 days and 400 mg on the third day 1 h before proguanil). Cimetidine significantly increased Cmax (P < 0.05), AUCo-alpha (P < 0.005) and elimination half-life t 1/2b of proquanil in plasma of healthy subjects. In ulcer patients, cimetidine significantly increased, AUCo-alpha (P < 0.05), elimination half life (P < 0.005) and Cmax. Cimetidine significantly reduced (P < 0.05) Total body clearance in both healthy subjects and in peptic ulcer patients. The Cmax and AUCo-alpha of the active metabolite cycloguanil was significantly decreased (P < 0.05) in both the healthy subjects and in the peptic ulcer patients. The Cmax of the inactive metabolite, 4-CPB was significantly decreased in healthy subjects and AUCo-alpha significantly decreased in peptic ulcer patients.

Review article: drug interactions with agents used to treat acid-related diseases

Patients with acid-related diseases often need to take multiple medications. Treatment of Helicobacter pylori infection often includes either a histamine type 2 (H2)-receptor antagonist or a proton pump (H+,K(+)-ATPase) inhibitor (proton pump inhibitor), administered in conjunction with one or more antimicrobials. Also, treatment for acid-related diseases often requires extended therapy during which many concomitant medications may be administered for concurrent disease states. Polypharmacy may be the result, particularly in elderly patients, who are at increased risk for both acid-related and many other diseases. Thus, it is important to understand the potential for clinically significant drug-drug interactions in this setting. H2-receptor antagonists and proton pump inhibitors can influence the pharmacokinetic profiles of other commonly administered medications by elevating intragastric pH, which can alter drug absorption, and by interacting with the cytochrome P (CYP) 450 enzyme system, which can affect drug metabolism and clearance. Such interactions are particularly important when they affect the pharmacokinetics of drugs with narrow therapeutic ranges (e.g. warfarin, digoxin). In these cases, drug-drug interactions can result in significant toxicity and even death. There are marked differences among H2-receptor antagonists and proton pump inhibitors in their potential for such interactions. The oldest drugs in each class, cimetidine and omeprazole, respectively, have the greatest potential to alter CYP activity and change the pharmacokinetics of other drugs. The most recently developed H2-receptor antagonist, famotidine, and the newer proton pump inhibitors, rabeprazole and pantoprazole, are much less likely to induce or inhibit CYP and thereby change the metabolism of other medications. These differences are important when choosing medications for the safe treatment of patients with acid-related diseases.

Effect of tacrine hydrochloride on hepatic drug metabolism

The aim was to assess tacrine hydrochloride (THA) as an inhibitor of rat hepatic oxidative enzymes. A model of hepatic microsome oxidative metabolism was established using antipyrine (AP) incubated with NADPH. AP and its metabolites, 3-hydroxymethyl antipyrine (HMA). 4-hydroxy antipyrine (OHA) and norantipyrine (NORA) were measured by high performance liquid chromatography (HPLC). Aliquots of 200, 400 and 600 microg/ml antipyrine were incubated with the microsomal preparation alone, with 20 microg/ml cimetidine or with 40, 80 or 200 microg/ml THA. Cimetidine inhibited HMA production by 35-38% (P<0.001) and OHA production by 49-52% (P<0.001). Incubation with the 3 concentrations of THA inhibited HMA production by 17%, 24% and 41% (P<0.001) and OHA production by 52%, 55% and 79%, respectively (P<0.001). NORA was identifiable when antipyrine was incubated with NADPH alone, but could not be identified after incubation with either cimetidine or THA. This study has shown that THA causes the inhibition of AP metabolism to HMA, OHA and possibly NORA. We suggest THA is an inhibitor of three different hepatic microsomal cytochrome P-450 enzyme sub-families.

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.

Inhibition and induction of cytochrome P450 and the clinical implications

The cytochrome P450s (CYPs) constitute a superfamily of isoforms that play an important role in the oxidative metabolism of drugs. Each CYP isoform possesses a characteristic broad spectrum of catalytic activities of substrates. Whenever 2 or more drugs are administered concurrently, the possibility of drug interactions exists. The ability of a single CYP to metabolise multiple substrates is responsible for a large number of documented drug interactions associated with CYP inhibition. In addition, drug interactions can also occur as a result of the induction of several human CYPs following long term drug treatment. The mechanisms of CYP inhibition can be divided into 3 categories: (a) reversible inhibition; (b) quasi-irreversible inhibition; and (c) irreversible inhibition. In mechanistic terms, reversible interactions arise as a result of competition at the CYP active site and probably involve only the first step of the CYP catalytic cycle. On the other hand, drugs that act during and subsequent to the oxygen transfer step are generally irreversible or quasi-irreversible inhibitors. Irreversible and quasi-irreversible inhibition require at least one cycle of the CYP catalytic process. Because human liver samples and recombinant human CYPs are now readily available, in vitro systems have been used as screening tools to predict the potential for in vivo drug interaction. Although it is easy to determine in vitro metabolic drug interactions, the proper interpretation and extrapolation of in vitro interaction data to in vivo situations require a good understanding of pharmacokinetic principles. From the viewpoint of drug therapy, to avoid potential drug-drug interactions, it is desirable to develop a new drug candidate that is not a potent CYP inhibitor or inducer and the metabolism of which is not readily inhibited by other drugs. In reality, drug interaction by mutual inhibition between drugs is almost inevitable, because CYP-mediated metabolism represents a major route of elimination of many drugs, which can compete for the same CYP enzyme. The clinical significance of a metabolic drug interaction depends on the magnitude of the change in the concentration of active species (parent drug and/or active metabolites) at the site of pharmacological action and the therapeutic index of the drug. The smaller the difference between toxic and effective concentration, the greater the likelihood that a drug interaction will have serious clinical consequences. Thus, careful evaluation of potential drug interactions of a new drug candidate during the early stage of drug development is essential.

Examination of metabolic pathways and identification of human liver cytochrome P450 isozymes responsible for the metabolism of barnidipine, a calcium channel blocker

1. In a human liver microsomal system, barnidipine was converted into three primary metabolites, an N-debenzylated product (M-1), a hydrolyzed product of the benzyl-pyrrolidine ester (M-3) and an oxidized product of the dihydropyridine ring (M-8). 2. Involvement of CYP3A in the three primary metabolic pathways was revealed by the following studies: (a) inhibition of CYP3A, (b) a correlation study using 10 individual human liver microsomes and (c) cDNA-expression studies. The secondary metabolites, M-2 and M-4 (pyridine forms of M-1 and M-3), were most likely generated from M-8 but were unlikely from M-1 or M-3. Involvement of CYP3A in the secondary pathways of metabolism is also suggested. 3. The possibility of interactions between barnidipine and coadministered drugs was examined in vitro. The formation rate of the primary metabolites was little affected by warfarin, theophylline, phenytoin, diclofenac and amitriptyline at concentrations of 200 microM, but was inhibited by glibenclamide, simvastatin and cyclosporin A. IC50 for the latter drugs was estimated to be > 200, 200 and 20 microM respectively, which was roughly > 200, 6000 and 50 times higher than their respective therapeutic plasma levels, suggesting that interactions with cyclosporin A, a CYP3A inhibitor, are of possible clinical relevance.