Drug interactions with antacids. Mechanisms and clinical significance

Cardiovascular Medications in Pregnancy: A Primer

Cardiovascular disease and cardiovascular disease-related disorders remain among the most common causes of maternal morbidity and mortality in the United States. Due to increased rates of obesity, delayed childbearing, and improvements in medical technology, greater numbers of women are entering pregnancy with preexisting medical comorbidities. Use of cardiovascular medications in pregnancy continues to increase, and medical management of cardiovascular conditions in pregnancy will become increasingly common. Obstetricians and cardiologists must familiarize themselves with the pharmacokinetics of the most commonly used cardiovascular medications in pregnancy and how these medications respond to the physiologic changes related to pregnancy, embryogenesis, and lactation.

Pharmacotherapeutics in older adults

The process of aging influences both pharmacodynamics and pharmacokinetics. In addition to this, the issue of the increased incidence of chronic diseases as the age of people and the effects of medications in older adults becomes very complex. This article will review the influence of the aging process on the absorption, distribution, metabolism, and excretion of drugs. Specific concerns of older adults, including drug groups and side effects of concern, drug-induced geriatric syndromes, and medication adherence, are also discussed.

Drug–drug interactions in oncology: Why are they important and can they be minimized?

Adverse drug-drug interactions are a major cause of morbidity and mortality. Cancer patients are at particularly high risk of such interactions because they commonly receive multiple medications, including cytotoxic chemotherapy, hormonal agents and supportive care drugs. In addition, the majority of cancer patients are elderly, and so require medications for co-morbid conditions such as cardiovascular, gastrointestinal, and rheumatological diseases. Furthermore, the age-related decline in hepatic and renal function reduces their ability to metabolize and clear drugs and so increases the potential for toxicity. Not all drug-drug interactions can be predicted, and those that are predictable are not always avoidable. However, increased awareness of the potential for these interactions will allow healthcare providers to minimize the risk by choosing appropriate drugs and also by monitoring for signs of interaction. This review considers the basic principles of drug-drug interactions, and presents specific examples that are relevant to oncology.

Adverse drug interactions and reactions in dermatology: current issues of clinical relevance

This article highlights several adverse drug interactions and reactions relevant to current dermatologic practice. Absorption interactions between drugs and compounds containing polyvalent cations, potential interactions between herbal and conventional medicines, the meaning of sulfa allergy, and adverse cutaneous reactions caused by epidermal growth factor receptor inhibitors are discussed.

pH-Dependent Dissolution in Vitro and Absorption in Vivo of Weakly Basic Drugs: Development of a Canine Model

PURPOSE

The aim of this research was to develop a pH-dependent canine absorption model for studying pH effect on both dissolution in vitro and pharmacokinetics in vivo using the weak bases ketoconazole and dipyridamole as model drugs.

METHODS

Ketoconazole and dipyridamole pH-dependent dissolution profiles in vitro were determined by dissolution test at different pH values using USP apparatus II and an Opt-Diss Fiber Optic UV System. In vivo absorption studies for ketoconazole and dipyridamole were performed with crossover design in three groups of beagle dogs under control (no treatment), pentagastrin, and famotidine treatments. Ketoconazole and dipyridamole plasma concentrations were quantified by gradient high performance liquid chromatography mass spectroscopy (HPLC MS/MS). Pharmacokinetic parameters were determined from individual plasma concentration vs. time profiles.

RESULTS

Ketoconazole and dipyridamole displayed pH-dependent dissolution. Increasing the pH of the dissolution medium from 1.2 to 6.8 reduced the extent of dissolution of ketoconazole and dipyridamole at 1 h by 96% and 92%, respectively. In vivo studies in dogs under control (no treatment), pentagastrin, and famotidine treatments show marked differences in systemic ketoconazole and dipyridamole exposure. Area under the concentration-time curve (AUC) increased more than 4-fold as compared to control group, whereas it increased nearly 30-fold for ketoconazole and 9-fold for dipyridamole with pentagastrin (gastric pH approximately 2-3) as compared to famotidine (gastric pH approximately 5-7.5) treatment.

CONCLUSIONS

This work demonstrates a pH-dependent dissolution in vitro and absorption in vivo for the weak bases ketoconazole and dipyridamole independent of food effects. This model is useful to examine pH-dependent effects on oral drug absorption and for screening formulations to overcome the pH dependency.

Comparison of pharmacokinetics and metabolism of desloratadine, fexofenadine, levocetirizine and mizolastine in humans

Abstract Absorption, distribution, metabolism and excretion of desloratadine, fexofenadine, levocetirizine, and mizolastine in humans have been compared. The time required to reach peak plasma levels (tmax) is shortest for levocetirizine (0.9 h) and longest for desloratadine (> or =3 h). Steady-state plasma levels are attained after about 6 days for desloratadine, 3 days for fexofenadine, 2-3 days for mizolastine and by the second day for levocetirizine. The apparent volume of distribution is limited for levocetirizine (0.4 L/kg) and mizolastine (1-1.2 L/kg), larger for fexofenadine (5.4-5.8 L/kg) and particularly large for desloratadine (approximately 49 l/kg). Fexofenadine and levocetirizine appear to be very poorly metabolized (approximately 5 and 14% of the total oral dose, respectively). Desloratadine and mizolastine are extensively metabolized. After administration of 14C-levocetirizine to healthy volunteers, 85 and 13% of the radioactivity are recovered in urine and faeces, respectively. In contrast, faeces are the preferential route of excretion for 14C-fexofenadine (80% vs. 11% of the radioactive dose in urine). The corresponding values are 41% (urine) and 47% (faeces) for 14C-desloratadine, 84-95% (faeces) and 8-15% (urine) for 14C-mizolastine. The absolute bioavailability is 50-65% for mizolastine; it is high for levocetirizine as the percentage of the drug eliminated unchanged in the 48 h urine is 77% of the oral dose; the estimation for fexofenadine is at least 33%; no estimation was found for desloratadine. Fexofenadine is a P-glycoprotein (P-gp) substrate and P-gp is certainly involved both in the poor brain penetration by the compound and, at least partially, in a number of observed drug interactions. An interaction of desloratadine with P-gp has been suggested in mice, whereas the information on mizolastine is very poor. The fact that levocetirizine is a substrate of P-gp, although weak in an in vitro model, could contribute to prevent drug penetration into the brain, whereas it is unlikely to be of any clinical relevance for P-gp-mediated drug interactions.

Clinically significant drug interactions with cholinesterase inhibitors: a guide for neurologists

Cholinesterase inhibitors are the only pharmacological class indicated for the treatment of mild to moderate Alzheimer's disease. These drugs are also being used off label to treat severe cases of Alzheimer's disease or vascular dementia and other disorders. The widespread use of cholinesterase inhibitors raises the possibility of their use in combination regimens, with the subsequent risk of deleterious drug-drug interactions in high-risk populations. The purpose of this review is to present the possible sources of pharmacokinetic or pharmacodynamic drug-drug interactions involving cholinesterase inhibitors. The four cholinesterase inhibitors (tacrine, donepezil, rivastigmine and galantamine) that are currently available have different pharmacological properties that expose patients to the risk of several types of drug interactions of nonequivalent clinical relevance. The principal proven clinically relevant drug interactions involve tacrine and drugs metabolised by the cytochrome P450 (CYP) 1A2 enzyme, as well as tacrine or donepezil and antipsychotics (which results in the appearance of parkinsonian symptoms). The bioavailability of galantamine is increased by coadministration with paroxetine, ketoconazole and erythromycin. It is of interest to note that because rivastigmine is metabolised by esterases rather than CYP enzymes, unlike the other cholinesterase inhibitors, it is unlikely to be involved in pharmacokinetic drug-drug interactions. Care must be taken to reduce the risk of inducing central (excitation, agitation) or peripheral (e.g. bradycardia, loss of consciousness, digestive disorders) hypercholinergic effects via drug interactions with cholinesterase inhibitors. A review of the literature does not reveal any alarming data but does highlight the need for prudent prescription, particularly when cholinesterase inhibitors are given in combination with psychotropics or antiarrhythmics. Possible interactions involving other often coprescribed antidementia agents (e.g. memantine, antioxidants, cognitive enhancers) remain an open area requiring particularly prudent use.

Pharmacokinetics and electrocardiographic pharmacodynamics of artemether-lumefantrine (Riamet) with concomitant administration of ketoconazole in healthy subjects

AIMS

To evaluate whether the potent CYP3A4 inhibitor ketoconazole has any influence on the pharmacokinetic and electrocardiographic parameters of the antimalarial co-artemether (artemether-lumefantrine) in healthy subjects.

METHODS

Sixteen subjects were randomized in an open-label, two period crossover design study. Subjects received a single dose of co-artemether (day 1) either alone or in combination with multiple oral doses of ketoconazole (400 mg on day 1 followed by 200 mg o.d. for 4 additional days). Serial blood samples were taken and assayed for artemether and its main active metabolite dihydroartemisinin (DHA), and lumefantrine.

RESULTS

The pharmacokinetics of artemether, its metabolite DHA, and lumefantrine were influenced by the presence of ketoconazole. AUC(0, infinity ) was increased from 320 to 740 ng ml-1 h (ratio 2.4, 90% CI 2.00, 2.86) for artemether, from 331 to 501 ng ml-1 h (ratio 1.7, 90% CI 1.40, 1.98) for DHA, and from 207 to 333 micro g ml-1 h (ratio 1.7, 90% CI 1.23, 2.21) for lumefantrine in the presence of ketoconazole. Cmax also increased in similar proportions for the three compounds (ratio 2.2 (90% CI 1.78, 2.83), 1.4 (90% CI 1.12, 1.74), and 1.3 (90% CI 0.96, 1.64), respectively). The terminal elimination half-life was increased for artemether (2.5 vs 1.9 h, 90% CI 1.12, 1.72) and DHA (3.1 vs 2.1 h, 90% CI 0.02, 3.36), but remained unchanged for lumefantrine (88 vs 95 h, 90% CI 0.81, 1.04). These increases in exposure to the antimalarial combination were much smaller than observed with food intake (up to 16 fold), and were not associated with increased side-effects or changes in electrocardiographic parameters. The study medications were well tolerated.

CONCLUSIONS

The concurrent administration of ketoconazole with co-artemether led to modest increases in artemether, DHA, and lumefantrine exposure in healthy subjects. Dose adjustment of co-artemether is probably unnecessary in falciparum malaria patients when administered in association with ketoconazole or other potent CYP3A4 inhibitors.

Pharmacokinetics and gamma scintigraphy evaluation of two enteric coated formulations of didanosine in healthy volunteers

AIMS

The aims of the study were to evaluate the bioavailability of didanosine from the encapsulated enteric coated beads (1 x 200 mg; enteric beads) and enteric coated mini-tablets (4 x 50 mg; enteric tablet) formulations relative to the chewable/dispersible buffered tablets (2 x 100 mg; buffered tablet), and to study their rate of gastrointestinal transit.

METHODS

This was a single-dose, randomized, three-way crossover study in 18 healthy male volunteers. A 200 mg dose of didanosine was given in each period and each formulation contained a gamma radiation-emitting isotope. Pharmacokinetic parameters determined were Cmax, tmax, AUC(0, infinity ) and t1/2. Bioequivalence was assessed using the confidence interval (CI) of 0.80, 1.25 for Cmax and AUC(0, infinity ). Scintigraphic images were recorded and gastrointestinal transit profiles were generated.

RESULTS

The point estimate and 90% CI of the ratio of Cmax for the enteric beads and enteric tablet relative to the buffered tablet was 0.71 (0.59, 0.85) and 0.55 (0.46, 0.66), respectively. The tmax was significantly different for the enteric beads (median, 1.33 h) and the enteric tablet (median, 2.83 h) than for the buffered tablet (median, 0.67 h). The AUC(0, infinity ) satisfied the bioequivalence criteria, and the point estimate and 90% CI of the ratio were 1.02 (0.91, 1.15) and 0.92 (0.82, 1.04) for the enteric beads and enteric tablet, respectively. The AUC(0, infinity ) values appeared to be less variable with the enteric beads (% CV = 19%) than with the enteric tablet (% CV = 33%). The t1/2 values were not significantly different between formulations, and the mean values ranged from 1.82 to 1.92 h. Inspection of the individual scintigraphy profiles and concentration-time curves suggested that didanosine was absorbed throughout the small intestine. Gastrointestinal transit parameters were higher for both enteric formulations than for the buffered tablet, indicating slower transit of the enteric formulations. Between the enteric formulations, gastric emptying was slower for the enteric beads than for the enteric tablet; however, plasma didanosine concentrations were observed sooner for the enteric beads, suggesting that the enteric coat for the beads dissolved more rapidly.

CONCLUSIONS

The enteric beads and enteric tablet formulations of didanosine were equivalent to the buffered tablet in their extent of absorption. Although the gastric emptying of the enteric tablet was faster, based on the rapid uncoating and the lower variability in AUC, the enteric beads were chosen for further clinical development.

Misconceptions of older adults with hypertension concerning OTC medications and alcohol

Knowledge and self-efficacy concerning interactions of antihypertensives with over-the-counter (OTC) analgesics and alcohol were assessed in 51 adults aged 60 and older taking antihypertensives and attending a blood pressure clinic. The subjects had low self-efficacy about how to prevent interactions of antihypertensives with OTC analgesics and alcohol. Inspection of knowledge item responses revealed eight general misconceptions about OTC medications. These data guide educating those with hypertension about potential drug interactions arising from self-medication.

Drug interactions with irbesartan

Irbesartan is an angiotensin II receptor antagonist indicated for the treatment of patients with hypertension. Although irbesartan does not require biotransformation for its pharmacological activity, it does undergo metabolism via the cytochrome P450 (CYP) 2C9 isoenzyme and negligible metabolism by the CYP3A4 isoenzyme. The long term treatment of patients with hypertension is generally required for effective management of the disease, and the use of concurrent medications is usually inevitable. This paper reviews the drug and food interaction trials involving irbesartan that have been conducted to date. Based on the available literature, no significant interactions have been identified between irbesartan and hydrochlorothiazide, nifedipine, simvastatin, tolbutamide, warfarin, magnesium and aluminum hydroxides, digoxin or food. Fluconazole did increase the steady-state peak plasma concentration (by 19%) and area under the concentration-time curve (by 63%) of irbesartan, but these increases are not likely to be clinically significant. In summary, irbesartan has demonstrated minimal potential for drug or food interactions in trials conducted to date.

Bioavailability of the oral selective phosphodiesterase 4 inhibitor cilomilast

STUDY OBJECTIVE

To determine the absolute bioavailability of cilomilast, and assess the effects of food, dosing time, and coadministration of antacid agents on its bioavailability and pharmacokinetics in healthy volunteers.

SETTING

Clinical pharmacology unit.

DESIGN

Five prospective pharmacokinetic studies: one single-blind, dose-escalating, placebo-controlled trial; four open-label, randomized studies.

SUBJECTS

Ninety-six healthy adult volunteers who were nonsmokers.

INTERVENTION

In the first study, four subjects received intravenous cilomilast 1, 2, and 4 mg. In the second study, 16 subjects received oral cilomilast 15 mg or intravenous cilomilast 4 mg. In the other three studies, a total of 76 subjects were given single oral 15-mg doses; one study compared its effects in fed versus fasted subjects, one looked for differences of morning versus evening dosing, and one examined coadministration with aluminum hydroxide-magnesium hydroxide.

MEASUREMENTS AND MAIN RESULTS

After intravenous administration of cilomilast, plasma concentrations increased in an approximately dose-proportional manner; the half-life, approximately 6.5 hours, was dose independent. Cilomilast clearance and volume of distribution were small. After oral dosing, the absolute bioavailability was consistently close to 100%. Absorption was slower in fed subjects than in fasted (median 2-hr delay in time to reach maximum plasma concentration, average 39% reduction in maximum plasma concentration), but the area under the concentration-time curve from time zero to infinity (systemic availability) was unaffected. Pharmacokinetic parameters were not influenced by time of dosing or coadministration of antacid.

CONCLUSION

The absolute bioavailability of oral cilomilast was 100%; it was not adversely affected by time of dosing or coadministration with food or antacid.

The effects of ketoconazole on ziprasidone pharmacokinetics--a placebo-controlled crossover study in healthy volunteers

AIMS

To assess the effects of multiple oral doses of ketoconazole on the single-dose pharmacokinetics of oral ziprasidone HCl.

METHODS

This was a 14-day, open-label, randomized, crossover study in 14 healthy subjects aged 18-31 years. Group 1 received oral ketoconazole 400 mg once daily for 6 days, followed by a 2 day wash-out period and 6 days of placebo administration. Group 2 received placebo followed by ketoconazole. Single oral doses of ziprasidone HCl 40 mg were administered on days 5 and 13 in both groups. Ziprasidone pharmacokinetic parameters were compared between placebo and ketoconazole administration periods.

RESULTS

Co-administration of ziprasidone with ketoconazole was associated with a modest increase in ziprasidone exposure; mean ziprasidone AUC(0,infinity) increased by 33%, from 899 ng ml(-1) h with placebo to 1199 ng ml(-1) h with ketoconazole. Mean Cmax increased by 34%, from 89 ng ml(-1) to 119 ng ml(-1), respectively. The treatment effect on both of these parameters was statistically significant (P<0.02). Most adverse events were of mild intensity. There were no serious adverse events, laboratory abnormalities, abnormal ECGs, or clinically significant alterations in vital signs throughout the study.

CONCLUSIONS

The concurrent administration of ketoconazole and ziprasidone led to modest, statistically significant increases in ziprasidone exposure, although the changes seen were not considered clinically relevant. This suggests that other inhibitors of CYP3A4 are unlikely to significantly affect the pharmacokinetics of ziprasidone.

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.

Drug, meal and formulation interactions influencing drug absorption after oral administration. Clinical implications

Drug-drug, drug-formulation and drug-meal interactions are of clinical concern for orally administered drugs that possess a narrow therapeutic index. This review presents the current status of information regarding interactions which may influence the gastrointestinal (GI) absorption of orally administered drugs. Absorption interactions have been classified on the basis of rate-limiting processes. These processes are put in the context of drug and formulation physicochemical properties and oral input influences on variable GI physiology. Interaction categorisation makes use of a biopharmaceutical classification system based on drug aqueous solubility and membrane permeability and their contributions towards absorption variability. Overlaying this classification it is important to be aware of the effect that the magnitudes of drug dosage and volume of fluid administration can have on interactions involving a solubility rate limits. GI regional differences in membrane permeability are fundamental to the rational development of extended release dosage forms as well as to predicting interaction effects on absorption from immediate release dosage forms. The effect of meals on the regional-dependent intestinal elimination of drugs and their involvement in drug absorption interactions is also discussed. Although the clinical significance of such interactions is certainly dependent on the narrowness of the drug therapeutic index, clinical aspects of absorption delays and therapeutic failures resulting from various interactions are also important.

Drug interactions and anti-infective therapies

Drug interactions are an important and often underappreciated cause of adverse clinical outcomes. This review considers the mechanisms for several clinically important drug interactions that involve the major classes of anti-infective agents. This approach is intended to complement the use of text-based references and computer databases so that physicians and pharmacists can avoid prescribing and dispensing drugs that have adverse interactions.

Concurrent administration of donepezil HCl and ketoconazole: assessment of pharmacokinetic changes following single and multiple doses

AIM

The aim of this study was to examine the pharmacokinetics of donepezil HCl and ketoconazole separately, and in combination, following administration of single and multiple oral doses.

METHODS

This was an open-label, randomized, three-period crossover study in healthy volunteers (n=21). During each treatment period, subjects received single daily doses of either donepezil HCl (5 mg), ketoconazole (200 mg), or a combination of both drugs for 7 consecutive days. Pharmacokinetic comparisons were made between treatment groups for the day 1 and day 7 profiles. Each treatment period was followed by a 3-week, drug-free washout period.

RESULTS

On both day 1 and day 7, a statistically significant difference was observed between the donepezil and the donepezil + ketoconazole treatment groups in terms of Cmax and AUC(0-24) of donepezil. The concurrent administration of both drugs resulted in a 12% greater Cmax (9.5 ng ml(-1) versus 8.4 ng ml(-1); P=0.01) and a 12% greater AUC(0-24) (135.2 ng h ml(-1) versus 118.7 ng h ml(-1); P=0.001) than donepezil alone on day 1, and a 26.8% greater Cmax (37.7 ng ml(-1) versus 27.6 ng ml(-1); P < 0.0001) and a 26.4% greater AUC(0-24) (680.9 ng h ml(-1) versus 501.0 ng h ml(-1); P<0.0001) than donepezil alone on day 7. In contrast, ketoconazole plasma concentrations were unaffected by the concurrent administration of donepezil, and there were no statistically significant differences in ketoconazole pharmacokinetics when ketoconazole administered alone was compared with ketoconazole administered with donepezil.

CONCLUSIONS

The concurrent administration of ketoconazole and donepezil produces no change in ketoconazole plasma concentrations, but a statistically significant change in donepezil plasma concentrations. These observed changes, which are smaller than those produced by ketoconazole for other agents sharing the CYP-3A4 pathway, are most likely the result of donepezil also being metabolized by CYP-2D6, as well as its slow rate of clearance from plasma.