Pharmacokinetic interactions with digoxin

Drug Interactions Affecting Antiarrhythmic Drug Use

Antiarrhythmic drugs (AAD) play an important role in the management of arrhythmias. Drug interactions involving AAD are common in clinical practice. As AADs have a narrow therapeutic window, both pharmacokinetic as well as pharmacodynamic interactions involving AAD can result in serious adverse drug reactions ranging from arrhythmia recurrence, failure of device-based therapy, and heart failure, to death. Pharmacokinetic drug interactions frequently involve the inhibition of key metabolic pathways, resulting in accumulation of a substrate drug. Additionally, over the past 2 decades, the P-gp (permeability glycoprotein) has been increasingly cited as a significant source of drug interactions. Pharmacodynamic drug interactions involving AADs commonly involve additive QT prolongation. Amiodarone, quinidine, and dofetilide are AADs with numerous and clinically significant drug interactions. Recent studies have also demonstrated increased morbidity and mortality with the use of digoxin and other AAD which interact with P-gp. QT prolongation is an important pharmacodynamic interaction involving mainly Vaughan-Williams class III AAD as many commonly used drug classes, such as macrolide antibiotics, fluoroquinolone antibiotics, antipsychotics, and antiemetics prolong the QT interval. Whenever possible, serious drug-drug interactions involving AAD should be avoided. If unavoidable, patients will require closer monitoring and the concomitant use of interacting agents should be minimized. Increasing awareness of drug interactions among clinicians will significantly improve patient safety for patients with arrhythmias.

A New Score to Assess the Perioperative Period of the Cancer Patient Undergoing Non-Palliative Elective Surgery: A Retrospective Evaluation of a Case Report by PERIDIA Score

The complexity of cancer patients and the use of advanced and demolitive surgical techniques frequently need post-operatory ICU hospitalization. To increase safety and to select the best medical strategies for the patient, a multidisciplinary team has performed a new peri-operatory assessment, arising from evidence-based literature data. Verifying that most of the cancer patients, admitted to the intensive care unit, undergo major surgery with localizations in the supramesocolic thoraco-abdominal area, the team focused the attention on supramesocolic peridiaphragmatic cancer surgery. Some scores already in use in clinical practice were selected for the peri-operatory evaluation process. None of them evaluate parameters relating to the entire peri-operative period. In detail, only a few study models were found that concern the assessment of the intra-operative period. Therefore, we wanted to see if using a mix of validated scores, it was possible to build a single evaluation score (named PERIDIAphragmatic surgery score or PERIDIA-score) for the entire peri-operative period that could be obtained at the end of the patient's hospitalization period in post-operative ICU. The main property sought with the creation of the PERIDIA-score is the proportionality between the score and the incidence of injuries, deaths, and the length of stay in the ward. This property could organize a tailor-made therapeutic path for the patient based on pre-rehabilitation, physiotherapy, activation of social assistance services, targeted counseling, collaborations with the continuity of care network. Furthermore, if the pre-operative score is particularly high, it could suggest different or less invasive therapeutic options, and if the intra-operative score is particularly high, it could suggest a prolongation of hospitalization in ICU. The retrospective prospective study conducted on 83 patients is still ongoing. The first data would seem to prove an increase of clinical complications in patients who were assigned a one-third score with respect to the maximum (16/48) of PERIDIA-score. Moreover, patients with a 10/16 score within each phase of the evaluation (pre, peri, and post) more frequently develop injuries. In the light of these evidence, the 29-point score assigned to our patient can be considered as predictive for the subsequent critical and fatal complications the patient faced up.

The pharmacokinetics of para-aminosalicylic acid and its relationship to efficacy and intolerance

Following its introduction as an antituberculosis agent close to 75 years ago, the use of para-aminosalicylic acid (PAS) has been limited by gastrointestinal intolerance and multiple formulations were produced in attempts to reduce its occurrence. More recently, an enteric-coated, granular, slow-release PAS formulation (PASER) was introduced and is now in wide-spread use for the treatment of drug-resistant tuberculosis. The current PASER dosing regimen is based on recommendations derived from older studies using a variety of different PAS formulations and relegate PAS to a role as an exclusively bacteriostatic agent. However, there is ample evidence that if sufficiently high serum concentrations are reached, PAS can be bactericidal and that intolerance following once daily dosing, that aids the achievement of such concentrations, is no worse than that following intermittent daily dosing. In particular, prevention of resistance to companion drugs appears to be dependent on the size of the single dose, and hence the peak concentrations, and not on maintaining serum levels consistently above minimum inhibitory concentration. We present a narrative review of the development of PAS formulations, dosing practices, and published data regarding pharmacokinetics and pharmacodynamics and the relationship of PAS dosage to intolerance and efficacy. Our conclusions suggests that we are at present not using PAS to its maximum ability to contribute to regimen efficacy and protect companion drugs.

Digoxin: Pharmacology and toxicology-A review

Digoxin is a cardiac glycoside used as drug in case of heart problems, including congestive heart failure, atrial fibrillation or flutter, and certain cardiac arrhythmias. It has a very narrow therapeutic window of the medication. Digoxin is toxic substance with well known cardiotoxic effect. In this work, pharmacology and toxicology of digoxin are summarized; Its pharmacokinetics, pharmacodynamics, available acute toxicity data (different species, different administration routes) are summarized in this article. Moreover, its treatment side effect and human poisonings are thoroughly discussed. Finally, appropriate therapy regimen is proposed.

A physiologically based pharmacokinetic - pharmacodynamic modelling approach to predict incidence of neutropenia as a result of drug-drug interactions of paclitaxel in cancer patients

Paclitaxel is the backbone of standard chemotherapeutic regimens used in a number of malignancies and is frequently given with concomitant medications. Newly developed oncolytic agents, including tyrosine kinase inhibitors are often shown to be CYP3A4 and P-gp inhibitors. The aim of this study was to develop a PBPK model for intravenously administered paclitaxel in order to predict the incidence of neutropenia and to estimate the DDI potential as a victim drug. The dose-dependent effects on paclitaxel plasma protein binding, volume of distribution and drug clearance were considered for dose levels of 80 mg/m², 135 mg/m² and 175 mg/m². A pharmacodynamics model that incorporate the impact of paclitaxel on the neutrophil was developed. The relative metabolic clearance via CYP3A4 and CYP2C8, the renal clearance as well as P-gp mediated biliary clearance were incorporated in the model in order to assess the neutropenia in the presence of DDI. The developed PBPK-PD model was able to recover the drop in neutrophils observed after the administration of 175mg/m2 of paclitaxel over a 3-h duration. The mean nadir observed was 1.9 × 10⁹ neutrophils/L and was attained after 10 days of treatment, and a fraction of 47% of the population was predicted to have at some point a neutropenia including 12% with severe neutropenia. In the case of concomitant administration of ketoconazole, 39% of the population was predicted to suffer from severe neutropenia. In summary, PBPK-PD modeling allows a priori prediction of DDIs and safety events involving complex combination therapies which are often utilized in an oncology setting.

Effect of Chronic Kidney Diseases on Mortality among Digoxin Users Treated for Non-Valvular Atrial Fibrillation: A Nationwide Register-Based Retrospective Cohort Study

PURPOSE

This study investigated the impact of chronic kidney disease on all-causes and cardiovascular mortality in patients with atrial fibrillation treated with digoxin.

METHODS

All patients with non-valvular atrial fibrillation and/or atrial flutter as hospitalization diagnosis from January 1, 1997 to December 31, 2012 were identified in Danish nationwide administrative registries. Cox proportional hazard model was used to compare the adjusted risk of all-causes and cardiovascular mortality among patients with and without chronic kidney disease and among patients with different chronic kidney disease stages within 180 days and 2 years from the first digoxin prescription.

RESULTS

We identified 37,981 patients receiving digoxin; 1884 patients had the diagnosis of chronic kidney disease. Cox regression analysis showed no statistically significant differences in all-causes (Hazard Ratio, HR 0.89; 95% confident interval, CI 0.78-1.03) and cardiovascular mortality (HR 0.88; 95%CI 0.74-1.05) among patients with and without chronic kidney disease within 180 days of follow-up period. No statistically significant differences was found using a 2 years follow-up period neither for all causes mortality (HR 0.90; 95%CI 0.79-1.03), nor for cardiovascular mortality (HR 0.87; 95%CI 0.74-1.02). No statistically significant differences was found comparing patients with and without estimated Glomerular Filtration Rate <30ml/min/1.73m2 and patients with different stages of chronic kidney disease, for all-causes and cardiovascular mortality within 180 days and 2 years from the first digoxin prescription.

CONCLUSIONS

This study suggest no direct effect of chronic kidney disease and chronic kidney disease stages on all-causes and cardiovascular mortality within both 180 days and 2 years from the first digoxin prescription in patients treatment-naïve with digoxin for non-valvular atrial fibrillation.

Application of permeability-limited physiologically-based pharmacokinetic models: part I-digoxin pharmacokinetics incorporating P-glycoprotein-mediated efflux

A prerequisite for the prediction of the magnitude of P-glycoprotein (P-gp)-mediated drug-drug interactions between digoxin and P-gp inhibitors (e.g. verapamil and its metabolite norverapamil) or P-gp inducers (e.g. rifampicin) is a predictive pharmacokinetic model for digoxin itself. Thus, relevant in vitro metabolic, transporter and inhibitory data incorporated into permeability-limited models, such as the "advanced dissolution, absorption and metabolism" (ADAM) module and the permeability-limited liver (PerL) module, integrated with a mechanistic physiologically-based pharmacokinetic (PBPK) model such as that of the Simcyp Simulator (version 12.2) are necessary. Simulated concentration-time profiles of digoxin generated using the developed model were consistent with observed data across 31 independent studies [13 intravenous single dose (SD), 12 per oral SD and six multiple dose studies]. The fact that predicted tmax (time of maximum plasma concentration observed) and Cmax (maximum plasma concentration observed) of oral digoxin were similar to observed values indicated that the relative contributions of permeation and P-gp-mediated efflux in the model were appropriate. There was no indication of departure from dose proportionality over the dose range studied (0.25-1.5 mg). All dose normalised area under the plasma concentration-time curve profiles (AUCs) for the 0.25, 0.5, 0.75 and 1 mg doses resembled each other. Thus, PBPK modelling in conjunction with mechanistic absorption and distribution models and reliable in vitro transporter data can be used to assess the impact of dose on P-gp-mediated efflux (or otherwise).

No Pharmacokinetic or Pharmacodynamic Interaction Between Digoxin and the Oral Direct Thrombin Inhibitor Ximelagatran in Healthy Volunteers

The interaction potential of digoxin and ximelagatran, an oral direct thrombin inhibitor being developed for the prevention and treatment of thromboembolic disease, was investigated in this randomized, double-blind, 2-way crossover study. On 2 separate occasions, healthy female and male volunteers (n = 16) received ximelagatran 36 mg or placebo twice daily for 8 days separated by a 4- to 14-day washout period. All volunteers received a single oral dose of digoxin 0.5 mg on day 4 of both study periods. No interaction between ximelagatran and digoxin was detected in the pharmaco-kinetic parameters (using a 90% confidence interval [CI] of least squares mean estimate ratios), including melagatran (the active form of ximelagatran) AUC(tau) and C(max) and digoxin AUC(t) and C(max). Digoxin did not alter the melagatran-induced prolongation of the activated partial thromboplastin time, and both drugs were well tolerated when administered in combination. In conclusion, no pharmacokinetic or pharmacodynamic interaction between digoxin and ximelagatran was observed in this study.

Absence of Clinically Relevant Interactions between Rivaroxaban - an Oral, Direct Factor Xa Inhibitor - and Digoxin or Atorvastatin in Healthy Subjects

Objective:

To investigate potential interactions between rivaroxaban, an oral direct Factor Xa inhibitor approved for the management of thromboembolic disorders, and digoxin or atorvastatin.

Methods:

Two randomized, phase 1 clinical trials were undertaken in healthy men to assess pharmacokinetic and pharmacodynamic interactions between rivaroxaban and digoxin or atorvastatin, and the safety of these drug combinations.

Results:

Steady-state rivaroxaban did not affect the pharmacokinetic profile of steady-state digoxin ( n = 17). Digoxin did not significantly influence the pharmacokinetic profile of single-dose rivaroxaban and had minimal effects on rivaroxaban-induced inhibition of Factor Xa activity and prolongation of clotting time. Similarly, steady-state atorvastatin did not affect the pharmacokinetic profile or the pharmacodynamics of rivaroxaban and vice versa ( n = 19). All drugs (alone or in combination) were well tolerated.

Conclusions:

There were no clinically relevant pharmacokinetic or pharmaco - dynamic interactions between rivaroxaban and digoxin, or between rivaroxaban and atorvastatin, suggesting that rivaroxaban can be coadministered with either drug. This study also confirmed that rivaroxaban does not interact with substrates for permeability (P)-glycoprotein alone (digoxin) or P-glycoprotein and cytochrome P450(CYP)3A4 (atorvastatin).

Common drug interactions leading to adverse drug events in the intensive care unit: management and pharmacokinetic considerations

Critically ill patients are predisposed to drug interactions because of the complexity of the drug regimens they receive in the intensive care setting. Drugs may affect the absorption, distribution, metabolism, and/or elimination of an object drug and consequently alter the intended pharmacologic response and potentially lead to an adverse event. Certain disease states that afflict critically ill patients may also amplify an intended pharmacologic response and potentially result in an unintended effect. A team approach is important to identify, prevent, and address drug interactions in the intensive care setting and optimize patient outcomes.

Evaluation of the pharmacokinetics of digoxin in healthy subjects receiving etoricoxib

AIMS

Digoxin is a commonly prescribed cardiac glycoside with a narrow therapeutic index. The aim was to investigate whether the cyclooxygenase-2 selective nonsteroidal anti-inflammatory drug etoricoxib affects the steady-state pharmacokinetics of digoxin.

METHODS

This was a double-blind, randomized, placebo-controlled, two-period cross-over study. In each period, 14 healthy volunteers ranging in age from 21 to 35 years received oral digoxin 0.25 mg daily and were randomized to either etoricoxib 120 mg or matching placebo tablets once daily for 10 days. Trough digoxin plasma concentrations were analysed by linear regression to examine digoxin accumulation over time.

RESULTS

The geometric mean ratios (etoricoxib/placebo) for AUC(0-24h), C(max) and urinary excretion were 1.06 (90% confidence interval 0.97, 1.17), 1.33 (1.21, 1.46) and 1.10 (1.00, 1.20), respectively. The median (range) for digoxin T(max) (h) values with etoricoxib and placebo were 0.5 (0.5, 1.5) and 1.0 (0.5, 1.5), respectively. Steady-state digoxin plasma concentrations were achieved by day 7 in each treatment period. No serious adverse experiences were reported.

CONCLUSIONS

Although etoricoxib 120 mg did produce an approximately 33% increase in digoxin C(max), this increase does not appear to be clinically meaningful, as cardiotoxicity with digoxin has been associated with elevations in steady-state rather than peak concentrations. From these results, it appears that etoricoxib does not cause any changes in digoxin steady-state pharmacokinetics that would necessitate a dose adjustment.

Medications for Extensively Drug-Resistant Tuberculosis: Back to the Future?

Objective:

To reexamine the existing medications for the potential treatment of extensively drug-resistant tuberculosis (XDR-TB), based on susceptibility data, and to identify potential future medications from the literature.

Data Sources:

Relevant information was identified through a search of MEDLINE (1966–November 2007), PubMed (1955–November 2007), American Search Premier (1975–November 2007), International Pharmaceutical Abstracts (1960–November 2007), Science Citation Index Expanded (1996–November 2007), Cochrane Databases (publications archived until November 2007), and various tertiary sources as listed in the references, using the terms extensively drug-resistant tuberculosis (XDR-TB), ethambutol, pyrazinamide, para-aminosalicylic acid, cycloserine, linezolid, diarylquinoline, nitroimidazopyran, fluoroquinolones, β-lactams, new treatments, and ethionamide alone or in combination regimens.

Study Selection and Data Extraction:

After identification of the relevant information, the data presented in this article were selected based on clinical relevance and value of information.

Data Synthesis:

Based on susceptibility data, pyrazinamide, ethambutol, para-aminosalicylic acid, cycloserine, and ethionamide may be used for the treatment of tuberculosis. However, due to the emergence of XDR-TB, many of these agents are no longer successful treatment regimens. We have found limited data supporting potential future use of β-lactams, clarithromycin, and linezolid in resistant TB infections. TMC207, nitroimidazopyran, and SQ109 compounds may also prove to be viable options in the near future for treatment of tuberculosis, especially in cases with resistance to mainstay medications.

Conclusions:

Extensively resistant tuberculosis appears to be a potentially catastrophic disease if allowed to spread. Due to its resistance profile, very few potentially effective agents are available, calling attention to this growing problem.

Effect of macrolide antibiotics on uptake of digoxin into rat liver

The objective of this study was to examine the effect of macrolide antibiotics, clarithromycin, erythromycin, roxithromycin, josamycin and azithromycin, on the hepatic uptake of digoxin. The uptake of [(3)H]digoxin was studied in rats in vivo, using the tissue-sampling single-injection technique, and in isolated rat hepatocytes in vitro. The uptake of [(3)H]digoxin into rat hepatocytes was concentration-dependent with a Michaelis constant (K(m)) of 445 nM. All the macrolide antibiotics inhibited the uptake of [(3)H]digoxin into rat hepatocytes in a concentration-dependent manner. However, clarithromycin did not affect the in vivo hepatic uptake of digoxin in rats. The in vivo permeability-surface area product of digoxin for hepatic uptake (PS(inf)) was estimated to be 12.5 ml/min/g liver from the present in vitro data, which is far larger than the hepatic blood flow rate (1.4 ml/min/g liver). Macrolide antibiotics at clinically relevant concentrations inhibit digoxin uptake by rat hepatocytes in vitro, but not in vivo, probably because hepatic uptake of digoxin in rats is blood flow-limited. Clinically observed digoxin-macrolide interaction in humans could be due to macrolide inhibition of hepatic digoxin uptake, if the uptake is permeation-limited.

The complexity of intestinal absorption and exsorption of digoxin in rats

The potential multiple carrier-mediated mechanisms involved in the transport of digoxin in rat intestine were investigated by the rapid filtration method in rat intestinal brush-border vesicles (BBMV) and in vitro Ussing chambers. The uptake of digoxin showed a typical overshoot phenomenon in the presence of an inward proton gradient and an outward bicarbonate gradient, or an outward glutathione gradient in BBMV. Good fitting to an equation consisting of both saturable and linear terms was obtained using non-linear regression analysis. GF120918, a specific P-gp inhibitor, significantly increased the absorptive permeability of digoxin in rat ileum (7.02 x 10(-7) cm/s versus 2.11 x 10(-6) cm/s with GF120918) but the addition of DIDS (0.5 mM), an anionic transporter inhibitor, or bromosulfophthalein (0.1 mM), an Oatp inhibitor, in the presence of GF120918 decreased the absorptive permeability compared with GF120918 alone (2.11 x 10(-6) cm/s versus 1.46 x 10(-6) cm/s, p<0.01 and 2.11 x 10(-6) cm/s versus 1.60 x 10(-6) cm/s, p<0.05, respectively). The above results suggest the involvement of carrier-mediated uptake mechanism, possibly Oatp, in digoxin absorption. Interestingly, GF120918 (1 microM) did not abolish the polarized transport of digoxin in rat jejunum and ileum, and DIDS (0.5 mM), not a P-gp inhibitor, and MK571 (50 microM), an MRP-selective inhibitor, can also significantly decrease the exsorptive permeability of digoxin. This result indicates the involvement of non-P-gp efflux transporter in digoxin secretion and this transporter is DIDS and MK571-sensitive. Contrary to conventional concept, our studies show that intestinal absorption of digoxin may involve both active uptake and efflux transporters. Our study may have clinical implications in drug-drug or drug-food interactions involving transporters.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

Vascular binding, blood flow, transporter, and enzyme interactions on the processing of digoxin in rat liver

The roles of vascular binding, flow, transporters, and enzymes as determinants of the clearance of digoxin were examined in the rat liver. Digoxin is metabolized by Cyp3a and utilizes the organic anion transporting polypeptide 2 (Oatp2) and P-glycoprotein (Pgp) for influx and excretion, respectively. Uptake of digoxin was found to be similar among rat periportal (PP) and perivenous (PV) hepatocytes isolated by the digitonin-collagenase method. The Km values for uptake were 180 +/- 112 and 390 +/- 406 nM, Vmax values were 13 +/- 8 and 18 +/- 4.9 pmol/min/mg protein, and nonsaturable components were 9.2 +/- 1.3 and 10.7 +/- 2.5 microl/min/mg for PP and PV, respectively. The evenness of distribution of Oatp2 and Pgp was confirmed by Western blotting and confocal immunofluorescent microscopy. When digoxin was recirculated to the rat liver preparation in Krebs-Henseleit bicarbonate (KHB) for 3 h in absence or presence of 1% bovine serum albumin (BSA) and 20% red blood cell (rbc) at flow rates of 40 and 10 ml/min, respectively, biexponential decays were observed. Fitted results based on compartmental analyses revealed a higher clearance (0.244 +/- 0.082 ml/min/g) for KHB-perfused livers over the rbc-albumin-perfused livers (0.114 +/- 0.057 ml/min/g) (P < 0.05). We further found that binding of digoxin to 1% BSA was modest (unbound fraction = 0.64), whereas binding to rbc was associated with slow on (0.468 +/- 0.021 min(-1)) and off (1.81 +/- 0.12 min(-1)) rate constants. We then used a zonal, physiologically based pharmacokinetic model to show that the difference in digoxin clearance was attributed to binding to BSA and rbc and not to the difference in flow rate and that clearance was unaffected by transporter or enzyme heterogeneity.

Role of Organic Anion Transporting Polypeptide 2 in Pharmacokinetics of Digoxin and β-Methyldigoxin in Rats

Recently, we found that potent P-glycoprotein (P-gp) inhibitors, such as verapamil and cyclosporin A, markedly modulated the pharmacokinetics of digoxin in rats, whereas they did not affect beta-methyldigoxin pharmacokinetics significantly. Digoxin is also a substrate of rat organic anion transporting polypeptide 2 (Oatp2). Here, we compared the magnitude of Oatp2-mediated drug interaction of digoxin and beta-methyldigoxin using amiodarone as an Oatp2 inhibitor in rats. Amiodarone (20 mg/kg) given intravenously significantly increased plasma levels and decreased biliary excretion, liver distribution, and intestinal distribution of digoxin administered intravenously at a dose of 10 mug/kg. Amiodarone also significantly decreased biliary excretion and liver distribution of beta-methyldigoxin, but the change in plasma levels of beta-methyldigoxin was quite small. These findings may give a clue in selecting these cardiac glycosides in clinical pharmacotherapy for patients receiving multiple drugs towards escape from Oatp2-mediated drug interactions.

Voriconazole does not affect the steady-state pharmacokinetics of digoxin

AIMS

Voriconazole is a triazole antifungal agent with potent fungicidal activity against Aspergillus species. Digoxin is a commonly prescribed cardiac glycoside with a narrow therapeutic index. This aim of this study was to investigate the effect of multiple-dose voriconazole on the steady-state pharmacokinetics of digoxin in healthy male volunteers.

METHODS

This was a double-blind, randomized, placebo-controlled, parallel-group study. All subjects received daily administration of oral digoxin for a total of 22 days (0.5 mg twice daily on day 1, 0.25 mg twice daily on day 2 and 0.25 mg once daily on days 3-22). In addition, on days 11-22 the subjects were randomized to receive either voriconazole (200 mg twice daily) or matching placebo.

RESULTS

Concomitant administration with voriconazole did not significantly alter the Cmax, AUCtau, tmax or CLR of digoxin at steady state. The ratios between groups for Cmax and AUCtau at day 22, corrected for baseline (day 10) were 109.8%[90% confidence interval (CI) 97.1, 124.1] and 100.5% (90% CI 91.4, 110.5), respectively. In addition, group mean Cmin values were similar in both treatment groups throughout the study. There were no significant differences between treatments with respect to the incidence of adverse events, all of which were classified as mild and transient in nature.

CONCLUSIONS

The steady-state pharmacokinetics of digoxin are not affected in a clinically relevant manner by the concomitant administration of voriconazole.

Common single nucleotide polymorphisms of the MDR1 gene have no influence on its mRNA expression level of normal kidney cortex and renal cell carcinoma in Japanese nephrectomized patients

In this study, we have quantified the mRNA expression levels of multidrug resistance gene 1 (MDR1) in the normal kidney cortex and renal cell carcinoma (RCC) segments from 24 Japanese nephrectomized patients by real-time polymerase chain reaction (PCR). The mRNA expression level of MDR1 in RCC segments was significantly decreased in comparison with each normal segment (P=0.0042, by Student's paired t-test). In addition, the ten common single nucleotide polymorphisms (SNPs) of the MDR1 gene in the patients were assessed using the PCR-restriction enzyme fragment length polymorphism method to investigate the influence of these SNPs on its mRNA expression levels. The allele frequencies of these SNPs were comparable with our previous report in the Japanese recipients of living-donor liver transplantation (Goto et al., Pharmacogenetics 12:451-457; 2002). MDR1 expression levels in the normal kidney cortex were independent on the five SNPs, which were polymorphic in the Japanese population. Furthermore, the effect of the SNPs on expression levels of MDR1 mRNA in RCC segments was not recognized. These findings suggest that the common SNPs in the MDR1 gene have no influence on the expression of its transcript in RCC segments as well as in the normal kidney cortex.

Ex Situ Inhibition of Hepatic Uptake and Efflux Significantly Changes Metabolism: Hepatic Enzyme-Transporter Interplay

The disposition of digoxin and the influence of the organic anion transporting polypeptide (Oatp)2 inhibitor rifampicin and the P-glycoprotein (P-gp) inhibitor quinidine on its hepatic disposition were examined in the isolated perfused rat liver. Livers from groups of rats were perfused in a recirculatory manner after a bolus dose of digoxin (10 microg), a dual substrate for Oatp2 and P-gp as well as CYP3A. Perfusions of digoxin were also examined in groups of rats in the presence of the inhibitors: rifampicin (100 microM) or quinidine (10 microM). In all experiments, perfusate samples were collected for 60 min. Digoxin and its primary metabolite were determined in perfusate and liver by liquid chromatography/mass spectrometry. The area under the curve (AUC) from 0 to 60 min was determined. The AUC +/- S.D. of digoxin was increased from control (3880 +/- 210 nM x min) by rifampicin (5200 +/- 240 nM x min; p < 0.01) and decreased by quinidine (3220 +/- 340 nM x min; P < 0.05). It is concluded that rifampicin limits the hepatic entrance of digoxin and reduced the hepatic exposure of digoxin to CYP3A by inhibiting the basolateral Oatp2 uptake transport, whereas quinidine increased the hepatic exposure of digoxin to CYP3A by inhibiting the canalicular P-gp transport. These data emphasize the importance of uptake and efflux transporters on hepatic drug metabolism.

A guide to selection and appropriate use of macrolides in skin infections

Dermatologists must be aware of the adverse effects of antimicrobial agents as well as various drug interactions that may influence the choice of drug as well as specific drug schedules. The development of modern antibacterials has improved the treatment of cutaneous bacterial infections. Macrolide antibacterials continue to be an important therapeutic class of drugs with established efficacy in a variety of skin infections. All macrolides inhibit protein synthesis by reversibly binding to the 23S ribosomal RNA in the 50S-subunit. Erythromycin, the prototype of macrolide antibacterials, was isolated from the metabolic products of a strain of Streptomyces erytherus in 1952. Originally, erythromycin was introduced as an alternative to penicillin because of its activity against the Gram-positive organisms. Numerous studies have demonstrated the efficacy and safety of erythromycin for various infectious diseases. Unfortunately, erythromycin is associated with a number of drawbacks including a narrow spectrum of activity, unfavorable pharmacokinetic properties, poor gastrointestinal tolerability, and a significant number of drug-drug interactions. Newer macrolides have been developed to address these limitations. The pharmacokinetics of azithromycin and clarithromycin allow for shorter dosing schedules because of prolonged tissue levels. The efficacy of azithromycin for the treatment of skin and soft tissue infections in adults and children is well established. The unique pharmakinetics of azithromycin makes it a suitable agent for the treatment of acne. Clarithromycin represents a clear advance in the macrolide management of patients with leprosy and skin infections with atypical mycobacteria. Dirithromycin and roxithromycin display no clinical or bacteriological adcantage over erythromycin despite a superior pharmacokinetic profile. An area of concern is the increasing macrolide resistance that is being reported with some of the common pathogens which may limit the clinical usefulness of this class of antimicrobial agents in future.

Impact of ginkgo biloba on the pharmacokinetics of digoxin

Many medications are known to alter digoxin pharmacokinetics, including the herbal medication St. John's wort. An open-labeled, randomized, crossover trial was conducted in eight healthy human volunteers to determine if ginkgo biloba (GB) also alters the pharmacokinetics of digoxin. On two occasions separated by 2 weeks, subjects ingested digoxin, 0.5 mg. One week prior to each study phase, half of the volunteers were randomly initiated on GB therapy, 80 mg three times daily, that continued until the end of the study phase. Immediately prior to and for 36 hours following digoxin ingestion, multiple blood samples were collected for digoxin plasma concentration determination. No significant difference between treatments was observed with respect to AUC(0- infinity ) (digoxin alone: 21.0 +/- 8.6 [ng/mL] x h; digoxin + GB: 25.6 +/- 13.2 [ng/mL] x h). Additionally, no significant difference between therapies was observed with respect to C(max), T(max), or Cl(o). In six subjects, k(e) and t(1/2) were able to be determined. These parameters also did not differ significantly between treatments. In conclusion, within the context of the specific GB product used during this investigation, the concomitant use of GB and digoxin did not appear to have any significant effect on the pharmacokinetics of orally administered digoxin in healthy volunteers.

Role of P-glycoprotein in pharmacokinetics and drug interactions of digoxin and beta-methyldigoxin in rats

Digoxin and beta-methyldigoxin were evaluated pharmacokinetically in terms of P-glycoprotein (P-gp)-mediated drug interactions in rats. Evaluation was made by measuring the effects of a potent P-gp inhibitor (verapamil, cyclosporin A) on in vitro efflux transport of these compounds across the everted small intestine, on in situ absorption from the small intestine, and on in vivo total plasma clearance (CL(total)) as well as biliary and urinary excretions after intravenous administration. Both the intestinal efflux transport and absorption of beta-methyldigoxin were approximately 1.5-fold greater than those of digoxin, probably due to its higher lipophilicity. Addition of verapamil (300 microM) significantly decreased the intestinal efflux transport and increased the intestinal absorption of digoxin. In contrast, the influence of verapamil on beta-methyldigoxin was small. Intravenous cyclosporin A (30 mg/kg) significantly decreased in vivo CL(total) and biliary excretion of digoxin, but affected little on beta-methyldigoxin clearances. These results suggest that P-gp-mediated drug interactions can easily occur in digoxin, but hardly in beta-methyldigoxin. These findings may give a clue in selecting these digitalis compounds in clinical use, towards escape from P-gp-mediated drug interactions or reduction of interindividual variations.

Comparison of in vitro P-glycoprotein screening assays: recommendations for their use in drug discovery

The ATP-dependent drug efflux pump P-glycoprotein (P-gp) affects the absorption and disposition of many compounds. P-gp may also play role in clinically significant drug-drug interactions. Therefore, it is important to find potential substrates or inhibitors of P-gp early in the drug discovery process. To identify compounds that interact with this transporter, several P-gp assays were validated and compared by testing a set of 28 reference compounds, including inhibitors of cytochrome P450 3A4 (CYP3A4). The assays included in silico predictions, inhibition assays (based on cellular uptake of rhodamine-123 or calcein AM), and functional assays (ATPase activity assay and transcellular transport assay, the latter for a subset of compounds). In addition, species differences were studied in an indirect fluorescence indicator screening assay and test systems expressing porcine, mouse, or human P-gp. Our results suggest that several P-gp assays should be used in combination to classify compounds as substrates or inhibitors of P-gp. Recommendations are given on screening strategies which can be applied to different phases of the drug discovery and development process.

Food-drug interactions

Interactions between food and drugs may inadvertently reduce or increase the drug effect. The majority of clinically relevant food-drug interactions are caused by food-induced changes in the bioavailability of the drug. Since the bioavailability and clinical effect of most drugs are correlated, the bioavailability is an important pharmacokinetic effect parameter. However, in order to evaluate the clinical relevance of a food-drug interaction, the impact of food intake on the clinical effect of the drug has to be quantified as well. As a result of quality review in healthcare systems, healthcare providers are increasingly required to develop methods for identifying and preventing adverse food-drug interactions. In this review of original literature, we have tried to provide both pharmacokinetic and clinical effect parameters of clinically relevant food-drug interactions. The most important interactions are those associated with a high risk of treatment failure arising from a significantly reduced bioavailability in the fed state. Such interactions are frequently caused by chelation with components in food (as occurs with alendronic acid, clodronic acid, didanosine, etidronic acid, penicillamine and tetracycline) or dairy products (ciprofloxacin and norfloxacin), or by other direct interactions between the drug and certain food components (avitriptan, indinavir, itraconazole solution, levodopa, melphalan, mercaptopurine and perindopril). In addition, the physiological response to food intake, in particular gastric acid secretion, may reduce the bioavailability of certain drugs (ampicillin, azithromycin capsules, didanosine, erythromycin stearate or enteric coated, and isoniazid). For other drugs, concomitant food intake may result in an increase in drug bioavailability either because of a food-induced increase in drug solubility (albendazole, atovaquone, griseofulvin, isotretinoin, lovastatin, mefloquine, saquinavir and tacrolimus) or because of the secretion of gastric acid (itraconazole capsules) or bile (griseofulvin and halofantrine) in response to food intake. For most drugs, such an increase results in a desired increase in drug effect, but in others it may result in serious toxicity (halofantrine).

Comparative safety of the different macrolides

The macrolides are generally well tolerated when used for the treatment of acute infections. Even when given long term for prophylaxis, there are few discontinuations due to side-effects. There are isolated reports of QT(c) prolongation in patients treated with erythromycin and other 14-membered-ring macrolides. Since the 14-membered-ring macrolides are metabolized by P450 isoenzymes, there is the potential for interaction with other therapeutic agents also metabolized in this way. Pharmacokinetic studies have demonstrated interactions between either erythromycin or clarithromycin and cyclosporin, cisapride, pimozide, disopyramide, astemizole, carbamazepine, midazolam, digoxin, hydroxymethylglutaryl-coenzyme A reductase inhibitors (i.e. 'statins') and warfarin. In patients receiving such concurrent therapy, azithromycin may be superior to erythromycin and clarithromycin.

Cilomilast: pharmacokinetic and pharmacodynamic interactions with digoxin

BACKGROUND

Cilomilast is an orally active, selective phosphodiesterase 4 inhibitor currently in clinical development for the treatment of chronic obstructive pulmonary disease.

OBJECTIVE

The purpose of this study was to examine the tolerability and steady-state pharmacokinetics of cilomilast and digoxin when coadministered at standard therapeutic doses in healthy adults.

METHODS

In an initial, open-label phase, healthy young adults received cilomilast 15 mg BID for 5 days. After a 7-day washout period, subjects entered a double-blind, crossover phase during which they received oral digoxin (375 microg once daily) for 2 consecutive 14-day periods with no intervening washout period. Cilomilast 15 mg BID or placebo was coadministered during the first 14-day period. Subjects then crossed over to the alternative treatment for the second 14-day period. Blood and urine samples were collected at appropriate times for evaluation of digoxin and cilomilast steady-state pharmacokinetic parameters. The size of the study was sufficient to achieve 90% power to correctly exclude an effect of cilomilast.

RESULTS

Twelve of the 16 subjects enrolled completed the study. There were 4 withdrawals--1 due to noncompliance, 1 due to a positive drug screening, and 2 due to adverse events. At steady state, cilomilast 15 mg BID had no significant effect on the steady-state pharmacokinetic parameters of digoxin, with 90% CIs for both primary end points--area under the plasma concentration-time curve (AUC) over a 24-hour dosing interval and minimum plasma concentration--completely contained within the specified interval for equivalence (0.80-1.25). A mean reduction in maximum observed plasma concentration of digoxin of 11% was observed during coadministration with cilomilast, and time to maximum concentration was delayed by a median of 1 hour, suggesting a small reduction in the rate of digoxin absorption. Digoxin did not appear to markedly affect cilomilast steady-state pharmacokinetics. The most frequently reported adverse event was headache.

CONCLUSIONS

Cilomilast 15 mg BID had no clinically significant effect on steady-state AUC or on predose trough plasma concentrations of digoxin (375 microg once daily). The steady-state pharmacokinetics of cilomilast 15 mg BID were similar whether administered alone or with digoxin at steady state (375 microg once daily).

Effect of rofecoxib on the pharmacokinetics of digoxin in healthy volunteers

The authors examined the effect of the cyclooxygenase-2 (COX-2) inhibitor, rofecoxib, at steady state on the pharmacokinetics of digoxin following a single dose in healthy subjects. Each healthy subject (N = 10) received rofecoxib (75 mg once daily) or placebo for 11 days in a double-blind, randomized, balanced, two-period crossover study. A single 0.5 mg oral dose of digoxin elixir was administered on the 7th day of each 11-day period. Each treatment period was separated by 14 to 21 days. Samples for plasma and urine immunoreactive digoxin concentrations were collected through 120 hours following the digoxin dose. No statistically significant differences between treatment groups were observed for any of the calculated digoxin pharmacokinetic parameters. For digoxin AUC(0-infinity), AUC(0-24), and Cmax, the geometric mean ratios (90% confidence interval) for (rofecoxib + digoxin/placebo + digoxin) were 1.04 (0.94, 1.14), 1.02 (0.94, 1.09), and 1.00 (0.91, 1.10), respectively. The digoxin median tmax was 0.5 hours for both treatments. The harmonic mean elimination half-life was 45.7 and 43.4 hours for rofecoxib + digoxin and placebo + digoxin treatments, respectively. Digoxin is eliminated renally. The mean (SD) cumulative urinary excretion of immunoreactive digoxin after concurrent treatment with rofecoxib or placebo was 228.2 (+/- 30.8) and 235.1 (+/- 39.1) micrograms/120 hours, respectively. Transient and minor adverse events occurred with similar frequency on placebo and rofecoxib treatments, and no treatment-related pattern was apparent. Rofecoxib did not influence the plasma pharmacokinetics or renal elimination of a single oral dose of digoxin.

Differential pharmacokinetics of digoxin in elderly patients

Digoxin remains one of the most commonly prescribed of all cardiac medications. The main indications for digoxin usage include atrial fibrillation and heart failure; both these conditions are more prevalent in older patients. Given the aging population and the increasing incidence of heart failure we would expect prescribing of digoxin to remain as frequent or to even increase in older patients. Older patients are also more likely to develop toxicity and diagnosis of digoxin toxicity can be difficult in this group. Numerous components contribute to the development of toxicity in older patients, ranging from aging-related changes in renal function or body mass to polypharmacy and possible interactions with digoxin. It is therefore important to understand how the pharmacokinetics of digoxin may be altered in the older population. Application of basic pharmacological principles may be helpful in anticipating these problems. This review describes the pharmacokinetics of digoxin, the changes in pharmacokinetics with increasing age and how concomitant disease states or drug interactions may affect the pharmacokinetics of digoxin. Greater knowledge about the causes and prevention of digoxin toxicity should further reduce the morbidity and mortality arising from digoxin toxicity, especially in the elderly population.

Effect of aging on the incidence of digoxin toxicity

OBJECTIVE

To evaluate the relationship of the therapeutic serum digoxin concentration (SDC) range (0.5-2 ng/mL, as recommended in previous clinical studies) with the incidence of digoxin toxicity during digoxin maintenance therapy.

METHODS

Subjects included all inpatients (n = 462) and outpatients (n = 437) receiving digoxin oral maintenance therapy for heart failure and/or atrial fibrillation with tachycardia at Kosei Hospital, Anjo, Japan. SDC and blood chemistry analysis were determined, and a 24-hour Holter electrocardiographic recording was performed when the SDC was at the presumed steady-state concentration.

RESULTS

Analysis of clinical data showed that there was an overlapping (toxic and nontoxic) range of SDCs in which the incidence of digoxin toxicity was patient-dependent (1.4-2.9 ng/mL). No patient exhibited signs or symptoms of digoxin toxicity when the SDC was <1.4 ng/mL; all patients had evidence of toxicity when the SDC was >3 ng/mL. Additionally, it was shown that the concentration range of this overlapping range tended to broaden and shift to lower concentrations with increasing age. Patients with signs of toxicity when their SDCs were in the overlapping range had normal serum creatinine, blood urea nitrogen, digoxin clearance, creatinine clearance, and potassium concentrations, except for a significantly higher mean age than patients without toxicity. The incidence of digoxin toxicity was dependent on increasing age in patients whose SDCs were within the recommended therapeutic range. Moreover, clinical evidence of digoxin toxicity in patients >71 years old was 26.5%, despite their SDCs falling between 1.4 and 2 ng/mL.

CONCLUSIONS

Increased age is most likely associated with enhanced susceptibility to digoxin toxicity, possibly due to unknown pharmacodynamic changes. This raises the possibility that patients >71 years show clinical evidence of digoxin toxicity despite having SDCs within the recommended therapeutic range.

Mechanisms underlying variability in response to drug therapy: implications for amiodarone use

Drug disposition can be described by the traditional processes of absorption, distribution, metabolism, and elimination. A contemporary view of these processes includes the concept that they are determined by the regulated activity of specific gene products. Such a view is an important step to an increased understanding of interindividual variability in drug disposition and in response to drug therapy. In addition, molecular mechanisms underlying common drug interactions are now being elucidated. Despite this new knowledge, little is understood about the molecular mechanisms determining the unusual pharmacokinetic and pharmacodynamic profile of amiodarone. These unusual characteristics include incomplete bioavailability, distribution to multiple tissue sites, extreme lipophilicity, biotransformation to an active metabolite, and very slow elimination of both parent drug and active metabolite. The drug also produces a range of important pharmacologic effects, including antiadrenergic effects that are apparent early during therapy, changes in cardiac repolarization that take longer to develop, and important extracardiac actions, including side effects and drug interactions. As a consequence of these pharmacokinetic and pharmacodynamic complexities, individualization of dose during long-term therapy with amiodarone has not been systematically explored.

Effect of multiple doses of montelukast, a CysLT1 receptor antagonist, on digoxin pharmacokinetics in healthy volunteers

The effect of multiple oral doses of montelukast, a cysLT1 receptor antagonist, on the pharmacokinetics of oral digoxin was studied in healthy male volunteers in a randomized double-blind two-period crossover study. Subjects received 10 mg of montelukast or placebo daily for 11 days. On day 7, they received a single 0.5 mg oral dose of digoxin elixir. The pharmacokinetic parameters of digoxin (AUC0-->24' AUC0-->infinity' Cmax' tmax' t1/2) and cumulative urinary excretion over 120 hours were not affected by the multiple doses of montelukast. The 90% confidence interval for each of these parameters fell within prespecified clinically acceptable bounds. Side effects were mild and transient. This suggests that concurrent administration of montelukast and digoxin was well tolerated. Concurrent treatment with montelukast has no effect on the pharmacokinetics of digoxin.

Pharmacokinetic and pharmacodynamic drug interactions between digoxin and macrogol 4000, a laxative polymer, in healthy volunteers

AIMS

The aim of this study was to examine the bioequivalence between a single oral dose of digoxin administered alone and with a coadministration of macrogol 4000 (a laxative polymer) in 18 healthy volunteers.

METHODS

This was an open, randomised, two-way cross-over study, with a single dose oral administration of 0.5 mg digoxin administered alone or in combination with macrogol 4000, 20 g day-1 during 8 days. Pharmacokinetics of digoxin, heart rate and PR ECG interval at rest were assessed.

RESULTS

Macrogol 4000 coadministration was associated with a 30% decrease of digoxin AUC and a 40% decrease in its Cmax (P<0.05). Digoxin tmax and t1/2,z were not significantly altered. Heart rate and PR interval did not differ during the two therapeutic sequences, digoxin alone and digoxin in combination.

CONCLUSIONS

Macrogol 4000 coadministration interacts with single-dose digoxin pharmacokinetics. This is most likely due to a reduction of the intestinal absorption of digoxin. However, there was no consequence of this interaction on heart rate and AV conduction.

Lack of mutual pharmacokinetic interaction between cerivastatin, a new HMG-CoA reductase inhibitor, and digoxin in healthy normocholesterolemic volunteers

The potential mutual interaction between cerivastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, and digoxin was assessed in this nonmasked, nonrandomized, multiple-dose study. The effect of cerivastatin 0.2 mg on mean plasma digoxin levels and the effect of digoxin on the single-dose pharmacokinetics of cerivastatin were assessed in 20 healthy normocholesterolemic men between 18 and 45 years of age weighing 140 to 200 lbs (63.3 to 90.0 kg). Subjects were given a single dose of cerivastatin 0.2 mg. After a 2-day washout period, subjects were given a loading dose of digoxin 0.5 mg for 3 days followed by 0.25 mg daily for 5 additional days (period 1-digoxin alone). Concurrent dosing with cerivastatin 0.2 mg continued for 14 days (period 2-digoxin and cerivastatin), followed by an 8-day course of digoxin-only administration and an optional 6-day extension of digoxin-only treatment for a total of 14 days (period 3). Safety was assessed through physical examination, electrocardiography, laboratory tests, and ophthalmologic examination. Ratio analyses of mean digoxin plasma trough levels, 24-hour urinary digoxin levels, and digoxin clearance with and without concurrent cerivastatin dosing also were carried out. In addition, single-dose pharmacokinetic variables for cerivastatin, including area under the curve (AUC(0-24)), peak concentration (C(max)), time to peak concentration (T(max)), and elimination half-life (t1/2), were examined with and without concurrent digoxin dosing. Eleven of the 20 subjects completed the entire study. Seven subjects discontinued the study because of treatment-emergent adverse events or laboratory abnormalities that were mostly unrelated to cerivastatin, and 2 subjects were discontinued because of protocol violations. Treatment-emergent adverse events developed in 12 subjects receiving cerivastatin; 11 of these subjects were receiving digoxin concurrently. Six adverse events that led to discontinuation of treatment were unrelated to cerivastatin but were related to digoxin or to a preexisting condition. The most commonly reported event was headache, which occurred with equal frequency compared with placebo groups in large cerivastatin clinical trials. Other events were mild or moderate and resolved without intervention. Mild and transient elevations in hepatic transaminase and creatine kinase values (all <2 times the upper limit of normal) were observed in 7 subjects. After 14 days of concurrent dosing of cerivastatin and digoxin, steady-state digoxin plasma levels, urinary digoxin levels, and urinary digoxin clearance were unchanged compared with steady-state digoxin levels when digoxin was given alone. Compared with dosing with digoxin alone, the AUC(0-24), Cmax, and t1/2 for cerivastatin increased 3%, 20%, and 7%, respectively, while the T(max) was reduced by 18% during concurrent treatment with digoxin. These changes are minimal and would not be expected to be clinically relevant. These results demonstrate that when cerivastatin is administered concurrently with digoxin, neither digoxin nor cerivastatin plasma levels are altered. The combination therapy was generally well tolerated.

Inhibition of P-glycoprotein-mediated drug transport: A unifying mechanism to explain the interaction between digoxin and quinidine [seecomments]

BACKGROUND

Although quinidine is known to elevate plasma digoxin concentrations, the mechanism underlying this interaction is not fully understood. Digoxin is not extensively metabolized, but it is known to be transported by the drug efflux pump P-glycoprotein, which is expressed in excretory tissues (kidney, liver, intestine) and at the blood-brain barrier. Accordingly, we tested the hypothesis that inhibition of P-glycoprotein-mediated digoxin transport by quinidine contributes to the digoxin-quinidine interaction.

METHODS AND RESULTS

First, we demonstrated active transcellular transport of both digoxin and quinidine in cultured cell lines that express P-glycoprotein in a polarized fashion. In addition, 5 micromol/L quinidine inhibited P-glycoprotein-mediated digoxin transport by 57%. Second, the effect of quinidine on digoxin disposition was studied in wild-type and in mdr1a(-/-) mice, in which the gene expressing the major digoxin-transporting P-glycoprotein has been disrupted. Because the in vitro data showed that quinidine itself is a P-glycoprotein substrate, quinidine doses were reduced in mdr1a(-/-) mice to produce plasma concentrations similar to those in wild-type control animals. Quinidine increased plasma digoxin concentrations by 73.0% (P=0.05) in wild-type animals, compared with 19.5% (P=NS) in mdr1a(-/-) mice. Moreover, quinidine increased digoxin brain concentrations by 73.2% (P=0.05) in wild-type animals; by contrast, quinidine did not increase digoxin brain concentrations in mdr1a(-/-) mice but rather decreased them (-30.7%, P<0.01).

CONCLUSIONS

Quinidine and digoxin are both substrates for P-glycoprotein, and quinidine is a potent inhibitor of digoxin transport in vitro. The in vivo data strongly support the hypothesis that inhibition of P-glycoprotein-mediated digoxin elimination plays an important role in the increase of plasma digoxin concentration occurring with quinidine coadministration in wild-type mice and thus support a similar mechanism in humans.

Ventricular rate control in chronic atrial fibrillation during daily activity and programmed exercise: a crossover open-label study of five drug regimens

OBJECTIVES

We compared the effects of five pharmacologic regimens on the circadian rhythm and exercise-induced changes of ventricular rate (VR) in patients with chronic atrial fibrillation (CAF).

BACKGROUND

Systematic comparison of standardized drug regimens on 24 h VR control in CAF have not been reported.

METHODS

In 12 patients (11 male, 69+/-6 yr) with CAF, the effects on VR by 5 standardized daily regimens: 1) 0.25 mg digoxin, 2) 240 mg diltiazem-CD, 3) 50 mg atenolol, 4) 0.25 mg digoxin + 240 mg diltiazem-CD, and 5) 0.25 mg digoxin + 50 mg atenolol; were studied after 2 week treatment assigned in random order. The VR data were analyzed by ANOVA with repeated measures. The circadian phase differences were evaluated by cosinor analysis.

RESULTS

The 24-h mean (+/-SD) values of VR (bpm) were - digoxin: 78.9 +/- 16.3, diltiazem: 80.0+/-15.5, atenolol: 75.9+/-11.7, digoxin + diltiazem: 67.3+/-14.1 and digoxin + atenolol: 65.0+/-9.4. Circadian patterns were significant in each treatment group (p < 0.001). The VR on digoxin + atenolol was significantly lower than that on digoxin (p < 0.0001), diltiazem (p < 0.0002) and atenolol (p < 0.001). The time of peak VR on Holter was significantly delayed with regimens 3 and 5 which included atenolol (p < 0.03). During exercise, digoxin and digoxin + atenolol treatments resulted in the highest and lowest mean VR respectively. The exercise Time-VR plots of all groups were nearly parallel (p = ns). The exercise duration was similar in all treatment groups (p = ns).

CONCLUSIONS

This study indicates that digoxin and diltiazem, as single agents at the doses tested, are least effective for controlling ventricular rate in atrial fibrillation during daily activity. Digoxin + atenolol produced the most effective rate control reflecting a synergistic effect on the AV node. The data provides a basis for testing the effects of chronic suppression of diurnal fluctuations of VR on left atrial and ventricular function in CAF.

Metabolism of digoxin and digoxigenin digitoxosides in rat liver microsomes: involvement of cytochrome P4503A

1. The sequential metabolism of digoxin (Dg3) to digoxigenin bis-digitoxoside (Dg2), digoxigenin mono-digitoxoside (Dg1) and digoxigenin (Dg0) was investigated in rat liver microsomes. 2. Kinetic studies produced results consistent with a single enzyme mechanism describing the successive oxidative cleavages. Formation of Dg2 was catalysed with mean (+/-SD) Km and Vmax of 125 +/- 22 microM and 362 +/- 37 pmol/min/mg protein, respectively. The corresponding values for the formation of Dg1 were 61 +/- 5 microM and 7 +/-1 pmol/min/mg protein. Dg0 formation was catalysed with the apparent values of 30 +/- 9 microM and 310 +/- 30 pmol/min/mg protein. 3. Chemical inhibition of cytochrome P450 (CYP) 3A subfamily with ketoconazole and triacetyoleandomycin decreased the formation of Dg2 and Dg1 by up to 90%. Antibodies specific to rat CYP3A2 lowered the rate of oxidative cleavage of Dg3 and Dg2 by up to 85%. Inhibition of CYP2E1, CYP2C subfamily and CYP1A2 by chemical and immuno-inhibition did not affect initial rates of metabolism of Dg3 and Dg2. In contrast, Dg1 metabolism was not affected by triacetyloleandomycin as well as by antibodies to CYP3A2, CYP2C11, CYP2E1, CYP2B1/2B2 and CYP1A2. It was however inhibited by >80% by gestodene and 17alpha-ethynylestradiol (selective inhibitors of human CYP3A). 4. Collectively, these data support the involvement of CYP3A in the cleavage of Dg3 and Dg2 in rat liver microsomes. The enzyme-metabolizing Dg1 remains to be identified.

Comparative tolerability of erythromycin and newer macrolide antibacterials in paediatric patients

The macrolides are a well established group of antibacterials frequently used in general practice. The most frequently used macrolides in paediatric patients are erythromycin, a naturally occurring compound, and clarithromycin and azithromycin, recently developed macrolides. Overall adverse effect rates of 7 to 26% for erythromycin, 14 to 26% for clarithromycin, and 6 to 27% for azithromycin have been described in children. Adverse gastrointestinal effects, including nausea, vomiting, diarrhoea and abdominal cramps, are the most common problems in children. Allergic reactions, hepatotoxicity, ototoxicity and adverse effects involving the central and peripheral nervous systems have also been observed in children. Stevens-Johnson, Schonlein-Henoch and Churg-Strauss syndromes have been rarely described in children. Treatment-related laboratory abnormalities have been recorded in 2 to 4% of erythromycin- and in 0 to 1% of both clarithromycin- and azithromycin-treated children. Elevation in liver function tests was the most common abnormality cited. Increased macrolide use in children in recent years has resulted in a growing potential for drug interactions between them and other pharmacologically active agents via the inhibition of cytochrome P450 (CYP) microsomal enzymes. Drug interactions with theophylline, cyclosporin, carbamazepine, terfenadine and warfarin limit erythromycin use. Clarithromycin is a weak inducer of CYP and exhibits fewer drug-drug interactions than erythromycin. However, its use with theophylline, carbamazepine and terfenadine is contraindicated. In contrast, no significant interactions have been reported with azithromycin to date. Macrolides have been proven to be well tolerated in the treatment of upper and lower respiratory tract infections, skin and soft tissue infections, and also in less frequent infections occurring in paediatric patients. In addition, clarithromycin and azithromycin have shown good tolerability profiles in immunocompromised paediatric patients. In conclusion, macrolides antibacterials have proven to be well tolerated in paediatric patients. Although the incidence of adverse effects is similar with the use of erythromycin and the newer macrolides, drug interactions occur significantly less when clarithromycin or azithromycin are administered.

Effect of troglitazone on steady-state pharmacokinetics of digoxin

Twelve healthy subjects participated in a study to determine the effect of multiple doses of troglitazone on the steady-state pharmacokinetics of digoxin. Subjects received digoxin 0.25 mg orally once daily on days 1 through 20 and 400 mg of troglitazone orally once daily on days 11 through 20. Serial plasma samples and 24-hour urine samples collected before and after the doses on days 10 and 20 were analyzed for digoxin using a radioimmunoassay method. Eleven subjects completed the study. Administration of multiple oral doses of digoxin and troglitazone was well tolerated. Mean values for maximum concentration (Cmax), time to Cmax (tmax), and area under the concentration-time curve from 0 to 24 hours (AUC0-24) of digoxin on day 10 were similar to those on day 20. Mean day 10 digoxin values for minimum concentration (Cmin), apparent oral clearance (Cl/F), total urinary excretion from 0 to 24 hours (Ae0-24), and renal clearance (Clr) were also similar to corresponding values on day 20. Thus, concomitant administration of multiple-dose troglitazone does not alter the steady-state pharmacokinetics of digoxin.

Lack of interaction between valaciclovir, the L-valyl ester of aciclovir, and digoxin

AIMS

Changes in both digoxin and aciclovir renal clearance following coadministration with some other renally eliminated drugs have been reported. The potential interaction of valaciclovir, with its antiherpetic metabolite aciclovir, and digoxin was investigated.

METHODS

Twelve healthy volunteers (seven males, five females) participated in an open, randomized, four-period crossover study. Valaciclovir, 1000 mg, was given alone on one occasion, and on another, after the second of two 0.75 mg digoxin doses administered 12 h apart. Blood samples and all urine were collected up to 12 h following the valaciclovir dose for aciclovir radioimmunoassay. On a third occasion, digoxin was given alone and on a fourth, with 1000 mg valaciclovir three times/day for 8 days starting 12 h before the first digoxin dose. Blood samples were taken up to 168 h and all urine collected up to 24 h following the second dose for digoxin radioimmunoassay.

RESULTS

There were no clinically significant differences in digoxin or aciclovir pharmacokinetic parameters when digoxin or valaciclovir was given alone or in combination.

CONCLUSIONS

No dosage adjustment is required when valaciclovir and digoxin are coadministered.