Digoxin Population Pharmacokinetics from Routine Clinical Data: Role of Patient Characteristics for Estimating Dosing Regimens

Population Pharmacokinetics of Digoxin in Nonagenarian Patients: Optimization of the Dosing Regimen

OBJECTIVE

The aim of this study was to develop a population pharmacokinetic model of digoxin in patients over 90 years old and to propose an equation for adjusting digoxin dose in this population.

METHODS

We included 326 nonagenarian patients admitted to Severo Ochoa University Hospital (Spain) who received digoxin and were under therapeutic drug monitoring. All data were retrospectively collected, and population modeling was performed with non-linear mixed-effect modeling software (NONMEM®). One- and two-compartment models were tested to calculate digoxin clearance (Cl), volume of distribution (Vd), absorption rate constant (Ka), and bioavailability (bioavailable fraction, F). The covariates were evaluated by stepwise covariate model building, and the final model was internally validated by bootstrap analysis with 1000 resamples. External validation was performed with another population of 95 patients with the same characteristics as the modeling group.

RESULTS

The population was 26% males, with a mean age of 93.2 years (90-103 years), mean creatinine 1.11 mg/dL (0.42-3.81 mg/dL), and mean total body weight 61.2 kg (40-100 kg). The pharmacokinetics of digoxin were best described by a one-compartment model (ADVAN2 TRANS2), with first-order conditional estimation with interaction. The covariates with influence on our model were creatinine clearance based on the Cockcroft-Gault equation (CG), serum potassium (K), co-administration of loop diuretics, and sex: Cl/F = 4.55 · (CG/36.4)0.468 · 0.83LD · 1.21SEX; Vd/F = 355 · (K/4.3)-0.849; Ka = 1.22 h-1 [where LD indicates loop diuretics (1 for administered, 0 for otherwise) and SEX indicates patient sex (1 for male, 0 for female)]. Based on our results, we proposed an equation to adjust the digoxin dosing regimen in nonagenarian patients: dose (mg) = 0.144 · (CG/36.4)0.468 · 0.83LD · 1.21SEX.

CONCLUSIONS

The greatest influence on digoxin clearance came from renal function calculated by the Cockcroft-Gault equation. Vd was decreased by K. The model developed showed a precise predictive performance to be applied for therapeutic drug monitoring.

The role of sex, age and genetic polymorphisms of CYP enzymes on the pharmacokinetics of anticholinergic drugs

There is evidence that use of drugs with anticholinergic properties increases the risk of cognitive impairment, and increased exposure to these drugs potentiates this risk. Anticholinergic drugs are commonly used even with associated risk of adverse events. Aging, sex, and genetic polymorphisms of cytochrome P450 (CYP) enzymes are associated with alterations in pharmacokinetic processes, which increase drug exposure and may further increase the risk of adverse drug events. Due to the increasing burden of cognitive impairment in our aging population and the future of personalized medicine, the objective of this review was to provide a critical clinical perspective on age, sex, and CYP genetic polymorphisms and their role in the metabolism and exposure to anticholinergic drugs. Age-related changes that may increase anticholinergic drug exposure include pseudocapillarization of liver sinusoidal endothelial cells, an approximate 3.5% decline in CYP content for each decade of life, and a reduction in kidney function. Sex-related differences that may be influenced by anticholinergic drug exposure include women having delayed gastric and colonic emptying, higher gastric pH, reduced catechol-O-methyl transferase activity, reduced glucuronidation, and reduced renal clearance and men having larger stomachs which may affect medication absorption. The overlay of poor metabolism phenotypes for CYP2D6 and CYP2C19 may further modify anticholinergic drug exposure in a significant proportion of the population. These factors help explain findings of clinical trials that show older adults and specifically older women achieve higher plasma concentrations of anticholinergic drugs and that poor metabolizers of CYP2D6 experience increased drug exposure. Despite this knowledge neither age, sex nor CYP phenotype are routinely considered when making decisions about the use or dosing of anticholinergic medications. Future study of anticholinergic medication needs to account for age, sex and CYP polymorphisms so that we may better approach personalized medicine for optimal outcomes and avoidance of medication-related cognitive impairment.

Population Pharmacokinetic Studies of Digoxin in Adult Patients: A Systematic Review

BACKGROUND

Digoxin is a cardiac glycoside that was introduced to cardiovascular medicine more than 200 years ago. Its use is associated with large variability, which complicates achieving the desired therapeutic outcomes.

OBJECTIVES

To present a synthesis of the available literature on the population pharmacokinetics of digoxin in adults and to identify the sources of variability in its pharmacokinetics.

METHODS

This is a PROSPERO registered systematic review (CRD42018105300). A literature search was conducted using the ISI Web of Science, Science Direct, PubMed, and SCOPUS databases to identify digoxin population pharmacokinetic studies of adults that utilized the nonlinear mixed-effect modeling approach.

RESULTS

Sixteen articles were included in the present analysis. Only two studies were conducted in elderly subjects as a separate population. Both the pharmacokinetics and pharmacodynamics of digoxin were investigated in one study. Furthermore, the reviewed studies were mostly conducted in East Asian populations (68.8%). Digoxin's pharmacokinetics were usually described by a one-compartment model because of the nature of the collected data. Weight, age, kidney function, presence of heart failure, and co-administered medications such as calcium channel blockers were the most commonly identified predictors of digoxin clearance. The value of apparent clearance in a typical study individual ranged from 0.005 to 0.2 l/h/kg, while the value of the apparent volume of distribution ranged from 3.14 to 15.2 l/kg. The quality of model evaluation was deemed excellent only in 31.3% of the studies.

CONCLUSION

This review provides information about variables that need to be considered when prescribing digoxin. The results highlight the need for prospective studies that allow two-compartment pharmacokinetic/pharmacodynamic models to be established, with a special focus on the elderly subpopulation.

Sex and gender differences in polypharmacy in persons with dementia: A scoping review

Purpose:

To date, research studies in most disciplines have not made sex-based analysis a priority despite increasing evidence of its importance. We now understand that both sex and gender impact medication prescribing, use, and effect. This is particularly true for older adults with dementia who have alterations in drug metabolism, drug response, and the permeability of the blood–brain barrier. To better understand the influence of sex and gender on drug use in older adults with dementia, we conducted a scoping review.

Methods:

This scoping review systematically searched the Medline, Embase, Web of Science, CINAHL, and ProQuest databases to find published reports on polypharmacy in populations of older adults with dementia that included a sex- or gender-based analysis.

Results:

A total of 12 published reports were identified. Findings were cohort studies and case-control trials that commented on sex-related differences in medication use as a secondary analysis to the studies’ primary objective. These studies showed that community-dwelling women received more potentially inappropriate medications and more psychotropic medications, while nursing home dwelling men received more potentially inappropriate medications, cholinesterase inhibitors, and antipsychotics. None of the identified studies explicitly examined gender-related differences in medication use.

Conclusion:

This scoping review supports that there is inadequate understanding of both sex and gender differences in drug use in older men and women with dementia. To tailor medication-specific interventions to improve drug therapy for older adults with dementia, it is important that future work includes sex- or gender-based analysis of drug use.

Population pharmacokinetics and optimization of the dosing regimen of digoxin in adult patients

Background

This study aimed to evaluate the population pharmacokinetics of digoxin in Japanese patients and establish a dosage regimen based on the pharmacokinetic data.

Methods

We analyzed 287 serum digoxin samples from 192 individuals by using the nonlinear mixed effects model. We used simulations to optimize the dosage regimen of digoxin to achieve a high likelihood of the target concentration (0.5–0.8 ng/mL).

Results

The total body clearance (CL/F ([L/h]) was calculated using the following formula: CL/F = (1.21 + 0.0532 × CLcr [(mL/min]) × (1 + 0.787 × AMD), where CLcr is the creatinine clearance and AMD is 0 in the case of concomitant administration of amiodarone and 1 otherwise. To achieve the target concentration (0.5–0.8 ng/mL), the dosage of digoxin was 0.0625 mg/day (CLcr < 35 mL/min and AMD = 0); 0.125 mg/day (CLcr, 35–65 mL/min and AMD = 0); 0.1875 mg/day (CLcr, 65–100 mL/min and AMD = 0); 0.0625 mg/every other day (CLcr < 30 mL/min and AMD = 1); and 0.0625 mg/day (CLcr, 30–85 mL/min and AMD = 1).

Conclusions

Our findings suggest that population parameters are useful for evaluating digoxin pharmacokinetics.

Gender differences in pharmacokinetics of lumefantrine and its metabolite desbutyl-lumefantrine in rats

Lumefantrine has been reported to be mainly bio-transformed by cytochrome P450 isozyme 3A4 to desbutyl-lumefantrine (DLF) in human liver microsomes. Since CYP3A is expressed in a sex specific manner in rats, it could be expected that the pharmacokinetics of lumefantrine would be changed in male rats compared with those in female rats. The pharmacokinetics of lumefantrine and its active metabolite DLF were evaluated after intravenous (0.5 mg/kg) and oral (20 mg/kg) administration of lumefantrine to male and female Sprague-Dawley rats. The quantitative bioanalysis was carried out by the liquid chromatography tandem mass spectrometry method. After intravenous and oral administration of lumefantrine the area under the curve (AUC) of lumefantrine was significantly higher in female rats than that in male rats, whereas the AUC of DLF was significantly lower in female rats in comparison with male rats. This lower AUC of DLF in female rats could have been due to reduced metabolism of lumefantrine in female rats. The bioavailability (%F) of lumefantrine was 1.66 times higher in male rats than that in female rats.

Impact of pharmacokinetics and pharmacogenetics on the efficacy of pranlukast in Japanese asthmatics

BACKGROUND AND OBJECTIVE

Wide inter-individual variability in therapeutic effects limits the efficacy of leukotriene (LT) receptor antagonists in the treatment of asthma. We have reported that genetic variability in the expression of LTC(4) synthase is associated with responsiveness to pranlukast in Japanese asthmatic patients. However, the effects of pharmacokinetic variability are less well known. This was an analysis of the pharmacokinetics of pranlukast in a population of adult asthmatics, and its effect on clinical responses. Other factors that may be related to the therapeutic effects of pranlukast, including LTC(4) synthase gene polymorphisms, were also investigated.

METHODS

The population pharmacokinetics of pranlukast was analysed in a one-compartment model, using data collected in 50 Japanese adults with moderate to severe asthma, who were treated with pranlukast, 225 mg bd for 4 days. In 32 of these patients, in whom the clinical response to pranlukast (increase in FEV(1) after 4 weeks of treatment) was measured in a previous study, a combined pharmacokinetic and pharmacogenetic analysis was performed.

RESULTS

Using the population pharmacokinetic model, the estimated the mean oral clearance (CL/F) of pranlukast was 16.4 L/h, and the inter-individual variability was 30.1%. Univariate and multivariate analyses showed that LTC(4) synthase polymorphisms, but not the CL/F of the drug, predicted an improvement in pulmonary function with pranlukast treatment (P < 0.05).

CONCLUSIONS

There was marked inter-individual variability in the pharmacokinetics of pranlukast among adult asthmatics, but this had little impact on the clinical effectiveness of the drug.

Population pharmacokinetic investigation of digoxin in Japanese neonates

OBJECTIVE

To establish the role of patient characteristics in estimating doses of digoxin for neonates using routine therapeutic drug monitoring data.

METHOD

The steady-state blood level data (n = 129) after repetitive oral administration in 71 hospitalized neonates were analysed using Nonlinear Mixed Effects Modelling (nonmem), a computer program designed for analysing drug pharmacokinetics in study populations through pooling of data. Analysis of the pharmacokinetics of digoxin was accomplished using a one-compartment open pharmacokinetic model. The effect of a variety of developmental and demographic factors on digoxin disposition was investigated.

RESULTS

Estimates generated by nonmem indicated that the clearance of digoxin (CL/F; L/h) was influenced by the demographic variables: total body weight (TBW), gestational age (GA) and neonate clearance factor (trough serum concentration of digoxin; Conc). These influences could be modelled by the equation CL/F = 0.0261 x TBW (kg)0.645 x Conc (ng/mL)(-0.724) x GA (weeks)0.8. The interindividual variability in digoxin clearance was modelled with proportional errors. The estimated coefficient of variation was 7.0%, and the residual variability was 13.1%.

CONCLUSION

Clinical application of the model to patient care may permit selection of an appropriate initial maintenance dose, thus enabling the clinician to achieve the desired therapeutic effect. However, the digoxin dosage regimen for the individual patient should be based on a careful appraisal of their clinical need for the drug.

Evaluation of a Sex-Based Difference in the Pharmacokinetics of Digoxin

STUDY OBJECTIVE

To determine whether a sex-based difference in digoxin pharmacokinetics exists in patients receiving long-term digoxin therapy for chronic heart failure or atrial fibrillation.

DESIGN

Single-center, retrospective review of medical records.

SETTING

University-based teaching hospital and outpatient clinic.

PATIENTS

Sixty-seven adults (32 men, 35 women) with chronic heart failure or atrial fibrillation who were receiving digoxin therapy.

MEASUREMENTS AND MAIN RESULTS

Serum digoxin concentrations and daily digoxin doses were obtained from patients' medical records. Daily doses were adjusted for patients' actual and ideal body weight and body mass index (BMI). The ratio between the serum digoxin concentration and each of the adjusted daily doses of digoxin was compared between men and women. The mean +/- SD serum digoxin concentration was 0.85 +/- 0.51 ng/ml for men compared with 1.02 +/- 0.51 ng/ml for women. Mean +/- SD unadjusted doses of digoxin were 0.180 +/- 0.063 and 0.164 +/- 0.059 mg/day for men and women, respectively; the difference was not statistically significant. Ratios of serum digoxin concentration to daily digoxin doses did not differ by sex when doses were estimated with actual or ideal weight. Only the ratio of the digoxin concentration to the BMI-adjusted dose was significantly different between men and women (0.14 +/- 0.09 and 0.19 +/- 0.11, respectively, p<0.05).

CONCLUSION

Sex-based differences in digoxin pharmacokinetics were absent when actual or ideal body weight was used. However, the ratio of serum digoxin concentration to daily digoxin dose adjusted for BMI differed by sex. Because digoxin is distributed to lean body mass, use of the BMI could have overadjusted body weight, leading to inaccurate pharmacokinetic assumptions and calculations. The pharmacokinetics of digoxin do not appear to differ by sex.

Sex and racial differences in pharmacological response: where is the evidence? Pharmacogenetics, pharmacokinetics, and pharmacodynamics

The Food and Drug Administration (FDA) reviewed 300 new drug applications between 1995 and 2000. Of the 163 that included a sex analysis, 11 drugs showed a >40% difference in pharmacokinetics between males and females, which was listed on the product label, yet no dosing recommendations were made based on sex. Female sex has been shown to be a risk factor for clinically relevant adverse drug reactions. Would simply dosing females based on their different pharmacokinetics decrease the incidence of adverse events? The answer is not known. Sex-dependent pharmacodynamic effects have been identified. The role of pharmacokinetics vs. pharmacodynamics is unclear, as is the impact of pharmacogenetics on both. This review highlights a few specific examples in each area in which sex differences in pharmacokinetics and pharmacodynamics are important and provides recommendations for additional needed research.

Population Pharmacokinetics I: Background, Concepts, and Models

OBJECTIVE

To present and emphasize the background, foundations, utility, and conceptual underlying theory of the population pharmacokinetic (PPK) approach with an examination of the advantages when compared with other approaches of pharmacokinetic modeling.

DATA SOURCES

Information on PPK was retrieved from a MEDLINE search (1979—June 2002) of literature and a bibliographic review of review articles and books.

STUDY SELECTION AND DATA EXTRACTION

All articles identified from data sources were evaluated and relevant information was included in this review.

DATA SYNTHESIS

PPK plays a pivotal role in developing dosing strategies for direct patient care and in drug development. PPK is valuable because it targets the patient group that will eventually receive the drug of interest, quantitates pharmacokinetic variability at several levels, and seeks to explain those sources of variability.

CONCLUSIONS

PPK models have great utility and the applications are many. They are very different from single-subject pharmacokinetic models and therefore require different approaches to model development.

Gender-specific implications for cardiovascular medication use in the elderly optimizing therapy for older women

Sex should be considered during the selection of cardiovascular medications and dosages of cardiovascular medications. There is mounting evidence that clinically important differences between the sexes exist in the pharmacokinetic processes that determine drug concentrations and in the pharmacodynamic processes that determine physiologic responses to pharmacologic agents. Although aging also affects these processes, aging does not eliminate the sex-related differences. The major pharmacokinetic differences between the sexes, on average, are lower weight and distribution volumes in women compared with men and lower renal drug clearance in women compared with men. Sex-related differences in hepatic drug clearance are less predictable. Pharmacodynamic responses that differ between the sexes include increased adverse cardiovascular drug effects in women compared with men (torsade de pointes arrhythmias, increased risk of hemorrhagic consequences of anticoagulation or thrombolytic therapy, electrolyte abnormalities with diuretics, myopathy with HMG Co-A reductase inhibitors, cough with ACE inhibitors, and increased incidence of thrombosis). Recommendations for optimizing cardiovascular drug therapy for the older women include individualization of drug selection to minimize the number of medications and side effects; dosage adjustment based on age, size, and sex; close monitoring for side effects; and consideration of cost and access to medications. Optimal care for the older woman with cardiovascular disease will also require investigation of cardiovascular medications in older women and of therapies for cardiovascular diseases that are more common in women than men.

The influence of sex on pharmacokinetics

Biologic differences exist between men and women that can result in differences in responses to drugs. Both pharmacokinetic and pharmacodynamic differences between the sexes exist, with more data on pharmacokinetic differences. On average, men are larger than women. Body size differences results in larger distribution volumes and faster total clearance of most medications in men compared to women. Greater body fat in women (until older ages) may increase distribution volumes for lipophilic drugs in women. Total drug absorption does not appear to be significantly affected by sex although absorption rates may be slightly slower in women. Bioavailability after oral drug dosing, for CYP3A substrates in particular, may be somewhat higher in women compared to men. Bioavailability after transdermal drug administration does not appear to be significantly affected by gender; nor does protein binding. Renal processes of glomerular filtration, tubular secretion, and tubular reabsorption appear to be faster in men compared to women whether considered on a mg/kg basis or total body weight basis. Algorithms to estimate glomerular filtration rate incorporate sex as a factor; some also include weight. For hepatic processes, drugs metabolized by Phase I metabolism (oxidation, reduction, and hydrolysis via cytochrome P450's 1A, 2D6, 2E1), Phase II conjugative metabolism (glucuronidation, conjugation, glucuronyltransferases, methyltransferases, dehydrogenases) and by combined oxidative and conjugation processes are usually cleared faster in men compared to women (mg/kg basis). Metabolism by CYP2C9, CYP2C19, and N-acetyltransferase, appear to be similar in men and women (mg/kg). Clearance of p-glycoprotein substrates appear to be similar in men and women. In contrast, total clearance of a number of CYP3A substrates appear to be mildly or moderately faster (mg/kg) in women compared to men. The clinical significance of reported differences warrants consideration. Clearance reported on a per kg basis directly addresses organ or enzyme clearance. The difference in size between men and women means translating these results to clinical dosage rates should include an adjustment for body size. Unfortunately, this is not standard. Reports of sex differences that persist after considering weight may warrant further dosage adjustments. In addition, investigations are often performed in healthy fasting individuals yet medications are prescribed to patients with confounding influences of disease, co-medications, diet, and social habits. The relative role of sex on pharmacokinetics as compared to genetics, age, disease, social habits and their potential interactions in the clinical setting is not yet fully known but should be routinely considered and further studied.

Population pharmacokinetics of digoxin in Egyptian pediatric patients: impact of one data point utilization

A population pharmacokinetic (PK) study was designed to estimate the PK parameters of digoxin among a selected group of Egyptian pediatric patients (n = 30) with mean age +/- SD and body weight +/- SD of 8.88 +/- 3.01 years and 23.9 +/- 5.8 kg, respectively. All patients had heart failure and were maintained on digoxin given orally. Nonlinear mixed effect modeling software version 5 (NONMEM Project Group, San Francisco, CA) and one-compartment modeling were used for fitting the data. A one-trough steady-state plasma concentration level of digoxin was used in the analysis. The population mean estimates for clearance (CL/f) and volume of distribution (V/f), in which f represents oral bioavailability, were 8.61 L/h and 450 L, respectively. Because of the limited number of samples per patient, regression analysis could not detect a correlation between patient covariates and estimated PK parameters. The analysis did not converge to obtain good parameter estimates. At least two samples per patient should be used to improve the PK estimation and allow better analysis of the relation between the potential covariates and estimated PK parameters.

How important are gender differences in pharmacokinetics?

Gender-related differences in pharmacokinetics have frequently been considered as potentially important determinants for the clinical effectiveness of drug therapy. The mechanistic processes underlying gender-specific pharmacokinetics can be divided into molecular and physiological factors. Major molecular factors involved in drug disposition include drug transporters and drug-metabolising enzymes. Men seem to have a higher activity relative to women for the cytochrome P450 (CYP) isoenzymes CYP1A2 and potentially CYP2E1, for the drug efflux transporter P-glycoprotein, and for some isoforms of glucuronosyltransferases and sulfotransferases. Women were suggested to have a higher CYP2D6 activity. No major gender-specific differences seem to exist for CYP2C19 and CYP3A. The often-described higher hepatic clearance in women compared with men for substrates of CYP3A and P-glycoprotein, such as erythromycin and verapamil, may be explained by increased intrahepatocellular substrate availability due to lower hepatic P-glycoprotein activity in women relative to men. Physiological factors resulting in gender-related pharmacokinetic differences include the generally lower bodyweight and organ size, higher percentage of body fat, lower glomerular filtration rate and different gastric motility in women compared with men. Although gender disparity in pharmacokinetics has been identified for numerous drugs, differences are generally only subtle. For a few drugs, e.g. verapamil, beta-blockers and selective serotonin reuptake inhibitors, gender-related differences in pharmacokinetics have been shown to result in different pharmacological responses, but their clinical relevance remains unproven. In contrast, gender differences of clinical importance have clearly been identified for pharmacodynamic processes such as QTc prolongation, and intensive future research efforts are needed to assess the full scope and impact of pharmacodynamic gender disparity on applied pharmacotherapy.

Pharmacoepidemiologic detection of calcium channel blocker-induced change on digoxin clearance using multiple trough screen analysis

Nonlinear mixed effects modeling was used to estimate the effects of digoxin-calcium channel blockers (verapamil, diltiazem, nifedipine) in 336 serum levels gathered from 172 patients (104 male and 68 female) receiving oral digoxin as hospital in-patients. The final model describing digoxin clearance (CL) was CL (l/day)=(87.9+2.71C(cr))0.872(CCB)0.880(CHF)0.937(SPI)0.854(DFAC), where C(cr) is the estimated creatinine clearance (ml/min); CCB=1 for concomitant administration of calcium channel blockers and CCB=zero otherwise; CHF=1 for the patients with congestive heart failure and CHF=zero otherwise; SPI=1 for concomitant administration of spironolactone and SPI=zero otherwise; DFAC=1 for administration of a half-tablet of digoxin and DFAC=zero otherwise. Concomitant administration of calcium channel blockers resulted in a 13% decrease in digoxin relative clearance.

Gender-related differences in pharmacokinetics and their clinical significance

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

Population analysis for the optimization of digoxin treatment in Japanese paediatric patients

BACKGROUND AND OBJECTIVES

Information about the pharmacokinetics of digoxin in paediatric patients is limited. We therefore aimed to investigate the effects of physiological factors on the digoxin clearance in Japanese paediatric patients.

METHOD

We used routinely collected therapeutic drug monitoring data (n=544), derived from the steady-state serum concentrations of digoxin in 181 hospitalized paediatric patients.

RESULTS

Of those physiological factors which have been examined in this study, age and total body weight were most closely correlated with digoxin clearance. Data on neonates within the first postnatal month indicated a tendency towards lower clearance for premature neonates than full-term neonates (P<0.01). Digoxin clearance was reduced by spironolactone in patients younger than 4 months (P<0.05). Patients with congestive heart failure showed a lower digoxin clearance than the others (P<0.001). Serum creatinine and gender did not have a statistically significant effect on digoxin clearance.

CONCLUSION

Age and total body weight are important factors influencing digoxin clearance in children. Spironolactone affected digoxin clearance and needs to be considered when dosing paediatric subjects.

Population-based investigation of relative clearance of digoxin in Japanese patients by multiple trough screen analysis: an update

The steady-state concentrations of digoxin at trough levels were studied to reestablish the role of patient characteristics in estimating doses for digoxin using routine therapeutic drug-monitoring data. The data (n = 548) showing steady-state serum concentrations of digoxin after repetitive oral administration in 385 hospitalized patients were analyzed using NONMEM, a computer program designed to analyze pharmacokinetics in study populations by allowing pooling of data. Analysis of the pharmacokinetics of digoxin was accomplished with a simple steady-state pharmacokinetic model. The effect of a variety of developmental and demographic factors on the clearance of digoxin was investigated. Estimates generated by NONMEM indicated that clearance of digoxin was influenced by the demographic variables of age, total body weight, serum creatinine, estimated creatinine clearance, gender, the coadministration of spironolactone, the presence or absence of congestive heart failure, and the administration of a half-tablet. The interindividual variability in the clearance of digoxin was modeled with proportional error with an estimated coefficient of variation of approximately 22%; the residual variability was approximately 25.0%. An a priori method, based on the value for clearance of digoxin obtained by NONMEM analysis, was proposed as a useful adjunct for the prediction of the steady-state concentration of digoxin at trough level as a function of the maintenance dose of digoxin.

Mixed-effect modeling for detection and evaluation of drug interactions: digoxin-quinidine and digoxin-verapamil combinations

Mixed-effect modeling has been suggested as a possible tool to detect and describe drug interactions in patient populations receiving drug combinations for the treatment of disease states. The mixed-effect modeling program, NONMEM, was used to measure the effects of the well-known digoxin-quinidine and digoxin-verapamil drug interactions in 294 patients receiving oral digoxin as hospital inpatients. Fourteen percent of the population took either quinidine or verapamil concurrently with digoxin (mean quinidine dose = 857 +/- 397 mg/day, verapamil = 261 +/- 110 mg/day). Two regression models for digoxin oral clearance were used. Model 1 used the knowledge that digoxin is eliminated by both renal and nonrenal routes (TVCL = ClNR+m.CrCl, where TVCL is the population digoxin oral clearance, ClNR is the nonrenal clearance, and m is the slope of the line that relates creatinine clearance (CrCl) to digoxin clearance); model 2 used a more conventional regression approach with a simple series of multipliers. For both models, quinidine administration decreased population digoxin oral clearance by approximately 45% and verapamil therapy decreased population digoxin oral clearance by approximately 30%. These values are similar to those found by traditional drug interaction studies conducted in small patient or normal subject populations. Mixed-effect modeling can detect clinically relevant drug interactions and produce information similar to that found in traditional pharmacokinetic crossover study designs.

A feasibility study of the multiple-peak approach for pharmacokinetic screening: population-based investigation of valproic acid relative clearance using routine clinical pharmacokinetic data

The multiple-peak approach was used to evaluate the effect of age, body mass, dose and gender on the population estimates of valproic acid relative clearance. Routine clinical pharmacokinetic data (n = 474) was collected from 250 patients receiving valproic acid and no other drug. The data was analysed according to a simple steady-state pharmacokinetic model using NONMEM, a computer program designed for population pharmacokinetic analysis allowing pooling of data. The final regression model for relative clearance (CL, mL kg-1 h-1) was: CL = 18.9 x body weight(-0.276) x daily dose(0.142) x gender where gender = 1 for males, 0.877 for females. NONMEM estimates suggested that the rate of valproic acid clearance decreased nonlinearly with increasing body weight in the maturation process, and increased nonlinearly with increasing valproic acid dose (mg kg-1 day-1). The clearance in females was about 11% less than in males. These estimates were similar to those detected from previous studies.

Predictive performance of four pharmacokinetic methods for calculating digoxin dosage

An evaluation was made of the performance of the Sheiner, Koup, Dobbs and Paulson methods for predicting total body clearance of digoxin in 59 patients with and without congestive heart failure (CHF). The predicted clearances were then used to predict the steady-state serum concentrations of digoxin. Actual serum concentrations were measured at steady state following single oral daily doses of digoxin for at least 1 month. Predictive performance was determined for each method by calculating the mean prediction error (ME) and the mean squared prediction error (MSE). Ninety-five per cent confidence intervals and correlation coefficients were also calculated. The prediction bias and precision of the methods were compared statistically by calculating the 95% confidence intervals of the delta ME and delta MSE. For patients with CHF, the Sheiner method was the least biased and the most precise for predicting observed serum levels. For patients without CHF, no significant difference between the Sheiner and Koup methods, and between the Dobbs and Paulson methods were seen, although the former two methods were less biased and more precise than the latter two. The Sheiner method was used as the basis for evaluating the performance of the three other methods in predicting digoxin clearance from patients with and without CHF. This revealed that the Koup, Dobbs and Paulson methods tended to overpredict clearances in patients with CHF; on the other hand, for patients without CHF, the Koup method was the least biased and the most precise of the three methods.

Interaction between structural, statistical, and covariate models in population pharmacokinetic analysis

The influence of the choice of pharmacokinetic model on subsequent determination of covariate relationships in population pharmacokinetic analysis was studied using both simulated and real data sets. Simulations and data analysis were both performed with the program NONMEM. Data were simulated using a two-compartment model, but at late sample times, so that preferential selection of the two-compartment model should have been impossible. A simple categorical covariate acting on clearance was included. Initially, on the basis of a difference in the objective function values, the two-compartment model was selected over the one-compartment model. Only when the complexity of the one-compartment model was increased in terms of the covariate and statistical models was the difference in objective function values of the two structural models negligible. For two real data sets, with which the two-compartment model was not selected preferentially, more complex covariate relationships were supported with the one-compartment model than with the two-compartment model. Thus, the choice of structural model can be affected as much by the covariate model as can the choice of covariate model be affected by the structural model; the two choices are interestingly intertwined. A suggestion on how to proceed when building population pharmacokinetic models is given.