Determination of Digoxin Clearance in Japanese Elderly Patients for Optimization of Drug Therapy A Population Pharmacokinetics Analysis Using Nonlinear Mixed-Effects Modelling

Background: Optimal use of digoxin in the elderly population requires information about the drug's pharmacokinetics and the influence of various factors on the drug's disposition. However, because of sampling restrictions, it is often difficult to perform traditional pharmacokinetic studies in elderly patients. Objective: This study was conducted to determine the apparent total clearance of digoxin from serum after oral administration (CL/F) and to establish the role of patient characteristics in estimating doses of digoxin for elderly patients (age >= 65 years), using routine therapeutic drug monitoring data. Methods: Analyses of the pharmacokinetics of digoxin were conducted using the nonlinear mixed-effects modelling (NONMEM (R)) software, a computer program designed to analyse pharmacokinetics in study populations by allowing pooling of data. Steady-state data (140 observations) obtained by routine therapeutic drug monitoring following repeated oral administration of digoxin in 94 hospitalized elderly patients (age >= 65 years) were analysed to establish the role of patient characteristics in estimating doses of digoxin for elderly patients. Results: Estimates generated by NONMEM (R) indicated that digoxin CL/F was influenced by the demographic variables of total bodyweight (TBW), serum creatinine (SCr), age (AGE), presence of congestive heart failure (CHF), concomitant administration of the calcium channel antagonists (calcium channel blockers [CCBs]: verapamil, diltiazem or nifedipine), sex (SEX) and elderly clearance factor (trough serum concentration of digoxin; [C(trough)]boolean AND theta). The full version of the final NONMEM (R) model was CL/F [L/h] = (0.588 x TBW [kg]boolean AND(0.189) x SCr [mg/dL]boolean AND(-0.163) x (AGE [years]/65 boolean AND (-0.152) x 0.957 boolean AND(CCB) x 0.941 boolean AND(CHF) x 0.965 boolean AND(SEX) x C(trough) [ng/mL]boolean AND (-0.180), where CCB is 1 for concomitant administration of a CCB and is 0 otherwise; CHF is 1 for patients with CHF and is 0 otherwise; SEX is 0 for male and is 1 for female; and the elderly clearance factor C(trough)(-0.180) is 1 for digoxin C(trough) < 1.7 ng/mL. Conclusions: We developed a new model for elderly patient dosing of digoxin with good predictive performance. Clinical application of the findings of the present study to patient care may permit selection of an appropriate initial digoxin maintenance dose, thus enabling the clinician to achieve a desired therapeutic effect. However, the digoxin dosage regimen should be based on an appraisal of the individual patient's clinical need for the drug.