Pharmacokinetic modelling of digoxin and theophylline: Effect of volume of distribution and plasma concentration

Objectives: This study explores the pharmacokinetic relationship between volume of distribution (Vd), plasma concentration and maintenance dose (MD) for two drugs with narrow therapeutic indices: Digoxin and theophylline at constant bioavailability, half-life of elimination and dosing interval. Materials and Methods: Using three-dimensional surface plots, the effect of Vd and plasma concentration on the MD of digoxin and theophylline was analysed. For digoxin, the plot demonstrates that a larger Vd necessitates a higher MD to achieve therapeutic plasma levels, essential for treating conditions such as heart failure and arrhythmias. Similarly, for theophylline, which is commonly used to manage asthma and chronic obstructive pulmonary disease, the interaction between Vd and plasma concentration affects the required MD. Results: These findings highlight the importance of individualised dosing strategies, considering the pharmacokinetic properties of each drug, to ensure efficacy and minimise toxicity. Conclusion: The visualised models offer valuable insights for clinical dosing adjustments based on patient-specific factors such as body composition and organ function, which can significantly influence the distribution and clearance of these drugs. Keywords: Digoxin, Maintenance dose, Pharmacokinetics, Plasma concentration, Theophylline, Volume of distribution