Functional half-life is a meaningful descriptor of steady-state pharmacokinetics of an extended-release formulation of a rapidly cleared drug - As shown by once-daily divalproex-ER

Background: For many drugs, steady-state concentration-time profiles are often not optimally characterised by the intrinsic terminal elimination half-life for various reasons, including multiexponential disposition with minimal contribution of the terminal phase to steady-state exposure or use of control I ed-release formulations with extended zero- or mixed zero-/first-order absorption. In such cases, 'effective' or 'functional' half-life (t 1/2F) has often been used to characterise steady-state pharmacokinetics. Valproic acid, commonly used in neuropsychiatry, has an elimination half-life of 4-16 hours in different populations (children vs adults, enzyme-induced vs uninduced). Divalproex-ER, a once-daily extended-release divalproex sodium formulation, is designed to release valproic acid over > 18 hours. Hence the steady-state divalproex-ER concentration-time profiles have small peak-trough fluctuations that are not optimally characterised by valproic acid elimination half-life. In this study, the value of t 1/2F was calculated to characterise divalproex-ER steady-state concentration-time profiles. Methods: The value of t 1/2F, defined as the time taken for the concentration to drop by one-half during a dosing interval (tau) at steady state, was derived using steady-state maximum (C-max) and minimum (C-min) plasma concentration and T values, and calculated as In(2)/(In [C-max/C-min]/tau). The t 1/2F values of valproic acid in adult hepatic enzyme-uninduced healthy subjects and enzyme-induced epilepsy patients were calculated from five pharmacokinetic studies in which divalproex-ER was administered once daily for 6-14 days. Results: The estimated geometric mean t 1/2F in uninduced adults was 40.0 hours versus the expected elimination half-life of 12-16 hours in this population (including patients on valproic acid monotherapy); for induced patients, t 1/2F was 26.9 hours versus the expected elimination half-life of 6-12 hours. Conclusion: The t 1/2F of valproic acid optimally characterises the expected steady-state C-max to C-min decrease of 33% in uninduced and 45% in induced adults following once-daily administration of divalproex-ER.