Estimation of Attainment of Steady-State Conditions for Compounds With a Long Half-Life

Half-life is a standard result reported with analysis of pharmacokinetic data. Different definitions such as noncompartmental half-life, terminal half-life, effective half-life, and context-sensitive half-life can yield substantially different estimates of the quantity "half-life." Time to attainment of steady-state conditions is generally derived from (terminal) half-life and therefore sensitive toward the definition of half-life. Thus, estimates of the time to attain steady state must be provided with a precise definition of steady state and the method used for estimation, particularly for drugs with long (terminal) half-life. For clinical purposes, terminal half-life can have limited relevance if drug concentrations in the terminal elimination phase are low. A general rule for which half-life to use is infeasible. While limited accumulation can be negligible if a plateau in pharmacokinetics/pharmacodynamics is reached or with a wide therapeutic window (ie, exposure range), small additional drug accumulation can be highly relevant for drugs with a narrow therapeutic window. Beyond the average, estimation of individual time to attainment of steady state can add highly relevant information about the variability between subjects. Simulations from population models and the use of different definitions of steady state provide an assessment of robustness of the results. The relevance of accurate estimation of time to attainment of steady state is illustrated with cenerimod, an sphingosine-1-phosphate 1 receptor modulator with long half-life currently in clinical development for which estimates of time to steady state ranged from 35 to 110 days with different calculations.