title: Population pharmacokinetic analysis of diurnal and seasonal variations of plasma concentrations of cilostazol in healthy volunteers.
abstract: BACKGROUND
  The background of this study was (1) to examine factors influencing cilostazol pharmacokinetics by developing a population model incorporating diurnal variation and other covariate effects and (2) to assess the feasibility of applying the developed model to determine the optimal dosing times.
  
  
  METHODS
  Data obtained from a cilostazol pharmacokinetic study consisting of 2 clinical trials (a single twice-a-day (BID) dosing trial in winter and a multiple BID dosing trial in summer) conducted in healthy Korean subjects were used for model building. A basic model was built, followed by a diurnal variation model, and then a final model was built incorporating covariates, including a seasonal difference. The optimal morning and evening dosing times were determined from simulations.
  
  
  RESULTS
  Diurnal variation in cilostazol pharmacokinetics was explained by the morning absorption rate constant being faster than in the evening, yielding values of 0.278 versus 0.234/h in summer, when 24- and 12-hour circadian rhythms were included in the model. The seasonal variation was explained by a 26.9% and a 31.8% decrease in the absorption rate constant and clearance, respectively, in winter compared with summer. Based on twice-a-day (BID) dosing, dosing times of 9 AM and 5 PM in summer and 10 AM and 7 PM in winter were expected to produce the smallest peak-to-peak fluctuations in cilostazol concentration, possibly minimizing unwanted effects of the drug.
  
  
  CONCLUSIONS
  This study demonstrated the intraday and interseasonal time-varying nature of cilostazol pharmacokinetics using a population modeling approach and developed a strategy for optimizing dosing times. It is suggested that these methods can be similarly applied to analyses and controls of other drugs that exhibit characteristics of time-varying pharmacokinetics.
authors.affiliation: *Department of Pharmacology, Yonsei University College of Medicine; and †Brain Korea 21 Plus Project for Medical Science, Yonsei University College of Medicine, Seoul, Korea.
authors.forename: Donghwan
authors.initials: D
authors.lastname: Lee
authors.forename: Hankil
authors.initials: H
authors.lastname: Son
authors.forename: Lay A
authors.initials: LA
authors.lastname: Lim
authors.forename: Kyungsoo
authors.initials: K
authors.lastname: Park
mesh_terms: D000284:Administration, Oral; D000077407:Cilostazol; D002940:Circadian Rhythm / Q000187:drug effects / Q000502:physiology*; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D008297:Male; D008657:Metabolic Clearance Rate / Q000187:drug effects / Q000502:physiology; D008954:Models, Biological; D012621:Seasons; D013777:Tetrazoles / Q000008:administration & dosage* / Q000097:blood*
chemical_list: D013777:Tetrazoles; D000077407:Cilostazol