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Wide inter-individual variability in therapeutic effects limits the efficacy of leukotriene (LT) receptor antagonists in the treatment of asthma. We have reported that genetic variability in the expression of LTC4 synthase is associated with responsiveness to pranlukast in Japanese asthmatic patients. However, the effects of pharmacokinetic variability are less well known. This was an analysis of the pharmacokinetics of pranlukast in a population of adult asthmatics, and its effect on clinical responses. Other factors that may be related to the therapeutic effects of pranlukast, including LTC4 synthase gene polymorphisms, were also investigated.The population pharmacokinetics of pranlukast was analysed in a one-compartment model, using data collected in 50 Japanese adults with moderate to severe asthma, who were treated with pranlukast, 225 mg bd for 4 days. In 32 of these patients, in whom the clinical response to pranlukast (increase in FEV1 after 4 weeks of treatment) was measured in a previous study, a combined pharmacokinetic and pharmacogenetic analysis was performed.Using the population pharmacokinetic model, the estimated the mean oral clearance (CL/F) of pranlukast was 16.4 L/h, and the inter-individual variability was 30.1%. Univariate and multivariate analyses showed that LTC4 synthase polymorphisms, but not the CL/F of the drug, predicted an improvement in pulmonary function with pranlukast treatment (P< 0.05).There was marked inter-individual variability in the pharmacokinetics of pranlukast among adult asthmatics, but this had little impact on the clinical effectiveness of the drug.