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Background:The phosphorylation ofp70S6 kinase (p70S6K) represents an important target for sensitive detection on pharmacodynamic effects of sirolimus,but the methods of assessing p70S6K phosphorylation are still unclear.The aim of this study was to investigate p70S6K phosphorylation located down-stream of the mammalian target ofrapamycin (mTOR) pathway in peripheral blood mononuclear cells (PBMCs) of liver transplant patients through different methods.Methods:Seventy-five liver transplant recipients from Beijing Chaoyang Hospital of the Capital Medical University were analyzed in this study.Patients were divided into three groups,patient treated with sirolimus (n =22),patient treated with tacrolimus (n =30),patient treated with cyclosporine (n =23).The p70S6K phosphorylation of PBMCs in patients and healthy control (HC,n =12) were analyzed by phospho-flow cytometry and West blotting.A correlation analysis of data from phospho-flow cytometry and West blotting was performed.Intra-assay variability of p70S6K phosphorylation in HC and different patients were measured.Results:Intra-assay variability ofp70S6K phosphorylation in phospho-flow cytometry was from 4.1％ to 8.4％ and in West blotting was from 8.2％ to 18％.The p70S6K phosphorylation in patients receiving a sirolimus (19.5 ± 7.7) was significantly lower than in HC (50.1 ± 11.3,P ＜ 0.001),tacrolimus (37.7 ± 15.7,P ＜ 0.001) or cyclosporine treated patients (41.7 ± 11.7,P ＜ 0.001).The p70S6K phosphorylation in HC (50.1± 11.3) was significantly higher than in tacrolimus (37.7 ± 15.7,P ＜ 0.01) or cyclosporine-treated patients (41.7 ± 11.7,P ＜ 0.01).There was correlation between data from phospho-flow cytometry and data from Westem blotting (r =0.88,P ＜ 0.001).Conclusions:The degree of mTOR inhibition by assessing p70S6K phosphorylation was established by phospho-flow cytometry and Westem blotting.Assessment of p70S6K phosphorylation may play an adjunct role to on pharmacodynamically guide and individualize sirolimus based on immunosuppression.