Effect of confounding factors on a phospho-flow assay of ribosomal S6 protein for therapeutic drug monitoring of the mTOR-inhibitor everolimus in heart transplanted patients.

CONTEXT: Several assays of monitoring immune cell function have been developed to enhance therapeutic drug monitoring. OBJECTIVE: An in vitro-validated whole-blood assay of phosphorylated ribosomal protein S6 (pS6RP) was evaluated for confounders to monitor the mTOR-inhibitor everolimus (ERL). MATERIALS AND METHODS: Whole blood samples from 87 heart transplant recipients were analyzed for pS6RP-expression in CD3-positive T-cells by phospho-flow analysis. RESULTS: ERL blood concentration, laboratory parameters, co-medications, demographic and clinical data were reviewed. CONCLUSION: Evaluating the pS6RP-assay revealed that pS6RP is influenced by cyclosporine A (CsA) blood concentration, duration of ERL treatment, co-medication with thiazide diuretics and different metabolic parameters.

Assay validation of phosphorylated S6 ribosomal protein for a pharmacodynamic monitoring of mTOR-inhibitors in peripheral human blood.

BACKGROUND: Therapeutic drug monitoring (TDM) of immunosuppressive drugs after organ transplantation is based on measuring blood levels alone, which often results in under- or over-immunosuppression. Previous studies have shown the potential of measuring pharmacodynamic drug effects for TDM, but assessment of biomarkers for individual drugs is still not clinical routine. Therefore, we validated a specific assay to measure the pharmacodynamic effects of mammalian target of rapamycin (mTOR)-inhibitors on phosphorylated S6 ribosomal protein (p-S6RP), a downstream target of mTOR. METHODS: Clinical relevant concentrations of sirolimus (SRL, 0.9-91.4 µg/L), cyclosporine A (CsA, 75.1-1202 µg/L), mycophenolate acid (MPA, 0.08-3.2 mg/L), or dexamethasone (DEX, 0.5-200 ng/mL) were added to whole-blood from healthy volunteers. Activated whole-blood was analyzed by phospho-flow cytometry to measure p-S6RP in T cells. RESULTS: Phospho-flow analysis revealed that SRL suppressed p-S6RP in human T cells in a dose-dependent manner with a half-maximal inhibitory concentration (IC(50)) at 19.8 nM and a maximal inhibitory effect (I(max) %) at 91.9%. Neither CsA, MPA, nor DEX inhibited mTOR-related S6RP-phosphorylation. Coefficient of variations from 0.03 to 0.05, 0.12 to 0.25, and 0.14 to 0.38 for intra-, interassay, and interindividual variability respectively, showed robustness of our assay. Furthermore, samples can be stored at RT or 4°C up to 2 h after withdrawal. CONCLUSION: We validated a robust whole-blood assay that allows the specific measurement of SRL- and everolimus-induced inhibition of T cells' function through detection of p-S6RP. Future studies in organ transplanted recipients will show if this assay has the potential to enhance a TDM for mTOR-inhibitor drugs in combination therapies.