Assessment of a flow cytometry technique for studying signaling pathways in platelets: Monitoring of VASP phosphorylation in clinical samples.

A recently released kit (PerFix EXPOSE) was reported to improve the measurement of the degree of phosphorylation of proteins in leukocytes by flow cytometry. We tested its adaptation for platelets to monitor vasodilator-stimulated phosphoprotein (VASP) phosphorylation, which is the basis of a currently used test for the assessment of the pharmacological response to P2Y12 antagonists (PLT VASP/P2Y12). The PerFix EXPOSE kit was compared to the PLT VASP/P2Y12 kit by using blood samples drawn at 24 h post clopidogrel dose from 19 patients hospitalized for a non-cardio-embolic ischemic stroke and treated with clopidogrel monotherapy for at least five days in an observational study. The platelet PerFix method was based on adaptation of the volume of the sample, the centrifugation speed and the incubation temperature. Poor agreement between prevention by adenosine diphosphate (ADP) of PGE1-induced cAMP-mediated VASP phosphorylation and ADP induced aggregation assessed by Light Transmittance Aggregometry was found. We found a significant correlation between the PLT VASP/P2Y12 kit and the PerFix EXPOSE kit. The PerFix EXPOSE kit may also be helpful to monitor adverse effects of second-generation tyrosine kinase inhibitors on platelets.

Prediction of individual factor VIII or IX level for the correction of thrombin generation in haemophilic patients.

BACKGROUND: The thrombin generation (TG) assay can assess individual clotting potential. The thrombin generation potential is correlated with the patient's bleeding phenotype and varies from one patient to the other for the same degree of factor VIII or IX deficiency. OBJECTIVE: To define in vitro for individual haemophilic patients the target factor VIII or IX level required to normalize their TG. PATIENTS/METHODS: Plasmas from 20 haemophilic patients were spiked with increasing levels of the deficient coagulation factor and TG parameters were measured. The relationships between factor levels and TG parameters were determined by linear regression. The normal range of thrombin generation was defined in 39 healthy male volunteers. RESULTS: Despite inter-individual heterogeneity in basal TG and responses to spiking, a linear relationship was found between factor VIII or IX levels and TG parameters for individual patients. Based on the individual responses of patient plasmas to spiking, it is possible to define in vitro the target factor VIII or IX levels needed to normalize the TG parameters. For both haemophilic A and haemophilic B patients, significant correlations were found between basal peak values and their correction slopes. The correction slope was steeper in haemophilic B patients, so the factor IX level needed to normalize the TG parameters was lower than for haemophilic A patients. CONCLUSIONS: The TG assay could be used to determine in vitro the patient-specific factor VIII or IX level to be reached to effectively normalize their TG. These in vitro results should be confirmed by ex-vivo studies.

Pharmacokinetic/pharmacodynamic model for unfractionated heparin dosing during cardiopulmonary bypass.

BACKGROUND: High-dose heparin is used during cardiopulmonary bypass (CPB) to prevent thrombosis in the circuits used for extracorporeal circulation. The aim of this study was, initially, to develop a population pharmacokinetic/pharmacodynamic (PK/PD) model to assess the variability of PK/PD parameters and their correlation with the results of the routine haemostatic test activated clotting time (ACT) and thereafter to develop a Bayesian estimator enabling an individualized dosing strategy. METHODS: Fifty consecutive patients undergoing cardiac surgery with CPB were included in the study. Heparin was administered as an initial bolus of 300 IU kg -1 followed by additional boluses of 5000 IU to maintain ACT <400 s. In total, 361 blood samples were collected. The PK and PD data were analysed using a non-linear mixed effect model. RESULTS: A two-compartment model with a linear elimination link to an E max model best described heparin anti-factor Xa activities and ACT. Covariate analysis showed that body weight was positively correlated with clearance and central compartment volume. Inclusion of body weight with these parameters decreased their variability by 11 and 15%, respectively. The Bayesian estimator performed well in predicting individual parameters in an independent group of patients. CONCLUSIONS: A population PK/PD analysis of heparin during CPB, using a routine haemostatic test, shows that Bayesian estimation might help to predict ACT on the basis of only one or two blood samples.