Recombinant factor IX: discrepancies between one-stage clotting and chromogenic assays.

New and modified recombinant factor IX (rFIX) products are in development and accurate potency estimation is important to ensure the consistency of production and efficacy of these therapeutics. Collaborative study data obtained during the replacement of the 3rd International Standard (IS) for FIX concentrate suggested that there was a discrepancy between potency estimates for rFIX using clotting and chromogenic methods, when the rFIX candidate was measured against the plasma-derived FIX (pdFIX) IS. This study explores potential chromogenic and one-stage clotting method discrepancies in more detail. Five batches each of rFIX and pdFIX were assayed against the 4th IS FIX concentrate (a pdFIX) by activated partial thromboplastin time (APTT) one-stage clotting assay and specific functional chromogenic assay. The potency of rFIX by chromogenic assay was consistently around 70% of the one-stage clotting potency (average 78 and 108 IU mL(-1) respectively). These differences were not observed with pdFIX, which had similar potencies (average 96 IU mL(-1) ) by each assay method. In addition, different APTT reagents yielded different potency estimates for rFIX when assayed against the pdFIX IS, with a variation of up to 23%. In all cases, the differences were largely resolved when a rFIX reference was used as the standard. This study highlights some of the challenges associated with assay of rFIX products in the laboratory and that careful consideration needs to be given to the choice of reference material used. This is especially important with the imminent arrival of new and modified rFIX products.

Calibration of the WHO 5th IS for Blood Coagulation Factor IX, Concentrate and Ph. Eur. Human Coagulation Factor IX Concentrate Biological Reference Preparation Batch 3 and investigation of the suitability of an IS as potency standard for purified full-length recombinant FIX.

A joint World Health Organization (WHO) - European Directorate for the Quality of Medicines & HealthCare (EDQM) study was run to calibrate the WHO 5th International Standard (IS) for Blood Coagulation Factor IX (FIX), Concentrate, and European Pharmacopoeia (Ph. Eur.) Human Coagulation Factor IX concentrate Biological Reference Preparation (BRP) Batch 3. The suitability of the 4th IS as a potency standard for purified full-length recombinant FIX (rFIX) was also investigated. Forty-nine laboratories contributed data for the calibration of 2 plasma-derived FIX candidates, relative to the 4th IS, from clotting and chromogenic assays. The intra-laboratory variability was reasonably low; the inter-laboratory variation was lower for sample B (14/148) than for sample C (14/162). Although there were no discrepancies between clotting and chromogenic assays, a significantly lower potency was obtained for sample C with clotting assays when buffer rather than FIX-deficient plasma was used as pre-diluent. A significant assay discrepancy was observed with estimates for the 4th IS for Blood Coagulation Factors FII, VII, IX, X, Plasma against the 4th IS, resulting in a clotting to chromogenic activity ratio of 1.11. The study also investigated the comparability of the plasma-derived concentrate standard with the rFIX products and considered the establishment of an IS for rFIX. The 3 rFIX products currently licensed were represented in this study. Data from 49 laboratories for 2 rFIX candidates were received, with additional results for another full-length rFIX test sample returned by 6 laboratories. The intra-laboratory variability when the rFIX samples were assayed against the 4th IS was acceptably low. Although the full-length rFIX could be assayed against the plasma-derived 4th IS and provided statistically valid results, there were large discrepancies among the clotting assays using different APTT reagents. The inter-laboratory variability of the chromogenic assays was similarly high. There were also significant clotting and chromogenic assay discrepancies. The data from the present study indicate that a recombinant standard for rFIX products will minimise assay discrepancies and improve inter-laboratory agreement. However, they also underline that the value assignment of the 1st rFIX IS needs careful consideration. The Expert Committee on Biological Standardization (ECBS) of WHO was therefore not requested to consider the establishment of an IS for rFIX. In order to ensure continued harmonised standards, sample B (14/148) was established as the WHO 5th IS for Blood Coagulation Factor IX, Concentrate, and as Ph. Eur. Human Coagulation Factor IX, concentrate BRP Batch 3 with the functional activity of 10.5 IU/ampoule.

Assays and reference materials for current and future applications of heparins.

Heparin is widely used in the prevention and treatment of thrombosis. However, this complex polysaccharide is biologically active in many systems other than coagulation, due to its structural similarity to the cell surface and matrix glycan heparan sulphate. These properties give rise to a number of potential therapeutic applications, such as those involving the anti-inflammatory activity of heparin. The anticoagulant activity of heparin is used to determine the potency of heparin preparations for use as antithrombotics. Several types of assay are used, and reference materials are available for their calibration. There is no equivalent measure of heparin's activity in other applications. For new types of heparin preparation, physicochemical methods of ensuring consistency and stability will be important, and new in vitro assays will have to be developed, all of which will require reference materials.

The effect of increasing the sulfation level of chondroitin sulfate on anticoagulant specific activity and activation of the kinin system.

Oversulfated chondroitin sulfate (OSCS) was identified as a contaminant in certain heparin preparations as the cause of adverse reactions in patients. OSCS was found to possess both plasma anticoagulant activity and the ability to activate prekallikrein to kallikrein. Differentially sulfated chondroitin sulfates were prepared by synthetic modification of chondroitin sulfate and were compared to the activity of OSCS purified from contaminated heparin. Whilst chondroitin sulfate was found to have minimal anticoagulant activity, increasing sulfation levels produced an anticoagulant response which we directly show for the first time is mediated through heparin cofactor II. However, the tetra-sulfated preparations did not possess any higher anticoagulant activity than several tri-sulfated variants, and also had lower heparin cofactor II mediated activity. Activation of prekallikrein was concentration dependent for all samples, and broadly increased with the degree of sulfation, though the di-sulfated preparation was able to form more kallikrein than some of the tri-sulfated preparations. The ability of the samples to activate the kinin system, as measured by bradykinin, was observed to be through kallikrein generation. These results show that whilst an increase in sulfation of chondroitin sulfate did cause an increase in anticoagulant activity and activation of the kinin system, there may be subtler structural interactions other than sulfation at play given the different responses observed.

Mapping the platelet profile for functional genomic studies and demonstration of the effect size of the GP6 locus.

BACKGROUND: Evidence suggests the wide variation in platelet response within the population is genetically controlled. Unraveling the complex relationship between sequence variation and platelet phenotype requires accurate and reproducible measurement of platelet response. OBJECTIVE: To develop a methodology suitable for measuring signaling pathway-specific platelet phenotype, to use this to measure platelet response in a large cohort, and to demonstrate the effect size of sequence variation in a relevant model gene. METHODS: Three established platelet assays were evaluated: mobilization of [Ca(2+)](i), aggregometry and flow cytometry, each in response to adenosine 5'-diphosphate (ADP) or the glycoprotein (GP) VI-specific crosslinked collagen-related peptide (CRP). Flow cytometric measurement of fibrinogen binding and P-selectin expression in response to a single, intermediate dose of each agonist gave the best combination of reproducibility and inter-individual variability and was used to measure the platelet response in 506 healthy volunteers. Pathway specificity was ensured by blocking the main subsidiary signaling pathways. RESULTS: Individuals were identified who were hypo- or hyper-responders for both pathways, or who had differential responses to the two agonists, or between outcomes. 89 individuals, retested three months later using the same methodology, showed high concordance between the two visits in all four assays (r(2) = 0.872, 0.868, 0.766 and 0.549); all subjects retaining their phenotype at recall. The effect of sequence variation at the GP6 locus accounted for approximately 35% of the variation in the CRP-XL response. CONCLUSION: Genotyping-phenotype association studies in a well-characterized, large cohort provides a powerful strategy to measure the effect of sequence variation in genes regulating the platelet response.

EPCR Ser219Gly: elevated sEPCR, prothrombin F1+2, risk for coronary heart disease, and increased sEPCR shedding in vitro.

We have progressively analysed three studies of coronary heart disease (CHD) for a variant in EPCR (Ser219Gly). Initially, in a prospective study, NPHSII, while no overall CHD-risk was identified in heterozygotes, homozygotes for 219Gly exhibited a three-fold elevated risk (HR 3.3, CI 1.22-8.96). In diabetics within NPHSII, there was a suggestion that 219Gly+ was associated with elevated CHD-risk (HR 1.89, CI 0.39-9.06) although numbers were small. To further assess the effect of the variant in diabetes, a case-control study of MI, HIFMECH, was used, in which previous analysis had defined a group with metabolic syndrome, by factor analysis. A significant CHD-risk interaction was identified between genotype and the 'metabolic syndrome' factor (interaction p=0.009). To further assess CHD-risk for this variant in type-2 diabetes and to assess the effect of the variant upon thrombin generation and plasma levels of soluble EPCR, a cross-sectional study of type-2 diabetes was used. A significant CHD-risk was identified for European Whites (OR 2.84, CI 1.38-5.85) and Indian Asians in this study (OR 1.6, CI 1.00-2.57) and the frequency of 219Gly was two-fold higher in Indian Asians. Soluble EPCR levels were strongly associated with genotype, with homozygotes for 219Gly having four-fold higher levels (p<0.0001). In vitro studies of EPCR-transfected cells suggested increased basal release of sEPCR from cells expressing the 219Gly EPCR phenotype. Furthermore, in base-line samples from NPHSII and in the diabetic study, a significant increase in prothrombin F1+2 level was observed for 219Gly. The increased CHD-risk and thrombin generation appears to be acting through increased shedding of the Gly allele from the cell surface.

Standardisation of thrombin generation test--which reference plasma for TGT? An international multicentre study.

We have previously shown that standardisation and normalization of results improve the intercentre variability of the calibrated automated thrombin generation test (TGT). We suspected that the source of reference plasma (RP) might be a contributing factor to variability and compared 5 commercial RP and a RP provided by the NIBSC, in an international, multicentre study. The detailed composition of the 6 tested plasma samples was determined in the Haemostasis Laboratory in Lyon. The lot to lot consistency, intra-assay, inter-assay variability were calculated for all tested plasmas. The RP and 3 plasma samples (a normal control, a hypocoagulable and a hypercoagulable plasmas) were tested over 6 days, in 5 European centres. Results were normalised against each of the tested RP and intercentre variability of results was compared. All laboratories used the same reagents. Before normalization, the inter-centre variability was 19.8 to 27.3%. After normalization, we observed a significantly improved inter-laboratory variation with all tested RP, despite differences between them. These results clearly demonstrate that the inter-centre variability of TGT can be significantly reduced by using a reference plasma normalization, and that certain RP have a better capacity to reduce this variability than others.