This assay enables the pharmacodynamic and pharmacokinetic properties of bypassing therapies to be monitored, thus helping to optimize treatment.

Factor VIII (FVIII)-bypassing agents have complex modes of action but all control bleeding in inhibitor patients by triggering the generation of thrombin. No routine test is available for monitoring this therapy in patients with inhibitors against FVIII. We present an assay that records FEIBA- or FVIIa-mediated changes in thrombin generation (TG) in FVIII inhibitor plasma samples. In plasma samples spiked with FEIBA TG was normalized above 0.4 U/ml, while for recombinant FVIIa (rFVIIa) more than 12.5 µg/ml were required to induce TG in the absence of tissue factor (TF). Addition of TF increased the TG potential of rFVIIa in vitro. This assay seems suitable for monitoring the pharmacokinetics of inhibitor bypassing agents during treatment and possibly for predicting responses to treatment.

Introduction: FEIBA â consists of zymogens and traces of activated forms of procoagulant factors II, VII, IX, X, anticoagulants protein C and TFPI, and small amounts of cofactors FV, FVIII and protein S, in a balanced ratio. As shown previously, FII-FXa complex plays a key role in FEIBA's mode of action (MoA). Methods: Thrombin generation (TG) was measured by spiking coagulation factors, cofactors and inhibitors to high titer FVIII inhibitor plasma, and in plasma samples from patients in a phase 3 clinical study evaluating the safety and efficacy of FEIBA prophylaxis in haemophilia A patients with inhibitors. Results: Increasing the FXa/FII ratio improved TG, while adding coagulation enzyme components had a negligible effect. Adding FX, FIX, and FVII increased the peak thrombin and decreased the lag time. The presence of FV and phospholipids led to faster TG, while protein C and protein S reduced the amount of peak thrombin. TFPI appeared to have no effect. Patients on prophylaxis with FEIBA â showed higher peak thrombin and AUC with elevated FII, FX, FIX, FVIIa, and protein C levels, and experienced significantly less bleeding episodes than those receiving on-demand treatment. Conclusion: These experiments showed that although the FII-FXa complex induced immediate thrombin formation on the activated platelet surface, other procoagulant components of FEIBA were necessary to achieve an optimal thrombin burst. The presence of the pro-and anti-coagulants in FEIBA provides a haemostatic balance, and is thus expected to prevent thrombotic events. Recent clinical data verified the postulated MoA of FEIBA in prophylaxis treatment.

SummaryIn humans, type III von Willebrand disease is caused by deletions or nonsense mutations. In dogs, the underlying genetic defects have not been determined yet. We searched for the genetic defect in four related type III deficient Dutch Kooiker dogs obtained from one breeder. Mutation analysis was performed with total RNA isolated from platelets or whole blood. The complete coding region of the vWf gene was amplified by RT-PCR and sequenced by the cycle sequencing technique. Two homozygous mutations were found, a G→A transition at the first position of the donor splice site sequence of intron 16 (TGgtaagt→TGataagt) and a missense mutation at nt 208 (G→A) (1). The splice site defect resulted in the generation of a transcript containing 46bp of intron sequence and a stop codon at amino acid position 729 in the propeptide region of the vWf protein. This mutation seems to be causative for the type III phenotype. The effect of the missense mutation in exon 3 which causes a change of Val to Ile on the vWD phenotype is unclear. Probably, this transition represents a polymorphism occurring in Dutch Kooiker dogs. Both mutations were not present in 5 healthy mongrel dogs.Parts of this paper were presented at the 39th annual meeting of the American Society of Hematology (ASH), December 5-9, San Diego, USA

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SummaryWe have worked out an efficient and time saving procedure for the expression of recombinant human prothrombin. The glycoprotein was expressed in the vaccinia virus expression system in several mammalian cell lines. The kidney cell lines Vero and BHK and the human cell line Hela were found to efficiently secrete prothrombin. Expression levels of 3–4 µg of factor II per 106 cells per day corresponding to 18–23 mU per 106 cells per day were achieved. Since the expression levels obtained with the vaccinia virus/Vero cell system were comparable to those obtained in amplified transformed CHO cells it provides an alternative system for the efficient expression of human prothrombin and may allow to further elucidate structure-function relationships of (pro)thrombin and its various effectors.