Development of a flow cytometric assay for detection of coated platelets in dogs and evaluation of binding of coated platelets to recombinant human coagulation factor VIIa.

ABSTRACT

OBJECTIVE: To develop an antibody-based flow cytometric assay to detect coated platelets in dogs and to characterize the interaction of recombinant human coagulation factor VIIa with activated platelets from dogs with hemophilia A. SAMPLE Platelets from 4 dogs with hemophilia A, 4 dogs with hemophilia B, 4 dogs with von Willebrand disease, and 6 hemostatically normal dogs. PROCEDURES Freshly isolated platelets were activated with thrombin, convulxin, or a thrombin-convulxin combination. Resulting platelet phenotypes were resolved on the basis of P-selectin and fibrinogen expression, and binding of recombinant human coagulation factor VIIa to these distinct platelet subpopulations was measured by use of a flow cytometric assay. RESULTS: Coated platelets were identified on the basis of expression of α-granule fibrinogen and were generated in response to stimulation with the thrombin-convulxin combination but not to stimulation with either agonist alone. Approximately 70% of the platelets from dogs with hemophilia A, hemophilia B, and von Willebrand disease and from the control dogs had the coated platelet phenotype. Recombinant human coagulation factor VIIa bound preferentially to coated platelets with a mean ± SD binding equilibrium constant of 2.6 ± 0.5µM. CONCLUSIONS AND CLINICAL RELEVANCE Formation of coated platelets in dogs was similar to that in humans. Recombinant human coagulation factor VIIa bound preferentially to coated platelets from dogs. IMPACT FOR HUMAN MEDICINE A similar mechanism of action for recombinant human coagulation factor VIIa may exist in dogs and humans. The potential for use of dogs in the study of bleeding disorders in humans was strengthened.