PS1492 ASSESSING TREATMENT RESPONSE IN CONGENITAL THROMBOTIC THROMBOCYTOPENIC PURPURA WITH AN INNOVATIVE SHEAR FLOW‐BASED ASSAY

Background: Congenital Thrombotic Thrombocytopenic Purpura (TTP) is defined by severe ADAMTS13 activity (<10%) and absence of anti‐ADAMTS13 antibodies, confirmed by mutational analysis. ADAMTS13 deficiency results in increased ultra large Von Willebrand Factor (VWF) multimers and disseminated microvascular ischemia. Current replacement therapy is primarily using plasma infusion (PI). Dose and PI interval are selected empirically for each patient. At present there is no method to assess disease and treatment response in patients with cTTP. Aims: We developed an innovative shear flow‐based assay to assess cTTP, with normal platelet counts, receiving plasma infusion (PI) or intermediate purity factor VIII concentrate (BPL‐8Y). Methods: A VenaFlux semi‐automated microfluidic system provides high shear flow. Citrated whole blood was treated with DiOC6 for platelet fluorescence. Thrombus formation on collagen‐coated microchannels was analysed by an epifluorescence microscope and an automated calculation of total surface coverage was developed. Surface coverage represented increasing thrombus formation (VWF‐platelets). Total surface coverage within 180 seconds equalled 100% coverage. Normal range for surface coverage, using 50 normal controls, was 6–39%. cTTP samples were taken 30 minutes before and after prophylaxis (PI or BPL‐8Y) and surface coverage determined. Further re‐measurement was undertaken after initiation of antiplatelet therapy. Results: 21 confirmed cTTP patients: 18 female and 3 male, median age: 33 years (range 16–69). Median pre‐treatment surface coverage was 89% (range 47–100%), with no significant difference considering mutation type (homozygous 98%, heterozygous 83%, p = 0.097) or age of presentation (childhood onset 73%, adult onset 90%, p = 0.19). PI improved surface coverage results (post PI coverage 44%, p = 0.0005). Antiplatelet therapy further improved median surface coverage, pre and post prophylaxis. Median pre‐treatment surface coverage for patients receiving PI and antiplatelet therapy was 50% ( p  < 0.0001). This improvement was confirmed by the median post‐treatment coverage (19% for patients on antiplatelets vs. 44% for patients not on antiplatelets, p = 0.0011). Post‐treatment surface coverage returned to normal range in 100% patients who received PI and antiplatelet therapy compared to 50% in patients who received PI alone (p = 0.04). Summary/Conclusion: This advanced shear flow‐based assay showed increased thrombi formation in cTTP patients, despite ‘remission’, confirming severe ADAMTS13 deficiency is associated with increased circulating ultra large VWF multimers and platelet thrombi. This assay provides additional information on treatment assessment in cTTP patients, optimised with regular PI and antiplatelet therapy, and it guides treatment.