Open ADAMTS-13 conformation index predicts earlier relapse in immune-mediated thrombotic thrombocytopenic purpura

Background: ADAMTS-13 adopts an open conformation in patients with immunemediated thrombotic thrombocytopenic purpura (iTTP) in acute phase while being closed in healthy donors. We reported that a substantial number of patients with iTTP in remission with restored ADAMTS-13 activity (>50%) still had an open ADAMTS-13 conformation, although a closed conformation is expected given the extent of remission. Objectives: To investigate whether open ADAMTS-13, represented by a conformation index >0.5, is associated with a risk of earlier ADAMTS-13 and/or clinical relapse. Methods: We collected follow-up data (ADAMTS-13 parameters, ADAMTS-13 and clinical relapse, and treatment) from 81 patients with iTTP in remission with ADAMTS- 13 activity >50%. Results: During follow-up, 19 ADAMTS-13 and 10 clinical relapses were reported (median follow-up period, 20 months). First, open or closed ADAMTS-13 conformation was dichotomized based on the 0.5 conformation index cutoff. Open ADAMTS-13 (conformation index, >0.5) was not identified as a risk factor for ADAMTS-13 and clinical relapse (log -rank test and Cox regression model). In contrast, by identifying the optimal conformation index cutoff for relapse prediction, using classification and regression tree analysis, a conformation index >0.645 and >0.835 was shown to be a risk factor for ADAMTS-13 relapse (hazard ratio, 3.3; 95% CI, 1.3-8.3; P = .01) and clinical relapse (hazard ratio, 4.4; 95% CI, 1.3-15.3; P = .02), respectively. Conclusion: Patients with open ADAMTS-13 with a conformation index >0.645 and >0.835 have a >3- and >4 -fold higher risk of earlier ADAMTS-13 and clinical relapse, respectively. Hence, ADAMTS-13 conformation index could be used to complement ADAMTS-13 activity monitoring to timely notice ADAMTS-13 relapse and prevent clinical relapse.