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The quantification of residual plasmatic ADAMTS13 activity in congenital thrombotic thrombocytopenic purpura (TTP) patients is constrained by limitations in sensitivity and reproducibility of commonly used assays at low levels of ADAMTS13 activity, blunting efforts to establish genotype-phenotype correlations. In the present study, the residual plasmatic activity of ADAMTS13 was measured centrally by surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (limit of detection = 0.5%) in 29 congenital TTP patients. The results were used to study correlations among ADAMTS13 genotype, residual plasmatic activity, and clinical phenotype severity. An ADAMTS13 activity above 0.5% was measured in 26 (90%) patients and lower levels of activity were associated with earlier age at first TTP episode requiring plasma infusion, more frequent recurrences, and prescription of fresh-frozen plasma prophylaxis. Receiver operating characteristic curve analysis showed that activity levels of less than 2.74% and 1.61% were discriminative of age at first TTP episode requiring plasma infusion < 18 years, annual rate of TTP episodes > 1, and use of prophylaxis. Mutations affecting the highly conserved N-terminal domains of the protein were associated with lower residual ADAMTS13 activity and a more severe phenotype in an allelic-dose dependent manner. The results of the present study show that residual ADAMTS13 activity is associated with the severity of clinical phenotype in congenital TTP and provide insights into genotype-phenotype correlations.

Abstract 2219 ABSTRACT Congenital thrombotic thrombocytopenic purpura (TTP) (OMIM #274150) is a rare, recessively inherited thrombotic microangiopathy characterized by the congenital deficiency of ADAMTS13 due to mutations in the corresponding gene. The clinical phenotype of congenital TTP is variable, encompassing neonatal-onset disease and adult-onset disease, forms with a single disease episode and chronic-relapsing forms. A review of the ∼100 published cases of congenital TTP showed similar age of first disease episode in patients carrying the same ADAMTS13 gene suggesting that different ADAMTS13 mutations might influence the severity of clinical phenotype, conceivably by determining different patterns of residual plasmatic activity of ADAMTS13. However, the quantification of residual ADAMTS13 activity in severely deficient patients was blunted by the relatively high limit of detection (LOD) of commonly used ADAMTS13 activity assays (varying between 3% and 6%). We report 29 cases of congenital TTP from 4 European cohorts, with 32 mutations of ADAMTS13 including 8 not previously published. To assess if residual ADAMTS13 activity was detectable in these patients and correlated with their clinical history, ADAMTS13 activity was measured by SELDI-TOF mass spectrometry (LOD=0.5%). ADAMTS13 activity above 0.5% was measurable in 26 (90%) patients, and lower levels of activity correlated with an earlier age of disease onset (r=0.51; p=0.006) and with the need for regular fresh frozen plasma prophylaxis (2.24% vs 3.88%; p=0.03). Mutations at the highly conserved N-terminal domains of the protein were associated with lower residual ADAMTS13 activity (1.48% vs 4.89%, p=0.02) and earlier disease onset (3.2 years vs 24.2 years, p=0.003). Our results show that residual ADAMTS13 activity correlates with the severity of clinical phenotype in congenital TTP and provide insights into genotype-phenotype correlations. Disclosures: No relevant conflicts of interest to declare.

We previously reported that concurrent cyclosporine and plasma exchange(CSA/PE) is superior to corticosteroids and PE for acute TTP in terms of a decreased exacerbation rate (0% v. 47%) after stopping PE.(ASH 2005,Abstract# 1235) We now present similar data and serial analysis of ADAMTS13 activity and inhibitor concentration from two patient cohorts treated with CSA/PE. Patients: Ten patients failing initial therapy with corticosteroids and PE (exacerbations or refractory) were treated with CSA/PE. This study led to a second cohort study of upfront therapy with CSA/PE in 11 patients with acute TTP. Methods: CSA (2–3 mg/kg) orally was administered concurrent with daily PE. After remission (normal platelet count and LDH), 2 additional exchanges were performed on an every other day schedule. CSA was administered for 6 months and discontinued. Results: In the exacerbation/refractory patients, 9/10 achieved remission after a median of 11 exchanges. One patient suffered an exacerbation and required one additional exchange to achieve a sustained (>30 d.) remission. Four patients relapsed 2,3,7, and 10 months after completing 6 months of CSA. In the CSA/PE upfront group, 10/11 achieved remission. One patient died of infectious and disease-related complications. Remission occurred after a median of 5 exchanges. No patients(0/10) suffered an exacerbation. Three patients recurred, 2 while taking CSA and one 2 weeks after stopping CSA. Median ADAMTS13 activity and the change in inhibitor concentration over time for all CSA/PE patients is shown in Figure 1. Conclusions: CSA/PE improves clinical outcome and prevents exacerbations after tapering PE. CSA/PE also increased in vitro ADAMTS13 activity and suppressed the inhibitor, with both occurring well after the last PE. These findings coupled with the decline in median ADAMT13 activity after discontinuing CSA(Fig. 2) suggest the improvement in ADAMTS13 activity was a result of CSA therapy. Table 1. Presenting ADAMTS13 and Inhibitor Data N Med. ADAMTS13% Prior to Initial Therapy Med. ADAMTS13% Prior to CSA and PE Med. Inhibitor Conc. Prior to Initial Therapy(ug/mL) Med. Inhibitor Conc. Prior to CSA and PE(ug/mL) Remission Rate Exacerbation (30 d. Recurrence) Rate CSA and PE Exacerbation and Refractory 10 2 (0–8) 4 (0–20) 587 (275–1,111) 3,173 (372–9,845) 9/10 1/10 CSA and PE Upfront Therapy 11 N/A 2 (0–60) N/A 571 (147–4,988) 10/11 0/10 Table 2. Effect CSA and PE on ADAMTS13 Activity and Inhibitor Concentration. Time (weeks) 0 4 10 24 CSA and PE Exac. and Refractory N 9 9 7 8 Median ADAMTS13 % 4 (0–20) 24 (4–39) 42 (5–72) 49 (7–116) Median Inhibitor Conc. (ug/mL) 3,173 (372–9,845) 294 (165–6,548) 191 (41–4,228) 204 (55–756) CSA and PE Upfront Therapy N 10 8 5 4 Median ADAMTS13% 2 (0–60) 20 (5–104) 32 (4–37) 49 (7–116) Median Inhibitor Conc. (ug/mL) 571 (147–4,988) 270 (82–2,637) 302 (110–378) 204 (55–756) Figure Figure