Recurrent Thrombocytopenia During Caplacizumab Therapy in Acute Immune‐Mediated Thrombotic Thrombocytopenic Purpura

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is a fatal thrombotic microangiopathy (TMA) caused by autoantibodies against disintegrin and metalloprotease with thrombospondin type 1 motif member 13 (ADAMTS13) [1]. Until recently, the standard treatments for iTTP were therapeutic plasma exchange (TPE) and immunosuppression. Caplacizumab, a humanized nanobody targeting the A1 domain of von Willebrand factor (VWF), demonstrated superiority over placebo in faster normalization of platelet count, a reduction in the composite endpoint (TTP-related death, exacerbation, or at least one thrombotic event), and a decrease in the need for TPE in an integrated analysis of the phase 2 and 3 trials for acute iTTP [2]. Treatment evaluation in the era of caplacizumab was proposed by the International Working Group (IWG) for iTTP [3]. Updated definitions for exacerbation and remission were evaluated based on the suspension of both TPE and anti-VWF therapy, namely, caplacizumab. However, thrombocytopenic events during caplacizumab administration have not yet been analyzed so far. This correspondence reports the findings of our study on the frequency of thrombocytopenic events and their association with relevant clinical parameters in caplacizumab-treated Japanese acute iTTP patients. Briefly, 33 episodes in 31 Japanese patients with acute iTTP who were administered caplacizumab between December 2022 and February 2024 were identified in the Nara Medical University Registry on TMA [4]. iTTP diagnosis was as follows: (1) hemolytic anemia and thrombocytopenia, (2) severely decreased ADAMTS13 activity (< 10%), and (3) presence of detectable ADAMTS13 inhibitor (≥ 0.5 Bethesda units [BU]/mL) [5]. Recurrent thrombocytopenia was defined as a decrease in the platelet count to < 150 × 109/L during caplacizumab therapy after initially achieving a clinical response (CR) with a platelet count of ≥ 150 × 109/L. Twenty patients met the criteria for recurrent thrombocytopenia (thrombocytopenia group), whereas the remaining patients were classified into the non-thrombocytopenia group. Additionally, we defined the event at which the platelet count reached its lowest point after achieving CR as the second dip (Figure S1). Of the 20 patients, 9 underwent intensified treatment, such as TPE or immunosuppressive therapies (intensified treatment group); the remaining 11 patients were classified into the non-intensified treatment group (Figure S2). Citrated patient plasma samples taken at diagnosis or during the treatment course were prepared and stored at −30°C until analysis. A sensitive chromogenic enzyme-linked immunosorbent assay (ELISA) was used to measure ADAMTS13 activity (Kainos Laboratories, Tokyo, Japan) [6]. ADAMTS13 inhibitor titer was evaluated using a plasma mixing assay (Bethesda assay); a titer of ≥ 0.5 BU/mL was considered positive. Anti-ADAMTS13 IgG levels were measured using a TECHNOZYM ADAMTS13 INH ELISA kit (Technoclone, Vienna, Austria). VWF antigen (VWF:Ag) and VWF ristocetin cofactor activity (VWF:RCo) (Siemens Healthineers) were analyzed using an automated coagulation analyzer CN-3000 (Sysmex, Kobe, Japan). Time-course evaluations of parameters related to ADAMTS13 and VWF were conducted at the following time points: (1) initial presentation, (2) initial CR with a platelet count ≥ 150 × 109/L, (3) at the second dip, (4) at the end of caplacizumab treatment, and (5) 1 week after the end of caplacizumab treatment (Figure S1). The time from the first TPE to sustained ADAMTS13 activity ≥ 10% was analyzed for correlations with initial ADAMTS13 inhibitor and anti-ADAMTS13 IgG levels. Categorical variables are presented as frequencies and continuous variables as medians with interquartile ranges. Comparisons between groups were made using Fisher's exact test or the Mann–Whitney U test, temporal changes by the Friedman test, and correlations by Spearman's rank test. p < 0.05 was considered significant. Analyses were performed using EZR [7]. Overall, 33 episodes in 31 patients with acute iTTP treated with caplacizumab were included for evaluation. 17 of 20 patients in the thrombocytopenia group and 12 of 13 patients in the non-thrombocytopenia group were enrolled during their initial iTTP episode. We retrospectively compared the differences in clinical characteristics, laboratory parameters, and treatment methods between patients with and without thrombocytopenia (Table S1). Laboratory data, the proportion of initial neurological symptoms, and treatment methods were not significantly different between the two groups. All patients achieved CR. Patients with thrombocytopenia were categorized into the intensified (n = 9) and the non-intensified group (n = 11). Clinical parameters and treatment details are summarized in Table 1. In the intensified treatment group, additional therapies were required mainly for recurrent thrombocytopenia, and included TPE (n = 4), FFP infusion (n = 1), rituximab (n = 3), prednisolone pulse therapy (n = 2), and cyclophosphamide (n = 2) (Table S2). Median initial titers of ADAMTS13 inhibitor by Bethesda assay were significantly higher in the intensified-treatment group (13.5 vs. 4.6 BU/mL, p = 0.015), whereas initial anti-ADAMTS13 IgG levels did not differ significantly. Median initial VWF:RCo/VWF:Ag was lower in the intensified treatment group (0.38 vs. 0.62, p = 0.026). The number of TPE sessions and the proportion or timing of rituximab were comparable between groups. By contrast, the median time to achieve ADAMTS13 activity ≥ 10% (43 vs. 19 days, p = 0.003) and the duration of caplacizumab treatment (47 vs. 36 days, p = 0.014) were significantly longer in the intensified treatment group. We next compared laboratory parameters at the second dip between groups. The median platelet count at the second dip was significantly lower in the intensified group (66 × 109/L vs. 109 × 109/L, p = 0.018). Hemoglobin and lactate dehydrogenase levels did not differ between groups. By contrast, the median ADAMTS13 activity was significantly lower (< 0.5% vs. 23.7%, p = 0.015), and both median ADAMTS13 inhibitor titers (3.6 vs. < 0.5 BU/mL, p = 0.036) and anti-ADAMTS13 IgG antibody levels (69.3 vs. 8.0 U/mL, p = 0.012) were significantly higher in the intensified group. Changes in the VWF:RCo and VWF:Ag levels were assessed separately in the intensified and non-intensified treatment groups (Figure 1). At nearly all time points during treatment, VWF:RCo decreased to the lower limit of quantification (12.13%) in both groups. Both parameters varied significantly over time in the intensified group without pairwise changes, whereas they were lower during treatment than at baseline or post-treatment in the non-intensified group. Among all patients included in this study, the initial ADAMTS13 inhibitor titer positively correlated with the time from the first TPE to achieving ADAMTS13 activity ≥ 10% (r = 0.672, p < 0.001) (Figure S3A), whereas the correlation with the initial anti-ADAMTS13 IgG level was 0.388 (n = 30, p = 0.034) (Figure S3B). Recurrent thrombocytopenia was observed in more than half of patients with iTTP treated with caplacizumab. Some patients received additional therapies such as re-initiation of plasma exchange or escalation of immunosuppression. The 2021 IWG consensus report redefined clinical outcomes as follows [3]: clinical exacerbation as a fall in platelet count < 150 × 109/L within 30 days after discontinuation of TPE or anti-VWF therapy, and clinical relapse as a similar decrease after clinical remission with documented severe ADAMTS13 deficiency. However, recurrent thrombocytopenia during caplacizumab, observed in our study, was not included, and no management strategy was proposed. Notably, among real-world reports on caplacizumab use, only the German–Austrian group documented exacerbation during caplacizumab in ~10% of first-line cases [8]. In cases of recurrent thrombocytopenia requiring intensified treatment, patients showed a more pronounced decrease in platelet count, lower ADAMTS13 activity, and higher ADAMTS13 inhibitor titer at the time of the second dip compared with the non-intensified treatment group. These patients also had higher inhibitor titers at presentation and delayed ADAMTS13 recovery, suggesting that the immunological status of iTTP was not sufficiently controlled. Previously, we speculated that persistent inhibitors might be related to fewer TPE sessions in caplacizumab-treated acute iTTP patients [9]. In the present study, although the initial inhibitor titer was higher in the intensified-treatment group, the number of TPE sessions did not differ significantly. The Japanese TTP guideline recommends continuing TPE until ≥ 2 days after platelet count normalization [5]. For patients with higher inhibitor titers, extending TPE beyond CR may help facilitate ADAMTS13 activity recovery by clearing anti-ADAMTS13 antibodies. In the non-intensified treatment group, two patients exhibited a marked reduction in ADAMTS13 activity with detectable anti-ADAMTS13 inhibitors, while in the other two patients, intestinal bleeding was identified. These were considered potential causes of recurrent thrombocytopenia in these patients. For the remaining seven patients, ADAMTS13 activity was preserved at the time of the second dip, suggesting the involvement of miscellaneous factors, including drug-related causes, in thrombocytopenic events. Stage-specific changes in VWF parameters indicate disease activity and treatment response. At initial presentation, the median initial VWF:RCo/VWF:Ag was significantly lower in the intensified treatment group, suggesting that more extensive thrombus formation led to the consumption of high-molecular-weight VWF multimers in patients requiring additional treatment. In contrast, during episodes of recurrent thrombocytopenia, VWF:RCo and VWF:Ag also declined but to a similar extent in both groups. This pattern is considered to reflect the therapeutic effect of caplacizumab, thereby excluding the possibility of inadequate drug efficacy as a cause of recurrent thrombocytopenia. This study is limited by its small size, retrospective design, and restriction to Japanese patients. Moreover, variability in test selection and plasma sampling produced incomplete data, such as those of initial anti-ADAMTS13 IgG levels. Recurrent thrombocytopenia was observed in over half of patients with acute iTTP receiving caplacizumab. Measuring ADAMTS13-related markers may help assess disease status and guide treatment decisions, which should be confirmed in prospective studies. We extend our gratitude to the primary care physicians who sent the specimens to our laboratory. This study was approved by the ethics committee of Nara Medical University and conducted in accordance with the Declaration of Helsinki. Information about the study was publicly available on a specific laboratory website, and patients were considered to have provided informed consent unless they declined to participate (the opt-out method). Information about the study was publicly available on a specific laboratory website, and patients were considered to have provided informed consent unless they declined to participate (the opt-out method). Kazuya Sakai received speaker fees from Sanofi and participated on the advisory boards for Takeda. Masayuki Kubo and Yasunori Ueda received speaker fees from Sanofi. Masanori Matsumoto provided consultancy services for Takeda, Alexion Pharma, and Sanofi; received speaker fees from Takeda, Alexion Pharma, Asahi Kasei Pharma, and Sanofi; and received research funding from Alexion Pharma, Chugai Pharmaceutical, Asahi Kasei Pharma, and Sanofi. For original data, please contact [email protected]. Data S1: ajh70139-sup-0001-Supinfo.docx. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.