Bortezomib for rituximab-refractory immune-mediated thrombotic thrombocytopenic purpura in the caplacizumab era: an Italian multicenter study

Background: Immune-mediated thrombotic thrombocytopenic purpura (iTTP) patients are not responsive to standard rituximab in approximately 10-15% of cases, and oral immunosuppressants showed controversial results with significant toxicity. Targeting plasma cells with bortezomib appears promising, but available evidence is scarce and stems only from isolated reports in the pre-caplacizumab era. Objectives: To evaluate the safety and efficacy of bortezomib in rituximab-refractory iTTP patients. Methods: We conducted a retrospective observational multicenter study among 13 Italian iTTP-treating centers, collecting data from May 2017 to May 2023 (caplacizumab licensed in Italy in January 2020). Results: Bortezomib was effective in 10/17 patients (59%). Eleven were treated in the acute phase (9/11 responders, 82%, allowing discontinuation of caplacizumab in 5/6 treated patients), and 7 during clinical remission (2/7 responders, 28%). Responses occurred at a median time of 30 days, but 3 patients responded after 4 months. The median duration of response was 22 months (IQR 10-38), still ongoing in 6 patients at the time of data cut-off. Responders had fewer previous acute iTTP episodes than non-responders [median (IQR) 1 (1-2) vs 5.5 (2-7), p=0.03]. Eight subjects (47%) reported toxicities, mostly in those treated with ≥2 cycles. Conclusion: Durable responses to bortezomib were registered in about 60% of multi-refractory iTTP patients, with mild-to-moderate toxicities. The occurrence of late responses (i.e., after 30 days) suggests a "watchful waiting" approach after bortezomib treatment.