Characterization of Breakthrough Hemolysis in PatientsWith Paroxysmal Nocturnal Hemoglobinuria: An International Multicenter Experience

Breakthrough hemolysis (BTH) is defined as a hemolytic exacerbation in a patient with paroxysmal nocturnal hemoglobinuria(PNH) treated with complement inhibitors (CIs). In the current era of several terminal and proximal inhibitors, there are no guide-lines for defining BTH and its severity, and clinical management is not standardized. This retrospective, observational, and multi-centric study evaluated BTH frequency and severity in PNH patients treated with complement inhibitors from 2007 until February2025 at 10 centers across Italy (9 centers) and the United Kingdom (1 center). A total of 271 BTH events occurred in 102 out of 198patients (51%), with 36/198 (18%) patients experiencing 3 or more BTH episodes over the 18-year follow-up. Most events (55%) wereprompted by infections, and 24 (10%) were linked to poor compliance with oral alternative pathway inhibitors. BTH was clinicallysevere in about half of patients, requiring transfusion in 46%, CI dosing adjustment in 17%, and anticoagulant prophylaxis in 16%.Overall, 5 (2%) breakthrough thrombotic events occurred in patients not on prophylaxis, all during an infection. BTH incidencewas 0.19 events per patient-year, maximal with proximal inhibitors (0.4 events per patient-year), and lower for anti-C5 (0.18 perpatient-year). By logistic regression analysis, the main predictors of BTH were more severe disease at diagnosis (increased LDH anda shorter time to first complement inhibitor), treatment with proximal inhibitors, and poorer EBMT response category.