The complement system is an important defense mechanism against pathogens; however, in certain pathologies, the system also attacks human cells, such as red blood cells (RBCs). In paroxysmal nocturnal hemoglobinuria (PNH), RBCs lack certain complement regulators which sensitize them to complement-mediated lysis, while in autoimmune hemolytic anemia (AIHA), antibodies against RBCs may initiate complement-mediated hemolysis. In recent years, complement inhibition has improved treatment prospects for these patients, with eculizumab now the standard of care for PNH patients. Current complement inhibitors are however not sufficient for all patients, and they come with high costs, patient burden, and increased infection risk. This review gives an overview of the underlying pathophysiology of complement-mediated hemolysis in PNH and AIHA, the role of therapeutic complement inhibition nowadays, and the high number of complement inhibitors currently under investigation, as for almost every complement protein, an inhibitor is being developed. The focus lies with novel therapeutics that inhibit complement activity specifically in the pathway that causes pathology or those that reduce costs or patient burden through novel administration routes.

Cold antibody-mediated autoimmune hemolytic anemia (cAIHA) is a rare disease, usually related to an underlying indolent clonal B-cell lymphoproliferative disorder. Jalink and colleagues describe an international retrospective analysis of Bruton tyrosine kinase (BTK) inhibition in patients with cAIHA. The data indicate a rapid and notable improvement in both the hemolytic anemia as well as acrocyanosis, suggesting that this may be an effective approach, regardless of underlying pathology or MYD88 mutational status.

Background In cold agglutinin mediated autoimmune hemolytic anemia (cAIHA), anti-red blood cell autoantibodies lead to complement-mediated hemolysis with or without symptoms of acrocyanosis after exposure at low temperatures. cAIHA can be divided into cold agglutinin disease (primary CAD) and cold agglutinin syndrome (CAS). The latter is secondary to diseases such as B-cell malignancies including CLL, infections or autoimmune disorders. In primary CAD, more than 90% of patients have a monoclonal IgM (mostly low level) and often a small bone marrow B-cell clone. There is no approved treatment. For patients with significant hemolytic anemia or acrocyanosis despite thermal protection, rituximab is the most accepted first line treatment with an overall response rate of 50% and median duration of response <1 year. Cytotoxic combinations such as rituximab-bendamustine produce more sustained remissions, although with concerns for long-term adverse effects and stem cell toxicity. Studies involving complement inhibitors are showing promising results on hemolysis, although cold induced peripheral symptoms (IgM mediated rather than complement-mediated) will not improve. Recent international guidelines on cAIHA suggest treatment with the Bruton tyrosine kinase (BTK)-inhibitor ibrutinib in refractory patients with cAIHA (Jäger et al Blood Rev 2020). Indeed, the underlying pathophysiology of cAIHA suggest that BTK inhibition could be effective. Aims To evaluate the efficacy of ibrutinib on anemia, hemolysis and acrocyanosis in patients with cold agglutinin-mediated AIHA (CAD/CAS). Methods An international retrospective study was undertaken of cAIHA patients (CAD/CAS) treated with BTK inhibition using a preformed questionnaire. For eligible patients, laboratory and clinical data regarding underlying disease, bone marrow pathology, hemolytic parameters and patient-reported acrocyanosis were collected at diagnosis, 30 days, 3 months, 6 months and 12 months and last date of follow up. Hemoglobin (Hb) response was considered none (NR), partial (PR, >2 g/dL Hb increase or >10g/dL) or complete (CR, >12g/dL). Adverse events were graded according to the Common Terminology Criteria, version-5.0 (2017). Results So far, 10 patients with cAIHA treated with a BTK-inhibitor (all involving ibrutinib) could be included in the study. Patients were followed from April 2014 until June 2020 at 5 centers (Italy (2), Norway, The United Kingdom and The United States). Median duration of follow up was 20 months (1-74 months). The main findings are summarized in table 1. The indication to start treatment was cAIHA based in all but 1 case (CLL). Median previous number of therapies was 2. All patients had a complement-mediated hemolytic anemia, 7 were transfusion-dependent, and 7 reported symptoms of acrocyanosis at the initiation of ibrutinib. After initiation of ibrutinib, all patients showed an improvement in hemoglobin (Median rise: 4.4 g/dL) resulting in 1 PR and 9 CR. All 7 transfusion-dependent patients became transfusion independent (5 within 30 days). In all but 1 patient, markers of hemolysis (LDH, bilirubin) improved after initiation of ibrutinib (see Figure 1). All 7 patients with acrocyanosis reported clear clinical improvement, with complete resolution of symptoms in 5. There was 1 adverse event (grade 1 bleeding). Data collection is still ongoing and future updates are expected. Conclusion Data show that ibrutinib is effective in the treatment of cAIHA with a notable and brisk improvement of both the hemolytic anemia as well as the cold induced peripheral symptoms. Although preliminary, these promising data support further research of BTK-inhibitor based treatment of cAIHA (CAD/CAS) in a prospective study. Disclosures Berentsen: Alexion, Apellis, Bioverativ and Janssen-Cilag: Other: Travel grants ; Alexion, Apellis, Bioverativ, Janssen-Cilag, True North Therapeutics: Honoraria; Apellis, Bioverativ, Momenta Pharmaceuticals and True North Therapeutics: Consultancy; Mundipharma: Research Funding. Castillo:TG Therapeutics: Research Funding; Pharmacyclics: Consultancy, Research Funding; Beigene: Consultancy, Research Funding; Kymera: Consultancy; Abbvie: Research Funding; Janssen: Consultancy, Research Funding. Treon:Bristol-Meyer-Squibb: Honoraria, Research Funding; Pharmacyclics: Honoraria, Research Funding. D'Sa:Sanofi: Honoraria; BeiGene: Honoraria, Research Funding; Janssen: Honoraria, Research Funding. OffLabel Disclosure: BTK-inhibitors (ibrutinib/acalabrutinib) are not yet indicated for the use in (primairy) cold autoimmune hemolytic anemia (cAIHA). However it is indicated for use in Waldenstrom macroglobulinemia (WM) and chronic lymphatic leukemia (CLL). Here we report retrospective data on a cohort of cases treated with ibrutinib for cAIHA mostly secondary to WM or CLL.

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Complement-mediated autoimmune hemolytic anemia (CM-AIHA) is characterized by destruction of red blood cells (RBCs) by autoantibodies that activate the classical complement pathway. These antibodies also reduce transfusion efficacy via lysis of donor RBCs. Since C1-inhibitor (C1-INH) is an endogenous regulator of the classical complement pathway, we hypothesized that peritransfusional C1-INH in patients with severe CM-AIHA reduces complement activation and hemolysis, and thus enhance RBC transfusion efficacy. We conducted a prospective, single center, phase 2 open-label trial (EudraCT2012-003710-13). Patients with confirmed CM-AIHA and indication for the transfusion of two RBC units were eligible for inclusion. Four intravenous C1-INH doses (6000, 3000, 2000 and 1000 U) were given with 12 h intervals around RBC transfusion. Serial blood samples were analyzed for hemolytic activity, RBC opsonization, complement activation and inflammation markers. Ten patients were included in the study. C1-INH administration increased plasma C1-INH antigen and -activity, peaking at 48 h after the first dose, accompanied by a significant reduction of RBC C3d deposition. Hemoglobin levels increased briefly after transfusion but returned to baseline within 48 h. Overall, markers of hemolysis, inflammation and complement activation remained unchanged. Five grade 3 and one grade 4 adverse event occurred, but were considered unrelated to the study medication. In conclusion, peritransfusional C1-INH temporarily reduced complement activation. However, C1-INH failed to halt hemolytic activity in severe transfusion-dependent CM-AIHA. We cannot exclude that posttransfusional hemolytic activity would have been even higher without C1-INH. The potential of complement inhibition on transfusion efficacy in severe CM-AIHA remains to be determined.

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