Complement Inhibitors in the Management of Complement-Mediated Hemolytic Uremic Syndrome and Paroxysmal Nocturnal Hemoglobinuria
Authors: Begum Farhana , Khan Nida , Boisclair Stephanie , Malieckal Deepa A. , Chitty David 
Published: April 2023

Background:
Complement-mediated HUS (CM-HUS) and paroxysmal nocturnal hemoglobinuria (PNH) are rare hematologic disorders that cause dysregulation and hyperactivation of the complement system. Historically, treatment of CM-HUS involved plasma exchange (PLEX), often with limited benefit and variable tolerance. Conversely, PNH was treated with supportive care or hemopoietic stem cell transplant. Within the last decade, monoclonal antibody therapies that block terminal complement pathway activation, have emerged as less invasive and more efficacious options for management of both disorders. This manuscript seeks to discuss a relevant clinical case of CM-HUS and the evolving landscape of complement inhibitor therapies for CM-HUS and PNH.

Areas of Uncertainty:
Eculizumab, the first humanized anti-C5 monoclonal antibody, has been the standard of care in treating CM-HUS and PNH for over a decade. Although eculizumab has remained an effective agent, the variability in ease and frequency of administration has remained an obstacle for patients. The development of novel complement inhibitor therapies with longer half-lives, has allowed for changes in frequency and route of administration, thus improving patient QOL. However, there are limited prospective clinical trial data given disease rarity, and limited information on variable infusion frequency and length of treatment.

Therapeutic Advances:
Recently, there has been a push to formulate complement inhibitors that improve QOL while maintaining efficacy. Ravulizumab, a derivative of eculizumab, was developed to allow for less frequent administration, while remaining efficacious. In addition, the novel oral and subcutaneous therapies, danicopan and crovalimab, respectively, along with pegcetacoplan are currently undergoing active clinical trials, and poised to further reduce treatment burden.

Conclusion:
Complement inhibitor therapies have changed the treatment landscape for CM-HUS and PNH. With a significant emphasis on patient QOL, novel therapies continue to emerge and require an in-depth review of their appropriate use and efficacy in these rare disorders.

Clinical Case:
A 47-year-old woman with hypertension and hyperlipidemia presented with shortness of breath and was found to have hypertensive emergency in the setting of acute renal failure. Her serum creatinine was 13.9 mg/dL; elevated from 1.43 mg/dL 2 years before. The differential diagnosis for her acute kidney injury (AKI) included infectious, autoimmune, and hematologic processes. Infectious work-up was negative. ADAMTS13 activity level was not low at 72.9%, ruling out thrombotic thrombocytopenic purpura (TTP). Patient underwent a renal biopsy, which revealed acute on chronic thrombotic microangiopathy (TMA). A trial of eculizumab was initiated with concurrent hemodialysis. The diagnosis of CM-HUS was later confirmed by a heterozygous mutation in complement factor I (CFI), resulting in increased membrane attack complex (MAC) cascade activation. The patient was maintained on biweekly eculizumab and was eventually transitioned to ravulizumab infusions as an outpatient. Her renal failure did not recover, and the patient remains on hemodialysis while awaiting kidney transplantation.