The introduction of eculizumab, a monoclonal antibody targeting C5 of the complement cascade, revolutionized the treatment of paroxysmal nocturnal hemoglobinuria (PNH), a rare hematologic disorder characterized by complement-mediated intravascular hemolysis, bone marrow failure, and thrombophilia. Treatment options for PNH were limited before eculizumab was approved by the Food and Drug Administration in 2007.
Its use resulted in the inhibition of intravascular hemolysis, hemoglobin stabilization, and substantial reductions in transfusion requirements. Moreover, eculizumab had the unexpected effect of reducing the risk of thromboembolic complications, the most severe complication of PNH. Patients treated with eculizumab experienced fewer thrombotic events (4%), compared with historical cohorts (27%). Importantly, 5-year overall survival rates for patients with PNH taking eculizumab improved more than 90%, compared wity the 80% reported historically.
More than 10 years later, we are tasked with assessing the impact of this drug. Unquestionably, eculizumab has done more for PNH than we could have hoped for. However, 10 years of additional data reveal the limitations of this groundbreaking therapy. Despite the overall sustained response and survival benefit, hematologic response remains variable. Complete normalization of hemoglobin occurs in less than one-third of patients. Transfusion requirements persist in many patients. Residual anemia during eculizumab therapy is at least partly attributed to bone marrow failure, a feature the complement inhibition does not address. Still, pharmacokinetic limitations of the drug also contribute to the lack of complete responses. There is residual intravascular hemolysis because of insufficient inhibition of C5 and the emergence of C3-mediated extravascular hemolysis constitutes an unanticipated mechanistic complication of all C5-mediated therapies.
The last few years have seen a surge in novel anticomplement agents, which improve upon the already well-established inhibition of C5 but also explore the efficacy of targeting earlier aspects of the complement pathway. During the American Society of Hematology (ASH) annual meeting, we had exciting updates on some of the promising new kids on the block.
Ravulizumab, the newest C5 monoclonal antibody approved by the FDA for PNH, displays more robust C5 inhibition, thereby reducing the breakthrough hemolysis still seen with eculizumab use. Crovalimab, also an anti-C5 humanized antibody, is engineered with Sequential Monoclonal Antibody Recycling Technology that improves the half-life of the drug and facilitates subcutaneous dosing while still achieving complete C5 inhibition. Some of the most exciting data is on danicopan, a small-molecule factor D inhibitor that targets the alternative pathway thereby inhibiting C3 convertase and blocking extravascular hemolysis. It has shown promise as a stand-alone agent, as well as with combined C5 inhibition, while promising safety, a reasonable concern as we explore the long-term risks of targeting the proximal complement pathway.
I was recently asked to comment on how the new complement inhibitors are addressing unmet needs in PNH. While the recent presentations at ASH demonstrate an improvement on the efficacy of C5 inhibition, pharmacokinetics, and drug delivery – all which translate to improved hemoglobin and reduced breakthrough hemolysis for PNH patients – I am most excited at the promise this new generation of drugs holds for other diseases. Since its approval for PNH, eculizumab has also been approved for use in atypical hemolytic uremic syndrome (aHUS), myasthenia gravis, and neuromyelitis optica spectrum disorder.
Perhaps the greatest potential I envision for the new generation of drugs is in aHUS, a chronic disease characterized by hemolytic anemia, thrombocytopenia, and end-stage renal disease that cannot be cured with renal transplantation. The pathophysiology involves dysregulation of complement activation because of genetic mutations or autoantibodies to key proteins in the complement cascade. Though we have experienced some success with eculizumab, responses can be incomplete, particularly in patients with C5 mutations. The newer drugs offer the opportunity to inhibit complement activation at both proximal and distal aspects of the cascade, which may prove ideal in a disease in which the affected protein is not consistent. Moreover, preclinical and clinical trials have shown promise for these novel complement inhibitors in other autoimmune diseases: antibody-mediated vasculitis, C3 glomerulopathy, catastrophic antibody syndrome, membranous nephropathy, and lupus nephritis.
The surge of new complement inhibitors could revolutionize our strategy for treatment of autoimmune-mediated diseases, in which downstream complement activation can manifest with life-threatening tissue injury. Inhibition of complement offers a promising strategy for blocking downstream immune-mediated effector mechanisms of injury common in several autoimmune diseases.
As the results from various clinical trials come to fruition, it will be exciting to determine how to best use this new generation of drugs to target new diseases and whether the next decade is poised to eclipse the progress in complement therapy already established by eculizumab.
Dr. Sosa is a benign hematologist at Fox Chase Cancer Center in Philadelphia. Her research interests are in thromboembolic disease, with a focus in racial and gender disparities.