Pharmacological Therapies in Paroxysmal Nocturnal Haemoglobinuria: Focus on Complement Inhibition

Paroxysmal nocturnal haemoglobinuria (PNH) is an ultra-rare acquired genetic stem cell disorder based on a mutation in the PIGA gene that results in susceptibility of resulting blood cells to complement-mediated intravascular haemolysis (IVH). In countries where anti-complement therapy is available, pharmacological treatments have transformed this disease from a highly morbid and sometimes lethal disorder. The first treatment developed was the terminal complement (C5) monoclonal antibody inhibitor eculizumab, in 2002. This has been largely supplanted by a longer-acting antibody, ravulizumab, targeting the same binding site on C5. These agents significantly modify the natural history of the disease by reducing the risk of thrombosis, the most lethal complication of PNH, as well as reducing transfusion dependence and improving renal function, quality of life and probably, survival. Other terminal inhibitors available include eculizumab biosimilars, crovalimab, pozelimab and cemdisiran (combination). Despite this, a proportion of patients develop extravascular haemolysis (EVH) based on the accumulation of C3 components on these PNH blood cells, which no longer undergo IVH because of C5 inhibition. This has led to the development of proximal complement inhibitors, which have been generally successful at reducing this iatrogenic complication, improving haemoglobin concentrations, reducing transfusion dependency and improving quality of life. Currently available proximal inhibitors (and their targets) are pegcetacoplan (C3), danicopan (Factor D) and iptacopan (Factor B). While effective, as with all other complement inhibitors, there is a risk of breakthrough IVH with their use and approaches to manage this complication are being developed.