This study is researching an experimental drug called ALN-CFB. The study is focused on people with Paroxysmal Nocturnal Hemoglobinuria (PNH) who are currently taking a complement component C5 inhibitor ("C5-inhibitor") and continue to have anemia (low red blood cell count).
The aim of the study is to see how tolerable ALN-CFB is compared to placebo. A placebo looks like the study drug but does not contain any drug.
The study is looking at several other research questions, including:
* What side effects may happen from taking ALN-CFB * How much ALN-CFB is in the blood at different times * How much Complement Factor B (CFB) protein levels in the blood are affected by ALN-CFB
Has been diagnosed with PNH confirmed by a history of high flow cytometry from prior testing
Treated with a stable dose of C5 inhibitor (eculizumab or approved eculizumab biosimilar, ravulizumab, or crovalimab) for at least 24 weeks prior to screening visit, as described in the protocol
Has hemoglobin ≤10.5 g/dL at screening visit 1, with evidence of anemia prior to this visit, as described in the protocol
Has peripheral blood reticulocyte count of ≥100 x 10^9/L at screening visit 1
Has history of bone marrow transplantation or receipt of an organ transplant
Has history of meningococcal infection or similar recurrent infections by other encapsulated bacterial organisms
Has any active, ongoing infection or a recent infection requiring ongoing systemic treatment with antibiotics, antivirals, or antifungals within 2 weeks of screening or during the screening period
Has laboratory evidence of bone marrow failure, as described in the protocol