PNH on Eculizumab: When Controlled Labs Aren't the Whole Story
A single patient, well-controlled by the number that usually matters most. The disagreement isn't whether her therapy is working — it's whether the wrong number is the one being watched.
Grace L. has spent the past four months training for her first half-marathon, a goal she set the same week her paroxysmal nocturnal hemoglobinuria was finally stable enough on eculizumab that she stopped organizing her weekends around fatigue. She was diagnosed five years ago after an episode of dark, tea-colored morning urine sent her to an emergency department that correctly recognized the pattern rather than dismissing it, and flow cytometry confirmed a large PNH clone affecting both her red cells and granulocytes.
Two years into eculizumab therapy, her intravascular hemolysis is well controlled — her LDH sits near normal, and she hasn't had a dark-urine episode since starting therapy — but she has needed a transfusion every two to three months regardless, and her indirect bilirubin and reticulocyte count have stayed persistently elevated even on schedule with her biweekly infusions. Her training log shows exactly what that anemia costs her: her easy long-run pace has stalled for six weeks despite consistent effort, which is what actually prompted this visit rather than a scheduled recheck.
That combination, a controlled LDH alongside ongoing anemia and transfusion need, is the specific signature of extravascular hemolysis: C3 fragments opsonizing PNH red cells for clearance by splenic macrophages, a process that a C5 inhibitor like eculizumab was never designed to prevent, since it blocks the terminal complement pathway downstream of where C3 opsonization already occurs. PEGASUS, the head-to-head trial that tested this exact scenario, enrolled eculizumab-treated PNH patients with persistent anemia much like hers and found a significantly larger hemoglobin improvement after switching to pegcetacoplan, a C3 inhibitor, than those who stayed on eculizumab — a result that speaks directly to her own labs rather than to PNH in general.
A lab pattern that names its own fix
Her pattern — controlled LDH but persistent anemia, elevated indirect bilirubin, ongoing transfusion need — is the textbook signature of extravascular hemolysis that eculizumab can't touch, since it acts downstream of C3. I'd switch her to pegcetacoplan; PEGASUS enrolled exactly this population and found a significantly larger hemoglobin gain after switching than staying on eculizumab.
Her half-marathon goal isn't incidental to this decision — a patient this motivated to reclaim her stamina is exactly who benefits most from actually closing the gap PEGASUS identified, rather than accepting a partially-controlled baseline as good enough.
I agree the mechanism argument is sound, but switching to a C3 inhibitor isn't risk-free in exchange for the hemoglobin gain. Pegcetacoplan carries its own risk of breakthrough hemolysis, sometimes acute and severe, if a dose is missed or if a concurrent infection transiently overwhelms C3 inhibition — a different failure mode than anything she's experienced on eculizumab.
Before switching, I'd want her eculizumab dosing interval double-checked for under-dosing first — a straightforward, lower-risk fix if that turns out to be the actual gap, rather than assuming the mechanism must be extravascular without ruling out simple underdosing.
Agreed: confirm her eculizumab trough level and dosing interval first; if adequately dosed, proceed to pegcetacoplan given the PEGASUS-matched clinical picture.
Not agreed: how closely to monitor for breakthrough hemolysis in the first weeks after switching — the pharmacologist wanted a scheduled early recheck built into the plan given pegcetacoplan's own breakthrough risk, which the hematologist considered reasonable but not urgent given her currently stable intravascular markers.