BACKGROUND AND OBJECTIVE: Eculizumab is an expensive therapeutic monoclonal antibody inhibiting complement C5 and approved for various indications, including the rare disease paroxysmal nocturnal hemoglobinuria. Eculizumab is administered in a “one-dose-fits-all” dosing paradigm in adults, which is inflexible and suboptimal in some patients. Therefore, the aim of this study was to develop alternative dosing regimens that may improve patient friendliness or improve cost effectiveness. METHODS: A prospective observational pharmacokinetic study was conducted in 27 patients with paroxysmal nocturnal hemoglobinuria. The dataset was enriched with pharmacokinetic and pharmacodynamic data of a previous study of patients with atypical hemolytic uremic syndrome. A population pharmacokinetic/pharmacodynamic model was developed and this model was used to explore alternative and individualized dosing regimens. RESULTS: A two-compartment model with parallel linear and non-linear elimination best described the data. No intra-individual variability in clearance could be observed for patients with paroxysmal nocturnal hemoglobinuria in contrast to patients with atypical hemolytic uremic syndrome. An inhibitory Emax model described the relationship between plasma concentrations and complement activity. We predicted that only 52.0% of patients with paroxysmal nocturnal hemoglobinuria have adequate complement inhibition on day 7 with the standard loading dose, compared with 99.9% of the patients with an alternative weight-based loading dose, without an increase in treatment costs. A 4-weekly dosing regimen was developed and therapeutic drug monitoring will enable interval prolongation to 4 weeks without relevant increases in cumulative drug use compared to the approved dose. CONCLUSIONS: The pharmacokinetics of eculizumab are similar in patients with atypical hemolytic uremic syndrome and patients with paroxysmal nocturnal hemoglobinuria, yet less variable in patients with paroxysmal nocturnal hemoglobinuria. Alternative dosing regimens can improve treatment in terms of efficacy and patient friendliness. CLINICAL TRIAL REGISTRATION: ClincialTrials.gov identifier: NCT04079257. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s40262-025-01536-x.