This study investigated how cyclodextrin excipients interact with relaxin, a therapeutic polypeptide being developed for heart-failure treatment. Affinity capillary electrophoresis and Flow Induced Dispersion Analysis (FIDA) were used to measure the binding between relaxin and two cyclodextrin variants, SBE-β-CD and HP-β-CD. Relaxin showed higher affinity for SBE-β-CD than for HP-β-CD, though both interactions were considerably weaker than relaxin's interaction with human serum albumin. The findings support sulfobutylether-β-cyclodextrin as an excipient that can improve solubility while preserving pharmacokinetic performance.