Protein‐Engineered Iron Nanoclusters as Hybrid Theranostic Materials for MRI Monitoring and Treatment of Cardiac Fibrosis

Integrating diagnostic imaging and therapy within a single platform remains a major challenge in precision medicine. Existing approaches mainly rely on multi‐component nanoparticles, increasing formulation complexity and limiting modularity. However, versatile platforms combining imaging and therapeutic functionalities remain limited. Here, we present protein‐engineered iron nanoclusters (Prot‐FeNCs) as hybrid protein‐based systems combining Hsp90‐targeted antifibrotic activity with high‐performance T 1 ‐weighted Magnetic Resonance Imaging (MRI) contrast. Using engineered proteins displaying cysteine‐based coordination sites, we develop a green aqueous synthesis route to ultrasmall iron nanoclusters that provide strong positive T 1 MRI contrast, comparable to gadolinium‐based agents in a biocompatible iron‐based system. Physicochemical characterization confirms metal coordination and structural and functional integrity of the protein scaffold, supporting its use as a template for hybrid materials fabrication. Prot‐FeNCs exhibit minimal iron release, robust stability, low cytotoxicity, and efficient cellular uptake, enabling sustained T 1 contrast in vitro. In a physiologically relevant 3D human engineered cardiac connective tissue model, Prot‐FeNCs selectively accumulate in fibrotic regions, downregulate fibrotic markers, and attenuate tissue remodeling while preserving biomechanical properties and enable reliable MRI‐based quantification of fibrosis severity. Our results position engineered Prot‐FeNCs as a promising modular hybrid theranostic platform whose scalable synthesis, tunable design, and biocompatibility support translation into next‐generation protein‐based nanomedicines.