Endothelial Progenitor Cell Therapy for Fracture Healing: A Dose-Response Study in a Rat Femoral Defect Model

Endothelial progenitor cell (EPC) therapy has been successfully used in orthopaedic preclinical models to heal bone defects. However, no previous studies have investigated the dose-response relationship between EPC therapy and bone healing. This study aimed to assess the effect of different EPC doses on bone healing in a rat model to define an optimal dose. Five-millimeter segmental defects were created in the right femora of Fischer 344 rats, followed by stabilization with a miniplate and screws. Rats were assigned to one of six groups (control, 0.1 M, 0.5 M, 1.0 M, 2.0 M, and 4.0 M; n = 6), receiving 0, 1 × 10⁵, 5 × 10⁵, 1 × 10⁶, 2 × 10⁶, and 4 × 10⁶ EPCs, respectively, delivered into the defect on a gelatin scaffold. Radiographs were taken every two weeks until the animals were euthanized 10 weeks after surgery. The operated femora were then evaluated using micro-computed tomography and biomechanical testing. Overall, the groups that received higher doses of EPCs (0.5 M, 1.0 M, 2.0 M, and 4.0 M) reached better outcomes. At 10 weeks, full radiographic union was observed in 67% of animals in the 0.5 M group, 83% of animals in the 1.0 M group, and 100% of the animals in the 2.0 M and 4.0 M groups, but none in the control and 0.1 M groups. The 2.0 M group also displayed the strongest biomechanical properties, which significantly improved relative to the control and 0.1 M groups. In summary, this study defined a dose-response relationship between EPC therapy and bone healing, with 2 × 10⁶ EPCs being the optimal dose in this model. Our findings emphasize the importance of dosing considerations in the application of cell therapies aimed at tissue regeneration and will help guide future investigations and clinical translation of EPC therapy.