Rationale: The success of cardiac stem cell therapies is limited by low cell retention, due at least in part to washout via coronary veins.

Objective: We sought to counter the efflux of transplanted cells by rendering them magnetically responsive and imposing an external magnetic field on the heart during and immediately after injection.

Methods and Results: Cardiosphere-derived cells (CDCs) were labeled with superparamagnetic microspheres (SPMs). In vitro studies revealed that cell viability and function were minimally affected by SPM labeling. SPM-labeled rat CDCs were injected intramyocardially, with and without a superimposed magnet. With magnetic targeting, cells were visibly attracted toward the magnet and accumulated around the ischemic zone. In contrast, the majority of nontargeted cells washed out immediately after injection. Fluorescence imaging revealed more retention of transplanted cells in the heart, and less migration into other organs, in the magnetically targeted group. Quantitative PCR confirmed that magnetic targeting enhanced cell retention (at 24 hours) and engraftment (at 3 weeks) in the recipient hearts by ≈3-fold compared to nontargeted cells. Morphometric analysis revealed maximal attenuation of left ventricular remodeling, and echocardiography showed the greatest functional improvement, in the magnetic targeting group. Histologically, more engrafted cells were evident with magnetic targeting, but there was no incremental inflammation.

Conclusions: Magnetic targeting enhances cell retention, engraftment and functional benefit. This novel method to improve cell therapy outcomes offers the potential for rapid translation into clinical applications.