Itoo, A. | bioRxiv Science

bioRxiv2026

Magnetic particle imaging reveals heterogeneous retention, leakage and redistribution of nanoparticles following intratumoral injection

Intratumoral (i.t.) delivery of nanoparticles (NPs) is widely used to achieve high local NP concentrations. However, the temporal fate of i.t.-injected NPs remains poorly understood. We present a quantitative approach using whole-body magnetic particle imaging (MPI) to track magnetic NPs (MNPs) following i.t. injection. Using fiducial-calibrated imaging, we quantified MNP mass over time in subcutaneous 4T1 breast tumors. Longitudinal imaging revealed progressive loss of i.t. MNP content and heterogeneous systemic redistribution across animals despite standardized delivery conditions. Ex vivo MPI confirmed off-target accumulation primarily in the liver and spleen, consistent with reticuloendothelial clearance pathways. Histological analysis demonstrated spatially heterogeneous i.t. MNP deposition, potentially associated with local vascular features and tumor microenvironmental heterogeneity that may influence i.t. MNP retention or MNP clearance from the tumor. These findings highlight the importance of quantitative longitudinal whole-body MPI for understanding the fate of MNPs for informing localized nanotherapy.

bioengineering↗

bioRxiv2025

Fast Dynamic Whole-Body In Vivo Cytometry Using Magnetic Particle Imaging

Rapid quantification of the immediate organ accumulation of injected stem cells remains a major challenge. We performed in vivo cytometry (non-invasive cell counting) with magnetic particle imaging (MPI) to track magnetically labeled cells in real-time on a time scale of minutes with high sensitivity, zero background signal, and simple linear quantification. Human mesenchymal stem cells (hMSCs, [~]25 {micro}m in diameter) and human neural precursor cells (hNPCs, [~]10 {micro}m in diameter) were labeled with ferucarbotran or Synomag(R)-D70 as superparamagnetic iron oxide (SPIOs), and tracked with MPI in mice to map their whole-body cell biodistribution after intravenous (IV) or intra-arterial (IA) injection. The organ site of cell accumulation and retention were dependent on cell type, injection route, and frequency of administration, with the lung and liver acting as the major entrapment organs. In vivo MPI enabled quantitative tracking of the dynamic clearance and redistribution of labeled cells, showing major differences between larger hMSCs and smaller hNPCs. Co-registered MRI/CT and histological validation confirmed the anatomical localization of SPIO-labeled cells including the brain following IA injection. Integrating MPI cytometry with preclinical and translational studies may aid in further optimization of the route, dose, and frequency of stem cell administration. One Sentence SummaryFast whole-body in vivo cytometry using MPI is able to dynamically track and quantify therapeutic stem cell accumulation.

bioengineering↗