Toxicity of Magnetic Nanoparticles in Medicine: Contributing Factors and Modern Assessment Methods.

With the rapid evolution of nanotechnology, magnetic iron oxide nanoparticles (MNPs)-primarily FeO and γ-FeO-have gained prominence in biomedicine. Their extensive specific surface area, tunable surface functionalities, and intrinsic magnetic characteristics render them highly versatile for diverse clinical applications, including tumor visualization through Magnetic Resonance Imaging (MRI), radiolabeling, targeted radiotherapy, hyperthermia, gene transfer, drug delivery, Magnetic Particle Imaging (MPI), magnetic blood filtration and theranostic strategies. Nevertheless, ensuring the biocompatibility and non-toxicity of these nanostructures remains a fundamental prerequisite for their medical implementation. Hence, it is essential to continuously refine our understanding of MNP-related toxicity and pursue comprehensive research on this front. This article consolidates up-to-date insights into the evaluation of MNPs' toxicological profiles, emphasizing the influence of physicochemical properties such as morphology, surface modifications, and electrostatic characteristics, along with operational factors like dosage and administration routes. Traditional toxicity testing strategies, including in vitro assays as first-line screening tools, together with standard ex vivo and in vivo models, are discussed. Special attention is given to the emerging role of New Approach Methodologies (NAMs), such as organoid formation, 3D bioprinting, in ovo chicken embryo assays, and image cytometry. These techniques offer ethical, human-relevant, and informative alternatives to animal testing, supporting more predictive and translationally relevant toxicity assessment of MNPs. Taken together, the integration of conventional assays with innovative NAMs, alongside careful consideration of physicochemical and operational factors, is essential to translate the laboratory promise of MNPs into safe and clinically effective nanomedicines.