Macrophages sequester and aggregate ultrasmall superparamagnetic iron oxide nanoparticles (USPIO) in their lysosomes. The amount of USPIO loading of macrophages depends upon the route and dose of administration, and the pharmacokinetics of accumulation and removal. Both fixed macrophages and activated macrophages associated with inflammatory diseases and cancer phagocytize USPIOs, and the loaded macrophages can serve to identify the extent of a macrophage-dependent disease as well as to direct treatment options. Furthermore, the absorption of energy from incident electromagnetic waves by the aggregated nanoparticles can be used for conformal thermotherapy. The USPIOs can further be used to carry drugs to the same activated macrophages. The co-administered drugs can be bound to the USPIO by condition-dependent releasing links that are responsive to local pH or heating.
