Targeting of ultrasmall superparamagnetic iron oxide (USPIO) particles to tumor cells in Vivo by using transferrin receptor pathways

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Abstract
Human transferrin was covalently coupled to ultrasmall superparamagnetic iron oxide (USPIO) particles, and the trans‐ferrin‐USPIO obtained was investigated in vivo in experimental SMT/2A tumor‐bearing rats (rat mammary carcinoma). Physicochemical characterization showed an overall size of 36 nm (DLS) with a core size of 5 nm (TEM). Relaxivities were R,1 = 23.6 and R2 = 52.1 liter/mmol · s (0.47 T). Bound transferrin was 280 μg/mg of iron. Pharmacokinetic investigations revealed a half‐life of 17 min in normal rats. The MR evaluation of tumor signal intensity over time showed a 40% (range 25–55%) signal reduction 150 min after injection with the reduction persisting for at least 8 h. Control experiments using the parent USPIO compound or USPIO labeled with a nonspecific human serum albumin (HSA‐USPIO) showed a change of only 10% (range 5–15%) in tumor signal intensity over time. The results demonstrate that a combination of the USPIO relaxivity properties with the specificity of transferrin‐medi‐ated endocytosis allows in vivo detection of tumors by MR imaging.