Intratumoral injection of ¹⁸⁸Re labeled cationic polyethylenimine conjugates: A preliminary report

¹⁸⁸Re (Rhenium) is easily obtained from an in-house ¹⁸⁸W/¹⁸⁸Re generator that is similar to the current ⁹⁹Mo/^99mTc generator, making it very convenient for clinical use. This characteristic makes this radionuclide a promising candidate as a therapeutic agent. Polyethylenimine (PEI) is a cationic polymer and has been used as a gene delivery vector. Positively charged materials interact with cellular blood components, vascular endothelium, and plasma proteins. In this study, the authors investigated whether intratumoral injection of ¹⁸⁸Re labeled transferrin (Tf)-PEI conjugates exert the effect of radionuclide therapy against the tumor cells. When the diameters of the Ramos lymphoma (human Burkitt's lymphoma) xenografted tumors reached approximately 1 cm, 3 kinds of ¹⁸⁸Re bound compounds (HYNIC-PEI-Tf, HYNIC-PEI, ¹⁸⁸Re perrhenate) were injected directly into the tumors. There were increases in the retention of ¹⁸⁸Re inside the tumor when PEI was incorporated with ¹⁸⁸Re compared to the use of free ¹⁸⁸Re. The ¹⁸⁸Re HYNIC-Tf-PEI showed the most retention inside the tumor (retention rate=approximately 97%). H&E stain of isolated tumor tissues showed that ¹⁸⁸Re labeled HYNIC-PEI-Tf caused extensive tumor necrosis. These results support ¹⁸⁸Re HYNIC-PEI-Tf as being a useful radiopharmaceutical agent to treat tumors when delievered by intratumoral injection.