Targeted delivery of activatable ¹³¹I-radiopharmaceutical for sustained radiotherapy with improved pharmacokinetics

Targeted radionuclide therapy (TRT) is an effective treatment for tumors. Self-condensation strategies can enhance the retention of radionuclides in tumors and enhance the anti-tumor effect. Considering legumain is overexpressed in multiple types of human cancers, a ¹³¹I-labeled radiopharmaceutical ([¹³¹I]MAAN) based on the self-condensation reaction between 2-cyanobenzothiazole (CBT) and cysteine (Cys) was developed by us recently for treating legumain-overexpressed tumors. However, liver enrichment limits its application. In this study, a new radiopharmaceutical [¹³¹I]IM(HE)₃AAN was designed and synthesized by introducing a hydrophilic peptide sequence His-Glu-His-Glu-His-Glu ((HE)₃) into [¹³¹I]MAAN to optimize the pharmacokinetics. Upon activation by legumain under a reducing environment, hydrophilic [¹³¹I]IM(HE)₃AAN could react with its precursor to form heterologous dimer [¹³¹I]H-Dimer that is highly hydrophobic. Cerenkov imaging revealed that [¹³¹I]IM(HE)₃AAN displayed superior tumor selectivity and longer tumor retention time as compared with [¹³¹I]MAAN, with a significant reduction in the liver uptake. After an 18-day treatment with [¹³¹I]IM(HE)₃AAN, the tumor proliferation was obviously inhibited, while no obvious injury was observed in the normal organs. These findings suggest that [¹³¹I]IM(HE)₃AAN could serve as a promising drug candidate for treating legumain-overexpressed tumors.