Synthesis and Evaluation of a Cathepsin B–Recognizing Trifunctional Chelating Agent to Improve Tumor Retention of Radioimmunoconjugates

Cathepsin B (CTSB) is a lysosomal protease that is overexpressed in tumor cells. Radioimmunoconjugates (RICs) composed of CTSB‐recognizing chelating agents are expected to increase the molecular weights of their radiometabolites by forming conjugates with CTSB in cells, resulting in their improved retention in tumor cells. We designed a novel CTSB‐recognizing trifunctional chelating agent, azide‐[ 111 In]In‐DOTA‐CTSB‐substrate ([ 111 In]In‐ADCS), to synthesize a RIC, trastuzumab‐[ 111 In]In‐ADCS ([ 111 In]In‐TADCS), and evaluated its utility to improve tumor retention of the RIC. [ 111 In]In‐ADCS and [ 111 In]In‐TADCS were synthesized with satisfactory yield and purity. [ 111 In]In‐ADCS was markedly stable in murine plasma until 96 h postincubation. [ 111 In]In‐ADCS showed binding to CTSB in vitro, and the conjugation was blocked by the addition of CTSB inhibitor. In the internalization assay, [ 111 In]In‐TADCS exhibited high‐level retention in SK‐OV‐3 cells, indicating the in vitro utility of the CTSB‐recognizing unit. In the biodistribution assay, [ 111 In]In‐TADCS showed high‐level tumor accumulation, but the retention was hardly improved. In the first attempt to combine a CTSB‐recognizing unit and RIC, these findings show the fundamental properties of the CTSB‐recognizing trifunctional chelating agent to improve tumor retention of RICs.