Preparation of 177Lu‐labeled Nimotuzumab for radioimmunotherapy of EGFR‐positive cancers: Comparison of DOTA and CHX‐A″‐DTPA as bifunctional chelators

This study was aimed at evaluating the role of bifunctional chelators DOTA‐NCS and CHX‐A″‐DTPA‐NCS used for conjugating ¹⁷⁷Lu with Nimotuzumab on the radiochemical yields, purity, in vitro stability, and specificity of the radioimmunoconjugates to EGFR. Two immunoconjugates were prepared wherein Nimotuzumab was conjugated with the acyclic ligand p‐NCS‐Bn‐CHX‐A″‐DTPA and macrocyclic ligand p‐NCS‐Bn‐DOTA. These were radiolabeled with ¹⁷⁷Lu, purified on PD‐10 column, and characterized by SE‐HPLC. In vitro stability was determined up to 4 days post preparation. Specificity of the radioimmunoconjugates was ascertained by in vitro studies in A431 cells while the biodistribution patterns were studied in normal Swiss mice up to 96 hours post injection. Four to five molecules of CHX‐A″‐DTPA/DOTA were attached to one molecule of Nimotuzumab. Radiochemical purity of both ¹⁷⁷Lu‐CHX‐A″‐DTPA‐Nimotuzumab and ¹⁷⁷Lu‐DOTA‐Nimotuzumab was determined to be greater than 98%. Both the radioimmunoconjugates exhibited good in vitro stability at 37°C up to 4 days post preparation in saline, and their clearance was largely by the hepatobiliary route. The DOTA‐ and CHX‐A″‐DTPA‐based radioimmunoconjugates could be prepared with good radiochemical purity, in vitro stability, and specificity to EGFR. Further studies in EGFR‐positive cancers would pave way for them for use in the clinics.