Clinically Relevant Radioactive Dose Formulation of 177Lu‐Labeled Cetuximab‐Fab Fragment for Potential Use in Cancer Theranostics

Abstract The goal of this study is to generate Fab fragment of cetuximab, a chimeric monoclonal antibody targeting epidermal growth factor receptor (EGFR) on the surface of cancer cells, and radiolabel it with 177 Lu for potential use in cancer theranostics. Cetuximab‐Fab was produced with high purity through papain digestion of cetuximab, and used for formulation of clinically relevant radioactive dose (∼ 1.3 GBq) of 177 Lu‐labeled cetuximab‐Fab after conjugation with a suitable bifunctional chelator, N‐[(R)‐2‐amino‐3‐(para‐isothiocyanato‐phenyl) propyl]‐trans‐(S,S)‐cyclohexane‐1,2‐diamine‐N,N,N′,N′′,N′′‐pentaacetic acid (CHX−A′′‐DTPA). The radiolabeled agent could be prepared with adequate specific activity (50.2 ± 0.6 GBq/μmol), high radiochemical purity (95.3 ± 0.5%) and appreciable in vitro stability under physiological conditions. Biodistribution studies with 177 Lu‐CHX−A′′‐DTPA‐cetuximab‐Fab carried out in Swiss mice bearing fibrosarcoma tumors demonstrated significant tumor uptake (10.7 ± 2.2 %ID/g) within 4 h post‐injection (p.i.) with good tumor‐to‐background contrast. While rapid clearance of the radiolabeled agent from non‐target organs/tissues through hepatobiliary and renal routes was observed, tumor uptake remained consistently high up to 7 d p.i. (13.3 ± 3.6 %ID/g). Blocking studies with unlabeled cetuximab confirmed EGFR specificity of the radiotracer in vivo . This study might aid toward future clinical translation of radiolabeled antibody‐fragments in cancer theranostics.