<i>In Vitro</i> and <i>In Vivo</i> Evaluation of ⁸⁹Zr-DS-8273a as a Theranostic for Anti-Death Receptor 5 Therapy

Abstract

<b>Background:</b> DS-8273a, an anti-human death receptor 5 (DR5) agonistic antibody, has cytotoxic activity against human cancer cells and induces apoptosis after specific binding to DR5. DS-8273a is currently being used in clinical Phase I trials. This study evaluated the molecular imaging of DR5 expression <i>in vivo</i> in mouse tumor models using SPECT/CT and PET/MRI, as a tool for drug development and trial design. <b>Methods:</b> DS-8273a was radiolabeled with indium-111 and zirconium-89. Radiochemical purity, immunoreactivity, antigen binding affinity and serum stability were assessed <i>in vitro</i>. <i>In vivo</i> biodistribution and pharmacokinetic studies were performed, including SPECT/CT and PET/MR imaging. A dose-escalation study using a PET/MR imaging quantitative analysis was also performed to determine DR5 receptor saturability in a mouse model. <b>Results:</b>¹¹¹In-CHX-A″-DTPA-DS-8273a and ⁸⁹Zr-Df-Bz-NCS-DS-8273a showed high immunoreactivity (100%), high serum stability, and bound to DR5 expressing cells with high affinity (K_a, 1.02-1.22 × 10¹⁰ M^-1). The number of antibodies bound per cell was 32,000. <i>In vivo</i> biodistribution studies showed high and specific uptake of ¹¹¹In-CHX-A″-DTPA-DS-8273a and ⁸⁹Zr-Df-Bz-NCS-DS-8273a in DR5 expressing COLO205 xenografts, with no specific uptake in normal tissues or in DR5-negative CT26 xenografts. DR5 receptor saturation was observed <i>in vivo</i> by biodistribution studies and quantitative PET/MRI analysis. <b>Conclusion:</b>⁸⁹Zr-Df-Bz-NCS-DS-8273a is a potential novel PET imaging reagent for human bioimaging trials, and can be used for effective dose assessment and patient response evaluation in clinical trials.