Micro-flow photosynthesis of new dienophiles for inverse-electron-demand Diels–Alder reactions. Potential applications for pretargeted in vivo PET imaging

Abstract

Pretargeted PET imaging has emerged as an effective two-step <i>in vivo</i> approach that combines the superior affinity and selectivity of antibodies with the rapid pharmacokinetics and favorable dosimetry of smaller molecules radiolabeled with short-lived radionuclides. This approach can be based on the bioorthogonal inverse-electron-demand Diels-Alder (IEDDA) reaction between tetrazines and <i>trans</i>-cyclooctene (TCO) derivatives. We aimed to develop new [¹⁸F]TCO-dienophiles with high reactivity for IEDDA reactions, and favorable <i>in vivo</i> stability and pharmacokinetics. New dienophiles were synthesized using an innovative micro-flow photochemistry process, and their reaction kinetics with a tetrazine were determined. <i>In vivo</i> stability and biodistribution of the most promising ¹⁸F-radiolabeled-TCO-derivative (<b>[¹⁸F]3</b>) was investigated, and its potential for <i>in vivo</i> pretargeted PET imaging was assessed in tumor-bearing mice. We demonstrated that <b>[¹⁸F]3</b> is a suitable dienophile for IEDDA reactions and for pretargeting applications.