Leveraging Bioorthogonal Click Chemistry to Improve ²²⁵Ac-Radioimmunotherapy of Pancreatic Ductal Adenocarcinoma.

Abstract

Interest in targeted alpha-therapy has surged due to α-particles' high cytotoxicity. However, the widespread clinical use of this approach could be limited by on-/off-target toxicities. Here, we investigated the inverse electron-demand Diels-Alder ligation between an ²²⁵Ac-labeled tetrazine radioligand and a <i>trans</i>-cyclooctene-bearing anti-CA19.9 antibody (5B1) for pretargeted α-radioimmunotherapy (PRIT) of pancreatic ductal adenocarcinoma (PDAC). This alternative strategy is expected to reduce nonspecific toxicities as compared with conventional radioimmunotherapy (RIT).<b>Experimental Design:</b> A side-by-side comparison of ²²⁵Ac-PRIT and conventional RIT using a directly ²²⁵Ac-radiolabeled immunoconjugate evaluates the therapeutic efficacy and toxicity of both methodologies in PDAC murine models. A comparative biodistribution study of the PRIT versus RIT methodology underscored the improved pharmacokinetic properties (e.g., prolonged tumor uptake and increased tumor-to-tissue ratios) of the PRIT approach. Cerenkov imaging coupled to PRIT confirmed the <i>in vivo</i> biodistribution of ²²⁵Ac-radioimmunoconjugate but-importantly-further allowed for the <i>ex vivo</i> monitoring of ²²⁵Ac's radioactive daughters' redistribution. Human dosimetry was extrapolated from the mouse biodistribution and confirms the clinical translatability of ²²⁵Ac-PRIT. Furthermore, longitudinal therapy studies performed in subcutaneous and orthotopic PDAC models confirm the therapeutic efficacy of ²²⁵Ac-PRIT with the observation of prolonged median survival compared with control cohorts. Finally, a comparison with conventional RIT highlighted the potential of ²²⁵Ac-PRIT to reduce hematotoxicity while maintaining therapeutic effectiveness. The ability of ²²⁵Ac-PRIT to deliver a radiotherapeutic payload while simultaneously reducing the off-target toxicity normally associated with RIT suggests that the clinical translation of this approach will have a profound impact on PDAC therapy.