Metal-Free Cycloaddition Chemistry Driven Pretargeted Radioimmunotherapy Using α-Particle Radiation

The pretargeted radioimmunotherapy approach (PRIT) decouples the administration of tumor targeting monoclonal antibodies (mAbs) from that of the radiolabeled ligand. This multistep strategy allows delivery of high doses of radiation to tumor cells while minimizing nonspecific normal tissue irradiation. In this study, we evaluated the potential of pretargeted α-particle radioimmunotherapy based on the inverse electron demand Diels-Alder (IEDDA) reaction between trans-cyclooctene (TCO) and tetrazine (Tz). Two tetrazine based chelators, DOTA-Tz and TCMC-Tz, were synthesized and compared for their radiolabeling efficiency with ²¹²Pb, radiochemical stability, and in vivo pharmacokinetics. Dosimetry was determined from pretargeted biodistribution studies. The PRIT study was carried out in LS174T tumor bearing mice pretargeted with CC49-TCO mAb. After removing unbound mAbs from the blood using two doses of clearing agent, mice were treated with various doses of (0, 2.78, 4.63, 7.40, and 2 × 2.78 MBq) of ²¹²Pb-DOTA-Tz. ²¹²Pb-DOTA-Tz displayed better in vivo biodistribution than ²¹²Pb-TCMC-Tz and was selected for PRIT study. All the mouse groups receiving treatment displayed a dose dependent reduction in tumor size, while the control groups showed exponential tumor growth. Treatment with 2.78, 4.63, and 2 × 2.78 MBq of ²¹²Pb-DOTA-Tz resulted in statistically significant improvement in median survival (26, 35, and 39 days, respectively). Groups receiving 7.40 MBq of ²¹²Pb-DOTA-Tz and 0.55 MBq of direct labeled CC49 exhibited acute radiation associated toxicity. This study successfully demonstrated that pretargeted ²¹²Pb α-particle therapy resulted in reduced tumor growth rates and improved survival with minimal normal tissue toxicity.