Pretargeted ¹⁷⁷Lu/²²⁵Ac combination therapy of colorectal cancer

Combining targeted alpha and beta therapy may address challenges such as toxicity, treatment resistance, and tumor heterogeneity. We evaluated the feasibility and therapeutic effectiveness of a DOTA-PRIT approach using a ¹⁷⁷Lu/²²⁵Ac radioisotope cocktail, directly compared with monotherapies targeting GPA33 in human colorectal cancer (CRC) xenografts in mice.

A three-step pretargeting regimen was employed: an anti-GPA33/anti-DOTA bispecific antibody (BsAb), a dendrimeric clearing agent, and radioligands labeled with ¹⁷⁷Lu, ²²⁵Ac, alone or in combination. Serial biodistribution studies in GPA33(+) SW1222 xenografts evaluated how co-injection of ¹⁷⁷Lu and ²²⁵Ac radioligands affected tumor uptake and biodistribution. iQID digital autoradiography was used to visualize isotope distribution in tumor and kidney samples. Mice bearing SW1222 and LS174T xenografts received mono- or combination-therapy regimens delivering 37-38 Gy to tumors. Dose-escalation studies, histopathology, and qPCR analysis of DNA-damage and apoptosis-related genes were also performed.

Biodistribution and autoradiography confirmed that the ¹⁷⁷Lu- and ²²⁵Ac-labeled ligand effectively bound to pretargeted GPA33(+) xenografts when co-administered. High therapeutic indices were maintained across treatment groups, with autoradiography showing general overlap of co-injected probes. Combination therapy demonstrated comparable efficacy to monotherapies. At 150 d post-treatment, no treatment group had reached median survival; 5/9 mice receiving the cocktail (62.9 MBq ¹⁷⁷Lu + 18.5 kBq ²²⁵Ac) were alive, including two tumor-free. In comparison, 4/8 mice in the ¹⁷⁷Lu group and 8/10 in the ²²⁵Ac group survived, with 3 and 5 tumor-free animals, respectively. Combination therapy was well tolerated, showing no significant adverse effects on body weight or blood cell counts compared to healthy controls. Combined administration was safe up to 62.9 MBq ¹⁷⁷Lu + 37 kBq ²²⁵Ac, resulting in 10/10 histological cures.

Our findings confirm the feasibility of a combined ¹⁷⁷Lu and ²²⁵Ac DOTA-PRIT in mice with established SW1222 xenografts, demonstrating tolerability and effectiveness comparable to monotherapy at equivalent average absorbed tumor doses.