Pretargeted radioimmunotherapy with 225Ac-proteus-DOTA hapten.

Objectives: We have recently reported a novel anti-tumor/anti-DOTA hapten bispecific approach to pretargeted radioimmunotherapy (DOTA-PRIT), demonstrating complete responses including cures in several solid tumor types using the beta-emitting lutetium-177-DOTA-hapten (¹⁷⁷Lu-DOTA-hapten; for e.g., please see Cheal SM, et al. (2017) J Nucl Med, 58(11):1735-42). We now seek to expand DOTA-PRIT to the use of alpha-emitting isotopes, specifically actinium-225. Herein we report the testing of the hypothesis that a DOTA-chelator (proteus-DOTA) that consists of a free (unchelated) high efficiency DOTA-chelator for ²²⁵Ac, tethered via a short polyethylene glycol (PEG) linker to a lutetium-complexed DOTA with pM affinity, could be used for optimal pretargeting of ²²⁵Ac.

Methods: For effective tumor targeting with DOTA-PRIT, an anti-DOTA antibody/DOTA hapten binding affinity of at least 100 pM is needed (Orcutt KD, et al. (2012) Mol Cancer Ther, 11(6): 1365-72). We synthesized proteus-DOTA, radiolabeled with ²²⁵Ac, and conducted in vitro and in vivo studies to determine if pretargeting of ²²⁵Ac-proteus-DOTA hapten to tumor was feasible. In addition, a toxicity study was performed in normal tumor-free athymic nude mice with varying dose levels of ²²⁵Ac-proteus-DOTA (0, 9.25, 18.5, 37, 74, 148, or 296 kBq/mouse) given as a single intravenous (i.v.) injection. Mice were monitored daily and weighed up to twice weekly for evidence of treatment induced toxicity for 145 days post-injection (p.i.).

Results: In mouse biodistribution experiments, i.v. administered ²²⁵Ac-proteus-DOTA showed rapid renal clearance and minimal normal tissue retention, with a majority of the total injected dose (97%) excreted by 4 h. When applied to DOTA-PRIT directed at GPA33 on human colorectal cancer SW1222 xenografted in athymic mice, ²²⁵Ac-proteus-DOTA showed specific tumor uptake (e.g., ~17 percentage of injected ²²⁵Ac-dose per gram (%ID/g) of tumor at 24 h p.i.; 1.86 kBq, 182 pmol) with minimal normal tissue accumulation including blood and kidney (both ~1 %ID/g 24 h p.i.). The tumor-targeting efficiency and resultant renal-based whole body clearance of DOTA-PRIT + ²²⁵Ac-proteus-DOTA mimics the ¹⁷⁷Lu-DOTA-hapten. Dose-escalation toxicity studies with activities as high as 296 kBq/mouse (29 nmol) showed no acute toxicity, as well as no chronic radiation damage. Conclusion: ²²⁵Ac-proteus-DOTA mimics the ¹⁷⁷Lu-DOTA-hapten in vivo behavior (tumor uptake and whole body clearance). The ability to selectively deliver ²²⁵Ac-proteus-DOTA will greatly expand the potential of delivering precision radioimmunotherapy to human solid tumors in laboratory animals, and ultimately man. Research Support: The Center for Targeted Radioimmunotherapy and Diagnosis, Ludwig Center for Cancer Immunotherapy, Mr. William H. Goodwin and Mrs. Alice Goodwin and the Commonwealth Foundation for Cancer Research and The Experimental Therapeutics Center, MSKCC, Kids Walk for Kids with Cancer NYC, and the Robert Steel Foundation.