Curative Multicycle Radioimmunotherapy Monitored by Quantitative SPECT/CT-Based Theranostics, Using Bispecific Antibody Pretargeting Strategy in Colorectal Cancer

Abstract

Radioimmunotherapy of solid tumors using antibody-targeted radionuclides has been limited by low therapeutic indices (TIs). We recently reported a novel 3-step pretargeted radioimmunotherapy (PRIT) strategy based on a glycoprotein A33 (GPA33)-targeting bispecific antibody and a small-molecule radioactive hapten, a complex of ¹⁷⁷Lu and S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (¹⁷⁷Lu-DOTA-Bn), that leads to high TIs for radiosensitive tissues such as blood (TI = 73) and kidney (TI = 12). We tested our hypothesis that a fractionated anti-GPA33 DOTA-PRIT regimen calibrated to deliver a radiation absorbed dose to tumor of more than 100 Gy would lead to a high probability of tumor cure while being well tolerated by nude mice bearing subcutaneous GPA33-positive SW1222 xenografts. Methods: We treated groups of nude mice bearing 7-d-old SW1222 xenografts with a fractionated 3-cycle anti-GPA33 DOTA-PRIT regimen (total administered ¹⁷⁷Lu-DOTA-Bn activity, 167 MBq/mouse; estimated radiation absorbed dose to tumor, 110 Gy). In randomly selected mice undergoing treatment, serial SPECT/CT imaging was used to monitor treatment response and calculate radiation absorbed doses to tumor. Necropsy was done on surviving animals 100-200 d after treatment to determine frequency of cure and assess select normal tissues for treatment-related histopathologies. Results: Rapid exponential tumor progression was observed in control treatment groups (i.e., no treatment or ¹⁷⁷Lu-DOTA-Bn only), leading to euthanasia due to excessive tumor burden, whereas 10 of 10 complete responses were observed for the DOTA-PRIT-treated animals within 30 d. Treatment was well tolerated, and 100% histologic cure was achieved in 9 of 9 assessable animals without detectable radiation damage to critical organs, including bone marrow and kidney. Radiation absorbed doses to tumor derived from SPECT/CT (102 Gy) and from biodistribution (110 Gy) agreed to within 6.9%. Of the total dose of approximately 100 Gy, the first dose contributes 30%, the second dose 60%, and the third dose 10%. Conclusion: In a GPA33-positive human colorectal cancer xenograft mouse model, we validated a SPECT/CT-based theranostic PRIT regimen that led to 100% complete responses and 100% cures without any treatment-related toxicities, based on high TIs for radiosensitive tissues. These studies support the view that anti-GPA33 DOTA-PRIT will be a potent radioimmunotherapy regimen for GPA33-positive colorectal cancer tumors in humans.

Keywords: SPECT; colorectal cancer; pretargeting; radioimmunotherapy.

FIGURE 1.

Anti-GPA33 DOTA-PRIT concept. (A) Schematic representation of anti-GPA33 BsAb huA33-C825 (left) and M-DOTA-Bn hapten (right), vehicle by which ¹⁷⁷Lu treatment is administered. Anti-(M)-DOTA-Bn scFv antibody C825 has approximately equal low-pM affinity for either Y-DOTA-Bn or Lu-DOTA-Bn. (B) Schematic representation of 3-step anti-GPA33 DOTA-PRIT protocol. (Copyright 2016, reprinted with permission from Memorial Sloan Kettering Cancer Center [MSK].)

FIGURE 2.

Timeline of theranostic anti-GPA33 DOTA-PRIT.

FIGURE 3.

Efficacy of DOTA-PRIT evaluated as tumor growth and survival in mice bearing SW1222 subcutaneous xenografts. (A) Tumor growth presented as change in tumor volume for each individual mouse over time (green, no treatment; blue, ¹⁷⁷Lu-DOTA-Bn only; red, DOTA-PRIT; red arrows, 3-cycle treatment times). (B) Kaplan–Meier plot of DOTA-PRIT (shown as time to tumor burden of 1,000 mm³).

FIGURE 4.

Effective (non–decay-corrected) ¹⁷⁷Lu activity concentrations (as MBq/g) determined using noninvasive theranostic SPECT/CT imaging during treatment.

FIGURE 5.

(Left) Representative SPECT/CT images obtained from same mouse (M1) at 24 h after injection of cycle 1 (top left; CT images) and 24 h after injection of cycle 3 (bottom left; SPECT/CT fusion images). (Right) A graph depicting the effective tumor activity (as MBq/g) derived from SPECT image analysis (right axis) and tumor volume (as mm³) derived from CT image analysis (left axis) for this particular animal is provided for reference.

FIGURE 6.

Representation of fitted exponential decay curves on 3-cycle treatment regimen. Curves were calculated out to complete removal of radioactive label and truncated at 496 h after injection of cycle 1, when tumors were no longer grossly palpable. Cumulated dose for cycle 1 represents dose of blue region, and cumulated dose for cycle 2 includes both blue and red regions. Of total dose of ∼100 Gy, first dose contributes 30%, second dose 60%, and third dose 10% (Table 2).