Single Chelator–Minibody Theranostic Agents for⁸⁹Zr PET Imaging and¹⁷⁷Lu Radiopharmaceutical Therapy of PSMA-Expressing Prostate Cancer

Here we describe an anti-prostate-specific membrane antigen (PSMA) minibody (IAB2MA) conjugated to an octadentate, macrocyclic chelator based on four 1-hydroxypyridin-2-one coordinating units (Lumi804 [L804]) labeled with ⁸⁹Zr (PET imaging) and ¹⁷⁷Lu (radiopharmaceutical therapy), with the goal of developing safer and more efficacious treatment options for prostate cancer. <b>Methods:</b> L804 was compared with the current gold standard chelators, DOTA and deferoxamine (DFO), conjugated to IAB2MA for radiolabeling with ¹⁷⁷Lu and ⁸⁹Zr in cell binding, preclinical biodistribution, imaging, dosimetry, and efficacy studies in the PSMA-positive PC3-PIP tumor-bearing mouse model of prostate cancer. <b>Results:</b> Quantitative radiolabeling (>99% radiochemical yield) of L804-IAB2MA with ¹⁷⁷Lu or ⁸⁹Zr was achieved at ambient temperature in under 30 min, comparable to ⁸⁹Zr labeling of DFO-IAB2MA. In contrast, DOTA-IAB2MA was radiolabeled with ¹⁷⁷Lu for 30 min at 37°C in approximately 90% radiochemical yield, requiring further purification. Using europium(III) as a luminescent surrogate, high binding affinity of Eu-L804-IAB2MA to PSMA was demonstrated in PC3-PIP cells (dissociation constant, 4.6 ± 0.6 nM). All 4 radiolabeled constructs showed significantly higher levels of internalization after 30 min in the PC3-PIP cells than in PSMA-negative PC3-FLU cells. The accumulation of ¹⁷⁷Lu- and ⁸⁹Zr-L804-IAB2MA in PC3-PIP tumors and all organs examined (i.e., heart, liver, spleen, kidney, muscle, salivary glands, lacrimal glands, carcass, and bone) was significantly lower than that of ¹⁷⁷Lu-DOTA-IAB2MA and ⁸⁹Zr-DFO-IAB2MA at 96 and 72 h after injection, respectively. Generally, SPECT/CT and PET/CT imaging data showed no significant difference in the SUV_mean of the tumors or muscle between the radiotracers. Dosimetry analysis via both organ-level and voxel-level dose calculation methods indicated significantly higher absorbed doses of ¹⁷⁷Lu-DOTA-IAB2MA in tumors, kidney, liver, muscle, and spleen than of ¹⁷⁷Lu-L804-IAB2MA. PC3-PIP tumor-bearing mice treated with single doses of ¹⁷⁷Lu-L804-IAB2MA (18.4 or 22.2 MBq) exhibited significantly prolonged survival and reduced tumor volume compared with unlabeled minibody control. No significant difference in survival was observed between groups of mice treated with ¹⁷⁷Lu-L804-IAB2MA or ¹⁷⁷Lu-DOTA-IAB2MA (18.4 or 22.2 MBq). Treatment with ¹⁷⁷Lu-L804-IAB2MA resulted in lower absorbed doses in tumors and less toxicity than that of ¹⁷⁷Lu-DOTA-IAB2MA. <b>Conclusion:</b> ⁸⁹Zr- and ¹⁷⁷Lu-L804-IAB2MA may be a promising theranostic pair for imaging and therapy of prostate cancer.