Single Chelator-Minibody Theranostic Agents for 89Zr PET Imaging and 177Lu Radiopharmaceutical Therapy of PSMA-Expressing Prostate Cancer

Abstract

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. Methods: 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. Results: 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. Conclusion: ⁸⁹Zr- and ¹⁷⁷Lu-L804-IAB2MA may be a promising theranostic pair for imaging and therapy of prostate cancer.

Keywords: L804; minibody; prostate cancer; targeted radiotherapy; theranostics.

Graphical abstract

FIGURE 1.

(A) Biodistribution of ⁸⁹Zr-L804-IAB2MA and ⁸⁹Zr-DFO-IAB2MA at 72 h after injection in PC3-PIP tumor–bearing mice. (B–E) Time–activity curves of ⁸⁹Zr-L804-IAB2MA (B and D) and ⁸⁹Zr-DFO-IAB2MA (C and E) in PC3-PIP tumor–bearing mice. Data are mean ± SEM (n = 4). *P < 0.05. **P < 0.01. ***P < 0.001.

FIGURE 2.

(A) Biodistribution of ¹⁷⁷Lu-L804-IAB2MA and ¹⁷⁷Lu-DOTA-IAB2MA at 96 h after injection in PC3-PIP tumor–bearing mice. (B–E) Time–activity curves of ¹⁷⁷Lu-L804-IAB2MA (B and D) and ¹⁷⁷Lu-DOTA-IAB2MA (C and E) in PC3-PIP tumor–bearing mice. Data are mean ± SEM (n = 4). *P < 0.05. **P < 0.01. ***P < 0.001.

FIGURE 3.

(A and B) Selected PET/CT images of ⁸⁹Zr-L804-IAB2MA (A) and ⁸⁹Zr-DFO-IAB2MA (B) at 4, 24, and 48 h after injection in PC3-PIP tumor–bearing mice. (C–E) Quantified SUV_mean for tumor (C) and muscle (D), and tumor-to-muscle SUV ratio (E). Data are mean ± SEM (n = 3). There were no significant differences (ns) in SUV_mean and SUV_ratio between ⁸⁹Zr-L804-IAB2MA and ⁸⁹Zr-DFO-IAB2MA.

FIGURE 4.

(A and B) Selected SPECT/CT images of ¹⁷⁷Lu-L804-IAB2MA (A) and ¹⁷⁷Lu-DOTA-IAB2MA (B) at 4, 24, 48, and 72 h after injection in PC3-PIP tumor–bearing mice. (C–E) Quantified SUV_mean for tumor (C) and muscle (D), and tumor-to-muscle SUV ratio (E). Data are mean ± SEM (n = 3). ns = not statistically significant. **P < 0.01. ***P < 0.001.

FIGURE 5.

¹⁷⁷Lu-L804-IAB2MA therapy in PC3-PIP tumor-bearing mice. (A) Kaplan–Meier survival curves of mice treated with ¹⁷⁷Lu-L804-IAB2MA, ¹⁷⁷Lu-DOTA-IAB2MA (14.8 and 22.2 MBq), or unlabeled minibody (control). (B) Average tumor volume of mice at 2, 6, 20, 34, and 48 d after treatment. Bars with different letters are significantly different (P < 0.05). Data are mean ± SEM.

FIGURE 6.

Analysis of white blood cells (WBC) (A) and lymphocytes (LYM) (B) in tail vein blood of PC3-PIP tumor–bearing mice. Bars with different letters are significantly different (P < 0.05). Data are mean ± SEM.