Evaluation of 134Ce/134La-PSMA-617 for PET Imaging and Auger Electron Therapy of Prostate Cancer

Abstract

The ¹³⁴Ce/¹³⁴La radionuclide pair has been proposed as a PET imaging surrogate for targeted α-radiotherapeutics. ¹³⁴Ce decays to ¹³⁴La via electron capture, emitting Auger electrons (AEs), which could be used for targeted radionuclide therapy. Additionally, the positron emission during this transition enables PET imaging, making ¹³⁴Ce/¹³⁴La a promising theranostic pair for prostate cancer. In this work, we investigated the potential of ¹³⁴Ce for AE capture therapy using prostate-specific membrane antigen (PSMA)-617 for targeted radionuclide delivery. Methods: Radiolabeling of [¹³⁴Ce]Ce-PSMA-617 proceeded as previously described, and C18 cartridge purification was optimized. In vitro, cell-binding and toxicity assays were performed on PSMA-positive PC3 PIP cells. In vivo PET imaging and ex vivo biodistribution studies were conducted on mice bearing dual PSMA-positive PC3 PIP and PSMA-negative PC3 flu tumor xenografts at various time points ranging from 1 to 72 h after injection. Additionally, an in vivo single-dose-treatment study was performed using 37- and 111-MBq doses in nude mice with PC3 PIP tumor xenografts. Results: PSMA-617 was successfully radiolabeled with ¹³⁴Ce/¹³⁴La and purified using the C18 cartridge method, achieving high molar activity (21.02 ± 0.11 MBq/nmol). Stability studies showed more than 95% stability in mouse serum at day 5. PSMA-positive PC3 PIP cells demonstrated 89.6% ± 0.55% cell binding, 55.45% ± 0.96% internalization at 24 h, and a dissociation constant of 32.9 ± 3.9 nM, comparable to other reported [¹⁷⁷Lu]Lu/[²²⁵Ac]Ac-PSMA-617 radiocomplexes. In contrast, no cellular uptake or internalization was observed in PSMA-negative PC3 flu cells. Clonogenic assay of [¹³⁴Ce]Ce-PSMA-617 showed a significant dose-dependent reduction in cell proliferation (P = 0.002). PET imaging revealed high tumor-specific uptake at early time points (1 and 4 h), followed by a gradual decline from 24 to 72 h, with rapid clearance from normal tissues. These results were corroborated by ex vivo biodistribution studies. In vivo therapy with [¹³⁴Ce]Ce-PSMA-617 in tumor-bearing mice demonstrated a significant increase in median survival compared with control animals (saline, 33 d; 37 MBq, 50 d; and 111 MBq, 80 d, end of the study). Conclusion: [¹³⁴Ce]Ce-PSMA-617 exhibited excellent in vitro and in vivo characteristics, providing significant survival benefits in mice. Collectively, these findings suggest that [¹³⁴Ce]Ce-PSMA-617 is an effective theranostic agent for PET imaging and AE therapy of prostate cancer.

Keywords: Auger electrons; [134Ce]Ce-PSMA-617; prostate cancer; targeted radiopharmaceutical therapy.

Graphical abstract

FIGURE 1.

[¹³⁴Ce]Ce-PSMA-617 can be efficiently prepared and demonstrates high cell binding and internalization in vitro. (A) Reaction scheme for radiolabeling and purification of [¹³⁴Ce]Ce-PSMA-617. (B) Cell binding and blocking assay for [¹³⁴Ce]Ce-PSMA-617 in PC3 PIP and flu cells (n = 3). High cell binding was observed in PSMA-positive PC3 PIP cells, whereas such binding was not seen in PSMA-negative PC3 flu cells (Supplemental Fig. 2). (C) Membrane-bound and internalization assay for [¹³⁴Ce]Ce-PSMA-617 in PC3 PIP cells (n = 3) revealed rapid binding and internalization.

FIGURE 2.

In vitro toxicity study for [¹³⁴Ce]Ce-PSMA-617 in PC3 PIP cells. (A) Clonogenic survival assay for [¹³⁴Ce]Ce-PSMA-617 showed dose-dependent decrease in colonies after treatment (n = 3). **P = 0.002. ***P = 0.0002. ****P < 0.0001. (B) DNA damage after treatment with various doses (0–1 MBq/mL) of [¹³⁴Ce]Ce-PSMA-617 in PC3 PIP cells showing dose-dependent increase in DNA damage (n = 3). **P = 0.0036. ****P < 0.0001. ns = nonsignificant by Sidak multiple-comparison test; Untr = untreated.

FIGURE 3.

Dynamic coronal fused small-animal PET/CT images obtained after injection of [¹³⁴Ce]Ce-PSMA-617 in PC3 PIP (left shoulder) and PC3 flu (right shoulder) tumor xenografts in athymic nude mice (n = 3). High liver uptake was observed at early time points because of free ¹³⁴La caused by nuclear recoil effect. This uptake gradually decreased over time, showing minimal or no liver uptake at later time points because of short half-life of ¹³⁴La (∼6.45 min).

FIGURE 4.

Transverse and coronal PET/CT images obtained up to 72 h after [¹³⁴Ce]Ce-PSMA-617 injection in mouse bearing PSMA-positive PC3 PIP (left shoulder) and PSMA-negative PC3 flu (right shoulder) tumor xenografts, demonstrating gradual increase in tumor uptake up to 4 h and slow decline from 24 to 72 h (n = 4).

FIGURE 5.

Multiple-time-point ex vivo biodistribution analysis. (A) Ex vivo biodistribution of [¹³⁴Ce]Ce-PSMA-617 in mouse bearing PSMA-positive PC3 PIP (left shoulder) and PSMA-negative PC3 flu (right shoulder) tumor xenografts until 72 h after injection (n = 4). (B) Tumor-to-kidney ratios derived from ex vivo biodistribution analysis.

FIGURE 6.

Single-dose-treatment study of [¹³⁴Ce]Ce-PSMA-617 in PC3 PIP tumor xenografts. (A) Schematic illustration for single-dose-treatment study of [¹³⁴Ce]Ce-PSMA-617 with 37-MBq dose, 111-MBq dose, and saline as control. (B) Average tumor volumes. (C) Body weight. (D) Kaplan–Meier plot for treatment and vehicle cohorts. Dotted lines indicate 95% CI for survival probability. (E) Treatment group’s median survival days and statistical analysis (n = 6). High dose (111 MBq) showed prolonged survival, and 37 MBq showed significant difference vs. control saline group. ***P = 0.0005. H&E = hematoxylin and eosin; Undefined = not reached endpoint.