Exploring the Therapeutic Potential of 177Lu-PSMA-617 in a Mouse Model of Prostate Cancer Bone Metastases

Abstract

Prostate cancer is the second leading cause of cancer-related death in men, with metastatic castration-resistant prostate cancer (mCRPC) and bone metastases representing a critical clinical challenge. Although radium-223 (Ra-223) is approved for treating mCRPC with bone metastases, its efficacy remains limited, necessitating the development of more effective therapies. This study investigates the therapeutic potential of ¹⁷⁷Lu-PSMA-617, a PSMA-targeted radiopharmaceutical, in a murine model of prostate cancer bone metastases. To our knowledge, this is the first study to systematically evaluate ¹⁷⁷Lu-PSMA-617 in an orthotopic bone metastatic prostate cancer model, providing a clinically relevant preclinical platform to assess both imaging and therapeutic performance. We conducted comprehensive preclinical evaluations, including synthesis, stability analysis, cell binding assays, nuclear imaging, in vivo biodistribution, pharmacokinetics, and antitumor efficacy. The synthesis of ¹⁷⁷Lu-PSMA-617 demonstrated high radiochemical yield (99.2%), molar activity (25.5 GBq/μmol), and purity (>98%), indicating high product quality. Stability studies confirmed minimal release of free Lutetium-177, maintaining the compound's integrity under physiological conditions. In vitro assays showed selective binding and internalization in PSMA-positive LNCaP prostate cancer cells, with negligible uptake in PSMA-negative PC-3 cells. In vivo biodistribution studies demonstrated efficient tumor targeting, with peak uptake in LNCaP tumors (23.31 ± 0.94 %IA/g) at 4 h post-injection. The radiopharmaceutical exhibited favorable pharmacokinetics, with high tumor-to-background ratios (tumor-to-blood, 434.4; tumor-to-muscle, 857.4). Therapeutic efficacy was confirmed by significant survival extension in treated mice (30.7% for 37 MBq and 53.8% for 111 MBq), with median survival times of 34 and 40 days, respectively, compared to 26 days in the control group. Radiation dosimetry analysis indicated a favorable safety profile with a calculated effective dose of 0.127 mSv/MBq. These findings highlight the novelty and translational relevance of using ¹⁷⁷Lu-PSMA-617 in a clinically relevant bone metastasis model, reinforcing its potential as a dual-purpose agent for both targeted therapy and molecular imaging in advanced prostate cancer.

Keywords: 177Lu-PSMA-617; bone metastasis; preclinical model; prostate cancer; targeted radionuclide therapy.

Figure 1

Radiochemical purity (RCP, %) of ¹⁷⁷Lu-PSMA-617 was measured by (A) radio-HPLC and (B) iTLC. (C) The radiochemical stability of ¹⁷⁷Lu-PSMA-617 in human serum.

Figure 2

The cell surface binding and internalization of ¹⁷⁷Lu-PSMA-617 in (A) PSMA-positive LNCaP cells were compared to those in (B) PSMA-negative PC-3 cells. Results are presented as %IA per 10⁶ cells (mean ± SD, n = 3).

Figure 3

(A) Saturation binding study and (B) competitive inhibition assay of ^177/natLu-PSMA-617 on intact LNCaP cells. Increasing concentrations of ^natLu-PSMA-617 were used, ranging from 0.01 to 10,000 nM. All radiotracers exhibited high affinity for the PSMA-positive LNCaP cells. The dissociation constant (K_d) and the maximum number of binding sites (B_max) were calculated from nonlinear regression analysis using GraphPad Prism. Data are presented as counts per minute (CPM).

Figure 4

(A) Bioluminescence imaging of ASID mice with bone metastasis of LNCaP-luc prostate cancer, (B) SPECT/CT images, and (C) biodistribution of ¹⁷⁷Lu-PSMA-617 (%IA/g: percentage of injected activity per gram of tissue) in mice after intravenous injection (n = 3 at each time point). (D) Tumor-to-normal tissue (T/N) ratios of ¹⁷⁷Lu-PSMA-617.

Figure 5

Undecalcified whole-mount cryosection and autoradiography of intratibial LNCaP prostate cancer xenograft models. (A) En bloc color macrophotograph of an embedded sample at the cutting surface. (B) Hematoxylin and eosin (H&E) stain of the whole-mount adhesive bound section. (C) Immunohistochemistry with the anti-PSMA antibody. Insets are images of the respective stained section of the tumor. Scale bars, 100 μm. (D) Autoradiography of ¹⁷⁷Lu-PSMA-617 and daughter emission. Areas of intense uptake colocalize with active bone metastasis sites in the distal femur and proximal tibia.

Figure 6

The antitumor efficacy of ¹⁷⁷Lu-PSMA-617 in mice with bone metastasis of prostate cancer. (A) Time-lapse bioluminescence imaging of mice bearing LNCaP/Luc prostate cancer after the treatment with saline (control), 1mCi of ¹⁷⁷Lu-PSMA-617 (n = 8), and 3mCi of ¹⁷⁷Lu-PSMA-617 (n = 9). (B) Quantification of bioluminescence in (A). (C) Kaplan–Meier survival curves of mice with bone metastasis of prostate cancer after treatment. The survival rate data comparisons were performed using the log-rank (Mantel–Cox) test; p < 0.05 was compared with the control.