Is Copper-61 the New Gallium-68? Automation and Preclinical Proof-of-Concept of 61Cu-Based Radiopharmaceuticals for Prostate Cancer Imaging

Abstract

Background: While gallium-68 has traditionally dominated PET imaging in oncology, copper radionuclides have sparked interest for their potential applications in nuclear medicine and theranostics. Considering the advantageous physical decay properties of copper-61 compared to those of gallium-68, we describe a fully automated GMP-compliant synthesis process for ⁶¹Cu-based radiopharmaceuticals and demonstrate their in vivo application for targeting the overexpressed PSMA by PET/MR imaging. Methods: Copper-61 was obtained through the irradiation of natural zinc liquid targets in a biomedical cyclotron. [⁶¹Cu]Cu-DOTAGA-PSMA-I&T and [⁶¹Cu]Cu-NODAGA-PSMA-I&T were produced without manual intervention in two Synthera^® Extension modules. Radiochemical purity was analyzed by radio-HPLC and iTLC. Cellular uptake was evaluated in LNCaP and DU145 cells. In vivo PET/MRI was performed in control mice to evaluate the biodistribution of both radiopharmaceuticals, and in tumor-bearing mice to assess the targeting ability towards PSMA. Results: The fully automated process developed proved to be effective for the synthesis of ⁶¹Cu-based radiopharmaceuticals, with appropriate molar activities. The final products exhibited high radiochemical purity (>98%) and remained stable for up to 6 h after the EOS. A time-dependent increase in cellular uptake was observed in LNCaP cells, but not in DU145 cells. As opposed to [⁶¹Cu]Cu-NODAGA-PSMA-I&T, [⁶¹Cu]Cu-DOTAGA-PSMA-I&T exhibited poor kinetic stability in vivo. Subsequent PET/MR imaging with [⁶¹Cu]Cu-NODAGA-PSMA-I&T showed tumor uptake lasting up to 4 h post-injection, predominant renal clearance, and no detectable accumulation in non-targeted organs. Conclusions: These results demonstrate the feasibility of the implemented process, which yields adequate amounts of high-quality radiopharmaceuticals and can be adapted to any standard production facility. This streamlined approach enhances reproducibility and scalability, bringing copper-61 closer to widespread clinical use, to the detriment of the conventionally accepted gallium-68.

Keywords: automation; copper-61; positron emission tomography; prostate cancer; prostate specific membrane antigen; radiopharmaceutical.

Figure 1

Representative (A) radio-HPLC and (B) iTLC chromatograms attesting the radiochemical purity of the final radiopharmaceuticals for [⁶¹Cu]Cu-DOTAGA-PSMA-I&T (upper row; left column) and [⁶¹Cu]Cu-NODAGA-PSMA-I&T (lower row; right column), in which “a” represents free copper-61 and “b” represents the radiolabeled compounds.

Figure 2

Stability results for (A) [⁶¹Cu]Cu-DOTAGA-PSMA-I&T and (B) [⁶¹Cu]Cu-NODAGA-PSMA-I&T in the final formulation and after incubation with mice serum over a period of 6 h after the EOS. Data are expressed as mean ± SEM (N = 3–6). FF: final formulation.

Figure 3

Cellular uptake of (A) [⁶¹Cu]Cu-DOTAGA-PSMA-I&T and (B) [⁶¹Cu]Cu-NODAGA-PSMA-I&T in PSMA-positive (LNCaP) and PSMA-negative (DU145) cells. Data are presented as mean ± SEM (N = 2–5).

Figure 4

Representative coronal whole-body PET/MR images acquired at 1 h (upper row) and 4 h (lower row) post-injection of (A) [⁶¹Cu]Cu-DOTAGA-PSMA-I&T (N = 2) and (B) [⁶¹Cu]Cu-NODAGA-PSMA-I&T (N = 1) into the tail vein of control mice. Images were normalized to % ID/g.

Figure 5

Representative axial (upper row) and coronal (lower row) whole-body PET/MR images acquired at 1 h (left column) and 4 h (right column) post-injection of [⁶¹Cu]Cu-NODAGA-PSMA-I&T into the tail vein of LNCaP tumor-bearing mice. Images were normalized to % ID/g.

Figure 6

Schematic flow of Synthera^® Extension module for post-processing of the irradiated solution. The red dot connects to the red dot in Figure 7. CU: CU cartridge; SAX: strong anion exchange cartridge; FPV: final product vial; cyc: cyclotron; V01–V11: valves 1–11.

Figure 7

Schematic flow of Synthera^® Extension module for the synthesis of [⁶¹Cu]Cu-PSMA-I&T radiopharmaceuticals. The red dot connects to the red dot in Figure 6. Strata-X: cartridge used for purification; FPV: final product vial; V01–11: valves 1–11.