Simple new method for labelling of PSMA-11 with Ga in NaHCO.

Background: Prostate specific membrane antigen (PSMA) is a type II membrane protein widely expressed on the surface of prostate cancer cells. One of its functions is to act as a receptor mediating the ligand internalization. This PSMA property is employed in the diagnostics and therapy of prostate cancer. Over the years, small molecules with high affinity for PSMA have been developed and labelled with positron emitters (e.g. Ga, F, C, Cu, or Y). One of these radiolabelled ligands, [Ga] PSMA-11, is one of the most widespread tracers for PET imaging of the prostate cancer. Many techniques have been proposed and tested for the Ga labelling of PSMA-11. The aim of our work was to design a labelling method of PSMA-11 that minimizes number of the used chemicals and steps, providing quantitative labelling yield at laboratory temperature and may be easily automated.

Methodology: AGe/Ga generator eluate in 0.1 M HCl was loaded on an activated Oasis MCX cartridge, and the cartridge was then thoroughly washed with water. The radionuclide Ga was eluted from the cartridge with 0.1 M NaHCO (pH = 8.5, n = 36) or with the same solution with pH adjusted to 7.2-9.0 (n = 38). Precursor PSMA-11 was mixed directly with the cartridge eluate of Ga in 0.1 M NaHCO of given pH. For the stability test, samples of GaPSMA-11 in 0.1 M NaHCO (pH 8.5) were mixed in ratio 1 : 1 with the following solutions: 0.1 M NaHCO (pH 8.5), human serum, PBS and 0.9% NaCl. In order to estimate an effect of the time elapsed between Ga elution from the cartridge in 0.1 M NaHCO (pH 8.5) and the labelling onset of PSMA-11, the latter was initiated 0, 5, 10 and 20 min post elution and radiochemical yield was monitored. All the PSMA-11 labelled samples were subjected to radiochemical purity test using HPLC. The whole process starting from generator elution up to HPLC analysis commencement took 10-15 min.

Results: Recovery of Ga from cartridge Oasis MCX using 0.1 M NaHCO at pH 8.5 was 71.5 ± 1.4%. Thirty six PSMA-11 samples (10 μg in reaction mixture) were labelled at pH 8.5 with total average radiochemical yield of 98 ± 2%. Recovery of Ga from cartridge Oasis MCX using 0.1 M NaHCO at variable pH of 7.2-9.0 was 62.5 ± 1.8% showing certain decrease with decreasing pH. A total of 138 samples of PSMA-11 were labelled with Ga at variable pH (7.2-9.0) and four different amounts of PSMA-11 (1, 2.5, 5 and 10 μg) resulting in the labelling yields of 54.0 ± 5.3%, 88.2 ± 3.2%, 99.4 ± 0.3% and 99.9 ± 0.1%, respectively. Irrespective of the pH, the radiolabelling yield was quantitative for the molar ratio PSMA-11: Ga > 5000 : 1 in the reaction mixture. Stability tests in 0.1 M NaHCO (pH 8.5), human serum, PBS and 0.9% NaCl revealed no observable release of Ga from the Ga-PSMA-11 complex within 3 h. Similarly, the delay between the Ga elution from the Oasis MCX cartridge in 0.1 M NaHCO (pH 8.5) and start of the labelling of PSMA-11 labelling has no effect on the radiochemical yield.

Conclusion: A new method of labelling PSMA-11 ligand with Ga in 0.1 M NaHCO using Oasis MCX cartridges was proposed, developed and tested. The results demonstrated that it is rapid, simple, reproducible and easy to automate.