Synthesis of ²²⁵Ac-PSMA-617 for Preclinical Use

The recent approval of radiopharmaceuticals for diagnosis and treatment of cancer is ushering nuclear medicine into a new era of theranostics and alpha therapy using radiopharmaceuticals labeled with ²²⁵Ac shows remarkable results in clinical trials. As such, reliable methods for the synthesis and quality control of ²²⁵Ac-radiopharmaceuticals are needed.

²²⁵Ac-PSMA-617 is being used for targeted alpha therapy in patients with prostate cancer, and we had cause to synthesize the agent for preclinical use. However, technology transfer proved cumbersome owing to the paucity of information available on synthesizing and analyzing ²²⁵Ac-radiotherapeutics. To address this need, we describe a straightforward synthesis of ²²⁵Ac-PSMA- 617 as well as suitable approaches for quality control analysis using standard equipment in a modern PET Center.

PSMA-617 precursor was dissolved in 25 μL metal-free water (0.67 mg/mL) and combined with 500 μL 0.05M Tris buffer, pH 9. Actinium stock solution (~65 μCi in 15 μL) was added and the reaction was heated at 120°C for 40-50 min. The reaction was cooled and 0.6 mL gentisic acid solution (4 mg/mL in 0.2 M NH₄OAc) was added. To formulate the dose for injection, sterile saline, USP (8 mL) was added and the pH was adjusted by the addition of 100 μL 0.05 M Tris buffer (pH 9) to give a final pH of ~7.2. The final solution was filtered using a 0.22 μm GV sterile filter into a sterile dose vial. Radiochemical purity was determined by radio-TLC (eluent: 50mM Sodium Citrate, pH 5), and plates were analyzed using an AR2000 scanner.

The method provided ²²⁵Ac-PSMA-617 in high radiochemical yield (57 ± 3 μCi, >99%) and radiochemical purity (98 ± 1%), formulated for preclinical studies (9 mL, pH = 7.2), n=3.

A straightforward synthesis of ²²⁵Ac-PSMA-617 is described that will facilitate production for (pre)clinical studies. The approach could also be applicable to the synthesis of other alpha radiotherapeutics incorporating ²²⁵Ac.