The clinical impact and accessibility of (68)Ga tracers for the prostate-specific membrane antigen (PSMA) and other targets would be greatly enhanced by the availability of a simple, one-step kit-based labeling process. Radiopharmacy staff are accustomed to such procedures in the daily preparation of (99m)Tc radiopharmaceuticals. Currently, chelating agents used in (68)Ga radiopharmaceuticals do not meet this ideal.
AIM: To develop and evaluate preclinically a (68)Ga radiotracer for imaging PSMA expression that could be radiolabeled simply by addition of (68)Ga generator eluate to a cold kit. Methods: A conjugate of a tris(hydroxypyridinone) (THP) chelator with the established urea-based PSMA inhibitor was synthesized and radiolabeled with (68)Ga by adding generator eluate directly to a vial containing the cold precursors THP-PSMA and sodium bicarbonate, with no further manipulation. It was analyzed after 5 min by instant thin layer chromatography and HPLC. The product was subjected to in vitro cell-binding studies to determine PSMA affinity using PSMA-expressing DU145-PSMA cells, with their non-expressing analog DU145 as a control. In vivo PET imaging and ex vivo biodistribution studies were carried out in mice bearing xenografts of the same cell lines, with (68)Ga-HBED-CC-PSMA as a comparator. Results: Radiolabeling was complete (>95%) within 5 min at room temperature, showing a single radioactive species by HPLC that was stable in human serum for >6 hours and showed specific binding to PSMA-expressing cells with an IC50 of 361 ± 60 nM. In vivo PET imaging showed specific uptake in PSMA-expressing tumors, reaching 5.6 ± 1.2 % ID/cm(3) at 40-60 min and rapid clearance from blood to kidney and bladder. The tumor uptake, biodistribution and pharmacokinetics were not significantly different to those of (68)Ga-HBED-CC-PSMA except for reduced uptake in the spleen. Conclusion: Conjugation of THP to the PSMA pharmacophore produces a (68)Ga tracer with equivalent imaging properties but greatly simplified radiolabeling compared to other (68)Ga-PSMA conjugates. THP offers the prospect of rapid, simple, one-step, room temperature "syringe-and-vial" radiolabeling of (68)Ga radiopharmaceuticals.