Production of Multimeric Prostate-Specific Membrane Antigen Small Molecule Radiotracers using a Solid-Phase 99m Tc Pre-

Small molecule ligands specific for prostate-specific membrane antigen (PSMA) have the potential to improve prostate cancer imaging. However, highly charged ligands are difficult to label with 99m Tc and to purify. In this study, we present an adamantane-trimerized small molecule that has nanomolar binding to PSMA, but that also has 12 negative charges. Methods—To convert this molecule into a clinically viable SPECT diagnostic, we have developed a simple, cartridge-based, solid-phase pre-labeling strategy that, within 25 minutes, converts readily available and inexpensive 99m Tc-pertechnetate into a chemically pure complex, with a reactive Nhydroxysuccinimide (NHS) ester, in neat organic solvent. This stable intermediate can label any amine-containing small molecule or peptide with 99m Tc in one step, with high specific activity, and without the need for HPLC. Results—Solid-phase conversion of 99m Tc-pertechnetate to 99m Tc-MAS3-NHS could be completed in 25 minutes, with >99% radiochemical purity and with no co-ligands present. This intermediate was then conjugated to adamantane-trimerized GPI in one step with >95% yield, and no need for HPLC purification. The final molecule bound specifically to living human tumor cells expressing PSMA on their surface. Quantitative comparison was made among GPI monomer, GPI trimer, and their 99m Tc derivatives. Conclusion—Our study describes a simple cartridge-based conversion of 99m Tc pertechnetate to a useful, pre-loaded NHS ester intermediate that takes only 25 minutes to prepare and results in >99% radiochemical purity. Using this chemistry, we produce a high specific activity, 99m Tc-labeled, PSMA-targeted small molecule, and demonstrate gamma ray radioscintigraphic imaging of living human prostate cancer cells.