Production of Multimeric Prostate-Specific Membrane Antigen Small-Molecule Radiotracers Using a Solid-Phase 99mTc Preloading Strategy

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 adamantanetrimerized small molecule that has nanomolar binding to PSMA and 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 prelabeling strategy that, within 25 min, converts readily available and inexpensive 99m Tc-pertechnetate into a chemically pure complex, with a reactive N-hydroxysuccinimide (NHS) ester, in neat organic solvent. This stable intermediate can label any aminecontaining small molecule or peptide with 99m Tc in 1 step, with high specific activity and without the need for high-performance liquid chromatography (HPLC). Results: Solid-phase conversion of 99mTc-pertechnetate to 99mTc-MAS3-NHS (MAS3 is S-acetylmercaptoacetyltriserine) could be completed in 25 min, with .99% radiochemical purity and with no coligands present. This intermediate was then conjugated to adamantane-trimerized GPI (2[(3-amino-3-carboxypropyl)(hydroxy)(phosphinyl)methyl]pentane-1,5-dioic acid) in 1 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 99mTc-derivatives. Conclusion: Our study describes a simple cartridge-based conversion of 99m Tcpertechnetate to a useful, preloaded NHS ester intermediate that takes only 25 min to prepare and results in .99% radiochemical purity. Using this chemistry, we produced a highspecific-activity, 99m Tc-labeled, PSMA-targeted small molecule