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 99mTc and to purify. In this study, we present an adamantane-trimerized 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 de-veloped a simple, cartridge-based, solid-phase prelabeling strategy that, within 25 min, converts readily available and inex-pensive 99mTc-pertechnetate into a chemically pure complex, with a reactive N-hydroxysuccinimide (NHS) ester, in neat or-ganic solvent. This stable intermediate can label any amine-containing small molecule or peptide with 99mTc in 1 step, with high specific activity and without the need for high-performance liquid chromatography (HPLC). Results: Solid-phase conver-sion 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-trimer-ized 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 specif-ically to living human tumor cells expressing PSMA on their sur-face. Quantitative comparison was made among GPI monomer, GPI trimer, and their 99mTc-derivatives. Conclusion: Our study describes a simple cartridge-based conversion of 99mTc-pertechnetate to a useful, preloaded NHS ester intermediate that takes only 25 min to prepare and results in.99 % radio-chemical purity. Using this chemistry, we produced a high-specific-activity, 99mTc-labeled, PSMA-targeted small molecule and demonstrate g-ray radioscintigraphic imaging of living hu-man prostate cancer cells