Production of Multimeric Prostate-Specific Membrane Antigen Small-Molecule Radiotracers Using a Solid-Phase 99mTc Preloading Strategy.
Journal of nuclear medicine : official publication, Society of Nuclear Medicine 2007 Jul 13; 48(8):1379-89
Misra P P, Humblet V V, Pannier N N, Maison W W, Frangioni J JV
Division of Hematology/Oncology, Beth Israel Deaconess Medical Center, Boston, Massachusetts.
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 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 amine-containing 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 (99m)Tc-pertechnetate to (99m)Tc-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 (99m)Tc-derivatives. CONCLUSION: Our study describes a simple cartridge-based conversion of (99m)Tc-pertechnetate 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 high-specific-activity, (99m)Tc-labeled, PSMA-targeted small molecule and demonstrate gamma-ray radioscintigraphic imaging of living human prostate cancer cells.