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 developed a simple, cartridge-based, solid-phase prelabeling strategy that, within 25 min, converts readily available and inexpensive 99mTc-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 99mTc 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.


CONCLUSIONS
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% radiochemical purity. Using this chemistry, we produced a high-specific-activity, 99mTc-labeled, PSMA-targeted small molecule and demonstrate gamma-ray radioscintigraphic imaging of living human prostate cancer cells.
Misra
Preeti
P
Division of Hematology/Oncology, Beth Israel Deaconess Medical Center, Boston, Massachusetts 02215, USA.
Humblet
Valerie
V
Pannier
Nadine
N
Maison
Wolfgang
W
Frangioni
John V
JV
D000954:Antigens, Surface / Q000032:analysis*; D045744:Cell Line, Tumor; D043425:Glutamate Carboxypeptidase II / Q000032:analysis*; D006801:Humans; D007553:Isotope Labeling; D008297:Male; D013670:Sodium Pertechnetate Tc 99m; D015899:Tomography, Emission-Computed, Single-Photon
D000954:Antigens, Surface; D013670:Sodium Pertechnetate Tc 99m; C473848:FOLH1 protein, human; D043425:Glutamate Carboxypeptidase II