Synthesis and Evaluation of Technetium-99m- and Rhenium-Labeled Inhibitors of the Prostate-Specific Membrane Antigen (PSMA)

Abstract

The prostate-specific membrane antigen (PSMA) is increasingly recognized as a viable target for imaging and therapy of cancer. We prepared seven ^99mTc/Re-labeled compounds by attaching known Tc/Re chelating agents to an amino-functionalized PSMA inhibitor (lys-NHCONH-glu) with or without a variable length linker moiety. K_i values ranged from 0.17 to 199 nM. Ex vivo biodistribution and in vivo imaging demonstrated the degree of specific binding to engineered PSMA+ PC3 PIP tumors. PC3-PIP cells are derived from PC3 that have been transduced with the gene for PSMA. Despite demonstrating nearly the lowest PSMA inhibitory potency of this series, [^99mTc(CO)₃(L1)]⁺ (L1 = (2-pyridylmethyl)₂N(CH₂)₄CH(CO₂H)NHCO-(CH₂)₆CO-NH-lys-NHCONH-glu) showed the highest, most selective PIP tumor uptake, at 7.9 ± 4.0% injected dose per gram of tissue at 30 min postinjection. Radioactivity cleared from nontarget tissues to produce a PIP to flu (PSMA-PC3) ratio of 44:1 at 120 min postinjection. PSMA can accommodate the steric requirements of ^99mTc/Re complexes within PSMA inhibitors, the best results achieved with a linker moiety between the ε amine of the urea lysine and the chelator.