Synthesis and biological evaluation of substrate-based imaging agents for the prostate-specific membrane antigen

Abstract

Prostate-specific membrane antigen (PSMA) is an attractive target for the imaging of prostate cancer (PCa) due to the elevated expression on the surface of prostate tumor cells. Most PSMA-targeted low molecular weight imaging agents are inhibitors of PSMA. We have synthesized a series of substrate-based PSMA-targeted imaging agents by mimicking poly-gamma-glutamyl folic acid, an endogenous substrate of PSMA. In vitro the gamma-linked polyglutamate conjugates proved to be better substrates than the corresponding alpha-linked glutamates. However, in vivo imaging studies of gamma-ray-emitting and gamma-linked glutamates did not demonstrate selective uptake in PSMA-positive over PSMA-negative tumors. Subsequent chromatographic studies and in silico molecular dynamics simulations indicated that hydrolysis of the substrates is slow and access to the enzymatic active site is limited. These results inform the design of future substrate-based imaging agents for PSMA.