The effects of mode of binding on clickable PSMA-targeted imaging agents

Objectives PSMA (prostate specific membrane antigen) is an enzyme biomarker that is highly up-regulated and expressed in prostate tumor cells providing an ideal target for imaging and therapeutic applications for prostate cancer. Our first generation PSMA targeted molecule CTT-54 (IC50 = 14 nM) is an irreversible phosphoramidate inhibitor of PSMA and when labeled withh 18F or 99mTc demonstrated promising in vivo PET and SPECT imaging of prostate tumors in mice models. The SPECT agent also exhibited rapid internalization in PSMA positive cells, presumably through the PSMA enzyme-inhibitor complex. Our lab has previously shown that mode of binding for PSMA inhibitiros affects their rate and extent of internalization. The focus of this study was to determine how the mode of binding affects the uptake and clearance of clickable PSMA-targeted imaging agents in vitro and in vivo.

Methods The in vitro percent uptake and internalization as well as the biodistribution in mice models were determined for both an irreversible and slowly reversible radiolabeled PSMA inhibitor. The PSMA inhibitors were radiolabeled through a DBCO pendant group with chelated 99mTc functionalized with an azide group.

Results Initial in vitro studies demonstrated a significant difference in uptake between an irreversible (25%, 4h) and slowly reversible (9.2%, 4h) clickable PSMA-targeted SPECT imaging agent despite having similar IC50 values.

Conclusions There was a significant effect upon the in vitro performance of an irreversible and slowly reversible clickable PSMA-targeted SPECT imaging agent due to the differences in mode of binding. Further comparison of clickable PSMA-targeted imaging agents is expected to elucidate the effects of mode of binding on in vitro and in vivo performance.

Research Support Washington State Life Sciences Discovery Fund (08-012374880), National Institutes of Health (R01CA140617