PSMA-targeted SPECT agents: Mode of binding effect on in vitro performance

BACKGROUND The enzyme-biomarker prostate-specific membrane antigen (PSMA) is an active target for imaging and therapeutic applications for prostate cancer. The internalization of PSMA has been shown to vary with inhibitors' mode of binding: irreversible, slowly reversible, and reversible. METHODS In the present study, PSMA-targeted clickable derivatives of an irreversible phosphoramidate inhibitor DBCO-PEG₄-CTT-54 (IC₅₀ = 1.0 nM) and a slowly reversible phosphate inhibitor, DBCO-PEG₄-CTT-54.2 (IC₅₀ = 6.6 nM) were clicked to ^99mTc(CO) ₃-DPA-azide to assemble a PSMA-targeted SPECT agent. The selectivity, percent uptake, and internalization of these PSMA-targeted SPECT agents were evaluated in PSMA-positive and PSMA-negative cells. RESULTS In vitro studies demonstrated that PSMA-targeted SPECT agents exhibited selective cellular uptake in the PSMA-positive LNCaP cells compared to PSMA-negative PC3 cells. More importantly, it was found that ^99mTc(CO)₃-DPA-DBCO- PEG₄-CTT-54 based on an irreversible PSMA inhibitor core, exhibited greater uptake and internalization than ^99mTc(CO)₃-DPA- DBCO-PEG₄-CTT-54.2 constructed from a slowly reversible PSMA inhibitor core. CONCLUSIONS We have demonstrated that a PSMA-targeted SPECT agent can be assembled efficiently using copper-less click chemistry. In addition, we demonstrated that mode of binding has an effect on internalization and percent uptake of PSMA-targeted SPECT agents; with the irreversible targeting agent demonstrating superior uptake and internalization in PSMA+ cells. The approach demonstrated in this work now supports a modular approach for the assembly of PSMA-targeted imaging and therapeutic agents. Prostate 73: 355-362, 2013. © 2012 Wiley Periodicals, Inc.