A high-affinity [18F]-labeled phosphoramidate peptidomimetic PSMA-targeted inhibitor for PET imaging of prostate cancer

Abstract

Introduction In this study, a structurally modified phosphoramidate scaffold, with improved prostate-specific membrane antigen (PSMA) avidity, stability and in vivo characteristics, as a PET imaging agent for prostate cancer (PCa), was prepared and evaluated. Methods p-Fluorobenzoyl-aminohexanoate and 2-(3-hydroxypropyl)glycine were introduced into the PSMA-targeting scaffold yielding phosphoramidate 5. X-ray crystallography was performed on the PSMA/5 complex. [18F]5 was synthesized, and cell uptake and internalization studies were conducted in PSMA(+) LNCaP and CWR22Rv1 cells and PSMA(−) PC-3 cells. In vivo PET imaging and biodistribution studies were performed at 1 and 4 h post injection in mice bearing CWR22Rv1 tumor, with or without blocking agent. Results The crystallographic data showed interaction of the p-fluorobenzoyl group with an arene-binding cleft on the PSMA surface. In vitro studies revealed elevated uptake of [18F]5 in PSMA(+) cells (2.2% in CWR22Rv1 and 12.1% in LNCaP) compared to PSMA(−) cells (0.08%) at 4 h. In vivo tumor uptake of 2.33% ID/g and tumor-to-blood ratio of 265:1 was observed at 4 h. Conclusions We have successfully synthesized, radiolabeled and evaluated a new PSMA-targeted PET agent. The crystal structure of the PSMA/5 complex highlighted the interactions within the arene-binding cleft contributing to the overall complex stability. The high target uptake and rapid non-target clearance exhibited by [18F]5 in PSMA(+) xenografts substantiates its potential use for PET imaging of PCa. Advances in Knowledge The only FDA-approved imaging agent for PCa, Prostascint®, targets PSMA but suffers from inherent shortcomings. The data acquired in this manuscript confirmed that our new generation of [18F]-labeled PSMA inhibitor exhibited promising in vivo performance as a PET imaging agent for PCa and is well-positioned for subsequent clinical trials. Implications for Patient Care Our preliminary data demonstrate that this tracer possesses the required imaging characteristics to be sensitive and specific for PCa imaging in patients at all stages of the disease.

Structure–Activity Relationship of 18F-Labeled Phosphoramidate Peptidomimetic Prostate-Specific Membrane Antigen (PSMA)-Targeted Inhibitor Analogues for PET Imaging of Prostate Cancer

“…24 Compound 1 was prepared in a final step by global deprotection of benzyl esters in precursor 7 (Scheme 1). Radiolabling precursor 1 represents a common structural core of the phosphoramidate analogues examined in the present study.…”

“…Compound 5 , where the inhibitor core was separated from the FB ring by one AH unit, has previously shown desirable PSMA targeting properties. 24 [ 18 F] 5 retained high affinity for PSMA and showed consistent uptake and rapid internalization into CW22Rv1 (PSMA+) cells. The in vitro properties translated well to mouse models implanted with CW22Rv1 tumors xenografts.…”

Glu-Ureido–Based Inhibitors of Prostate-Specific Membrane Antigen: Lessons Learned During the Development of a Novel Class of Low-Molecular-Weight Theranostic Radiotracers

“…However, all such substitutions reported to date have failed to provide viable leads and instead have resulted in compounds with substantially lower PSMA affinities (9,(44)(45)(46). Likewise, a search for a new "ultimate" zinc-binding group has not been successful; consequently, ureido-based PSMA inhibitor scaffolds are currently the most prevalent theranostic PSMA-targeting vectors used, followed only by transition-state mimetics, such as phosphoramidates (47).…”

Prostate-specific membrane antigen targeted protein contrast agents for molecular imaging of prostate cancer by MRI

“…2 [ 64 Cu]6, a 64 Cu labeled PSMA inhibitor developed by the conjugation of 64 Cu chelated CB-TE2A to the lysine–glutamate urea scaffold, shows high tumor penetration, and specific distribution to the PSMA positive tumor implanted in the mice model compared with the control PSMA negative tumor and background tissue. 16,49–52 …”