Identifying Potential Off-Target Protein Binders of Glutamate-Ureido-Lysine, a Prostate Cancer Specific Imaging Agent

Prostate-specific membrane antigen (PSMA) is highly overexpressed in most prostate cancers and is clinically visualized using PSMA-specific probes incorporating Glutamate-Ureido-Lysine (GUL). PSMA is effectively absent from certain high-mortality, treatment-resistant subsets of prostate cancers, such as neuroendocrine prostate cancer (NEPC); however, GUL-based probes still sometimes identify NEPC metastatic tumours. These probes may bind unknown proteins associated with PSMA-suppressed cancers. We identified the upregulation of PSMA-like aminopeptidase NAALADaseL and the metabotropic glutamate receptors (mGluRs) in NEPC; we found that the expression levels inversely correlate with PSMA expression and are associated with poor clinical prognosis indicating they may participate in NEPC disease progression. Computationally predicting that GUL-based probes bind well to these targets, we designed and synthesized a new fluorescent probe to investigate these proteins in vitro, where it shows excellent affinity for PSMA, NAALADaseL and specific mGluRs associated with poor prognosis.