Preclinical Evaluation of Novel Glutamate-Urea-Lysine Analogues That Target Prostate-Specific Membrane Antigen as Molecular Imaging Pharmaceuticals for Prostate Cancer

Prostate-specific membrane antigen (PSMA) is expressed in normal human prostate epithelium and is highly up-regulated in prostate cancer. We previously reported a series of novel small molecule inhibitors targeting PSMA. Two compounds, MIP-1072, ( S )-2-(3-(( S )-1-carboxy-5-(4-iodobenzylamino)pentyl)ureido)pentanedioic acid, and MIP-1095, ( S )-2-(3-(( S )-1carboxy-5-(3-(4-iodophenyl)ureido)pentyl)ureido)pentanedioic acid, were selected for further evaluation. MIP-1072 and MIP-1095 potently inhibited the glutamate carboxypeptidase activity of PSMA ( K i = 4.6 ± 1.6 nmol/L and 0.24 ± 0.14 nmol/L, respectively) and, when radiolabeled with 123 I, exhibited high affinity for PSMA on human prostate cancer LNCaP cells ( K d = 3.8 ± 1.3 nmol/L and 0.81 ± 0.39 nmol/L, respectively). The association of [ 123 I]MIP-1072 and [ 123 I]MIP-1095 with PSMA was specific; there was no binding to human prostate cancer PC3 cells, which lack PSMA, and binding was abolished by coincubation with a structurally unrelated NAALADase inhibitor, 2-(phosphonomethyl)pentanedioic acid (PMPA). [ 123 I]MIP-1072 and [ 123 I]MIP-1095 internalized into LNCaP cells at 37°C. Tissue distribution studies in mice showed 17.3 ± 6.3% (at 1 hour) and 34.3 ± 12.7% (at 4 hours) injected dose per gram of LNCaP xenograft tissue, for [ 123 I]MIP-1072 and [ 123 I]MIP-1095, respectively. [ 123 I]MIP-1095 exhibited greater tumor uptake but slower washout from blood and nontarget tissues compared with [ 123 I]MIP-1072. Specific binding to PSMA in vivo was shown by competition with PMPA in LNCaP xenografts, and the absence of uptake in PC3 xenografts. The uptake of [ 123 I]MIP-1072 and [ 123 I]MIP-1095 in tumor-bearing mice was corroborated by single-photon emission computed tomography/computed tomography (SPECT/CT) imaging. PSMA-specific radiopharmaceuticals should provide a novel molecular targeting option for the detection and staging of prostate cancer. [Cancer Res 2009;69(17):6932–40]