Preclinical evaluation of novel glutamate-urea-lysine analogues that target prostate-specific membrane antigen as molecular imaging pharmaceuticals for prostate cancer

Abstract

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 degrees 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.

Conflict of interest statement

Disclosure of potential conflicts of interest

This work was conducted at Molecular Insight Pharmaceuticals, Inc. S. Hillier, K. Maresca, F. Femia, J. Marquis, C. Zimmerman, J. Barrett, J. Joyal, and J. Babich are employees of Molecular Insight Pharmaceuticals, Inc. W. Eckelman and M. Pomper are consultants for Molecular Insight Pharmaceuticals, Inc.

Figures

Figure 1

A. Binding of [123I]MIP-1072 or [123I]MIP-1095 to LNCaP and PC3 cells. Cells were incubated for 1 hr with each compound in the absence or presence of unlabeled compound or PMPA. B. Saturation binding analysis of [123I]MIP-1072 and [123I]MIP-1095. LNCaP cells were incubated at 4 °C for 1 hr with 30–300,000 pM [123I]MIP-1072 or [123I]MIP-1095. The Kd and Bmax were determined by non-linear regression analysis. C. LNCaP cellular internalization of [123I]MIP-1072 and [123I]MIP-1095. LNCaP cells were incubated with 100 nM radiolabeled compound for the indicated time, washed, and treated with a mild acid buffer to separate cell surface bound (dashed lines) from total bound material (solid lines). The results are representative of two independent experiments.

Figure 2

Selective targeting of PSMA in vivo with [123I]-MIP-1072 and [123I]-MIP-1095. Radiolabeled compound was injected into mice bearing LNCaP xenografts and imaged by SPECT/CT at 4 hr (top) or mice bearing PC3 PIP (PSMA +) or PC3 flu (PSMA −) xenografts and imaged by SPECT/CT at 2 hr (bottom). Each mouse was injected with approximately 1 mCi of radiolabeled compound at a specific activity >1000 mCi/µmol.

Figure 3

Specific binding of [123I]MIP-1072 (A) and [123I]MIP-1095 (B) to PSMA in vivo. Radiolabeled compound (2 µCi/mouse at >1000 mCi/µmol) was injected alone (LNCaP tumor , PC3 tumor ) or co-injected with 50 mg/kg PMPA (LNCaP tumor , or PC3 tumor ) via the tail vein. Data are expressed as %ID/g.