Design and evaluation of novel glutamate-urea heterodimers that target PSMA as molecular imaging agents for prostate cancer.

B143 The expression of distinct proteins on the surface of tumor cells offers the opportunity to diagnose and characterize disease by probing the phenotypic identity and biochemical composition of a tumor. Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein that is primarily expressed in normal human prostate epithelium and is highly upregulated in prostate cancer, including metastatic disease. The development of small molecule ligands that interact specifically with PSMA and carry an appropriate radionuclide may provide a promising and novel molecular targeting option for the detection, staging, and potential treatment of prostate cancer. Therefore, a series of halogen-containing glutamate-urea heterodimers (glu-urea-X) originally described by Kozikowski et al J Med Chem. 2001 Feb 1;44(3):298-301, where X = ϵ-N-(o-I, m-I, p-I, p-Br, or p-F)-benzyl-Lys or ϵ-(p-I)-phenyl ureido-Lys, were designed, synthesized and shown to bind to the N-acetyl-α-linked-acidic-dipeptidase (NAALADase) region of PSMA. The affinity of the analogs was determined by screening in a competitive binding assay against N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-(S)-3-[131I]iodo-L-tyrosine for binding to PSMA on LnCaP prostate cancer cells. The most potent compounds, 2-{3-[1-carboxy-5-(4-iodo-benzylamino)-pentyl]-ureido}-pentanedioic acid (MIP-1072) and (S)-2-(3-((R)-1-carboxy-5-(3-(4-iodophenyl)ureido)pentyl)ureido)pentanedioic acid (MIP-1095), exhibited an IC50 of 25 ± 8 and 10 ± 3 nM, respectively (IC50 values of the series ranged from 8 to >1,000 nM). MIP-1072 and MIP-1095 inhibited the NAALADase enzymatic activity of PSMA with Ki values of 6 ± 2 and 0.3 ± 0.1 nM, respectively. In a saturation binding experiment both compounds bound to PSMA with high affinity (Kd of [123I]MIP-1072 = 0.25 ± 0.11 nM, Kd of [123I]MIP-1095 = 1.08 ± 0.03 nM). An acid-insensitive time and temperature dependent increase in cell association of both compounds indicated cellular internalization. Both compounds were stable in plasma and liver microsomes for 1 hour. In nude mice bearing PSMA (+) xenografts, the peak uptake of MIP-1095 in the tumor was 34% injected dose per gram (ID/g) at 4 hours and the compound persisted in the tumor for 24 hours (29% ID/g). MIP-1072 achieved peak uptake at 1 hour (17 %ID/g) and washed out more rapidly (5% ID/g at 24 hr). Both compounds exhibited excellent target to background ratios with >30:1 for tumor:blood and >60:1 for tumor:muscle by 2 hr. Co-injection of the animals with a structurally unrelated NAALADase inhibitor reduced in vivo tumor uptake of both compounds >30-fold. SPECT imaging demonstrated localization of [123I]MIP-1072 and [123I]MIP-1095 in PSMA(+) tumors and kidney tissue but not PSMA(-) tumors or other tissues. In conclusion, 2 novel iodine containing Glu-urea-Lys analogs were synthesized that bind with high affinity to PSMA; upon binding, the compounds are internalized in prostate cancer cells that express PSMA and accumulate in human prostate cancer xenografts in vivo. As MIP-1095 and MIP-1072 exhibit distinct pharmacokinetic profiles, both compounds will be further investigated in patients for localization in prostate cancer.