Halogen Replacement on the Lysine Side Chain of Lys-Urea-Glu-Based PSMA Inhibitors Leads to Significant Changes in Targeting Properties

Purpose Investigate the impact of various halogens on pharmacokinetics, biodistribution, and micro positron emission tomography/computed tomography (PET/CT) imaging of Glu-urea-Lys-based prostate-specific membrane antigen (PSMA) inhibitors. Procedures Based on the modification of SC691, a small molecule inhibitor of PSMA previously developed by our group, we synthesized 68 Ga-labeled compounds by modifying the lysine terminal amino with different halogenated phenyl substituents. After complete characterization, in vitro and in vivo properties were studied. Results The [ 68 Ga]Ga-DOTA-SC691-R possesses a high radiochemical yield (98–99%). The internalization values of [ 68 Ga]Ga-DOTA-SC691-H, [ 68 Ga]Ga-DOTA-SC691-Cl, and [ 68 Ga]Ga-DOTA-SC691-Br in LNCaP cells all displayed time-dependent pattern enhanced with time. The results of in vitro competitive inhibition assay showed that the affinity of nat Ga-DOTA-SC691-R for PSMA had a trend of H < F < Cl < Br < I. The blocking imaging and dynamic imaging on micro-PET/CT of male non-obese diabetic/severe combined immunodeficiency mice with LNCaP tumors showed the rapid tumor targeting properties of [ 68 Ga]Ga-DOTA-SC691-R with specificity for PSMA. Static imaging of micro-PEC/CT of these compounds could rapidly localize LNCaP tumors with decent image quality (except for [ 68 Ga]Ga-DOTA-SC691-H). Biodistribution data showed that [ 68 Ga]Ga-DOTA-SC691-R were metabolized via the kidney and tumor accumulation followed the order of H ≈ F ≈ Cl < I < Br uptake values at 1 h. [ 68 Ga]Ga-DOTA-SC691-Br showed the highest tumor accumulation and retention (15.21 ± 5.57%ID/g at 30 min, 20.39 ± 4.38%ID/g at 60 min, and 13.30 ± 4.39%ID/g at 120 min), which is consistent with the results of the competitive inhibition assay and cell binding assay. Conclusions It was demonstrated that the halogen substituent on the lysine terminal amino group on the Glu-urea-Lys backbone did positively affect the binding of [ 68 Ga]Ga-DOTA-SC691-R to PSMA. The bulkier and less electronegative Br (or I) elements are preferred for structural modifications here.