Effects of Linker Modification on Tumor-to-Kidney Contrast of 68Ga-Labeled PSMA-Targeted Imaging Probes

⁶⁸Ga-PSMA-11 is currently the most popular prostate-specific membrane antigen (PSMA) radioligand used in the clinic to detect prostate cancer and metastases. However, the high uptake of ⁶⁸Ga-PSMA-11 in kidneys can create halo-artifacts resulting in lower detection sensitivity for lesions adjacent to the kidneys. In this study, we developed two ⁶⁸Ga-labeled PSMA-targeted tracers, ⁶⁸Ga-HTK01166 and ⁶⁸Ga-HTK01167, based on ⁶⁸Ga-PSMA-617 with the goal of improving tumor-to-kidney ratio compared to ⁶⁸Ga-PSMA-11. The 2-naphthylalanine (2-Nal) in PSMA-617 was replaced with 2-indanylglycine (Igl) or 3,3-diphenylalanine (Dip) to synthesize HTK01166 and HTK01167, respectively. Binding affinities ( K_i) of Ga-PSMA-11, Ga-PSMA-617, Ga-HTK01166, and Ga-HTK01167 to PSMA were 3.13 ± 0.40, 1.23 ± 0.08, 5.74 ± 2.48, and 25.7 ± 9.84 nM, respectively, as determined by in vitro competition binding assays. ⁶⁸Ga labeling was performed in HEPES buffer with microwave heating, and ⁶⁸Ga-labeled PSMA-11, PSMA-617, HTK01166, and HTK01167 were obtained in 46-69% average decay-corrected radiochemical yield with >99% radiochemical purity and 62.9-152 GBq/μmol average specific activity. PET imaging and biodistribution studies were performed in mice bearing PSMA-expressing LNCap prostate cancer xenografts. All tracers enabled clear visualization of tumors in PET images with excellent tumor-to-background contrast. The uptake values (%ID/g) for tumor and kidneys at 1 h postinjection were 8.91 ± 0.86 and 204 ± 70.6 for ⁶⁸Ga-PSMA-11, 16.7 ± 2.30 and 29.2 ± 5.14 for ⁶⁸Ga-PSMA-617, 14.1 ± 4.40 and 147 ± 59.6 for ⁶⁸Ga-HTK01166, and 7.79 ± 1.65 and 4.30 ± 1.80 for ⁶⁸Ga-HTK01167. The tumor-to-kidney ratios for ⁶⁸Ga-labeled PSMA-11, PSMA-617, HTK01166, and HTK01167 were 0.05 ± 0.02, 0.63 ± 0.10, 0.10 ± 0.02, and 1.98 ± 0.63, respectively. Compared with ⁶⁸Ga-PSMA-617, ⁶⁸Ga-HTK01166 showed comparable tumor uptake and almost 5-fold higher kidney uptake, whereas ⁶⁸Ga-HTK01167 exhibited lower tumor and kidney uptake. Compared with ⁶⁸Ga-PSMA-11, ⁶⁸Ga-HTK01167 had similar tumor uptake and tumor-to-blood contrast ratio (23.8 ± 6.71 vs 20.4 ± 4.98) but higher tumor-to-background contrast ratios for other background organs especially for kidneys. Our data indicate that substitution of 2-Nal in PSMA-617 with other lipophilic amino acid can modulate PSMA binding affinity and their pharmacokinetics in vivo.