Optimizing the Performance of 68Ga Labeled FSHR Ligand in Prostate Cancer Model by Means of Aprotinin

Purpose: Radiolabeled FSH1 peptides are potential specific probes for FSHR imaging. However, moderate uptakes and fast washout from the tumors may limit its widespread use. In this study, 68Ga labeled modified FSH1 analogs was prepared and the imaging properties were determined in the prostate cancer model with or without aprotinin.Methods: NOTA-MAL-FSH4 was synthesized and labeled with 68Ga. The pharmacokinetic profile of the peptide after co-administration with aprotinin was determined through metabolism analyses and microPET imaging.Results: 68Ga-NOTA-MAL-FSH4 was successfully prepared. The IC50 value of displacement 68Ga-NOTA-MAL-FSH4 with FSH1 was 139.4±1.16 nM. The PC-3 prostate tumor was visible after administration of the 68Ga labeled tracer. In vitro RP-HPLC analysis revealed that the average percentage of intact peptide in the plasma, liver and tumor was 8.30, 9.57 and 7.06 % respectively. In presence of aprotinin, the amounts of intact peptide increased to 34.32%, 20.63 % and 15.39 % in the counterparts respectively. MicroPET imaging showed that the uptakes of PC-3 tumors at 60mins after co-administration of 100μg, 200μg or 400μg enzyme inhibitors were 2.91±0.21%ID/g, 3.89±0.16%ID/g and 9.21±0.22%ID/g respectively.Conclusion: With the aid of a serine protease inhibitor, the performance of the 68Ga labeled peptide was optimized, which may benefit further clinical application.

Influence of the Position and Composition of Radiometals and Radioiodine Labels on Imaging of Epcam Expression in Prostate Cancer Model Using the DARPin Ec1

The epithelial cell adhesion molecule (EpCAM) is intensively overexpressed in 40–60% of prostate cancer (PCa) cases and can be used as a target for the delivery of drugs and toxins. The designed ankyrin repeat protein (DARPin) Ec1 has a high affinity to EpCAM (68 pM) and a small size (18 kDa). Radiolabeled Ec1 might be used as a companion diagnostic for the selection of PCa patients for therapy. The study aimed to investigate the influence of radiolabel position (N- or C-terminal) and composition on the targeting and imaging properties of Ec1. Two variants, having an N- or C-terminal cysteine, were produced, site-specifically conjugated to a DOTA chelator and labeled with cobalt-57, gallium-68 or indium-111. Site-specific radioiodination was performed using ((4-hydroxyphenyl)-ethyl)maleimide (HPEM). Biodistribution of eight radiolabeled Ec1-probes was measured in nude mice bearing PCa DU145 xenografts. In all cases, positioning of a label at the C-terminus provided the best tumor-to-organ ratios. The non-residualizing [125I]I-HPEM label provided the highest tumor-to-muscle and tumor-to-bone ratios and is more suitable for EpCAM imaging in early-stage PCa. Among the radiometals, indium-111 provided the highest tumor-to-blood, tumor-to-lung and tumor-to-liver ratios and could be used at late-stage PCa. In conclusion, label position and composition are important for the DARPin Ec1.