68Ga-DOTA-NT-20.3 Neurotensin receptor 1 positron emission tomography imaging as a surrogate for neuroendocrine differentiation of prostate cancer

Abstract

PSMA-negative neuroendocrine prostate cancer (NEPC) is likely to be a lethal subtype of prostate cancer (PCa) with limited clinical diagnostic and therapeutic options. High expression of neurotensin receptor subtype 1 (NTR1) associated with neuroendocrine differentiation of PCa, which makes NTR1 a potential target for NEPC. In this study, NTR1-targeted tracer 68Ga-DOTA-NT-20.3 was synthesized and evaluated by determining its affinity to androgen-dependent (LNCap) and androgen-independent (PC3) xenografts. Methods: 68Ga-DOTA-NT-20.3 was labeled with an automated iQS-theranostics synthesizer module and its stability, labeling yield, and radiochemical purity were analyzed by radio-HPLC. Receptor binding affinity was evaluated in NTR1-positive PC3 cells by competitive binding assay. The biodistribution of 68Ga-DOTA-NT-20.3 in vivo was evaluated in PC3 and LNCap xenografts by micro-PET imaging. NTR1 expression was identified by immunohistochemistry and immunofluorescence. Results: 68Ga-DOTA-NT-20.3 was synthesized successfully with a yield rate of 88.07 ± 1.26 %, radiochemical purity ≥ 99% and favorable stability. The NTR1 affinity (IC50) for 68Ga-DOTA-NT-20.3 was 7.59 ± 0.41 nM. Micro-PET/CT in PC3 xenografts showed high contrast images with intense tumor uptake, which revealed specific NTR1 expression. The tumor showed significant radioactivity (4.95 ± 0.67 percentage of injected dose per gram of tissue [%ID/g]) at 1h, which fell to 1.95 ± 0.17 %ID/g (P < 0.01, t = 8.72) after specific blockage by neurotensin. LNCap xenografts had no significant accumulation (0.81 ± 0.06 %ID/g) of 68Ga-DOTA-NT-20.3 at 1 h. In contrast, 68Ga-PSMA-11 was mainly concentrated in LNCap xenografts (8.60 ± 2.11 %ID/g), with no significant uptake in PC3 tumors (0.53 ± 0.05 %ID/g), consistent with the in vitro immunohistochemistry findings. Biodistribution showed rapid clearance from the blood and main organs (brain, heart, lung, liver, muscle, bone) with significantly high tumor/liver (4.41 ± 0.73) and tumor/muscle ratios (12.34 ± 1.32) at 60 min post-injection. Conclusion: 68Ga-DOTA-NT-20.3 can be efficiently prepared with a high yield and radiochemical purity. Its favorable biodistribution and prominent NTR1 affinity make 68Ga-DOTA-NT-20.3 a potential radiopharmaceutical for the detection of PSMA-negative PCa and identification of neuroendocrine differentiation.