The Synthesis and Evaluation of 64Cu-DOTA-NT, 64Cu-NOTA-NT and 64Cu-AmBaSar-NT for PET Imaging of Neurotensin Receptor in Prostate Cancer

Objectives Secreted by neuroendocrine-like prostate cells, neurotensin (NT) has numerous physiological effects predominantly mediated through its high affinity receptor neurotensin receptor-1 (NTR-1). In particular, NTR-1 was found to be expressed and activated in aggressive prostate cancer cells, but not in normal prostate epithelial cells. In this study, we developed three 64Cu labeled NT analogs for positron emission tomography (PET) imaging of NTR expression in prostate cancers.

Methods Lys-NT20.3 peptide was conjugated with three 64Cu chelators: DOTA, NOTA, or AmBaSar, respectively. NTR binding affinities of these probes were evaluated using in vitro cell binding assay. The in vivo behaviors of radiolabeled probes were compared in well-established NTR positive HT-29 tumor model. The selected agents were then employed to image NTR expression in prostate cancers.

Results All three NT conjugates retained the majority of NTR binding affinity of the unmodified NT analog (2.9 ± 1.2 nM, 3.0 ± 1.4 nM, 4.8 ± 1.0 nM and 2.4 ± 0.3 nM for DOTA-NT, NOTA-NT, AmBaSar-NT, and Lys-NT analog, respectively). In HT-29 tumor, all conjugates demonstrated prominent tumor uptake based on biodistribution study. Although comparable stability profile was observed for all conjugates, 64Cu-NOTA-NT and 64Cu-AmBaSar-NT demonstrated improved tumor to background contrast compared with 64Cu-DOTA-NT. PET/CT imaging of NTR expression was performed on androgen independent PC-3 tumor. Consistent with in vitro NTR expression profile, PC-3 xenografts showed high and specific uptake of the probes which can be blocked by un-labeled NT peptide. Most normal organs low NTR expression levels and accordingly low radiotracer uptakes.

Conclusions 64Cu-DOTA-NT, 64Cu-NOTA-NT, and 64Cu-AmBaSar-NT are promising imaging agents for NTR positive tumors, which have comparable tumor imaging capability in vivo. These agents may help us identify NTR positive lesions and predict which patients and individual tumors are likely to respond to novel interventions targeting NTR-1.