Synthesis and evaluation of [68Ga]Ga-DOTA-Pip-PEP-4 derivatives for imaging gastrin-releasing peptide receptor expression with positron emission tomography.

BACKGROUND: Gastrin-releasing peptide receptor (GRPR) is overexpressed in several malignancies and is an attractive target for imaging and radioligand therapy. We recently reported [68Ga]Ga-DOTA-Pip-PEP-4 with favorable tumor uptake but also high pancreas uptake. In this study, we investigated whether replacing the piperidine moiety at AA14 in [68Ga]Ga-DOTA-Pip-PEP-4 with structurally distinct six-membered ring-containing derivatives could improve GRPR targeting and reduce pancreas uptake. METHODS: SP01010, SP01011, and SP01022 were synthesized by solid-phase synthesis and labeled with natGa/68Ga. GRPR binding affinities were determined by in vitro competition binding assays. Positron emission tomography (PET) imaging and biodistribution studies were performed in PC-3 tumor-bearing mice at 1 h post-injection. RESULTS: SP01010, SP01011, and SP01022 were synthesized in 39-51% yield, and their nonradioactive Ga-complexed standards were obtained in 69-82% yield. Ki(GRPR) values of Ga-SP01010, Ga-SP01011 and Ga-SP01022 were 4.29 ± 0.71, 2.79 ± 0.77, and 36.7 ± 5.11 nM, respectively. 68Ga-labeled tracers were obtained in 27-57% decay-corrected radiochemical yield with >95% radiochemical purity. All tracers enabled clear visualization of PC-3 tumor xenografts in PET images and the tumor uptake was highest for [68Ga]Ga-SP01011 (9.63 ± 1.07%ID/g), followed by [68Ga]Ga-SP01022 (4.17 ± 1.15%ID/g) and [68Ga]Ga-SP01010 (3.98 ± 0.40%ID/g). Their pancreas uptake ranged from 0.83 to 4.96%ID/g, significantly lower than that of [68Ga]Ga-DOTA-Pip-PEP-4 (24.8 ± 1.77%ID/g). CONCLUSION: Substitution of the piperidine moiety in [68Ga]Ga-DOTA-Pip-PEP-4 significantly influences GRPR binding and pharmacokinetics. [68Ga]Ga-SP01011 has enhanced tumor uptake and minimal pancreas uptake, highlighting its potential as a promising PET tracer for detecting GRPR-expressing cancer.