Evaluation of agonist and antagonist radioligands for somatostatin receptor imaging of breast cancer using positron emission tomography

Background The somatostatin receptor subtype 2 (sstr2) is expressed on a majority of luminal breast cancers, however SPECT and scintigraphy imaging with agonistic sstr2 probes has been sub-optimal. High affinity antagonists can access more binding sites on the cell surface, resulting in higher tumor uptake and improved sensitivity. We compared the tumor uptake and biodistribution of the antagonist 68 Ga-NODAGA-JR11 with two agonists 68 Ga-DOTA-Tyr 3 -octreotide ( 68 Ga-DOTATOC) and 68 Ga-DOTA-Tyr 3 -octreotate ( 68 Ga-DOTATATE), in the human, sstr2-positive, luminal breast cancer model: ZR-75-1. Results Peptides were assayed for binding affinity using a filtration-based competitive assay to sstr2. nat Ga-DOTATOC and nat Ga-DOTATATE had excellent affinity (inhibition constant K i : 0.9 ± 0.1 nM and 1.4 ± 0.3 nM respectively) compared to nat Ga-NODAGA-JR11 (25.9 ± 0.2 nM). The number of binding sites on ZR-75-1 cells was determined in vitro by saturation assays. Agonist 67/nat Ga-DOTATOC bound to 6.64 ± 0.39 × 10 4 sites/cells, which was 1.5-fold higher than 67/nat Ga-NODAGA-JR11 and 2.3-fold higher than 67/nat Ga-DOTATATE. All three 68 Ga-labeled peptides were obtained in good decay-corrected radiochemical yield (61-68%) and were purified by high performance liquid chromatography to ensure high specific activity (137 – 281 MBq/nmol at the end of synthesis). NOD scid gamma mice bearing ZR-75-1 tumors were injected intravenously with the labeled peptides and used for PET/CT imaging and biodistribution at 1 h post-injection. We found that 68 Ga-DOTATOC had the highest tumor uptake (18.4 ± 2.9%ID/g), followed by 68 Ga-DOTATATE (15.2 ± 2.2%ID/g) and 68 Ga-NODAGA-JR11 (12.2 ± 0.8%ID/g). Tumor-to-blood and tumor-to-muscle ratios were also higher for the agonists (>40 and >150 respectively), compared to the antagonist (15.6 ± 2.2 and 45.2 ± 11.6 respectively). Conclusions The antagonist 68 Ga-NODAGA-JR11 had the lowest tumor uptake and contrast compared to agonists 68 Ga-DOTATOC and 68 Ga-DOTATATE in ZR-75-1 xenografts. The main contributing factor to this result could be the use of an endogenously expressing cell line, which may differ from previously published transfected models in the number of low-affinity, antagonist-specific binding sites. The relative merit of agonists versus antagonists for sstr2 breast cancer imaging warrants further investigation, first in preclinical models with other sstr2-positive breast cancer xenografts, and ultimately in luminal breast cancer patients.