Dude Iulia, Zhang Zhengxing, Rousseau Julie, Hundal-Jabal Navjit, Colpo Nadine, Merkens Helen, Lin Kuo-Shyan, Bénard François
1Department of Molecular Oncology, BC Cancer Agency Research Centre, 675 West 10th Ave, Vancouver, V5Z 1 L3 BC Canada.
2Department of Radiology, University of British Columbia, Vancouver, BC Canada.
EJNMMI Radiopharm Chem. 2017;2(1):4. doi: 10.1186/s41181-017-0023-y. Epub 2017 Apr 17.
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 Ga-NODAGA-JR11 with two agonists Ga-DOTA-Tyr-octreotide (Ga-DOTATOC) and Ga-DOTA-Tyr-octreotate (Ga-DOTATATE), in the human, sstr2-positive, luminal breast cancer model: ZR-75-1.
Peptides were assayed for binding affinity using a filtration-based competitive assay to sstr2. Ga-DOTATOC and Ga-DOTATATE had excellent affinity (inhibition constant K: 0.9 ± 0.1 nM and 1.4 ± 0.3 nM respectively) compared to 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 Ga-DOTATOC bound to 6.64 ± 0.39 × 10 sites/cells, which was 1.5-fold higher than Ga-NODAGA-JR11 and 2.3-fold higher than Ga-DOTATATE. All three 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 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 Ga-DOTATOC had the highest tumor uptake (18.4 ± 2.9%ID/g), followed by Ga-DOTATATE (15.2 ± 2.2%ID/g) and 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).
The antagonist Ga-NODAGA-JR11 had the lowest tumor uptake and contrast compared to agonists Ga-DOTATOC and 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.
生长抑素受体2型（sstr2）在大多数管腔型乳腺癌中表达，然而，使用激动剂sstr2探针进行单光子发射计算机断层扫描（SPECT）和闪烁扫描成像的效果并不理想。高亲和力拮抗剂可作用于细胞表面更多的结合位点，从而提高肿瘤摄取率并改善敏感性。我们在人sstr2阳性管腔型乳腺癌模型ZR-75-1中，比较了拮抗剂镓标记的NODAGA-JR11与两种激动剂镓标记的DOTA-酪氨酰奥曲肽（Ga-DOTATOC）和镓标记的DOTA-酪氨酰奥曲瑞林（Ga-DOTATATE）的肿瘤摄取和生物分布情况。
使用基于过滤的竞争性分析法检测肽段与sstr2的结合亲和力。与Ga-NODAGA-JR11（25.9±0.2 nM）相比，Ga-DOTATOC和Ga-DOTATATE具有优异的亲和力（抑制常数K分别为0.9±0.1 nM和1.4±0.3 nM）。通过饱和分析法在体外测定ZR-75-1细胞上的结合位点数。激动剂Ga-DOTATOC与6.64±0.39×10个位点/细胞结合，比Ga-NODAGA-JR11高1.5倍，比Ga-DOTATATE高2.3倍。所有三种镓标记的肽段均以良好的衰变校正放射化学产率（61%-68%）获得，并通过高效液相色谱法纯化以确保高比活度（合成结束时为137-281 MBq/nmol）。将携带ZR-75-1肿瘤的无特定病原体（NOD）γ小鼠静脉注射标记肽段，并在注射后1小时用于正电子发射断层扫描/计算机断层扫描（PET/CT）成像和生物分布研究。我们发现，Ga-DOTATOC的肿瘤摄取率最高（18.4±2.9%ID/g），其次是Ga-DOTATATE（15.2±2.2%ID/g）和Ga-NODAGA-JR11（12.2±0.8%ID/g）。与拮抗剂（分别为15.6±2.2和45.2±11.6）相比，激动剂的肿瘤与血液和肿瘤与肌肉的比值也更高（分别>40和>150）。
在ZR-75-1异种移植模型中，与激动剂Ga-DOTATOC和Ga-DOTATATE相比，拮抗剂Ga-NODAGA-JR11的肿瘤摄取率和对比度最低。导致该结果的主要因素可能是使用了内源性表达细胞系，其低亲和力、拮抗剂特异性结合位点的数量可能与先前发表的转染模型不同。对于sstr2乳腺癌成像，激动剂与拮抗剂的相对优势值得进一步研究，首先在其他sstr2阳性乳腺癌异种移植的临床前模型中进行研究，最终在管腔型乳腺癌患者中进行研究。
