Abstract：
PET imaging of the glucagon-like peptide-1 receptor (GLP-1R) using radiolabeled exendin is a promising imaging method to detect insulinomas. However, high renal accumulation of radiolabeled exendin could hamper the detection of small insulinomas in proximity to the kidneys and limit its use as a radiotherapeutic agent. Here, we report two new exendin analogues for GLP-1R imaging and therapy, designed to reduce renal retention by incorporating a cleavable methionine-isoleucine (Met-Ile) linker. We examined the renal retention and insulinoma targeting properties of these new exendin analogues in a nude mouse model bearing subcutaneous GLP-1R-expressing insulinomas. NOTA or DOTA was conjugated via a methionine-isoleucine linker to the C-terminus of exendin-4 (NOTA-MI-exendin-4 or DOTA-MI-exendin-4). NOTA- and DOTA-exendin-4 without the linker were used as references. The affinity for GLP-1R was determined in a competitive binding assay using GLP-1R transfected cells. Biodistribution of [68Ga]Ga-NOTA-exendin-4, [68Ga]Ga-NOTA-MI-exendin-4, [177Lu]Lu-DOTA-exendin-4, and [177Lu]Lu-DOTA-MI-exendin-4 was determined in INS-1 tumor-bearing BALB/c nude mice, and PET/CT was acquired to visualize renal retention and tumor targeting. For all tracers, dosimetric calculations were performed to determine the kidney self-dose. The affinity for GLP-1R was in the low nanomolar range (<11 nM) for all peptides. In vivo biodistribution revealed a significantly lower kidney uptake of [68Ga]Ga-NOTA-MI-exendin-4 at 4 h post-injection (p.i.) (34.2 ± 4.2 %IA/g), compared with [68Ga]Ga-NOTA-exendin-4 (128 ± 10 %IA/g). Accumulation of [68Ga]Ga-NOTA-MI-exendin-4 in the tumor was 25.0 ± 8.0 %IA/g 4 h p.i., which was similar to that of [68Ga]Ga-NOTA-exendin-4 (24.9 ± 9.3 %IA/g). This resulted in an improved tumor-to-kidney ratio from 0.2 ± 0.0 to 0.8 ± 0.3. PET/CT confirmed the findings in the biodistribution studies. The kidney uptake of [177Lu]Lu-DOTA-MI-exendin-4 was 39.4 ± 6.3 %IA/g at 24 h p.i. and 13.0 ± 2.5 %IA/g at 72 h p.i., which were significantly lower than those for [177Lu]Lu-DOTA-exendin-4 (99.3 ± 9.2 %IA/g 24 h p.i. and 45.8 ± 3.9 %IA/g 72 h p.i.). The uptake in the tumor was 7.8 ± 1.5 and 11.3 ± 2.0 %IA/g 24 h p.i. for [177Lu]Lu-DOTA-MI-exendin-4 and [177Lu]Lu-DOTA-exendin-4, respectively, resulting in improved tumor-to-kidney ratios for [177Lu]Lu-DOTA-MI-exendin-4. The new exendin analogues with a Met-Ile linker showed 2-3-fold reduced renal retention and improved tumor-to-kidney ratios compared with their reference without the Met-Ile linker. Future studies should demonstrate whether [68Ga]Ga-NOTA-MI-exendin-4 results in improved detection of small insulinomas in close proximity to the kidneys with PET/CT. [177Lu]Lu-DOTA-MI-exendin-4 might open a window of opportunity for exendin-based radionuclide therapy.
摘要：
使用放射性标记的exendin对胰高血糖素样肽1受体（GLP-1R）进行PET成像是检测胰岛素瘤的有前途的成像方法。然而,放射性标记的exendin的高肾脏积累可能会阻碍肾脏附近小胰岛素瘤的检测，并限制其作为放射治疗剂的使用。这里,我们报道了两种新的用于GLP-1R成像和治疗的exendin类似物，旨在通过掺入可切割的甲硫氨酸-异亮氨酸（Met-Ile）接头来减少肾脏保留。我们在带有皮下表达GLP-1R的胰岛素瘤的裸鼠模型中检查了这些新的exendin类似物的肾脏保留和胰岛素瘤靶向特性。NOTA或DOTA通过甲硫氨酸-异亮氨酸接头与exendin-4的C端缀合(NOTA-MI-exendin-4或DOTA-MI-exendin-4)。没有接头的NOTA-和DOTA-exendin-4用作参考。使用GLP-1R转染的细胞在竞争性结合测定中测定对GLP-1R的亲和力。[68Ga]Ga-NOTA-exendin-4，[68Ga]Ga-NOTA-MI-exendin-4，[177Lu]Lu-DOTA-exendin-4和[177Lu]Lu-DOTA-MI-exendin-4的生物分布在INS-1荷瘤BALB/c裸鼠中确定，获得PET/CT以可视化肾脏滞留和肿瘤靶向。对于所有示踪剂，进行剂量学计算以确定肾脏自身剂量。对于所有肽,GLP-1R的亲和力在低纳摩尔范围(<11nM)。体内生物分布显示，在注射后4小时（p.i.）（34.2±4.2％IA/g）时，[68Ga]Ga-NOTA-MI-exendin-4的肾脏摄取显着降低，与[68Ga]Ga-NOTA-exendin-4(128±10%IA/g)相比。[68Ga]Ga-NOTA-MI-exendin-4在肿瘤中的积累为25.0±8.0％IA/g4hp.i.，这类似于[68Ga]Ga-NOTA-exendin-4(24.9±9.3%IA/g)。这导致肿瘤与肾脏的比率从0.2±0.0提高到0.8±0.3。PET/CT证实了生物分布研究中的发现。[177Lu]Lu-DOTA-MI-exendin-4的肾脏摄取在24hp.i.时为39.4±6.3％IA/g，在72hp.i.时为13.0±2.5％IA/g，显着低于[177Lu]Lu-DOTA-exendin-4（99.3±9.2％IA/g24hp.i.和45.8±3.9％IA/g72hp.i.）。[177Lu]Lu-DOTA-MI-exendin-4和[177Lu]Lu-DOTA-exendin-4在肿瘤中的摄取分别为7.8±1.5和11.3±2.0％IA/g，导致[177Lu]Lu-DOTA-MI-exendin-4的肿瘤肾比率改善。与没有Met-Ile接头的参考相比，具有Met-Ile接头的新的exendin类似物显示2-3倍减少的肾保留和改善的肿瘤-肾比率。未来的研究应证明[68Ga]Ga-NOTA-MI-exendin-4是否可以改善PET/CT对肾脏附近的小胰岛素瘤的检测。[177Lu]Lu-DOTA-MI-exendin-4可能为基于exendin的放射性核素治疗打开机会之窗。
