Läppchen Tilman, Tönnesmann Roswitha, Eersels Jos, Meyer Philipp T, Maecke Helmut R, Rylova Svetlana N
Department of Nuclear Medicine, Medical Center-University of Freiburg, Faculty of Medicine, University of Freiburg, Germany.
Department of Nuclear Medicine, Inselspital, Bern University Hospital and University of Bern, Bern, Switzerland.
PLoS One. 2017 Jan 19;12(1):e0170435. doi: 10.1371/journal.pone.0170435. eCollection 2017.
GLP-1 receptors are ideal targets for preoperative imaging of benign insulinoma and for quantifying the beta cell mass. The existing clinical tracers targeting GLP-1R are all agonists with low specific activity and very high kidney uptake. In order to solve those issues we evaluated GLP-1R agonist Ex-4 and antagonist Ex(9-39) radioiodinated at Tyr40 side by side with [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4 (68Ga-Ex-4) used in the clinic. The Kd, Bmax, internalization and binding kinetics of [Nle14,125I-Tyr40-NH2]Ex-4 and [Nle14,125I-Tyr40-NH2]Ex(9-39) were studied in vitro using Ins-1E cells. Biodistribution and imaging studies were performed in nude mice bearing Ins-1E xenografts. In vitro evaluation demonstrated high affinity binding of the [Nle14,125I-Tyr40-NH2]Ex-4 agonist to the Ins-1E cells with fast internalization kinetics reaching a plateau after 30 min. The antagonist [Nle14,125I-Tyr40-NH2]Ex(9-39) did not internalize and had a 4-fold higher Kd value compared to the agonist. In contrast to [Nle14,125I-Tyr40-NH2]Ex(9-39), which showed low and transient tumor uptake, [Nle14,125I-Tyr40-NH2]Ex-4 demonstrated excellent in vivo binding properties with tumor uptake identical to that of 68Ga-Ex-4, but substantially lower kidney uptake resulting in a tumor-to-kidney ratio of 9.7 at 1 h compared to 0.3 with 68Ga-Ex-4. Accumulation of activity in thyroid and stomach for both peptides, which was effectively blocked by irenat, confirms that in vivo deiodination is the mechanism behind the low kidney retention of iodinated peptides. The 124I congener of [Nle14,125I-Tyr40-NH2]Ex-4 demonstrated a similar favourable biodistribution profile in the PET imaging studies in contrast to the typical biodistribution pattern of [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4. Our results demonstrate that iodinated Ex-4 is a very promising tracer for imaging of benign insulinomas. It solves the problem of high kidney uptake of the radiometal-labelled tracers by improving the tumor-to-kidney ratio measured for [Nle14,Lys40(Ahx-DOTA-68Ga)NH2]Ex-4 by 32 fold.
胰高血糖素样肽-1（GLP-1）受体是良性胰岛素瘤术前成像及定量胰岛β细胞量的理想靶点。现有的靶向GLP-1受体的临床示踪剂均为激动剂，比活度低且肾脏摄取非常高。为解决这些问题，我们将在Tyr40位点放射性碘化的GLP-1受体激动剂艾塞那肽-4（Ex-4）和拮抗剂Ex(9 - 39)与临床使用的[Nle14,Lys40(Ahx - DOTA - 68Ga)NH2]Ex - 4（68Ga - Ex - 4）进行了对比评估。使用Ins - 1E细胞在体外研究了[Nle14,125I - Tyr40 - NH2]Ex - 4和[Nle14,125I - Tyr40 - NH2]Ex(9 - 39)的解离常数（Kd）、最大结合容量（Bmax）、内化作用及结合动力学。对携带Ins - 1E异种移植瘤的裸鼠进行了生物分布和成像研究。体外评估表明，[Nle14,125I - Tyr40 - NH2]Ex - 4激动剂与Ins - 1E细胞具有高亲和力结合，内化动力学快，30分钟后达到平台期。拮抗剂[Nle14,125I - Tyr40 - NH2]Ex(9 - 39)不发生内化，与激动剂相比Kd值高4倍。与显示出低且短暂肿瘤摄取的[Nle14,125I - Tyr40 - NH2]Ex(9 - 39)不同，[Nle14,125I - Tyr40 - NH2]Ex - 4在体内具有优异的结合特性，肿瘤摄取与68Ga - Ex - 4相同，但肾脏摄取显著降低，1小时时肿瘤与肾脏的比值为9.7，而68Ga - Ex - 4为0.3。两种肽在甲状腺和胃中的活性积聚均被艾瑞那肽有效阻断，这证实体内脱碘是碘化肽肾脏潴留低的背后机制。在PET成像研究中，[Nle14,125I - Tyr40 - NH2]Ex - 4的124I类似物显示出与[Nle14,Lys40(Ahx - DOTA - 68Ga)NH2]Ex - 4典型生物分布模式不同的类似良好生物分布特征。我们的结果表明，碘化Ex - 4是用于良性胰岛素瘤成像的非常有前景的示踪剂。它通过将[Nle14,Lys40(Ahx - DOTA - 68Ga)NH2]Ex - 4的肿瘤与肾脏比值提高32倍来解决放射性金属标记示踪剂肾脏高摄取的问题。
