Olberg Dag E, Bauer Nadine, Andressen Kjetil W, Hjørnevik Trine, Cumming Paul, Levy Finn O, Klaveness Jo, Haraldsen Ira, Sutcliffe Julie L
Norsk medisinsk syklotronsenter AS, Postboks 4950 Nydalen, 0424 Oslo, Norway; School of Pharmacy, University of Oslo, P.O.Box 1068, Blindern, 0316 Oslo, Norway.
Department of Biomedical Engineering, University of California Davis, Davis, CA, USA.
Nucl Med Biol. 2016 Aug;43(8):478-89. doi: 10.1016/j.nucmedbio.2016.05.003. Epub 2016 May 10.
The gonadotropin releasing hormone receptor (GnRH-R) has a well-described neuroendocrine function in the anterior pituitary. However, little is known about its function in the central nervous system (CNS), where it is most abundantly expressed in hippocampus and amygdala. Since peptide ligands based upon the endogenous decapetide GnRH do not pass the blood-brain-barrier, we are seeking a high-affinity small molecule GnRH-R ligand suitable for brain imaging by positron emission tomography. We have previously reported the radiosynthesis and in vitro evaluation of two novel [(18)F]fluorinated GnRH-R ligands belonging to the furamide class of antagonists, with molecular weight less than 500 Da. We now extend this work using palladium coupling for the synthesis of four novel radioligands, with putatively reduced polar surface area and hydrophilicity relative to the two previously described compounds, and report the uptake of these (18)F-labeled compounds in brain of living rats.
We synthesized reference standards of the small molecule GnRH-R antagonists as well as mesylate precursors for (18)F-labeling. The antagonists were tested for binding affinity for both human and rat GnRH-R. Serum and blood stability in vitro and in vivo were studied. Biodistribution and PET imaging studies were performed in male rats in order to assess brain penetration in vivo.
A palladium coupling methodology served for the synthesis of four novel fluorinated furamide GnRH receptor antagonists with reduced heteroatomic count. Radioligand binding assays in vitro revealed subnanomolar affinity of the new fluorinated compounds for both human and rat GnRH-R. The (18)F-GnRH antagonists were synthesized from the corresponding mesylate precursors in 5-15% overall radiochemical yield. The radiolabeled compounds demonstrated good in vivo stability. PET imaging with the (18)F-radiotracers in naive rats showed good permeability into brain and rapid washout, but absence of discernible specific binding in vivo.
The novel small molecule (18)F-fluorinated GnRH-R antagonist compounds show high receptor affinity in vitro, and may prove useful for quantitative autoradiographic studies in vitro. The compounds were permeable to the blood-brain barrier, but nonetheless failed to reveal significant specific binding in brain of living rats. Nonetheless, our approach may serve as a foundation for designing PET ligands suitable to image the GnRH-R distribution in brain.
促性腺激素释放激素受体（GnRH-R）在前脑垂体中具有明确的神经内分泌功能。然而，对于其在中枢神经系统（CNS）中的功能却知之甚少，而在中枢神经系统中，它在海马体和杏仁核中表达最为丰富。由于基于内源性十肽GnRH的肽配体无法通过血脑屏障，我们正在寻找一种适用于正电子发射断层扫描脑成像的高亲和力小分子GnRH-R配体。我们之前报道了两种属于呋喃酰胺类拮抗剂的新型[(18)F]氟化GnRH-R配体的放射性合成及体外评估，其分子量小于500 Da。我们现在扩展这项工作，利用钯偶联反应合成四种新型放射性配体，相对于之前描述的两种化合物，其推测的极性表面积和亲水性降低，并报告这些(18)F标记化合物在活体大鼠脑中的摄取情况。
我们合成了小分子GnRH-R拮抗剂的参考标准品以及用于(18)F标记的甲磺酸盐前体。测试了拮抗剂对人和大鼠GnRH-R的结合亲和力。研究了其在体外和体内的血清及血液稳定性。在雄性大鼠中进行了生物分布和PET成像研究，以评估其在体内的脑渗透情况。
钯偶联方法用于合成四种新型氟化呋喃酰胺GnRH受体拮抗剂，其杂原子数减少。体外放射性配体结合试验表明，新型氟化化合物对人和大鼠GnRH-R均具有亚纳摩尔亲和力。(18)F-GnRH拮抗剂由相应的甲磺酸盐前体合成，总放射化学产率为5 - 15%。放射性标记化合物在体内表现出良好的稳定性。用(18)F放射性示踪剂对未处理大鼠进行PET成像显示，其对脑具有良好的渗透性且快速清除，但在体内未观察到明显的特异性结合。
新型小分子(18)F-氟化GnRH-R拮抗剂化合物在体外显示出高受体亲和力，可能对体外定量放射自显影研究有用。这些化合物可透过血脑屏障，但在活体大鼠脑中仍未显示出明显的特异性结合。尽管如此，我们的方法可为设计适合脑内GnRH-R分布成像的PET配体奠定基础。
