de Vries Erik F J, Kortekaas Rudie, van Waarde Aren, Dijkstra Durk, Elsinga Philip H, Vaalburg Willem
PET Center, University Medical Center Groningen, The Netherlands.
J Nucl Med. 2005 Aug;46(8):1384-92.
While searching for a PET method to determine the density and occupancy of the dopamine D(3) receptor, we found evidence that suggested that the dopamine D(3) antagonist GR218231 could be a substrate of the P-glycoprotein efflux pump. P-glycoprotein protects the brain against toxic substances and xenobiotics, but it also hampers the delivery of various drugs into the brain. In this study, we aimed to explore whether radiolabeled GR218231 could be applied as a PET tracer for monitoring P-glycoprotein activity in the blood-brain barrier. Such an imaging technique could be useful for the development of new drugs and novel strategies to deliver drugs to the brain and for identification of undesirable drug-drug interactions.
As a potential PET tracer, GR218231 was labeled with (11)C by reaction of the newly synthesized desmethyl precursor with (11)C-methyl triflate. The biodistribution of (11)C-GR218231 was determined in rats. To assess specific binding to the dopamine D(3) receptor, blocking experiments with unlabeled GR218231 (0.2 and 2.5 mg/kg) were performed. To demonstrate the influence of P-glycoprotein on cerebral uptake of (11)C-GR218231, the efflux pump was modulated with 50 mg/kg cyclosporine A. The sensitivity of (11)C-GR218231 for P-glycoprotein modulation was assessed in dose-response studies, using escalating cyclosporine A dosages.
(11)C-GR218231 was prepared in 53% +/- 8% decay-corrected radiochemical yield and had a specific activity of 15 +/- 10 GBq/micromol (mean +/- SD). Biodistribution studies in rats revealed a low and homogeneous uptake in the brain. Pretreatment of the animals with unlabeled GR218231 did not demonstrate any specific binding. Modulation of P-glycoprotein with cyclosporine A caused a 12-fold higher (11)C-GR218231 uptake in the brain, indicating that the low cerebral tracer uptake was caused by the P-glycoprotein efflux pump in the blood-brain barrier. Cyclosporine A dose-escalation studies showed a dose-dependent sigmoidal increase in (11)C-GR218231 uptake in brain and spleen (median effective dose [ED(50)], 23.3 +/- 0.6 and 38.4 +/- 2.4 mg/kg, respectively), whereas a dose-dependent decrease was observed in the pancreas (ED(50), 36.0 +/- 4.4 mg/kg).
Although (11)C-GR218231 is unsuited for dopamine D(3) receptor imaging with PET, it appears to be an attractive PET tracer for visualization and quantification of P-glycoprotein activity in the blood-brain barrier.
在寻找一种用于确定多巴胺D(3)受体密度和占有率的正电子发射断层扫描（PET）方法时，我们发现有证据表明多巴胺D(3)拮抗剂GR218231可能是P-糖蛋白外排泵的底物。P-糖蛋白可保护大脑免受有毒物质和外源性物质的侵害，但它也会阻碍各种药物进入大脑。在本研究中，我们旨在探讨放射性标记的GR218231是否可作为PET示踪剂用于监测血脑屏障中P-糖蛋白的活性。这样一种成像技术可能有助于开发新药和将药物输送到大脑的新策略，以及识别不良的药物-药物相互作用。
作为一种潜在的PET示踪剂，通过新合成的去甲基前体与(11)C-甲基三氟甲磺酸酯反应，用(11)C标记GR218231。在大鼠中测定(11)C-GR218231的生物分布。为了评估与多巴胺D(3)受体的特异性结合，用未标记的GR218231（0.2和2.5mg/kg）进行阻断实验。为了证明P-糖蛋白对(11)C-GR218231脑摄取的影响，用50mg/kg环孢素A调节外排泵。在剂量反应研究中使用递增的环孢素A剂量评估(11)C-GR218231对P-糖蛋白调节的敏感性。
(11)C-GR218231的制备放射性化学产率经衰变校正后为53%±8%，比活度为15±10GBq/μmol（平均值±标准差）。大鼠体内生物分布研究显示大脑摄取低且均匀。用未标记的GR218231预处理动物未显示任何特异性结合。用环孢素A调节P-糖蛋白导致脑内(11)C-GR218231摄取增加12倍，表明脑内示踪剂摄取低是由血脑屏障中的P-糖蛋白外排泵所致。环孢素A剂量递增研究显示脑和脾中(11)C-GR218231摄取呈剂量依赖性S形增加（半数有效剂量[ED(50)]分别为23.3±0.6和38.4±2.4mg/kg），而胰腺中观察到剂量依赖性降低（ED(50)为36.0±4.4mg/kg）。
尽管(11)C-GR218231不适合用于PET多巴胺D(3)受体成像，但它似乎是一种用于可视化和定量血脑屏障中P-糖蛋白活性的有吸引力的PET示踪剂。
