Comparison of In Vitro Assays in Selecting Radiotracers for In Vivo P-Glycoprotein PET Imaging
作者:Renske Raaphorst、Heli Savolainen、Mariangela Cantore、Evita van de Steeg、Aren van Waarde、Nicola Colabufo、Philip Elsinga、Adriaan Lammertsma、Albert Windhorst、Gert Luurtsema
DOI:10.3390/ph10030076
日期:——
Positron emission tomography (PET) imaging of P-glycoprotein (P-gp) in the blood-brain barrier can be important in neurological diseases where P-gp is affected, such as Alzheimer´s disease. Radiotracers used in the imaging studies are present at very small, nanomolar, concentration, whereas in vitro assays where these tracers are characterized, are usually performed at micromolar concentration, causing often discrepant in vivo and in vitro data. We had in vivo rodent PET data of [11C]verapamil, (R)-N-[18F]fluoroethylverapamil, (R)-O-[18F]fluoroethyl-norverapamil, [18F]MC225 and [18F]MC224 and we included also two new molecules [18F]MC198 and [18F]KE64 in this study. To improve the predictive value of in vitro assays, we labeled all the tracers with tritium and performed bidirectional substrate transport assay in MDCKII-MDR1 cells at three different concentrations (0.01, 1 and 50 µM) and also inhibition assay with P-gp inhibitors. As a comparison, we used non-radioactive molecules in transport assay in Caco-2 cells at a concentration of 10 µM and in calcein-AM inhibition assay in MDCKII-MDR1 cells. All the P-gp substrates were transported dose-dependently. At the highest concentration (50 µM), P-gp was saturated in a similar way as after treatment with P-gp inhibitors. Best in vivo correlation was obtained with the bidirectional transport assay at a concentration of 0.01 µM. One micromolar concentration in a transport assay or calcein-AM assay alone is not sufficient for correct in vivo prediction of substrate P-gp PET ligands.
对血脑屏障中的 P-糖蛋白(P-gp)进行正电子发射断层扫描(PET)成像,对于 P-gp 受影响的神经系统疾病(如阿尔茨海默病)非常重要。成像研究中使用的放射性示踪剂的浓度非常小,只有纳摩尔级,而体外检测这些示踪剂时,通常是在微摩尔浓度下进行的,因此体内和体外数据往往不一致。我们已获得[11C]维拉帕米、(R)-N-[18F]氟乙基维拉帕米、(R)-O-[18F]氟乙基-去甲维拉帕米、[18F]MC225 和 [18F]MC224 的体内啮齿动物 PET 数据,本研究还包括两种新分子[18F]MC198 和 [18F]KE64 。为了提高体外检测的预测价值,我们用氚标记了所有示踪剂,并在 MDCKII-MDR1 细胞中以三种不同的浓度(0.01、1 和 50 µM)进行了双向底物转运检测,还用 P-gp 抑制剂进行了抑制检测。作为比较,我们在 Caco-2 细胞中使用了浓度为 10 µM 的非放射性分子进行转运试验,在 MDCKII-MDR1 细胞中使用了钙黄绿素-AM 进行抑制试验。所有 P-gp 底物的转运都与剂量有关。在最高浓度(50 µM)下,P-gp 饱和度与 P-gp 抑制剂处理后的情况相似。双向转运试验在 0.01 µM 浓度时获得了最佳体内相关性。仅靠转运试验或钙蓝蛋白-AM 试验中的一个微摩尔浓度不足以正确预测底物 P-gp PET 配体的体内浓度。