Center for Radiopharmaceutical Sciences, Paul Scherrer Institute, Villigen-PSI, Switzerland; and.
Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland.
J Nucl Med. 2021 Oct;62(10):1475-1481. doi: 10.2967/jnumed.120.255760. Epub 2021 Jan 15.
The aim of this study was to identify a folate receptor-α (FRα)-selective PET agent potentially suitable for the selection of patients who might profit from FRα-targeted therapies. The 6 and 6 isomers of F-aza-5-methyltetrahydrofolate (MTHF) were assessed regarding their binding to FRα and FRβ, expressed on cancer and inflammatory cells, respectively, and compared with F-AzaFol, the folic acid-based analog. FR selectivity was investigated using FRα-transfected (RT16) and FRβ-transfected (D4) CHO cells. The cell uptake of F-folate tracers was investigated, and receptor-binding affinities were determined with the nonradioactive analogs. In vitro autoradiography of the F-folate tracers was performed using RT16 and D4 tissue sections. Biodistribution studies and PET/CT imaging of the radiotracers were performed on mice bearing RT16 and D4 xenografts. The uptake of F-6-aza-5-MTHF was high when using RT16 cells (62% ± 10% of added activity) but much lower when using D4 cells (5% ± 2%). The FRα selectivity of F-6-aza-5-MTHF was further demonstrated by its approximately 43-fold higher binding affinity to FRα (half-maximal inhibitory concentration [IC], 1.8 ± 0.1 nM) than to FRβ (IC, 77 ± 27 nM). The uptake of F-6-aza-5-MTHF and F-AzaFol was equal in both cell lines (52%-70%), with similar affinities to FRα (IC, 2.1 ± 0.4 nM and 0.6 ± 0.3 nM, respectively) and FRβ (0.8 ± 0.2 nM and 0.3 ± 0.1 nM, respectively). The autoradiography signal obtained with F-6-aza-5-MTHF was 11-fold more intense for RT16 than for D4 tissue sections. Biodistribution data showed high uptake of F-6-aza-5-MTHF in RT16 xenografts (81% ± 20% injected activity per gram [IA]/g 1 h after injection) but significantly lower accumulation in D4 xenografts (7.3% ± 2.1% IA/g 1 h after injection), which was also visualized using PET. The uptake of F-6-aza-5-MTHF and F-AzaFol was similar in RT16 (53% ± 10% IA/g and 45% ± 2% IA/g, respectively) and D4 xenografts (77% ± 10% IA/g and 52% ± 7% IA/g, respectively). This study demonstrated FRα selectivity for F-6-aza-5-MTHF but not for F-6-aza-5-MTHF or F-AzaFol. This characteristic, together with its favorable tissue distribution, makes F-6-aza-5-MTHF attractive for clinical translation to enable detection of FRα-positive cancer while preventing undesired accumulation in FRβ-expressing inflammatory cells.
本研究旨在鉴定一种叶酸受体-α（FRα）选择性 PET 探针，其可能适用于选择可能受益于 FRα 靶向治疗的患者。评估了 F-aza-5-甲基四氢叶酸（MTHF）的 6 和 6 异构体对分别表达于癌细胞和炎症细胞上的 FRα 和 FRβ 的结合能力，并与叶酸基类似物 F-AzaFol 进行了比较。使用 FRα 转染（RT16）和 FRβ 转染（D4）CHO 细胞研究 FR 选择性。研究了 F-叶酸示踪剂的细胞摄取情况，并使用非放射性类似物测定了受体结合亲和力。使用 RT16 和 D4 组织切片进行了 F-叶酸示踪剂的体外放射自显影。在携带 RT16 和 D4 异种移植物的小鼠中进行了放射性示踪剂的生物分布研究和 PET/CT 成像。当使用 RT16 细胞时，F-6-aza-5-MTHF 的摄取量很高（加入的放射性活度的 62%±10%），而当使用 D4 细胞时则低得多（5%±2%）。F-6-aza-5-MTHF 对 FRα 的选择性结合能力约为 FRβ 的 43 倍（FRα 的半数最大抑制浓度 [IC]为 1.8±0.1 nM，FRβ 的 IC 为 77±27 nM），进一步证明了其 FRα 选择性。F-6-aza-5-MTHF 和 F-AzaFol 在两种细胞系中的摄取量相等（52%-70%），对 FRα 的亲和力相似（IC 分别为 2.1±0.4 nM 和 0.6±0.3 nM）和 FRβ（0.8±0.2 nM 和 0.3±0.1 nM）。F-6-aza-5-MTHF 的放射自显影信号在 RT16 组织切片上比 D4 组织切片强 11 倍。生物分布数据显示，F-6-aza-5-MTHF 在 RT16 异种移植物中的摄取量很高（注射后 1 小时每克组织 [IA]/g 为 81%±20%），而在 D4 异种移植物中的摄取量明显较低（注射后 1 小时为 7.3%±2.1%IA/g），这也可以通过 PET 成像观察到。F-6-aza-5-MTHF 和 F-AzaFol 在 RT16（分别为 53%±10%IA/g 和 45%±2%IA/g）和 D4 异种移植物（分别为 77%±10%IA/g 和 52%±7%IA/g）中的摄取量相似。本研究表明 F-6-aza-5-MTHF 对 FRα 具有选择性，但对 F-6-aza-5-MTHF 或 F-AzaFol 没有选择性。这种特性，加上其有利的组织分布，使 F-6-aza-5-MTHF 具有吸引力，可用于临床转化，以实现对 FRα 阳性癌症的检测，同时防止在表达 FRβ 的炎症细胞中产生不必要的蓄积。
