Laboratory of Molecular Imaging and Nanomedicine (LOMIN), National Institute of Biomedical Imaging and Bioengineering (NIBIB), National Institutes of Health (NIH), Bethesda, MD, 20892, USA.
Mol Imaging Biol. 2013 Dec;15(6):758-67. doi: 10.1007/s11307-013-0640-0.
CXCR4 is overexpressed on tumor cells from many types of human cancers. A high level of CXCR4 expression often correlates with poor prognosis, chemotherapy resistance, and metastasis. The development of CXCR4-specific radiotracers for positron emission tomography (PET) imaging will allow in vivo evaluation of receptor expression level for diagnosis or therapeutic evaluation.
Two new (18)F-labeled radiotracers based on an Ac-TC14012 peptide, [(18)F]FP-Ac-TC14012 and [(18)F]FB-Ac-TC14012, were synthesized and characterized. The affinities of the 2-fluoropropionate (FP)-conjugated or 4-fluorobenzoate (FB)-conjugated peptides to CXCR4-transfected Chinese hamster ovarian (CHO) cells were evaluated in a competitive binding assay with [(125)I]CXCL12 radioligand. The cell uptake and retention of [(18)F]FP-labeled and [(18)F]FB-labeled peptides were measured. The tumor targetability and pharmacokinetics of these two tracers were also evaluated by microPET imaging and biodistribution studies.
The labeled peptides retained high binding affinity to CXCR4 and showed much higher uptake in CXCR4-positive CHO cells than in CXCR4-negative cells in vitro. The smaller and more hydrophilic [(18)F]FP prosthetic group resulted in higher affinity and lower nonspecific cell uptake compared to the [(18)F]FB-labeled peptide. Both radiotracers showed much higher accumulation in CXCR4-positive than CXCR4-negative tumor xenografts in mice and allowed clear visualization of CXCR4 expression by PET. Among the two, [(18)F]FP-Ac-TC14012 showed higher tumor uptake and better tumor-to-background contrast. Unlike their N-terminal 4-F-benzoate analogs, these two tracers had minimal blood retention, likely due to reduced red blood cell binding. Metabolic organs, such as the liver and kidney, also showed high uptake. When blocked with low-dose cold peptide (10 μg), the tumor uptake was significantly increased, most likely due to the increased concentration in blood circulation, as evidenced by decreased liver uptake.
These results demonstrate that the [(18)F]FP-labeled Ac-TC14012 peptide with high tumor uptake, low nonspecific binding, and good tumor-to-background contrast promises [(18)F]FP-Ac-TC14012 as a PET tracer for in vivo PET imaging of CXCR4 expression.
CXCR4 在多种人类癌症的肿瘤细胞中过度表达。高水平的 CXCR4 表达通常与预后不良、化疗耐药和转移有关。开发用于正电子发射断层扫描 (PET) 成像的 CXCR4 特异性放射性示踪剂将允许体内评估受体表达水平以进行诊断或治疗评估。
基于 Ac-TC14012 肽，合成并表征了两种新的 (18)F 标记的放射性示踪剂 [(18)F]FP-Ac-TC14012 和 [(18)F]FB-Ac-TC14012。使用 [(125)I]CXCL12 放射性配体在竞争性结合测定中评估了 2-氟丙酸 (FP) 缀合或 4-氟苯甲酸 (FB) 缀合肽与 CXCR4 转染的中国仓鼠卵巢 (CHO) 细胞的亲和力。测量了 [(18)F]FP 标记和 [(18)F]FB 标记肽的细胞摄取和保留。通过 microPET 成像和生物分布研究还评估了这两种示踪剂的肿瘤靶向性和药代动力学。
标记的肽保留了对 CXCR4 的高结合亲和力，并显示出比体外 CXCR4 阴性细胞更高的 CXCR4 阳性 CHO 细胞摄取率。与 [(18)F]FB 标记的肽相比，较小且更亲水性的 [(18)F]FP 取代基导致更高的亲和力和更低的非特异性细胞摄取率。两种示踪剂在 CXCR4 阳性肿瘤异种移植小鼠中均表现出比 CXCR4 阴性肿瘤更高的积聚，并且可以通过 PET 清楚地观察到 CXCR4 表达。在这两种情况下，[(18)F]FP-Ac-TC14012 表现出更高的肿瘤摄取率和更好的肿瘤与背景对比。与它们的 N 端 4-F-苯甲酸类似物不同，这两种示踪剂具有最小的血液保留，可能是由于红细胞结合减少所致。代谢器官，如肝脏和肾脏，也有很高的摄取率。当用低剂量冷肽 (10μg) 阻断时，肿瘤摄取显著增加，这很可能是由于循环血液中的浓度增加，这一点从肝脏摄取减少得到了证明。
这些结果表明，具有高肿瘤摄取率、低非特异性结合和良好肿瘤与背景对比的高 [(18)F]FP 标记 Ac-TC14012 肽有望成为 CXCR4 表达的体内 PET 成像的 [(18)F]FP-Ac-TC14012 PET 示踪剂。
