Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA.
J Nucl Med. 2012 Jul;53(7):1127-34. doi: 10.2967/jnumed.111.099390. Epub 2012 Jun 12.
Folic acid has been frequently exploited to target attached drugs to cells that overexpress a folate receptor (FR). Unfortunately, folic acid and folate-linked drugs bind equally well to both major isoforms of the FR-that is, FR-α, which is primarily expressed on malignant cells, and FR-β, which is upregulated on activated monocytes and macrophages. Because both major isoforms of FR can be expressed simultaneously in the same organism, folic acid cannot enable selective targeting of therapeutic and imaging agents to either tumor masses or sites of inflammation. In an effort to develop a targeting ligand that can selectively deliver attached imaging and therapeutic agents to tumor cells, we constructed a reduced and alkylated form of folic acid, N(5), N(10)-dimethyl tetrahydrofolate (DMTHF) that exhibits selectivity for FR-α.
DMTHF-(99m)Tc was injected into mice bearing FR-α-expressing tumor xenografts and imaged by γ-scintigraphy. The selectivity for FR-α over FR-β in vivo was examined by γ-scintigraphic images of animal models of various inflammatory diseases such as apolipoprotein E-deficient mice with atherosclerosis, DBA/1 LacJ mice with induced arthritis, C57BL/6J mice with muscle injury, and BALB/C mice with both FR-α tumor and ulcerative colitis, by administration of equal doses of DMTHF-(99m)Tc and EC20-(99m)Tc. The uptake of radiochelates in various organs was quantified by biodistribution studies. DMTHF-near-infrared dye conjugate and DMTHF-Oregon green dye conjugates were synthesized and evaluated for FR-α selectivity over FR-β in rat peritoneal macrophages and human peripheral blood monocytes, respectively, by flow cytometry. Fluorescence-guided imaging was also performed using folate and DMTHF dye conjugates.
The new targeting ligand was found to bind malignant cells in mice with solid tumor xenografts but not peripheral blood monocytes or inflammatory macrophages in animal models of atherosclerosis, rheumatoid arthritis, muscle injury, or ulcerative colitis. Results from optical and radioimaging studies and biodistribution experiments confirm the differential specificity of this new ligand for malignant masses.
The new targeting ligand DMTHF enables selective noninvasive imaging and therapy of tumor tissues in the presence of inflammation.
叶酸经常被用来将附着的药物靶向到过度表达叶酸受体 (FR) 的细胞。不幸的是，叶酸和叶酸连接的药物与 FR 的两种主要同工型结合得同样好，即主要表达于恶性细胞的 FR-α，以及在激活的单核细胞和巨噬细胞中上调的 FR-β。因为 FR 的两种主要同工型可以同时在同一生物体中表达，所以叶酸不能使治疗和成像剂选择性地靶向肿瘤团块或炎症部位。为了开发一种能够选择性地将附着的成像和治疗剂递送到肿瘤细胞的靶向配体，我们构建了叶酸的一种还原和烷基化形式，N(5)，N(10)-二甲基四氢叶酸 (DMTHF)，它对 FR-α具有选择性。
将 DMTHF-(99m)Tc 注射到表达 FR-α 的肿瘤异种移植小鼠中，并通过γ闪烁照相进行成像。通过给予等量的 DMTHF-(99m)Tc 和 EC20-(99m)Tc，通过动脉粥样硬化的载脂蛋白 E 缺陷小鼠、诱导关节炎的 DBA/1 LacJ 小鼠、肌肉损伤的 C57BL/6J 小鼠以及同时具有 FR-α 肿瘤和溃疡性结肠炎的 BALB/C 小鼠的各种炎症性疾病动物模型的γ闪烁成像，研究 DMTHF-(99m)Tc 在体内对 FR-β 的选择性。通过生物分布研究定量测定各种器官中放射性螯合物的摄取。合成了 DMTHF-近红外染料缀合物和 DMTHF-Oregon 绿染料缀合物，并通过流式细胞术分别评估其在大鼠腹腔巨噬细胞和人外周血单核细胞中对 FR-β 的选择性。还使用叶酸和 DMTHF 染料缀合物进行了荧光引导成像。
新的靶向配体被发现与患有实体瘤异种移植的小鼠中的恶性细胞结合，但与动脉粥样硬化、类风湿关节炎、肌肉损伤或溃疡性结肠炎动物模型中的外周血单核细胞或炎症性巨噬细胞不结合。光学和放射性成像研究以及生物分布实验的结果证实了这种新配体对恶性肿块的差异特异性。
新的靶向配体 DMTHF 能够在炎症存在的情况下选择性地进行肿瘤组织的非侵入性成像和治疗。
