Shan Liang
National Center for Biotechnology Information, NLM, NIH
The lactoferrin (Lf)-conjugated poly(ethylene glycol) (PEG)-coated FeO nanoparticles, abbreviated as FeO-Lf, was synthesized by Qiao et al. for use as a contrast agent for magnetic resonance imaging (MRI) of the brain (1). As a ligand, Lf acts to enhance the blood–brain barrier (BBB) penetration and Lf receptor-targeting of FeO-Lf. BBB is composed of tight junction-sealed brain capillary endothelial cells (BCECs) and supporting pericytes and astrocytic endfeet (2). Exogenous compounds are prevented by the BBB from reaching the brain by passive transport or through the paracellular route (3). Because many proteins including Lf could effectively cross the BBB through transcytosis, an active transport mechanism of BCECs, this mechanism has been actively used to design brain-targeted delivery systems for targeted imaging and therapy of neurological diseases (4, 5). Mammalian Lf is a cationic iron-binding glycoprotein (80 kDa), and its receptor expresses on the endothelial cells of BBB (6). Upon binding with its receptor, Lf could cross the BBB through receptor-mediated transcytosis (5). The transport of Lf across the BBB is unidirectional, from the apical side to the basolateral side, with no apparent intraendothelial degradation (1, 7). Studies with membrane preparations of mouse brains have shown that the Lf receptor in the BCECs has two classes of binding sites: a high-affinity site that has a dissociation constant (K) of 10.61 nM and a of 410 fmol bound/µg protein; and a low-affinity site that has a K of 2,228 nM and a of 51,641 fmol bound/µg protein (5). The plasma concentration of endogenous Lf (~5 nM) is lower than the K of Lf receptors in the BBB, which avoids the competitive inhibition of endogenous Lf to exogenous Lf-conjugated agents (1). Lf receptor has also been shown to be overexpressed in various tumors including brain glioma (8). Because of these features, Lf has been applied as a ligand in designing brain-targeted delivery systems. Xie et al. conjugated Lf to superparamagnetic iron oxide nanoparticles (SPIONs) to develop the contrast agent Lf-SPION (8), while Qiao et al. developed the agent FeO-Lf by conjugating Lf to FeO nanoparticles through PEG (1). For the former agent, Lf was designed to target Lf-SPIONs to tumors expressing the Lf receptor. For the latter agent, Lf served to enhance the BBB-penetrating ability of FeO-Lf by targeting Lf receptors expressed in the BCECs. Studies with the two agents showed that Lf-SPIONs were able to effectively enhance the glioma contrast, and FeO-Lf could effectively penetrate the BBB in healthy rats because of the conjugation of Lf (1, 8). This chapter summarizes the data obtained by Qiao et al. with FeO-Lf.
乔等人合成了乳铁蛋白（Lf）共轭聚乙二醇（PEG）包被的FeO纳米颗粒，简称为FeO-Lf，用作脑部磁共振成像（MRI）的造影剂（1）。作为一种配体，Lf可增强FeO-Lf的血脑屏障（BBB）穿透能力和Lf受体靶向性。血脑屏障由紧密连接封闭的脑毛细血管内皮细胞（BCECs）以及支持性周细胞和星形胶质细胞终足组成（2）。血脑屏障可阻止外源性化合物通过被动转运或细胞旁途径进入大脑（3）。由于包括Lf在内的许多蛋白质可通过BCECs的主动转运机制——转胞吞作用有效穿过血脑屏障，因此该机制已被积极用于设计针对神经系统疾病的靶向成像和治疗的脑靶向递送系统（4,5）。哺乳动物Lf是一种阳离子铁结合糖蛋白（80 kDa），其受体在血脑屏障的内皮细胞上表达（6）。与受体结合后，Lf可通过受体介导的转胞吞作用穿过血脑屏障（5）。Lf穿过血脑屏障的转运是单向的，从顶端侧到基底外侧，且在内皮细胞内无明显降解（1,7）。对小鼠脑细胞膜制剂的研究表明，BCECs中的Lf受体有两类结合位点：一类高亲和力位点，解离常数（K）为10.61 nM，结合量为410 fmol/μg蛋白质；另一类低亲和力位点，K为2228 nM，结合量为51641 fmol/μg蛋白质（5）。内源性Lf的血浆浓度（约5 nM）低于血脑屏障中Lf受体的K，这避免了内源性Lf对外源性Lf共轭剂的竞争性抑制（1）。Lf受体在包括脑胶质瘤在内的各种肿瘤中也被证明过表达（8）。由于这些特性，Lf已被用作设计脑靶向递送系统的配体。谢等人将Lf与超顺磁性氧化铁纳米颗粒（SPIONs）共轭，开发出造影剂Lf-SPION（8），而乔等人通过PEG将Lf与FeO纳米颗粒共轭，开发出制剂FeO-Lf（1）。对于前一种制剂，Lf旨在将Lf-SPIONs靶向到表达Lf受体的肿瘤。对于后一种制剂，Lf通过靶向BCECs中表达的Lf受体来增强FeO-Lf的血脑屏障穿透能力。对这两种制剂的研究表明，Lf-SPIONs能够有效增强胶质瘤的对比度，而FeO-Lf由于Lf的共轭作用能够有效穿透健康大鼠的血脑屏障（1,8）。本章总结了乔等人使用FeO-Lf获得的数据。
