中枢神经系统(CNS)是人体血管分布最丰富的器官系统之一,脑血管及相关脑细胞共同构成神经血管单元,该结构可将神经活动与血流动态耦合,同时参与血脑屏障(BBB)的形成与稳态维持[1]。BBB是一种特化生理屏障,可分隔血液与脑组织,主要由脑微血管内皮细胞、星形胶质细胞、周细胞、基底膜,以及包含紧密连接(TJs)、黏附连接(AJs)在内的连接复合体构成,能够大幅限制血液中绝大多数大分子物质的跨屏障转运,是外周给药(蛋白类药物)递送至CNS的核心难题[2]。CNS药物递送技术策略有受体介导转胞吞(RMT)、血脑屏障短暂可逆开放(transient BBB disruption)、局部靶向治疗(local therapies)和脑脊液介导递送(CSF-mediated delivery)等。基于RMT的递送技术具有非侵入式给药的优势,可借助血脑屏障界面高表达的细胞膜表面受体(如TfR1/CD71、CD98hc、IGF1R等),将大分子药物递送至脑实质[3]。当前,脑部药物靶向递送研究常以TfR1为核心靶点。
TfR1
结构与功能
结构
转铁蛋白受体 1(TfR1,又称 CD71)是同源二聚体II型跨膜糖蛋白,包括N端胞内段、跨膜区以及胞外区,胞外区由茎区(stalk region)、顶端结构域(apical domain)、蛋白酶样结构域(protease-like domain)和螺旋结构域(helix domain)组成,茎区的2个半胱氨酸(Cys89、Cys98)通过共价键将两个大片段同源二聚胞外结构域交联为完整受体。胞外段负责结合配体——转铁蛋白(Transferrin,Tf)并介导其内吞入细胞,完成铁离子递送。Tf的N lobe与受体的protease-like domain相互作用,C lobe则结合受体的helix domain。
Tf-TfR1 complex示意图[4]
TfR1参与细胞铁摄取
TfR1在细胞铁摄取中发挥核心不可替代作用,是细胞从血液摄取铁的主要通道。血液中铁以三价铁(Fe³⁺)形式紧密结合Tf。载铁转铁蛋白(双铁转铁蛋白,holo-Tf)结合细胞膜表面的TfR1同源二聚体,二者形成的复合物聚集于网格蛋白包被凹穴,凹穴向内凹陷启动胞吞过程。Tf-TfR1复合物内化形成网格蛋白包被囊泡,成熟为早期内体;内体酸性环境(pH≈5.5)诱导Tf、TfR1构象改变,Fe³⁺从Tf蛋白释放,释放的Fe³⁺经STEAP铁还原酶还原为Fe²⁺,并经二价金属转运蛋白1(DMT1)转运至细胞质,用于供给亚铁血红素、核糖核苷酸还原酶(DNA合成的限制酶)等发挥功能,或储存于铁蛋白中。铁释放后,脱铁转铁蛋白(apo-Tf)仍在低pH内体中结合TfR1,随后apo-Tf-TfR1复合物循环运回到细胞膜表面,在胞外中性pH环境下二者发生解离,apo-Tf释放至血液循环中,TfR1重新参与下一轮铁摄取[5,6]。
细胞摄取铁的完整过程[5]
TfR1可作为穿越BBB的递送靶点
TfR1广泛表达于肝细胞、红细胞与增殖细胞表面[7],且高表达于血脑屏障守门细胞——脑微血管内皮细胞管腔侧膜,远高于外周内皮细胞,这一分布特点使TfR1成为大分子穿越血脑屏障递送的理想靶点。阿尔茨海默病相关模型研究进一步凸显了该靶点的优势:病理状态下脑内皮细胞的TfR1表达水平显著上调,而LRP1、GLUT1和胰岛素受体等其他屏障受体会随疾病发生表达下调,大幅削弱递送效率[8]。
基于TfR1
BBB-crossing药物
部分基于TfR1穿越BBB的技术平台[9]
目前,多家药企布局了以TfR1作为递送靶点、介导药物穿越BBB的递送技术平台。代表性平台有Denali的Transport VehicleTM(TV)、Roche的BrainshuttleTM、JCR Pharma的J-Brain Cargo®以及Aliada/Abbvie合作的Modular Delivery(MODELTM)平台。从各平台公开的结构信息可见,TfR1递送模块的设计策略存在明显差异,主流方案依据适应症需求,将各类适配的治疗模块(如抗体、酶、寡核苷酸、蛋白等)与TfR1靶向抗体融合,药物分子一端作用于疾病治疗靶点,另一端特异性靶向TfR1,依靠TfR1介导的胞吞途径实现药物高效穿越血脑屏障。Denali开发的TV递送平台为差异化路线,该平台通过对human IgG1 Fc结构域的loop区进行工程化改造,嵌入TfR1结合位点并优化亲和力。TV架构设计简洁,无附加序列,在维持IgG1天然结构与FcRn结合活性的基础上,能够灵活偶联抗体、酶或寡核苷酸,分别构建为抗体转运载体(ATV)、酶转运载体(ETV)及寡核苷酸转运载体(OTV),支持不同类型生物大分子的脑内高效递送[10]。
现阶段已开发的TfR1介导递送药物,其靶向元件主要识别TfR1的apical domain及protease-like domain,其中靶向apical domain的研发管线占主导。
J-Brain Cargo®技术平台及代表药物
JCR Pharma的J-Brain Cargo®技术平台赋能的产品Pabinafusp alfa(JR-141)是当前进展最快的TfR1 based的BBB-crossing药物,该药物已在日本获批上市,全球范围内处于phase III阶段。它是一款可穿透血脑屏障的融合蛋白,由完整人源IDS酶与抗TfR1抗体融合而成,适应症为II型粘多糖贮积症(MPSII,又叫做Hunter综合征,是一种由于缺乏艾杜糖醛酸-2-硫酸酯酶(IDS酶)导致糖类在体内(尤其是CNS)积累的遗传病)。该药物抗TfR1抗体部分特异性靶向TfR1的apical domain,不与Tf的结合位点重叠,从而避免影响TfR1的铁摄取功能且能触发TfR1介导的转胞吞作用[11],IDS酶部分携带含6-磷酸甘露糖(M6P)的多糖链,能够识别6-磷酸甘露糖受体(M6PR)。Pabinafusp alfa与TfR1结合后,会与TfR1一同经历内吞作用,随后在细胞内转运至对侧,通过胞吐作用进入脑实质,之后被神经元、星形胶质细胞及周细胞表面的M6PR受体识别并摄取,从而将IDS酶递送至脑内以产生疗效。
Pabinafusp alfa的结构及其细胞摄取途径示意图[12]
Transport VehicleTM(TV)技术平台及代表药物
TV技术平台代表药物[13]
Denali依托自主TV平台开发了多款靶向TfR1的CNS治疗药物,TV技术的核心创新是对IgG1 Fc的CH3结构域进行蛋白工程改造,引入TfR1结合表位。为筛选性能最优的Fc 突变体,团队搭建酵母表面定向进化高通量文库,依次开展软突变、NNK周边扫描、聚焦文库亲和力多轮迭代优化,最终筛选出TV35.21.16、TV35.21.17两大成熟候选序列。筛选环节中,研究团队使用环形重排的apical domain(circularly permutated apical domain)作为筛选抗原,截除天然apical domain内部的柔性环,重新排布蛋白的N/C端。相较于天然二聚全长TfR1,这种结构可降低酵母细胞表面发生的多价结合作用,大幅提升筛选的严谨度,有效规避假阳性克隆,精准富集可同时结合人、食蟹猴TfR1的TV突变克隆。晶体结构证实,TV与TfR1的结合区域完全独立于Tf、FcRn的天然结合位点,不会干扰机体正常铁转运并保留FcRn循环功能。
Tividenofusp alfa/DNL310是Denali基于TV平台开发的首款候选药物,该药物为IDS酶-Fc融合蛋白,适应症为MPSII,已于2025年提交BLA申请。该融合蛋白可特异性结合脑内皮细胞上的TfR1,借助受体介导转胞吞作用穿透血脑屏障,将IDS酶递送至脑实质,降解MPSII患者CNS中积累的糖类,以缓解MPSII引发的神经认知与躯体症状。
TV筛选过程及apical domain抗原的特殊设计[14]
BrainshuttleTM技术平台及代表药物
Trontinemab是Roche基于其独有的Brainshuttle™技术平台开发的的一款双特异性2+1结构Aβ抗体,该技术将抗Aβ的单克隆抗体与TfR1穿梭模块(具有1个TfR1结合位点)融合。由于其独特的结构设计,Trontinemab能实现高效穿越BBB,靶向Aβ的聚集形式并清除脑中的淀粉样斑块,在低剂量下即可实现CNS暴露,在阿尔茨海默症患者体内快速减少Aβ,延缓疾病进展[15]。在2026 AAIC大会上,罗氏公布了Trontinemab Brainshuttle™ AD Ib/IIa期开放标签延展研究的全新长期安全性、淀粉样蛋白清除及生物标志物数据。基于该研究建立的模型,已为当前正在开展的两项 III 期TRONTIER1、TRONTIER早期症状型阿尔茨海默病临床试验确定给药方案[16]。
Trontinemab的作用机理[17]
恺佧生物供应高品质Apical domain 和 TfR1 ECD蛋白
随着各类TfR1脑穿梭递送技术快速成熟,CNS药物研发的瓶颈被逐步打破,相关临床管线近年来迎来爆发式增长。Apical domain与Tf在TfR1上的结合位点空间分离,不与循环Tf产生直接竞争,因此,apical domain成为TfR1介导转胞吞递送药物开发的主流靶向结构域。
恺佧生物可提供野生型(wild type)与环形重排(circularly permutated)两款apical domain重组蛋白,分别适配动物免疫、分子筛选等不同研发场景;其中circularly permutated apical domain 是高通量酵母/噬菌体筛选的重要抗原,可有效规避多价结合带来的假阳性干扰。同时公司配套提供TfR1 ECD蛋白,全方位支撑TfR1 based的CNS药物的研发进程。
产品数据
Immobilized Human Transferrin R Apical Domain, Llama IgG2b Fc Tag at 0.5μg/ml (100μl/well) on the plate. Dose response curve for Anti-Transferrin R Antibody, hFc Tag with the EC50 of 9.4ng/ml determined by ELISA.
Immobilized Biotinylated Human Transferrin R Apical Domain, His Avi Tag at 1μg/ml (100μl/well) on the streptavidin precoated plate (5μg/ml). Dose response curve for Anti-Transferrin R Antibody, hFc Tag with the EC50 of 4.2ng/ml determined by ELISA.
Immobilized Biotinylated Human Circularly Permuted Apical Domain, His Avi Tag at 1μg/ml(100μl/well) on the streptavidin precoated plate(5μg/ml). Dose response curve for Anti-Transferrin R Antibody, hFc Tag with the EC50 of 3.4ng/ml determined by ELISA.
Immobilized Biotinylated Cynomolgus Circularly Permuted Apical Domain, His Avi Tag at 0.5μg/ml(100μl/well) on the streptavidin precoated plate(5μg/ml). Dose response curve for Anti-Transferrin R Antibody, hFc Tag with the EC50 of 0.11μg/ml determined by ELISA.
Immobilized Human Transferrin R, His Tag at 2μg/ml (100μl/well) on the plate. Dose response curve for Biotinylated Human Transferrin, His Avi Tag with the EC50 of 14.3ng/ml determined by ELISA.
Loaded Anti-Transferrin R Antibody, hFc Tag on ProA-Biosensor can bind Human Transferrin R, His Tag with an affinity constant of 0.65nM as determined in BLI assay (Gator® Prime).
产品列表
产品货号
产品名称TFR-HM6AD
Human Transferrin R/CD71 Apical Domain Protein, Llama IgG2b Fc TagTFR-HM4ADB
Biotinylated Human Transferrin R/CD71 Apical Domain Protein, His-Avi TagAPC-HM4DDB
Biotinylated Human Circularly Permuted Apical Domain Protein, His-Avi Tag
TFR-CM4ADB
Biotinylated Cynomolgus Transferrin R Apical Domain Protein, Avi-His Tag
TFR-CM4ADB
Biotinylated Cynomolgus Circularly Permuted Apical Domain Protein, His-Avi TagTFR-HM102
Human Transferrin R Protein, His TagTFR-HM102-UL
Human Transferrin R, Ultra Low Endotoxin, His TagTFR-HM201
Human Transferrin R, hFc (IgG1) TagTFR-HM401B
Biotinylated Human Transferrin R, His-Avi TagTFR-MM101
Mouse Transferrin R, His TagTFR-MM101-UL
Mouse Transferrin R, Ultra Low Endotoxin, His TagTFR-CM101
Cynomolgus Transferrin R, His TagTFR-CM101-UL
Cynomolgus Transferrin R, Ultra Low Endotoxin, His TagTFN-HM101
Human Transferrin, His TagTFN-HM401B
Biotinylated Human Transferrin, His-Avi TagTFN-HM101F
FITC-Labeled Human Transferrin, His Tag
参考文献
[1] Zuchero YJ, Silverman A, Dennis M, et al. Brain delivery of therapeutic proteins using an Fc fragment blood-brain barrier transport vehicle in mice and monkeys Science Translational Medicine. 2020;12(545). doi:10.1126/scitranslmed.aay1359
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[4] Wessling-Resnick, Marianne. (2018). Crossing the Iron Gate: Why and How Transferrin Receptors Mediate Viral Entry. Annual Review of Nutrition. 38. 431-458. 10.1146/annurev-nutr-082117-051749.
[5] Sabo, S.L., & Buxbaum, J.D. (2002). The role of the transferrin-transferrin-receptor system in drug delivery and targeting.
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[8] Shen, X.; Li, H.; Zhang, B.; Li, Y.; Zhu, Z. Targeting Transferrin Receptor 1 for Enhancing Drug Delivery Through the Blood–Brain Barrier for Alzheimer’s Disease. Int. J. Mol. Sci. 2025, 26, 9793. https://doi.org/10.3390/ijms26199793
[9]https://investors.denalitherapeutics.com
[10] Chew KS, Wells RC, Moshkforoush A, Chan D, Lechtenberg KJ, Tran HL, Chow J, Kim DJ, Robles-Colmenares Y, Srivastava DB, Tong RK, Tong M, Xa K, Yang A, Zhou Y, Akkapeddi P, Annamalai L, Bajc K, Blanchette M, Cherf GM, Earr TK, Gill A, Huynh D, Joy D, Knight KN, Lac D, Leung AW, Lexa KW, Liau NPD, Becerra I, Malfavon M, McInnes J, Nguyen HN, Lozano EI, Pizzo ME, Roche E, Sacayon P, Calvert MEK, Daneman R, Dennis MS, Duque J, Gadkar K, Lewcock JW, Mahon CS, Meisner R, Solanoy H, Thorne RG, Watts RJ, Zuchero YJY, Kariolis MS. CD98hc is a target for brain delivery of biotherapeutics. Nat Commun. 2023 Aug 19;14(1):5053. doi: 10.1038/s41467-023-40681-4. Erratum in: Nat Commun. 2023 Sep 7;14(1):5516. doi: 10.1038/s41467-023-41355-x. PMID: 37598178; PMCID: PMC10439950.
[11] Yamamoto R, Yoden E, Tanaka N, Kinoshita M, Imakiire A, Hirato T, Minami K. Nonclinical safety evaluation of pabinafusp alfa, an anti-human transferrin receptor antibody and iduronate-2-sulfatase fusion protein, for the treatment of neuronopathic mucopolysaccharidosis type II. Mol Genet Metab Rep. 2021 Apr 18;27:100758. doi: 10.1016/j.ymgmr.2021.100758. PMID: 33981582; PMCID: PMC8081988.
[12] Fukatsu T, Morio H, Furihata T, Sonoda H. Transferrin receptor-targeting property of pabinafusp alfa facilitates its uptake by various types of human brain-derived cells in vitro. Front Drug Deliv. 2023 Jul 3;3:1082672. doi: 10.3389/fddev.2023.1082672. PMID: 40838062; PMCID: PMC12363330.
[13]https://investors.denalitherapeutics.com/static-files/bacd8b61-af8a-4f84-91fa-2c20ce531548
[14] Kariolis MS, Wells RC, Getz JA, Kwan W, Mahon CS, Tong R, Kim DJ, Srivastava A, Bedard C, Henne KR, Giese T, Assimon VA, Chen X, Zhang Y, Solanoy H, Jenkins K, Sanchez PE, Kane L, Miyamoto T, Chew KS, Pizzo ME, Liang N, Calvert MEK, DeVos SL, Baskaran S, Hall S, Sweeney ZK, Thorne RG, Watts RJ, Dennis MS, Silverman AP, Zuchero YJY. Brain delivery of therapeutic proteins using an Fc fragment blood-brain barrier transport vehicle in mice and monkeys. Sci Transl Med. 2020 May 27;12(545):eaay1359. doi: 10.1126/scitranslmed.aay1359. PMID: 32461332.
[15]Roche presents novel therapeutic and diagnostic advancements in Alzheimer’s at AD/PD 2025
[16]Roche presents new data in Alzheimer’s disease from across its integrated pharmaceutical and diagnostics portfolio at AAIC[17]Brainshuttle™ AD: New interim results of a randomized, placebo-controlled Phase Ib/IIa proof-of-concept study with trontinemab, a novel anti-amyloid monoclonal bispecific antibody for the treatment of Alzheimer’s disease - CTAD-2025-presentation-kulic-brainshuttle-tm-ad-new-interim-results.pdf
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