生物工程学报  2023, Vol. 39 Issue (9): 3628-3643
http://dx.doi.org/10.13345/j.cjb.230006
中国科学院微生物研究所、中国微生物学会主办
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文章信息

吴小波, 赵婕, 高远, 姚庆鑫, 谢建军
WU Xiaobo, ZHAO Jie, GAO Yuan, YAO Qingxin, XIE Jianjun
靶向递送蛋白水解靶向嵌合体在癌症治疗中的研究进展
Advances in targeted delivery of proteolysis targeting chimeras in cancer therapy
生物工程学报, 2023, 39(9): 3628-3643
Chinese Journal of Biotechnology, 2023, 39(9): 3628-3643
10.13345/j.cjb.230006

文章历史

Received: January 4, 2023
Accepted: March 21, 2023
Published: March 23, 2023
靶向递送蛋白水解靶向嵌合体在癌症治疗中的研究进展
吴小波1,2 , 赵婕1,2 , 高远2 , 姚庆鑫2,3 , 谢建军1     
1. 中南林业科技大学材料科学与工程学院, 湖南 长沙 410004;
2. 国家纳米科学中心 中国科学院纳米生物效应与安全性重点实验室, 北京 100190;
3. 北京化工大学材料科学与工程学院, 北京 100029
摘要:小分子抗癌药物通过靶向特定蛋白来抑制肿瘤生长,但大部分致病蛋白被认为是“不可成药”的。蛋白水解靶向嵌合体(proteolysis targeting chimeras, PROTAC)通过靶向降解目标蛋白来抑制肿瘤细胞生长,是一项非常有潜力的新技术。本文在介绍传统多肽型PROTAC和小分子型PROTAC基础上,详细总结了靶向递送型PROTAC的最新研究进展,主要包括识别分子介导靶向PROTAC、纳米材料介导靶向PROTAC和可控激活小分子PROTAC前药。研究表明,靶向递送型PROTAC在提高肿瘤细胞特异性、减少脱靶效应和降低生物毒性等方面具有潜在应用价值。最后,本文对PROTAC的成药性进行了展望。
关键词蛋白水解靶向嵌合体    蛋白降解    靶向递送    癌症    抗癌药物    
Advances in targeted delivery of proteolysis targeting chimeras in cancer therapy
WU Xiaobo1,2 , ZHAO Jie1,2 , GAO Yuan2 , YAO Qingxin2,3 , XIE Jianjun1     
1. School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, Hunan, China;
2. CAS Key Laboratory for Biomedical Effects of Nanomaterials & Nanosafety, National Center for Nanoscience and Technology, Beijing 100190, China;
3. School of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
Abstract: Small-molecule anticancer drugs inhibited tumor growth based on targeted inhibition of specific proteins, while most of oncogenic proteins are "undruggable". Proteolysis targeting chimeras (PROTAC) is an attractive and general strategy for treating cancer based on targeted degradation of oncogenic proteins. This review briefly describes the peptide-based PTOTAC and small molecule-based PROTAC. Subsequently, we summarize the development of targeted delivery of PROTAC, such as targeting molecule-mediated targeted delivery of PROTAC, nanomaterial-mediated targeted delivery of PROTAC and controllable activation of small-molecular PROTAC prodrug. Such strategies show potential application in improving tumor selectivity, overcoming off-target effect and reducing biotoxicity. At the end, the druggability of PROTAC is prospected.
Keywords: proteolysis targeting chimeras    protein degrades    targeted delivery    cancer    anticancer drugs    

2022年癌症统计数据显示,癌症是威胁人类的主要问题之一[1]。在抗癌药物开发中,小分子药物大多通过非共价结合来抑制致病蛋白活性,所需药物浓度较高,且癌细胞可能通过靶蛋白过表达或突变而产生耐药性[2]。蛋白水解靶向嵌合体(proteolysis targeting chimeras, PROTAC)通过选择性降解目标蛋白来实现抗肿瘤功效[3-5]。PROTAC由目标蛋白(protein of interest, POI)配体、连接链(linker)和E3连接酶(E3 ligase)配体3部分组成,是一种能特异性识别目标蛋白并招募E3连接酶的双功能分子。通过形成“目标蛋白-PROTAC-E3连接酶” (POI-PROTAC-E3)三元复合物,使目标蛋白被泛素分子标记,并通过泛素-蛋白酶体系统(ubiquitin-proteasome system, UPS)完成降解。在完成目标蛋白的泛素化后,PROTAC分子从三元复合物中解离,用于靶向降解下一个目标蛋白,由此进入迭代循环[6] (图 1)。与小分子抑制剂相比,PROTAC在较低药物浓度下即可实现目标蛋白降解,降低了脱靶效应和对正常组织毒副作用[7-8]。Arvinas公司研发的针对雌激素受体和雄激素受体的PROTAC分子ARV-110和ARV-471,在临床Ⅱ期中表现出理想安全性和临床活性[9-10]。PROTAC技术的出现为新药开发带来了新机遇,引发了学术界和工业界积极探索[11-14]

图 1 PROTAC的作用机理[6] Fig. 1 The mechanism of PROTAC[6].
1 多肽型PROTAC

作为概念验证,2001年Crews和Deshaies团队[15]首次将募集E3连接酶Skp1-Cullin-F boxβ-TRCP (SCFβ-TRCP)的IkBα磷酸肽和血管生成抑制剂卵假散囊菌素结合用于甲硫氨酰氨肽酶-2 (methionine aminopeptidase-2, MetAP-2)降解。由于设计的PROTAC-1分子共价结合靶蛋白MetAP-2,限制了其循环使用特性,此外,分子量较大的IkBα磷酸肽导致PROTAC-1无法穿透细胞膜,限制了该分子的应用。随后,他们设计了非共价天然底物的PROTAC-2和PROTAC-3分子[16],分别将雌二醇(estradiol, E2)和二氢睾脂酮(dihydroxytestosterone, DHT)与IkBα磷酸肽相连,通过显微注射,在10 µmol/L浓度下诱导胞内雌激素受体和雄激素受体降解,但该类分子极易被细胞内磷酸酶水解,导致稳定性降低。2004年,Crews团队[17]基于E3连接酶冯·希佩尔-林道(von Hippel-Lindau, VHL)最小识别域,以及缺氧诱导因子1α (hypoxia-inducible factor 1α, HIF1α)衍生肽ALAPYIP,将多聚d-精氨酸标签偶联在ALAPYIP羧基末端,设计了首个穿膜PROTAC分子,在稳定表达雄激素受体绿色荧光融合蛋白的人胚肾细胞中检测到了雄激素受体降解。细胞穿透性PROTAC的开发,为实现体内致病蛋白降解提供了可能。多肽型PROTAC采用多肽序列作为E3连接酶配体,导致该类PROTAC分子具有较大分子量,因而细胞渗透性较差,且肽键在体内极易水解,使PROTAC分子稳定性降低。

2 小分子型PROTAC

2008年,Crews团队[18]报道了第一个基于小分子的PROTAC,以可溶性聚乙二醇(polyethylene glycol, PEG)为连接链,将选择性雄激素受体调节剂(selective androgen receptor modular, SARM)和E3连接酶小鼠双微体2 (murine double minute 2, MDM2)的抑制剂Nutlin-3组合到一起,制备得到较低分子量的PROTAC分子,在10 μmol/L浓度下与人宫颈癌细胞共孵育7 h,可明显观察到雄激素受体降解。同期,Naito团队[19]发现细胞凋亡抑制蛋白1 (cellular inhibitor of apoptosis protein 1, cIAP1)的配体贝他定(bestatin)衍生物可促进cIAP1自泛素化和降解。基于此,Hashimoto团队[20]报道了第一个利用E3连接酶cIAP1介导细胞视黄醇结合蛋白(cellular retinoic acid binding proteins-Ⅱ, CRABP-Ⅱ)降解的例子,在1 µmol/L浓度下与人纤维肉瘤细胞孵育3 h,即可观察到CRABP-Ⅱ降解,表现出足够细胞渗透性和降解活性,用其处理人神经母细胞瘤细胞时,发现细胞迁移明显受到抑制。但cIAP1配体脱靶会导致cIAP1自降解,因此亟需开发高亲和力的配体或新的E3连接酶。自基于E3连接酶VHL多肽配体的PROTAC报道以来[17, 21],研究者一直在寻找替代HIF1α衍生肽的小分子配体。Ciulli团队[22-24]保留了羟脯氨酸部分结构域,通过不断优化构效关系,得到一系列高亲和力和特异性的VHL小分子配体。通过将VHL的配体VHL-1与溴结构域蛋白4 (bromodomain containing protein 4, BRD4)的抑制剂JQ1连接生成MZ1分子,在HeLa细胞中以0.1 µmol/L浓度诱导BRD4降解,显著提高了降解效率[24-25]。而后,免疫调节药物(immunomodulatory drugs, IMiDs)沙利度胺及其衍生物来那度胺、泊马度胺被确认可作用于E3连接酶羟脑苷脂(cereblon, CRBN)[26-28],Bradner团队[29]将CRBN的配体泊马度胺和JQ1相连合成了dBET1分子,同样在0.1 µmol/L浓度下成功诱导BRD4降解,并建立了白血病小鼠模型来研究其体内治疗效果,显示出良好药代动力学特性,有效抑制了肿瘤生长。小分子型PROTAC由全小分子配体合成,具有较低分子量,有效提高了细胞渗透性,但该类PROTAC选择性较差,无法特异性累积在肿瘤部位,脱靶效应可能给生物体带来潜在毒副作用。

3 靶向递送PROTAC

虽然PROTAC提供了一种通用且高特异性靶向降解目标蛋白的方法,给药物发现领域带来极大改变,但PROTAC仍然面临较大挑战,限制了PROTAC的临床效用。第一,PROTAC分子量较大,通常超过800 Da,导致其在水中溶解度较差,生物利用度较低;第二,PROTAC的高极性表面限制了其组织通透性,阻碍了其跨越生理屏障和细胞膜的能力;第三,由于疾病部位和正常组织中E3连接酶的非选择性表达,导致PROTAC的脱靶效应,往往会产生严重的副作用[30]。虽然分子结构优化有助于改善PROTAC某些物理化学性质,但很难同时满足其在活体应用的所有需要。靶向药物递送系统可以成为一种有效替代策略,通过将PROTAC分子负载到输送载体上,靶向递送到病变部位,并在病变部位时空可控释放PROTAC分子,可最大限度提高PROTAC成药性[31]。药物递送系统在过去几十年的快速增长,得益于载体材料在药物负载能力、长循环特性、病灶靶向能力以及刺激响应释放等特性上的不断改善,以及治疗药物在疏水性、病灶选择性累积,高生物利用度、低毒副作用等性能上的不断提高[32]。类似于传统小分子药物性能的改善,近年,研究人员不断整合PROTAC和药物递送系统的概念[33],开发了一系列靶向递送及刺激响应释放的PROTAC递送系统。

3.1 识别分子介导靶向递送PROTAC

配体-受体相互作用是指配体选择性识别受体并实现信号传递,以此执行特定生理功能的过程[34]。在抗肿瘤领域,利用配体和受体之间的相互作用能提高药物分子肿瘤靶向性[35]。例如,利用癌细胞过表达的叶酸受体、抗原等[36-37],将相应的配体与PROTAC偶联,可实现PROTAC主动靶向肿瘤组织,减少正常细胞毒性。此外,对于细胞内一些难以用小分子配体靶向的蛋白,如RNA结合蛋白、转录因子等[38-39],通过适配体结合其特定位点,亦可实现PROTAC递送并降解。

叶酸受体α (folate receptor α, FRα)在正常组织中表达有限,但在乳腺癌、卵巢癌、肺癌等肿瘤组织中过表达[36]。Wei团队[40]在ARV-771分子的基础上,将叶酸分子与其相连,合成了Folate-ARV-771分子。叶酸受体介导Folate-ARV-771进入癌细胞,通过内源性水解酶去除叶酸基团释放ARV-771,实现胞内溴结构域蛋白(bromodomains, BRDs)降解。正常细胞由于叶酸受体含量较低,Folate-ARV-771不能结合到其表面,因此不会干扰其生理活性(图 2A)。在人宫颈癌细胞、卵巢癌细胞和乳腺癌细胞中,Folate-ARV-771对BRDs的降解效率与ARV-771几乎一致,IC50无明显变化。正常细胞中,BRDs的降解效率降低了10倍以上,IC50由1.1 μmol/L变至10 μmol/L以上,细胞毒性明显减弱。而后对丝裂原活化蛋白激酶(mitogen-activated protein kinase, MEK)和间变性淋巴瘤激酶(anaplasticlymphoma kinase, ALK)融合蛋白进行验证,发现叶酸偶联的PROTAC均能实现蛋白选择性降解。Jin团队[43]也通过叶酸介导策略在间变性大细胞淋巴瘤细胞系和非小细胞肺癌细胞系中观察ALK融合蛋白降解。但添加叶酸基团后,ALK融合蛋白降解效率降低,IC50由16 nmol/L变为465 nmol/L,需更高浓度来抑制癌细胞增殖。叶酸偶联PROTAC,能显著提高PROTAC分子细胞选择性,降低正常细胞毒副作用,但叶酸基团引入增加了PROTAC分子量,不利于口服给药,且细胞内部环境对PROTAC分子释放速率影响较大,目前还没有体内实验证明其药代动力学特性及抗肿瘤效率。

图 2 识别分子介导靶向递送PROTAC Fig. 2 Targeting molecule-mediated targeted delivery of PROTAC. A:叶酸介导[40]. B:适配体介导[41]. C:抗体介导[42] Folate (A)[40], aptamer (B)[41], and antibody (C)[42].

适配体是具有独特三级结构的短链核苷酸序列,能形成稳定三维构象,特异性识别靶标分子[44]。Sheng团队[41]通过将E3连接酶VHL的配体和JQ1相连,合成了靶向降解BRD4的PROTAC分子,而后在二硫键作用下将PROTAC连接到核酸适配体上,合成了第一个适配体介导靶向的PROTAC。适配体特异性识别并结合过表达核仁蛋白,进而介导PROTAC分子内吞,在谷胱甘肽作用下破坏二硫键,释放PROTAC分子,进而介导BRD4降解(图 2B)。相比于未改性的PROTAC分子,适配体介导靶向的PROTAC选择性降解乳腺癌细胞中BRD4,DC50为22 nmol/L,且有效抑制癌细胞生长,IC50为56.9 nmol/L。而在正常乳腺细胞中,适配体介导靶向PROTAC的IC50为3.13 μmol/L,而未改性PROTAC的IC50为0.67 μmol/L,显著降低了正常细胞毒性。小鼠肿瘤模型中,适配体介导靶向的PROTAC特异性靶向肿瘤组织,增强抗肿瘤活性,对正常组织器官不产生影响。Hall团队[45]借助sRNA模拟物对RNA结合蛋白(RNA binding protein, RBP)的识别,并通过E3连接酶VHL的配体HIF-1α衍生肽引导其通过蛋白酶体降解。在癌细胞和正常细胞中对2种RBPs蛋白,即LIN28和RBFOX1进行降解,发现不同RNA-PROTAC对LIN28和RBFOX1蛋白的降解活性差异显著。Wei团队[46]通过DNA与E3连接酶VHL配体结合生成TF-PROTAC,通过DNA选择性结合,使其特异性降解目标转录因子(transcription factor, TF)。相比于RNA、DNA寡聚体更稳定,且TF与DNA结合特异性比RBP与RNA强。在HeLa细胞中验证发现TF-PROTAC使P65蛋白几乎完全降解,而后对E2F转录因子1 (E2F transcription factor 1, E2F1)进行验证,发现E2F1水平显著降低,同时抑制了癌细胞增殖。适配体偶联PROTAC,增加了PROTAC水溶性和肿瘤靶向能力,但适配体偶联物在体内代谢不稳定,易被核酸酶水解和肾脏过滤排出。

抗体能特异性识别抗原和病原体,对靶标具有高亲和力和特异性[37]。Dragovich团队[47]在研究BRDs降解剂时发现一个有效的BRD4抑制剂,将其与E3连接酶VHL配体结合生成GNE-987分子,在人急性髓系白血病细胞中表现出显著降解活性,DC50为0.03 nmol/L,对细胞活性进行评估,发现该分子能有效抑制细胞增殖,IC50为0.02 nmol/L,但在体内实验中,口服生物利用度和药代动力学特性均较差,GNE-987水溶性小于1 μmol/L。为改善GNE-987性质,作者将其偶联到抗CLL1抗体上,静脉注射到小鼠体内,发现GNE-987半衰期显著延长,表现出良好体内稳定性和药代动力学特性,一周后仍有超过70%的GNE-987与抗体偶联。后期该课题组系统研究了抗体-PROTAC偶联物中衍生位点、Linker组分和响应特性等对降解效率的影响,发现抗体偶联PROTAC抗肿瘤效果表现出强大抗原依赖性[48-50]。Tate团队[42]设计并合成了曲妥珠单抗-PROTAC (trastuzumab-PROTAC, Ab-PROTAC),由人表皮生长因子受体-2 (human epidermal growthfactor receptor 2, HER2)介导内吞,通过溶酶体作用释放活性PROTAC,而不表达HER2受体的细胞中则不会引起蛋白降解(图 2C)。HER2阳性乳腺癌细胞系中,仅需4 h即可实现BRD4完全降解,DCmax为100 nmol/L,HER2阴性细胞系中观察不到BRD4降解,由此实现癌细胞选择性抑制。Melo团队[51]基于抗体直接将细胞表面E3连接酶环指蛋白RNF43 (ring finger protein 43, RNF43)与跨膜蛋白联系起来,介导体内体外目标蛋白降解。抗体偶联PROTAC,通过抗原依赖选择性递送PROTAC,将PROTAC的催化性与抗体偶联药物的组织特异性结合,有效发挥了两者优势,但当抗原抗体数量关系不匹配时,可能造成脱靶,促发机体免疫,增加给药风险[52]

3.2 纳米材料介导靶向递送PROTAC

纳米材料因其具有小尺寸效应、体积效应和表面效应等特点而被广泛用于药物递送,具有增强运载药物稳定性和溶解性、促进跨膜运输和延长循环时间等优势[31, 53]。常用纳米材料有:脂质体、高分子纳米材料和无机纳米材料等[54-55]。将PROTAC分子与纳米材料结合,利用实体瘤的高通透性和滞留效应(enhanced permeability and retention effect, EPR)使PROTAC分子被动累积到肿瘤部位,实现肿瘤靶向性。

脂质体是由亲水基团和亲脂基团组成的球型囊泡,具有生物相容性好、负载能力强和便于设计等特点[56]。Wang团队[57]为克服dGPX4分子水溶性差和体内稳定性低等问题,将其封装在脂质体纳米颗粒中递送至肿瘤组织处,在活性氧响应下释放,有效降低内源性谷胱甘肽过氧化酶4 (glutathione peroxidase 4, GPX4)含量,抑制肿瘤生长(图 3A)。相对于PBS对照组而言,脂质体递送dGPX4使肿瘤体积减少了80%,且小鼠体重无明显变化,表现出有效抗肿瘤活性和较高生物安全性。游离dGPX4对肿瘤抑制作用为60%,但小鼠肝肾功能明显损伤,体重下降。Xu团队[61]利用脂质体将预先与E3连接酶VHL结合的ARV-771分子封装,通过内吞作用进入细胞后,在溶酶体作用下释放出ARV-771,实现蛋白降解。由于脂质体的作用,细胞渗透性增强,游离ARV-771的DC50约为100 nmol/L,而与VHL融合并由脂质体递送,在25 nmol/L浓度下即可实现BRD4完全降解。E3连接酶和ARV-771的预融合及脂质体递送协同提高了目标蛋白降解效率。此外,Patel团队[62]开发了携带ARV-825的纳米脂质体(loaded PEGylated nanoliposomes, LARPC),相比于游离ARV-825,LARPC对血管长度抑制提高近5倍,细胞迁移抑制提高近2倍,显著增强了ARV-825的肿瘤靶向性。脂质体介导递送PROTAC,可有效提高PROTAC分子细胞渗透性,但由于PROTAC分子水溶性较差,通常只能装载到磷质层,限制了其装载率,当治疗剂量较高时,阳离子脂质体可能会对正常细胞造成毒副作用[63]

图 3 纳米材料介导靶向递送PROTAC Fig. 3 Nanomaterial-mediated targeted delivery of PROTAC. A:脂质体纳米颗粒介导[57]. B:聚合物纳米颗粒介导[58]. C:肽修饰纳米颗粒介导[59]. D:金纳米颗粒介导[60] A: Liposome[57]. B: Polymer[58]. C: Peptide-modified[59]. D: Gold nanoparticle[60].

高分子纳米颗粒由天然或合成的单体或聚合物组成,可形成不同结构和特性的载体[64]。Pu团队[58]报道了一种两亲性半导体聚合物纳米PROTAC (semiconducting polymer nano-PROTAC, SPNpro),通过全身给药后,SPNpro在小鼠肿瘤组织中选择性积累。在小鼠乳腺癌细胞中,用SPNpro与细胞共孵育后,发现细胞内Anti-IDO抗体含量降低了89.3%,相比于直接靶向吲哚胺2, 3-双加氧酶(indoleamine 2, 3-dioxygenase1, IDO)的IPP分子,SPNpro的效率约提高了10%。此外,协同光照治疗,诱导肿瘤相关抗原释放和免疫原性细胞死亡,抗肿瘤效率提高了50% (图 3B)。随后,该团队[65]进一步开发了智能聚合物纳米PROTAC (smart nano-PROTAC targeting COX-1/2, SPNCOX)。通过在肿瘤部位累积并与肿瘤过表达组织蛋白酶B (cathepsin B, CatB)作用释放PROTAC,靶向降解环加氧酶1/2,减少下游产物前列腺素E2 (prostaglan-din E2, PGE2),抑制免疫抑制细胞激活,进而提高抗肿瘤免疫力。此外,Zhang团队[66]构建了用工程肺癌细胞膜进行伪装的聚合物纳米颗粒(CRV-engineered Lewis lung carcinoma cell membranes/DS-PLGA/dBET6, CREATE),发现该分子能同时靶向肺癌细胞和肿瘤相关巨噬细胞,刺激诱导dBET6分子释放,促进细胞凋亡并抑制肿瘤生长。在小鼠肿瘤模型中,尽管小鼠体重均未观察到明显变化,但相对于对照组而言,CREATE对肿瘤的抑制效果达到95%以上,而游离dBET6仅为50%,且CREATE诱导细胞凋亡程度显著高于dBET6和其他对照组。最近,Gao团队[59]设计合成了靶向降解BRD4的肽修饰纳米颗粒[cyclo(Arg-Gly-Asp-D-Phe-Lys)-poly-(ethylene glycol)-poly(ε-caprolactone), cRGD-PEG-PCL],通过主动靶向性环肽cRGD的修饰和抗瘤药物阿霉素的偶联,使其主动靶向肿瘤组织,提高结直肠癌的抗肿瘤活性(图 3C)。相比于游离的ARV-825,cRGD-PEG-PCL显著促进细胞凋亡,抗肿瘤活性提高10倍以上。聚合物介导递送PROTAC,方便构建不同响应类型纳米颗粒,协同促进抗肿瘤效果,但可能存在颗粒聚集等问题[67],且目前只有少量聚合物纳米颗粒药物被美国食品药品监督管理局(Food and Drug Administration, FDA)批准用于临床。

无机纳米材料因其制备简单、贮存稳定,在生物成像和药物递送领域有较大应用前景[68]。He团队[69]为克服PROTAC药理屏障,合成了聚丙烯酰基巯基咪唑金纳米颗粒-肽聚集体(supramolecular gold(I)-thiol-peptide complex@ polyacryl sulfydryl imidazole, Nano-MP@PSI)。在谷胱甘肽作用下发生断键,被分解成更小纳米颗粒,同时释放PROTAC分子。相比于对照组而言,在视网膜母细胞瘤和胰腺癌小鼠模型经过Nano-MP@PSI给药治疗后,显著抑制肿瘤生长,肿瘤体积和重量均减少了90%左右,小鼠重量和组织切片等均无显著变化。Liu团队[60]也利用金-硫醇之间的相互作用,以PEG为连接链将ALK配体塞利替尼和E3连接酶CRBN配体泊马度胺分别连接到金纳米颗粒上,开发了首个多头PROTAC (pegylated GNPs loaded with both ceritinib and pomalidomide molecules, Cer/Pom-PEG@GNPs),Au-S键间的强相互作用增强了金纳米颗粒与配体之间的整体稳定性。进入细胞后,具有柔性连接链的Cer/Pom-PEG@GNPs可灵活招募ALK和E3连接酶CRBN,促进两者之间相互作用,实现ALK降解(图 3D)。在人非小细胞肺癌细胞中,Cer/Pom-PEG@GNPs以剂量依赖有效降低ALK融合蛋白水平,在4 μmol/L浓度下超过80%的ALK融合蛋白被降解。Wang团队[70]基于碳点的生物相容性和便于修饰等特征,合成了靶向降解细胞程序性死亡-配体1 (programmed cell death 1 ligand 1, PD-L1)的碳点PROTAC (carbon-dot (CD)-based PROTACs, CDTACs)。通过CDTACs的EPR效应和PD-L1主动靶向,CDTACs与肿瘤细胞膜上PD-L1结合,内吞进入细胞,实现PD-L1降解。在小鼠黑色素瘤细胞和小鼠结肠癌细胞中对PD-L1的降解效率分别达到90%和99%。无机纳米材料递送PROTAC,得益于无机材料独特的物理化学特性,但由于溶解度较低和毒性问题,尤其是重金属使用,使无机纳米载体的临床应用受到限制[66]

3.3 可控激活小分子PROTAC前药

由于PROTAC自身分子结构复杂,因此,在PROTAC前药策略中通常采用结构简单且能够有效脱除的小分子基团修饰PROTAC,以改善PROTAC成药性。例如,采用亲脂性基团修饰PROTAC分子结构中的关键单元,可以提高PROTAC的口服利用度[71]、细胞穿透性[72]等。PROTAC前药通过刺激响应在目标区域激活释放,进而特异性靶向降解目标蛋白。常见的刺激响应条件可分为外源性条件(如光、热、超声等)和内源性条件(如酶、活性氧、谷胱甘肽、pH等)。

光化学治疗是一种非侵入性治疗手段,新型高效光敏剂一直是人们追求的目标[73]。Pan团队[74]在dBET1基础上建立了光敏笼状PROTAC (photo-caged PROTACs, pc-PROTAC)。在dBET1分子上引入“光笼”基团后,pc-PROTAC与BRD4几乎没有结合亲和力,IC50为7.6 μmol/L,而dBET1的IC50仅22 nmol/L。在人B淋巴细胞瘤细胞中,dBET1在0.1 μmol/L浓度下即可有效降解BRD4,而黑暗环境中,pc-PROTAC在3 μmol/L浓度下仍不能降解BRD4。紫外照射后,pc-PROTAC快速转变为具有活性的dBET1,在0.3 μmol/L浓度下显著降低了BRD4水平,显示出强大的光控降解特性。在斑马鱼模型中,紫外光照后,pc-PROTAC与dBET1具有一样的效果,均显著降低BRD4浓度,干扰胚胎早期发育。Wei团队[75]利用E3连接酶CRBN的配体设计合成了一系列光控PROTAC (light-inducible switch on PROTACs, opto-PROTAC),通过光控介导IKZF1/3、BRDs和ALK降解。此外,通过紫外光还可实现分子构型的改变,可控特异性激活PROTAC。Jiang团队[76]基于邻四氟偶氮苯单元构型的变化,设计了光诱导可逆PROTAC (photoswitchable azobenzene-proteolysis targeting chimeras, Azo-PROTAC)。在可见光下,Azo-PROTAC分子为反式构型,能特异性识别并降解BCR-ABL融合蛋白(breakpoint cluster region-abelson leukemia virus, BCR-ABL),紫外光照射后,分子变为顺式构型,降解活力显著降低或消失(图 4A)。在人慢性髓原白血病细胞中,Azo-PROTAC反式构型的IC50为68 nmol/L。为评估Azo-PROTAC可逆性,将反式构型的Azo-PROTAC与细胞共孵育24 h后,将细胞转移到新鲜培养基中并分为2组,一组每隔4 h进行紫外照射,而另一组则不处理。结果发现,紫外照射的实验组BCR-ABL水平升高,而另一组则一直保持在较低水平。Trauner团队[78]也通过靶向BET家族蛋白证明了光照对PROTAC分子活性的影响。光控PROTAC对时间和空间的可逆调控,提高了蛋白降解的精准性,但光的组织穿透性仍是一个问题。此外,正常组织与肿瘤组织间缺乏明显界限,很难找到仅在肿瘤部位激活的光控PROTAC分子。

图 4 可控激活小分子PROTAC前药 Fig. 4 Controllable activation of small-molecular PROTAC prodrug. A:光诱导可逆PROTAC[76]. B:活性氧诱导PROTAC释放[77] A: Light-induced reversible on/off PROTAC[76]. B: ROS-induced release of PROTAC[77].

肿瘤微环境(tumor microenvironment, TME)是肿瘤细胞赖以生存的复杂环境,通常表现为缺氧、弱酸性、高浓度谷胱甘肽和高水平活性氧等[79]。基于此,Chen团队[77]利用活性氧响应激活PROTAC分子降解BET家族蛋白,在人乳腺导管癌细胞中以10 nmol/L的浓度显著降低溴结构域蛋白3 (bromodomain containing protein 3, BRD3)的水平,IC50为48 nmol/L,在人胚肾细胞中未发现BRD3降解,IC50为4 073 nmol/L,表现出显著特异性(图 4B)。Zhu团队[80]利用肿瘤细胞在缺氧条件下,硝基还原酶(nitroreductase, NTR)水平高的特点,开发了用于表皮生长因子受体(epidermal growth factor receptor, EGFR)降解的PROTAC 17-1分子。未添加NTR时,PROTAC 17-1分子对人非小细胞肺癌细胞的IC50为201 nmol/L,添加NTR后,PROTAC 17-1被激活,IC50为18 nmol/L,抗增殖活性提高了10倍以上。在小鼠模型中,PROTAC 17-1表现出良好血浆稳定性,给药0.08 h后达到最大血浆浓度,且药物半衰期达到11 h。治疗21 d后,小鼠肿瘤抑制率为86%,未观察到明显体重下降和组织毒性,生物安全性显著提高。Xing团队[81]在E3连接酶CRBN的配体泊马度胺上修饰半胱氨酸和硝基还原酶响应基团,其在肿瘤缺氧条件下暴露半胱氨酸基团,随后与2-氰基苯并噻唑(2-cyanobenzothiazole, CBT)修饰的JQ1 (JQ1-CBT)发生生物正交反应,原位生成具有降解能力的PROTAC分子J252。局部生成的J252特异性降低BRD4的水平,进而改变遗传靶标表达,精准调控细胞活性。在小鼠肿瘤模型中,NTR激活的JW4+JQ1-CBT分子,显著降低了BRD4水平,并伴随HIF-1α和c-Myc癌基因的下调,显著抑制肿瘤生长和减少肿瘤血管面积。利用肿瘤微环境差异,特异性激活PROTAC分子,能最大限度减少对正常细胞的毒性,提高PROTAC分子靶向性。

4 总结与展望

PROTAC技术经过20多年的不断完善和发展,目前已发展出VHL和CRBN两种最主要的E3泛素连接酶配体,同时结合新型小分子抑制剂的开发,越来越多致病蛋白可以通过PROTAC技术成功降解。近年来,利用靶向药物递送系统可进一步改善PROTAC成药性,具有一定优势的同时也存在如下不足:(1) 识别分子介导靶向递送PROTAC充分利用了抗体等的靶向性和药代动力学特性,选择性将药物直接输送到病变部位并响应性释放PROTAC分子,实现目标蛋白降解,减少正常组织毒副作用。然而,目前难以找到在靶细胞上显著高表达但在正常组织中低表达的理想抗原,且抗体与相关抗原结合后存在有效内化问题,以及内化后有效释放PROTAC问题。(2) 纳米材料介导靶向递送PROTAC可提高PROTAC稳定性和溶解性,促进药物透过生理屏障,延长血液循环时间,使药物获得更好的有效性和安全性。但目前进入临床试验的例子很少,主要原因为纳米载体的组成复杂,质量难以把控。此外,纳米材料的EPR效应对临床肿瘤的适用性有待进一步考察。(3) 可控激活小分子PROTAC前药在细胞层面可提高细胞穿透性、降低细胞毒性,但目前大多研究缺乏体内实验数据。PROTAC本身结构庞大,在平衡前药释放特异性和稳定性时需大量化学合成工作,额外修饰基团的引入可能进一步恶化其物理化学特性。

综上所述,分子量大是PROTAC成药性差的主要原因,需进一步平衡PROTAC分子的三段式结构。通过PROTAC降解目标蛋白需在空间上拉近E3连接酶与目标蛋白距离,生成稳定三元复合物。在选择合适E3连接酶配体、目标蛋白配体的同时,优化PROTAC分子结构中连接链的长短和类型,可确保目标蛋白降解效率的同时降低PROTAC的整体分子量。此外,Heightman团队[82]利用点击化学在胞内原位生成PROTAC (in-cell click-formed proteolysis targeting chimeras, CLIPTAC)为降低PROTAC分子量提供了较好的研究思路。利用生物正交基团和反式基团分别修饰E3连接酶配体和目标蛋白配体,得到2种PROTAC前体。当第一种PROTAC前体分子在细胞中累积后,在加入第二种PROTAC前体分子,使两者在胞内原位发生生物正交反应,生成具有活性的完整PROTAC分子。尽管减少了PROTAC的分子量,但PROTAC前体分子可能在细胞外发生正交反应,使PROTAC的整体治疗效率降低。因此,明确2种PROTAC前体的药代动力学特性,进而确定最优的给药方案,在未来值得进一步探索。此外,由于2种PROTAC前体缺乏细胞靶向能力,易造成脱靶毒性,需进一步提高2种PROTAC前体的靶向性。

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