摘要
以顺铂为代表的二价铂[Pt(Ⅱ)]类药物是现在活跃于一线的抗肿瘤药物,但Pt(Ⅱ)类药物存在不良反应大、生物利用度不佳以及耐药性等问题。四价铂[Pt(Ⅳ)]络合物是Pt(Ⅱ)在轴向位置进行不同取代的衍生物,在肿瘤还原性物质的作用下Pt(Ⅳ)可被还原为Pt(Ⅱ),因此Pt(Ⅳ)可作为Pt(Ⅱ)的前药。Pt(Ⅳ)中轴向取代基的引入可改善Pt(Ⅱ)类药物的药代动力学性质、选择性和生物活性,以及实现除DNA交联之外的附加细胞毒机制,可一定程度上克服Pt(Ⅱ)类药物的耐药性。本文归纳总结了铂类药物的耐药机制,包括铂转运增加、解毒能力增加、自噬增强和DNA修复增强等方面;综述了Pt(Ⅳ)前药的构效关系、主要类型及研究进展,并提出了克服铂类药物耐药性的可能途径。
20世纪60年代顺铂抗肿瘤活性的发现成为无机化学领域的一个里程

Figure 1 Structures of platinum drugs
顺铂主要通过主动运输(通过铜转运蛋白ctr1)或被动转运进入细胞后靶向核DN
顺铂虽使用至今,但也存在诸多缺点,比如顺铂水解产生的活性Pt(Ⅱ)水络合物与蛋白质反应,可能会导致生物利用度不佳、难以穿过细胞膜、出现不良反应乃至失活等问题。此外,如何将顺铂或者铂类药物靶向递送至患者的肿瘤组织仍是亟待解决的问题。此外,顺铂最大缺点是其耐药性,耐药性取决于癌症的类型,例如睾丸癌、卵巢癌、头颈癌和小细胞肺癌对顺铂较为敏感,而非小细胞肺癌和结直肠癌则较容易对顺铂产生耐药,耐药性产生后顺铂的治疗效果急速下
四价铂[Pt(Ⅳ)]前药是解决这些问题的有效方法之
1.1.1 溶质载体超家族(solute carrier superfamily, SLCs)铂类药物在细胞内的积累是细胞毒性的必要保证,因此铂类药物产生耐药性的主要原因之一是减少药物内流或增加药物外排。SLCs包含300多个成员和65个亚家
铜(Cu)是细胞增殖和血管生成所必需的元素,其转运需要高度特异性的蛋白质-蛋白质相互作用。在人体内,抗氧化剂1铜伴侣蛋白(antioxidant 1 copper chaperone,atox1)介导Cu(I)向P型ATP酶atp7a和atp7b的转运,后两种蛋白负责铜转运和过量的铜外流。Lasorsa
谷胱甘肽(glutathione,GSH)在细胞氧化还原状态中起着至关重要的作用,它可以清除自由基,保护细胞免受外源性物质的侵害并维持多种蛋白质的巯基处于还原状态。GSH的活性巯基基团对铂具有很高的亲和力,从而使GSH成为一个简单有效的铂清除
自噬(autophagy)是一种将受损的细胞器、错误折叠的蛋白及其他大分子物质等运送至溶酶体降解并再利用的过程。自噬是广泛存在于真核细胞的现象,并且可分为巨自噬、微自噬和分子伴侣介导的自噬3大类。这是一个受到精密调控的步骤,此步骤是细胞生长、发育与稳态中的常规步骤,帮助细胞产物在合成、降解以及接下来的循环中保持一个平衡状态。铂类药物治疗后,在铂耐药细胞中观察到药物诱导的自噬增加,同时基础自噬也有增
最近的研究发现硫氧还原蛋白(thioredoxin-1,Trx-1)是缺氧诱导因子1α(hypoxia-inducible factor 1-alpha,HIF1α)的上游调节剂。Trx-1通过增强与Ref-1的相互作用来增加HIF1α的表达和结合能力,抑制Trx-1/Ref-1可以加强氧化磷酸化,并逆转对顺铂的耐药
顺铂是一种方形平面Pt(Ⅱ)络合物,具有两个顺式胺配体和两个氯代配体。Pt-N键在热力学上是稳定的,而Pt-Cl键是半不稳定的。因此,该氯代配体可以被水(H2O)或被其他亲核试剂缓慢取

Figure 2 Coordination chemistry of platinum (II) and platinum (IV)
尽管细胞中有许多还原剂,但通常认为抗坏血酸和GSH是还原Pt(Ⅳ)前药的细胞还原剂。抗坏血酸是一种双电子还原剂,具有还原水溶液中的Pt(Ⅳ)配合物的能力。抗坏血酸在血浆中的浓度较低(50 ~ 150 μmol/L),在中性粒细胞的细胞质中浓度较高(约1 mmol/L
Pt(Ⅳ)络合物的抗癌活性通常归因于它们还原为Pt(Ⅱ)产物。因此,Pt(Ⅳ)到Pt(Ⅱ)的转化对于Pt(Ⅳ)络合物的生物学活性来说是十分重要的。细胞内还原剂(例如GSH和抗坏血酸),通常作为还原性物质促进Pt(Ⅳ)的还原。Pt(Ⅳ)脱除两个轴向配体还原为Pt(Ⅱ)是不可逆的过程,Pt(Ⅳ)络合物的还原速率取决于轴向配体和平面配体的性
首先,轴向配体还原速率由小到大依次为:OH ˂ OCOCH3 ˂ Cl ˂ OCOCF3(
* en= ethylenediamine; k: Reduction rate constant
其次,平面配体的大小影响Pt(Ⅳ)络合物的还原速率,平面配体越大则还原速率越快。例如,抗坏血酸不会还原[Pt(en)2(OH)2Cl2],但很容易还原[Pt(ipa)2(OH)2Cl2](ipa=异丙胺)配体, 原因是异丙胺的基团更大。需要指出的是,具有更快还原速率的化合物有着更高的细胞毒性活性,但可能也会带来了较差的代谢表现。
传统Pt(Ⅳ)前药的轴向配体一般无细胞毒性,主要注重于改善Pt(Ⅳ)前药的摄取、代谢及生物分布。奥马铂(ormaplatin)是第一个开展临床试验Pt(Ⅳ)前药(

Figure 3 Structures of ormaplatin, iproplatin and LA-12
另一个进入临床研究的Pt(Ⅳ)配合物是异丙铂(iproplatin),也称为JM9或CHIP(
LA-12是一种金刚烷胺铂衍生物,相比顺铂,LA-12的氨基被金刚胺取代,轴向配体为乙酸根(
上述进入临床的3个Pt(Ⅳ)前药均没有进入到上市阶段,原因是高毒性以及疗效未超过顺铂。传统的Pt(Ⅳ)前药只通过平面或轴向配体来改善其水溶性或者脂溶性,依旧是主要依靠铂离子与DNA的结合来实现对肿瘤细胞的抑制,在安全性和疗效上均有待实现突破。
近年来,具有新颖结构的新一代Pt(Ⅳ)前药和不同作用机制的Pt(Ⅳ)前药进展迅速,受到了众多研究者的青睐。
Zheng

Figure 4 Pt(IV) prodrug designed to bind non-covalently to HSA
研究人员设计了许多Pt(Ⅳ)类化合物,以最大限度地提高细胞对前药的吸收,进而导致DNA铂化的增加,提高细胞毒性。增强肿瘤细胞中药物的积累、减少顺铂失活的一个重要方法是促进Pt(Ⅳ)前药摄取并减少外排。耐药癌细胞系与高GSH水平有关,GSH与顺铂共价结合,结合物从细胞中排出。谷胱甘肽S-转移酶(glutathione S-transferase,GST)催化还原型谷胱甘肽与外源性物质的结合,因此被认为与顺铂耐药有关。Zanellato

Figure 5 Structures of ethacrynic acid and ethacraplatin
组蛋白去乙酰化酶(histone deacetylase,HDAC)是一种锌金属酶,可从组蛋白的赖氨酸残基中去除乙酰基,从而使DNA更紧密地包裹在组蛋白周围,形成封闭、不易接近的染色质形式。抑制HDAC可促进染色质的开放形式,从而使核小体DNA更容易接近。HDAC抑制剂(HDACi)与顺铂联合给药可产生开放结构DNA,促进和增强DNA铂化,从而提高细胞毒性。Kasparkova

Figure 6 Structures of compound 2 and 3
顺铂可以与同一DNA链上相邻的两个鸟嘌呤结合,从而将铂嵌合到DNA中。DNA损伤修复系统检测到畸变,便通过核苷酸切除修复(nucleotide excision repair,NER)从DNA中去除铂。许多对顺铂耐药的癌细胞系具有比顺铂敏感癌细胞系更有效的DNA修复系

Figure 7 Structures of NERi, NERi-Pt(IV), CX-Pt, mitaplatin and COX-Pt
酪蛋白激酶Ⅱ(casein kinase II,CK2)在许多肿瘤的发生、发展和转移中起重要作用,它通过调节β-连环蛋白(β-catenin)的表达来调节Wnt信号传导。有文献表明CK2在修复DNA的单链和双链断裂中起关键作
由于癌细胞的新陈代谢改变,即使在有氧条件下也偏向于进入无氧酵解通路。
二氯乙酸(dichloroacetic acid,DCA)通过抑制丙酮酸脱氢酶激酶(pyruvate dehydrogenase kinases,PDK),从而提高丙酮酸脱氢酶的活性,使得丙酮酸变成乙酰辅酶A从而进入三羧酸循环,抑制了无氧酵解通路进而促进癌细胞凋亡。Xue
环氧合酶(cyclooxygenase, COX)是一种参与肿瘤发生的酶,与肿瘤细胞对铂类药物的耐药性有关。顺铂与COX 抑制剂(吲哚美辛、布洛芬)在联合治疗中观察到优异的协同作用。Neumann
Oliveira

Figure 8 Platinum-triggered bond-cleavage of pentynoyl amide and N-propargyls
与此同时,本课题组也发现了Pt(Ⅱ)触发的丙炔基生物正交裂解反应,有意思的是同时还发现Pt(Ⅳ)无法催化此生物正交反应。基于此提出了一种一体化生物正交Pt(Ⅳ)前药的策略,并设计了整合型前药化合物4,其轴向配体为

Figure 9 Integrated bioorthogonal prodrugs
到目前为止,顺铂、卡铂和奥沙利铂等铂类药物已经在临床使用多年并取得了较好的肿瘤治疗效果。但是在长期服用铂类药物的过程中,耐药性是铂类药物面临的主要难题。在对铂类药物耐药机制的研究中发现,其耐药机制主要分为铂的转运增加和解毒能力增加,其中Pt转运蛋白和DNA损伤修复受到了最多的关注。科研工作者对铂类药物耐药性的解决做出了积极探索,Pt(Ⅳ)前药有望成为铂类药物的一种新选择,新兴的Pt(Ⅳ)前药能实现双重乃至三重细胞毒作用,无论是抑制DNA修复的Pt(Ⅳ)前药,或者是抑制PDK、COX等与肿瘤密切相关蛋白的Pt(Ⅳ)前药,这些工作都对克服铂类药物耐药性进行了积极的探索。虽然目前尚未有Pt(Ⅳ)类药物上市,但相关研究正不断地取得进展,不仅提高了Pt(Ⅳ)前药的稳定性,改善了体内代谢,而且部分Pt(Ⅳ)药物已具有较高的成药性。另外,笔者所在课题组发现铂不仅可以作为抗肿瘤药,Pt(Ⅱ)还可以在体内催化生物正交反应的发生,而Pt(Ⅳ)则不能,并基于此提出了一体化生物正交Pt(Ⅳ)前药的概念,丰富了Pt(Ⅳ)类药物的应用范围,为克服铂类药物的耐药性提供了新的思路。
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