急性髓系白血病(acute myeloid leukemia, AML)是由于造血干细胞分化障碍及增殖过度所致异常骨髓造血前体细胞聚集的恶性克隆性疾病,在传统治疗下,近年来大部分临床研究协作组的儿童AML总体生存率(overall survival, OS)可达60%,5年无事件生存率(event-free survival, EFS)超过50%[1]。但常规化疗对于进一步提高初治完全缓解率(complete remission, CR)以及克服缓解后复发的作用有限,且治疗所引起相关合并症的发生率和死亡率较高[2]。因此,了解AML细胞遗传学特征和相关基因突变对预后的影响,并靶向针对性地改进治疗方案,有助于增加疗效、减轻毒性,进而减少复发和改善长期生存率。本文就AML关键信号靶点和表观遗传修饰等治疗机制及其靶向治疗药物作一综述。
1 信号靶点及靶向治疗Fms样酪氨酸激酶3(Fms-like tyrosine kinase 3, FLT3)属于Ⅲ型受体酪氨酸激酶,高表达于造血干细胞,并通过与配体结合发生二聚体化磷酸化,从而激活下游信号通路以调节细胞生长和凋亡。20%~25%的AML存在FLT3突变,约2/3的突变发生于跨膜区内部的串联重复序列(internal tandem duplication, ITD),使得FLT3可在无配体情况下异常激活[3-4]。FLT3突变者化疗完全缓解率低、复发率高,总体生存期短,且预后与等位基因突变比例相关[5-6]。
酪氨酸激酶抑制剂(tyrosine kinase inhibitor, TKI)是临床应用较多的靶向治疗药物,Ⅰ代药物中包括索拉非尼、米哚妥林、来他替尼等,但其有效性和靶向性不如Ⅱ代FLT3-ITD抑制剂奎扎替尼和喹诺酮苄脒衍生物(crenolanib)[7]。儿童白血病及淋巴瘤治疗进展协会(therapeutic advances in childhood leukemia and lymphoma, TACL)将奎扎替尼与阿糖胞苷及依托泊苷联合用于1个月以上21岁以下的复发AML患者,在化疗第1~5天静脉使用阿糖胞苷及依托泊苷,第7~28天口服奎扎替尼,结果存在FLT3-ITD突变的AML患者中有42%达到CR或部分缓解,其余患者病情也无明显进展[8]。体外研究表明,crenolanib作为FLT3抑制剂,对存在FLT3/ITD D835点突变的AML抑制作用优于奎扎替尼,且可降低骨髓抑制风险,关于crenolanib联合索拉菲尼用于儿童复发/难治性AML治疗的安全剂量和毒性目前正在进行临床Ⅰ期试验[9]。临床Ⅰ/Ⅱ期试验显示FLT3抑制剂ASP-2215在AML的作用明显强于上述药物,还可减少骨髓抑制风险,但在儿童AML中的疗效还有待研究[10]。
其他靶点及相关通路包括KIT、RAS、JAK-STAT的突变或异常激活与AML发生发展密切相关。KIT突变常提示预后不良,其酪氨酸激酶抑制剂伊马替尼和达沙替尼在成人AML中应用较多[11-12]。有学者将RAS-ERK信号通路高选择性抑制剂曲马替尼治疗57例复发/难治性成人AML患者,结果显示21%的患者可达到CR,其用于儿童AML的Ⅰ期试验正在进行中[13]。JAK2突变对于AML的发病及预后有重要意义,相应抑制剂鲁索替尼对骨髓增殖性肿瘤继发的AML成人患者有着明显疗效,现已进入Ⅳ期临床试验,而对于儿童AML的疗效有待研究[14]。此外,选择性较高的JAK2抑制剂AZD1480可作为ATP竞争性抑制剂靶向攻击AML细胞而不影响正常造血干细胞,还在动物实验研究阶段[15]。
Polo样激酶在细胞有丝分裂及凋亡中起着重要作用,且在AML细胞高表达,作为潜在治疗靶点,相应的小分子抑制剂volasertib单药或者与小剂量阿糖胞苷联合用于成人复发/难治性AML的治疗均显示出一定疗效[16-17]。近年来,Aurora激酶抑制剂alisertib在联合传统化疗用于成人AML治疗的临床Ⅰ期试验中显示出良好的耐受性,而其在儿童复发/难治性白血病的Ⅱ期试验目前正在进行[18-19]。
2 蛋白酶体抑制剂蛋白酶体可调节胞内诸多调控细胞增殖、凋亡的蛋白质降解,其活性改变与肿瘤发生相关,其抑制剂可干扰细胞增殖信号通路,并且与传统化疗药物具有细胞毒性协同作用[20]。蛋白酶体抑制剂中研究较多的是硼替佐米和卡非佐米。硼替佐米联合传统化疗用于儿童初诊AML患者的III期临床试验显示其并未明显改善患者的EFS或OS,相反可能增加化疗药物的毒性[21]。近来有研究显示,存在FLT3-ITD突变或奎扎替尼耐药的AML患者对硼替佐米较为敏感,可能与其诱导FLT3表达下调进而导致细胞死亡等机制相关[22]。
卡非佐米属于第2代磷酸激酶抑制剂,抑制作用强于硼替佐米,与依托泊苷或环磷酰胺联合治疗儿童复发/难治性白血病及实体瘤的研究尚处于Ⅰ期临床试验阶段[23]。
3 表观遗传修饰表观遗传修饰是在不改变DNA序列的情况下调节基因表达的过程,其调节异常如DNA过甲基化或组蛋白去乙酰化均可能加速癌变,因此可作为靶向治疗的发展方向[24-25]。
甲基转移酶抑制剂阿扎胞苷和地西他滨已被FDA批准用于骨髓增生异常综合征和AML的治疗,单药常用于不宜进行强化疗的AML患者,在临床试验中多与化疗药物联用观察疗效。TACL开展的阿扎胞苷联合传统化疗治疗儿童AML的临床试验显示其具有良好疗效,且作为维持治疗药物用于索拉非尼诱导缓解后的FLT3-ITD突变AML患者,可延长持续缓解时间,提示两者具有协同作用[26]。目前正在开展关于阿扎胞苷联合沙莫司亭用于儿童AML维持治疗的Ⅱ期试验。地西他滨已被批准用于AML治疗,临床试验显示其单药或者与传统化疗联合可使部分患者达到CR,作为初治缓解儿童AML维持治疗的Ⅱ期临床试验正在进行[27]。第二代去甲基化药物SGI-110正在进行Ⅱ期试验,评估其对成人复发/难治性AML的临床疗效[28]。
组蛋白去乙酰化酶抑制剂的Ⅱ期临床试验显示,伏立诺他联合去甲氧基柔红霉素及阿糖胞苷可提高初诊儿童AML患者的缓解率[29];联合去甲基化药物治疗AML的Ⅱ期临床试验显示,两者也可能存在协同作用[28, 30]。且组蛋白甲基化过程也参与基因表达和AML的发生发展,其Ⅰ代抑制剂EPZ4777和Ⅱ代衍生物EPZ5676主要用于混合谱系白血病的治疗[30-31]。
4 免疫治疗单克隆抗体靶向药物可识别肿瘤细胞特定抗原,直接作用于肿瘤细胞。CD33抗原表达于85%~90%的AML细胞表面,且低表达于正常造血细胞,因此成为AML靶向治疗极具价值的研究目标[32]。吉妥珠单抗(gemtuzumab ozogamacin, GO)是人源化抗CD33单抗与细胞毒药物刺孢霉素组成的免疫偶联毒素,可切割DNA双链的特定序列而诱导细胞凋亡[14]。COG开展的III期临床试验将GO与传统化疗联合应用于825例儿童AML患者,结果显示CD33高表达者的EFS为53%,而CD33低表达者为41%;并且以183例FLT3-ITD突变的AML儿童患者为研究对象,其中接受传统化疗的复发率59%,联合GO治疗的复发率37%,显示GO可减少FLT3-ITD阳性患儿的复发风险[33-35]。但GO易发生耐药,且存在诸如肝静脉闭塞等药物不良反应,而正在临床开展I期试验的SGN-CD33A对成人AML的治疗疗效是GO的3倍以上,不良反应仅限于粒细胞减少,可能作为GO的替代药物,但其在儿童中的作用尚待进一步研究[14]。去甲基化药物可增加AML细胞CD33表达,SGN-CD33A联合阿扎胞苷或地西他滨用于成人AML治疗的III期临床试验也在开展中[36]。此外,CD3和CD33抗原的双特异性T细胞衔接抗体AMG330在AML体外试验中也显示出良好效应[37-38]。
近年来嵌合抗原受体T细胞免疫疗法(chimeric antigen receptor T-cell immunotherapy, CAR-T)成为靶向治疗研究热点,CD123作为AML相关抗原,因其高表达于髓系白血病干细胞而作为AML潜在治疗靶点,CAR-T 123可特异性杀伤AML细胞,但存在骨髓抑制风险,因此目前正在开展以CD3作为免疫效应细胞靶向抗原的CD3*CD123双亲和性多重靶向免疫治疗的I期临床试验[39]。此外,Chien等[40]研究提示,针对FLT3的新CAR-T疗法在体内外试验中均显示出良好的抗白血病效应,但有诱发再生障碍性贫血的风险。目前CAR-T技术用于AML患者的治疗在疗效方面存在诸多不确定性,特异性高表达于AML的抗原还有待进一步发掘。
程序性细胞凋亡1受体(programmed death-1 receptor, PD-1)及其配体PD-L1表达于肿瘤细胞表面,其异常调控与肿瘤免疫逃逸、发生及发展相关,免疫调定点抑制剂的I/II期临床试验目前正在成人AML患者开展[41]。免疫药物来那度胺联合阿扎胞苷用于儿童AML患者缓解后维持治疗,以及WT1基因和放射免疫疗法等也可作为AML靶向治疗的发展方向[42]。
5 靶向治疗副作用分子靶向药物在AML治疗中展现出良好的疗效和耐受性,大部分靶向治疗药物的毒副作用可控且小于传统细胞毒类药物,在应用过程中产生诸如皮肤毒性、胃肠道损害、高血压、骨髓抑制及肝静脉闭塞等不良反应,但也有部分靶向药物在临床试验中产生致命性不良反应,如高强度的免疫激活可引起细胞因子大量释放,进而导致细胞因子释放综合征(cytokine release syndrome, CRS),对呼吸、心血管及中枢神经等系统造成不同程度的损害,且有文献报道CRS的程度可能与患者肿瘤负荷相关[43]。
6 结语AML治疗的最终目标是最大限度地提高生存率。靶向治疗对复发/难治性AML患者预后的改善提供了更多更好的选择。
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