Kaufmann S H
Oncology Center, Johns Hopkins Hospital, Baltimore, Maryland.
Cancer Res. 1991 Feb 15;51(4):1129-36.
To search for possible synergy between topoisomerase (topo) II-directed chemotherapeutic agents and topo I-directed agents, IL-60 human progranulocytic leukemia cells were incubated with etoposide in the absence or presence of camptothecin (CPT). Treatment of HL-60 cells for 1 h with 15-20 microM etoposide resulted in the death of 99-99.9% of the cells as assessed by colony formation in soft agar. Unexpectedly, simultaneous incubation with 1 microM CPT increased the survival of etoposide-treated cells as much as 30-fold. Inhibition of etoposide cytotoxicity was observed at CPT concentrations as low as 0.01 microM and was one-half maximal at 0.1 microM. CPT also antagonized the cytotoxicity of 4'-(9-acridinylamino)methanesulfon-M-anisidide and daunorubicin, two structurally unrelated topo II-directed agents. Topotecan, a CPT analogue currently undergoing Phase I clinical trials, had a similar effect. Studies using an alkaline unwinding assay (to measure DNA strand breaks) and Western blotting (to assess formation of covalent adducts involving topo II) revealed that CPT did not alter the ability of etoposide to stabilize topo II-DNA adducts. CPT is a potent inhibitor of both DNA and RNA synthesis. To further assess the mechanism by which CPT diminished the cytotoxicity of topo II-directed agents, inhibitors of DNA synthesis or RNA synthesis were substituted for CPT. Aphidicolin, an inhibitor of replicative DNA polymerases, enhanced the survival of etoposide-treated HL-60 cells less than 3-fold. In contrast, inhibitors of RNA synthesis (cordycepin or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) enhanced the survival of etoposide-treated HL-60 cells as much as 20-fold. The potential biological and therapeutic implications of these results are discussed.
为了探寻拓扑异构酶（topo）II靶向化疗药物与topo I靶向药物之间可能存在的协同作用，将IL-60人早幼粒细胞白血病细胞在不存在或存在喜树碱（CPT）的情况下与依托泊苷一起孵育。用15 - 20微摩尔的依托泊苷处理HL-60细胞1小时，通过软琼脂中的集落形成评估，导致99% - 99.9%的细胞死亡。出乎意料的是，与1微摩尔CPT同时孵育使依托泊苷处理的细胞存活率提高了多达30倍。在低至0.01微摩尔的CPT浓度下就观察到对依托泊苷细胞毒性的抑制作用，在0.1微摩尔时达到最大抑制作用的一半。CPT还拮抗了4'-（9-吖啶基氨基）甲磺酰基-M-茴香胺和柔红霉素这两种结构不相关的topo II靶向药物的细胞毒性。拓扑替康，一种目前正在进行I期临床试验的CPT类似物，也有类似作用。使用碱性解旋测定法（测量DNA链断裂）和蛋白质印迹法（评估涉及topo II的共价加合物的形成）进行的研究表明，CPT不会改变依托泊苷稳定topo II - DNA加合物的能力。CPT是DNA和RNA合成的有效抑制剂。为了进一步评估CPT降低topo II靶向药物细胞毒性的机制，用DNA合成抑制剂或RNA合成抑制剂替代CPT。阿非迪霉素，一种复制性DNA聚合酶的抑制剂，使依托泊苷处理的HL-60细胞存活率提高不到3倍。相比之下，RNA合成抑制剂（虫草素或5,6 - 二氯 - 1 - β - D - 呋喃核糖基苯并咪唑）使依托泊苷处理的HL-60细胞存活率提高多达20倍。讨论了这些结果潜在的生物学和治疗意义。
