Catalytic Inhibitors of Topoisomerase II Differently Modulate the Toxicity of Anthracyclines in Cardiac and Cancer Cells

Anthracyclines (such as doxorubicin or daunorubicin) are among the most effective anticancer drugs, but their usefulness is hampered by the risk of irreversible cardiotoxicity. Dexrazoxane (ICRF-187) is the only clinically approved cardioprotective agent against anthracycline cardiotoxicity. Its activity has traditionally been attributed to the iron-chelating effects of its metabolite with subsequent protection from oxidative stress. However, dexrazoxane is also a catalytic inhibitor of topoisomerase II (TOP2). Therefore, we examined whether dexrazoxane and two other TOP2 catalytic inhibitors, namely sobuzoxane (MST-16) and merbarone, protect cardiomyocytes from anthracycline toxicity and assessed their effects on anthracycline antineoplastic efficacy. Dexrazoxane and two other TOP2 inhibitors protected isolated neonatal rat cardiomyocytes against toxicity induced by both doxorubicin and daunorubicin. However, none of the TOP2 inhibitors significantly protected cardiomyocytes in a model of hydrogen peroxide-induced oxidative injury. In contrast, the catalytic inhibitors did not compromise the antiproliferative effects of the anthracyclines in the HL-60 leukemic cell line; instead, synergistic interactions were mostly observed. Additionally, anthracycline-induced caspase activation was differentially modulated by the TOP2 inhibitors in cardiac and cancer cells. Whereas dexrazoxane was upon hydrolysis able to significantly chelate intracellular labile iron ions, no such effect was noted for either sobuzoxane or merbarone. In conclusion, our data indicate that dexrazoxane may protect cardiomyocytes via its catalytic TOP2 inhibitory activity rather than iron-chelation activity. The differential expression and/or regulation of TOP2 isoforms in cardiac and cancer cells by catalytic inhibitors may be responsible for the selective modulation of anthracycline action observed.

蒽环类药物（anthracyclines，如多柔比星（doxorubicin）或柔红霉素（daunorubicin））是目前临床应用最广泛的高效抗肿瘤药物之一，但其临床使用受到不可逆心脏毒性风险的严重限制。右雷佐生（dexrazoxane, ICRF-187）是目前唯一获批用于对抗蒽环类药物心脏毒性的临床心脏保护剂。传统观点认为，其心脏保护活性源于其代谢产物的铁螯合效应，进而阻断氧化应激损伤。然而，右雷佐生同时也是拓扑异构酶II（TOP2）的催化抑制剂。为此，本研究考察了右雷佐生与另外两种TOP2催化抑制剂——索布佐生（sobuzoxane, MST-16）和美巴隆（merbarone），能否保护心肌细胞免受蒽环类药物的毒性侵害，并评估了这些化合物对蒽环类药物抗肿瘤活性的影响。

实验结果表明，右雷佐生与另外两种TOP2抑制剂均可有效保护分离的新生大鼠心肌细胞，抵御多柔比星与柔红霉素诱导的细胞毒性。但在过氧化氢诱导的心肌细胞氧化损伤模型中，所有受试的TOP2抑制剂均未表现出显著的保护作用。与之形成对比的是，这些TOP2催化抑制剂并未削弱蒽环类药物在HL-60白血病细胞系中的抗增殖效果，反而大多呈现出协同交互作用。此外，TOP2抑制剂对蒽环类药物诱导的半胱天冬酶激活的调控作用，在心肌细胞与癌细胞中存在显著差异。

进一步实验发现，右雷佐生经水解后可有效螯合细胞内不稳定铁离子，而索布佐生与美巴隆均未表现出此类铁螯合活性。综上，本研究数据提示，右雷佐生对心肌细胞的保护作用可能主要通过其TOP2催化抑制活性实现，而非铁螯合能力。催化抑制剂对心肌细胞与癌细胞中TOP2同工型的差异化表达及/或调控，或许是造成本研究中观察到的蒽环类药物作用选择性调控的核心机制。