Siim B G, Menke D R, Dorie M J, Brown J M
Department of Radiation Oncology, Stanford University School of Medicine, California 94305-5468, USA.
Cancer Res. 1997 Jul 15;57(14):2922-8.
Tirapazamine (TPZ) is a hypoxia-selective bioreductive drug currently in Phases II and III clinical trials with both radiotherapy and chemotherapy. The response of tumors to TPZ is expected to depend both on the levels of reductive enzymes that activate the drug to a DNA-damaging and toxic species and on tumor oxygenation. Both of these parameters are likely to vary between individual tumors. In this study, we examined whether the enhancement of radiation damage to tumors by TPZ can be predicted from TPZ-induced DNA damage measured using the comet assay. DNA damage provides a functional end point that is directly related to cell killing and should be dependent on both reductive enzyme activity and hypoxia. We demonstrate that TPZ potentiates tumor cell kill by fractionated radiation in three murine tumors (SCCVII, RIF-1, and EMT6) and two human tumor xenografts (A549 and HT29), with no potentiation observed in a third xenograft (HT1080). Overall, there was no correlation of radiation potentiation and TPZ-induced DNA damage in the tumors, except that the nonresponsive tumor xenograft had significantly lower levels of DNA damage than the other five tumor types. However, there was a large tumor-to-tumor variability in DNA damage within each tumor type. This variability appeared not to result from differences in activity of the reductive enzymes but largely from differences in oxygenation between individual tumors, measured using fluorescent detection of the hypoxia marker EF5. The results, therefore, suggest that the sensitivity of individual tumors to TPZ, although not necessarily the response to TPZ plus radiation, might be assessed from measurements of DNA damage using the comet assay.
替拉扎明（TPZ）是一种对缺氧具有选择性的生物还原药物，目前正处于放疗和化疗联合应用的II期和III期临床试验阶段。预计肿瘤对TPZ的反应既取决于将该药物激活为具有DNA损伤性和毒性的还原酶水平，也取决于肿瘤的氧合作用。这两个参数在不同个体肿瘤之间可能会有所不同。在本研究中，我们检测了是否可以通过彗星试验测定的TPZ诱导的DNA损伤来预测TPZ对肿瘤辐射损伤的增强作用。DNA损伤提供了一个与细胞杀伤直接相关的功能性终点，并且应该取决于还原酶活性和缺氧情况。我们证明，TPZ在三种小鼠肿瘤（SCCVII、RIF-1和EMT6）和两种人肿瘤异种移植模型（A549和HT29）中增强了分次放疗对肿瘤细胞的杀伤作用，而在第三种异种移植模型（HT1080）中未观察到增强作用。总体而言，肿瘤中的辐射增强作用与TPZ诱导的DNA损伤之间没有相关性，只是无反应的肿瘤异种移植模型的DNA损伤水平明显低于其他五种肿瘤类型。然而，每种肿瘤类型内的DNA损伤在肿瘤之间存在很大差异。这种差异似乎不是由还原酶活性的差异导致的，而是主要源于使用缺氧标志物EF5的荧光检测法测定的个体肿瘤之间氧合作用的差异。因此，结果表明，尽管不一定是对TPZ加放疗的反应，但可以通过彗星试验测量DNA损伤来评估个体肿瘤对TPZ的敏感性。
