Low-Dose Metronomic Daily Cyclophosphamide and Weekly Tirapazamine: A Well-Tolerated Combination Regimen with Enhanced Efficacy That Exploits Tumor Hypoxia
The recent clinical successes of antiangiogenic drug-based therapies have also served to highlight the problem of acquired resistance because, similar to other types of cancer therapy, tumors that initially respond eventually stop doing so. Consequently, strategies designed to delay resistance or treat resistant subpopulations when they arise have assumed considerable importance. This requires a better understanding of the various possible mechanisms for resistance. In this regard, reduced oxygenation is thought to be a key mediator of the antitumor effects of antiangiogenic therapies; accordingly, increased hypoxia tolerance of the tumor cells presents a potential mechanism of resistance. However, hypoxia can also be exploited therapeutically through the use of hypoxic cell cytotoxins, such as tirapazamine. With this in mind, we measured the oxygenation of PC-3 human prostate cancer xenografts subjected to chronic low-dose metronomic (LDM) antiangiogenic chemotherapy using cyclophosphamide given through the drinking water. We found that LDM cyclophosphamide impairs the oxygenation of PC-3 xenografts even during relapse, coinciding with reduced microvessel density. Combination of LDM cyclophosphamide with tirapazamine results in significantly improved tumor control in the PC-3, HT-29 colon adenocarcinoma, and MDA-MB-231 breast cancer human xenograft models without having a negative effect on the favorable toxicity profile of LDM cyclophosphamide. These results provide further evidence that reduced vascular dependence/increased hypoxia tolerance may be a basis for eventual resistance of tumors exposed to long-term LDM chemotherapy.