A comparative analysis of low-dose metronomic cyclophosphamide reveals absent or low-grade toxicity on tissues highly sensitive to the toxic effects of maximum tolerated dose regimens.

The survival benefits of traditional maximum tolerated dose (MTD) cytotoxic therapy have been modest for the treatment of most types of metastatic malignancy and, moreover, often come with increased acute and chronic toxicity. Recent studies have demonstrated that the frequent administration of comparatively low doses of cytotoxic agents, with no extended breaks [low-dose metronomic (LDM) chemotherapy], may not only be at least as efficient as MTD therapy but also less toxic. This coincides with an apparent selectivity for "activated" endothelial cells of the tumor vasculature. However, the impact of LDM chemotherapy on the most sensitive target cell populations normally affected by MTD therapy (i.e., bone marrow progenitors, gut mucosa, and hair follicle cells) has not been analyzed in experimental detail. Therefore, we compared effects of LDM and MTD cyclophosphamide (CTX) on bone marrow and gut mucosa. Furthermore, we studied the potential impact of LDM CTX on angiogenesis in the context of wound healing and evidence of organ toxicity. We show absent or moderate hematologic and intestinal toxicity of LDM as opposed to MTD CTX. Of note was the finding of sustained lymphopenia, which is not unexpected given the use of CTX as immunosuppressive drug. There was no negative impact on wound healing or evidence of organ toxicity. LDM offers clear safety advantages over conventional MTD chemotherapy and therefore would appear to be ideal for long-term combination therapy with targeted antiangiogenic drugs.