Less is more, regularly: metronomic dosing of cytotoxic drugs can target tumor angiogenesis in mice

Chemotherapeutic drugs, long the mainstay of cancer treatment, cause DNA damage and disrupt DNA replication in proliferating cells. Drug regimens have been designed to kill as many tumor cells as possible by treating with “maximum tolerated doses” (MTDs) of these cytotoxic agents. Side effects such as neurotoxicity and damage to proliferating cells in healthy tissues pose serious constraints on the use of chemotherapy. In an effort to balance toxicity with efficacy, a conventional dosing schedule calls for episodic application of a cytotoxic drug at or near the MTD, followed by periods of rest to allow normal tissues to recover. Many such chemotherapy regimens are initially efficacious, resulting in tumor regression or stabilization and prolonged survival. In rare cases, cures are achieved. In general, however, responses are short-lived, with relapses often marked by aggressive cancers that are resistant to the cytotoxic drug. Furthermore, the standard MTD regimen as a rule seriously impairs quality of life. The harsh side effects and the ultimate failures of most chemotherapies have fueled broad investigation of alternatives, including drugs that target not the transformed tumor cells themselves, but rather a genetically stable constituent cell type of tumors, the endothelial cells that form blood vessels. Angiogenesis, the process by which new blood vessels are formed, is a hallmark capability of cancer (1); a compelling body of evidence argues that tumor growth depends on the vasculature, and, in particular, on continuing angiogenesis (2, 3). More than two dozen new drugs that are in or soon to enter clinical trials appear to interfere with tumor angiogenesis (3, 4); there is considerable anticipation about their benefits in treating cancer. Now, two studies suggest a potentially complementary strategy of rescheduling the administration of classical cytotoxic drugs in order to target tumor endothelial cells. It is well established that tumor-associated endothelial cells proliferate during chronic angiogenesis in tumors, albeit at lower frequencies than the tumor cells themselves. Apparently because of their lower rate of cell division, replication of these endothelial cells is only weakly disrupted by the episodic regimens of standard chemotherapeutic protocols. In these two new studies, however, cytotoxic drugs were administered routinely, to target the slowly proliferating tumor endothelial cells and abrogate their apparent capability to repair and recover during the usual rest periods. Both groups worked with mice bearing subcutaneous tumors, and each presents data suggesting that “metronomic” dosing regimens—either continuous infusion or frequent administration without extended rest periods—could have real value in the clinic. Both also demonstrated combinatorial effects of such altered cytotoxic drug regimens with newer, more specific angiogenesis inhibitors.