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Research Articles: Therapeutics, Targets, and Development

Modulation of the antitumor activity of metronomic cyclophosphamide by the angiogenesis inhibitor axitinib

Division of Cell and Molecular Biology, Department of Biology, Boston University, Boston, Massachusetts

Requests for reprints: David J. Waxman, Division of Cell and Molecular Biology, Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215. Fax: 617-353-7404. E-mail: djw{at}bu.edu

Abstract

The promising but still limited efficacy of angiogenesis inhibitors as monotherapies for cancer treatment indicates a need to integrate these agents into existing therapeutic regimens. Presently, we investigate the antitumor activity of the small-molecule angiogenesis inhibitor axitinib (AG-013736) and its potential for combination with metronomic cyclophosphamide. Axitinib significantly inhibited angiogenesis in rat 9L tumors grown s.c. in scid mice but only moderately delayed tumor growth. Combination of axitinib with metronomic cyclophosphamide fully blocked 9L tumor growth on initiation of drug treatment. In contrast, metronomic cyclophosphamide alone required multiple treatment cycles to halt tumor growth. However, in contrast to the substantial tumor regression that is ultimately induced by metronomic cyclophosphamide, the axitinib/cyclophosphamide combination was tumor growth static. Axitinib did not inhibit hepatic activation of cyclophosphamide or export of its activated metabolite, 4-hydroxy-cyclophosphamide (4-OH-CPA), to extrahepatic tissues; rather, axitinib selectively decreased 9L tumor uptake of 4-OH-CPA by 30% to 40%. The reduced tumor penetration of 4-OH-CPA was associated with a decrease in cyclophosphamide-induced tumor cell apoptosis and a block in the induction of the endogenous angiogenesis inhibitor thrombospondin-1 in tumor-associated host cells, which may contribute to the absence of tumor regression with the axitinib/cyclophosphamide combination. Finally, axitinib transiently increased 9L tumor cell apoptosis, indicating that its effects are not limited to the endothelial cell population. These findings highlight the multiple effects that may characterize antiangiogenic agent/metronomic chemotherapy combinations and suggest that careful optimization of drug scheduling and dosages will be required to maximize antitumor responses. [Mol Cancer Ther 2008;7(1):79–89]

Grant support: NIH grant CA49248 (D.J. Waxman).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Supplementary material for this article is available at Molecular Cancer Therapeutics Online (http://mct.aacrjournals.org/).

Received 8/21/07; revised 11/ 9/07; accepted 12/ 4/07.