No abstract

The contribution of bone marrow-derived circulating endothelial progenitor cells (CEPs) to tumor angiogenesis has been controversial, primarily because of their low numbers in blood vessels of untreated tumors. We show that treatment of tumor-bearing mice with vascular disrupting agents (VDAs) leads to an acute mobilization of CEPs, which home to the viable tumor rim that characteristically remains after such therapy. Disruption of this CEP spike by antiangiogenic drugs or by genetic manipulation resulted in marked reductions in tumor rim size and blood flow as well as enhanced VDA antitumor activity. These findings also provide a mechanistic rationale for the enhanced efficacy of VDAs when combined with antiangiogenic drugs.

SUMMARY Several hypotheses have been proposed to explain how antiangiogenic drugs enhance the treatment efficacy of cytotoxic chemotherapy including impairing the ability of chemotherapy-responsive tumors to regrow after therapy. With respect to the latter, we show that certain chemotherapy drugs, e.g. paclitaxel, can rapidly induce pro-angiogenic bone marrow derived circulating endothelial cell (CEP) mobilization, and subsequent tumor homing, whereas others, e.g. gemcitabine, did not. Acute CEP mobilization was mediated, at least in part, by systemic induction of SDF-1α and could be prevented by various procedures such as treatment with anti-VEGFR2 blocking antibodies or by paclitaxel treatment in CEP-deficient Id-mutant mice, both of which resulted in enhanced anti-tumor effects mediated by paclitaxel, but not gemcitabine.

An enduring problem in cancer research is the failure to reproduce highly encouraging preclinical therapeutic findings using transplanted or spontaneous primary tumours in mice in clinical trials of patients with advanced metastatic disease. There are several reasons for this, including the failure to model established, visceral metastatic disease. We therefore developed various models of aggressive multi-organ spontaneous metastasis after surgical resection of orthotopically transplanted human tumour xenografts. In this Opinion article we provide a personal perspective summarizing the prospect of their increased clinical relevance. This includes the reduced efficacy of certain targeted anticancer drugs, the late emergence of spontaneous brain metastases and the clinical trial results evaluating a highly effective therapeutic strategy previously tested using such models.

Metronomic antiangiogenic chemotherapy, the prolonged administration of relatively low drug doses, at close regular intervals with no significant breaks, has been mainly studied at the preclinical level using single chemotherapeutic drugs, frequently in combination with a targeted antiangiogenic drug, and almost always evaluated on primary localized tumors. We tested a ''doublet'' combination metronomic chemotherapy treatment using two oral drugs, UFT, a 5-fluorouracil (5-FU) prodrug administered by gavage, and cyclophosphamide, for efficacy and toxicity in a new mouse model of advanced, terminal, metastatic human breast cancer. The optimal biological dose of each drug was first determined by effects on levels of circulating endothelial progenitor cells as a surrogate marker for angiogenesis, which was assessed to be 15 mg/kg for UFT and 20 mg/kg for cyclophosphamide. A combination treatment was then evaluated in mice with advanced metastatic disease using a serially selected metastatic variant of the MDA-MB-231 breast cancer-cell line, 231/ LM2-4. UFT or cyclophosphamide treatment showed only very modest survival advantages whereas a combination of the two resulted in a remarkable prolongation of survival, with no evidence of overt toxicity despite 140 days of continuous therapy, such that a significant proportion of mice survived for over a year. In contrast, this striking therapeutic effect of the combination treatment was not observed when tested on primary orthotopic tumors. We conclude that combination oral low-dose daily metronomic chemotherapy, using cyclophosphamide and UFT, is superior to monotherapy and seems to be a safe and highly effective experimental antimetastatic therapy, in this case, for advanced metastatic breast cancer. (Cancer Res 2006; 66(7): 3386-91)

Cancer stem cells (CSC) are predicted to be critical drivers of tumor progression due to their self-renewal capacity and limitless proliferative potential. An emerging area of research suggests that CSC may also support tumor progression by promoting tumor angiogenesis. To investigate how CSC contribute to tumor vascular development, we used an approach comparing tumor xenografts of the C6 glioma cell line containing either a low or a high fraction of CSC. Compared with CSC-low tumors, CSC-high tumors exhibited increased microvessel density and blood perfusion and induced increased mobilization and tumor recruitment of bone marrow–derived endothelial progenitor cells (EPC). CSC-high C6 cell cultures also induced higher levels of endothelial cell proliferation and tubule organization in vitro compared with CSC-low cultures. CSC-high cultures and tumors expressed increased levels of the proangiogenic factors vascular endothelial growth factor and stromal-derived factor 1, and when signaling by either factor was blocked, all aspects of angiogenesis observed in CSC-high cultures and tumors, including microvessel density, perfusion, EPC mobilization/recruitment, and stimulation of endothelial cell activity, were reduced to levels comparable with those observed in CSC-low cultures/tumors. These results suggest that CSC contribute to tumor angiogenesis by promoting both local endothelial cell activity and systemic angiogenic processes involving bone marrow–derived EPC in a vascular endothelial growth factor–dependent and stromal-derived factor 1–dependent manner.

Chemotherapeutic drugs chronically administered to tumor-bearing mice, using a frequent schedule at doses substantially lower than the maximum tolerated dose (MTD) (i.e., metronomic dosing), can cause sustained and potent antiangiogenic effects by targeting the endothelial cells of newly growing tumor blood vessels. These effects appear to occur in the absence of an increase in the severity of side effects caused by destruction of other cell types normally sensitive to MTD chemotherapy, suggesting a marked and selective sensitivity of activated endothelial cells, the basis of which is unknown. Here we report that protracted exposure of endothelial cells in vitro to low concentrations of several different anticancer agents, including microtubule inhibitors and an alkylating agent, caused marked induction of gene and protein expression of TSP-1, a potent and endothelial-specific inhibitor of angiogenesis. Increases in circulating TSP-1 were also detected in the plasma of human tumor-bearing severe combined immunodeficient mice treated with metronomic low-dose cyclophosphamide. Most importantly, the antiangiogenic and antitumor effects of low-dose continuous cyclophosphamide were lost in TSP-1-null C57BL͞6 mice, whereas, in contrast, these effects were retained by using a MTD schedule of the same drug. Taken together, the results implicate TSP-1 as a secondary mediator of the antiangiogenic effects of at least some low-dose metronomic chemotherapy regimens.endothelial cells ͉ tumor angiogenesis ͉ endogenous inhibitors ͉ TSP-1-null mice

Vascular endothelial cells have been identified as a critical component of the neural stem cell niche, raising the possibility that brain tumor stem-like cells (TSLC) may also rely on signaling interactions with nearby tumor vasculature to maintain their stem-like state. The disruption of such a TSLC vascular niche by an antiangiogenic therapy could result in loss of stemness characteristics associated with intrinsic drug resistance and, thus, preferentially sensitize TSLC to the effects of chemotherapy. Considering these possibilities, we investigated the impact of antiangiogenic anticancer therapy on the TSLC fraction of glioma tumors. Athymic nude mice bearing s.c. tumor xenografts of the C6 rat glioma cell line were treated with either a targeted antiangiogenic agent, antiangiogenic schedules of low-dose metronomic chemotherapy, combination therapies of antiangiogenic agents and chemotherapy, or, for the purpose of comparison, a conventional cytotoxic schedule of maximum tolerated dose chemotherapy using cyclophosphamide. Targeted antiangiogenic therapy or cytotoxic chemotherapy did not reduce the fraction of tumor sphere-forming units (SFU) in the tumor, whereas all treatment groups that combined both antiangiogenic and cytotoxic drug effects caused a significant reduction in SFU. This work highlights the possibility that selective eradication of TSLC may be achieved by targeting the tumor microenvironment (and potentially a supportive TSLC niche) rather than the TSLC directly. Furthermore, this work suggests a possible novel effect of antiangiogenic therapy, namely, as a chemosensitizer of TSLC, and thus represents a possible new mechanism to explain the ability of antiangiogenic therapy to enhance the efficacy of chemotherapy. [Cancer Res 2007;67(8):3560-4]