Therapy-induced acute recruitment of circulating endothelial progenitor cells to tumors

Pathways mediating VEGF-independent tumor angiogenesis

FDA approval of several inhibitors of the VEGF pathway has enabled significant advances in the therapy of cancer and neovascular age-related macular degeneration. However, similar to other therapies, inherent/acquired resistance to anti-angiogenic drugs may occur in patients, leading to disease progression. So far the lack of predictive biomarkers has precluded identification of patients most likely to respond to such treatments. Recent suggest that both tumor and non-tumor (stromal) cell types are involved in the reduced responsiveness to the treatments. The present review examines the role of tumor- as well as stromal cell-derived pathways involved in tumor growth and in refractoriness to anti-VEGF therapies.

Predominant expression of CCL2 at the tumor site of prostate cancer patients directs a selective loss of immunological tolerance to CCL2 that could be amplified in a beneficial manner

We have previously shown that, during inflammatory autoimmune diseases in humans, the immune system develops a neutralizing auto-Ab-based response to a very limited number of inflammatory mediators, and that amplification of each response could be beneficial for the host. Our working hypothesis has been that this selective breakdown of immunological tolerance is due to a predominant expression of an inflammatory mediator at an immune-restricted site undergoing a destructive process. All three conditions also take place in cancer diseases. In this study, we delineate this hypothesis for the first time in a human cancer disease and then explore its clinical implications. We show that in primary tumor sections of prostate cancer subjects, CCL2 is predominantly expressed at the tumor site over other chemokines that have been associated with tumor development, including: CXCL12, CXCL10, CXCL8, CCL3, and CCL5. Subsequently, the immune response selectivity mounts an Ab-based response to CCL2. These Abs are neutralizing Abs. These findings hold diagnostic and therapeutic implications. The current diagnosis of prostate cancer is based on prostate-specific Ag measurements that do not distinguish benign hypertrophy from malignancy. We show in this study that development of anti-CCL2 Abs is selective to the malignant stage. From a clinically oriented perspective, we show, in an experimental model of the disease, that DNA-based amplification of this response suppresses disease, which has implications for a novel way of therapy in humans.

Using optical spectroscopy to longitudinally monitor physiological changes within solid tumors

The feasibility of using quantitative diffuse reflectance spectroscopy to longitudinally monitor physiological response to cancer therapy was evaluated in a preclinical model. This study included two groups of nude mice bearing 4T1 flank tumors (N = 50), half of which were treated with a maximum tolerated dose of doxorubicin (DOX). Diffuse reflectance spectra were collected from tumors during a period of 2 weeks using a fiber-optic probe coupled to a spectrometer. These spectra were quantified using an inverse scalable Monte Carlo model of light transport in tissue to extract the concentrations of oxygenated, deoxygenated hemoglobin (dHb), and a wavelength mean reduced scattering coefficient (<micro(s)'>). The tumor growth rates of the treated and control groups were nearly identical, as were changes in the scattering parameter <micro(s)'> during this time frame. However, tumors treated with DOX showed a transient but significant increase in blood oxygen saturation. A comparison between the optically derived and immunohistochemical end points in a subset of the 50 animals showed that the temporal kinetics of dHb concentration and <micro(s)'> were highly concordant with those of hypoxic and necrotic fractions, respectively. In conclusion, optical methods could function as a "screening" technology in longitudinal studies of small animal tumor models to accelerate development and testing of new anticancer drugs. This technique could isolate specific landmark time points at which more expensive and sophisticated imaging methods or immunohistochemistry could be performed.

Bone marrow derived cells in tumor angiogenesis and growth: are they the good, the bad or the evil?

Increasing evidence implicates an important role for a variety of bone marrow derived cells (BMDCs) in tumor angiogenesis and metastatic tumor growth. These cells are derived either from the hematopoietic or mesenchymal cell lineage, and they are distinguished, in part, by the expression of the panhematopoietic marker - CD45. Some of these cell populations can colonize tumors perivascularily, and appear to promote angiogenesis and tumor cell proliferation by paracraine mechanisms, whereas others can contribute "directly" to the growth of tumor vessel capillaries or metastases. In this review we focus in particular on the role of hemangiocytes or recruited bone marrow derived circulating cells (RBCCs) in neovascularization, the contribution of VEGFR1+ hematopoietic stem cells and endothelial precursor cells in metastasis, and the involvement of myeloid derived suppressor CD11b+/Gr-1+ cells in the resistance of tumors to certain antiangiogenic drugs, e.g., VEGF blocking antibodies.

High levels of circulating VEGFR2+ Bone marrow-derived progenitor cells correlate with metastatic disease in patients with pediatric solid malignancies

PURPOSE

Pediatric solid malignancies display important angiogenic potential, and blocking tumor angiogenesis represents a new therapeutic approach for these patients. Recent studies have evidenced rare circulating cells with endothelial features contributing to tumor neovascularization and have shown the pivotal role of bone marrow-derived (BMD) progenitor cells in metastatic disease progression. We measured these cells in patients with pediatric solid malignancies as a prerequisite to clinical trials with antiangiogenic therapy.

PATIENTS AND METHODS

Peripheral blood was drawn from 45 patients with localized (n = 23) or metastatic (n = 22) disease, and 20 healthy subjects. Subsets of circulating vascular endothelial growth factor receptor (VEGFR)2+-BMD progenitor cells, defined as CD45-CD34+VEGFR2(KDR)+7AAD- and CD45(dim)CD34+VEGFR2+7AAD- events, were measured in progenitor-enriched fractions by flow cytometry. Mature circulating endothelial cells (CEC) were measured in whole blood as CD31+CD146+CD45-7AAD- viable events. Data were correlated with VEGF and sVEGFR2 plasma levels.

RESULTS

The CD45-CD34+VEGFR2(KDR)+7AAD- subset represented <0.003% of circulating BMD progenitor cells (< or =0.05 cells/mL). However, the median level (range) of the CD45(dim)CD34+VEGFR2+7AAD- subset was higher in patients compared with healthy subjects, 1.5% (0%-10.3%) versus 0.3% (0%-1.6%) of circulating BMD progenitors (P < 0.0001), and differed significantly between patients with localized and metastatic disease, 0.7% (0%-8.6%) versus 2.9% (0.6%-10.3%) of circulating BMD progenitors (P < 0.001). Median CEC value was 7 cells/mL (0-152 cells/mL) and similar in all groups. Unlike VEGFR2+-BMD progenitors, neither CECs, VEGF, or sVEGFR2 plasma levels correlated with disease status.

CONCLUSION

High levels of circulating VEGFR2+-BMD progenitor cells correlated with metastatic disease. Our study provides novel insights for angiogenesis mechanisms in pediatric solid malignancies for which antiangiogenic targeting of VEGFR2+-BMD progenitors could be of interest.

Endothelial progenitor cells do not contribute to tumor endothelium in primary and metastatic tumors

Despite extensive research, the contribution of bone-marrow-derived endothelial progenitor cells (BM-EPC) to tumor angiogenesis remains controversial. In previous publications, the extent of incorporation of BM-EPCs into the endothelial cell (EC) layer in different tumor models has been reported as significant in some studies but undetectable in others. Here, we studied the differentiation of BM-EPCs and its contribution to tumor vessels in experimental and spontaneous lung metastasis (B16 melanoma and prostate carcinoma), in an autochthonous transgenic model of prostate tumorigenesis, in orthotopically implanted lung tumors [Lewis lung carcinoma (LLC)], in heterotopic subcutaneous models (LLC and C1 prostate carcinoma) growing in green fluorescent protein (GFP)-expressing bone marrow (BM) chimeras. Immunofluorescence was performed with a set of endothelial and hematopoietic markers and confocal microscopy was used to generate 3D reconstruction images. By performing rigorously conducted morphological studies, we found no evidence of BM-EPCs differentiation into tumor endothelium independently of tumor type, grade and organ site in primary and metastatic tumors. The vast majority of GFP(+) cells were trafficking leucocytes or periendothelial myeloid cells. To explore the possibility that local overexpression of vascular endothelial growth factor (VEGF) might increase the numbers of incorporated BM-EPCs, we analyzed tumors genetically manipulated to overexpress VEGF(164). Local VEGF production induces a massive infiltration of bone-marrow-derived cells, but did not lead to vessel wall integration of these cells. Collectively, these findings suggest that during tumor progression vascularization occurs primarily via classical tumor angiogenesis (e.g., sprouting of pre-existing ECs), whereas BM-EPCs do not incorporate into the vessel wall to any significant extent.

Glioma tumor stem-like cells promote tumor angiogenesis and vasculogenesis via vascular endothelial growth factor and stromal-derived factor 1

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.

Contribution of granulocyte colony-stimulating factor to the acute mobilization of endothelial precursor cells by vascular disrupting agents

Vascular disrupting agents (VDA) cause acute shutdown of abnormal established tumor vasculature, followed by massive intratumoral hypoxia and necrosis. However, a viable rim of tumor tissue invariably remains from which tumor regrowth rapidly resumes. We have recently shown that an acute systemic mobilization and homing of bone marrow-derived circulating endothelial precursor (CEP) cells could promote tumor regrowth following treatment with either a VDA or certain chemotherapy drugs. The molecular mediators of this systemic reactive host process are unknown. Here, we show that following treatment of mice with OXi-4503, a second-generation potent prodrug derivative of combretastatin-A4 phosphate, rapid increases in circulating plasma vascular endothelial growth factor, stromal derived factor-1 (SDF-1), and granulocyte colony-stimulating factor (G-CSF) levels are detected. With the aim of determining whether G-CSF is involved in VDA-induced CEP mobilization, mutant G-CSF-R(-/-) mice were treated with OXi-4503. We found that as opposed to wild-type controls, G-CSF-R(-/-) mice failed to mobilize CEPs or show induction of SDF-1 plasma levels. Furthermore, Lewis lung carcinomas grown in such mice treated with OXi-4503 showed greater levels of necrosis compared with tumors treated in wild-type mice. Evidence for rapid elevations in circulating plasma G-CSF, vascular endothelial growth factor, and SDF-1 were also observed in patients with VDA (combretastatin-A4 phosphate)-treated cancer. These results highlight the possible effect of drug-induced G-CSF on tumor regrowth following certain cytotoxic drug therapies, in this case using a VDA, and hence G-CSF as a possible therapeutic target.

Guiding the optimal translation of new cancer treatments from canine to human cancer patients

On June 20, 2008, a meeting entitled "Translation of new cancer treatments from canine to human cancer patients," sponsored by the National Cancer Institute in Bethesda, Maryland, was convened to discuss the potential value, opportunity, risks, and rewards of an integrated and comparative drug development path for new cancer therapeutics that includes naturally occurring cancers in pet animals. A summary of this meeting and subsequent discussion are provided here to afford clarity on the conduct of these studies so as to optimize the opportunities provided by this novel drug development and modeling strategy.

Bone marrow stem and progenitor cell contribution to neovasculogenesis is dependent on model system with SDF-1 as a permissive trigger

Adult bone marrow (BM) contributes to neovascularization in some but not all settings, and reasons for these discordant results have remained unexplored. We conducted novel comparative studies in which multiple neovascularization models were established in single mice to reduce variations in experimental methodology. In different combinations, BM contribution was detected in ischemic retinas and, to a lesser extent, Lewis lung carcinoma cells, whereas B16 melanomas showed little to no BM contribution. Using this spectrum of BM contribution, we demonstrate the necessity for site-specific expression of stromal-derived factor-1alpha (SDF-1alpha) and its mobilizing effects on BM. Blocking SDF-1alpha activity with neutralizing antibodies abrogated BM-derived neovascularization in lung cancer and retinopathy. Furthermore, secondary transplantation of single hematopoietic stem cells (HSCs) showed that HSCs are a long-term source of neovasculogenesis and that CD133(+)CXCR4(+) myeloid progenitor cells directly participate in new blood vessel formation in response to SDF-1alpha. The varied BM contribution seen in different model systems is suggestive of redundant mechanisms governing postnatal neovasculogenesis and provides an explanation for contradictory results observed in the field.

CD133-expressing stem cells associated with ovarian metastases establish an endothelial hierarchy and contribute to tumor vasculature

Recruitment and localization of endothelial precursors within tumors is a potential area for the development of therapeutics, because their functional contribution to tumor vasculature is realized to be important for cancer cell survival. However, the exact nature of the recruited cell type and cellular events orchestrating the entire phenomenon remains obscure. We report that human ovarian cancer is frequently associated with cells expressing the stem cell surface marker CD133. We further show that these CD133-expressing cells are nontumorigenic in nature, and they augment tumor development through their vasculogenic potential. This cell population is attracted by cancer stem cells (CSCs) and retains a direct physical association within the CSC-derived spheroids. Our study further delineates the contribution of these vasculogenic CD133(+) stem cells, termed by us as endothelial stem cells (EnSCs) to the developing tumor vasculature during disease progression. In support of their being stem cells, the EnSCs have a capability of establishing an entire endothelial cell hierarchy. We conclude that such EnSCs play a crucial role in ensuring the development of long-term tumor vasculature to complement CSC-driven tumor development and disease progression.

The role of bone-marrow-derived cells in tumor growth, metastasis initiation and progression

Emerging evidence from murine models suggests that tumor-specific endocrine factors systemically stimulate the quiescent bone marrow (BM) compartment, resulting in the expansion, mobilization and recruitment of BM progenitor cells. Discrete subsets of tumor-instigated BM-derived progenitor cells support tumor progression and metastasis by regulating angiogenesis, inflammation and immune suppression. Notably, clinical studies have begun to reveal that increased BM recruitment in tumors is associated with poor prognosis. Thus, the BM-derived tumor microenvironment is an attractive therapeutic target, and drugs targeting the components of the microenvironment are currently in clinical trials. Here, we focus on recent advances and emerging concepts regarding the intriguing role of BM-derived cells in tumor growth, metastasis initiation and progression, and we discuss future directions in the context of novel diagnostic and therapeutic opportunities.

Dead or alive? Autofluorescence distinguishes heat-fixed from viable cells

PURPOSE

A proof-of-concept study to evaluate a new autofluorescence method to differentiate necrotic thermally fixed cells from viable tissue following thermal ablation.

METHODS

A conductive interstitial thermal therapy (CITT) device was used to ablate swine mammary tissue and rabbit VX-2 carcinomas in vivo. The ablated regions and 10-mm margins were resected 24 h following treatment, embedded in HistOmer and sectioned at 3 mm. The fresh sections were evaluated for gross viability with triphenyl tetrazolium chloride, 1 h post-resection. Representative non-viable and viable areas were then processed and embedded into paraffin, and sectioned at 5 microm. Standard H&E staining and proliferating cell nuclear antigen (PCNA) immunohistochemistry were compared against autofluorescence intensity, at 488-nm wavelength, for cellular viability.

RESULTS

Heat-fixed cells in non-viable regions exhibit increased autofluorescence intensity compared to viable tissue (area under receiver operating characteristics (ROC) curve = 0.96; Mann-Whitney P < 0.0001). An autofluorescence intensity-based classification rule achieved 92% sensitivity with 100% specificity for distinguishing non-viable from viable samples. In contrast, PCNA staining did not reliably distinguish heat-fixed, dead cells from viable cells.

CONCLUSIONS

Examination of H&E-stained sections using autofluorescence intensity-based classification is a reliable and readily available method to accurately identify heat-fixed cells in ablated surgical margins.

Tie2-expressing monocytes (TEMs): novel targets and vehicles of anticancer therapy?

There is a growing interest in understanding the complex interactions between bone marrow-derived myeloid-lineage cells and angiogenesis in tumors. Such interest has been revived recently by the observation that tumor-infiltrating myeloid cells convey proangiogenic programs that can counteract the activity of antiangiogenic drugs in mouse tumor models. Among myeloid cells, Tie2-expressing monocytes (TEMs) appear to have nonredundant function in promoting tumor angiogenesis and growth in mouse models. The identification and functional characterization of TEMs in mice and humans may provide novel molecular targets for anticancer therapy. Moreover, TEMs may be exploited to deliver antitumor drugs specifically to the tumor microenvironment.

Endothelial progenitor cells: identity defined?

In the past decade, researchers have gained important insights on the role of bone marrow (BM)-derived cells in adult neovascularization. A subset of BM-derived cells, called endothelial progenitor cells (EPCs), has been of particular interest, as these cells were suggested to home to sites of neovascularization and neoendothelialization and differentiate into endothelial cells (ECs) in situ, a process referred to as postnatal vasculogenesis. Therefore, EPCs were proposed as a potential regenerative tool for treating human vascular disease and a possible target to restrict vessel growth in tumour pathology. However, conflicting results have been reported in the field, and the identification, characterization, and exact role of EPCs in vascular biology is still a subject of much discussion. The focus of this review is on the controversial issues in the field of EPCs which are related to the lack of a unique EPC marker, identification challenges related to the paucity of EPCs in the circulation, and the important phenotypical and functional overlap between EPCs, haematopoietic cells and mature ECs. We also discuss our recent findings on the origin of endothelial outgrowth cells (EOCs), showing that this in vitro defined EC population does not originate from circulating CD133⁺ cells or CD45⁺ haematopoietic cells.

Increased antitumor activity of bevacizumab in combination with hypoxia inducible factor-1 inhibition

Inhibition of hypoxia inducible factor-1 (HIF-1) is an attractive therapeutic strategy to target the tumor microenvironment. However, HIF-1 inhibitors may have limited activity as single agents and combination therapies may be required. We tested the hypothesis that HIF-1 inhibition in a hypoxic-stressed tumor microenvironment, which could be generated by administration of antiangiogenic agents, may result in a more pronounced therapeutic effect. The activity of bevacizumab, either alone or in combination with the HIF-1alpha inhibitor topotecan, was evaluated in U251-HRE xenografts. Tumor tissue was collected at the end of treatment and changes in tumor oxygenation, angiogenesis, proliferation, apoptosis, HIF-1alpha levels, HIF-1 target genes, and DNA damage were evaluated. Bevacizumab decreased microvessel-density and increased intratumor-hypoxia, but did not induce apoptosis. Moreover, bevacizumab alone caused a significant increase of HIF-1-dependent gene expression in tumor tissue. Addition of a low dose of daily topotecan to bevacizumab significantly inhibited tumor growth, relative to mice treated with topotecan or bevacizumab alone (P < 0.01). The addition of topotecan to bevacizumab was also associated with profound inhibition of HIF-1 transcriptional activity, significant inhibition of proliferation, and induction of apoptosis. Importantly, DNA damage induced by topotecan alone was not augmented by addition of bevacizumab, suggesting that increased cytotoxic activity did not account for the increased antitumor effects observed. These results strongly suggest that combination of anti-vascular endothelial growth factor antibodies with HIF-1 inhibitors is an attractive therapeutic strategy targeting in the hypoxic tumor microenvironment.

White adipose tissue cells are recruited by experimental tumors and promote cancer progression in mouse models

The connection between obesity and accelerated cancer progression has been established, but the mediating mechanisms are not well understood. We have shown that stromal cells from white adipose tissue (WAT) cooperate with the endothelium to promote blood vessel formation through the secretion of soluble trophic factors. Here, we hypothesize that WAT directly mediates cancer progression by serving as a source of cells that migrate to tumors and promote neovascularization. To test this hypothesis, we have evaluated the recruitment of WAT-derived cells by tumors and the effect of their engraftment on tumor growth by integrating a transgenic mouse strain engineered for expansion of traceable cells with established allograft and xenograft cancer models. Our studies show that entry of adipose stromal and endothelial cells into systemic circulation leads to their homing to and engraftment into tumor stroma and vasculature, respectively. We show that recruitment of adipose stromal cells by tumors is sufficient to promote tumor growth. Finally, we show that migration of stromal and vascular progenitor cells from WAT grafts to tumors is also associated with acceleration of cancer progression. These results provide a biological insight for the clinical association between obesity and cancer, thus outlining potential avenues for preventive and therapeutic strategies.

Nitric oxide synthase inhibition enhances the tumor vascular-damaging effects of combretastatin a-4 3-o-phosphate at clinically relevant doses

PURPOSE

The therapeutic potential of combining the prototype tumor vascular-disrupting agent combretastatin A-4 3-O-phosphate (CA-4-P) with systemic nitric oxide synthase (NOS) inhibition was investigated preclinically.

EXPERIMENTAL DESIGN

Vascular response (uptake of (125)I-labeled iodoantipyrine; laser Doppler flowmetry) and tumor response (histologic necrosis; cytotoxicity and growth delay) were determined.

RESULTS

Inducible NOS selective inhibitors had no effect on blood flow in the P22 rat sarcoma. In contrast, the non-isoform-specific NOS inhibitor N(omega)-nitro- l-arginine (l-NNA; 1 and 10 mg/kg i.v. or chronic 0.1 or 0.3 mg/mL in drinking water) decreased the P22 blood flow rate selectively down to 36% of control at 1 hour but did not induce tumor necrosis at 24 hours. CA-4-P, at clinically relevant doses, decreased the P22 blood flow rate down to 6% of control at 1 hour for 3 mg/kg but with no necrosis induction. However, l-NNA administration enhanced both CA-4-P-induced tumor vascular resistance at 1 hour (chronic l-NNA administration) and necrosis at 24 hours, with 45% or 80% necrosis for 3 and 10 mg/kg CA-4-P, respectively. Bolus l-NNA given 3 hours after CA-4-P was the most effective cytotoxic schedule in the CaNT mouse mammary carcinoma, implicating a particular enhancement by l-NNA of the downstream consequences of CA-4-P treatment. Repeated dosing of l-NNA with CA-4-P produced enhanced growth delay over either treatment alone in P22, CaNT, and spontaneous T138 mouse mammary tumors, which represented a true therapeutic enhancement.

CONCLUSIONS

The combination of NOS inhibition with CA-4-P is a promising approach for targeting tumor vasculature, with relevance for similar vascular-disrupting agents in development.

Role of the hypoxic tumor microenvironment in the resistance to anti-angiogenic therapies

Angiogenesis, a key process for the growth of human cancers, has recently been exploited for the development of a novel class of cancer therapeutics that was thought to have wide applications and not to induce resistance in the clinical setting. Indeed, anti-angiogenic therapy has become an important option for the management of several human malignancies. However, a significant number of patients either do not respond to anti-angiogenic agents or fairly rapidly develop resistance. In addition, the benefit of anti-angiogenic therapy is relatively short-lived and the majority of patients eventually relapses and progresses. Several mechanisms of resistance to anti-angiogenic therapy have been recently proposed. The current review focuses on the role of intra-tumor hypoxia as a mechanism of resistance to anti-angiogenic agents and speculates on therapeutic approaches that might circumvent resistance and thereby improve clinical outcome.

Microtubule depolymerizing vascular disrupting agents: novel therapeutic agents for oncology and other pathologies

Vascular disrupting agents (VDAs) are a relatively new group of 'vascular targeting' agents that exhibit selective activity against established tumour vascular networks, causing severe interruption of tumour blood flow and necrosis to the tumour mass. Microtubule depolymerizing agents form by far the largest group of small molecular weight VDAs many of which, including lead compound disodium combretastatin A-4 3-O-phosphate (CA-4-P), are under clinical development for cancer. Although distinct from the angiogenesis inhibitors, VDAs can also interfere with angiogenesis and therefore constitute a potential group of novel drugs for the treatment of pathological conditions characterized by excessive angiogenesis, in addition to cancer. The endothelial cytoskeleton is the primary cellular target of this family of drugs, and some progress in understanding the molecular and signalling mechanisms associated with their endothelial disrupting activity has been made in the last few years. Susceptibility of tumour vessels to VDA damage is ascribed to their immature pericyte-defective nature, although the exact molecular mechanisms involved have not been clearly defined. Despite causing profound damage to tumours, VDAs fail to halt tumour growth unless used together with conventional treatments. This failure is attributed to resistance mechanisms, primarily associated with cells that remain viable within the tumour rim, and enhanced angiogenesis. The focus is now to understand mechanisms of susceptibility and resistance to identify novel molecular targets and develop strategies that are more effective.

Cyclophosphamide enhances human tumor growth in nude rat xenografted tumor models

The effect of the immunomodulatory chemotherapeutic agent cyclophosphamide (CTX) on tumor growth was investigated in primary and metastatic intracerebral and subcutaneous rat xenograft models. Nude rats were treated with CTX (100 mg/kg, intraperitoneally) 24 hours before human ovarian carcinoma (SKOV3), small cell lung carcinoma (LX-1 SCLC), and glioma (UW28, U87MG, and U251) tumor cells were inoculated subcutaneously, intraperitoneally, or in the right cerebral hemisphere or were infused into the right internal carotid artery. Tumor development was monitored and recorded. Potential mechanisms were further investigated. Only animals that received both CTX and Matrigel showed consistent growth of subcutaneous tumors. Cyclophosphamide pretreatment increased the percentage (83.3% vs 0%) of animals showing intraperitoneal tumors. In intracerebral implantation tumor models, CTX pretreatment increased the tumor volume and the percentage of animals showing tumors. Cyclophosphamide increased lung carcinoma bone and facial metastases after intra-arterial injection, and 20% of animals showed brain metastases. Cyclophosphamide transiently decreased nude rat white blood cell counts and glutathione concentration, whereas serum vascular endothelial growth factor was significantly elevated. Cyclophosphamide also increased CD31 reactivity, a marker of vascular endothelium, and macrophage (CD68-positive) infiltration into glioma cell-inoculated rat brains. Cyclophosphamide may enhance primary and metastatic tumor growth through multiple mechanisms, including immune modulation, decreased response to oxidative stress, increased tumor vascularization, and increased macrophage infiltration. These findings may be clinically relevant because chemotherapy may predispose human cancer subjects to tumor growth in the brain or other tissues.

Bone marrow-derived endothelial progenitor cells contribute to the angiogenic switch in tumor growth and metastatic progression

Emerging evidence indicates that bone marrow (BM)-derived endothelial progenitor cells (EPCs) contribute to angiogenesis-mediated growth of certain tumors in mice and human. EPCs regulate the angiogenic switch via paracrine secretion of proangiogenic growth factors and by direct luminal incorporation into sprouting nascent vessels. While the contributions of EPCs to neovessel formation in spontaneous and transplanted tumors and to the metastatic transition have been reported to be relatively low, remarkably, specific EPC ablation in vivo has resulted in severe angiogenesis inhibition and impaired primary and metastatic tumor growth. The existence of a BM reservoir of EPCs, and the selective involvement of EPCs in neovascularization, have attracted considerable interest because these cells represent novel target for therapeutic intervention. In addition, EPCs are also being used as pharmacodynamic surrogate markers for monitoring cancer progression, as well as for optimizing efficacy of anti-angiogenic therapies in the clinic. This review will focus primarily on recent advances and emerging concepts in the field of EPC biology and discuss ongoing debates involving the role of EPCs in tumor neovascularization. For detailed information on the in vitro characterization of EPCs contribution to non-tumor pathologies, the reader is directed towards several excellent reviews and publications [F. Bertolini, Y. Shaked, P. Mancuso and R.S. Kerbel, Nat. Rev., Cancer 6 (2006) 835-845. [1]] [J.M. Hill, T. Finkel and A.A. Quyyumi, Vox Sang. 87 Suppl 2 (2004) 31-37. [2]] [A.Y. Khakoo and T. Finkel, Annu. Rev. Med. 56 (2005) 79-101. [3]] [H.G. Kopp, C.A. Ramos and S. Rafii, Curr. Opin. Hematol. 13 (2006) 175-181. [4]; K.K. Hirschi, D.A. Ingram and M.C. Yoder, Arterioscler. Thromb. Vasc. Biol. 28 (2008) 1584-1595. [5]; F. Timmermans, J. Plum, M.C. Yoder, D.A. Ingram, B. Vandekerckhove and J. Case, J. Cell. Mol. Med. 13 (2009) 87-102. [6]] and reviews by Bertolini, Voest and Yoder in this issue.

Translating preclinical findings of (endothelial) progenitor cell mobilization into the clinic; from bedside to bench and back

It is generally accepted that angiogenesis plays a major role in tumor growth and numerous targeting agents directed against angiogenesis pathways have been developed and approved for clinical use. In the past years the concept of angiogenesis has developed into a multi-faceted process in which, besides local activation and division of endothelial cells, bone marrow derived progenitor cells (BMDPCs) contribute to neovascularization. A multitude of preclinical and clinical data indicates that the release of BMDPCs influences the response to certain anti-cancer modalities. In this review we provide an overview of all the preclinical and clinical studies contributing to this hypothesis and translate these findings to the clinic by pointing out the clinical implications these findings might have. The recent insight in the mechanism of a systemic host response, in response to various treatment modalities has shed new light on the mechanism of tumor regrowth, early recurrence and metastasis formation during or after treatment. This provides various new targets for therapy which can be used to improve conventional chemotherapy. Furthermore it provides a potential explanation why bevacizumab selectively enhances the effectiveness of only certain types of chemotherapy.

The multiple personality disorder phenotype(s) of circulating endothelial cells in cancer

Circulating endothelial cells (CECs) and circulating endothelial progenitors (CEPs) are currently being investigated in a variety of diseases as markers of vascular turnover or damage and, also in the case of CEPs, vasculogenesis. CEPs appear to have a "catalytic" role in different steps of cancer progression and recurrence after therapy, and there are preclinical and clinical data suggesting that CEC enumeration might be useful to select and stratify patients who are candidates for anti-angiogenic treatments. In some types of cancer, CECs and CEPs might be one of the possible hidden identities of cancer stem cells. The definition of CEC and CEP phenotype and the standardization of CEC and CEP enumeration strategies are highly desirable goals in order to exploit these cells as reliable biomarkers in oncology clinical trials.

Endothelial progenitor cells and their potential clinical implication in cardiovascular disorders

Risk factors associated with cardiovascular diseases reduce the availability of endothelial progenitor cells (EPC) by affecting their mobilization and integration into injured vascular sites. The existence of a bone marrow reservoir of EPC has attracted interest, especially as target for therapeutic intervention in different pathological settings. Among the cardiovascular risk factors, hypertension has been shown to be a strongest predictor of EPC migratory impairment. However, at present, data concerning EPC biology are still limited. In this article we provide an overview of data relevant to their potential clinical implications in cardiovascular disorders. In addition, the recent advances in understanding the role of EPC in the pathophysiology of hypertension are discussed.

Microenvironmental regulation of metastasis

Metastasis is a multistage process that requires cancer cells to escape from the primary tumour, survive in the circulation, seed at distant sites and grow. Each of these processes involves rate-limiting steps that are influenced by non-malignant cells of the tumour microenvironment. Many of these cells are derived from the bone marrow, particularly the myeloid lineage, and are recruited by cancer cells to enhance their survival, growth, invasion and dissemination. This Review describes experimental data demonstrating the role of the microenvironment in metastasis, identifies areas for future research and suggests possible new therapeutic avenues.

Beyond chemotherapy: targeted therapies in ovarian cancer

Ovarian cancer is the leading cause of death from gynaecological malignancies in the Western world. Despite the evolution of surgical techniques and meticulously designed chemotherapy regimens, relapse remains almost inevitable in patients with advanced disease. In an age when great advances have been made in understanding the genetics and molecular biology of this heterogeneous disease, it is likely that the introduction of novel targeted therapies will have a major impact on the management of ovarian cancer. Importantly, such strategies might allow selection of treatments based on the molecular characteristics of tumours and bring us closer to an era of personalized medicine.

Granulocyte macrophage colony-stimulating factor inhibits breast cancer growth and metastasis by invoking an anti-angiogenic program in tumor-educated macrophages

Tumor-educated macrophages facilitate tumor metastasis and angiogenesis. We discovered that granulocyte macrophage colony-stimulating factor (GM-CSF) blocked macrophages vascular endothelial growth factor (VEGF) activity by producing soluble VEGF receptor-1 (sVEGFR-1) and determined the effect on tumor-associated macrophage behavior and tumor growth. We show GM-CSF treatment of murine mammary tumors slowed tumor growth and slowed metastasis. These tumors had more macrophages, fewer blood vessels, and lower oxygen concentrations. This effect was sVEGFR-1 dependent. In situ hybridization and flow cytometry identified macrophages as the primary source of sVEGFR-1. These data suggest that GM-CSF can re-educate macrophages to reduce angiogenesis and metastases in murine breast cancer.

Angiogenesis and antiangiogenic therapy in non-Hodgkin's lymphoma

Angiogenesis, the growth of new blood vessels, requires dynamic expansion, assembly and stabilization of vascular endothelial cells in response to proangiogenic stimuli. Antiangiogenic strategies have become an important therapeutic modality for solid tumors. While many aspects of postnatal pathological angiogenesis have been extensively studied in the context of nonhematopoietic neoplasms, the precise role of these processes in lymphoma pathogenesis is under active investigation. Lymphoma growth and progression is potentiated by at least two distinct angiogenic mechanisms: autocrine stimulation of tumor cells via expression of vascular endothelial growth factor (VEGF) and VEGF receptors by lymphoma cells, as well as paracrine influences of proangiogenic tumor microenvironment on both local neovascular transformation and recruitment of circulating bone marrow-derived progenitors. Lymphoma-associated infiltrating host cells including hematopoietic monocytes, T cells and mesenchymal pericytes have increasingly been associated with the pathogenesis and prognosis of lymphoma, in part providing perivascular guidance and support to neoangiogenesis. Collectively, these distinct angiogenic mechanisms appear to be important therapeutic targets in selected non-Hodgkin's lymphoma (NHL) subtypes. Understanding these pathways has led to the introduction of antiangiogenic treatment strategies into the clinic where they are currently under assessment in several ongoing studies of NHL patients.

Positive and negative modulation of angiogenesis by VEGFR1 ligands

Vascular endothelial growth factor-A (VEGF-A) is a key target for new antiangiogenic drugs for the treatment of both malignant and nonmalignant human diseases. Vascular effects of VEGF family members are mainly mediated by VEGF receptor 2 (VEGFR2). Conversely, the function and signaling of VEGFR1, which is present on endothelial and nonendothelial cells, are poorly understood. Intriguingly, two of five members in the VEGF family--VEGF-B and placental growth factor (PlGF)--are exclusive ligands for VEGFR1 and do not interact with the other VEGFRs, VEGFR2 and VEGFR3. These VEGFR1-specific ligands may be important therapeutic targets for the treatment of cancer. This Review discusses the distinctive roles of VEGFR1 and its ligands PlGF and VEGF-B in the mediation of angiogenic signaling and considers the therapeutic potential of targeting these particular vascular factors.

Role of endothelial progenitors and other bone marrow-derived cells in the development of the tumor vasculature

Increasing evidence suggests the importance of bone marrow-derived cells for blood vessel formation (neovascularization) in tumors, which can occur in two mechanisms: angiogenesis and vasculogenesis. Angiogenesis results from proliferation and sprouting of existing blood vessels close to the tumor, while vasculogenesis is believed to arise from recruitment of circulating cells, largely derived from the bone marrow, and de novo clonal formation of blood vessels from these cells. Although bone marrow-derived cells are crucial for neovascularization, current evidence suggests a promotional role of these cells on the existing blood vessels rather than de novo neovascularization in tumors. This is believed to be due to the highly proangiogenic features of these cells. The bone marrow-derived cells are heterogeneous, consisting of many different cell types including endothelial progenitor cells, myeloid cells, lymphocytes, and mesenchymal cells. These cells are highly orchestrated under the influence of the specific tumor microenvironment, which varies depending on the tumor type, thereby tightly regulating neovascularization in the tumors. In this review, we highlight some of the recent findings on each of these cell types by outlining some of the essential proangiogenic cytokines that these cells secrete to promote tumor angiogenesis and vasculogenesis.

Dominant effect of antiangiogenesis in combination therapy involving cyclophosphamide and axitinib

PURPOSE

Antiangiogenic drug treatment inhibits tumor growth by decreasing blood supply, which can also reduce the delivery of other therapeutic agents. Presently, we investigated the effect of the vascular endothelial growth factor receptor tyrosine kinase inhibitor axitinib (AG-013736) on tumor vascular patency and chemotherapeutic drug uptake. Furthermore, the effect of axitinib on the antitumor activity of combination treatments with cyclophosphamide was examined.

EXPERIMENTAL DESIGN

Prostate cancer PC-3 xenografts were used to evaluate the effect of axitinib treatment on tumor vascular morphology, fluorescent dye perfusion, hypoxia, and uptake of 4-hydroxycyclophosphamide, the active metabolite of the chemotherapeutic prodrug cyclophosphamide. Sequential or simultaneous schedules for axitinib and cyclophosphamide administration were evaluated in both PC-3 tumors and 9L gliosarcoma xenograft models.

RESULTS

Axitinib monotherapy induced sustained growth stasis in PC-3 tumors in association with extensive apoptotic cell death. A substantial decrease in tumor vascular patency was observed, exemplified by a near complete loss of Hoechst 33342 perfusion and the absence of pimonidazole staining in the increasingly hypoxic tumors. Antitumor activity was significantly enhanced in both PC-3 and 9L tumors treated using an optimized schedule of sequential, intermittent axitinib-cyclophosphamide combination therapy despite a 40% to 70% decrease in tumor tissue uptake of 4-hydroxycyclophosphamide.

CONCLUSIONS

In axitinib-cyclophosphamide combination therapy, enhanced anticancer activity can be achieved when the reduced tumor cell exposure to the cancer chemotherapeutic agent is compensated by antiangiogenesis-induced tumor cell starvation. This intrinsic antitumor effect was particularly evident in PC-3 tumor xenografts, where tumor blood flow deprivation dominates the overall therapeutic response.

Elevated levels of circulating endothelial progenitor cells in head and neck cancer patients

BACKGROUND AND OBJECTIVES

Measurement of circulating endothelial cells (CECs) and progenitor cells (EPCs) has potential as a surrogate marker for monitoring anticancer treatment. This study evaluated the significance of CECs and EPCs in the blood of patients with head and neck squamous cell carcinoma.

METHODS

In a prospective trial fresh blood samples from 22 tumor patients and 18 controls were tested using multiparametric flow-cytometry. CECs were defined as CD31(+)/CD146(+) and CD45(-)/7AAD(-). EPCs were defined as CD133(+)/KDR(+) and CD3(-)/CD19(-)/CD33(-)/7AAD(-).

RESULTS

Median levels (min/max) of CECs in the tumor group were 2 (0/5) at the time of diagnosis, 1 (0/5) 1 year after therapy and 2 (0/6) in the control cohort. Median levels of EPCs were 5 (1/41) before and 10 (0/21) after treatment in the tumor group compared to 2 (0/7) in the control cohort (P < 0.001 and P = 0.03). CEC and EPC levels showed no apparent correlation with tumor size and response to radiotherapy after 18 months of observation.

CONCLUSIONS

In this pilot study CD133(+)/KDR(+) EPCs were significantly elevated in head and neck tumor patients before and after therapy. Our results warrant further studies on the use of EPCs as a surrogate marker for anticancer therapies in these patients.

[Tumor vasculature as a therapeutic target in non-small cell lung cancer]

Despite developments in conventional (chemo)radiotherapy and surgery, survival of non-small cell lung cancer (NSCLC) patients remains poor. Treatments with targeted molecular drugs offer novel therapeutic strategies. Bevacizumab, a recombinant anti-vascular endothelial growth factor (VEGF) antibody, is the antiangiogenic drug at the most advanced stage of development in the therapy of NSCLC. However, a number of questions and future challenges relating to the use of bevacizumab in NSCLC remain. Furthermore, novel agents targeting the pre-existing NSCLC vasculature (i.e. vascular disrupting agents, VDAs) or multiple tyrosine kinase inhibitors have emerged as unique drug classes delivering promising results in several preclinical and clinical studies. Herein, we review the most recent data using these novel targeted agents either alone or in combination with chemotherapy in NSCLC.

Combination of antiangiogenesis with chemotherapy for more effective cancer treatment

Angiogenesis is a hallmark of tumor development and metastasis and is now a validated target for cancer treatment. However, the survival benefits of antiangiogenic drugs have thus far been rather modest, stimulating interest in developing more effective ways to combine antiangiogenic drugs with established chemotherapies. This review discusses recent progress and emerging challenges in this field; interactions between antiangiogenic drugs and conventional chemotherapeutic agents are examined, and strategies for the optimization of combination therapies are discussed. Antiangiogenic drugs such as the anti-vascular endothelial growth factor antibody bevacizumab can induce a functional normalization of the tumor vasculature that is transient and can potentiate the activity of coadministered chemoradiotherapies. However, chronic angiogenesis inhibition typically reduces tumor uptake of coadministered chemotherapeutics, indicating a need to explore new approaches, including intermittent treatment schedules and provascular strategies to increase chemotherapeutic drug exposure. In cases where antiangiogenesis-induced tumor cell starvation augments the intrinsic cytotoxic effects of a conventional chemotherapeutic drug, combination therapy may increase antitumor activity despite a decrease in cytotoxic drug exposure. As new angiogenesis inhibitors enter the clinic, reliable surrogate markers are needed to monitor the progress of antiangiogenic therapies and to identify responsive patients. New targets for antiangiogenesis continue to be discovered, increasing the opportunities to interdict tumor angiogenesis and circumvent resistance mechanisms that may emerge with chronic use of these drugs.

FLT1 and its ligands VEGFB and PlGF: drug targets for anti-angiogenic therapy?

Less than 5 years ago, it was still not clear whether anti-angiogenic drugs would prove successful in the clinic. After numerous patients with cancer or age-related macular degeneration have been treated with these drugs, they have now become part of the standard range of therapeutic tools. Despite this milestone, anti-angiogenic therapy still faces a number of clinical hurdles, such as improving efficacy, avoiding escape and resistance, and minimizing toxicity. Hopefully, other agents with complementary mechanisms, such as those that target placental growth factor, will offer novel opportunities for improved treatment.

Role of the microenvironment in tumor growth and in refractoriness/resistance to anti-angiogenic therapies

Angiogenesis is critical for growth of many tumor types and the development of anti-angiogenic agents opened a new era in cancer therapy. However, similar to other anti-cancer therapies, inherent/acquired resistance to anti-angiogenic drugs may occur in cancer patients leading to disease recurrence. Recent studies in several experimental models suggest that both tumor and non-tumor (stromal) cell types may be involved in the reduced responsiveness to the treatments. The current review focuses on the role of stromal cells in tumor growth and in refractoriness to anti-VEGF treatment.

Human endothelial precursor cells express tumor endothelial marker 1/endosialin/CD248

Angiogenesis occurs during normal physiologic processes as well as under pathologic conditions such as tumor growth. Serial analysis of gene expression profiling revealed genes [tumor endothelial markers (TEM)] that are overexpressed in tumor endothelial cells compared with normal adult endothelial cells. Because blood vessel development of malignant tumors under certain conditions may include endothelial precursor cells (EPC) recruited from bone marrow, we investigated TEM expression in EPC. The expression of TEM1 or endosialin (CD248) and other TEM has been discovered in a population of vascular endothelial growth factor receptor 2+/CD31+/CD45-/VE-cadherin+ EPC derived from human CD133+/CD34+ cells. EPC share some properties with fully differentiated endothelial cells from normal tissue, yet reverse transcription-PCR and flow cytometry reveal that EPC express higher levels of endosialin at the molecular and protein levels. The elevated expression of endosialin in EPC versus mature endothelial cells suggests that endosialin is involved in the earlier stages of tumor angiogenesis. Anti-endosialin antibodies inhibited EPC migration and tube formation in vitro. In vivo, immunohistochemistry indicated that human EPC continued to express endosialin protein in a Matrigel plug angiogenesis assay established in nude mice. Anti-endosialin antibodies delivered systemically at 25 mg/kg were also able to inhibit circulating murine EPC in nude mice bearing s.c. SKNAS tumors. EPC and bone marrow-derived cells have been shown previously to incorporate into malignant blood vessels in some instances, yet they remain controversial in the field. The data presented here on endothelial genes that are up-regulated in tumor vasculature and in EPC support the hypothesis that the angiogenesis process in cancer can involve EPC.

Preoperative bevacizumab combined with letrozole and chemotherapy in locally advanced ER- and/or PgR-positive breast cancer: clinical and biological activity

The antiangiogenic agent bevacizumab showed synergistic effects when combined with chemotherapy in advanced breast cancer. We presently investigated the activity of bevacizumab in combination with chemotherapy, including capecitabine and vinorelbine, and endocrine therapy, including letrozole (+triptorelin in premenopausal women), as primary therapy for patients with ER and/or PgR ⩾10% T2–T4a-c, N0–N2, M0 breast cancer. Biological end point included the proliferative activity (Ki67), whereas clinical end points were clinical response rate, pathological complete response (pCR) and tolerability. Circulating endothelial cells (CECs) and their progenitors, as surrogate markers of antiangiogenic activity, were measured at baseline and at surgery.Thirty-six women are evaluable. A clinical response rate of 86% (95% CI, 70–95) and no pCR were observed; Ki67 was significantly decreased by 71% (interquartile range, −82%, −62%). Toxicity was manageable: two grade 3 hypertension, four grade 3 deep venous thrombosis and no grade >2 proteinuria were observed. Treatment significantly decreased the percentage of viable CECs and prevented the chemotherapy-induced mobilisation of circulating progenitors. Basal circulating progenitors were positively associated with clinical response. In conclusion, bevacizumab is feasible and active in association with primary chemoendocrine therapy for ER-positive tumours in terms of proliferation inhibition, clinical response and antiangiogenic activity.

The isolation of novel mesenchymal stromal cell chemotactic factors from the conditioned medium of tumor cells

Bone marrow-derived mesenchymal stromal cells (MSCs) localize to solid tumors. Defining the signaling mechanisms that regulate this process is important in understanding the role of MSCs in tumor growth. Using a combination of chromatography and electrospray tandem mass spectrometry we have identified novel soluble signaling molecules that induce MSC chemotaxis present in conditioned medium of the breast carcinoma cell line MDA-MB231. Previous work has employed survey strategies using ELISA assay to identify known chemokines that promote MSC chemotaxis. While these studies provide valuable insights into the intercellular signals that impact MSC behavior, many less well-described, but potentially important soluble signaling molecules could be overlooked using these methods. Through the less directed method of column chromatography we have identified novel candidate MSC chemotactic peptides. Two proteins, cyclophilin B and hepatoma-derived growth factor were then further characterized and shown to promote MSC chemotaxis.

Tumour vascular disrupting agents: combating treatment resistance

A large group of tubulin-binding microtubule-depolymerizing agents act as tumour vascular disrupting agents (VDAs). Several members of this group are now in clinical trials in combination with conventional anticancer drugs and radiotherapy. Here we briefly update on the development of tubulin-binding combretastatins as VDAs, summarize what is known of their mechanisms of action and address issues relating to treatment resistance, using disodium combretastatin A-4 3-O-phosphate (CA-4-P) as an example. Characteristically, VDAs cause a rapid shutdown of blood flow to tumour tissue with much less effect in normal tissues. However, the tumour rim is relatively resistant to treatment. Hypoxia (or hypoxia reoxygenation) induces upregulation of genes associated with angiogenesis and drug resistance. It may be possible to take advantage of treatment-induced hypoxia by combining with drugs that are activated under hypoxic conditions. In summary, VDAs provide a novel approach to cancer treatment, which should effectively complement standard treatments, if treatment resistance is addressed by judicious combination treatment strategies.