The vascular targeting agent combretastatin A-4-phosphate induces neutrophil recruitment to endothelial cells in vitro.

BACKGROUND: Combretastatin A-4-phosphate (CA-4-P) is a microtubule depolymerising agent currently in clinical trial as a tumour vascular-targeting agent. In vivo, CA-4-P causes rapid shutdown of tumour blood flow (within minutes) and a significant neutrophil infiltration at later times. MATERIALS AND METHODS: Using an in vitro flow-cell assay, we investigated neutrophil-endothelial cell interactions and associated mechanisms, following endothelial cell exposure to CA-4-P. Cellular adhesion molecule (CAM) expression was examined using immunoblotting and immunofluorescence, and the role of CAM in neutrophil recruitment was investigated using specific blocking antibodies. RESULTS: Exposure of HUVEC to CA-4-P, resulted in significant neutrophil recruitment, and increased expression of endothelial CAM. Results of antibody studies demonstrated that endothelial CAM expression induced by CA-4-P is responsible for the observed neutrophil recruitment. DISCUSSION: This study demonstrated that the tumour vascular targeting agent, CA-4-P, directly induces endothelial CAM expression and subsequent neutrophil recruitment. In vivo, neutrophil infiltration probably contributes to CA-4-P-induced tumour cell kill. Therefore, increasing neutrophil recruitment into tumours may have potential for optimising vascular-targeted strategies for cancer therapy.

The biology of the combretastatins as tumour vascular targeting agents.

The tumour vasculature is an attractive target for therapy. Combretastatin A-4 (CA-4) and A-1 (CA-1) are tubulin binding agents, structurally related to colchicine, which induce vascular-mediated tumour necrosis in animal models. CA-1 and CA-4 were isolated from the African bush willow, Combretum caffrum, and several synthetic analogues are also now available, such as the Aventis Pharma compound, AVE8062. More soluble, phosphated, forms of CA-4 (CA-4-P) and CA-1 (CA-1-P) are commonly used for in vitro and in vivo studies. These are cleaved to the natural forms by endogenous phosphatases and are taken up into cells. The lead compound, CA-4-P, is currently in clinical trial as a tumour vascular targeting agent. In animal models, CA-4-P causes a prolonged and extensive shut-down of blood flow in established tumour blood vessels, with much less effect in normal tissues. This paper reviews the current understanding of the mechanism of action of the combretastatins and their therapeutic potential.

Influence of hypoxia and tumour-conditioned medium on endothelial cell adhesion molecule expression in vitro.

BACKGROUND: Cell adhesion molecule expression by tumour endothelium is involved in the efficacy of several cancer therapies. This study investigated the effect of tumour microenvironmental conditions, i.e. reduced oxygenation and tumour-conditioned medium (TCM), on the expression of adhesion molecules by HUVECs. MATERIALS AND METHODS: E-selectin (CD62-E), VCAM-1 (CD106) and PECAM-1 (CD31) were measured using an ELISA assay. Reduced oxygen tension (approximately 0%, 1% vs. 20%) and the presence of TCM from human breast adenocarcinoma MDA-MB-231 was investigated following the treatment of HUVECs with TNFalpha. RESULTS: E-selectin (peak at 4 hours) and VCAM-1 (peak at 24 hours) expression were significantly increased by TNFalpha but unchanged by reduced oxygenation. TCM significantly attenuated E-selectin (71%), VCAM-1 (74%) and PECAM (62%) response to TNFalpha. CONCLUSION: Tumour-secreted factors have a greater inhibitory action than reduced oxygen tension on HUVECs adhesion molecule expression induced by TNFalpha. Reduced expression of adhesion molecules may limit cancer therapies that mediate their action by host cell infiltration.

Neutrophils as inflammatory and immune effectors in photodynamic therapy-treated mouse SCCVII tumours.

Neutrophils have become recognised as important contributors to the effectiveness of tumour eradication by photodynamic therapy (PDT). In this study, we have used the mouse SCCVII squamous cell carcinoma model to investigate the activity of neutrophils in tumours treated by PDT. Tumour levels of neutrophilic myeloperoxidase (MPO) demonstrated not only a massive and sustained sequestration of these cells in PDT-treated tumours but also revealed their activated state evidenced by the presence of released MPO. Among the adhesion molecules expressed on tumour vascular endothelium, ICAM-1 appears to be of primary importance in the invasion of neutrophils into PDT-treated tumours, because its functional blocking with monoclonal antibodies reduced the tumour cure rate. A marked upregulation of its ligands CD11b/CD18 and CD11c/CD18 found on neutrophils associated with PDT-treated tumours supports this assumption. To evaluate the role of inflammatory cytokines regulating neutrophil activity, neutralising antibodies were given to mice before PDT treatment. The results suggest that IL-1beta activity is critical for the therapeutic outcome, since its neutralisation diminished the cure rates of PDT-treated tumours. No significant effect was observed with anti-IL-6 and anti-TNF-alpha treatment. Further flow cytometry-based examination of neutrophils round in PDT-treated tumours revealed that these cells express MHC class II molecules, which suggests their engagement as antigen-presenting cells and involvement in the development of antitumour immune response.