Combination Therapies Targeting PDGF and VEGF Signaling Pathways in Solid Tumors

Vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) are independently involved in several cancer-associated mechanisms including autocrine stimulation of cancer cells, stimulation of tumor angiogenesis and regulation of interstitial fluid pressure (IFP). The scope of this thesis was to investigate the combinatory effect of anti-VEGF and anti-PDGF treatment on tumor angiogenesis and tumor IFP. Angiogenesis is a process of formation of blood vessels. Based on the tumors dependency on the blood vessels to supply them with oxygen and nutrients, several anti-angiogenic therapies have been tried and shown to have beneficial anti-tumor effects. More recently, anti-angiogenic treatment appeared to transiently “normalize” disorganized tumor vasculature and therefore to improve the uptake of cytotoxic agents. In the first study, treatment was performed on two tumor models that differ only with regard to the degree of maturation of the vasculature, reflected by different number of pericytes that are the target for anti-PDGF treatment in these tumors. The aim was to study the role of pericyte coverage in protecting endothelial cells from anti-VEGF therapies. In the pericyte-rich tumor model the combination treatment gave a more efficient anti-angiogenic effect. Interestingly, it was only a subset of pericytes that was sensitive for the treatment. In the second paper, the effect of anti-VEGF and anti-PDGF treatment on tumor IFP was measured. IFP is elevated in most solid tumors, which is linked to poor prognosis and higher recurrence rate. Additionally, this serves as a problem in ant-cancer therapies since it makes the uptake of cytotoxic agents inefficient. PDGF is known to actively regulate the IFP by regulating the contractile activity of fibroblasts, while VEGF regulates IFP primarily by affecting vessel leakiness. In the current study, combination of anti-VEGF and anti-PDGF therapies was shown to have an additive effect. However, the timing of administration of inhibitors appeared to be crucial. It was only short, but not long term combination treatment that further reduced IFP as compared to monotherapies. Surprisingly, the additive effect on IFP did not translate into an increased efficiency of chemotherapy when comparing combination treatment with monotherapies. The last paper is a follow up of the first study, where it was shown that combination of anti-VEGF and anti-PDGF treatment affect the tumor vasculature. Here we investigated if the anti-angiogenic effect improves treatment efficiency of a cytotoxic agent. There was a significant effect of the combination of anti-VEGF and anti-PDGF on Taxol treatment efficiency in this Taxol resistant tumor model. However, the mechanism for the treatment effect and the relative contribution of the targeted vasculature in the outcome of the therapy remains to be determined, since tumor cells were also sensitized for Taxol in vitro. In summary, we have shown that targeting of PDGF and VEGF signaling pathways simultaneously affect both vasculature and IFP to a higher extent than monotherapies