Angiogenesis, the formation of new blood vessels, is required for tumor growth and metastasis [
1,
2]. The ability of tumors to promote angiogenesis is driven by expression of pro-angiogenic factors such as VEGF, bFGF, PDGF and transforming growth factor-β (TGF-β) of which VEGF is the most important [
1,
3]. VEGF exerts its actions through two receptor tyrosine kinases, VEGFR-1 and VEGFR-2, but it is signaling via VEGFR-2 that is important for tumor angiogenesis [
4]. A third VEGF receptor VEGFR-3, is important for lymphangiogenesis and is activated by two different ligands, VEGF-C and VEGF-D [
5]. In view of the dependence of tumors on angiogenesis for sustained growth, targeting angiogenesis has been the focus of much research into new anti-cancer therapies in recent years [
6]. Direct angiogenesis inhibitors target endothelial cells by inhibiting their ability to proliferate, migrate and form new blood vessels. The first example of this was the use of an antibody to VEGF, which was able to inhibit the growth of tumors in mouse models [
7]. Subsequently a humanized version of this antibody, bevacizumab, was developed which showed promising evidence of efficacy in pre-clinical models. However, this was not translated into the clinical setting. With the exception of renal carcinoma and glioblastoma multiforme [
8,
9], bevacizumab appears to have little activity as a single agent, although it does confer significant benefit when combined with cytotoxic agents and is now approved by the US Food and Drug Administration for use in colorectal, breast, lung and renal cancer, and glioblastoma multiforme. Other approaches to target angiogenesis have centered on the use of small molecule kinase inhibitors, which target VEGFR-2. Many of these inhibitors are multi-targeted with additional activity against one or a number of other receptor tyrosine kinases. One such inhibitor is E7080, an orally active multi-targeted tyrosine kinase inhibitor, which is currently in clinical development. Three dose escalation phase I trials of E7080 have now been performed, examining different dosing schedules [
10‐
12]. In view of promising anti-tumor effects observed, a number of disease specific phase II and III trials, including melanoma, renal, thyroid, ovarian, hepatocellular and endometrial cancer, are now underway. E7080 is a potent inhibitor of VEGFR-2 and VEGFR-3 with IC
50s of 4 and 5.2 nM respectively, but also has activity against VEGFR-1, FGFR-1, and PDGFRα/β tyrosine kinases although the IC
50s are around 10 fold higher [
13]. E7080 shows anti-tumor activity in human cancer cell xenografts, which has been attributed to its ability to inhibit angiogenesis predominantly through effects on VEGFR-2 inhibition but also through inhibition of KIT and FGFR-1 [
13‐
15]. Inhibition of VEGFR-3 mediated lymphangiogenesis by E7080 also contributed to its ability to suppress lymph node and lung metastases in a mammary tumor model [
16]. Although these effects were not mediated via direct effects of E7080 on tumor cells two of the main targets of E7080, FGFR-1 and PDFGR, are expressed in a number of solid tumors. We therefore set out to determine whether E7080, in addition to effects on angiogenesis, could directly affect the behavior of epithelial tumor cells.
FGFR-1 amplification and over expression has been reported in a sub-set of breast cancers [
17], lung squamous cell carcinoma [
18] and also in oral squamous carcinoma, rhabodomyosarcoma, ovarian and bladder cancer [
19]. In melanoma there is evidence for a bFGF/FGFR-1 autocrine loop driving proliferation
in vivo [
20,
21] indicating that in some tumor types inhibitors such as E7080 may also have direct effects on the proliferation of tumor cells. The PDGFR and its ligands are also expressed in a number of different tumor types including gliomas, breast and ovarian cancer and there is evidence for PDGFR autocrine growth control in gliomas [
22,
23]. However, PDGFR signaling is also important in tumor stromal cells and many effects of PDGF in tumor cells may be mediated via paracrine activation of stromal cells [
24,
25] and in particular endothelial cells [
23,
26]. The effects of E7080 on tumor angiogenesis may therefore in part be mediated via inhibition of PDGFR signaling in endothelial cells. A number of small molecule tyrosine kinase inhibitors that target both the FGFR and the PDGFR are now in clinical development, although, like E7080, they also inhibit a number of other receptor tyrosine kinases making it difficult to determine the importance of targeting each individual receptor kinase to tumor development [
19,
22,
27].
Cancer cell proliferation is often regarded as the most important aspect of cancer progression, and traditionally, anti-cancer treatment has focused on preventing growth of the tumor. However, other key aspects play a crucial role in cancer progression, including tissue invasion and metastasis [
28]. A malignant tumor's ability to invade and migrate differentiates it from a benign tumor, and hence these processes are crucial for cancer progression. 90% of human cancer deaths are caused by distant metastases and therefore preventing cancer spread could have a huge impact on patient survival. Signaling via both the FGFR-1 and PDGFR has been associated with increased migration, invasion and metastatic potential [
29‐
33].
We therefore set out to determine whether E7080 has any direct effects on tumor cell proliferation, migration and invasion. We show that although there was very little effect of E7080 on the proliferation of a range of human tumor cell lines both their migration and invasion could be blocked at concentrations of E7080 that inhibited signaling through FGFR-1 and PDGFR-β. Furthermore, we show that U2OS osteosarcoma cell migration was inhibited by both E7080 and knock-down of PDGFR-β expression suggesting that E7080 may directly affect the migratory capacity of tumor cells by targeting PDGFR-β. The ability of E7080 to inhibit both migration and invasion in vitro implies that it could potentially have significant benefit in the prevention (or at least in the reduction) of development of tumor metastases in vivo.