Antimetastatic activity of a cyclooxygenase-2 inhibitor

In most cancer patients, metastases are already present at time of diagnosis (Fidler and Ellis, 1994) and while surgery remains the mainstay of treatment for primary tumours it may paradoxically enhance growth of residual or metastatic disease (Da Costa et al, 1998; Pidgeon et al, 1999; Demichelli et al, 2001). This may occur as a consequence of an alteration in the balance between pro- and antiangiogenic factors as part of the healing process and removal of the tumour can of itself stimulate tumour growth by removing the source of angiostatin (O'Reilly et al, 1997). There is an increase in growth factors in the immediate postoperative period (Nissen et al, 1998) and although angiogenesis is essential for wound healing it also plays a key role in the growth and metastasis of tumours (Folkman, 1997). In addition, surgical manipulation may also increase tumour cell dissemination into the bloodstream resulting in the seeding of tumour cells in distant organs and the establishment of metastases (Fisher and Fisher, 1965; Hansen et al, 1995). As most cancer patients ultimately die of metastatic disease, it is important to develop therapies that are effective against metastases. Several studies show that regularly taking aspirin or other conventional nonsteroidal anti-inflammatory drugs (NSAIDs) provides a 40–50% reduction in the relative risk of death by colon cancer, indicating that inhibition of cyclooxygenase (COX), both COX-1 and COX-2, has a chemopreventive effect (Dubois et al, 1998). In rodent models of Familial Adenomatous Polyposis (FAP), a genetic disease leading to colon carcinoma, blockade of COX-2, either by gene deletion or pharmacological inhibition of enzyme activity, suppresses intestinal polyp formation (Oshima et al, 2001). Cyclooxygenase-2 inhibition also demonstrates chemopreventive activity against colon carcinogenesis in rodent models (Reddy et al, 1996). NSAIDS inhibit the activity of both COX enzymes and consequently can inhibit or abolish the effects of prostaglandins (Bjorkman, 1998; Hawkey, 1999). Selective inhibition of the COX-2 isoform has reduced toxicity profile compared to inhibition of both isoforms (Scheiman, 2002). It has been suggested that arachidonic acid, the substrate for COX-2, induces apoptosis and that depletion of arachidonic acid by COX-2 activity decreases apoptosis (Cao et al, 2000). We previously demonstrated that COX inhibition reduced the growth of a primary tumour, number and incidence of spontaneous metastases accompanied by increased apoptosis and decreased microvessel density in the primary tumour (Connolly et al, 2002). Prostaglandin E2 (PGE2), is produced in large amounts by some tumours and has been implicated in the promotion and growth of malignant tumours (Honn et al, 1981). PGE2 is produced from arachidonic acid by either of two enzymes: COX-1 is expressed constitutively in most tissues whereas COX-2 is predominantly inducible but these classifications are not absolute. Cyclooxygenase-2 is markedly increased at sites of inflammation and at sites of proliferation such as within tumours (Hawkey, 1999) One of the mechanisms by which PGE2 may support tumour growth is by inducing angiogenesis necessary to supply oxygen and nutrients to tumours (Form and Auerbach, 1983). Cyclooxygenase-2 expression has been observed in newly formed blood vessels within tumours, whereas under normal physiological conditions the quiescent vasculature expresses only the constitutive COX-1 enzyme (Masferrer et al, 1999). Growth, invasion and metastasis of many cancers depend on angiogenesis (Folkman, 1990). The current view is that the net balance between endogenous angiogenesis stimulators and inhibitors regulates the ‘switch to the angiogenic phenotype’ (Hanahan and Folkman, 1996). Vascular endothelial growth factor (VEGF)/vascular permeability factor (VPF) is the most potent angiogenic factor identified. VEGF induces both the migration and proliferation of endothelial cells in vitro while inhibiting endothelial cell apoptosis (Connolly et al, 1989). Therapeutic blockade of VEGF has been shown to inhibit primary and metastatic tumour growth in animal models (Kim et al, 1993; Asano et al, 1995; Benjamin and Keshet 1997), which has been attributed to an antiangiogenic effect. We and others have recently shown that VEGF can also act as a survival factor for tumour cells, protecting them from apoptosis (Pidgeon et al, 2001; Beierle et al, 2002; Harmey and Bouchier-Hayes, 2002a). Although there are many studies showing that COX-2 inhibitors have antitumour activity many of these have used chemically induced tumours and there are few, if any, studies examining antimetastatic effects of COX inhibition. We examined the antimetastatic activity of COX-2 inhibitors in an orthotopic model following excision of the primary tumour and in an experimental metastasis model.