Phosphatidylinositol 3-kinase inhibitors are a triple threat to ovarian cancer.

In this issue of Clinical Cancer Research, Hu et al. assess the role of phosphatidylinositol 3-kinase (PI3K) inhibition in vascular permeability, angiogenesis, and vascular remodeling in both tumor vessels and the peritoneal lining an athymic mouse model of intraperitoneal human ovarian carcinoma (1). Ovarian cancer is a vicious disease. Often, ovarian cancer presents unexpectedly at an advanced stage with tumor cells widely infesting the peritoneal cavity. Current treatment for such patients is rarely adequate. Genetic analysis of tumor specimens led to the discovery that PIK3CA , a gene involved in numerous receptor signaling pathways, was a major oncogenic force in the development of f40% of sporadic ovarian cancers (1). In ovarian cancer, multiple tumor cell lines exist that have increased PIK3CA copy number and gene expression. Such cell lines are useful models of the disease that can be studied in vitro and in mouse peritoneal xenografts (1). The importance of PIK3CA to malignancy is not limited to ovarian cancer. PIK3CA was first recognized as an oncogene because of its ability to bind polyoma middle T antigen, a DNA tumor virus oncogene (2). Later, PIK3CA was found as a retroviral oncogene that transformed normal chicken embryo fibroblasts into tumors (3). Increased PIK3CA copy numbers are found not only in carcinomas of the ovary but also in carcinomas of the cervix, stomach, and thyroid (4–9). More recently, it has emerged that PIK3CA is one of the most frequently mutated human oncogenes in cancer (10, 11). Although missense mutations are prevalent in carcinomas of the colon and breast, they are rare in ovarian cancer (12). PIK3CA , therefore, seems to be a major oncogene in human epithelial malignancies, which is altered in ovarian cancer mainly through amplification. PIK3CA encodes the catalytic p110 subunit of PI3K-a (p110a), which produces phosphatidylinositol-3,4,5-triphosphate (PI-3,4,5-P3). PI-3,4,5-P3 is a second messenger that stimulates a variety of proteins in the cell that activate proliferation, migration, translation, energy consumption, and angiogenesis and repress apoptosis (13). In normal cells, p110a responds to a variety of stimuli to produce a pulse of PI-3,4,5-P3 and then returns to its dormant state within minutes of activation. However, ovarian tumor cell lines with amplification of PIK3CA have increased levels of p110a enzyme and enzyme activity (2). In general, tumor cells use the enhanced PI-3,4,5-P to proliferate, migrate, and avoid cell death signals and importantly secrete vascular endothelial growth factor (VEGF), a key mediator of angiogenesis and capillary permeability (14, 15). Recently, it has been shown that RNA interference knockdown of PIK3CA message has potent antitumor effects in ovarian lines with PIK3CA amplification (16). In addition, deletion of mutant alleles in colon cancer cells enhanced apoptosis substantially (17). A similar line of evidence has accumulated in tumor cells lacking the PTEN tumor suppressor, which have elevated PI-3,4,5-P, because PTEN is the phosphatase that inactivates PI-3,4,5-P. Here, reintroduction of PTEN inhibited p110a signaling and induced cell death (14, 18). This body of evidence strongly supports the hypothesis that p110a is critical to the viability of ovarian carcinomas with genetic alterations of the PIK3CA pathway. On these grounds, p110a would seem to be an excellent target for therapy in tumors that have a genetic activation of the gene. There are two readily available inhibitors of p110-a, wortmannin and LY294002. Both of these drugs are effective in cells; however, both drugs inhibit other members of the PI3K family at the same concentration (19, 20). In fact, LY294002 is known to inhibit the mammalian target of rapamycin (mTOR), an enzyme that is activated by p110a, and therefore may represent a synergistic off-target effect. Despite their lack of specificity, both of these inhibitors unequivocally shut down p110a and its downstream cascade of signals. When ovarian tumor cells with PIK3CA amplification are exposed to LY294002 in tissue culture, cell death is induced (2). Cells lacking such genetic changes are only modestly affected by the drug. It is clear from in vitro data that cell lines lacking genetic alterations to PIK3CA are far less susceptible to the effects of a p110a inhibitor than cells that have such alterations (2, 17). Given these positive findings in vitro, the next major question is whether these inhibitors could also work in an animal model. Can sufficient drug levels be achieved? Can a therapeutic window be identified in a whole animal, given the multitude of physiologic signals that the PI3K family of enzymes regulates? To study the potential therapeutic benefit of p110a inhibitors in vivo, Hu et al. at the University of California, San Francisco generated an intraperitoneal ovarian cancer mouse model using human cell lines in immunodeficient mice (3). With this model, they were the first to show that LY294002 inhibits tumor growth in vivo. Tumor inhibition occurred through the induction of apoptosis; when LY294002 was administered in combination with paclitaxel, almost no