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Aberrant genetic alternations in human gliomas such as amplification of EGFR, mutation and/or deletion of tumor suppressor gene PTEN, and mutation of PIK3CA contribute to constitutive activation of PI3K and Akt activity, an essential signal pathway controlling many pathways involved in cell proliferation, apoptosis, angiogenesis etc. These observations strongly support the PI3K/Akt pathway being a critical target for molecular therapeutic development.

We investigate the potential anti-tumor activity of a novel PI3K inhibitor in human glioma in vitro and in vivo. We first tested the effect of this compound on cell growth using a sulforhodamine B assay (SRB). Compound treatment suppressed glioma cell proliferation with IC50 at low nM concentration. Interestingly, wild-type PTEN cell lines appeared to be less sensitive to the PI3K inhibitor than their PTEN-negative counterpart. Compound treatment inhibited activity of target genes as assessed by the detection of phosphorylated protein level along PI3K/Akt/mTOR signaling pathway, including Akt, S6K1, S6, and 4EBP1 but not Erk1/2. Inhibition of the signaling led to G1 cell cycle arrest in all cell lines tested regardless of their PTEN status. Expression of the vascular endothelial growth factor (VEGF), an important angiogeneic modulator in glioma cells was significantly attenuated, suggesting a potential anti-angiogenic activity. Although the compound did not induce apoptosis, it however induced autophagy, a type II programmed cell death, in glioma cell lines demonstrated by acridine orange staining and flow cytometric analysis. These results established the effect of this inhibitor on glioma survival. Furthermore, a preclinical testing for the therapeutic efficacy of the compound demonstrated that in vivo oral daily administration at 25 mg/kg for 4 weeks significantly (p< 0.05) prolonged survival of intracranial U87 xenograft animals without weight loss, suggesting its effectiveness and safety. Results from analyzing tumor extracts harvested from animals at 2 and 4 weeks post-treatment recapitulated the in vitro findings in that the compound inhibited activity of target genes along the PI3K/Akt/mTOR cascade. Further, immunohistochemical analysis showed a great reduction of von Willebrand factor (Factor VIII) positive staining in compound-treated tumor section as compared to the controls, suggesting an anti-angiogenic effect of the compound in vivo.

In summary, by antagonizing its targeted signaling nodes, this PI3K inhibitor induced cell cycle arrest, down-regulation of VEGF, and autophagy. It is likely that the prolonged animal survival following compound treatment is attributed to its concerted activities. These results warrant further development of this compound for clinical trial for human gliomas or other advanced cancers with altered PTEN/PI3K signaling.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA