Modulating the phosphatidylinositol 3-kinase/mammalian target of rapamycin pathway for therapeutic enhancement of photodynamic therapy

Tumor recurrence due to incomplete eradication of tumor cells is a major problem facing all current cancer therapies. To overcome or minimize this problem, it is necessary to enhance cell killing capability of cancer therapy and/or prevent cell regrowth after treatment. Because phosphatidylinositol 3-kinases (PI3K) pathway plays an important role in stimulating cell survival and growth, this study evaluated the combination of PI3K pathway inhibitors (BEZ235 and LY294002) and photodynamic therapy (PDT) with the photosensitizer benzoporphyrin derivative (BPD) (verteporfinRTM).;We found that treatment with PDT or PI3K inhibitor alone significantly inhibited PI3K/mTOR signaling and cell growth in both SVEC endothelial and PC3 prostate cancer cells, although SVEC cells were more responsive to treatments than PC3 cells. Combination of either LY294002 or BEZ235 with PDT caused greater inhibition of PI3K signaling pathway, leading to enhanced cell growth inhibition in both cell lines. SVEC cells exhibited a higher sensitivity towards such a combination than PC3 cells.;Autophagy was also induced after both PI3K inhibition and after PDT. Autophagic flux assay revealed that PDT actually decreased basal as well as PI3K-inhibitor induced autophagic flux, especially at a high dose of BPD (400ng/m1; ∼LD60). Autophagy appeared to protect cells from PDT-induced cell death because inhibition of autophagy by combination of PDT with chloroquine significantly increased cell death.;Finally, we generated BEZ-resistant SVEC cells and assessed the effect of PDT in these cells. BEZ-adapted cells had higher basal PI3K/mTOR signaling. BEZ-adapted cells had lower basal or inducible autophagic flux. BEZ-adapted cells were more sensitive to disruption of autophagy by chloroquine or PDT, and more so when PDT was combined with chloroquine compared with the parental cell line.;In conclusion, these studies show that the endothelium may be a better acute target for PDT compared with tumor cells and lends more evidence in favor of vascular-targeted PDT in the treatment of cancer. Our results further suggest that although PDT inhibits PI3K/mTOR signaling, it concurrently disrupts autophagic flux. On this basis, PDT may offer a therapeutic option to enhance efficacy of other treatment modalities that induce autophagy or that may utilize autophagy as a survival mechanism.