Phosphatidylinositol-3-kinase as a therapeutic target in melanoma

Purpose: Phosphatidylinositol-3 kinases (PI3K) are critical for malignant cellular processes including growth, proliferation, and survival, and are targets of drugs in clinical development. We assessed expression of PI3K in melanomas and nevi, and studied associations between PI3K pathway members and in vitro response to a PI3K inhibitor, LY294002. Experimental Design: Using Automated Quantitative Analysis, we quantified expression of p85 and p110α subunits in 540 nevi and 523 melanomas. We determined the IC₅₀ for LY294002 for 11 melanoma cell lines and, using reverse phase protein arrays, assessed the association between levels of PI3K pathway members and sensitivity to LY294002. Results: p85 and p110α tend to be coexpressed (P < 0.0001); expression was higher in melanomas than nevi (P < 0.0001) for both subunits, and higher in metastatic than primary melanomas for p85 (P < 0.0001). Although phospho-Akt (pAkt) levels decreased in all cell lines treated with LY294002, sensitivity was variable. We found no association by t tests between baseline p85, p110α, and pAkt levels and sensitivity to LY294002, whereas pS6 Ser²³⁵ and Ser²⁴⁰ were lower in the more resistant cell lines (P = 0.01and P = 0.004, respectively). Conclusions: Expression of p85 and p110α subunits is up-regulated in melanoma, indicating that PI3K is a good drug target. Pretreatment pS6 levels correlated with sensitivity to the PI3K inhibitor, LY294002, whereas PI3K and pAkt did not, suggesting that full activation of the PI3K pathway is needed for sensitivity to PI3K inhibition. pS6 should be evaluated as a predictor of response in melanoma patients treated with PI3K inhibitors, as these drugs enter clinical trials.