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PURPOSE: The phosphatidylinositol 3'-kinase (PI3K)/AKT/molecular target of rapamycin (mTOR) pathway is involved in the development of tumor resistance to endocrine therapy in breast cancer cell lines and represents an attractive target for pharmacologic intervention. However, the effects of endocrine therapy with aromatase inhibitors on in vivo expression of this signaling cascade, and its relation to tumor response and patient outcome, is unknown. EXPERIMENTAL DESIGN: PI3K, phospho-AKT (pAKT) and phospho-mTOR were assessed by immunohistochemistry on tumor specimens collected at baseline and after 6 months of treatment in 113 elderly breast cancer patients consecutively enrolled in a randomized phase II trial of primary letrozole therapy and letrozole associated with metronomic cyclophosphamide. RESULTS: Basal expression of the pathway was not significantly correlated with response or patient outcome. Both letrozole alone and letrozole with cyclophosphamide resulted in a significant reduction of PI3K expression (P = 0.02 and P < 0.005, respectively) and phospho-mTOR expression (P = 0.0001 and P = 0.0001, respectively). pAKT showed no change in the letrozole arm, whereas it was significantly decreased in the letrozole plus cyclophosphamide arm (P < 0.005). pAKT expression reduction was associated with a greater response rate (P = 0.05) and greater reduction in Ki67 expression (P = 0.05). Phospho-mTOR expression reduction was associated with a significantly longer disease-free survival in a multivariate analysis (P = 0.02). CONCLUSIONS: Letrozole inhibits key molecules in the PI3K pathway that are important targets of new drugs being developed to overcome resistance. Changes in these molecules may have prognostic significance. These results should be taken into account when planning prospective trials testing up-front aromatase inhibitor with drugs targeting the PI3K/AKT/mTOR signaling pathway.

Original publication

DOI

10.1158/1078-0432.CCR-07-1046

Type

Journal article

Journal

Clin Cancer Res

Publication Date

01/05/2008

Volume

14

Pages

2673 - 2680

Keywords

Aged, Antineoplastic Agents, Antineoplastic Combined Chemotherapy Protocols, Biomarkers, Tumor, Breast Neoplasms, Cyclophosphamide, Down-Regulation, Female, Humans, Nitriles, Phosphatidylinositol 3-Kinases, Protein Kinases, Proto-Oncogene Proteins c-akt, TOR Serine-Threonine Kinases, Tissue Array Analysis, Triazoles