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Abstract

EXPERIMENTAL DESIGN: The prevalence of PI3K/MAPK pathway activation in RMS clinical samples was assessed using immunohistochemistry. Compensatory signaling and crosstalk between PI3K/MAPK pathways was determined in RMS cell lines following p110α shRNA-mediated depletion. Pharmacological inhibition of reprogrammed signaling in stable p110α knockdown lines was used to determine the target-inhibition profile inducing maximal growth inhibition. The in vitro and in vivo efficacy of inhibitors of TORC1/2(AZD8055), MEK(AZD6244) and P13K/mTOR(NVP-BEZ235) were evaluated alone and in pair-wise combinations.

RESULTS: PI3K pathway activation was seen in 82.5% rhabdomyosarcomas with co-activated MAPK in 36% and 46% of alveolar and embryonal sub-types respectively. p110α knockdown in cell lines over the short and long term was associated with compensatory expression of other p110 isoforms, activation of the MAPK pathway and cross-talk to reactivate the PI3K pathway. Combinations of PI3K pathway and MEK inhibitors synergistically inhibited cell growth in vitro. Treatment of RD cells with AZD8055 plus AZD6244 blocked reciprocal pathway activation, as evidenced by reduced AKT/ERK/S6 phosphorylation. In vivo, the synergistic effect on growth and changes in pharmacodynamic biomarkers was recapitulated using the AZD8055/AZD6244 combination but not NVP-BEZ235/AZD6244. Pharmacokinetic analysis provided evidence of drug-drug interaction with both combinations.

CONCLUSIONS: Dual PI3K/MAPK pathway activation and compensatory signaling in both rhabdomyosarcoma subtypes predicts a lack of clinical efficacy for single agents targeting either pathway, supporting a therapeutic strategy combining a TORC1/2 with a MEK inhibitor.