Compound 401 is a potent inhibitor of DNA-PK (IC ₅₀ =0.28 μM). Compound 401 is reported to be a poor inhibitor of PI3K, ATM, and ATR in vitro, but it is active against mTOR. Compound 401 shows activity against mTOR (IC ₅₀ =5.3 μM) but not p110α/p85α PI3K (IC ₅₀ >100 μM). Treatment of cells with Compound 401 blocks the phosphorylation of sites modified by mTOR-Raptor and mTOR-Rictor complexes (ribosomal protein S6 kinase 1 Thr ³⁸⁹ and Akt Ser ⁴⁷³ , respectively). By contrast, there is no direct inhibition of Akt Thr ³⁰⁸ phosphorylation, which is dependent on PI3K. Similar effects are also observed in cells that lack DNA-PK. Compound 401 inhibits immunoprecipitated epitope-tagged mTOR or endogenous mTOR in Raptor immunoprecipitates. In both cases, inhibition of 67% or 78% is obtained at 5 μM or 10 μM Compound 401, respectively. By contrast, dose response curves show that the p110α/p85α or p110β/p85α PI3K complexes are poorly inhibited by Compound 401 at these concentrations. The proliferation of TSC1 ^-/- fibroblasts is inhibited in the presence of Compound 401, but TSC1 ^+/+ cells are resistant.