CH5138303 is a triazine Hsp90 inhibitor. CH5138303 binds to the N-terminus of Hsp90α (Kd = 0.52 nM) and inhibits growth of HCT116 and NCI-N87 human tumor cell lines in vitro (IC50s = 89 and 66 nM, respectively). CH5138303 also inhibits growth of human NCI-N87 gastric cancer xenografts (ED50 = 3.9 mg/kg) and displays high oral bioavailability in mice.