LB-100, a small molecule PP2A inhibitor currently in phase 1 and 2 dose-finding and efficacy trials, is an amide that is stable at pH ≥ 10.5, but can hydrolyze at lower pH values to endothall and N-methylpiperazine. Endothall has been detected in plasma of patients after i.v. LB-100 administration, but in vitro LB-100 hydrolysis has not been studied in detail. LC-MS/MS assays of various LB-100 solutions showed that LB-100 rapidly hydrolyzes in aqueous solutions at room temperature (RT) at pH 5.6-6.5 to endothall (t