7-Oxa[2.2.1]bicycloheptane-2,3-dicarboxylic acid derivatives as phosphatase inhibitors

Cantharidin (1) as well as norcantharidin or endothall (2) have been described to be inhibitors of protein phosphatase 2A, but were reported to be inactive as inhibitors of calcineurin. Using an HPLC-assay, we have discovered that 1 and 2 are inhibitors of calcineurin-dependent (appKi = 10.8 and 3.26 μM, respectively) dephosphorylation of a phosphopeptide derived from the RII subunit of c-AMP dependent kinase, H-DLDVPIPGRFDRRVS(PO4)VAAE-OH (7), which is commonly used in calcineurin assays. The phosphatase inhibitor 6 also inhibits the production of IL-2 in Jurkat cells, a calcineurin dependent process, with IC50 = 4.7 μM, indicating that some of the biological activity of this substance class may be due to calcineurin inhibition, in addition to the established PP2A inhibition.