Synthesis and biological evaluation of norcantharidin analogues: towards PP1 selectivity.

Simple modifications to the anhydride moiety of norcantharidin have lead to the development of a series of analogues displaying modest PP1 inhibition (low muM IC(50)s) comparable to that of norcantharidin (PP1 IC(50)=10.3+/-1.37 microM). However, unlike norcantharidin, which is a potent inhibitor of PP2A (IC(50)=2.69+/-1.37 microM), these analogues show reduced PP2A inhibitory action resulting in the development of selective PP1 inhibitory compounds. Data indicates that the introduction of two ortho-disposed substituents on an aromatic ring, or para-substituent favours PP1 inhibition over PP2A inhibition. Introduction of a p-morphilinoaniline substituent, 35, affords an inhibitor displaying PP1 IC(50)=6.5+/-2.3 microM; and PP2A IC(50)=7.9+/-0.82 microM (PP1/PP2A=0.82); and a 2,4,6-trimethylaniline, 23, displaying PP1 IC(50)=48+/-9; and PP2A IC(5) 85+/-3 microM (PP1/PP2A=0.56). The latter shows a 7-fold improvement in PP1 versus PP2A selectivity when compared with norcantharidin. Subsequent analysis of 23 and 35 as potential PP2B inhibitors revealed modest inhibition with IC(50)s of 89+/-6 and 42+/-3 microM, respectively, and returned with PP1/PP2B selectivities of 0.54 and 0.15. Thus, these analogues are the simplest and most selective PP1 inhibitors retaining potency reported to date.