Heterocyclic substituted cantharidin and norcantharidin analogues - synthesis, protein phosphatase (1 and 2A) inhibition, and anti-cancer activity

Norcantharidin (3) is a potent PP1 (IC50 = 9.0 +/- 1.4 mu M) and PP2A (IC50 = 3.0 +/- 0.4 mu M) inhibitor with 3-fold PP2A selectivity and induces growth inhibition (GI(50) similar to 45 mu M) across a range of human cancer cell lines including those of colorectal (HT29, SW480), breast (MCF-7), ovarian (A2780), lung (H460), skin (A431), prostate (DU145), neuroblastoma (BE2-C), and glioblastoma (SJ-G2) origin. Until now limited modifications to the parent compound have been tolerated. Surprisingly, simple heterocyclic half-acid norcantharidin analogues are more active than the original lead compound, with the morphilino-substituted (9) being a more potent (IC50 = 2.8 +/- 0.10 mu M) and selective (4.6-fold) PP2A inhibitor with greater in vitro cytotoxicity (GI(50) similar to 9.6 mu M) relative to norcantharidin. The analogous thiomorpholine-substituted (10) displays increased PP1 inhibition (IC50 = 3.2 +/- 0 mu M) and reduced PP2A inhibition (IC50 = 5.1 +/- 0.41 mu M), to norcantharidin. Synthesis of the analogous cantharidin analogue (19) with incorporation of the amine nitrogen into the heterocycle further increases PP1 (IC50 = 5.9 +/- 2.2 mu M) and PP2A (IC50 = 0.79 +/- 0.1 mu M) inhibition and cell cytotoxicity (GI(50) similar to 3.3 mu M). These analogues represent the most potent cantharidin analogues thus reported. Crown Copyright (c) 2007 Published by Elsevier Ltd. All rights reserved.