Cantharidin analogues: synthesis and evaluation of growth inhibition in a panel of selected tumour cell lines.

Diels-Alder addition of furans (furan, furfuryl alcohol, and 3-bromofuran) to maelic anhydride yields three distinct 5,6-dehydronorcantharidins. Hydrogenation of (4,10-dioxatricyclo[5.2.1.0]decane-3,5-dione) (4a), in dry ethanol affords the monoester (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic aid monoethyl ester) (6). Subsequent transesterification affords a series of monoesters (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid monomethyl ester (7)), 7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid monopropyl ester (8), (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid monohexyl ester (9)) and differentially substituted diesters (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid 2-ethyl ester 3-isopropyl ester) (10), and (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid 2-ethyl ester 3-phenyl ester) (11). Analogues were firstly screened for their ability to inhibit protein phosphatases 1 (PP1) and 2A (PP2A) as the lead compounds cantharidin (1) and norcantharidin (2) are known PP1 and PP2A inhibitors. Only analogues 4a, 6-8 displayed good PP1 and PP2A inhibition (PP1 IC(50)'s=2.0, 2.96, 4.71, and 4.82 microM, respectively; PP2A IC(50)'s=0.2, 0.45, 0.41, and 0.47 microM, respectively). All analogues were also screened for their anti-cancer potential against a panel of tumour cell lines, HL60, L1210, SW480, WiDr, HT29, HCT116, A2780, ADDP, and 143B, producing GI(50) values ranging from 6 microM to >1000 microM. Analogues possessing good PP1 and/or PP2A inhibition also returned moderate to good anti-cancer activity. Analogues with substituents directly attached to the intact bicyclo[2.2.1]heptane skeleton were poor to moderate anti-cancer agents. This correlates well with their lack of PP1 or PP2A activity. Analogues capable of undergoing a facile ring opening of the anhydride or with a single carboxylate were good PP1 and PP2A inhibitors, largely correlating to the observed anti-cancer activity in all cases, except 11. Analogue 11, whist neither a PP1 nor a PP2A inhibitor shows anti-cancer activity comparable to 1 and 2. We believe that intracellular esterases generate the corresponding dicarboxylate, which is a potent PP1 and PP2A inhibitor, and that it is this species which is responsible for the observed anti-cancer activity.