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We previously reported the discovery of a simple conjugated cyano pharmacophore which had led to the development of (Z)-2-(3,4-dichlorophenyl)-3-(4-nitrophenyl)acrylonitrile, as a selective inhibitor of oestrogen receptor positive (ER+ve) human breast cancer cell line, MCF-7. Further exploration though modification of the acrylonitrile and aromatic substituents has highlighted key structural components necessary for broad spectrum cytotoxicity. The acrylic acid derivates (Z)-2-(3,4-dichlorophenyl)-3-(4-nitrophenyl)acrylic acid and (Z)-2-(3,4-dichlorophenyl)-3-(4-methoxyphenyl)acrylic acid (9) were inactive; confirming the importance of the cyanide moiety. The most potent 2-phenylacrylonitriles synthesized were (Z)-2-(3,4-dichlorophenyl)-3-(1H-indol-3-yl)acrylonitrile and (Z)-2-(3,4-dichlorophenyl)-3-(1H-indol-5-yl)acrylonitrile (20) with an average GI(50) values of 1.4 and 0.53 μM respectively. Five additional (Z)-2-(3,4-dichlorophenyl)-3-(indolyl)acrylonitriles also displayed average GI(50) values of ≤8.4 μM. In the case of indole, this represents a 32-fold increase in broad spectrum cytotoxicity relative to the lead.

Cantharidin (1) and norcantharidin (2) display high levels of anticancer activity against a broad range of tumour cell lines. Synthetic manipulation of norcantharidin yields (3S,3aR,4S,7R,7aS)-3-hydroxyhexahydro-4,7-epoxyisobenzofuran-1(3H)-one (3), which also displays a high level of anticancer activity against tumour cells but interestingly, shows selectivity towards HT29 (colon; GI(50) = 14 μM) and SJ-G2 (glioblastoma; GI(50) = 15 μM) cell lines. Substitution at the hydroxyl group of the cyclic lactone within (3) produces a diasteromeric pair of products that have no difference in cytotoxicity over the cell lines tested. Incorporation of an isopropyl tail at this position (16) produced the most promising compound of this series to date, with strong selectivity towards HT29 (colon; GI(50) = 19 μM) and SJ-G2 (glioblastoma; GI(50) = 21 μM) cell lines but completely void of any activity against the remaining tumour cell lines (GI(50) > 100 μM), as per the parent molecule. We also discovered that the introduction of a terminal phosphate moiety (28) at the same position produced a different trend in cytotoxicity with strong activity in BE2-C (neuroblastoma; GI(50) = 9 μM) cells; suggestive of an alternate mode of action.

A series of anhydride modified cantharidin analogues have been synthesised and screened for their ability to inhibit protein phosphatase 2A. Surprisingly only analogues capable of undergoing a facile ring opening of the anhydride moiety displayed any significant inhibition. Subsequent NMR experiments indicated that 7-oxobicyclo[2.2.1]heptane-2,3-dicarboxylic acid was the major (sole) species under assay conditions. The ability of these modified anhydro-cantharidin analogues to inhibit protein phosphatase 2A varies from 4 (16) to 100% (8) at 100 μM test concentration.

Cantharidin (1) and norcantharidin (2) are potent protein phosphatase 1 and 2A inhibitors that also display high levels of anticancer activity against a broad range of tumor cells lines. Surprisingly, Delta-5,6-ethyl norcantharidin (3, cis-tetrahydrofurano[3,4-c]furan-1,3-dione) displays neither phosphatase inhibition nor anticancer activity. This suggests that the 5,6-ethyl bridge is pivotal to both anti-cancer and protein phosphatase activity. Additionally bioisosteric replacement of the ethereal oxygen has no effect on biological activity nor does modification of the anhydride moiety. Unlike the parent norcantharidin, anhydride ring opening has no effect on either protein phosphatase inhibition or anti-cancer activity. Additionally, this work highlights the discovery of the octyl substituted, cis-5-benzyl-2-hexyltetrahydro-2H,3aH-pyrrolo[3,4-c]pyrrole-1,3-dione, 9p, and the octyl substituted, cis-octyltetrahydro-5H-furo[3,4-c]pyrrole-4,6-dione, 8p, as two new cytotoxic agents which are equipotent (9p) with, and more potent (8p) than norcantharidin.