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Anticancer Activity and Protein Phosphatase 1 and 2A Inhibition of a New Generation of Cantharidin Analogues

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Abstract

Cantharidin (Spanish Fly) is a naturaltoxin and an inhibitor of proteinphosphatases 1 (PP1) and 2A (PP2A), whichhave key roles in cell cycle progression.We have synthesised two series ofdemethylated cantharidin analogues, onedisplaying an open-ring lactoneconfiguration in solution (Novo-1 toNovo-5) similar to cantharidin, the othershowing a closed-ring lactone configuration(Novo-6 to Novo-10). In the present study,these ten agents were screened for invitro PP1 and PP2A inhibition and cellularcytotoxicity in nine cancer cell lines ofhaematopoietic (L1210, HL60), ovarian(A2780, ADDP), osteo (143B), and colon(HCT116, HT29, WiDr, SW480) origin and onenormal colon cell line (CCD-018).

The open-ring series (IC50,PP1=2.0−4.8 μM, PP2A=0.2−0.5 μM)maintained the PP2A selectivity ofcantharidin (IC50, PP1=1.8 μM,PP2A=0.2 μM), although some were lesspotent. The closed-ring series (IC50,PP1 = 12.5->1000 μM,PP2A=5->1000 μM) were considerablyless potent inhibitors, confirming the needof ring opening for inhibition. Thecytotoxicity (IC50, 72 h, MTT assay) ofcantharidin ranged from 6−15 μM, whilethe new analogues ranged from 14 to>1000 μM. Cytotoxicity of the agentsdid not consistently parallel the invitro potency of protein phosphataseinhibition. A number of analogues showedcolon cancer selectivity, particularlyNovo-6, where the cytotoxicity ranged from14−88 μM in the colon cancer cells and275−680 μM in all other cell linesincluding normal colon cells. The reasonfor this selectivity was not apparent andmay involve additional intracellulartargets. Cell cycle analysis showedcantharidin to enhance cell cycleprogression as evident from an increasedS-phase population and enhanced DNAsynthesis, culminating in G2/M arrestand apoptosis. With Novo-1 and Novo-6, thecell cycle changes paralleled thecytotoxicity responses, with thepredominant effect of G2/M cell cyclearrest followed by cell death. Inconclusion, we have synthesised newanticancer agents that show selectivecytotoxicity in colon cancer cells whileremaining inactive in normal colon cells,and which mediate their effects viathe G2/M phase of the cell cycle.

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Authors and Affiliations

  1. Department of Medical Oncology, Newcastle Mater Misericordiae Hospital, Australia

    Jennette A. Sakoff & Stephen P. Ackland

  2. Medicinal Chemistry Group, School of Biological and Chemical Sciences, The University of Newcastle, NSW, Australia

    Monique L. Baldwin, Mirella A. Keane & Adam McCluskey

Authors
  1. Jennette A. Sakoff
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  2. Stephen P. Ackland
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  3. Monique L. Baldwin
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  4. Mirella A. Keane
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  5. Adam McCluskey
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Sakoff, J.A., Ackland, S.P., Baldwin, M.L. et al. Anticancer Activity and Protein Phosphatase 1 and 2A Inhibition of a New Generation of Cantharidin Analogues. Invest New Drugs 20, 1–11 (2002). https://doi.org/10.1023/A:1014460818734

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