Structure-activity relationship of cantharidin derivatives to protein phosphatases 1, 2A1, and 2B

Synthesis and biological evaluation of new chloro/acetoxy substituted isoindole analogues as new tyrosine kinase inhibitors

We have developed a versatile synthetic approach for the synthesis of new isoindole derivatives via the cleavage of ethers from tricyclic imide skeleton compounds. An exo-cycloadduct prepared from the Diels-Alder reaction of furan and maleic anhydride furnished imide derivatives. The epoxide ring was opened with Ac₂O or Ac₂O/AcCl in the presence of a catalytic amount of H₂SO₄ in order to yield new isoindole derivatives 8a-d and 9a-d. The anticancer activity of these compounds was evaluated against the HeLa cell lines. The synthesized compounds showed inhibitory effects on the viability of HeLa cells and the degree of cytotoxicity was increased with the level of bigger branched isoindole derivatives. To better understand the acting mechanism of these molecules, western blot analysis was performed with using mTOR and its downstream substrates. In addition, human mTOR and ribozomal S6 kinase β1 (RS6Kβ1) have been investigated with molecular modelling studies as possible targets for compound series 8 and 9.

Focused library with a core structure extracted from natural products and modified: application to phosphatase inhibitors and several biochemical findings

Synthesis of a focused library is an important strategy to create novel modulators of specific classes of proteins. Compounds in a focused library are composed of a common core structure and different diversity structures. In this Account, we describe our design and synthesis of libraries focused on selective inhibitors of protein phosphatases (PPases). We considered that core structures having structural and electronic features similar to those of PPase substrates, phosphate esters, would be a reasonable choice. Therefore, we extracted core structures from natural products already identified as PPase inhibitors. Since many PPases share similar active-site structures, such phosphate-mimicking core structures should interact with many enzymes in the same family, and therefore the choice of diversity structures is pivotal both to increase the binding affinity and to achieve specificity for individual enzymes. Here we present case studies of application of focused libraries to obtain PPase inhibitors, covering the overall process from selection of core structures to identification and evaluation of candidates in the focused libraries. To synthesize a library focused on protein serine-threonine phosphatases (PPs), we chose norcantharidin as a core structure, because norcantharidin dicarboxylate shows a broad inhibition profile toward several PPs. From the resulting focused library, we identified a highly selective PP2B inhibitor, NCA-01. On the other hand, to find inhibitors of dual-specificity protein phosphatases (DSPs), we chose 3-acyltetronic acid extracted from natural product RK-682 as a core structure, because its structure resembles the transition state in the dephosphorylation reaction of DSPs. However, a highly selective inhibitor was not found in the resulting focused library. Furthermore, an inherent drawback of compounds having the highly acidic 3-acyltetronic acid as a core structure is very weak potency in cellulo, probably due to poor cell membrane permeability. Therefore, we next modified the core structure from acidic to neutral by transformation to the enamine derivative and constructed a second-generation focused library (RE derivatives). The resulting compounds showed dramatically improved cell membrane permeability and inhibitory selectivity and included VHR (vaccinia VH1-related)-selective RE12 and CDC25A/B (cell division cycle 25A/B)-selective RE44. These inhibitors act on target enzymes in cellulo and do not generate reactive oxygen species, which is a potential problem with quinoid-type inhibitors of CDC25s. The cellular activity of RE12 was further improved by replacement of the side chain to afford RE176, which showed more potent antiproliferative activity than RE12 against HeLa cells. The dramatic change of inhibitory selectivity obtained by core structure modification from 3-acyltetronic acid to its enamine derivative was associated with a change in the mode of action. Namely, RE derivatives were found to be noncompetitive inhibitors with respect to a small-molecular substrate of CDC25A/B, whereas RK-682 was a competitive inhibitor of VHR. We identified the binding site of RE derivatives on the CDC25A as a pocket adjacent to the active site; this appears to be a promising target site for development of further novel inhibitors of CDC25s.

Synthesis of novel lipophilic N-substituted norcantharimide derivatives and evaluation of their anticancer activities

This research attempted to study the effect of lipophilicity on the anticancer activity of N-substituted norcantharimide derivatives. Twenty-three compounds were synthesized and their cytotoxicities against five human cancer cell lines studied. The lipophilicity of each derivative was altered by its substituent, an alkyl, alkyloxy, terpenyl or terpenyloxy group at the N-position of norcantharimide. Further, among all synthesized derivatives studied, the compounds N-farnesyloxy-7-oxabicyclo[2.2.1]heptane-2,3-dicarboximide (9), and N-farnesyl-7-oxabicyclo[2.2.1]heptane-2,3-dicarboximide (18), have shown the highest cytotoxicity, anti-proliferative and apoptotic effect against human liver carcinoma HepG2 cell lines, yet displayed no significant cytotoxic effect on normal murine embryonic liver BNL CL.2 cells. Their overall performance led us to believe that these two compounds might be potential candidates for anticancer drugs development.

Synthesis of optically pure norcantharidin analogue NCA-01, a highly selective protein phosphatase 2B inhibitor, and its derivatives

An efficient synthetic route to optically pure norcantharidin analogue NCA-01, a highly selective inhibitor of protein phosphatase 2B (PP2B; calcineurin), has been developed. The absolute stereochemistry of the enantiomers was determined by X-ray crystallographic analysis. Optically pure NCA derivatives that had various substituents at the C1 position were synthesized in a similar manner. The PP2B-inhibitory activities of NCA-01 and its derivatives were independent of the enantiomeric form. NCA-01 dimethyl ester potently inhibited IL-2 production in Jurkat cells.

Norcantharidin analogues: a patent review (2006 - 2010)

INTRODUCTION

Norcantharidin (7-oxabicyclo [2.2.1] heptane-2,3-dicarboxylic anhydride) is the demethylated form of cantharidin. Norcantharidin not only has strong anticancer activity, but also eliminates most side-effects in the urinary system, does not cause myelosuppression and increases the number of white blood cells. With structural modification, norcantharidin analogues show potential anticancer activities.

AREAS COVERED

A comprehensive patent review of norcantharidin analogues from 2006 to 2010 is presented. Protein phosphatase 1, 2A, 2B and 5 inhibitors are described. The review summarizes the new compounds and lays the foundation for seeking more effective anticancer compounds.

EXPERT OPINION

Although norcantharidin has improved activity and toxicity, the effects routinely do not satisfy the current clinical need. Exploring better analogues is vital for changing the current situation, but norcantharidin is a good lead compound.

Novel endothall-containing platinum(IV) complexes: synthesis, characterization, and cytotoxic activity

Two platinum(IV) complexes (OC-6-33)-dichlorido(ethane-1,2-diamine)dihydroxidoplatinum(IV) and (OC-6-33)-diammine(dichlorido)dihydroxidoplatinum(IV) were carboxylated using demethylcantharidin as carboxylation agent. The complexes were characterized by elemental analysis, mass spectrometry, multinuclear (1H, 13C, 15N, and 195Pt) NMR spectroscopy, and, in case of (OC-6-33)-diamminebis(3-carboxy-7exo-oxabicyclo[2.2.1]heptane-2-carboxylato)dichloridoplatinum(IV) via X-ray diffraction. Cytotoxicity of the complexes was studied in seven human cancer cell lines representing five tumor entities, i.e., ovarian carcinoma (CH1, SK-OV-3), cervical carcinoma (HeLa), colon carcinoma (SW480, HCT-116), osteosarcoma (U-2 OS), and hepatocellular carcinoma (Hep G2) by means of the MTT (=3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium hydrobromide) assay.

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.

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

Norcantharidin (3) is a potent PP1 (IC(50)=9.0+/-1.4 microM) and PP2A (IC(50)=3.0+/-0.4 microM) inhibitor with 3-fold PP2A selectivity and induces growth inhibition (GI(50) approximately 45 microM) 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 (IC(50)=2.8+/-0.10 microM) and selective (4.6-fold) PP2A inhibitor with greater in vitro cytotoxicity (GI(50) approximately 9.6 microM) relative to norcantharidin. The analogous thiomorpholine-substituted (10) displays increased PP1 inhibition (IC(50)=3.2+/-0 microM) and reduced PP2A inhibition (IC(50)=5.1+/-0.41 microM), to norcantharidin. Synthesis of the analogous cantharidin analogue (19) with incorporation of the amine nitrogen into the heterocycle further increases PP1 (IC(50)=5.9+/-2.2 microM) and PP2A (IC(50)=0.79+/-0.1 microM) inhibition and cell cytotoxicity (GI(50) approximately 3.3 microM). These analogues represent the most potent cantharidin analogues thus reported.

Optically active cantharidin analogues possessing selective inhibitory activity on Ser/Thr protein phosphatase 2B (calcineurin): Implications for the binding mode

Cantharidin is well known as a potent serine/threonine protein phosphatase 1 and 2A (PP1 and PP2A) inhibitor, with less potent inhibitory activity for PP2B, which regulates T-cell proliferation. We synthesized and evaluated four optically pure stereoisomers of 1-substituted norcantharidin analogues. The absolute stereochemistry of each stereoisomer was determined based on X-ray crystal structure analysis. Remarkably, optically active cantharidin analogues having (1S)-configuration showed selective inhibition of PP2B, without inhibiting PP1 or PP2A.

Synergistic interaction between platinum-based antitumor agents and demethylcantharidin

A novel series of TCM-platinum complexes [Pt(C8H8O5)(NH2R)2] 1-5, designed from incorporating demethylcantharidin, a modified component from a traditional Chinese medicine (TCM) with a platinum moiety was found to circumvent cisplatin resistance in mouse leukemia and human hepatocellular carcinoma. These properties are most likely due to the inclusion of the protein phosphatase 2A (PP2A)-inhibiting demethylcantharidin in the novel compounds. We have investigated the potential synergistic effect of combining demethylcantharidin with a platinum-based antitumor agent, such as cisplatin, carboplatin, or oxaliplatin in vitro against L1210 mouse leukemia and SK-Hep-1 human hepatocellular carcinoma, and in vivo against a SK-Hep-1 subcutaneous-inoculated xenograft in nude mice, using median effect analysis. Demethylcantharidin and the platinum antitumor agents were synergistic in all cell lines tested in vitro, and the most effective antiproliferative regimen was when demethylcantharidin was added 24 h before cisplatin. Synergistic antitumor activity was also demonstrated in vivo without undue toxicity; no excessive loss in mouse body weight or overt pathology were observed at the effective doses. The results support a new approach for augmenting cytotoxic effect of established Pt-based drugs with demethylcantharidin in treating human hepatocellular carcinoma and other solid tumors.

Laurenditerpenol, a new diterpene from the tropical marine alga Laurenciaintricata that potently inhibits HIF-1 mediated hypoxic signaling in breast tumor cells

The degree of tumor hypoxia correlates with advanced disease stages and treatment resistance. The transcription factor hypoxia-inducible factor-1 (HIF-1) promotes tumor cell adaptation and survival under hypoxic conditions. Therefore, specific HIF-1 inhibitors represent an important new class of potential tumor-selective therapeutic agents. A T47D human breast tumor cell-based reporter assay was used to examine extracts of plants and marine organisms for inhibitors of HIF-1 activation. Bioassay-guided fractionation of the lipid extract of the red alga Laurencia intricata yielded a structurally novel diterpene, laurenditerpenol (1). The structure of 1 was determined spectroscopically. The relative configurations of the substituents of each ring system were assigned on the basis of NOESY correlations. The absolute configuration of position C-1 was determined by the modified Mosher ester procedure (directly in NMR tubes). Compound 1 potently inhibited hypoxia-activated HIF-1 (IC50: 0.4 microM) and hypoxia-induced VEGF (a potent angiogenic factor) in T47D cells. Compound 1 selectively inhibits HIF-1 activation by hypoxia but not iron chelator-induced activation. Further, 1 suppresses tumor cell survival under hypoxic conditions without affecting normoxic cell growth. Compound 1 inhibits HIF-1 by blocking the induction of the oxygen-regulated HIF-1alpha protein. Mitochondrial respiration studies revealed that 1 suppresses oxygen consumption.

Modified norcantharidins; synthesis, protein phosphatases 1 and 2A inhibition, and anticancer activity

Fourteen modified norcantharidin analogues have been synthesised and screened for their ability to inhibit the serine/threonine protein phosphatases 1 and 2A. The most potent compounds found were 10 (PP1 IC(50)=13+/-5 microM; PP2A IC(50)=7+/-3 microM) and 16 (PP1 IC(50)=18+/-8 microM; PP2A IC(50)=3.2+/-0.4 microM). Overall, only analogues possessing at least one acidic residue at the former anhydride warhead displayed any PP1 or PP2A inhibitory action. The ability of these analogues to inhibit PP1 and PP2A correlates well with their observed anti-cancer activity against a panel of five cancer cell lines: A2780 (human ovarian carcinoma), G401 (human kidney carcinoma), HT29 (human colorectal carcinoma), H460 (human lung carcinoma) and L1210 (murine leukemia).

Cantharimides: a new class of modified cantharidin analogues inhibiting protein phosphatases 1 and 2A

Cantharidin and its analogues have been of considerable interest as potent inhibitors of the serine/threonine protein phosphatases 1 and 2A (PP1 and PP2A). However, limited modifications to the parent compounds is tolerated. As part of an on-going study we have developed a new series of cantharidin analogues, the cantharimides. Inhibition studies indicate that cantharimides possessing a D- or L-histidine, are more potent inhibitors of PP1 and PP2A (PP1 IC(50)=3.22+/-0.7 microM; PP2A IC(50)=0.81+/-0.1 microM and PP1 IC(50)=2.82+/-0.6 microM; PP2A IC(50)=1.35+/-0.3 microM, respectively) than norcantharidin (PP1 IC(50)=5.31+/-0.76 microM; PP2A IC(50)=2.9+/-1.04 microM) and essentially equipotent with cantharidin (PP1 IC(50)=3.6+/-0.42 microM; PP2A IC(50)=0.36+/-0.08 microM). Cantharimides with non-polar or acidic amino acid residues are only poor inhibitors of PP1 and PP2A.

Anhydride modified cantharidin analogues. Is ring opening important in the inhibition of protein phosphatase 2A?

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 microM test concentration.

2-Carboxymethylendothal analogues as affinity probes for stabilized protein phosphatase 2A

Endothal (1diacid) and [3H]cantharidic acid ([3H]CA) bind with high affinity to the catalytic subunit of protein phosphatase 2A (PP2A). PP2A in liver cytosol was greatly stabilized with 30% glycerol as a preliminary step in the potential use of endothal-type derivatives for affinity chromatography. We report here the first introduction of a functionalizable group into endothal which allows retention of binding site affinity (assayed as [3H]CA binding in mouse liver cytosol). 2-Carboxymethylendothal anhydride (7) was prepared in two steps and 97% overall yield from cis-aconitic anhydride and furan. The potency of 7 was retained on conversion to two 2-carboxymethyl esters but not to two 2-(n-alkylcarboxamidomethyl) analogues.