Tannic Acid and Ethyl Gallate Potentialize Paclitaxel Effect on Microtubule Dynamics in Hep3B Cells.

Among broad-spectrum anticancer agents, paclitaxel (PTX) has proven to be one of the most effective against solid tumors for which more specific treatments are lacking. However, drawbacks such as neurotoxicity and the development of resistance reduce its therapeutic efficacy. Therefore, there is a need for compounds able to improve its activity by synergizing with it or potentiating its effect, thus reducing the doses required. We investigated the interaction between PTX and tannins, other compounds with anticancer activity known to act as repressors of several proteins involved in oncological pathways. We found that both tannic acid (TA) and ethyl gallate (EG) strongly potentiate the toxicity of PTX in Hep3B cells, suggesting their utility in combination therapy. We also found that AT and EG promote tubulin polymerization and enhance the effect of PTX on tubulin, suggesting a direct interaction with tubulin. Biochemical experiments confirmed that TA, but not EG, binds tubulin and potentiates the apparent binding affinity of PTX for the tubulin binding site. Furthermore, the molecular docking of TA to tubulin suggests that TA can bind to two different sites on tubulin, one at the PTX site and the second at the interface of α and ß-tubulin (cluster 2). The binding of TA to cluster 2 could explain the overstabilization in the tubulin + PTX combinatorial assay. Finally, we found that EG can inhibit PTX-induced expression of pAkt and pERK defensive protein kinases, which are involved in resistance to PXT, by limiting cell death (apoptosis) and favoring cell proliferation and cell cycle progression. Our results support that tannic acid and ethyl gallate are potential chemotherapeutic agents due to their potentiating effect on paclitaxel.

CLIP-170S is a microtubule +TIP variant that confers resistance to taxanes by impairing drug-target engagement.

Taxanes are widely used cancer chemotherapeutics. However, intrinsic resistance limits their efficacy without any actionable resistance mechanism. We have discovered a microtubule (MT) plus-end-binding CLIP-170 protein variant, hereafter CLIP-170S, which we found enriched in taxane-resistant cell lines and patient samples. CLIP-170S lacks the first Cap-Gly motif, forms longer comets, and impairs taxane access to its MT luminal binding site. CLIP-170S knockdown reversed taxane resistance in cells and xenografts, whereas its re-expression led to resistance, suggesting causation. Using a computational approach in conjunction with the connectivity map, we unexpectedly discovered that Imatinib was predicted to reverse CLIP-170S-mediated taxane resistance. Indeed, Imatinib treatment selectively depleted CLIP-170S, thus completely reversing taxane resistance. Other RTK inhibitors also depleted CLIP-170S, suggesting a class effect. Herein, we identify CLIP-170S as a clinically prevalent variant that confers taxane resistance, whereas the discovery of Imatinib as a CLIP-170S inhibitor provides novel therapeutic opportunities for future trials.

Structural model for differential cap maturation at growing microtubule ends.

Microtubules (MTs) are hollow cylinders made of tubulin, a GTPase responsible for essential functions during cell growth and division, and thus, key target for anti-tumor drugs. In MTs, GTP hydrolysis triggers structural changes in the lattice, which are responsible for interaction with regulatory factors. The stabilizing GTP-cap is a hallmark of MTs and the mechanism of the chemical-structural link between the GTP hydrolysis site and the MT lattice is a matter of debate. We have analyzed the structure of tubulin and MTs assembled in the presence of fluoride salts that mimic the GTP-bound and GDP•Pi transition states. Our results challenge current models because tubulin does not change axial length upon GTP hydrolysis. Moreover, analysis of the structure of MTs assembled in the presence of several nucleotide analogues and of taxol allows us to propose that previously described lattice expansion could be a post-hydrolysis stage involved in Pi release.

Structural Basis of Colchicine-Site targeting Acylhydrazones active against Multidrug-Resistant Acute Lymphoblastic Leukemia.

Tubulin is one of the best validated anti-cancer targets, but most anti-tubulin agents have unfavorable therapeutic indexes. Here, we characterized the tubulin-binding activity, the mechanism of action, and the in vivo anti-leukemia efficacy of three 3,4,5-trimethoxy-N-acylhydrazones. We show that all compounds target the colchicine-binding site of tubulin and that none is a substrate of ABC transporters. The crystal structure of the tubulin-bound N-(1'-naphthyl)-3,4,5-trimethoxybenzohydrazide (12) revealed steric hindrance on the T7 loop movement of ß-tubulin, thereby rendering tubulin assembly incompetent. Using dose escalation and short-term repeated dose studies, we further report that this compound class is well tolerated to >100 mg/kg in mice. We finally observed that intraperitoneally administered compound 12 significantly prolonged the overall survival of mice transplanted with both sensitive and multidrug-resistant acute lymphoblastic leukemia (ALL) cells. Taken together, this work describes promising colchicine-site-targeting tubulin inhibitors featuring favorable therapeutic effects against ALL and multidrug-resistant cells.

Synthesis, Microtubule-Binding Affinity, and Antiproliferative Activity of New Epothilone Analogs and of an EGFR-Targeted Epothilone-Peptide Conjugate.

A new simplified, epoxide-free epothilone analog was prepared incorporating an N-(2-hydroxyethyl)-benzimidazole side chain, which binds to microtubules with high affinity and inhibits cancer cell growth in vitro with nM potency. Building on this scaffold, a disulfide-linked conjugate with the purported EGFR-binding (EGFR, epidermal growth factor receptor) peptide GE11 was then prepared. The conjugate retained significant microtubule-binding affinity, in spite of the size of the peptide attached to the benzimidazole side chain. The antiproliferative activity of the conjugate was significantly lower than for the parent scaffold and, surprisingly, was independent of the EGFR expression status of cells. Our data indicate that the disulfide-based conjugation with the GE11 peptide is not a viable approach for effective tumor-targeting of highly potent epothilones and probably not for other cytotoxics.

Klebsiella oxytoca enterotoxins tilimycin and tilivalline have distinct host DNA-damaging and microtubule-stabilizing activities.

Establishing causal links between bacterial metabolites and human intestinal disease is a significant challenge. This study reveals the molecular basis of antibiotic-associated hemorrhagic colitis (AAHC) caused by intestinal resident Klebsiella oxytoca Colitogenic strains produce the nonribosomal peptides tilivalline and tilimycin. Here, we verify that these enterotoxins are present in the human intestine during active colitis and determine their concentrations in a murine disease model. Although both toxins share a pyrrolobenzodiazepine structure, they have distinct molecular targets. Tilimycin acts as a genotoxin. Its interaction with DNA activates damage repair mechanisms in cultured cells and causes DNA strand breakage and an increased lesion burden in cecal enterocytes of colonized mice. In contrast, tilivalline binds tubulin and stabilizes microtubules leading to mitotic arrest. To our knowledge, this activity is unique for microbiota-derived metabolites of the human intestine. The capacity of both toxins to induce apoptosis in intestinal epithelial cells-a hallmark feature of AAHC-by independent modes of action, strengthens our proposal that these metabolites act collectively in the pathogenicity of colitis.

Structure-activity relationships, biological evaluation and structural studies of novel pyrrolonaphthoxazepines as antitumor agents.

Microtubule-targeting agents (MTAs) are a class of clinically successful anti-cancer drugs. The emergence of multidrug resistance to MTAs imposes the need for developing new MTAs endowed with diverse mechanistic properties. Benzoxazepines were recently identified as a novel class of MTAs. These anticancer agents were thoroughly characterized for their antitumor activity, although, their exact mechanism of action remained elusive. Combining chemical, biochemical, cellular, bioinformatics and structural efforts we developed improved pyrrolonaphthoxazepines antitumor agents and their mode of action at the molecular level was elucidated. Compound 6j, one of the most potent analogues, was confirmed by X-ray as a colchicine-site MTA. A comprehensive structural investigation was performed for a complete elucidation of the structure-activity relationships. Selected pyrrolonaphthoxazepines were evaluated for their effects on cell cycle, apoptosis and differentiation in a variety of cancer cells, including multidrug resistant cell lines. Our results define compound 6j as a potentially useful optimized hit for the development of effective compounds for treating drug-resistant tumors.

Anticancer properties of the abietane diterpene 6,7-dehydroroyleanone obtained by optimized extraction.

AIM: 6,7-dehydroroyleanone (DHR) is a cytotoxic abietane present in the essential oil of Plectranthus madagascariensis. METHODS/RESULTS: Different extraction parameters were tested, and its extraction optimization was accomplished with a Clevenger apparatus-based hydrodistillation. After isolation, its effect on microtubules, P-glycoprotein and caspases was assessed on several cell lines and the compound was coupled with hybrid nanoparticles. The results show that DHR does not interfere with microtubule formation, but evades the resistance mechanisms of P-glycoprotein. Strong activation of caspases-3 and -9 indicates that DHR is able to induce apoptosis by triggering the intrinsic cell death pathway. Moreover, the assembly of DHR with hybrid nanoparticles was able to potentiate the effect of DHR in cancer cells. CONCLUSION: DHR seems to be a promising starting material with anticancer properties to further be explored.

Gallic acid sensitizes paclitaxel-resistant human ovarian carcinoma cells through an increase in reactive oxygen species and subsequent downregulation of ERK activation.

Paclitaxel (PTX) is currently used as a front-line chemotherapeutic agent for several types of cancer, including ovarian carcinoma; however, PTX-resistance frequently arises through multiple mechanisms. The development of new strategies using natural compounds and PTX in combination has been the aim of several prior studies, in order to enhance the efficacy of chemotherapy. In this study, we found the following: (i) gallic acid (GA), a phenolic compound, potentiated the capacity of PTX to decrease proliferation and to cause G2/M cycle arrest in the PTX-resistant A2780AD ovarian cancer cell line; (ii) GA exerted a pro-oxidant action by increasing the production of reactive oxygen species (ROS), and co-treatment with the antioxidant agent N­acetyl-L­cysteine (NAC) prevented GA+PTX-induced cell proliferation inhibition and G2/M phase arrest; (iii) PTX stimulated ERK phosphorylation/activation, and co-treatment with the MEK/ERK inhibitor PD98049 potentiated the proliferation inhibition and G2/M phase arrest; (iv) and finally, GA abrogated the PTX-induced stimulation of ERK phosphorylation, a response that was prevented by co-treatment with NAC. Taken together, these results indicate that GA sensitizes PTX-resistant ovarian carcinoma cells via the ROS­mediated inactivation of ERK, and suggest that GA could represent a useful co-adjuvant to PTX in ovarian carcinoma treatment.

High-affinity ligands of the colchicine domain in tubulin based on a structure-guided design.

Microtubule-targeting agents that bind at the colchicine-site of tubulin are of particular interest in antitumoral therapy due to their dual mechanism of action as antimitotics and vascular disrupting agents. Cyclohexanediones derivatives have been described as a new family of colchicine-domain binders with an association constant to tubulin similar to that of colchicine. Here, the high-resolution structures of tubulin in complex with cyclohexanediones TUB015 and TUB075 were solved by X-ray crystallography. A detailed analysis of the tubulin-TUB075 interaction by means of computational affinity maps allowed the identification of two additional regions at the binding site that were addressed with the design and synthesis of a new series of cyclohexanediones with a distal 2-substituted benzofurane. These new compounds showed potent antiproliferative activity with IC50 values in the nM range, arrested cell cycle progression at the G2/M phase and induced apoptosis at sub µM concentrations. Moreover, they caused the destruction of a preformed vascular network in vitro and inhibited the migration of endothelial cells at non-toxic concentrations. Finally, these compounds displayed high affinity for tubulin as substantiated by a K b value of 2.87 × 108 M-1 which, to the best of our knowledge, represents the highest binding constant measured to date for a colchicine-domain ligand.

Triazolopyrimidines Are Microtubule-Stabilizing Agents that Bind the Vinca Inhibitor Site of Tubulin.

Microtubule-targeting agents (MTAs) are some of the clinically most successful anti-cancer drugs. Unfortunately, instances of multidrug resistances to MTA have been reported, which highlights the need for developing MTAs with different mechanistic properties. One less explored class of MTAs are [1,2,4]triazolo[1,5-a]pyrimidines (TPs). These cytotoxic compounds are microtubule-stabilizing agents that inexplicably bind to vinblastine binding site on tubulin, which is typically targeted by microtubule-destabilizing agents. Here we used cellular, biochemical, and structural biology approaches to address this apparent discrepancy. Our results establish TPs as vinca-site microtubule-stabilizing agents that promote longitudinal tubulin contacts in microtubules, in contrast to classical microtubule-stabilizing agents that primarily promote lateral contacts. Additionally we observe that TPs studied here are not affected by p-glycoprotein overexpression, and suggest that TPs are promising ligands against multidrug-resistant cancer cells.

Modification of C-seco taxoids through ring tethering and substituent replacement leading to effective agents against tumor drug resistance mediated by ßIII-Tubulin and P-glycoprotein (P-gp) overexpressions.

In our efforts to improve the efficacy of taxane-based microtubule (MT) stabilizing agents against tumor drug resistance mediated by multiple mechanisms, two clinically relevant factors were focused: i.e., P-glycoprotein and ßIII-tubulin overexpression. Based on the structure of C-seco taxoid 1 m (IDN5390) which was believed to more selectively interact with ßIII-tubulin than paclitaxel, we prepared a series of C-seco taxoids bearing various 7,9-O-linkages and/or different substituents at C2 and C3' positions. Some of them exhibited much more potent binding affinity to MTs and cytotoxicity than their C-seco parent compounds in drug resistant cells with both mechanisms. SAR analysis indicated that C2 modifications significantly enhanced MT binding but brought ambiguous influence to cytotoxicity whereas 7,9-linkage and C3' modifications enhance cytotoxicity more efficiently than improve MT binding. These observations illustrate a better translation of molecular binding effect to cellular activity by C ring closure and C3' modification than C2 modification in C-seco taxoids.

Quinolin-6-Yloxyacetamides Are Microtubule Destabilizing Agents That Bind to the Colchicine Site of Tubulin.

Quinolin-6-yloxyacetamides (QAs) are a chemical class of tubulin polymerization inhibitors that were initially identified as fungicides. Here, we report that QAs are potent anti-proliferative agents against human cancer cells including ones that are drug-resistant. QAs act by disrupting the microtubule cytoskeleton and by causing severe mitotic defects. We further demonstrate that QAs inhibit tubulin polymerization in vitro. The high resolution crystal structure of the tubulin-QA complex revealed that QAs bind to the colchicine site on tubulin, which is targeted by microtubule-destabilizing agents such as colchicine and nocodazole. Together, our data establish QAs as colchicine-site ligands and explain the molecular mechanism of microtubule destabilization by this class of compounds. They further extend our structural knowledge on antitubulin agents and thus should aid in the development of new strategies for the rational design of ligands against multidrug-resistant cancer cells.

Synthesis and Anti-Proliferative Activity of Sulfanyltriazolylnaphthalenols and Sulfanyltriazolylnaphthalene-1,4-diones.

A series of new sulfanyltriazolylnaphthalenols (10a-f and 13a-f) and sulfanyltriazolylnaphthalene-1,4-diones (14a-f) were synthesized and evaluated against a panel of cancer cell lines. Among the tested compounds, 10b and 10d showed the best anti-proliferative activity with GI50 values ranging from 2.72 to 10 and 3.13 to 13.1 µM, respectively, in several of the tumor cell lines tested. Compound 10d is highly selective toward leukemia cell lines and can be regarded as a good model for the development of new anti-leukemic agents.

Structural Determinants of the Dictyostatin Chemotype for Tubulin Binding Affinity and Antitumor Activity Against Taxane- and Epothilone-Resistant Cancer Cells.

A combined biochemical, structural, and cell biology characterization of dictyostatin is described, which enables an improved understanding of the structural determinants responsible for the high-affinity binding of this anticancer agent to the taxane site in microtubules (MTs). The study reveals that this macrolide is highly optimized for MT binding and that only a few of the structural modifications featured in a library of synthetic analogues resulted in small gains in binding affinity. The high efficiency of the dictyostatin chemotype in overcoming various kinds of clinically relevant resistance mechanisms highlights its potential for therapeutic development for the treatment of drug-resistant tumors. A structural explanation is advanced to account for the synergy observed between dictyostatin and taxanes on the basis of their differential effects on the MT lattice. The X-ray crystal structure of a tubulin-dictyostatin complex and additional molecular modeling have allowed the rationalization of the structure-activity relationships for a set of synthetic dictyostatin analogues, including the highly active hybrid 12 with discodermolide. Altogether, the work reported here is anticipated to facilitate the improved design and synthesis of more efficacious dictyostatin analogues and hybrids with other MT-stabilizing agents.

The Mechanism of the Interactions of Pironetin Analog/Combretastatin A-4 Hybrids with Tubulin.

We here report an investigation of the interactions with tubulin of two types of molecules of a hybrid structural type consisting in a combretastatin A-4 moiety and a simplified pironetin fragment. The cytotoxicities of the molecules on two reference tumoral cell lines were measured. In addition, the effects of the compounds on the cell cycle and on microtubule assembly were observed. The dynamics of microtubule polymerization was investigated by means of immunofluorescence assays. It was thus established that at least some of the compounds under study exert their cytotoxic action by means of interaction with tubulin.

Synthesis, Characterization, and Application in HeLa Cells of an NIR Light Responsive Doxorubicin Delivery System Based on NaYF4:Yb,Tm@SiO2-PEG Nanoparticles.

Herein, we present a phototriggered drug delivery system based on light responsive nanoparticles, which is able to release doxorubicin upon NIR light illumination. The proposed system is based on upconversion fluorescence nanoparticles of ß-NaYF4:Yb,Tm@SiO2-PEG with a mean diameter of 52±2.5 nm that absorb the NIR light and emit UV light. The UV radiation causes the degradation of photodegradable ortho-nitrobenzyl alcohol derivates, which are attached on one side to the surface of the nanoparticles and on the other to doxorubicin. This degradation triggers the doxorubicin release. This drug delivery system has been tested "in vitro" with HeLa cells. The results of this study demonstrated that this system caused negligible cytotoxicity when they were not illuminated with NIR light. In contrast, under NIR light illumination, the HeLa cell viability was conspicuously reduced. These results demonstrated the suitability of the proposed system to control the release of doxorubicin via an external NIR light stimulus.

Synthesis and biological evaluation of new oxadiazoline-substituted naphthalenyl acetates as anticancer agents.

A series of new oxadiazoline-substituted naphthalenyl acetates 3a-e and oxadiazoline-substituted 4-methoxynaphthalenyl acetates 7b-e were synthesized and tested by the National Cancer Institute (NCI) for their in vitro anticancer activity. The two derivatives bearing acetoxy groups at the 1 and 3 positions of the phenyl ring 3c and 7c were the most active showing significant anticancer activity against all tested cancer cell lines, with GI50 values ranging from 0.175 to 3.91 µM, and 0.306-11.7 µM, respectively. The selectivity of compound 3c was greater for non-solid tumor cell lines. Computational prediction of molecular and pharmacokinetic properties revealed that both compounds are safe and compound 7c had a good drug-likeness score.

Design and synthesis of pironetin analogue/colchicine hybrids and study of their cytotoxic activity and mechanisms of interaction with tubulin.

We here report the synthesis of a series of 12 hybrid molecules composed of a colchicine moiety and a pironetin analogue fragment. The two fragments are connected through an ester-amide spacer of variable length. The cytotoxic activities of these compounds and their interactions with tubulin have been investigated. Relations between the structure and activity are discussed. Since the spacer is not long enough to permit a simultaneous binding of the hybrid molecules to the colchicine and pironetin sites on tubulin, a further feature investigated was whether these molecules would interact with the latter through the pironetin end (irreversible covalent binding) or through the colchicine end (reversible noncovalent binding). It has been found that binding to tubulin may take place preferentially at either of these ends depending on the length of the connecting spacer.

The impact of cyclopropane configuration on the biological activity of cyclopropyl-epothilones.

Two cis-12,13-cyclopropyl-epothilone B variants have been synthesized, differing only in the configuration of the stereocenters at C12 and C13. The syntheses were based on a common allylic alcohol intermediate that was converted into the corresponding diastereomeric hydroxymethyl-cyclopropanes by means of stereoselective Charette cyclopropanations. Macrocyclizations were accomplished through ring-closing metathesis (RCM). Substantial differences between the two compounds were found with regard to microtubule binding affinity, antiproliferative activity and their effects on the cellular microtubule network. While the analogue with the cyclopropane moiety oriented in a corresponding way to the epoxide configuration in natural epothilones was almost equipotent with epothilone A, the other was significantly less active. Based on these findings, natural epothilone-like activity of cis-fused 12,13-cyclopropyl-epothilone analogues is tightly linked to the natural orientation of the cyclopropane moiety.