Thiadiazole-, selenadiazole- and triazole-fused anthraquinones as G-quadruplex targeting anticancer compounds.

G-quadruplex (G4) ligands attract considerable attention as potential anticancer therapeutics. In this study we proposed an original scheme for synthesis of azole-fused anthraquinones and prepared a series of G4 ligands carrying amino- or guanidinoalkylamino side chains. The heterocyclic core and structure of the terminal groups strongly affect on binding to G4-forming oligonucleotides, cellular accumulation and antitumor potency of compounds. In particular, thiadiazole- and selenadiazole- but not triazole-based ligands inhibit the proliferation of tumor cells (e.g. K562 leukemia) and stabilize primarily telomeric and c-MYC G4s. Anthraselenadiazole derivative 11a showed a good affinity to c-MYC G4 in vitro and down-regulated expression of c-MYC oncogene in cellular conditions. Further studies revealed that anthraselenadiazole 11a provoked cell cycle arrest and apoptosis in a dose- and time-dependent manner inhibiting K562 cells growth. Taken together, this work gives a valuable example that the closely related heterocycles may cause a significant difference in biological properties of G4 ligands.

Perfluorocarbon Nanoemulsions with Fluorous Chlorin-Type Photosensitizers for Antitumor Photodynamic Therapy in Hypoxia.

The efficacy of photodynamic therapy (PDT) strictly depends on the availability of molecular oxygen to trigger the light-induced generation of reactive species. Fluorocarbons have an increased ability to dissolve oxygen and are attractive tools for gas delivery. We synthesized three fluorous derivatives of chlorin with peripheral polyfluoroalkyl substituents. These compounds were used as precursors for preparing nanoemulsions with perfluorodecalin as an oxygen depot. Therefore, our formulations contained hydrophobic photosensitizers capable of absorbing monochromatic light in the long wavelength region and the oxygen carrier. These modifications did not alter the photosensitizing characteristics of chlorin such as the generation of singlet oxygen, the major cytocidal species in PDT. Emulsions readily entered HCT116 colon carcinoma cells and accumulated largely in mitochondria. Illumination of cells loaded with emulsions rapidly caused peroxidation of lipids and the loss of the plasma membrane integrity (photonecrosis). Most importantly, in PDT settings, emulsions potently sensitized cells cultured under prolonged (8 weeks) hypoxia as well as cells after oxygen depletion with sodium sulfite (acute hypoxia). The photodamaging potency of emulsions in hypoxia was significantly more pronounced compared to emulsion-free counterparts. Considering a negligible dark cytotoxicity, our materials emerge as efficient and biocompatible instruments for PDT-assisted eradication of hypoxic cells.

Synthesis of Cobalt Bis(Dicarbollide)-Curcumin Conjugates for Potential Use in Boron Neutron Capture Therapy.

A series of novel cobalt bis(dicarbollide)-curcumin conjugates were synthesized. Two conjugates were obtained through the nucleophilic ring-opening reaction of the 1,4-dioxane and tetrahydropyran derivatives of cobalt bis(dicarbollide) with the OH group of curcumin, and using two equiv. of the oxonium derivatives, two other conjugates containing two cobalt bis(dicarbollide) units per molecule were obtained. In contrast to curcumin, the conjugates obtained were found to be non-cytotoxic against both tumor and normal cell lines. The analysis of the intracellular accumulation of the conjugates by flow cytometry showed that all cobalt bis(dicarbollide)-curcumin conjugates entered HCT116 colorectal carcinoma cells in a time-dependent manner. New non-cytotoxic conjugates contain a large amount of boron atoms in the biomolecule and can potentially be used for further biological research into boron neutron capture therapy (BNCT).

Pt(IV) Prodrugs with Non-Steroidal Anti-inflammatory Drugs in the Axial Position.

We report herein the design, synthesis, and biological investigation of a series of novel Pt(IV) prodrugs with non-steroidal anti-inflammatory drugs naproxen, diclofenac, and flurbiprofen, as well as these with stearic acid in the axial position. Six Pt(IV) prodrugs 5-10 were designed, which showed superior antiproliferative activity compared to cisplatin as well as an ability to overcome tumor cell line resistance to cisplatin. By tuning the drug lipophilicity via variation of the axial ligands, the most potent Pt(IV) prodrug 7 was obtained, with an enhanced cellular accumulation of up to 153-fold that of cisplatin and nanomolar cytotoxicity both in 2D and 3D cell cultures. Pt2+ species were detected at different depths of MCF-7 spheroids after incubation with Pt(IV) prodrugs using a Pt-coated carbon nanoelectrode. Cisplatin accumulation in vivo in the murine mammary EMT6 tumor tissue of BALB/c mice after Pt(IV) prodrug injection was proved electrochemically as well. The drug tolerance study on BALB/c mice showed good tolerance of 7 in doses up to 8 mg/kg.

Heterocyclic ring expansion yields anthraquinone derivatives potent against multidrug resistant tumor cells.

Chemical modifications of anthraquiones are aimed at novel derivatives with improved antitumor properties. Emergence of multidrug resistance (MDR) due to overexpression of transmembrane ATP binding cassette transporters, in particular, MDR1/P-glycoprotein (Pgp), can limit the use of anthraquinone based drugs. Previously we have demonstrated that annelation of modified five-membered heterocyclic rings with the anthraquinone core yielded a series of compounds with optimized antitumor properties. In the present study we synthesized a series of anthraquinone derivatives with six-membered heterocycles. Selected new compounds showed the ability to kill parental and MDR tumor cell lines at low micromolar concentrations. Molecular docking into the human Pgp model revealed a stronger interaction of 2-methylnaphtho[2,3-g]quinoline-3-carboxamide 17 compared to naphtho[2,3-f]indole-3-carboxamide 3. The time course of intracellular accumulation of compound 17 in parental K562 leukemia cells and in Pgp-positive K562/4 subline was similar. In contrast, compound 3 was readily effluxed from K562/4 cells and was significantly less potent for this subline than for K562 cells. Together with reported strategies of drug optimization of the anthracycline core, these results add ring expansion to the list of perspective modifications of heteroarene-fused anthraquinones.

Thiophene-2-carboxamide derivatives of anthraquinone: A new potent antitumor chemotype.

The anthraquinone scaffold has long been known as a source of efficacious antitumor drugs. In particular, the various chemical modifications of the side chains in this scaffold have yielded the compounds potent for the wild type tumor cells, their counterparts with molecular determinants of altered drug response, as well as in vivo settings. Further exploring the chemotype of anticancer heteroarene-fused anthraquinones, we herein demonstrate that derivative of anthra[2,3-b]thiophene-2-carboxamide, (compound 8) is highly potent against a panel of human tumor cell lines and their drug resistant variants. Treatment with submicromolar or low micromolar concentrations of 8 for only 30 min was sufficient to trigger lethal damage of K562 chronic myelogenous leukemia cells. Compound 8 (2.5 µM, 3-6 h) induced an apoptotic cell death as determined by concomitant activation of caspases 3 and 9, cleavage of poly(ADP-ribose) polymerase, increase of Annexin V/propidium iodide double stained cells, DNA fragmentation (subG1 fraction) and a decrease of mitochondrial membrane potential. Neither a significant interaction with double stranded DNA nor strong inhibition of the DNA dependent enzyme topoisomerase 1 by 8 were detectable in cell free systems. Laser scanning confocal microscopy revealed that some amount of 8 was detectable in mitochondria as early as 5 min after the addition to the cells; exposure for 1 h caused significant morphological changes and clustering of mitochondria. The bioisosteric analog 2 in which the thiophene ring was replaced with furan was less active although the patterns of cytotoxicity of both derivatives were similar. These results point at the specific role of the sulfur atom in the antitumor properties of carboxamide derivatives of heteroarene-fused anthraquinone.

Amides of pyrrole- and thiophene-fused anthraquinone derivatives: A role of the heterocyclic core in antitumor properties.

Heteroarene-fused anthraquinone derivatives represent a class of perspective anticancer drug candidates capable of targeting multiple vital processes including drug resistance. Taking advantage of previously demonstrated potential of amide derivatives of heteroarene-fused anthraquinones, we herein dissected the role of the heterocyclic core in antitumor properties. A new series of naphtho[2,3-f]indole-3- and anthra[2,3-b]thiophene-3-carboxamides was synthesized via coupling the respective acids with cyclic diamines. New compounds demonstrated a submicromolar antiproliferative potency close to doxorubicin (Dox) against five tumor cell lines of various tissue origin. In contrast to Dox, the new compounds were similarly cytotoxic for HCT116 colon carcinoma cells (wild type p53) and their isogenic p53 knockout counterparts. Modification of the heterocyclic core changed the targeting properties: the best-in-series naphtho[2,3-f]indole-3-carboxamide 8 formed more affine complexes with DNA duplex than furan and thiophene analogs, a property that can be translated into a stronger inhibition of topoisomerase 1 mediated DNA unwinding. At tolerable doses the water soluble derivative 8 significantly inhibited tumor growth (up to 79%) and increased the lifespan (153%) of mice bearing P388 lymphoma transplants. Together with better solubility for parenteral administration and well tolerance by animals of the indole derivative 8 indicates prospects for further search of new antitumor drug candidates among the heteroarene-fused anthraquinones.

New anthra[2,3-b]furancarboxamides: A role of positioning of the carboxamide moiety in antitumor properties.

Derivatives of the anthraquinone (anthracene-9,10-dione) such as doxorubicin, mitoxantrone and others have proved great clinical efficacy for decades. Currently the search in this exceptionally productive chemical class is aimed at optimization of antitumor properties including circumvention of drug resistance. Previously we have reported that heteroarene-fused anthraquinones fused to a 5-membered heterocyclic ring are advantageous in killing drug resistant tumor cells. Herein we present the synthesis and antitumor properties of a series of new anthra[2,3-b]furan-2-carboxamides. Vast majority of new derivatives were similarly cytotoxic to wild type tumor cell lines and their isogenic sublines with P-glycoprotein overexpression and/or p53 inactivation. Comparison of structurally close derivatives varying in their position relative to the furan moiety, that is, furan-3-carboxamide 1vs furan-2-carboxamides 5 and 6, revealed fundamental differences in the cytotoxicity profiles, formation of drug-DNA complexes, efficacy of topoisomerase 1 inhibition and mechanisms of tumor cell death. Together with previous SAR data on the role of individual substituents, these results provide evidence that regioisomerization of anthra[2,3-b]furancarboxamides generates the practically perspective derivatives whose properties may vary significantly.

Tri-armed ligands of G-quadruplex on heteroarene-fused anthraquinone scaffolds: Design, synthesis and pre-screening of biological properties.

Here, a combined molecular modelling methodology was used to identify the binding mode of 4,11-bis((2-guanidinoethyl)amino)anthra[2,3-b]thiophene-5,10-dione (1), a previously reported G4 ligand. After calculating the optimal interaction parameters 1 with the target, two series of tri-armed ligands based on furan- or thiophene-fused anthraquinone scaffolds were designed and synthesized. The new compounds bearing an additional side chain at the 2-position of the heterocycle and the 4,11-side chains with different spacer lengths and structures of terminal groups demonstrated much stronger affinity for telomeric G4 (4-15 times) versus the parental ligand. Moreover, the specificity to the quadruplex over duplex DNA was significantly improved (up to 75 times) when the 3-guanidinopropyl side chain was introduced at the 2-position of the heterocycle ring. All tri-armed ligands demonstrated modest antiproliferative potency, which is likely due to low intracellular penetration. Nevertheless, this work shows how computer-aided rational design of new potent compounds can be used for targeted anticancer therapy.

New antitumor anthra[2,3-b]furan-3-carboxamides: Synthesis and structure-activity relationship.

Chemical modifications of the anthraquinone scaffold are aimed at optimization of this exceptionally productive class of antitumor drugs. In particular, our previously reported anthra[2,3-b]furan-3-carboxamides demonstrated a high cytotoxic potency in cell culture and in vivo. In this study, we expanded our series of anthra[2,3-b]furan-3-carboxamides by modifying the key functional groups and dissected the structure-activity relationship within this chemotype. The majority of new compounds inhibited the growth of mammalian tumor cell lines at submicromolar to low micromolar concentrations. We found that 4,11-hydroxy groups as well as the carbonyl moiety in the carboxamide fragment were critical for cytotoxicity whereas the substituent at the 2-position of anthra[2,3-b]furan was not. Importantly, the new derivatives were similarly potent against wild type cells and their variants resistant to doxorubicin due to P-glycoprotein (Pgp) expression or p53 inactivation. The most cytotoxic derivatives 6 and 9 attenuated plasmid DNA relaxation by topoisomerase 1. Finally, we demonstrated that 6 and 9 at 1 µM induced intracellular oxidative stress, accumulation in G2/M phase of the cell cycle, and apoptosis in gastric carcinoma cell lines regardless of their p53 status. These results further substantiate the potential of anthra[2,3-b]furan-3-carboxamides as antitumor drug candidates.

Discovery of antitumor anthra[2,3-b]furan-3-carboxamides: Optimization of synthesis and evaluation of antitumor properties.

Anthraquinones and their analogues, in particular heteroarene-fused anthracendiones, are prospective scaffolds for new compounds with improved antitumor characteristics. We herein report the use of a 'scaffold hopping' approach for the replacement of the core structure in the previously discovered hit compound naphtho[2,3-f]indole-5,10-dione 2 with an alternative anthra[2,3-b]furan-5,10-dione scaffold. Among 13 newly synthesized derivatives the majority of 4,11-dihydroxy-2-methyl-5,10-dioxoanthra[2,3-b]furan-3-carboxamides demonstrated a high antiproliferative potency against a panel of wild type and drug resistant tumor cell lines, a property superior over the reference drug doxorubicin or lead naphtho[2,3-f]indole-5,10-dione 2. At low micromolar concentrations the selected derivative of (R)-3-aminopyrrolidine 3c and its stereoisomer (S)-3-aminopyrrolidine 3d caused an apoptotic cell death preceded by an arrest in the G2/M phase. Studies of intracellular targets showed that 3c and 3d formed stable intercalative complexes with the duplex DNA as determined by spectral analysis and molecular docking. Both 3c and 3d attenuated topoisomerase 1 and 2 mediated unwinding of the supercoiled DNA via a mechanism different from conventional DNA-enzyme tertiary complex formation. Furthermore, 3d decreased the activity of selected human protein kinases in vitro, indicating multiple targeting by the new chemotype. Finally, 3d demonstrated an antitumor activity in a model of murine intraperitoneally transplanted P388 leukemia, achieving the increase of animal life span up to 262% at tolerable doses. Altogether, the 'scaffold hopping' demonstrated its productivity for obtaining new perspective antitumor drug candidates.

Synthesis and cytotoxic potency of novel tris(1-alkylindol-3-yl)methylium salts: role of N-alkyl substituents.

Novel derivatives of tris(indol-3-yl)methane and tris(indol-3-yl)methylium salts with the alkyl substituents at the N-atoms of the indole rings were synthesized. An easy substitution of indole rings in trisindolylmethanes for other indoles under the action of acids is demonstrated, and the mechanism of substitution is discussed. To obtain trisindolylmethylium salts, the environmentally safe method of oxidation of trisindolylmethanes with air oxygen in acidic conditions was developed. Tris(1-alkylindol-3-yl)methanes and tris(1-alkylindol-3-yl)methylium salts represent three-bladed molecular propellers whose physico-chemical and biological properties strongly depend on the N-alkyl substituent. The cytotoxicity of novel compounds increased with the number of C atoms in the alkyl chains, with optimal number n=3-5 whereas the derivatives with longer side chains were less cytotoxic. The most potent novel compounds killed human tumor cells at nanomolar-to-submicromolar concentrations, being one order of magnitude more potent than the prototype antibiotic turbomycin A [tris(indol-3-yl)methylium salt]. Apoptosis in HCT116 colon carcinoma cell line induced by tris(1-pentyl-1H-indol-3-yl)methylium methanesulfonate was detectable at concentrations tolerable by normal blood lymphocytes. Thus, N-alkyl substituted tris(1-alkylindol-3-yl)methylium salts emerge as perspective anticancer drug candidates.