Natural Products as Anti-Cancerous Therapeutic Molecules Targeted towards Topoisomerases.

Topoisomerases are reported to resolve the topological problems of DNA during several cellular processes, such as DNA replication, transcription, recombination, and chromatin remodeling. Two types of topoisomerases (Topo I and II) accomplish their designated tasks by introducing single- or double-strand breaks within the duplex DNA molecules, and thus maintain the proper structural conditions of DNA to release the topological torsions, which is generated by unwinding of DNA to access coded information, in the course of replication, transcription, and other processes. Both the topoisomerases have been looked at as crucial targets against various types of cancers such as lung, melanoma, breast, and prostate cancers. Conceptually, targeting topoisomerases will disrupt both DNA replication and transcription, thereby leading to inhibition of cell division and consequently stopping the growth of actively dividing cancerous cells. Since the discovery of camptothecin (an alkaloid) as an inhibitor of Topo I in 1958, a number of derivatives of camptothecin were developed as potent inhibitors of Topo I. Two such derivatives of camptothecin, namely, topotecan and irinotecan, have been commonly used as US Food and Drug Administration (FDA) approved drugs against Topo I. Similarly, the first Topo II inhibitor, namely, etoposide, an analogue of podophyllotoxin, was developed in 1966 and got FDA approval as an anti-cancer drug in 1983. Subsequently, several other inhibitors of Topo II, such as doxorubicin, mitoxantrone, and teniposide, were developed. These drugs have been reported to cause accumulation of cytotoxic non-reversible DNA double-strand breaks (cleavable complex). Thus, the present review describes the anticancer potential of plant-derived secondary metabolites belonging to alkaloids, flavonoids and terpenoids directed against topoisomerases. Furthermore, in view of the recent advances made in the field of computer-aided drug design, the present review also discusses the use of computational approaches such as ADMET, molecular docking, molecular dynamics simulation and QSAR to assess and predict the safety, efficacy, potency and identification of these potent anti-cancerous therapeutic molecules.

Amexanthomycins A-J, pentangular polyphenols produced by Amycolatopsis mediterranei S699∆rifA.

Ten new pentangular polyphenols, namely amexanthomycins A-J (1-10) were isolated from the strain Amycolatopsis mediterranei S699∆rifA constructed by deleting the polyketide synthase genes responsible for the biosynthesis of rifamycins. Their structures were elucidated on the basis of 1D and 2D NMR spectroscopic data and high-resolution ESIMS. Amexanthomycins A-C (1-3) showed inhibitory activity against human DNA topoisomerases.

Bioactive Pentacyclic Triterpene Ester Derivatives from Alnus viridis ssp. viridis Bark.

Seven derivatives of pentacyclic triterpene acids (1-7) were isolated from the bark of Alnus viridis ssp. viridis using a combination of column chromatography and semipreparative HPLC. Compounds 1-3, 6, and 7 were determined to be new after spectroscopic data interpretation and were assigned as 27-hydroxyalphitolic acid derivatives (1-3), a 27-hydroxybetulinic acid derivative (6), and a 3-epi-maslinic acid derivative (7), respectively. Pentacyclic triterpenoids with a C-27 hydroxymethyl group have been found in species of the genus Alnus for the first time. These compounds were subjected to cytotoxicity testing against a number of cancer cell lines. Also, selected pentacyclic triterpenoids were selected as potential inhibitors of topoisomerases I and IIα for an in silico investigation.

Lanostane-type triterpenoids from Abies faxoniana and their DNA topoisomerase inhibitory activities.

Nine lanostane-type triterpenoids were isolated from branches and leaves of Abies faxoniana, along with 10 known compounds. Two were isolated as inseparable mixtures of epimers at C-23 of the γ-lactone ring that had a lactol structure. The structures of the nine compounds were established by spectroscopic analysis and circular dichroism (CD) data. The absolute configurations at the stereogenic centres of two of the known compounds were confirmed by X-ray crystallography. One compound showed cytotoxic activities against HCT-116, MCF-7, and A549 cells with IC50 values of 8.9, 7.6, and 4.2µM, respectively. The isolated compounds were tested for their effects on human DNA topoisomerases I and II. One was found to be a selective inhibitor of human topo II activity with an IC50 value of 53.5µM, which was comparable to that of the topo II inhibitor etoposide (IC50=49.6µM).

Lead selection and characterization of antitubercular compounds using the Nested Chemical Library.

Discovering new drugs to treat tuberculosis more efficiently and to overcome multidrug resistance is a world health priority. To find novel antitubercular agents several approaches have been used in various institutions worldwide, including target-based approaches against several validated mycobacterial enzymes and phenotypic screens. We screened more than 17,000 compounds from Vichem's Nested Chemical Library™ using an integrated strategy involving whole cell-based assays with Corynebacterium glutamicum and Mycobacterium tuberculosis, and target-based assays with protein kinases PknA, PknB and PknG as well as other targets such as PimA and bacterial topoisomerases simultaneously. With the help of the target-based approach we have found very potent hits inhibiting the selected target enzymes, but good minimal inhibitory concentrations (MIC) against M. tuberculosis were not achieved. Focussing on the whole cell-based approach several potent hits were found which displayed minimal inhibitory concentrations (MIC) against M. tuberculosis below 10 µM and were non-mutagenic, non-cytotoxic and the targets of some of the hits were also identified. The most active hits represented various scaffolds. Medicinal chemistry-based lead optimization was performed applying various strategies and, as a consequence, a series of novel potent compounds were synthesized. These efforts resulted in some effective potential antitubercular lead compounds which were confirmed in phenotypic assays.

Further constituents of Galianthe thalictroides (Rubiaceae) and inhibition of DNA topoisomerases I and IIα by its cytotoxic ß-carboline alkaloids.

A new cytotoxic ß-carboline alkaloid, 1-methyl-3-(2-hydroxypropan-2-yl)-2-(5-methoxy-9H-ß-carbolin-1-yl)-cyclopentanol (1), was isolated from roots of Galianthe thalictroides, together with the alkaloid 1-(hydroxymethyl)-3-(2-hydroxypropan-2-yl)-2-(5-methoxy-9H-ß-carbolin-1-yl)-cyclopentanol (2), the anthraquinones 1-methyl-alizarin and morindaparvin-A, the coumarin scopoletin, homovanillic alcohol, (-)-epicatechin, and the steroids stigmast-4-en-3-one, 4,22-stigmastadien-3-one, campest-4-en-3-one, stigmast-4-en-3,6-dione, 6-ß-hydroxy-stigmast-4-en-3-one, stigmasterol, campesterol, ß-sitosterol, and ß-sitosterol-3-O-ß-D-glucopyranoside. Among the previously known compounds, homovanillic alcohol is a novel finding in Rubiaceae, while 1-methyl-alizarin, morindaparvin-A, scopoletin, stigmast-4-en-3-one, 4,22-stigmastadien-3-one, campest-4-en-3-one, stigmast-4-en-3,6-dione, and 6-ß-hydroxy-stigmast-4-en-3-one is reported for the first time in the genus Galianthe. The cytotoxic ß-carboline alkaloids 1 and 2 exhibited potent antitopoisomerase I and IIα activities and strong evidence is provided for their action as topoisomerase IIα poisons and redox-independent inhibitors.

p-Terphenyl O-ß-glucuronides, DNA topoisomerase inhibitors from Streptomyces sp. LZ35ΔgdmAI.

The analysis of genome sequence indicated that Streptomyces sp. LZ35 has the potential of producing many types of secondary metabolites, including p-terphenyls and geldanamycins. The fermentation of LZ35 in laboratory produces geldanamycins as the major components, which hampers the isolation of minor compounds. To clean the background of geldanamycins, the mutant strain LZ35ΔgdmAI of Streptomyces sp. LZ35 was constructed by disrupting the first PKS module of geldanamycin gene cluster (gdm). From this mutant, five novel p-terphenyls bearing glucuronic acid moiety, namely echosides A-E (1-5), were isolated with the aid of chromophore-guided fractionation. The structures of 1-5 were elucidated by the analysis of their HR-ESI-MS and NMR spectroscopic data. DNA relaxation assay indicated that compound 1 had evident inhibitory activity against topoisomerase I. Moreover, the inhibitory activity of compound 3 against topoisomerase IIα is approximately equal to VP16, indicating that p-terphenyl O-ß-glucuronides are promising leads for the development of novel inhibitors of topoisomerases.

Identification of anziaic acid, a lichen depside from Hypotrachyna sp., as a new topoisomerase poison inhibitor.

Topoisomerase inhibitors are effective for antibacterial and anticancer therapy because they can lead to the accumulation of the intermediate DNA cleavage complex formed by the topoisomerase enzymes, which trigger cell death. Here we report the application of a novel enzyme-based high-throughput screening assay to identify natural product extracts that can lead to increased accumulation of the DNA cleavage complex formed by recombinant Yersinia pestis topoisomerase I as part of a larger effort to identify new antibacterial compounds. Further characterization and fractionation of the screening positives from the primary assay led to the discovery of a depside, anziaic acid, from the lichen Hypotrachyna sp. as an inhibitor for both Y. pestis and Escherichia coli topoisomerase I. In in vitro assays, anziaic acid exhibits antibacterial activity against Bacillus subtilis and a membrane permeable strain of E. coli. Anziaic acid was also found to act as an inhibitor of human topoisomerase II but had little effect on human topoisomerase I. This is the first report of a depside with activity as a topoisomerase poison inhibitor and demonstrates the potential of this class of natural products as a source for new antibacterial and anticancer compounds.