In vitro anticancer screening of South African plants

AIM OF THE STUDY

The purpose of the present study is to evaluate South African plants for their anticancer activity.

MATERIALS AND METHODS

Plant species were collected throughout South Africa and voucher specimens were deposited and identified at the South African National Biodiversity Institute. Plant extracts were prepared and screened for in vitro anticancer activity against a panel of three human cell lines (breast MCF7, renal TK10 and melanoma UACC62) at the CSIR. Plant extracts that exhibited anticancer activity against these three human cell lines were screened by the NCI against sixty human cancer cell lines organized into sub-panels representing leukaemia, melanoma, cancer of the lung, colon, kidney, ovary, central nervous system, breast and prostate.

RESULTS

A total of 7500 plant extracts were screened for in vitro anticancer activity against breast MCF7, renal TK10 and melanoma UACC62 human cell lines between the period 1999 and 2006. Hits were classified into four categories based on their total growth inhibition of the cell lines. A hit rate of 5.9% was obtained for extracts which showed moderate activity and these were screened by the NCI against a panel of sixty human cancer cell lines. The extracts of plant species with limited published information for their anticancer properties were subjected to bioassay-guided fractionation and the active constituents isolated and identified. The largest number of plant specimens in this study was from the family Asteraceae, which is rich in sesquiterpene lactones.

CONCLUSIONS

Although the extracts of the plants were randomly selected, 68% of these plant species which were hits in the screening programme are reported to be used medicinally. Based on our data, it appears that unrelated medicinal use of the source plants may serve as an initial guide to selection of plants for anticancer screening.

Antineoplastic agents. 536. New sources of naturally occurring cancer cell growth inhibitors from marine organisms, terrestrial plants, and microorganisms(1a,)

Bioassay-guided fractionation of extracts of various plants, marine organisms, and microorganisms has led to the discovery of new natural sources of a number of known compounds that have significant biological activity. The isolation of interesting and valuable cancer cell growth inhibitors including majusculamide C ( 1), axinastatin 5 ( 5), bengazoles A ( 6), B ( 7), and E ( 8), manzamine A ( 10), jaspamide ( 11), and neoechinulin A ( 19) has been summarized.

Ethnobotany and drug discovery: the experience of the US National Cancer Institute

Between 1960 and 1981 the National Cancer Institute (NCI) screened 114,000 extracts of 35,000 plants, mainly collected in temperate regions. Of the three clinically active anticancer drugs so far discovered in that programme, none was isolated from a plant collected on an ethnobotanical basis, though various Taxus species, which are the source of taxol, are reported to have been used medicinally. Since 1986, the NCI has focused its collections in tropical and subtropical regions worldwide; collections cover a broad taxonomic range, though priority is given to medicinal plants when relevant information is available. As of August 1993, 21,881 extracts derived from over 10,500 samples had been tested in a screen for activity against the human immunodeficiency virus (HIV); 2320 of these extracts were of medicinal plant origin. Approximately 18% of both the total number of extracts and the medicinal plant-derived extracts showed significant anti-HIV activity; in each instance about 90% of the active extracts were aqueous. The activity of the aqueous extracts has been attributed mainly to the presence of polysaccharides or tannins. Four plant-derived compounds are in preclinical development at the NCI; only one of the four sources plants, obtained from a noncontract source, was collected on an ethnobotanical basis. At this stage the results indicate that the current NCI collection policy offers the best chances for the discovery and development of agents for the treatment of AIDS (acquired immune deficiency syndrome) and cancer.

Natural products as sources of new drugs over the last 25 years

This review is an updated and expanded version of two prior reviews that were published in this journal in 1997 and 2003. In the case of all approved agents the time frame has been extended to include the 251/2 years from 01/1981 to 06/2006 for all diseases worldwide and from 1950 (earliest so far identified) to 06/2006 for all approved antitumor drugs worldwide. We have continued to utilize our secondary subdivision of a "natural product mimic" or "NM" to join the original primary divisions. From the data presented, the utility of natural products as sources of novel structures, but not necessarily the final drug entity, is still alive and well. Thus, in the area of cancer, over the time frame from around the 1940s to date, of the 155 small molecules, 73% are other than "S" (synthetic), with 47% actually being either natural products or directly derived therefrom. In other areas, the influence of natural product structures is quite marked, with, as expected from prior information, the antiinfective area being dependent on natural products and their structures. Although combinatorial chemistry techniques have succeeded as methods of optimizing structures and have, in fact, been used in the optimization of many recently approved agents, we are able to identify only one de novo combinatorial compound approved as a drug in this 25 plus year time frame. We wish to draw the attention of readers to the rapidly evolving recognition that a significant number of natural product drugs/leads are actually produced by microbes and/or microbial interactions with the "host from whence it was isolated", and therefore we consider that this area of natural product research should be expanded significantly.

Prenylated flavonoids from the root bark of Berchemia discolor, a Tanzanian medicinal plant

Five new prenylated flavonoids (1-5) were isolated from the root bark of Berchemia discolor, collected in Tanzania, along with 10 known compounds, by bioactivity-guided fractionation. The structures of compounds 1-5 were elucidated using various spectroscopic techniques. Of these isolates, compound 4, and the known compounds nitidulin (6), amorphigenin (7), and dabinol (8), exhibited cytotoxic activity when evaluated against a small panel of human cancer cells. Nitidulin (6) was further tested in an in vivo hollow fiber assay and found to be active against LNCaP (human hormone-dependent prostate cancer) cells implanted intraperitoneally, at doses of 10, 20, and 40 mg/kg.

Natural Products from Marine Invertebrates and Microbes as Modulators of Antitumor Targets

Over the last twenty-five to thirty years, exploration of the marine fauna and microbial flora has progressed from a random search by natural product chemists who liked to dive and wished to combine their hobby with their profession, to fully integrated programs of systemic investigation of the chemical agents elaborated by marine organisms of all phyla (as presumably defensive agents against predators) for their potential as leads to human-use drug candidates where the putative mechanisms have been identified as modulation of, and/or interaction with, potential molecular targets, rather than just exhibiting general cytotoxicity. This review is not exhaustive but is meant to cover the highlights of such agents and is arranged on a (nominal) target basis rather than by organism or chemical class.

Antineoplastic agents. 551. Isolation and structures of bauhiniastatins 1-4 from Bauhinia purpurea

Bioassay-guided (P388 lymphocytic leukemia cell line) separation of extracts prepared from the leaves, stems, and pods of Bauhinia purpurea, and, in parallel, its roots, led to the isolation of four new dibenz[b,f]oxepins (2a, 3-5) named bauhiniastatins 1-4, as well as the known and related pacharin (1) as cancer cell growth inhibitors. The occurrence of oxepin derivatives in nature is quite rare. Bauhiniastatins 1-4 were found to exhibit significant growth inhibition against a minipanel of human cancer cell lines, and bauhiniastatin 1 (2a) was also found to inhibit the P388 cancer cell line. Structures for these new cancer cell growth inhibitors were established by spectroscopic techniques that included HRMS and 2D NMR.

Natural product extracts of plant and marine origin having antileukemia potential. The NCI experience

While effective treatments exist for acute lymphocytic leukemia (ALL), particularly in the case of children, and for chronic mylogenous leukemia (CML), more efficacious treatments for other forms of acute and chronic forms of the disease are still needed. The National Cancer Institute has tested over 90,000 extracts of terrestrial plants and marine plants and invertebrates in its human cancer one-dose/60-cell-line prescreen, and the results for plants and marine organisms meeting criteria established for activity against selected leukemia cell lines are presented. Taxonomic data are limited to family and genus in the case of plants, and phylum for marine organisms, and those groups of organisms exhibiting significant activity (so-called "hot" families and genera) are discussed. The "hot" terrestrial plant families Myrsinaceae and Sapindaceae have not been studied to any extent and appear to merit special attention, although leukemia cell line selectivity is also noted for other families.

Rautandiols A and B, pterocarpans and cytotoxic constituents from Neorautanenia mitis

As part of a study on potential antitumor agents from rainforest plants, two new pterocarpans, rautandiol A (1) and rautandiol B (2), together with eight known compounds, were isolated from Neorautanenia mitis. Among the compounds isolated, rotenone (3) and 12-hydroxyrotenone (4) showed significant cytotoxic activity with IC(50) values of 0.008-0.010 and 0.04-0.06 microg/mL against MCF-7 and A-549 cells, respectively.

Tropane aromatic ester alkaloids from a large-scale re-collection of Erythroxylum pervillei stem bark obtained in Madagascar

Fractionation by pH zone-refining countercurrent chromatography of an extract of the stem bark of Erythroxylum pervillei, obtained on a kilogram scale in southern Madagascar, led to the isolation and characterization of four tropane aromatic ester alkaloids as minor constituents, namely, pervilleines G (5) and H (6) and cis-pervilleines B (7) and F (8). Their structures were determined by spectroscopic data interpretation.

Plants as a source of anti-cancer agents

Plant-derived compounds have been an important source of several clinically useful anti-cancer agents. These include vinblastine, vincristine, the camptothecin derivatives, topotecan and irinotecan, etoposide, derived from epipodophyllotoxin, and paclitaxel (taxol A number of promising new agents are in clinical development based on selective activity against cancer-related molecular targets, including flavopiridol and combretastin A4 phosphate, while some agents which failed in earlier clinical studies are stimulating renewed interest.

Advanced preclinical and clinical trials of natural products and related compounds from marine sources

The marine environment has proven to be a very rich source of extremely potent compounds that have demonstrated significant activities in anti-tumor, anti-inflammatory, analgesia, immuno-modulation, allergy and anti-viral assays. Although the case can and has been made that the nucleosides such as Ara-A and Ara-C are derived from knowledge gained from investigations of bioactive marine nucleosides, no drug directly from marine sources (whether isolated or by total synthesis) has yet made it to the commercial sector in any human disease. However, as shown in this review, there are now significant numbers of very interesting molecules that have come from marine sources, or have been synthesized as a result of knowledge gained from a prototypical compound, that are either in or approaching Phase III clinical trials in cancer, analgesia and allergy, with a very substantial number of other, quite different potential agents following in their wake, in these and in other diseases.

Marine natural products and related compounds in clinical and advanced preclinical trials

The marine environment has proven to be a very rich source of extremely potent compounds that have demonstrated significant activities in antitumor, antiinflammatory, analgesia, immunomodulation, allergy, and anti-viral assays. Although the case can and has been made that the nucleosides such as Ara-A and Ara-C are derived from knowledge gained from investigations of bioactive marine nucleosides, no drug directly from marine sources (whether isolated or by total synthesis) has yet made it to the commercial sector in any disease. However, as shown in this review, there are now significant numbers of very interesting molecules that have come from marine sources, or have been synthesized as a result of knowledge gained from a prototypical compound, that are either in or approaching Phase II/III clinical trials in cancer, analgesia, allergy, and cognitive diseases. A substantial number of other potential agents are following in their wake in preclinical trials in these and in other diseases.

A tale of two tumor targets: topoisomerase I and tubulin. The Wall and Wani contribution to cancer chemotherapy

The seminal discoveries of camptothecin and Taxol by Wall and Wani are discussed in a manner that demonstrates the influence that these two compounds has had on the further development of natural product, natural product-derived, and (some) synthetic entities as potential drug leads that interact either with tubulin or with topoisomerase I. The major categories of tubulin interactive agents in terms of inhibition and promotion of tubulin polymerization are briefly discussed. Likewise, a brief discussion of topoisomerase I inhibitors is presented. Lists of tubulin interactive agents and topoisomerase I inhibitors in preclinical and clinical development are given in Tables 2 and 3, respectively. This review is not meant to be exhaustive, but does illustrate the profound impact that these two plant-derived agents have had on cancer chemotherapy.

Investigations of the marine flora and fauna of the Islands of Palau

The Islands of Palau have proven to be an excellent source of bioactive marine natural products primarily as a result of the systematic studies from the late 1970s by the research groups of Scheuer at the University of Hawaii, Faulkner at the Scripps Oceanographic Institution/University of California at San Diego, and Paul at the University of Guam. Their efforts were materially aided by the excellent facilities provided by the Government of Palau and for the last 10 years, those of the NCI's shallow water collection contractor, the Coral Reef Research Foundation. This review covers the structures and biological activities where noted, of the multitudinous marine-derived natural products isolated from the marine flora and fauna of this nation and demonstrates the enormous variety of novel structures elaborated by these organisms.

Antitumor agents. 228. five new agarofurans, Reissantins A-E, and cytotoxic principles from Reissantia buchananii

Twenty-one compounds, including five new agarofuran sesquiterpenes, reissantins A-E (1-5), were isolated from Reissantia buchananii by means of bioassay-directed fractionation and their structures identified from spectral data. Reissantins A-C are the first reported simple agarofuran sesquiterpenes to contain a 5-carboxy-N-methyl-2-pyridone (CNMP) substituent, which has previously been found only in macroring agarofuran pyridine alkaloids. The major terpenoid components, celastrol (6) and its methyl ester derivative, pristimerin (7), were significantly active against nine cancer cell lines, including A549, MCF-7, HCT-8, KB, KB-VIN, U-87-MG, PC-3, 1A9, and PTX10 cell lines, with ED(50) values ranging from 0.076 to 0.34 microg/mL.

Natural products as sources of new drugs over the period 1981-2002

This review is an updated and expanded version of a paper that was published in this journal in 1997. The time frame has been extended in both directions to include the 22 years from 1981 to 2002, and a new secondary subdivision related to the natural product source but applied to formally synthetic compounds has been introduced, using the concept of a "natural product mimic" or "NM" to join the original primary divisions. From the data presented, the utility of natural products as sources of novel structures, but not necessarily the final drug entity, is still alive and well. Thus, in the area of cancer, the percentage of small molecule, new chemical entities that are nonsynthetic has remained at 62% averaged over the whole time frame. In other areas, the influence of natural product structures is quite marked, particularly in the antihypertensive area, where of the 74 formally synthetic drugs, 48 can be traced to natural product structures/mimics. Similarly, with the 10 antimigraine drugs, seven are based on the serotonin molecule or derivatives thereof. Finally, although combinatorial techniques have succeeded as methods of optimizing structures and have, in fact, been used in the optimization of a number of recently approved agents, we have not been able to identify a de novo combinatorial compound approved as a drug in this time frame.

Elicitation, a new window into plant chemodiversity and phytochemical drug discovery

Plant extracts collected from the wild are important sources for drug discovery. However, these extracts suffer from a lack of reproducible bioactivity and chemical composition caused by the highly inducible, variable, and transitory nature of plant secondary metabolism. Here, we demonstrate that exposing roots of hydroponically grown plants to chemical elicitors selectively and reproducibly induced the production of bioactive compounds, dramatically increased the hit rate, and more than doubled the number of plant species showing in vitro activity against bacteria, fungi, or cancer. Elicitation performed under controlled conditions dramatically improves reliability and efficiency of plant extracts in drug discovery while preserving wild species and their habitats.

Natural products and derivatives as leads to cell cycle pathway targets in cancer chemotherapy

The influence of natural products upon drug discovery in general has been quite impressive; one only has to look at the number of clinically active drugs that are in use in cancer therapy to see how many either are natural products or have a natural pro-duct pharmacophore. What is now becoming quite apparent is that materials from natural sources are excellent probes (indicators) for cellular targets that when modulated, may well have a deleterious effect upon the cycling of a tumor cell through the conventional cell cycle. If the particular target is not expressed in normal cell cycling, then a directed "perturbation" of the tumor cell's cycle may well lead to a novel method of treatment for specific tumor types. In this review we have not attempted to be exhaustive but have given a current overview of how natural products from marine, microbial and plant sources have permitted in-depth analyses of various parts of the cell cycle under varying conditions with the ultimate aims of attempting to "control or perturb" the cycling of tumor cells in a fashion that permits their ultimate removal via cellular death, with a minimum of trauma to the host.

Antineoplastic agents. 489. Isolation and structures of meliastatins 1-5 and related euphane triterpenes from the tree Melia dubia

The bark of the giant neem tree Melia dubia was found to contain 11 euphane-type triterpenes. Five new compounds, meliastatins 1-5 (1-5), proved to inhibit growth of the P388 lymphocytic leukemia cell line (ED(50) 1.7-5.6 microg/mL). Four of the others, the previously known methyl kulonate (8), kulinone (9), 16-hydroxybutyrospermol (10), and kulactone (11), were also found to inhibit (ED(50) 2.5-6.2 microg/mL) the P388 cancer cell line. In addition, two new euphane triterpenes were isolated and named dubione A (6) and dubione B (7). Structures for each of the 11 euphane triterpenes were established by spectral techniques that included HRMS and 2D NMR.

Anticancer drug discovery and development throughout the world

This year's American Society of Clinical Oncology International Symposium devoted 2 hours to a lively discussion of various aspects of anticancer drug discovery and development throughout the world. The scientific program started with an overview of efforts directed toward promoting international collaboration in natural product-derived anticancer drug discovery. This was followed by a discussion on the importance of interethnic differences and pharmacogenetics in anticancer drug development. Thereafter, this part of the program was completed by a description of the activities of the newly created Singapore-Hong Kong-Australia Drug Development Consortium and an overview of the contribution of Japan to anticancer drug development. The logistics and regulatory aspects of clinical trials with new anticancer agents in different parts of the world were then presented, with an emphasis on Europe, North America, and Japan. The program was completed with a panel discussion of the efforts to harmonize the exchange of clinical data originating from one region of the globe with other territories, with input from official representatives of the United States Food and Drug Administration and the Medical Devices Evaluation Center of Japan.