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.

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.

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.

Coral reefs, forests, and thermal vents: the worldwide exploration of nature for novel antitumor agents.

Nature has been a source of medicinal treatments for thousands of years, and plant-based systems continue to play an essential role in the primary health care of 80% of the world's population. Nature has provided many of the effective anticancer agents in current use, such as the microbially derived drugs, dactinomycin, bleomycin, and doxorubicin, and the plant-derived drugs vinblastine, irinotecan, topotecan, etoposide, and paclitaxel. The search for novel antitumor agents from natural sources continues with botanists, marine biologists, and microbiologists teaming up with chemists, pharmacologists, toxicologists, and clinicians in the investigation of coral reefs, rainforests, and deep subsurface thermal vents for novel bioactive compounds. The wealth of anticancer drugs of natural origin and critical aspects of the ongoing discovery and development process are discussed.

Anti-HIV michellamines from Ancistrocladus korupensis.

Here we report details of the isolation and determination of the absolute configurations and comparative anti-HIV activities of novel, atropisomeric naphthylisoquinoline alkaloid dimers, michellamines A, B, and C, from a newly described species of Ancistrocladus from the Korup rainforest of Cameroon. We further provide a more extensive analysis of the range of anti-HIV activity of michellamine B, the most potent and abundant member of the series. Michellamine B inhibited HIV-induced cell killing and viral replication in a variety of human cell lines, as well as in cultures of human peripheral blood leukocytes and monocytes. Michellamine B was active against a panel of biologically diverse laboratory and clinical strains of HIV-1, including the AZT-resistant strain G910-6 and the pyridinone-resistant strain A17; the compound also inhibited several strains of HIV-2.

The calanolides, a novel HIV-inhibitory class of coumarin derivatives from the tropical rainforest tree, Calophyllum lanigerum.

Eight new coumarin compounds (1-8) were isolated by anti-HIV bioassay-guided fractionation of an extract of Calophyllum lanigerum. The structures of calanolide A (1), 12-acetoxycalanolide A (2), 12-methoxycalanolide A (3), calanolide B (4), 12-methoxycalanolide B (5), calanolide C (6) and related derivatives 7 and 8 were solved by extensive spectroscopic analyses, particularly HMQC, HMBC, and difference NOE NMR experiments. The absolute stereochemistry of calanolide A (1) and calanolide B (4) was established by a modified Mosher's method. Calanolides A (1) and B (4) were completely protective against HIV-1 replication and cytopathicity (EC50 values of 0.1 microM and 0.4 microM, respectively), but were inactive against HIV-2. Some of the related compounds also showed evidence of anti-HIV-1 activity. Studies with purified bacterial recombinant reverse transcriptases (RT) revealed that the calanolides are HIV-1 specific RT inhibitors. Moreover, calanolide A was active not only against the AZT-resistant G-9106 strain of HIV-1 but also against the pyridinone-resistant A17 strain. This was of particular interest since the A17 virus is highly resistant to previously known HIV-1 specific, non-nucleoside RT inhibitors (e.g., TIBO; BI-RG-587; L693,593) which comprise a structurally diverse but apparently common pharmacologic class. The calanolides represent a substantial departure from the known class and therefore provide a novel new anti-HIV chemotype for drug development.

Antineoplastic agents from natural sources: achievements and future directions.

The influence of natural products upon anticancer drug discovery and design cannot be overestimated. Approximately 60% of all drugs now in clinical trials for the multiplicity of cancers are either natural products, compounds derived from natural products, contain pharmacophores derived from active natural products or are 'old drugs in new clothes', where (modified) natural products are attached to targeting systems. This review covers those materials that the authors are aware of as being in clinical trials through early 2000 and demonstrates how, even today, in the presence of massive numbers of agents from combinatorial libraries, the compounds produced by 'Mother Nature' are still in the forefront of cancer chemotherapeutics as sources of active chemotypes.

Medicinals for the millennia: the historical record.

Nature has been a source of medicinal agents for thousands of years, and an impressive number of modern drugs have been isolated from natural sources, many based on their use in traditional medicine. The use of herbal drugs is once more escalating in the form of complementary and alternative medicine. The past century, however, has seen an increasing role played by microorganisms in the production of the antibiotics and other drugs for the treatment of some serious diseases. With less than 1% of the microbial world currently known, advances in procedures for microbial cultivation and the extraction of nucleic acids from environmental samples from soil and marine habitats, and from symbiotic and endophytic microbes associated with terrestrial and marine macro-organisms, will provide access to a vast untapped reservoir of genetic and metabolic diversity. By use of combinatorial chemical and biosynthetic technology, novel natural product leads will be optimized on the basis of their biological activities to yield effective chemotherapeutic and other bioactive agents.

Paclitaxel (Taxol): a success story with valuable lessons for natural product drug discovery and development.

The discovery and development of paclitaxel, which covered a time span of some 30 years, has provided some important lessons for those involved in natural product drug discovery and development. These include the adoption of novel screens as they become available, the elucidation of mechanisms of action, and addressing the supply issue at an early stage of development. These issues, as applied to paclitaxel, are illustrated. The development of the NCI human cancer cell line screen, and its application to mechanistic studies through use of COMPARE analyses, are discussed, as is the production of the marine-derived anticancer agent, bryostatin 1, which provides another illustration of a successful approach to solving a supply issue. The history of the development of paclitaxel also illustrates the importance of multidisciplinary collaboration, and the various mechanisms used by the NCI Developmental Therapeutics Program for promoting such collaboration are presented.

Discovery and development of antineoplastic agents from natural sources.

Nature has provided many effective anticancer agents in current use, such as the microbially derived drugs; dactinomycin; bleomycin and doxorubicin; and the plant-derived drugs, vinblastine, irinotecan, topotecan, etoposide, and paclitaxel. The search for novel antitumor agents from natural sources continues through collaboration among scientists worldwide in the investigation of coral reefs, rainforests, and deep subsurface thermal vents for novel bioactive compounds. The potential for drug discovery is being further enhanced by recent advances in procedures for microbial cultivation and the extraction of nucleic acids from environmental samples, resulting in the identification of novel microbes that provide a vast untapped reservoir of genetic and metabolic diversity. Manipulation of the biosynthetic pathways of microbial polyketides through genetic engineering permits the biosynthesis of bioactive polyketides not generated naturally.

Natural product drug discovery in the next millennium.

Nature has been a source of medicinal agents for thousands of years, and an impressive number of modern drugs have been isolated from natural sources, many based on their use in traditional medicine. In the past century, however, an increasing role has been played by microorganisms in the production of antibiotics and other drugs for the treatment of some serious diseases. Advances in the description of the human genome, as well as the genomes of pathogenic microbes and parasites, is permitting the determination of the structures of many proteins associated with disease processes. With the development of new molecular targets based on these proteins, there is an increasing demand for novel molecular diversity for screening. Natural products will play a crucial role in meeting this demand through the continued investigation of world's biodiversity, much of which remains unexplored. With less than 1% of the microbial world currently known, advances in procedures for microbial cultivation and the extraction of nucleic acids from environmental samples from soil and marine habitats, will provide access to a vast untapped reservoir of genetic and metabolic diversity. The same holds true for nucleic acids isolated from symbiotic and endophytic microbes associated with terrestrial and marine macroorganisms. By use of combinatorial chemical and biosynthetic technology, novel natural product leads will be optimized on the basis of their biological activities to yield effective chemotherapeutic and other bioactive agents. The investigation of these resources requires multi-disciplinary, national, and international collaboration in the discovery and development process.

Antineoplastic agents, 122. Constituents of Combretum caffrum.

An investigation of the South African tree Combretum caffrum (Combretaceae) for antineoplastic constituents was conducted by employing the astrocytoma bioassay (9ASK). By this approach and a combination of solvent partition, steric exclusion, and adsorption chromatographic procedures, a substance designated combretastatin [1a] was isolated and found to display significant (71-90% astrocyte reversal at 1-100 micrograms/ml dose levels) astrocyte reversal and murine P-388 lymphocytic leukemia (PS) cell growth inhibition (ED50 0.011 micrograms/ml). The structure of (-)-combretastatin was elucidated by X-ray crystal structure analysis and confirmed by total synthesis. The absolute configuration at C-10 was assigned the R-configuration on the basis of Horeau esterification techniques. Other prominent, albeit PS-inactive, constituents were found to be 3,3',4'-tri-O-methylellagic acid [2] and acacetin [3].

Cucurbitacins: differential cytotoxicity, dereplication and first isolation from Gonystylus keithii.

A characteristic pattern of differential cytotoxicity of extracts of Iberis amara seeds, predominantly toward renal tumor, brain tumor, and melanoma cell lines in the NCI human disease-oriented tumor screening panel, was traced to cucurbitacins E [1] and I [2]. This same differential cytotoxicity profile was detected in extracts of Begonia plebeja and Gonystylus keithii. Computer-assisted recognition of these profiles was followed by a rapid chemical fractionation, thus permitting the efficient dereplication of those extracts containing cucurbitacins B [3] and D [4], respectively. This is the first report of cucurbitacins from the genus Gonystylus.

Natural product-based anti-HIV drug discovery and development facilitated by the NCI developmental therapeutics program.

During the decade 1987-1996, the Developmental Therapeutics Program (DTP) of the National Cancer Institute (NCI) provided infrastructure support for both intramural and extramural anti-HIV (human immunodeficiency virus) drug discovery research and development. This retrospective review describes some of the anti-HIV lead discovery and development that took place under DTP auspices or which was substantially facilitated by resources made available through the DTP. Examples highlighted include leads identified through the initial screening of pure natural product derived compounds and those derived from bioassay-guided fractionation of crude natural product extracts, and these are classified according to the mechanism of action targeting the critical steps within the replication cycle of HIV.

Antineoplastic agents, 88. Pimelea prostrata.

The aerial portion of Pimelea prostrata (Thymelaeaceae) collected in New Zealand was evaluated as a source of substances that inhibit growth of the murine P-388 lymphocytic leukemia (PS). Simplexin (1) and Pimelea factor P2 (2) were found to strongly inhibit growth (ED50 5 X 10(-3) and 8 X 10(-4) micrograms/ml, respectively) of the PS in vitro cell line. The cyclic orthoester (2) was also found to inhibit growth (T/C 132 at 20 micrograms/kg) of the PS in vivo system. Detailed 1H-nmr (at 400 MHz) and 13C-nmr studies combined with fast atom bombardment mass spectral evidence were employed to confirm the structural assignments.

Antineoplastic agents, 178. Isolation and structure of lychnostatins 1 and 2 from the South American Lychnophora antillana.

Bioassay-guided (P388 lymphocytic leukemia cell line) separation of a CH2Cl2/MeOH extract of Lychnophora antillana led to the isolation of two cytostatic (P-388, ED50 2.0 and 0.19 micrograms/ml, respectively) germacranolides designated lychnostatins 1 [1] and 2 [2]. Structural elucidation was based initially upon high field (400 MHz) nmr and electron impact mass spectral interpretations and unequivocally completed by X-ray crystal structure determinations.

The taxol supply crisis. New NCI policies for handling the large-scale production of novel natural product anticancer and anti-HIV agents.

Over the past 30 years, the National Cancer Institute has been involved in the preclinical and/or clinical evaluation of the majority of those agents approved for the treatment of cancer. Many of the new agents under consideration in the NCI program are either natural products or derivatives of natural product leads, and of critical importance to their development is the issue of drug supply. In responding to the drug supply crisis which emerged with the demonstration of the clinical efficacy of taxol, the NCI has identified several important lessons for those interested in natural product drug discovery and development. As a result, the NCI has developed plans to avert similar supply crisis in the future by initiating exploratory research projects for large-scale production of promising agents at the earliest possible point following the demonstration of confirmed antitumor activity. These plans, together with a review of the development of taxol, are presented in this paper.

Cytotoxic quassinoids from Cedronia granatensis.

The NCI in vitro primary disease-oriented antitumor screen has been used to select and guide the fractionation of the organic and aqueous extracts of Cedronia granatensis. Two quassinoids, sergiolide [1] and isobrucein B [2], to which the screening panel cell lines exhibited up to a 1000-fold range of differential sensitivity, were isolated. At concentrations of 10(-5)-10(-8) M, the compounds typically produced LC50-level responses against a majority of the melanoma lines and several of the colon, lung, and other solid tumor lines. These and related quassinoids may, therefore, be of interest for in vivo evaluation in appropriate xenograft tumor models.