Plant Endophytes and Epiphytes: Burgeoning Sources of Known and "Unknown" Cytotoxic and Antibiotic Agents?

In the last 20 or so years, the influence of endophytes and, quite recently, epiphytes of plants upon the compounds found in those plants, which were usually assumed to be phytochemicals produced by the plant for a variety of reasons, often as a defense against predators, is becoming more evident, in particular in the case of antitumor agents originally isolated from plant sources, though antibiotic agents might also be found, particularly from epiphytes. In this review, we started with the first report in 1993 of a taxol-producing endophyte and then expanded the compounds discussed to include camptothecin, the vinca alkaloids, podophyllotoxin, and homoharringtonine from endophytic microbes and then the realization that maytansine is not a plant secondary metabolite at all, and that even such a well-studied plant such as Arabidopsis thaliana has a vast repertoire of potential bioactive agents in its leaf epiphytic bacteria. We have taken data from a variety of sources, including a reasonable history of these discoveries that were not given in recent papers by us, nor in other papers covering this topic. The sources included the Scopus database, but we also performed other searches using bibliographic tools, thus, the majority of the papers referenced are the originals, though we note some very recent papers that have built on previous results. We concluded with a discussion of the more modern techniques that can be utilized to "persuade" endophytes and epiphytes to switch on silent biosynthetic pathways and how current analytical techniques may aid in evaluating such programs. We also comment at times on some findings, particularly in the case of homoharringtonine, where there are repetitious data reports differing by a few years claiming the same endophyte as the producer.

Natural Products as Sources of New Drugs over the Nearly Four Decades from 01/1981 to 09/2019

This review is an updated and expanded version of the five prior reviews that were published in this journal in 1997, 2003, 2007, 2012, and 2016. For all approved therapeutic agents, the time frame has been extended to cover the almost 39 years from the first of January 1981 to the 30th of September 2019 for all diseases worldwide and from ∼1946 (earliest so far identified) to the 30th of September 2019 for all approved antitumor drugs worldwide. As in earlier reviews, only the first approval of any drug is counted, irrespective of how many "biosimilars" or added approvals were subsequently identified. As in the 2012 and 2016 reviews, we have continued to utilize our secondary subdivision of a "natural product mimic", or "NM", to join the original primary divisions, and the designation "natural product botanical", or "NB", to cover those botanical "defined mixtures" now recognized as drug entities by the FDA (and similar organizations). From the data presented in this review, the utilization of natural products and/or synthetic variations using their novel structures, in order to discover and develop the final drug entity, is still alive and well. For example, in the area of cancer, over the time frame from 1946 to 1980, of the 75 small molecules, 40, or 53.3%, are N or ND. In the 1981 to date time frame the equivalent figures for the N* compounds of the 185 small molecules are 62, or 33.5%, though to these can be added the 58 S* and S*/NMs, bringing the figure to 64.9%. In other areas, the influence of natural product structures is quite marked with, as expected from prior information, the anti-infective area being dependent on natural products and their structures, though as can be seen in the review there are still disease areas (shown in Table 2) for which there are no drugs derived from natural products. Although combinatorial chemistry techniques have succeeded as methods of optimizing structures and have been used very successfully in the optimization of many recently approved agents, we are still able to identify only two de novo combinatorial compounds (one of which is a little speculative) approved as drugs in this 39-year time frame, though there is also one drug that was developed using the "fragment-binding methodology" and approved in 2012. We have also added a discussion of candidate drug entities currently in clinical trials as "warheads" and some very interesting preliminary reports on sources of novel antibiotics from Nature due to the absolute requirement for new agents to combat plasmid-borne resistance genes now in the general populace. We continue to draw the attention of readers to the 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"; thus we consider that this area of natural product research should be expanded significantly.

Current Status of Marine-Derived Compounds as Warheads in Anti-Tumor Drug Candidates

In this review, we have attempted to describe all of the antibody–drug conjugates using a marine-derived compound as the “warhead”, that are currently in clinical trials as listed in the current version of the NIH clinical trials database (clinicaltrials.gov). In searching this database, we used the beta-test version currently available, as it permitted more specific search parameters, since the regular version did not always find trials that had been completed in the past with some agents. We also added small discussion sections on candidates that are still at the preclinical stage, including a derivative of diazonamide that has an unusual interaction with tubulin (DZ-23840), which may also be a potential warhead in the future.

Drugs and Drug Candidates from Marine Sources: An Assessment of the Current "State of Play"

The potential of the marine environment to produce candidate compounds (structures) as leads to, or even direct drugs from, has been actively discussed for the last 50 or so years. Over this time frame, several compounds have led to drugs, usually in the area of cancer (due to funding sources). This review is designed to show where there have been successes, but also to show that in a number of disease areas, there are structures originally isolated from marine invertebrates and free-living microbes that have potential, but will need to be "adopted" by pharmaceutical houses in order to maximize their potential.

Antineoplastic Agents. 585. Isolation of Bridelia ferruginea Anticancer Podophyllotoxins and Synthesis of 4-Aza-podophyllotoxin Structural Modifications

Cytotoxic constituents of the terrestrial plant Bridelia ferruginea were isolated using bioactivity-guided fractionation, which revealed the presence of the previously known deoxypodophyllotoxin (1), isopicrodeoxypodophyllotoxin (2), β-peltatin (3), β-peltatin-5-O-β-D-glucopyranoside (3a), and the indole neoechinulin (4). As an extension of previous podophyllotoxin research, SAR studies were undertaken focused on 4-aza-podophyllotoxin structural modifications. A number of such derivatives were synthesized following modifications to the A and E rings. Such structural modifications with alkyl and 4-fluorobenzyl substituents at the 4-aza position provided the most potent cancer cell growth inhibitory activity (GI50 0.1 to <0.03 μg/mL) against a panel of six human cancer cell lines and one murine cancer cell line. Several compounds corresponding to 4'-demethylated modifications were also synthesized and found to be significantly less potent.

Natural Products as Sources of New Drugs from 1981 to 2014

This contribution is a completely updated and expanded version of the four prior analogous reviews that were published in this journal in 1997, 2003, 2007, and 2012. In the case of all approved therapeutic agents, the time frame has been extended to cover the 34 years from January 1, 1981, to December 31, 2014, for all diseases worldwide, and from 1950 (earliest so far identified) to December 2014 for all approved antitumor drugs worldwide. As mentioned in the 2012 review, we have continued to utilize our secondary subdivision of a "natural product mimic", or "NM", to join the original primary divisions and the designation "natural product botanical", or "NB", to cover those botanical "defined mixtures" now recognized as drug entities by the U.S. FDA (and similar organizations). From the data presented in this review, the utilization of natural products and/or their novel structures, in order to discover and develop the final drug entity, is still alive and well. For example, in the area of cancer, over the time frame from around the 1940s to the end of 2014, of the 175 small molecules approved, 131, or 75%, are other than "S" (synthetic), with 85, or 49%, 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 anti-infective area being dependent on natural products and their structures. 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 it is considered that this area of natural product research should be expanded significantly.

Natural Products as a Vital Source for the Discovery of Cancer Chemotherapeutic and Chemopreventive Agents

Throughout history, natural products have played a dominant role in the treatment of human ailments. For example, the legendary discovery of penicillin transformed global existence. Presently, natural products comprise a large portion of current-day pharmaceutical agents, most notably in the area of cancer therapy. Examples include Taxol, vinblastine, and camptothecin. These structurally unique agents function by novel mechanisms of action; isolation from natural sources is the only plausible method that could have led to their discovery. In addition to terrestrial plants as sources for starting materials, the marine environment (e.g., ecteinascidin 743, halichondrin B, and dolastatins), microbes (e.g., bleomycin, doxorubicin, and staurosporin), and slime molds (e.g., epothilone B) have yielded remarkable cancer chemotherapeutic agents. Irrespective of these advances, cancer remains a leading cause of death worldwide. Undoubtedly, the prevention of human cancer is highly preferable to treatment. Cancer chemoprevention, the use of vaccines or pharmaceutical agents to inhibit, retard, or reverse the process of carcinogenesis, is another important approach for easing this formidable public health burden. Similar to cancer chemotherapeutic agents, natural products play an important role in this field. There are many examples, including dietary phytochemicals such as sulforaphane and phenethyl isothiocyanate (cruciferous vegetables) and resveratrol (grapes and grape products). Overall, natural product research is a powerful approach for discovering biologically active compounds with unique structures and mechanisms of action. Given the unfathomable diversity of nature, it is reasonable to suggest that chemical leads can be generated that are capable of interacting with most or possibly all therapeutic targets.

Endophytic and epiphytic microbes as "sources" of bioactive agents

Beginning with the report by Stierle and Strobel in 1993 on taxol^(R) production by an endophytic fungus (Stierle et al., 1993), it is possible that a number of the agents now used as leads to treatments of diseases in man, are not produced by the plant or invertebrate host from which they were first isolated and identified. They are probably the product of a microbe in, on or around the macroorganism. At times there is an intricate “dance” between a precursor produced by a microbe, and interactions within the macroorganism, or in certain cases, a fungus, that ends up with the production of a novel agent that has potential as a treatment for a human disease. This report will give examples from insects, plants, and marine invertebrates.

New horizons for old drugs and drug leads

There is mounting urgency to find new drugs for the treatment of serious infectious diseases and cancer that are rapidly developing resistance to previously effective drugs. One approach to addressing this need is through drug repurposing, which refers to the discovery of new useful activities for "old" clinically used drugs through screening them against relevant disease targets. A large number of potential drug that, for various reasons, have failed to advance to clinical and commercial use can be added to the candidates available for such purposes. The application of new techniques and methodology developed through the impressive progress made in multidisciplinary, natural product-related research in recent years should aid substantially in expediting the discovery and development process. This review briefly outlines some of these developments as applied to a number of selected natural product examples, which may also include advances in chemical synthesis of derivatives with extended biological activities.

Natural products: a continuing source of novel drug leads

BACKGROUND

Nature has been a source of medicinal products for millennia, with many useful drugs developed from plant sources. Following discovery of the penicillins, drug discovery from microbial sources occurred and diving techniques in the 1970s opened the seas. Combinatorial chemistry (late 1980s), shifted the focus of drug discovery efforts from Nature to the laboratory bench.

SCOPE OF REVIEW

This review traces natural products drug discovery, outlining important drugs from natural sources that revolutionized treatment of serious diseases. It is clear Nature will continue to be a major source of new structural leads, and effective drug development depends on multidisciplinary collaborations.

MAJOR CONCLUSIONS

The explosion of genetic information led not only to novel screens, but the genetic techniques permitted the implementation of combinatorial biosynthetic technology and genome mining. The knowledge gained has allowed unknown molecules to be identified. These novel bioactive structures can be optimized by using combinatorial chemistry generating new drug candidates for many diseases.

GENERAL SIGNIFICANCE

The advent of genetic techniques that permitted the isolation / expression of biosynthetic cassettes from microbes may well be the new frontier for natural products lead discovery. It is now apparent that biodiversity may be much greater in those organisms. The numbers of potential species involved in the microbial world are many orders of magnitude greater than those of plants and multi-celled animals. Coupling these numbers to the number of currently unexpressed biosynthetic clusters now identified (>10 per species) the potential of microbial diversity remains essentially untapped.

The impact of the United Nations Convention on Biological Diversity on natural products research

The discovery and development of novel, biologically active agents from natural sources, whether they be drugs, agrochemicals or other bioactive entities, involve a high level of interdisciplinary as well as international collaboration. Such collaboration, particularly at the international level, requires the careful negotiation of collaborative agreements protecting the rights of all parties, with special attention being paid to the rights of host (source) country governments, communities and scientific organizations. While many biodiversity-rich source countries currently might not have the necessary resources for in-country drug discovery and advanced development, they provide valuable opportunities for collaboration in this endeavor with research organizations from more high-income nations. This chapter discusses the experiences of the US National Cancer Institute and the US government-sponsored International Cooperative Biodiversity Groups program in the establishment of international agreements in the context of the Convention of Biological Diversity's objectives of promoting fair and equitable collaboration with multiple parties in many countries, and includes some specific lessons of value in developing such collaborations.

Natural products as sources of new drugs over the 30 years from 1981 to 2010

This review is an updated and expanded version of the three prior reviews that were published in this journal in 1997, 2003, and 2007. In the case of all approved therapeutic agents, the time frame has been extended to cover the 30 years from January 1, 1981, to December 31, 2010, for all diseases worldwide, and from 1950 (earliest so far identified) to December 2010 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 and have added a new designation, "natural product botanical" or "NB", to cover those botanical "defined mixtures" that have now been recognized as drug entities by the FDA and similar organizations. 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 175 small molecules, 131, or 74.8%, are other than "S" (synthetic), with 85, or 48.6%, 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 anti-infective area being dependent on natural products and their structures. Although combinatorial chemistry techniques have succeeded as methods of optimizing structures and have been used very successfully 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 30-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.

Therapeutic agents from the sea: biodiversity, chemo-evolutionary insight and advances to the end of Darwin's 200th year

Drugs from the sea? Darwin may not have considered this concept when he was thinking about mechanisms that drove diversification of life on earth. In recognition of his 200th year, and celebration of the publication in 1859 of his "On the origin of species", we review the global status of marine biodiscovery in medicinal fields, with a focus on the South Pacific. Furthermore, in the Darwinian spirit, we touch on putative evolutionary drivers and the chemical ecology of the successful leads. We argue that, for the relatively limited investment in effort to date, the success of marine leads as therapeutics promotes enhanced focus on marine biodiversity as a source of useful medicinal agents. The simple prime argument in support of this is the fact that we can exploit over four billion years of evolution in combinatorial chemistry in marine organisms, directed at relevant and effective biological activity.

Microbial antitumor drugs: natural products of microbial origin as anticancer agents

This review discusses the role of microbial secondary metabolites produced by organisms from all three domains of life (the Archaea, Prokarya and Eukarya) as sources of pure natural products or derivatives of natural products, and as leads for novel synthetic compounds that may have the potential to ameliorate cancer. Drugs for which a microbe has either been identified as the producer of the active component or has been associated with its production based on circumstantial evidence are discussed. In addition, the impact of the increase in available genomic information and manipulation is discussed, along with some relevant examples, leading to the realization that the potential of microbes as sources of new leads/agents or as biological probes is large.

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.