Preparación de los datos

A “paradigm shift” in drug discovery occurred in the early 19th century, marked by the isolation of pure bioactive entities from medicinal plants, beginning with the isoquinoline alkaloid, morphine.3–5The purification of plant drug molecules such as atropine, cocaine, codeine, digitoxin, and quinine, later in the same century, proved to be significant not only for the extensive medicinal uses of these isolates, but also for the crucial roles these molecules played in better understanding human disease and in the development of organic and medicinal chemistry.26

In the 20th century, additional important drugs were isolated from plants, including artemisinin, digoxin, paclitaxel, vinblastine, and vin- cristine. Also in this same century, as a result of advances made in pharma- cology and a better understanding of human diseases at the molecular level, physicians and pharmacists gradually shifted from the use of plant extrac- tives in prescriptions to pure naturally occurring and synthetically modified natural products, or totally synthetic compounds. Today, the effort to find new bioactive principles from medicinal plants may bring together sci- entists working in a diverse range of disciplines including biochemistry, botany, ethnobotany, medicinal chemistry, microbiology, molecular biol- ogy, organic chemistry, pharmaceutics, pharmacognosy, pharmacology, plant ecology, and taxonomy.9,12–16 The advances in molecular biology are duly reflected in the complexity of bioassays employed in the medicinal plant drug discovery field, and also provide the “mode of action” informa- tion at the molecular level in a rapid and accurate fashion.27Preliminary in vitro experiments may then be followed up with a variety of in vivo bioassays.28

Terrestrial plants and other organisms are known to be the sources of a plethora of small organic molecules, representing considerable struc- tural diversity, which may not be matched by the creativity of synthetic

chemists.15,29 When all currently known (both synthetic and natural)

chemical entities are taken into consideration, the area of chemical space occupied by bioactive molecules is a relatively limited one.15,29,30 When considered statistically, natural products have been found to account for much greater chemical diversity than compounds generated by both syn- thetic and combinatorial chemistry methods together. They also possess some unique structural differences, which afford them with greater drug- like qualities.31–35 Accordingly, libraries of pure natural product, when combined with the advantages of combinatorial chemistry, offer an even more effective and reliable avenue for exploring the “more bioactive part” of chemical space when compared with a purely synthetic approach.29,33,35 From a chemical informatics perspective, natural products are signif- icantly different from synthetic- and combinatorial chemistry-produced compounds in that they have more single bonds, and protonated amino and free hydroxyl groups, while having fewer aromatic rings.36Also, natural compounds possess a greater diversity of ring systems and tend to be more rigid than their synthetic counterparts.32,34 Natural products also exhibit more chiral centers and fewer rotatable bonds than substances produced by synthesis.32The “rule of five,” developed originally to guide the efforts of combinatorial chemistry, has spearheaded a new approach to drug discov- ery by defining the physicochemical characteristics a “drug-like” molecule should have.37In this respect, comparison studies have shown that natural products resemble proprietary drugs and show a high degree of “drug- likeness,” compared to their synthetic counterparts.34The intricate biosyn- thetic processes that lead to bioactive secondary metabolites from organisms with bioactivity continue to be a major research interest and there has been much thought as to how and why these compounds are biosynthesized. The most widely accepted and satisfactory explanations refer to an evolutionary perspective, as will be described in the next two paragraphs.30,38–40

For survival purposes, all living organisms rely on an ability to trans- form and interconvert a diverse set of organic and inorganic compounds in order to utilize them as a source of energy and as their structural building

blocks.3 The presence of these crucial building molecules, the “primary metabolites” (amino acids, fatty acids, nucleosides, and sugars) can be considered synonymous with “life” since they are ubiquitous among all organisms. While the role of primary metabolism for the survival of species can be appreciated readily, the rationale for the presence of some other compounds with no apparent role in the internal economy of the pro- ducing organism is not so clearly evident. These small-molecule organic compounds of considerable structural diversity and typically a limited tax- onomical distribution, the “secondary metabolites,” are believed to provide the producing organisms with a survival advantage. Support for this may be inferred from the observation that organisms lacking immune systems (e.g. higher and lower plants, algae, fungi and microorganisms) generally

show a high abundance of these compounds.38 The term “natural prod-

uct” is usually used interchangeably with “secondary metabolite” for the purposes of drug discovery from organisms.3,38It is found commonly that plants, like other organisms, tend to produce a series of analogues of a given structural type, rather than only one main secondary metabolite of a given class. It has been argued that over millions of years, the compounds pro- viding a survival advantage are preserved (and even fine-tuned structurally through biosynthetic modification), while the not-so-active analogues are eliminated through evolutionary pressure.30

Plants interact chemically with other organisms such as insects, microorganisms, other plants, and even mammals as a result of their sec- ondary metabolites, leading to a multitude of biological responses.41,42 Clearly, since secondary metabolites are produced at the expense of the producing organism, it would be expected that this “chemical artillery” must offer the plant some advantage against its potential adversaries.43 Secondary metabolites mostly exert their effects by acting through enzyme or protein interactions.15,24,44While some of these compounds act as sub- strates at the receptor level and mimic the endogenous substances in the target organism, others simply disrupt protein-protein interactions neces- sary for normal cell function (for examples, see23,45–50). Therefore, it might be concluded that it is this ability of secondary metabolites to interact with the physiology of other species that renders them as an impressive source of “evolutionarily fine-tuned” drug-like molecules.30

Drug discovery efforts from plants has been evolving continuously in response to a number of recent technological advances, such as the development of chromatographic methods that allow reproducible and fast purification steps for diverse compound classes; the availability of sensitive spectroscopic methods permitting the structural characterization of sam- ples in microgram quantities; efficient chemical methods that permit the synthesis, derivatization, and optimization of bioactive lead compounds; and the wide accessibility of diverse sets of bioassays that provide the natural products chemist with critical information to target bioactive compounds from the early stages of separation studies.11,17,28,33,35,51–54

The standard practice in plant natural products isolation chemistry for drug discovery can be broken down into five steps, namely, organism collection, extraction, compound isolation, structure determination, and bioassay.9,17,27,55–57 Plant samples are authenticated taxonomically and extracted with a solvent of choice and the resultant extracts are screened against pharmacologically relevant targets.27,28,57 Once a positive result is achieved, isolation studies follow, guided by the relevant bioassay stud- ies using the so-called “activity-guided fractionation” technique. When an active principle is isolated, thorough spectroscopic and spectrometric or X- ray crystallographic methods are employed as needed for the unambiguous assignment of its structure and configuration.9,16,57In the pharmaceutical industry and in other laboratories with considerable resources, many of the above-mentioned techniques are employed along with combinatorial and synthetic chemistry efforts, as well as computational modeling and chem- ical informatics studies coupled with specific high-throughput screening methods.18,29,35,42,44,51–54,58–62

2.3 EXAMPLES OF NATURALLY OCCURRING DRUGS