Drug discovery from medicinal plants has now become quite complex, as it reaches almost all areas of investigation and analytical methodology. It all starts with a botanist, ethnobotanist, ethnobotanist, or plant ecologist collecting and identifying plants of interest. This can be either species with confirmed biological activity but without isolation of the active compound (e.g., traditional herbal remedies) or taxa collected at random in a metric ton for a large screening program. Intellectual property rights must be respected in any country where plants of interest are collected (Baker et al., 1995). Phytochemists (natural product chemists) prepare extracts from plant material for biological screening using medicinally relevant tests and then pursue isolation and characterization of active compounds using bioassay-guided fractionation. Molecular biology has become a very important department related to fossil fuel medical drug discovery through defining and establishing appropriate screening assays on physiologically relevant molecular targets. Pharmacognosy is the discipline that incorporates all of these fields into a single, distinct interdisciplinary science.
There are several procurement strategies for drug discovery compounds such as: isolation from plants and other natural sources; synthetic chemistry; combinatorial chemistry; molecular modeling (Le and Baxendale 2002; Geisen et al. 2003; Lombardino and Lowe 2004). Although molecular modeling, combinatorial chemistry and many other techniques in synthetic chemistry are attracting the attention of pharmaceutical companies and funding organizations, natural products, especially medicinal plants, remain extremely important sources of new drugs, drug leads and chemical entities. (Newman et al., 2000; Newman et al., 2003; Butler, 2004). In both 2001 and 2002, about one-fourth of the best-selling drugs worldwide were natural products or derived from natural products (Butler, 2004). An example of this is arteether, a very strong anti-malarial drug. It is based on artemisinin, a substance derived from the plant Artemisia annua (Asteraceae), which is used in traditional Chinese medicine (TCM) ( Van Agtmael et al., 1999 ; Graul, 2001 ).
Despite the obvious success achieved in drug discovery from medicinal plants, there are still many hurdles to overcome for future efforts. Pharmacognosists, phytochemists, independent natural product scientists, etc. must step up their game with drug development so that the field can produce drugs that are equal to or better than other discovery efforts (Butler, 2004). It is estimated that drug development will take on average more than 10 years (Reichert, 2003) and cost more than US$800 million (Dixon and Gagnon, 2004).
A large portion of this time and money is spent on this, while innumerable leads are available to them and discarded during the drug discovery process. It has been estimated that only one out of 5000 lead compounds will successfully proceed through clinical trials and be approved for use. Lead identification is the first step in a very long drug development process. This includes lead optimization (involving medicinal and combinatorial chemistry), development, which includes toxicology, pharmacology, pharmacokinetics, ADME (absorption, distribution, metabolism, and excretion), and drug delivery, and multiple clinical trials, all of which take a tremendous amount of time.
Compared to drug discovery by other means, drug discovery with medicinal plants has been longer and more complex. This appears to be the reason why most pharmaceutical companies have discontinued or curtailed their natural product research altogether (Butler, 2004; Cohen and Carter, 2005).
Indeed, this has sparked interest in the “rediscovery of natural products”. As one authority has eloquently stated, “Today we would not have the biggest selling drug class, the statins; the entire field of angiotensin antagonists and angiotensin-converting enzyme inhibitors; the entire field of immunosuppressives, nor most anticancer and antibacterial drugs. Imagine if all these drugs were not available to physicians or patients today.” So it is clear that nature has and will continue to play its roles in drug discovery (Cragg and Newman, 2005).
