Bioactive Phytochemicals: Drug Discovery to Product Development

RegionIraq

Drug Discovery from Plant Sources: Scope, Approach and Challenges

Javed Ahmad¹, Javed Ahamad², *

¹ Department of Pharmaceutics, College of Pharmacy, Najran University, Kingdom of Saudi Arabia

² Department of Pharmacognosy, Faculty of Pharmacy, Tishk International University, Erbil, Kurdistan Region, Iraq

Abstract

Medicinal plants are recognized to fulfill human necessities like food, clothes, shelter and health. The search for eternal health and longevity to seek remedy to relieve discomfort prompted mankind to develop many ways and means of health care systems. Traditional medicines were practiced in ancient civilizations for the cure of ailments. In recent years, natural products play a very important role in drug discovery for life-threatening ailments like cancer, malaria, diabetes and cardiovascular problems. Recently, drug discovery from plants for the treatment of cancer gets more focused and leads to the discovery of novel anticancer drugs such as paclitaxel, docetaxel, topotecan, irinotecan, vincristine and vinblastine. Drug discovery from plants is a long and tedious process, and it requires selecting suitable plants, pre-clinical screening, clinical evaluation and drug approval for marketing. Herbal medicines obtained from plants are generally considered safe compared to synthetic drugs, and secondary metabolites obtained from plants have more chemical diversity and considered superior to synthetic combinatorial chemicals. In this book chapter, we comprehensively discussed the advantages and role of higher plants in drug discovery, steps and approaches of drug discovery from higher plants.

Keywords: Approaches of Drug Discovery, Cancer, Drug Discovery, Diabetes, Malaria, Natural Products, Plants.

* Corresponding author Dr. Javed Ahamad: Department of Pharmacognosy, Faculty of Pharmacy, Tishk International University, Erbil, Kurdistan Region, Iraq; E-mails: [email protected], [email protected]

1. INTRODUCTION

Since ancient times, plants are recognized to fulfill human necessities like food, clothes, shelter and remedies for ailments. Since the early days, mankind explored natural resources and used them as a remedy for the cure of diseases. The traditional systems of medicine such as Ayurveda, Unani, Chinese medicine and

Homeopathy are examples of ways of treatment of human diseases. In ancient times, the human beings used natural resources for various purposes, such as food and medicinal agents [1]. Traditional medicines were practiced in ancient civilizations; the traditional practitioner gathered knowledge from generation to generation, and sometimes they document this knowledge. Ancient Indian scholars such as Charaka and Sushrutha, examined and classified medicinal herbs based on their properties and they called it as Gunas. Charaka arranged 50 groups containing ten herbs in each group, and according to him, these herbs are sufficient to cure all types of human diseases. Sushrutha also prepared seven groups from 760 herbs based upon some common properties [2]. Chinese scholars also worked meticulously and developed remedies from natural resources. One of the oldest known literature by Pent Sao documented by ShenNung around 3000 B.C., and it contained 365 drugs, one for each day of the year [3]. Hippocrates "Father of medicine" (460-360 B.C.), Aristotle (384-322 B.C.), Dioscorides (40-80 A.D.), Galen (131-200 A.D.) and the early Arabian physicians like Rhazes (865-925 AD) and Avicenna (980-1037 AD) were recognized as pioneers in drug discovery from natural products [4].

Higher plants have advantages in drug discovery compared to synthetic drugs because they have a long history of use by humans as food and spices. Natural products obtained from plants are generally considered safe compared to synthetic drugs, and the secondary metabolites obtained from plants have more chemical diversity and considered superior to synthetic combinatorial chemicals [5]. Several natural products recently showed cytotoxic and antitumor activities in clinical trials and were successfully developed as anticancer drugs such as vincristine, vinblastine, paclitaxel and docetaxeletc. Drug discovery from plants today is an expensive and lengthy process. The drug discovery from plants requires a team effort consisting of experts from different disciplines such as pharmacognosists, pharmacologists, medicinal chemist and pharmaceutics. Phytochemicals from medicinal plants serve as a lead compound in drug discovery and are further used for synthetic or semi-synthetic drug development to ensure patent protection [6]. In ancient civilization, plants were used as medicinal agents for the treatment of various human ailments. The knowledge of medicinal values of these plants was inherited from generation to generation. Most of these traditional practitioners formulate and dispense their own medicines; hence this requires proper documentation and research to use natural products as therapeutically effective and safe medicinal agents [7]. In recent years, traditional medicines have emerged as an alternative treatment of chronic diseases and lifestyle disorders such as cancer, malaria, tuberculosis, diabetes, obesity and cardiovascular complications [8]. Drug discovery from the plant is a long and tedious process, and it requires selecting suitable plants, biological or pre-clinical screening, clinical evaluation and drug approval for marketing. In this book chapter, we comprehensively discussed the advantages and role of higher plants in drug discovery, steps and approaches of drug discovery from higher plants.

2. ADVANTAGES OF DRUG DISCOVERY FROM NATURAL RESOURCES

Newman and Cragg, reported that totally 1562 new drugs approved during 1981 to 2014, of which, those of plant origin comprised as follows: natural products (4%; N), derivatives of natural products (21%, ND), synthetic compounds with natural product-derived pharmacophores (10%; S*/NM), and synthetic drug with NP pharmacophore (11%; S/NM). It is clear from the above report that natural products contribute a lot in drug discovery [9]. Plants are considered as equal or superior in drug discovery because of their chemical diversity and human friendly in nature based on their long history of use as food by a human. Phytochemicals have been elaborated within living systems, they are often perceived as showing more drug-likeness and biological friendliness or tolerance than synthetic molecules, making bioactive phytochemicals good candidates for further drug discovery and development [6]. The data on new drugs approved during 1981-2014 is summarized in Table 1 and Fig. (1).

Table 1 New drugs approved during 1981-2014 from all sources (number = 1562).

Fig. (1))

New drugs approved during 1981-2014 from all sources (number = 1562) [9].

3. ROLE OF MEDICINAL PLANTS IN DRUG DISCOVERY

Higher medicinal plants considered a good candidate in drug discovery because of long use as food. A rough estimate of about 65% world population relies on traditional medicines for their primary health care needs. Newman et al., reported that drug discovered from the natural product or natural product derivatives comprises about 26% of all new chemical entities launched into the market between 1981 to 2014 [9]. WHO-Traditional Medicine Programme is based on the potential of ethnomedicine as a lead for drug discovery [10]. Farnsworth et al., conducted a survey worldwide and identified 122 phytocompounds from traditionally used plants, and he found that about 80% of these phytochemicals were used for the same or related ethnomedical purposes. The authors also found that these phytochemicals were derived only from 94 species of medicinal plants [1, 5]. Up to 122 phytochemicals are derived only from 94 species of plants, whereas there is an abundant number of plant species not explored phytochemically and pharmacologically [1]. Several workers estimated the number of plants on the planet, which is about 250,000, and out of these, only about 6% have been screened for biologic activity and 15% have been evaluated phytochemically, so there is an abundance of drugs remaining to be discovered in these plants [11]. Some important natural drugs/phytochemicals, which are derived from higher plants, are summarized in Table 2.

Table 2 Bioactive phytochemicals derived from higher plants [1, 5].

4. STEPS OF DRUG DISCOVERY FROM HIGHER PLANTS

Drug discovery and development from plants require multidisciplinary research that includes experts from the natural product, pharmacology and pharmaceutics. For drug discovery from higher medicinal plants, the following three important steps are required: (a) selection of appropriate plant candidates for biological evaluation; (b) development of specific methods for biological evaluations (pre-clinical and clinical models); and bioassay-guided fractionation and isolation of new bioactive phytocompounds and their structure elucidation.

The drug discovery and development takes a long time and is expensive. According to Shrager, drug discovery involves several steps. The first step is to find a ‘lead’ molecule. Lead from plants is a bioactive molecule, which exhibits desired biological or pharmacological activity and safety in cytotoxicity tests in the suitable in-vitro model [12]. The discovery of lead molecules from higher plants includes the screening of large numbers of plants pharmacologically and then fractionation and isolation of particular phytochemicals. This isolated new molecule with desired pharmacological activity serves as a lead molecule in drug discovery. In the next step, animal safety analysis is done to determine whether the lead compound is toxic (toxicity test), and then the in-vivo effectiveness of the lead molecule against disease in an animal model was evaluated (Fig. 2). The pre-clinical studies usually done in animal models and clinical studies in human subjects. If these pre-clinical and clinical tests give positive results, then the next step is to design proper dosage forms. In the last step of drug development, the new drug is applied for a license to get marketing rights [6].

5. DRUG DISCOVERY FROM HIGHER PLANTS: APPROACHES

Higher plants have immense medicinal values, and they should be pharmacologically evaluated for the treatment of complicated diseases like cancer, HIV/AIDS, CVD, diabetes, etc. Successful strategies for selecting plant candidate for isolation, characterization and drug discovery involves the selection of plants based upon ethnomedicinal use. There are four main approaches usually used to select proper medicinal plants in drug discovery and development [1, 6, 13].

Fig. (2))

Drug discovery from medicinal plants.

5.1. Ethno-Pharmacological Approach

Ethnomedicine may be defined as the use of medicinal plants by human. Since ancient times, humans preserved natural resources, which they used as a remedy for the cure of diseases. This traditional knowledge now became a very important criterion for the selection of plants in drug discovery. In this approach of drug discovery, a diversified means are applied, such as observation, description and experimental evaluation of indigenous plants. It also includes the role of ethnobotany, phytochemistry, and pharmacology that contribute to drug discovery [1, 5]. Farnsworth et al., identified 122 phytocompounds used worldwide are derived from traditionally used plants. He also suggested the following steps of drug discovery from traditional plants based upon their ethnopharmacological use (Fig. 3) [5].

Fig. (3))

Flow chart of drug discovery based upon ethno-pharmacology.

5.2. Follow-up of Pharmacological Reports

Pharmacological or biological activities are reported in the literature from the 1930s to 1970s and contain pharmacological data, and unfortunately, they were not screened or evaluated phytochemically. A rough estimate of a number of plants on the planet is about 250,000, and out of these, only about 6% have been screened for pharmacologic activity and 15% have been evaluated phytochemically, so there is an abundance of drugs yet to be discovered in these plants [11]. These literature reports are a very good basis for the selection of a plant candidate in drug discovery and the development of a new drug.

5.3. Random Selection Followed by Phytochemical Screening

This method of drug discovery, also called as phytochemical screening approach, it has been used in the past and is currently pursued mainly in the developing countries for initial stages of drug discovery for selection of the particular class of plant species based upon their secondary metabolites [1]. These phytochemical tests are simple to perform, but sometimes the accuracy of these phytochemicals tests is not reliable, and it makes it difficult to assess the correct idea about the pharmacological activity of medicinal plants [14].

5.4. Random Selection Followed by Pharmacological Screening

In this method of drug discovery, medicinal plants were selected randomly and evaluated against several diseases. This method reduces the bias in collecting plant samples based on ethnomedical data. In this approach, plant samples are submitted to routine extraction, isolation, and systemic pharmacological assay without a preconceived selection on the basis of ethnobotanical knowledge [5]. The National Cancer Institute (NCI) in the United States and Central Drug Research Institute (CDRI) in India are examples of such sponsored programs for the discovery of drugs from natural products. NCI screened about 35,000 species of medicinal plants from 1960 to 1981; this extensive research resulted in the discovery of two potent anticancer drugs: taxol and camptothecin [15]. In India, at CDRI, about 2,000 plants were screened pharmacologically for various biological activities, till date, no active biological molecule was identified from this research [16]. The disadvantage of this approach is the very low probability of finding a lead compound per plant species investigated.

6. CHALLENGES IN DRUG DISCOVERY FROM HIGHER PLANTS

The drug discovery and development from natural resources poses significant challenges in several areas. The first most important concern is the supply of the drug in sufficient quantities to allow pre-clinical and clinical evaluations and, ultimately, if the given drug is a success in clinical outcomes, then commercial production. The second major challenge is that of formulation, as natural products from plant origin are mostly insoluble in an aqueous phase, and such solubility is an important requirement for the administration of the drug to humans [17].

There are several other disadvantages of using plants in drug discovery, such as plants as biologic systems have inherent potential variability in their chemistry and resulting in pharmacological activity [18]. Since 1992, several countries prohibited the collection and export of indigenous plant materials. It was also reported that in areas where regulations permit plant collection and export, at least 2 years are required to negotiate and obtain permission to collect plant materials. Several other restrictions for drug discovery from natural resources include: (a) intellectual property rights, (b) preservation of genetic materials, and (c) compensation for drug discoveries arising from natural resources. The authors also reported that drug discovery from natural resources is time-consuming and costly [19]. The drug discovery and development of new drugs cost approximately 2.6 billion USD [20].

CONCLUDING REMARKS

Higher plants are considered equal or superior in drug discovery because of their chemical diversity and human-friendly nature based on their long history of use as food by a human. Phytochemicals have been elaborated within living systems; they are often perceived as showing more drug-likeness and biological friendliness than synthetic molecules, making bioactive phytochemicals good candidates for further drug discovery and development. Medicinal plants are considered as a novel source of phytochemicals, which are equal or superior in drug discovery and development program, e.g., artemisinin, vinblastine, vincristine, camptothecin, quinine, morphine, taxol, and digoxin. Natural products played a very important role in drug discovery and development, as a total of 1562 new drugs approved from 1981 to 2014, out of which natural products and natural product derivatives were about 51%. Due to recent advances in the field of extraction, isolation, characterization and pharmacological screening methods, drug discovery from natural products become very fast. In recent decades, more and more drugs were discovered from natural products.

ABBREVIATIONS

B = Biological drugs

CDRI = Central Drug Research Institute

N = Natural drugs

NB = Botanical drugs with defined mixture

ND = Natural product derivatives

NCI = National Cancer Institute

S = Synthetic drugs

S/NM = Synthetic drug with NP pharmacophore

S* = Synthetic drugs which mimic of natural product

S*/NM = Synthetic compounds with natural product-derived pharmacophores

V = Vaccine

WHO = World health organization

CONSENT FOR PUBLICATION

Not applicable.

CONFLICT OF INTEREST

The authors confirm that this chapter content has no conflict of interest.

ACKNOWLEDGEMENTS

Declared none.

REFERENCES