Keywords

1 Introduction

Modern drug discovery and development endeavors typically come from the basic research and then gradually move on to definite sequential activities, which if successful ends in a new drug for the treatment of a human disease. The entire pathway is systematized by well-defined mileposts, which include identification of the lead compound, selection of the drug target, its modification to a compound suitable for toxicity testing in experimental animals, and choosing a drug molecule for clinical evaluation. Even before the beginning of human studies, a drug molecule suitable for clinical testing is assumed to satisfy specific and challenging safety criteria. It should bind selectively to the receptor on the target and prompt for the preferred functional response. There must be satisfactory bioavailability and distribution inside the body to reach the site of action, and this should produce the desired responses in in vivo models. Most importantly, a drug molecule suitable for testing in human being must pass toxicity evaluations to show that humans contributing in the phase 1 clinical trials are showing negligible risks only (Hefti 2008).

Presently traditional medicines are used in primary health-care systems in most countries equivalent to conventional medicine. Therefore, traditional medicine should be subjected to research for their efficacy and safety for greater health care. At present there is a requirement for evidence-based drug development with fluctuation of global economic scene. When developing novel drugs using traditional medicines, it is essential to consider novel standard parameters whenever possible (Zhang 2015). Quality control of traditional medicines is also prerequisite of standard clinical trials. It is essential to follow current standard quality controlling methods, viz., Good Manufacturing Practice (GMP); Chemistry, Manufacturing and Controls (CMC); Good Clinical Practice (GCP); and Good Laboratory Practice (GLP).

There are numerous examples of emergence of new drugs from the plant sources. Morphine was isolated from opium produced from latex of the poppy plant (Papaver somniferum) about 200 years ago. A number of drugs developed from natural/plant sources have certainly revolutionized medicine, like antibiotics (e.g., erythromycin, penicillin, tetracycline), anti-parasitics (e.g., avermectin), anti-malarials (e.g., quinine, artemisinin), hypolipidemics (e.g., lovastatin and analogs), immuno-suppressants for organ transplants (e.g., rapamycins, cyclosporine), anticancer drugs (e.g., irinotecan, paclitaxel), and antidiabetic drugs (e.g., metformin) (Alamgir 2017).

There are about more than 100 plant-derived drugs and molecules/compounds that are in preclinical stage on which clinical trials are ongoing (Harvey 2008); undoubtedly, there are numerous species of plants in plant kingdom that contains the substance of medicinal value which will be discovered in the future; a lot of plants are continuously being screened for their possible pharmacological value (particularly for their hypotensive, anti-inflammatory, hypoglycemic, anti-fertility, antibiotic, anti-Parkinsonism, amebicidal, and cytotoxic properties) (Pan et al. 2013). The use of sole genuine compounds with synthetic drugs is also having lots of restrictions, and in the current years, there has been an immense resurgence of interest in the Ayurvedic and homeopathic systems of medicine, both of which rely profoundly on plant source (Kumar et al. 2017).

2 Drug Development from Natural Resources: Benefits and Drawbacks

Use of plant sources as preliminary point of the drug development program is related with few specific advantages:

  • Typically, the assortment of a plant candidate species for research can be done on the basis of long-term use of folklore medicines by humans. This methodology is based on the finding that active compounds isolated from such plants are likely to be safer than those obtained from plant species without a history of human use. Subsequently, the synthesis of lead molecules could be reducing the pressure on natural resources. Drug development from Cinchona officinalis, Rauwolfia serpentina, Digitalis purpurea, etc. in the past fall under this category (Atanasov et al. 2015).

  • The lead molecules isolated from natural source by using such methods can be of use with some limitations like low bioavailability, low toxicity, etc. Such type of limitations can be overcome through modification in the molecule like nanonization and by formulating their semisynthetic derivatives. For example, the bark of the willow tree (genus Salix) has been known from ancient times to have analgesic properties which is due to the presence of the natural product salicin and is hydrolyzed into salicylic acid (Jamshidi-Kia et al. 2018). A synthetic derivative acetylsalicylic acid (aspirin) is a widely used pain reliever. There are numerous examples of phyto-constituents which are obtained from natural sources and modified chemically, viz., morphine (Papaver somniferum), colchicine (Colchicum autumnale), penicillin G (Penicillium citrinum), paclitaxel (Taxus brevifolia), metformin (Galega officinalis), etc.

Although there is incredible growth in traditional system of health care globally, ITSM based on its different features of folklore medicines have also developed greatly. But there are several constrains in this development in a proper way which include:

  • Rules and regulations imposed for traditional medicines are just similar to chemical-based drugs.

  • Availability of raw material means dramatic depletion of wild populations of the plants; for example, the plants Panax ginseng, Artemisia annua, and Taxus brevifolia are now endangered due to the overexploitation.

  • Once isolated from their source, compounds may work differently than expected. Moreover, the approach could be more time-consuming and more costly and may be less sustainable.

Already 29 plants and their worthy products have been banned by the government of India as they are considered as endangered species (Sen and Chakraborty 2016).

3 Colligative Properties of an Isolated Phyto-constituent

Not all lead molecules generated by the drug discovery persons are tested in complete regulatory packages. This is because the regulatory testing is very time-consuming and costly affair. A series of tests are initially conducted to help select certain candidate molecules within the desired pharmacological possibilities and safety profile for further regulatory testing. Drugs that do not meet the necessary requirements in these initial assays are less likely to be taken for testing in more expensive, time-consuming regulatory tests (Koehn and Carter 2005).

Koehn and Carter have figured out the following some elite characteristics of the compounds isolated from natural sources. They are as follows.

3.1 Molecular Structure

The automatic screening technologies and wide range of chemical libraries and archives have made it reasonably easy to identify initial lead candidates for new drug targets. The chemists have developed specific rules that lead molecules must fulfill.

  • Molecular weight should be less than 500.

  • Not more than five hydrogen bond donors.

  • Not more than ten hydrogen bond acceptors.

3.2 Octanol/Water Partition Coefficient

The lipid solubility of drugs is stated as octanol/water coefficients of the uncharged molecules or log P. When the log P value is higher, drugs will be highly lipid soluble and believably accumulated in the body (Bergström and Larsson 2018).

3.3 Structure-Activity Relationship (SAR)

SAR gives information about the possible toxicity of a chemical based on chemical structure, when no experimental data is available. It can make predictions about a wide variety of toxicological properties of compounds such as neurotoxicity, carcinogenicity, skin sensitization, thyroid toxicity, teratogenicity, respiratory, and mutagenicity.

3.4 Cytotoxicity

An essential part of the drug discovery/approval process is determining the toxic effects of potential drugs. Following a toxic attack, cells may react with changes in size or morphology depending on the type of cell and compound. Some toxins can affect the cell’s functionality by changing the physiology of organs such as lysosomes and endosomes or by causing a rise in number of lysosomes seen in the case of phospholipidosis. There are several types of diagnostic kits that are available in the market that can be used to measure these types of parameters.

3.5 Parallel Artificial Membrane Permeability Assay (PAMPA)

The potential of a molecule is orally absorbed as one of the most important aspects in deciding whether a molecule is a probable lead candidate. Parallel artificial membrane permeability assay is a decent alternative to cellular models for the initial absorption, distribution, metabolism, and excretion (ADME) and primary investigations of the research compounds. This method is used to measure the effective permeability, P (e), as a function of pH from 4 to 10. This technique provides quick response, low-cost, and automation-friendly method to measure a chemical entity’s passive permeability (Kansy et al. 2004).

3.6 Derived Solubility

The water solubility of a drug is a crucial physical property that affects both its ADME profile and screenability in high-output systems.

3.7 Aqueous/Plasma Stability

The stability of lead compounds in plasma is an important parameter that can strongly affect the in vivo efficacy of a test compound. Drugs that are exposed to enzymatic processes (proteinase, esterase) in plasma may undergo intra-molecular rearrangement or bind covalently to the proteins. Thus the determination of plasma stability should be performed early in drug discovery phase. Measurement of plasma stability is performed at physiological pH level in plasma (Chung et al. 2015).

3.8 Protein Binding

In-depth understanding of plasma and tissue (brain, liver, etc.) protein binding is important for evaluating the distribution of drug molecules. The plasma or tissue homogenate is incubated with the test compound. The bound and unbound test compounds are separated using ultrafiltration or equilibrium dialysis, and the amount of test compound in both fractions is estimated using HPLC or LC/MS.

These unique characteristics of lead molecules of natural origin pose order of challenges for medicinal chemists as they start working upon development of analogs (Chung et al. 2015).

4 Benchmarks for Selecting the Plants for Research

It has been estimated that <10% of the approximately 300,000–500,000 species of plants worldwide have been studied for 1 or more bioactivities.

Success in identifying a new biologically active plant-based natural product can be influenced firstly by a clever choice of plant or secondly by how randomly the selection of plant extracts can be quickly and effectively screened. The following selection criteria are suggested for plant-related research (Dias et al. 2012). The sketch of possible approaches to the discovery of new drug leads has been mentioned in Fig. 1.1.

Fig. 1.1
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Sketch of possible approaches to the discovery of new drug leads

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4.1 Random Selection

In the random screening method, plant extracts, fractions, or isolated compounds are randomly selected on their convenience and availability. In the perception of plant-based drug discovery, this method can be highly beneficial when applied with samples originating from regions with high biodiversity and endemism. The random selection of test sample has the effectiveness in the identification of unpredicted biological activities that could not have been expected based on the existing information (Atanasov et al. 2015). Paclitaxel and camptothecin are the bio-actives which were isolated through this approach.

4.2 Selection Based on Traditional Use (Ethno-Medicinal Approach)

This is the widespread basis for selecting plants for investigation especially in societies and rural and ethnic communities where traditional medicine is whole sole part of human health care. If a traditional healer claims success in the treatment of a disease, the researcher can assume from the above selection criteria a chemical constituent with suitable pharmacological activity in the plant extract. The ethno-medicinal approach allows for better chance of finding an active compound as well as documenting and preserving local knowledge. This becomes of greater importance with the increased mobility among rural communities and the subsequent loss of local information of the use of native plant species (Lewis 2003). Regarding the ethno-medicinal approach for the selection of the plant, two important issues require attention. Firstly, the rights of the country of origin related to any drugs discovered need to be protected, as mentioned in the United Nations Convention on Biological Diversity (UNCBD) (Baker et al. 1995). Secondly, the prominence of any ethno-pharmacological field studies should be carried out before the plant selection which indicates an impact on the success of the research.

The ethno-medicinal approach has successfully been used by the researchers at Shaman Pharmaceuticals to verify the use of Cryptolepis sanguinolenta (Lindl.) as a treatment for type II diabetes as well as a source for the isolation of the active constituent, the alkaloid cryptolepine (Bierer et al. 1998; Luo et al. 1998). The dichloromethane and hot water extracts of the roots of C. sanguinolenta showed the ability to reduce the blood glucose in animal model. In vivo bioassay-guided fractionation using the same model results in the isolation of cryptolepine as an active constituent (Bierer et al. 1998; Luo et al. 1998).

4.3 Ecological Approach

In the ecological approach, the selection of plant candidate is dependent upon the observation of interactions between organisms and their surroundings from which bioactive natural compounds can be produced. The basic fundamental hypothesis of this approach is that secondary metabolites which possess ecological functions also exerts pharmacological activity. Different investigators have considered different phases of the ecological argument, including the relationship between biodiversity and chemodiversity (Ramesha et al. 2011), the apparency theory (de Almeida et al. 2011), the life-strategy theory (Coley et al. 2003), chemical defenses and herbivory (Albuquerque et al. 2012), animal behavior (Obbo et al. 2013), and phylogenetic trends (Zhu et al. 2012).

Another approach which is simultaneously linked with ecological approach is the zoopharmacognosy approach. In this approach, the activity of plant is sometimes evaluated through observation of animal behavior. Khaya species are common to Madagascar and Africa, and people use their bitter bark and seeds for treating fevers, microbial infections, and worm infestations. Baboons and chimpanzees in Western Uganda have been observed to eat the bark and seeds that are bitter in taste and have no nutritional value (Obbo et al. 2013). The petroleum extract of Khaya anthotheca evidenced for good activity against Plasmodium falciparum K1 (IC50 = 0.955 μg/mL) and Trypanosoma brucei rhodesiense STIB 900 (IC50 = 5.72 μg/mL). It appears that chimpanzees and baboons were using seeds and bark for self-medicating, in addition to evidence of the effectiveness of these plants used by traditional healers (Obbo et al. 2013).

4.4 Computational Approach: Virtual Screening and Reverse Pharmacognosy

Computational methods are supplementary knowledge-based approach that assists to select plant material with a high probability for pharmacological activity. These methods can also aid with the validation of biological activity of natural compounds and selection of test samples dependent on in silico bioactivity predictions for constituents of plant species. Virtual screening (VS) uses the availability of large compound libraries generated by combinatorial and high-throughput chemistry to select low number of potential candidates for experimental testing (López-Vallejo et al. 2011). Virtual screening can follow two general strategies: ligand-based virtual screening and structure-based virtual screening. In this method, molecular docking is broadly used to explain the mechanism of action and defend the SAR of natural products. The purpose of docking is to accurately predict the position of a ligand within the protein binding site and the ability of binding with a docking score (López-Vallejo et al. 2011).

Reverse pharmacognosy intends to find out new biological targets for natural products by either virtual screening or real screening and then to connect these findings to original or different plant sources (Do et al. 2005).

5 Biological Activity-Guided Fractionation for Compound Isolation

Isolation strategies for natural products are constantly evolving. Originally, all compounds that could be purified were isolated from a plant that was used traditionally to treat diseases without concern if the specific compound was responsible for activity. This ensures that the isolation of several inactive compounds and offered a way to bioassay-directed isolation, leading the disease to the responsible compound (Altemimi et al. 2017). Bioassay-guided isolations are currently the most common technique to purify the responsible compound for a certain bioactivity. Bioassay-guided fractionation starts with a crude extract from the dried plant material using either aqueous ethanol or aqueous methanol. The crude extract is then taken through a liquid-liquid extraction using solvents of increasing polarity, from hexanes to water, to produce five fractions (1–5) as shown in Fig. 1.2. Bioassay-guided fractionation of plant extracts can be achieved through chromatographic separation techniques which can lead to isolation of biological active molecules. If one of the fractions is bioactive, that fraction is further purified with either gravity column chromatography or flash column chromatography, depending on the complexity of the crude sample. The column fractions are then again screened with the bioassay to confirm the activity. This process is continued until a final pure compound is isolated responsible for the bioactivity. Final purification often requires high-performance liquid chromatography (HPLC) in order to obtain clean nuclear magnetic resonance (NMR) spectra and high-resolution mass spectrometric (HRMS) data for compound characterization. Even a small amount of an impurity can lead to mis-assignment of peaks in the spectrum. Advances in isolation and structural elucidation technologies provide a more comprehensive image of the entire plant extract.

Fig. 1.2
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Flow diagram of preliminary liquid-liquid extraction based on polarity

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6 Ayurvedic Perception in Selection of Plant Candidate for its Therapeutic (e.g., Antidiabetic) Activity

On the basis of the above-said approaches of screening of medicinal plants, it is possible to apply the traditional knowledge on a variety of herbs to identify the improved lead compounds or phyto-chemicals for research and development to find out better treatment of diabetes. In Ayurvedic literature, prameha is characterized with undue urination (in both quantity and frequency) and turbidity. The nature of the turbidity may differ depending upon the body reaction with the tridoshas. Understanding of prameha is not merely related only to the patho-physiology and clinical picture of diabetes mellitus as depicted in Fig. 1.3. From the pathological and etiological state of complications, prameha is almost common in obesity and metabolic syndrome (Sharma and Chandola 2011).

Fig. 1.3
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Correlation of different types/stages of prameha with diabetes mellitus

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The herbs from the authentic classical text Charak Samhita, Sushruta Samhita, and Astanga Hrdaya (Jadavaji 1992; Sharma 2001; Srikantha Murty 2000) are traditionally employed in the treatment of diabetes mentioned in Table 1.1 and scientifically explored by various investigators to evaluate the same in terms of Rasa, Veerya, Vipaka, Guna, and Karma. On the basis of the traditional elements for the herbs having pramehahara/tridoshhara effects, it can be assumed that these herbs have particular pharmacological traits in general. Going by the dominance investigation of these attributes, the following scenario appears.

  • Rasa: Kashaya, Tikta, and Katu

  • Guna: Laghu and Ruksha

  • Vipaka: Katu

  • Veerya: Ushna and Sheet

  • Dosha Karma: Kapha-Pitta-Vata Shamana (Tridoshahara)

Table 1.1 Pramehahara and madhumehahara (antidiabetic) drugs in Ayurveda
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7 Effects of Medicinal Plant Extract on Type II Diabetes Mellitus

Botanical agents show promise for the development of new compounds to treat type II diabetes mellitus. Till now, over 400 traditional and folklore plant treatments for diabetes have been reported, although only few of these have established a scientific and medical evaluation to assess their effectiveness. The anti-hyperglycemic effect of a number of plant extracts has been confirmed in individuals and animal models of type II diabetes (Modak et al. 2007). The WHO Expert Committee on Diabetes Mellitus has also recommended that traditional medicinal herbs should be further investigated. Several phyto-constituents including glycosides, flavonoids, alkaloids, saponins, glycolipids, dietary fibers, peptidoglycans, polysaccharides, carbohydrates, amino acids, and others obtained from various plant sources have been reported as antidiabetic agents with different mechanisms of action which are mentioned in Table 1.2, and scientifically explored plants as antidiabetics are mentioned in Table 1.3 (Mishra et al. 2010; Alam et al. 2019).

Table 1.2 Mechanism of action of phyto-chemicals of different chemical categories involved in diabetic pathway
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Table 1.3 Scientifically explored plants investigated for their antidiabetic activity (Mishra et al. 2010; Alam et al. 2019)
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8 Conclusion

There is a reasonable need to renew scientific interest toward natural products for inclusion in the drug discovery program. One of the vital concerns related to plant products is the prediction of hit rate during several stages of drug development. Such a prediction is expected to be lower in case of random selection of plant species considering the overall complexity of botanical sources for new chemical entities. The best drug of the future will come from a combination of a natural product research and synthetic approaches.

Clinical experience with herbal medicine as classified in traditional medicine may simplify issues associated with deprived prognosis. New functional leads taken from traditional knowledge and experiential databases can help to reduce the time, money, and toxicity, which are the three specific barriers to drug development. Furthermore, the trend today, especially in an industrial setting, is to seek biologically active compounds from plants that will serve as lead compounds for synthetic or semisynthetic development to assure patent protection.