Therapeutic Potential of Medicinal Plants: Current Situation and Outlook

Abstract

Medicinal plants constitute the major therapeutic sources for the treatment and management of diseases among the large population of the African continent. The application of these plant resources for the management of ailments such as diabetes, cancer, neurodegenerative disorder, malaria, hypertension among others is based on the historical evidence of effectiveness, acceptability, affordability, accessibility and cultural compatibility. Various parts of medicinal plants such as flowers, leaves, stems, bark, roots, seeds, fruits possess therapeutic potentials due to the presence of bioactive substances in them. The therapeutic potentials of these medicinal plants are due to the complex interactions of the bioactive substances with their molecular target structures, such as the transport systems, enzymes and receptors. Currently, many well-known active substances derived from plants are included in medicines through scientific processing procedures that ensure standardization resulting in an improvement over the traditional crude practices among the various African cultures.

1. Introduction

The potency of medicinal plants for healing can be traced back to prehistorical times. Man right in the past relied on nature for food and medicines [1]. Nature served as the repository of diverse biodiversity and a unique laboratory for natural active ingredients of medicinal plants synthesis which man looks to for rescue in times of ailment.

Medicinal plants as major components of the biodiversity have been used in African traditional medicines to provide herbal remedies to different kinds of ailments [2] among the various African cultures.

The application of medicinal plants as the active ingredients of African herbal medicines was rooted in African history, culture belief and religion [2, 3] accepted among the various subcultures before the advent of colonization [4] which introduced Western medicines and Western religion that tended to erode the African healing culture [5, 6].

The African healing culture involves the application of the knowledge of medicinal plant healing potentials gained through trial and error experiments, experience and many years of careful observation [1] passed across from one generation to another. This knowledge exists among cultures, tribes and families in the form of local folklore [2]. In preparing the herbal medicines, it was noted that depending upon the individual recipe, all parts of the plant can be utilized, such as the whole plants, seeds, bark, leaves, roots, flowers, stems and fruits [7].

2. Traditional methods of extraction/preparation of medicines from medicinal plants

Historically, the extraction of the therapeutic ingredients of medicinal plant involves variety of methods such as teas, powders, tinctures [2], decoctions, infusions, poultices and digestion. It also included applications like raw plant consumption (without any preparation), juice, salves, vinegars, baths, syrup and inhalation [7].

Teas are prepared by steeping the medicinal plant materials (leaves, roots, flowers or bark). This involved pouring hot water over the plant material in a teapot or infuser and letting it steep for the time recommended usually; 5–7 minutes for flowers and leaves, 10–15 minutes for bark, roots and 3–5 minutes for volatile herbs.

Powders involved drying the medicinal plant parts either the leaves, roots, flower, bark or stem and ground or pulverized then using fine mesh to sieved and removed the chaff [8]. It can be taken as a powder directly, in water, pap, alcohol, etc.

A tincture is prepared by steeping in alcohol from 4 to 6 weeks the medicinal plant materials for use in treatment to extract the medicinal properties.

In decortion, the plant material instead of steeping is boiled in water. A known volume of water is used as a solvent for continuous hot extraction. It involved pouring water onto dried, pulverized and powdered plant materials placed in a clean container and then stirred. The extraction process is hastened by applying heat throughout. Usually, a short duration of about 15 minutes is observed for the process. This process is used for heat stable and water soluble plant material extraction. Usually, the solvent to crude drug ratio is 16:1 or 4:1 [9].

Infusion extraction involves submersing the medicinal plant material in boiling or hot water for some hours to leaches out the therapeutic components from the plant.

Poultices preparations involve grinding into a moist clump, the portion of the medicinal plant materials (roots, leaves or bark) for application on the area affected. Poultices are usually for external applications in treating skin diseases and body swelling while tinctures and infusions are often for internal use except in a few situations where they may be externally applied [7].

Digestion is used for the extraction of readily soluble plant materials. It involves the application of moderate heat during the process of extraction. The mixture of solvent for extraction and powdered plant materials poured into a clean container is placed in an oven at a temperature about 50°C or over water bath with continuous heat application throughout the extraction process to decrease the extraction solvent viscosity and enhanced the bioactive compound removal [9].

2.1 Brief historical evidence of utilization of medicinal plant in Africa

Historical evidence revealed that medicinal plant have been used either as an extract or as a whole plant for the provision of valuable medicines for the treatment of ailments in Africa. Such medicinal plants include the following:

Gum Arabic (Acacia senegal L.), which is believed to originate from dried regions and semi-desert of sub-Saharan Africa. Other parts of the world, including North Africa, West Africa and Northern Nigeria, use it as a medicinal plant. For at least four thousand (4000) years ago, and as far back as (3400 B.C.) from the first Egyptian Dynasty, gum Arabic has been used in medicines for both man and animal. Infections such as gonorrhea, upper respiratory tract infections, bleeding, diarrhea, bronchitis, leprosy and typhoid fever have been treated using various parts of the plant [3].

Another important medicinal plant with a documented history of use in Africa which has contributed to poverty alleviation in South Africa is the Aloe ferox Mill. known as Bitter Aloe or Cape Aloe which originated from Lesotho and South Africa. The plant is used in Africa and Europe as laxative medicine due to the fact that it has anticancer, bitter tonic, antimicrobial, antioxidant and anti-inflammatory attributes [3, 10, 11].

Other African medicinal plants of historical therapeutic values include Artemisia herba-alba Asso (Med)—Asteraceae known as Worm-wood. The Worm-wood since ancient times has been used by many cultures in folk medicine. It is used to treat arterial hypertension and diabetes in Moroccan folk medicine [12, 13] and for the treatment of hypertension, neuralgias, diabetes, bronchitis and diarrhea in Tunisia [3]. In folk medicines, Artemisia herba-alba tea has been utilized as a hemostatic agents, antispasmodic, analgesic and antibacterial [3].

Worthy of note is Securidaca longipedunculata Fresen from the Polygalaceae family an African indigenous medicinal plant exploited for a variety of ailments. Ailments such as malaria, stomach disorder, toothache, headache, sleeping sickness, cough, chest complaints, snakebite and wound are treated using the bark and the root of the plant which are taken orally as a powder or as an infusion. The roots are used as pesticides against storage pests of seeds. Headache, fever and rheumatism are treated with the seeds. Snakebite, venereal diseases and coughs are treated with the leaves while arrow poison and stomach problems are treated with the bark in Nigeria [14]. In Tanzania, the dried bark and root are used for treating nervous system disorders as a purgative. Dried leaves are for the treatment of snakebite, venereal disease, coughs, sores and wounds in East Africa. In Malawi, bilharzia, venereal disease and snakebite, wounds and coughs are treated using the leaves. Ghana uses the root bark for the cure of epilepsy, while impotence and malaria are treated with the plant root in Rhodesia (Zimbabwe) and Bechuanaland now the Republic of Botswana [6].

3. Scientific secret of the therapeutic potentials of medicinal plant

The application of medicinal plants in the cure or treatment of ailments right from the past points to the fact that numerous bioactive substances possessing different therapeutic attributes are present in plants which confer on them the therapeutic potentials.

Most of these bioactive compounds are present in plants not necessarily for the propagation or growth of the plants but as a result of modification requires for the plants adaptation to their habitats, such as protection against diseases, insects, predators or other conditions like stress, drought among others [2].

These bioactive substances are called secondary metabolites and include tannins, lignins, flavonoides, sterols, terpenoids, phenolics, essential oils, alkaloids, glycosides, etc. [15, 16], which are products of primary metabolism (the breakdown and biosynthesis of fats, carbohydrates, nucleic acids and proteins).

To obtain the biosynthetic intermediates which, ultimately, results in the formation of secondary metabolites also known as the bioactive substance, the biosynthesis of the secondary metabolites requires the fundamental processes of glycolysis, Krebs cycle and photosynthesis. The intermediate molecules (acetyl coenzyme A (acetyl-CoA), shikimic acid, mevalonic acid and 1-deoxyxylulose-5-phosphate) are the most important building blocks involved in the biosynthesis of the bioactive compound [2, 17] in medicinal plant.

The therapeutic potentials of these medicinal plants is due to the complex interactions of the bioactive substances with their molecular target structures, such as the transport systems, enzymes and receptors.

With the transport systems, bioactive compounds can interact to influence the absorption, distribution and elimination of nutrients and organic compound and other organic cations [18, 19]. An example is polyphenols from medicinal plants, such as grapes, tea and cocoa, whose presence interferes with the activity and expression of several cell membrane transporters [19].

With the enzymes, bioactive substances can activate or inhibits enzymes [20], modulating metabolic pathways and cellular processes [21]. Plant derivative involved in modulating metabolic pathway and cellular processes is ∆⁹-tetrahydrocannabinol (THC), the main psychoactive component of cannabis (Cannabis sativa and Cannabis indica) which activate G Protein Couple Receptors (GCPRs) cannabinoid 1 (CB1) and cannabinoid 2 (CB2) to which the endocannabinoids (ECBs) bind to and activate [21].

While with the receptors, bioactive compounds can bind to receptors, triggering signaling cascades that regulate various physiological processes [21], such as hormone regulation, neurotransmission and inflammation. Medicinal plant-derived compounds involved in cell signaling as bioactive lipids include carotenoids and phenolics (including flavonoides), terpenoids (including sterols) and cannabinoids (terpeno-phenol compounds also known as phytocannabinoids) [21].

These bioactive substances interactions can result in several therapeutic effects including the following:

1. Immune system modulation: bioactive compounds such as derivatives of caffeic acid, polysaccharides and alkamides from Echinacea are found to possess immunomodulatory properties. It has been reported that Echinacea interactions with the natural killer (NK) cells, T cells and macrophages increase their immune activity. The two pro-inflammatory cytokines that may be stimulated that are essential for controlling immunological response are interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) [15]. Other bioactive compounds with immunomodulatory activities that regulate immune signaling pathways include triterpenoids, peptidoglycans and polysaccharides from Ganoderma lucidum (Reishi mushroom), andrographolides and flavonoids from Andrographis paniculata, while ginsenosides from Panax ginseng promote immune cell proliferation, and enhance immune responses against infections by modulating cytokine production by regulating function of the immune cell, including T cells, B cells and natural killer (NK) cells [15].

2. Analgesic and Anxiolytic activities: medicinal plants such as Ganoderma applanatum (mushroom), Citrus maxima (Burm.) and Echinops kebericho M. (Asteraceae) contain phytochemicals such as anthraquinone, glycosides, tannins, alkaloids, saponins, flavonoids, phenols and steroid [22], which anxiolytic activities is by mimicking gamma-aminobutyric acid (GABA) activity as a result of the interaction of the phytochemicals with the neurotransmitter receptors [22, 23]. While both the peripheral and central analgesic activities are attributed to the phytochemical inhibition and activation of receptors and transmitters [24].

3. Anti-inflammatory and antioxidant activities: bioactive compounds such as curcumin from turmeric (Curcuma longa) possess potent anti-inflammatory potentials by preventing the activation of nuclear factor-kappa B (NF-B) a crucial inflammation regulator. Also, gingerol from the ginger acts by decreasing inflammatory cytokines synthesis, including interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-) [15]. Anti-oxidant activities of medicinal plant bioactive compounds such as flavonoid, quercetin from onions and apples are by lipid inhibition and neutralization of reactive oxygen species. Curcumin from turmeric and resveratrol from grapes are other examples [15].

4. Cardiovascular protection: medicinal plants such as turmeric (Curcuma longa) has bioactive compounds such as curcumin that have anti-inflammatory, anti-oxidant and lipid lowering effects. It was reported to possess the potentials for stopping atherosclerosis development and lower indicators of inflammation and enhance endothelial functions [25] thereby beneficial to cardiovascular health. Bioactive compounds such as flavonoids, procyanidins and triterpenoids from Hawthorn (Crataegus spp.) have been found to demonstrate blood flow improvement, lower blood pressure and heart function improvement, allicin, flavonoids and sulfur compounds from garlic have demonstrated blood pressure lowering activity, and enhance endothelial functions and both LDL and total cholesterol reduction [15]. Others that contribute to cardiovascular health include catechins from green teas.

5. Antimicrobial [21]: bioactive compounds such as berberine from Berberis spp., sanguinarine derivatives of alkaloids from the Papaveraceae family. Flavonoids, for example, kaempferol and quercetin, found in plants like Allium cepa and Camellia sinensis exhibit antibacterial properties by inhibition of essential enzymes and bacterial cell membrane disruption. Growth of fungi like Candida spp. and Aspergillus spp. have been reported to have been prevented by polyphenols such as the epigallocatechin gallate (EGCG) in green tea (Camellia sinensis). Similarly, antiviral activity of some bioactive compounds from flavonoids such as quercetin and hesperidin has been reported. Quercetin and hesperidin are said to inhibit viral pathogen replication. Also, the growth of a number of viruses, including the respiratory syncytial virus (RSV) and the hepatitis C virus (HCV), has been reported to have been prevented by tannins from Punica granatum (pomegranate) and Camellia sinensis (green tea). The antimicrobial activities of the bioactive compounds from medicinal plants are either by microbial cell membrane disruption leading to lysis and death of cells, inhibition of growth and vital enzymes involved in microbial metabolism and enhancement of the body defense mechanisms against infections by modulating the immune response [15].

6. Anticancer properties: bioactive compounds such as resveratrol a derivative of polyphenols from berries and grapes, epigallocatechin gallate (EGCG) from green teas, alkaloids such as vincristine and vinblastine from Catharanthus roseus (Madagascar periwinkle), camptothecin from Camptotheca acuminata, paclitaxel a terpenoid from the Pacific yew tree (Taxus brevifolia), sulforaphane found in vegetables like cauliflower and broccoli, allyl isothiocyanate from garlic all have been reported to possess anticancer properties which therapeutic activities are through modulation of important signaling pathways for cell apoptosis, growth, inflammation, angiogenesis, metastasis and survival, enzyme systems for tumor growth inhibition, epigenetic modifications and promotion of DNA repair [15].

7. Neuroprotective and neuroregenerative effects: bioactive compounds such as curcumin from turmeric (Curcuma longa) have exhibited activity as a neuroprotective compound, its therapeutic activity involved the aggregation of amyloid-beta plaques inhibition and neuroinflammation reduction. Other bioactive compounds for neuroprotective and neuroregenerative disorders such as Parkinson’s and Alzheimer’s include flavonoids and terpenoids from Ginkgo biloba, resveratrol and bacosides from Bacopa monnieri [15].

For the development of new drugs and therapies as well as the optimization of traditional herbal remedies, understanding of the complex interactions between bioactive substances and molecular target structures is crucial.

4. Advances in the utilization of bioactive compounds of medicinal plants

The utilization of medicinal plants in the management and treatment of diseases has continue to witness tremendous developments and improvements. Prepared herbal medicines from medicinal plants have been criticized for being crude, lack regulation, lack standardization, have no dose as well as the in ability to describe the mechanisms of action. These mentioned challenges made the use of herbal medicines from plant neglected and it used attributed to the rural populace who were regarded as been poor and could not access Western medications. But today, the emergence of Covid-19 and other terminal diseases, as well as drug resistance ailment, has triggered the quest for herbal medicines from medicinal plant for both developing and developed countries alike. This quest for herbal drugs of natural origin has led to thought-provoking innovations in natural medicine research as well as technological advancement which tend to proffer solutions to those criticisms. Below are some of the advances made in medicinal plant utilization:

1. Technological advancement in bioactive compound extraction and characterization: extraction and characterization of medicinal plants bioactive compounds is currently being explored by the researchers using new and advanced technologies which is a great improvement over the crude practices of extraction methods utilized by the ancients. These technologies enable the development of standardized extraction and analysis protocols by the researchers, they allow for new compounds and their structures identification, ensuring bioactive compounds efficient extraction and characterizations, enhancement of medicinal plant-based products quality and efficacy and understanding of biosynthesis of bioactive compound. These technologies include Advanced microscopy and spectroscopy, Supercritical Fluid Extraction (SFE), Microwave-Assisted Extraction (MAE), Phytochemical Analysis Software, Artificial Intelligence (AI) and Machine Learning (ML), High-Performance Liquid Chromatography (HPLC), Mass Spectrometry (MS), Pressurized Liquid Extraction (PLE), Ultra-High-Performance Liquid Chromatography (UHPLC), Nanotechnology approach and Magnetic Resonance (NMR) Spectroscopy [26, 27, 28, 29].

2. Bioinformatic tools: to accelerate new bioactive compounds discovery and development of drugs from medicinal plant, bioinformatics tools and techniques have been developed to facilitate the analysis and interpretation of large datasets. Over the last two decades, the rapid development of omics-based plant studies has evolved due to the continuous introduction of novel omics concepts and rapid development of sequencing technologies which greatly facilitated the comprehensive dissection of biological processes occurring in plants at the genetic, transcriptional and metabolic levels [30]. At the genetic level, medicinal plants can be explored by the researchers using genome sequencing technology, sequencing and assembling of large medicinal plant genomes have been made easier using next-generation sequencing (NGS) and long-read sequencing technologies, molecular mechanism of medicinal plants and their impact on human health can be understood by genomic insight. These techniques allows for the identification of new bioactive compounds and the understanding of their mechanisms of action [31, 32].

3. Nanotechnology: the complaints against herbal medicines have been a lack of dose, standardization, adulteration among others but a novel means of overcoming these challenges is through nanotechnology. It is a “field of applied science and technology which aimed to develop devices and dosage forms in the range of 1 to 100 nm [28].” The field of nanotechnology presents a novel approach for the delivery of herbal drugs thereby overcoming the traditional drug delivery systems drawbacks. Herbal medicines were not given the due attention for development as a novel formulation for a long time as a result of scientific justification and processing difficulties, such as standardization, extraction and identification of individual drug components in complex polyherbal systems that were lacking. However, these scientific needs for herbal medicines in modern medicines have been resolved by modern phytopharmaceutical research which allows for the development of nanoparticles, microemulsions, matrix systems, solid dispersions, liposomes, solid lipid nanoparticles (SLNs), etc., as novel formulations [28].

4. Artificial intelligence and machine learning: the application of artificial intelligence and machine learning in medicinal plant research enables the therapeutic properties of medicinal plant extracts and bioactive compounds to be predicted. Identification of new bioactive compounds and extraction methods optimization is facilitated by these techniques [26, 27, 30].

5. Sustainable domestication and cultivation: because of the growing need for medicinal plants, their domestication and cultivation become very important because of their economic and ecological significance. More so, deliberate actions needed to be taken to ensure uniformity and consistency of the bioactive compounds supply for drug development. Researchers employed innovative methods of medicinal plant domestication and cultivation such as hydroponics, molecular breeding, agroforestry and vertical farming for the production of a large quantity of medicinal plants using good agricultural cultivation practices thereby, ensuring standard and quality of the bioactive compounds [26, 30].

6. Pharmacological and toxicological examination: more comprehensive pharmacological and toxicological studies on medicinal and aromatic plants to assess their efficacy and safety, are being conducted by researchers using in vitro and in vivo models for the evaluation of medicinal plant extracts and isolated compounds bioactivity and potential toxicity. This help to address the challenges of herbal medicines safety and efficacy issues [26].

4.1 The future outlook of medicinal plants

Large population of people are turning to herbal medicines for its therapeutic value from both developed and developing countries. This development is not necessarily because of poverty or lack of access to Western medicines but due to the realization that several advantages are presented by medicinal plants bioactive compounds such as the ability to cross blood-brain barriers [33], being of natural origin, little or no side effect compared to the Western medications, multiple target effects. Their ability for cellular process modulation, including immune responses, metabolic pathways, oxidative stress and inflammation all enhanced their therapeutic potentials [15].

However, the application of various advancements in medicinal plant research is essential to ensure that the full potential of medicinal plants bioactive compounds is fully realized.

Interdisciplinary collaborations combining traditional knowledge with modern scientific process is required for the future of utilizing medicinal plants bioactive substances. Novel therapies and personalized medicine approaches can be developed from the collaboration which can help in bridging the gap between traditional medicine and evidence-based medicine [15].

Additionally, exciting prospects for enhancing the therapeutic potential of these compounds are provided by advancements in nanotechnology, pharmacogenomics and phytopharmaceutical formulations. The therapeutic outcomes of these compounds can be optimized by improving their bioavailability, targeting specific tissues or cells and tailoring treatments based on individual genetic variations [34].

5. Conclusion

Medicinal plants hold immense therapeutic potentials due to the presence of bioactive compounds. These bioactive compounds conferred on the medicinal plants the healing powers for addressing various health challenges. Evidence of used of medicinal plant by the ancients to cure several ailments abound. With scientific and technological advancements improvements can now be made on the foundation laid by the ancients in the use of herbal medicines. Collaboration and integration of herbal medicine into the healthcare delivery system will further pave the ways for their utilization thus realizing the full therapeutic potentials of medicinal plants.