Abstract
Some plants used in traditional medicine to manage sickle cell disease have been validated pharmacologically to ascertain their anti-sickling properties. However, there is no global systematic review of the evidence in support of their use for sickle cell management. This research aimed to conduct a systematic review of the pharmacological evidence to highlight species, genera, and some phytochemicals which could be primed for novel anti-sickling medications. The Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines were used. Articles in English, published from 2000 to 2022 were included in the analysis. The literature search covered 411 publications of which 82 were found to be eligible. More than half (53.65%) of the articles were published from 2016 to 2022. Anti-sickling research from Africa accounted for 86.58% of the publications, with more than half coming from Nigeria, 12.20% from Asia and only 1.22% from Europe. The Fabaceae, Euphorbiaceae, and Annonaceae were the top three plant families whereas Zanthoxylum and Terminalia were the most reported genera. Carica papaya and Terminalia catappa were the most reported species. Sickling reversal (n = 33/82) and hemoglobin polymerization studies (n = 29/82) were the most reported assays. Benzoic acid derivatives, butyl stearate, ellagic acid derivatives, and some pentacyclic triterpenoids were the only plant derived compounds validated for anti-sickling activities. A total of 117 plant species with anti-sickling activities were documented. Studies on secondary metabolites with anti-sickling properties to serve as scaffolds for novel drug development for sickle cell management were limited.
Introduction
Sickle cell disease (SCD) belongs to a group of inherited blood disorders or hemoglobinopathies. These are genetic blood diseases arising from the inheritance of mutant hemoglobin genes from both parents. Approximately 5% of the world's population carry genes for hemoglobin disorders, mainly, sickle-cell disease and thalassemia. 1 Sickle cell anemia (SCA), also known as drepanocytosis, hemoglobin S or SS disease, is the most important hemoglobinopathy worldwide according to the World Health Organization.2,3 SCD affects about 50 million people throughout the world with a high prevalence in Africa and India. Few cases are reported among families originating from South America, Cuba, Central America, Saudi Arabia, and the Mediterranean countries of Turkey, Greece, and Italy.3–6 The occurrence of this condition in North European countries is reportedly due to migration. 7 The records show that of the 300,000 babies born each year with severe hemoglobin disorders, 200 000 come from Africa alone. 8 It is also reported that approximately 1 in 12 African Americans are heterozygous for the disorder, and approximately 1 in 500 African American newborns are diagnosed yearly with SCD. 9 In countries such as Cameroon, Republic of Congo, Gabon, Ghana, and Nigeria, the prevalence is between 20% and 30% while in some parts of Uganda, it is as high as 45%. 1
Management of SCD is focused on preventing and treating pain episodes and other complications. The treatment varies from person to person depending on the age, symptoms, and severity. The only known medication for SCA is hydroxyurea (HU), which is recommended to reduce the number of painful episodes, reduce hospital stays and the requirement for blood transfusions. 10 However, it is known to cause neutropenia, thrombocytopenia, or worsening anemia. 11 Antibiotics are given during sickling crisis if infections are implicated whereas blood transfusions are commonly used to treat worsening anemia and vaso-oclusive complications. Blood and marrow stem cell transplants may offer a cure for a small number of economically fortunate patients. 12 New medications such as Endari (L-glutamine oral powder) and Oxbryta (voxelotor) have been approved by the Food and Drug Administration (FDA) but are not cost-effective. Stem cell transplant can cure SCD but require a matched donor. 13 The health-care cost of the management of SCD patients is disproportionately high compared to the number of people afflicted by the disease. Most of the populace in Sub-Saharan Africa, where prevalence of the disease is extremely high, are mostly below the poverty line and unable to afford the high cost of treatment. People in this region have learnt to manage several diseases, including SCA, with medicinal plants.
Several medicinal plants have been documented in traditional medicine for the management of SCA. Examples include Cryptolepis nigrescens (Wennberg) L.Joubert & Bruyns, Carica papaya L. (unripe fruit and leaves), Cajanus cajan (L.) Huth seeds, Zanthoxylum zanthoxyloides (Lam.) B.Zepernick & Timler root, Boerhavia diffusa L., Uvaria chamae P.Beauv, and Elaeis guineensis Jacq.14–16 These plants are extensively used in home-based remedies among the under privileged classes of the society for SCA. In Nigeria, plants such as Syzygium aromaticum (L.) Merr. & L.M.Perry, Piper guineense Thonn., Aframomum melegueta K.Schum, Sorghum bicolor Kuntze, and Pterocarpus osun Craib, have been used for the management of SCA for several decades. 17 The polyherbal anti-sickling medication “NIPRISAN” was developed from these medicinal plants. The product passed phases IIA and IIB clinical trials and widely used in Nigeria, India and the USA. 18 Another product, “Ciklavit,” based on the anti-sickling medicinal plant C. cajan, was also subjected to a randomized placebo-controlled single-blind intervention trial to compare it safety and efficacy in Nigeria. 19 Thus, SCD management using medicinal plants is widespread, especially in developing countries. Several studies have validated medicinal plants used in traditional medicine for the management of sickle cell anemia. 20 However, there is no evidence-based systematic review of these medicinal plants and their compounds for sickle cell management. The aim of this research was to document such plants and highlight species, genera, and some phytochemicals which could be primed for further research to develop novel anti-sickling medications, whose development has stalled since the discovery of hydroxyurea over several decades ago.
Methodology
Inclusion and Exclusion Criteria
The scope of this review was restricted to scientific publications written in English language. Papers describing the pharmacological testing of plant materials for anti-sickling activities and published from 2000 to 2022 were included in the study. The study also covered articles describing the anti-sickling activities of plant-derived compounds. Ethno-botanical surveys of anti-sickling medicinal plants without their pharmacological testing were also excluded. Additionally, papers which were not written in English, abstract only and had plant names in a local dialect with no scientific names available were excluded
Literature Search
Research articles and general scientific publications regarding herbs or plant extracts that have significant effect in the treatment of SCD were looked up in databases such as Google Scholar, PubMed, Science Direct, DOAJ, Open Alex, Wiley, AJOL, BioMed Central, JSTOR, and Scopus. A supplementary search was made in relevant institutional repositories and the bibliography section of papers included in the study. Search keywords used in this search included “anti-sickling and medicinal plants,” “anti-sickling and herbs,” “anti-sickling and plant extracts,” “anti-sickle cell anaemia and medicinal plants,” “anti-sickle cell anaemia and herbs” and “anti-sickle cell anaemia and plant extracts.” The Boolean operators “AND” and “OR” were used to afford the intended results. There were 411 publications found and collected of which 53 were duplicates and 276 papers excluded as per the exclusion criteria. The search covered the period from January 2000 to December 2022.
Data Extraction, Presentation, and Analysis
The guidelines for the selection and inclusion of articles described by the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) 21 (Figure 1) were followed. After the literature search, all articles were extensively reviewed for relevance as defined by the inclusion and exclusion criteria. Duplicates and other irrelevant articles were excluded (Figure 1). All full-text articles to be included in the study were further evaluated to ascertain their relevance to the objectives of the study such as experimental protocols describing in vitro or in vivo testing and other standardized assays that evaluated the effectiveness of the medicinal plant, herbs, plant extracts, and compounds in treating SCD. The data collated from the publications included the scientific name of the plant, the plant family, method of extraction, solvent used, the testing method used to measure efficacy, the country of origin of the paper and the year of publication. The available information was extracted onto an excel sheet, organized into sections according to the above aspects. The data sets under each section were summarized in tables as frequency and percentages. The plant names were verified using the world flora and plant of the world online databases (worldfloraonline.org; https://powo.science.kew.org/).
PRISMA flow diagram for the conduct of the study.
Results
Background of Selected Studies
The literature search on medicinal plants validated pharmacologically for their anti-sickling effect yielded 411 publications of which 53 were duplicates. Application of the inclusion and exclusion criteria afforded 82 articles which were used in the analysis as shown in the modified PRISMA chart (Figure 1). The selected articles spanned two decades (from 2002 to 2022) of anti-sickling research using medicinal plants; three articles from 2002 to 2005 (3.82%, n = 3/82), thirteen from 2006 to 2010 (15.85%, n = 13/82), twenty-two from 2011 to 2015 (26.83%, n = 22/82) and forty-four from 2016 to 2022 (53.65%, n = 44/82). Thus, more than half of the selected articles were published in the last six years indicating that sickle cell research using medicinal plants is receiving much attention in recent years.
In all, the studies selected originated from three continents: 71 from Africa (86.58%, n = 71/82), 10 from Asia (12.20%, n = 10/82), and one from Europe (1.22%, n = 1/82). Nigeria dominated the list of African countries (54.93%, n = 39/71), followed by the democratic republic of Congo (n = 19/82) and Cameroon (n = 4/71). Benin, Morocco, and Togo, contributed an article each. Studies recorded from the Asian continent were made up of eight from India and two from Saudi Arabia. Belgium recorded the only study in Europe (Table 1).
Detailed Information on Medicinal Plants Used Worldwide and Validated for the Treatment of Sickle Cell Disease.
Anti-Sickling Plant Families and Species
A total of 117 anti-sickling medicinal plants belonging to 51 families were documented (Table 1). The family Fabaceae recorded the highest number of species (ten) and genera (nine). Other dominant families included Euphorbiaceae (nine species and six genera), Annonaceae (eight species and six genera), Apocynaceae (five species and five genera), Rubiaceae (five species and five genera), Phyllantaceae (five species and four genera), Combretaceae (five species and three genera) and Rutaceae (four species from one genus) (Figure 2). The most reported species were C. papaya L (n = 5), Terminalia catappa L (n = 3), and Annona muricata L. (n = 3) whereas Zanthoxylum (n = 4), Terminalia (n = 3), and Ocimum (n = 3) dominated the list of plant genera with anti-sickling properties (Figure 3).
Plant families with the highest reported anti-sickling species and genera.
Most reported anti-sickling plant species (a) and genera (b).
Plant Parts and Solvents Used for Extraction
Analysis of the plant parts used revealed that leaves were the most preferred (n = 51), followed by the roots (n = 17), seeds (n = 15), and stem (n = 18) (Figure 4). Eleven different solvents of varying polarities were used in the extraction of these plant materials prior to the anti-sickling assays. The most reported solvent used was water (32.89%) followed by methanol (25.50%), ethanol (18.79%), and ethyl acetate (6.71%) whereas butanol was the least used solvent (0.67%).
Percentage use of the different plant parts for sickle cell management.
Anti-Sickling Assays and Compounds Reported
Seven different types of anti-sickling assays were used in the evaluation of anti-sickling plants in the published documents. The sickling reversal assay was the most reported (n = 33/82), followed by hemoglobin solubility or polymerization studies (n = 29/82) and Emmel's test (n = 28). Others included the sickling inhibition (n = 22/82), osmotic fragility (12/82) and the Itano's test (n = 1/82). Out of the eighty-two papers reviewed, only six papers reported the antisickling activity of isolated compound(s) from the medicinal plants. This included two tannins from the roots of Combretum racemosum P.Beauv, 56 a stearate ester and a pentacyclic triterpenoid from Ocimum basilicum L. and Ocimum gratisimum L. respectively, 72 two flavonoids and a phytosterol from Woodfordia fruticosa (L.) Kurz, 10 two pentacyclic triterpenoids from Rhaphiostylis beninensis (Hook.f. ex Planch.) Planch. ex Benth., 81 betulinic acid and it acetate from Melaleuca bracteate, Melaleuca viminalis (Sol. ex Gaertn.) Byrnes and Syzygium guineense (Willd.) DC., 84 the flavonoid quercetin from Mitracarpus hirtus (L.) DC. 90 and three isomeric divanilloylquinic acids isolated from the root bark of Zanthoxylum zanthoxyloides Lam. (Rutaceae) 91 (Figure 5).
Plant metabolites with anti-sickling activity reported.
Discussion
Medicinal plants have been utilized as a therapy against SCD and its associated symptoms over centuries in various countries across the globe. In this study, the rate at which sickle cell research was conducted, increased gradually from 2002 but sharply between 2016 and 2022. More than half the number of articles were published recently (2016–2022) which may depict the efforts being made to develop novel drug candidates which may be affordable and yield the best treatment outcomes. It may also reflect the projected increase in the incidence of SCD in many countries. Most of the research was conducted in Africa. Nigeria, the Democratic Republic of Congo, and India were the countries with the most published plant species with anti-sickling activity. This may be related to the incidence of SCD in these countries. Nigeria has the highest recorded number of SCD cases compared to other countries in the world. 98 It has the highest disease incidence in the world with approximately 91 011 children born with the defect. The rate accounts for almost 2% of all newborns annually. Subsequently, Nigeria has earned the tag “the sickle cell capital of the world” with its residents accounting for about half of all new cases of severe hemoglobin disorders worldwide. 89 The Democratic Republic of Congo has the second highest rate of SCD cases with 39 743 sickle cell births per year. Studies indicate that the number in Nigeria and Congo is likely to increase to approximately 140 000 and 45,000, respectively by the year 2050. 98 India has also been reported to have one of the highest number (42 016) of predicted SCD births. 99 The only reported research from Europe came from Belgium and this may reflect the low incidence of the diseases in Europe and other Western countries. However, the report that migration has led to an increase in the occurrence of sickle-cell disease in countries with previously low incidence of the disease 100 is a call for more research exploring natural remedies as novel drug candidates in this region.
The current study revealed the plant families; Fabaceae, Euphorbiaceae, Annonaceae, Apocynaceae, Combretaceae, Phyllantaceae, and Rubiaceae as having the largest number of plants validated for the management of SCD. Considerable number of plants from the Rutaceae, Lamiaceae, Meliaceae, and Myrtaceae were also reported to show promising anti-sickling activities. This opens the families for exploration into the discovery of novel anti-sickling agents to mitigate the threat posed by this debilitating disease. The large number of species within these families and their widespread distribution across the globe could have accounted for their high incidence of use against SCD. For example, the top ranked family, Fabaceae, has 18 870 species and is rich in several bioactive metabolites which are being explored for the management of several diseases. 101 Families like the Acanthaceae, Poaceae, Melastomaceae, which had fewer reports, may also consist of species that may be very effective against SCD. Their low records for SCD management could be due to their limited distribution across the globe and thus outside the reach of countries conducting extensive research into the disease. Zanthoxylum, Terminalia, and Ocimum were the top three genera with most species validated for the management of SCD. Four species of the genus Zanthoxylum were reported in this study to have anti-sickling activities. Among them, Z. zanthoxyloides, was the most widely used species for sickle cell anemia. 102 The plant showed sickling reversibility, increase in hemoglobin gelling time, and improved rheological properties of drepanocytary blood. 103 The widespread use of Zanthoxylum species for SCD could also stem from their potent anti-arthritic and anti-inflammatory activities,104–106 two factors which are crucial for sickling crisis management. Carica papaya was the most widely reported plant species for SCD, followed by T. catapa L. and A. muricata L. The unripe fruit and dried leaf extracts of C. papaya have been widely reported to reduce hemolysis and maintain the integrity of erythrocyte membranes under osmotic stress conditions. Elsewhere, the leaf extract was found to modulate the expression of fetal hemoglobin and its regulator genes.107–109 Similarly, the leaf extracts of T. catapa and A. muricata inhibited sickling, hemoglobin polymerization and stabilized erythrocyte membranes under osmotic conditions.30,110,111 In most cases, the research focused on the crude extracts of these plants with no attempt to track down the bioactive metabolites.
All plant parts were targeted in the research to validate the anti-sickling properties of medicinal plants. In most cases, leaves were used (42%) followed by stem (15%), roots (14%), seeds (13%), fruits (7%) then 3% for flowers. Most of the reports indicated the use of dried powdered plant parts for extraction. This process breaks down the cells and tissue structures, increasing the surface area and exposing the cellular structures to compounds and chemicals in the solvent during extraction. Some papers also emphasized the varying effectiveness of some plant parts over others. For example in the report by Adejumo, 54 the leaves, seeds, and seed pods of Garcinia kola Heckel were tested for sickling inhibition and sickling reversal activities. The leaves exhibited the most anti-sickling activity, while the seed pods showed the least activity. In other studies, some parts were effective in managing the condition while other parts made it worse. This was observed in studying the anti-sickling activity of Newbouldia laevis (P.Beauv.) Seem. based on Emmel's test. The study demonstrated that the stem and root bark of Newbouldia laevis possess significant anti-sickling activity compared to the leaves which caused hemolysis of sickled cells. 46 Studies have also cited increased anti-sickling activities by combining different plant parts (polyherbal formulations). For instance, Oyennike and his colleagues reported on a “Mishenland” polyherbal mixture made up of Uvaria afzelii Scott Elliot roots, Securidaca longepedunculata Fresen. root barks, S. bicolor leaves, M. charantia L leaves and seeds, Phyllanthus amarus Schumach. & Thonn. leaves and Dialium guineense Willd leaves and barks. The formulation inhibited hemoglobin-S polymerization by increasing the hemoglobin oxygen affinity in a dose-dependent manner. 33 Thus, preference for a plant part in sickle cell management should be based on traditional medical knowledge backed by pharmacological evidence of efficacy. The high records of leaves used for SCD management is highly welcomed for conservation and sustainable utilization of the plants.
The anti-sickling assays reported in the reviewed studies included the hemoglobin polymerization, Emmel's test, osmotic fragility, sickling reversal, sickling inhibition, and the Itano's test. All these assays represent key pathophysiologic events in the pathogenesis of SCD. During conditions of low oxygen tension, valine hydrophobicity attracts hydrophobic regions of adjacent β-chains, facilitating the polymerization of hemoglobin molecules. 112 If conditions are conducive (deoxygenation, diphosphoglycerate concentration, saline concentration, dehydration), polymerization of hemoglobin molecules reaches a critical mass resulting in erythrocyte sickling. 113 The damaged red blood cells are highly adhesive, 114 trapping platelets and resulting in vaso-occlusion. 115 When erythrocyte deformability is compromised by sickling, the erythrocyte membrane is disrupted, releasing free-Hb polymers into the circulation which are removed through phagocytoses by macrophages. 8 These pathologic events highlight the significance of the inhibition of hemoglobin polymerization, sickling inhibition, sickling reversal and the osmotic fragility or erythrocyte membrane stabilization assays. Screening medicinal plants for sickle cell management should be aimed at these targets. Thus, using just one assay at the preliminary validation stages is not enough to elucidate the full spectrum of anti-sickling activity of these medicinal plants.
In the publications, most of the plant's materials were extracted with methanol, water, and ethanol. The choice of extraction solvent may have partly been by recommendations from traditional healers through ethno-botanical documentation that the anti-sickling compounds were easily soluble in those solvents. Other solvent extracts reported, such as diethyl ether, dichloromethane, hexane, ethyl acetate, chloroform, petroleum ether, butanol, and acetone, even though did not reflect traditional mode of preparation, were equally important in assessing the nature of anti-sickling compounds in a particular plant. For example, Nwaoguikpe and Uwakwe, 32 comparatively tested chloroform, butanol, water, and methanol extracts of Xylopia aethiopica (Dunal) A.Rich. fruit and Monodora myristica (Gaertn.) Dunal for anti-sickling activities and observed variations in their potencies. The use of different solvent extracts makes it easy to track down the antisickling compounds or elucidate the mechanism of action of the extract in subsequent studies.
Despite the widespread anti-sickling reports on the plant extracts, the current review only observed few cases where isolated compounds from the plants were evaluated for anti-sickling activities. Six pentacyclic triterpenoids; beta-sitosterol, ursolic acid, friedelin, friedelinol, betulinic acid and its acetate, were reported to show sickling inhibitory activities.81,84 Triterpenoids have reportedly demonstrated anti-sickling action by inducing the leucine zipper transcription factor nuclear erythroid derived-2 related factor 2 (Nrf2), a master regulator of inducible antioxidant responses, which can attenuate cellular injury from oxidative stress.116,117 Oxidative stress is known to cause hemolysis and inflammation as well as the downstream events including the increased sickling of red blood cells within the kidney and the pathogenesis of acute chest syndrome in SCD.116,118 Several supplements used in antioxidant therapy for SCD are known enhancers of cellular antioxidant capacity. Thus, the triterpenoids reported in this study and their medicinal plants could be harnessed and their use optimized for SCD management.
One of the most used plant in folklore medicine for SCD management, Z. zanthoxyloides, was reported to contain three isomeric divanilloylquinic acids (Figure 1), given the common names burkinabin A, burkinabin B and burkinabin C as the main anti-sickling agents. 91 Divanilloylquinic acids belong to the benzoic acid derivatives (BAD). Earlier report that the BAD protocatechuic acid, p-hydroxy benzoic acid, vanillic acid, syringic acid, and ferulic acid,119,120 were the main anti-sickling compounds of the root of Z. zanthoxyloides was refuted by Ouattara et al 91 who did not find these compounds in sufficient quantities. The structure and related anti-sickling activity of the BAD have been well elucidated with predictions by the Hansch lipophilicity pi (π) and the Hammett electronic sigma (σ) constants. Fasanmade et al 121 postulated that strong electron donating groups attached on the benzene ring with average lipophilicity are an important feature for such BAD to possess potent anti-sickling activity. Hydrophilic substituents on the phenyl ring are also necessary to facilitate interaction and binding to polar amino acid residues near the mutation site of the defective globin molecule. Secondly, a phenyl core is critically important to enhance hydrophobic interactions with several hydrophobic amino acids. 91
Ellagic acid derivatives 3,3′, 4′ -tri-O-methyl ellagic acid and 3,3′ -di-O- methyl ellagic acid isolated from Combretum racemosum were also reported to show sickling inhibitory activities. 56 The mechanism of anti-sickling effect of the hydrolysable tannin ellagic acid is well elucidated. The compound is a known potent inhibitor of hemoglobin polymerization, attenuates oxidative stress and abrogates neutropenia induced by hydroxy urea when used concomitantly. 122 Thus, the use of ellagic acid and its derivatives as novel scaffolds for anti-sickling drug discovery research and as adjuvant therapy in SCD management is highly recommended. This research is limited by some few publications which were not in English language and few others which were inaccessible.
Conclusion
This research provides the first study to systematically document medicinal plants and their compounds with evidence of anti-sickling activities. Most of the studies were conducted in Africa with few in Asia and Europe. A total of 117 plant species with anti-sickling activities were documented. In most cases, however, the validations were based on in vitro studies. Studies on the secondary metabolites responsible for anti-sickling properties to serve as scaffolds for novel drug development for SCD management were limited. Future research could focus on in vivo pre-clinical and clinical studies as well as tracking the anti-sickling metabolites of these plants.
Footnotes
Acknowledgments
Authors are indebted to Dr Bernard Appiah, Department of Public Health, Syracuse University for the training in systematic reviews.
Authors’ Contributions
Conceptualization, IKA, DOK and FAA.; methodology IKA, DOK, FAA, EK, formal analysis, IKA, JNA, EK, BKT; investigation, DOK, EK, IKA, BKT resources, IKA, BKT, JNA; writing—original draft preparation, DOK, EK, IKA; writing—review and editing, IKA, EK, DOK; supervision, IKA and JNA. All authors have read and agreed to the published version of the manuscript.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
