Antidiabetic Medicinal Plants Used in Democratic Republic of Congo: A Critical Review of Ethnopharmacology and Bioactivity Data

Several studies have been conducted and published on medicinal plants used to manage Diabetes Mellitus worldwide. It is of great interest to review available studies from a country or a region to resort to similarities/discrepancies and data quality. Here, we examined data related to ethnopharmacology and bioactivity of antidiabetic plants used in the Democratic Republic of Congo. Data were extracted from Google Scholar, Medline/PubMed, Scopus, ScienceDirect, the Wiley Online Library, Web of Science, and other documents focusing on ethnopharmacology, pharmacology, and phytochemistry antidiabetic plants used in the Democratic Republic of Congo from 2005 to September 2021. The Kew Botanic Royal Garden and Plants of the World Online web databases were consulted to verify the taxonomic information. CAMARADES checklist was used to assess the quality of animal studies and Jadad scores for clinical trials. In total, 213 plant species belonging to 72 botanical families were reported. Only one plant, Droogmansia munamensis, is typically native to the DRC flora; 117 species are growing in the DRC and neighboring countries; 31 species are either introduced from other regions, and 64 are not specified. Alongside the treatment of Diabetes, about 78.13% of plants have multiple therapeutic uses, depending on the study sites. Experimental studies explored the antidiabetic activity of 133 plants, mainly in mice, rats, guinea pigs, and rabbits. Several chemical classes of antidiabetic compounds isolated from 67 plant species have been documented. Rare phase II clinical trials have been conducted. Critical issues included poor quality methodological protocols, author name incorrectly written (16.16%) or absent (14.25%) or confused with a synonym (4.69%), family name revised (17.26%) or missing (1.10%), voucher number not available 336(92.05%), ecological information not reported (49.59%). Most plant species have been identified and authenticated (89.32%). Hundreds of plants are used to treat Diabetes by traditional healers in DRC. However, most plants are not exclusively native to the local flora and have multiple therapeutic uses. The analysis showed the scarcity or absence of high-quality, in-depth pharmacological studies. There is a need to conduct further studies of locally specific species to fill the gap before their introduction into the national pharmacopeia.


INTRODUCTION
Most African traditional healers who detain ancestral heritages are illiterate, and their knowledge transmitted verbally from generation to generation is at risk of disappearing. To minimize such risk, the World Health Organization (WHO) recommends scientists carry out ethnopharmacological and experimental studies to record folk knowledge, create databases, and validate scientifically traditional claims from the perspective of developing improved medications (WHO, 2013). WHO estimates that 80% of people rely on conventional medicine to meet primary health care needs, and most of them use remedies from plants (Surya et al., 2014). Ethnopharmacological surveys help gather holistic knowledge and practices of conventional healthcare systems. Experimental investigations evaluate efficacy and safety by developing suitable standardized pharmaceutical dosage forms that can complement, if not replace, current modern medicines. Medicinal plants used as complementary/alternative medicines (CAM) to manage various diseases provide a real opportunity in developed and developing societies. In this sense, herbal medications appear to offer readily available means of managing metabolic disorders by minimizing the risk of side effects and sometimes potentiating the treatment outcomes of modern drugs (Etxeberria et al., 2012). Medicinal plants are also used as food and contain several healthy dietary compounds. For example, some flavonoids interfere with metabolic events and play a crucial role in preventing and managing metabolic disorders through different pathways (Farzaei et al., 2019).
One of the most explored diseases is diabetes mellitus (DM). Over 800 plant species showing hypoglycemic activities can be essential sources for discovering and developing new types of antidiabetic molecules (Patel et al., 2012). The magnitude justifies this craze that Diabetes is gaining more and more globally, making it a severe public health problem. Not long ago, the disease was associated with industrialization. DM is no longer a disease of high-income countries but a global health pandemic. In 2013, according to the International Diabetes Federation, the global population of adults with both type-1(DMT1) and type-2(DMT2) was projected to increase from 382 million to 592 million by 2035, with DMT2 accounting for 90-95% of cases (Glezeva et al., 2017). In Africa, the number was expected to double from 14 million in 2015 to 34 million by 2040. With its continuous and rapid increase in its prevalence worldwide, it should be one of the leading causes of morbidity and mortality in the coming years (Glezeva et al., 2017).
Recent data show about 1.7 million people suffer from DM in the Democratic Republic of Congo (DRC), ranking fourth in the top ten countries by diabetes cases in Africa (Zhivov et al., 2015;Kasangana et al., 2018). Like other African countries, and not withdrawing modern medicines, 80% of people rely on traditional medicine to meet primary health care needs (Mahomoodally, 2013). Ethnopharmacological and pharmacological studies have been conducted globally; however, the related data are disparate and uncontrolled. A preliminary review reported vernacular names, parts used, and the formulation of 70 medicinal plants used to treat DM in DRC. A few phytoconstituents and antidiabetic mechanisms are also mentioned (Jacques et al., 2015).
This review aimed to describe what is known hitherto about ethnopharmacological, pharmacological, and clinical studies embracing medicinal plants used to manage DM in the traditional medicine of the DRC, to highlight which plants are native or introduced, how they are formulated and used, what valid experimental studies have been conducted in preclinical and clinical phases. A critical analysis is made to assess the quality of studies carried inside DRC and resort similarities/discrepancies with studies conducted outside.

Literature Search Process
The review was an Internet search on Google Scholar, Medline, PubMed, Scopus, ScienceDirect, the Wiley Online Library, Web of Science, and other documents focusing on ethnopharmacology, pharmacology phytochemistry of antidiabetic plants used in the Democratic Republic of Congo from 2005 to September 2021. The review was conducted following Preferred Regulatory Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines 2009. A total of 34 studies were included. Ethnopharmacological/Ethnobotanical/ Ethnomedicinal (n 24), preclinical bioactivity (n 9); and one clinical trial (n 1) studies. One paper includes both an in vivo study and an Ethnobotanical survey.

Quality Critical Assessment
Studies that reported ethnopharmacology, phytochemistry, experimental pharmacology, and related clinical data were assessed for eligibility. The Kew Botanic Royal Garden and Plants of the World Online web databases were consulted to verify the taxonomic information on the species mentioned. All species names were checked at the UOB University herbarium. The quality of animal experiments reported was evaluated by examining the peer-reviewed publication, statement of control of temperature, appropriate animal model, compliance with animal welfare regulations for preclinical experiments, random allocation to treatment or management, blinded assessment of outcome, allocation sample size calculation, statement of potential conflict of interests, concealment, use of cointerventions/co-morbid. We used a CAMARADES checklist to assess the quality. Each task was given a quality score out of a possible total of 10 points. Thus, studies were categorized into low quality for mean score 1-5 and high quality for mean score 6-10 (Hooijmans et al., 2014;Auboire et al., 2018). The quality assessment of clinical trials has been evaluated using the Jadad  scale for reporting randomized controlled trials based on randomization, blinding, withdrawals, and dropouts methods (Halpern and Douglas, 2007).

Statistical Values of Plant Species
Some indexes often express the frequency of quoting for botanical families and plant species. In the present review, the following indexes have been used: Frequency of citation (FC Number of times a particular species was mentioned/Total number of times that all species were mentioned x 100); Relative Frequency of Citation (RFC FC/N; 0<RFC<1): index, where FC is the number of informants who mentioned the use of the species and N, is the total number of informants (Tardío and Pardo-De-Santayana, 2008); Use Value (UV ƩU/n) where U is the number of usable reports for a given plant species cited by each informant and n is the total number of informants interviewed for a given plant (Bano et al., 2014). The Relative Importance Index (RII) of each plant species was calculated based on the normalized number of pharmacological properties attributed to it and the normalized number of body systems (BS) it affects (Bennett and Prance, 2000). shows different locations where 24 Ethnopharmacological/Ethnobotanical/Ethnomedicinal studies were conducted on the DRC map. The studies were done in Kisangani (R1) by Katemo et al. (2012), Mpiana et al. (2015); in Beni and Lubero (R2) by (Kasika et al., 2015); in Bukavu (R3) by (Karhagomba et al., 2013;Kasali et al., 2013Kasali et al., , 2021Chiribagula et al., 2020;Manya et al., 2020); in Mbandaka, Bagdolite, and Kungu (R4) by (Mongeke et al., 2018) ; in Lubumbashi, Kafubu, Kasumbalesa, Kipushi, Likasi and Sambwa (R5) by (Muya et al., 2014;Mbayo et al., 2016;Amuri et al., 2017Amuri et al., , 2018Bashige-Chiribagula et al., 2017;Mbuyi et al., 2019;Valentin et al., 2020)  ; in Kinshasa, Kwango and Kongo central (R6) by (Ngbolua et al., 2016a(Ngbolua et al., , 2016bLatham and Mbuta, 2017;Masunda et al., 2019;Pathy et al., 2021); in non-specified sites (R7) by (Moswa et al., 2005;Manzo, 2012). Table 1 describes the names, parts, forms used, locations, and some statistical values of plants cited. From 24 reviewed papers, we identified 213 plant species belonging to 72 botanical families.

Preclinical Pharmacological and Toxicological Investigations Inside Democratic Republic of Congo
Only seven plants presented in Figure 5 were exclusively studied in experimental animals inside DRC; Albizia adianthifolia, Azanza garckeana, Gladiolus klattianus, Panda oleosa, Raphia gentiliana, Rauvolfia caffra, and Vitex madiensis; five studied in DRC. It also reported the only plant species native exclusively from DRC.

Preclinical Toxicological Investigations Inside Democratic Republic of Congo
Many toxicological studies have been carried out in animals (rodents) using plant extracts. Some studies have been undertaken in mice, guinea pigs, and rabbits to explore the acute toxicity of Panda oleosa. Endpoints consisted mainly of mortality, pathophysiological syndromes, and microscopic examination of the pancreas and other vital organs pathological changes. The sub-chronic evaluation focused on assessing biochemical, hematological, and histopathological markers after a relatively long period (14 days and sometimes 90 days). The level of exposure to different organs, including the fetus, liver, kidney, heart, etc., of different doses of plant extracts was also determined. Thus, most plant extracts produce a toxic effect in specific organs or systems at high doses.                Mutoshi (

Clinical Trials Inside Democratic Republic of Congo
Data from the present study showed the lack of local clinical trials of antidiabetic plants used to manage Diabetes in the DRC. Of seven native herbals, only Raphia gentiliana fruit extract was given to 25 males and 20 females, aged 18-50 years old, with normal blood sugar levels (Mpiana et al., 2013). Thirty persons were submitted to the fruits of R. gentiliana as food (0.14 g/kg), while fifteen were introduced to the glucose solution (0.07 g/kg) (standard). The glycemia was measured by spectrophotometry, and the triangle surface area ratio method was used to calculate the glycemic and load index. The observed values of glycemic index and load were −3.1% and −1.36%. The approach followed by the authors did not comply with any clinical trial requirements, and instead, their behavior went like traditional healers themselves.

Phytochemical Investigations
Some studies have been undertaken to explore the chemical composition of Panda oleosa, Physalis peruviana, and Vernonia amygdalina.

Ethnobotanical Information Reported
The analysis presented in Table 2 showed that the ecological status was reported for 185(50.41%) plants and not for  (2017)  The errors in plant authors included entirely different authors, spelling mistakes, inappropriate use of the period, improper use of bracket, and incomplete author name.
The origin of plants was specified in 69.95% and not in 30.05% of species. However, 54.93% of plants with known origin were native to Africa, 14.55% species were introduced, and Droogmansia munamensis was the only species exclusively native to DRC flora ("Haut-Katanga"). Concerning the data quality, the author names of plant species were correctly written in 69.59% of cases, not correctly registered in 16.16%, or absent in 14.25%. Furthermore, 17.26% of plants had family names changed, and 81.64% not changed. In few cases (n 10), the main plant was confused with its synonym. For example, Antidesma metacarpus (A. membranaceum), Annona senegalensis (A. arenaria), Cassia alata (Senna alata), Chenopodium ambrosioides (Dysphania ambrosioides), Citrus x aurantium (C. sinensis), Hibiscus esculentus (Abelmoschus esculentus), Mitragyna stipulosa (Hallea stipulosa), Nauclea latifolia (Sarcocephalus latifolius), Solanum americanum (S. nigrum), and Solanum gilo (S. aethiopicum and S. subsessile). Most species (89.32%) from different sites were identified and authenticated in other herbariums or laboratories of ecology, but only a few (7.95%) had a voucher number published. This situation implicates the responsibilities of publishers and reviewers. Table 2 shows the quality analysis of findings compared to data from Plants of the World Online web (http://powo.science.kew.org) database and http:// plantsoftheworldonline.org via the Royal Botanic Garden Kew database.

Ethnopharmacological Data Reported
The country is home to different ethnic groups, making it one of the most diverse countries globally, with more than 200 other ethnic groups speaking an estimated 213 native languages. Sometimes referred to as the Baluba, the Luba people are the largest ethnic group. The community is native to the Kasai, Maniema, and Katanga regions. The Mongo people comprise several smaller constituent groups, including the Mbole, Ekonda, Boyela, Bolia, and Nkutu. The Kongo ethnic group is native to DRC and Angola, speaking Kongo alongside Lingala, Kyanzi, and Kintandu. The Mangbetu ethnicity is concentrated within the Orientale Province (Kisangani). The Zande people reside in the tropical rainforest and the savanna and speak nearly five dialects of the Azande language. The Pygmies are considered to have been some of the earliest peoples to inhabit the Congo River Basin. Their short stature characterizes them, they are mainly hunters and gatherers, and they occupy the rainforest. The plants are distributed within tropical and subtropical ecological regions, flooded grasslands, moist broad-leaf forests, savannas, and mangroves.
Fabaceae was the most representative family, consistent with other studies that showed this family is commonly found in tropical rain and dry forests in the Americans and Africa (Burnham and Johnson, 2004). Around 60% of the Congobasin lies in the DRC, the second-largest contiguous tract of tropical forests globally, and the greatest extent of tropical rainforests in Africa. It covers more than 100 million hectares (Abernethy et al., 2016).
The leaf was the most used part 122(39.23%), followed by roots 73(23.47%), and stem bark 43(13.83%). According to (Moshi et al., 2012), the frequent use of leaves is associated with ease of accessibility among the aboveground parts of plants in natural ecosystems. The formulations prepared consisted more often of decoction for 173(60.49%), followed by maceration 31(10.84%) and infusion 29(10.14%). However, in 24 cases (8.39%), the formulation has not been reported. Decoction has often been the effective formulation of herbal remedies as it is easy to prepare by mixing a drug with boiling water (Mahomoodally et al., 2016).
Out of 213 plants listed, 103(33.66%) were found at site R5 and 85(27.78%) at R6. The majority of plants had local vernacular names, except in few cases where the author did not mention the names. For instance, Catharanthus roseus was found at almost all locations (except site R4) and Allium cepa at five sites. However, A. sativum, Cassia alata, C. occidentalis, Mangifera indica, Persea americana, and Vernonia amygdalina were quoted at four locations.

Validation of Ethnopharmacological Data
Studies undertook outside DRC confirmed the use of the majority of plants cited as antidiabetic remedies. Albizia adianthifolia was the most reported antidiabetic with eight citations representing an RFC of 0.0063, followed by Catharanthus roseus 7(RFC 0.0055). However, Allium cepa, Annona senegalensis, and Cassia occidentalis were reported six times (RFC 0.0047), followed by Mangifera indica, Morinda morindoides, Phaseolus lunatus, and Vernonia amygdalina with five citations (RFC 0.0039).
Comparatively, Elaeis guineensis was endorsed by 28 uses; the (UV score of 0.0221 was the highest compared to Ocimum gratissimum (0.0150), Antidesma membranaceum (0.0142), Jatropha curcas (0.0126), Bridelia ferruginea (0.0118), and Quassia africana (0.0118). Also, Balanites aegyptiaca, which is employed to treat nine body systems, showed the highest Relative Importance Index (32.5%), compared to Vitis vinifera (28.7%), Zingiber officinale (27.1%), Solanum seretii (24.9%), Thomandersia hensii (24.9%), Lippia multiflora (23.2%) and Stachytarpheta indica (23.2%). The use-value indicators ( Table 1) are relative and susceptible to changing since, in the methodology, many authors generally limit themselves to the total number of participants in the studies. The lack of information on the number of informants interviewed was commonly observed in the reviewed studies. According to the Declaration of Helsinki, the direct consequence is that it is no longer possible to analyze the quantitative aspects of these studies (The World Medical Association, 2001). Consequently, it is not possible to quantify certain vital indexes such as the Cultural Importance Index (CII), Fidelity Level of Citation (FL), Family Use Value, Importance Consensus Factor (ICF), etc. It should be noted that some rare studies make an effort to investigate these parameters, although the information on the number of respondents remains a challenge. One of the weaknesses of ethnopharmacological surveys is that the respondents are often the healers themselves and not or seldom the users. The questionnaires do not scrutinize evidence on the number of people treated and outcomes.
The analysis of the accurate data for all 213 plants listed showed that most studies used rats and mice, and in a few cases, guinea pigs and rabbits. Both streptozocin (35.55%) and alloxan (24.64%) represented 60.19% of all in vivo reported study models. Streptozotocin presents many advantages over alloxan, including its longer half-life, more productive, stable, and selective to islet beta cells, less toxic, and causing less mortality in animal models   (Lenzen, 2008;Wang-fischer and Garyantes, 2018). The majority of the bioactivity investigations link the antioxidant or free radical-scavenging activity with the pathophysiology of Diabetes. However, currently, the in vitro antioxidant model using, for example, DPPH and the others, is not pharmacologically relevant. It can be used as a chemical screening tool. Only in vivo or cell-based models remain relevant (Heinrich et al., 2020). Enzymes are a frequent pharmacological target for establishing the mechanism of action of new drugs. Upon in vitro studies, alpha-glucosidase activity inhibition was the most common investigation (45.3%), followed by inhibition of PTP1B (13.8%), alpha-amylase (9.7%), DPP-4 (1.4%), and 11ß-HSD1 activity (1.0%). Additionally, in cell lines studies, glucose uptake (28.0%) was to be the most commonly used, followed by glucose uptake regulation markers such as GLUT4 translocation and expression levels (9.7%) and PPAR (9.6%) (Munhoz and Fröde, 2017). Some examples of studies are given below. Azadirachta indica aqueous leaf extract (400 mg/kg bw) improved levels of BG, serum insulin, lipid profile, insulin signaling molecules, and GLUT4 proteins in the tissue of high-fat fructose-induced type-2 diabetic male rat after 30 days of treatment, compared to the control group. In Goto-Kakizaki rats, the acetone extract of Syzygium cumini seed was a potent inhibitor of alpha-glucosidase hydrolysis of maltose compared to untreated control animals (Shinde et al., 2008). Moreover, hepatic tissue demonstrated increased PPARɣ and PPARα protein expressions (Sharma et al., 2012). Oryza sativa extracts significantly elevated glucose uptake, GLUT1, and GLUT4, mRNA levels (Boue et al., 2016). Ethanol extract induced a significant gain in GLUT4 on plasma membranes of L6-GLUT4myc muscle cells at no cytotoxic concentrations (Kadan et al., 2013). Choosing an experimental model is not easy and usually depends on many factors. Ideally, the experiments should be carried out in several different models, considering that none of them ultimately reflects the complexity of human diabetes mellitus type DMT2 and that precautions should be taken to extrapolate the findings to the clinical practice (Arias-Díaz and Balibrea, 2007).
A. adianthifolia is also used to treat syphilis, hiccups, diarrhea, malaria, indigestion, blueness, and an aphrodisiac. Oral administration of 500 mg/kg of plant extract reduced hyperglycemia by 57% in guinea pigs subject to OGTT . Albizia grandibracteata is used in filariasis and skin and urinary tract infections. A. garckeana is used in epilepsy and edema of the lower limbs. Oral administration of 500 mg/kg bw aqueous extract under OGTT conditions reduced fasting blood sugar to 36.9% compared to 49.6% of glibenclamide as the reference medicine . Certain parts of the plant may be toxic or contain cytotoxic compounds, particularly with gossypol for non-ruminant animals (Randel et al., 1992). Gladiolus gregarius is used to treat hemorrhoids and back pain. Gladiolus klattianus is used for gonorrhea and fever. Under OGTT conditions, the aqueous extract of G. klattianus reduced 35% of blood sugar after 60 min . Panda oleosa Pierre has been proposed for HIV/AIDS. The aqueous extract of P. oleosa (25-100 mg/kg) significantly reduced glucose levels in a dose-dependent manner in rabbits under OGTT conditions (Muhoya et al., 2017). Vitex madiensis is used in asthma, anemia, diarrhea, tuberculosis, cough, urinary tract infections, and intestinal amebiasis. The aqueous extract of V. madiensis (500 mg/kg bw) reduced hypoglycemia to 43% compared to 55% obtained with glibenclamide . Raffia gentiliana is used for inflammation. Oral administration of aqueous fruit extracts in mice under OGTT conditions demonstrated 27 and 56% reduction after one and 2 hours (Mpiana et al., 2013). a Only study carried out in DRC; Score ≥ 3(Good quality); Score < 3 (Poor quality).
Temperature and humidity were not reported. The effeteness of the treatment was based on the capacity of the extract to reduce baseline glycemia (hypoglycemia effect) or the capacity to reduce induced hyperglycemia; this varied between 25 and 75%, compared to reference drugs (glibenclamide and metformin). According to (Baker et al., 2014), over 85% of published animal
Frontiers in Pharmacology | www.frontiersin.org October 2021 | Volume 12 | Article 757090 studies do not describe randomization or blinding, and over 95% lack the estimation of sufficient sample size needed for detecting actual effects.

Toxicological Data
For acute toxicity, Figure 7 shows comparative values of LD 50 reported for Cola nitida, Sida acuta, Ficus sycomorus, Moringa oleifera, Panda oleosa, Alchornea cordifolia, Morinda lucida, Physalis peruviana, Musanga cecropioides, Vitis vinifera, Erythrina abyssinica, Persea americana, Jatropha curcas, Momordica charantia, and Rauwolfia caffra. Almost all plants but Jatropha curcas are relatively non-toxic (LD 50 > 500 mg/kg). The highest value of LD 50 was observed at 8,600 mg/kg bw in rodents with an oral administration of Alchornea cordifolia. The bark extract of Panda oleosa was practically non-toxic in guinea pigs with an LD 50 of approximately 7,892 mg/kg bw; no signs of intoxication were observed with oral doses less than 2,000 mg/kg. However, at doses above 6,000 mg/kg, poor mobility, poor appetite, anuria, and death have been noted in animals (Katemo et al., 2018). The administration of the aqueous extract from the bark of Ficus benghalensis suggested an LD 50 > 5,000 mg/kg. In some cases, the toxic effects depended on sex, like Alchornea cordifolia, which showed different values of LD 50 in mice male compared to female animals (8,600 mg/kg in male and 3,800 mg/kg female) (Djimeli et al., 2017). Despite low acute toxicity, many plants exhibit some significant sub-chronic toxicity. Caesalpinia bonduc extract showed hematological changes after a subchronic study for 28 days at a dose up to 400 mg/kg bw in rats (Ogunlana et al., 2013). Except for the ripe fruit, solanine and other alkaloids present in all parts of Solanum americanum are toxic (Kuete, 2014). Aloe-emodin (anthraquinone) from Aloe species could be mutagenic or/and genotoxic in organs (Lynch et al., 2011). A daily and prolonged administration (28 days) of resveratrol in Vitis vinifera exhibited nephrotoxicity in the rat at the high dose (3000 mg/kg bw). Extracts of Aframomum melegueta (286-345 mg/kg bw) and Artemisia annua (300 mg/kg bw) produced a toxic effect on the development of fetus by discontinuation of first trimester pregnancies in rats (Inegbenebor et al., 2009;Abolaji et al., 2012). Leaf methanol extract of Jatropha curcas decreased the number of live fetuses and increased placental weight (Teixeira et al., 2017). Bulbs' aqueous extract of Crinum ornatum had caused significant effects (Central Nervous System), including losing appetite, slow movement, depression, less aggression, and lying at the corners of the cage (Lawal and Dangoggo, 2014). Erythrina abyssinica showed similar nervous effects in mice, significantly decreasing motility, sedation, frequent urination, and tremors during the first 6 h after drug administration at different doses (Bunalema et al., 2011). Some compounds in Salvia officinalis (Camphor, thujone, and terpene ketones) are considered the most toxic. Their consumption is not recommended in pregnancy and lactation because they are harmful to the fetus and newborn (Ghorbani and Esmaeilizadeh, 2017). A methanol extract (500 and 1,000 mg/kg/day) of Catharanthus roseus in the subacute investigation for 14 days showed inevitable mortality and presented some of the signs of intoxication on the study of the liver and kidney rats (Kevin et al., 2012). Sometimes, there are some contradictions in findings from different authors on toxicological studies in animals. In Cassia occidentalis, Lagarto et al. (2011) and Mishra et al. (2018) are contradictory. The first group did not report any toxicological signs in biochemical, hematological, and morphological markers, while the second group noticed some changes.

Clinical Trials
Data from the present study showed the lack of local clinical trials of antidiabetic plants used to manage Diabetes in the DRC. Of seven native herbals, only Raphia gentiliana fruit extract was given to 25 males and 20 females, aged 18-50 years old, with normal blood sugar levels (Mpiana et al., 2013). The approach followed by the authors did not comply with any clinical trial requirements, and instead, their behavior went like traditional healers themselves. Table 5 illustrates the assessment of the quality of clinical trials of antidiabetic plants using the Jadad scale for reporting randomized controlled trials based on randomization, blinding, withdrawals, and dropout methods.
For example, a double-blind, placebo-controlled, randomized clinical trial conducted on 20-60 year-old DMT2 patients who did not receive insulin showed that 3 months supplementation of 3 g of ginger (Zingiber officinale) improved glycemic indices, total antioxidant capacity, malondialdehyde, C-reactive protein, serum paraoxonase, dietary intake, and physical activity, measured at the beginning and end of the study, and after 12 h fasting compared to control groups. A randomized, placebocontrolled, parallel-group study with 42 treated patients treated with leaf hydroethanolic extract (500 mg/8 h for 3 months) and 44 as placebo groups showed that the Salvia officinalis leaves lowered fasting glucose and HbA1c the baseline at the endpoint with no adverse effects reported. A clinical trial on a juice extract from the fruit of Morinda cordifolia (2 ml/kg bw once a day) in patients with DMT2, after 90 days of treatment, presented a significant reduction of morning BG in several cases, an improvement of hyperglycemia status. In a prospective, randomized, double-blind, placebocontrolled clinical investigation, the administration of Terminalia chebula (250 and 500 mg/kg bw, for 12 weeks) in 60 diabetic patients significantly improved the endothelial function (reflection index) compared to placebo (−2.55 ± 1.82%, and −5.21 ± 2.41%, respectively). In an 8-weeks randomized controlled clinical trial study of the effect of Trigonella foenum-graecum intake seed in 50 patients with T2DM, the plant significantly reduced fasting blood glucose. It improved some liver and kidney function compared with control interventions.
A randomized, double-blind, placebo-controlled clinical trial of Urtica dioica leaf extract (500 mg/8 h, 3 months) combined with conventional oral antihyperglycemic drugs was conducted in 46 treated patients vs 46 placebo groups. At the endpoint, the extract significantly lowered the blood levels of fasting glucose, 2 h postprandial glucose, and HbA1c, without significant effects on other hepatic or cardiovascular parameters, vs the placebo. All considered these results demonstrated that nettle is safe and may have a beneficial effect on glycemic control in patients with advanced DMT2 needing insulin therapy. Vernonia amygdalina elicited a significant reduction in BG levels at the most postprandial time points and area-under-curve.
Unfortunately, many studies were carried out in poor quality conditions, with unclear randomization methods, threats to blinding, and lack of baseline demographics (Rios et al., 2015). The interpretation of Jadad score on clinical trials reviewed showed that studies conducted on A. cepa, A. sativum, B. aegyptiaca, E. guineensis, M. charantia, R. vomitoria, S. officinalis, U. dioica, and Z. officinale presented excellent quality (Jadad score ≥ 3) (Hartling et al., 2011). In addition to the effectiveness of the plant materials (extracts, isolated compounds), clinical trials must include other vital parameters to an antidiabetic evaluation in particular glycosylated hemoglobin A1c (HbA1c), personal medication, insulin, glycogen, lipid and protein profiles, and severity of adverse effects, patient's risk factors, ease of use, patient's financial situation, etc. (Chaudhury et al., 2017).

Alkaloids
The sulfur compounds present in the onion can significantly control the blood glucose and lipids in serum and tissues and normalize liver hexokinase, glucose 6-phosphatase and HMG CoA reductase (Akash et al., 2014). It was shown that vindoline I, vindolidine II, vindolicine III, and vindolinine improve the hyperglycemia condition of type 2 diabetes by enhancing glucose uptake in pancreatic or muscle cells. In addition, they can inhibit in vitro PTP-1B, which lessens insulin resistance. Vindolicine III was the most potent (Tiong et al., 2013). Catharanthine, vindolinene, vinblastine, vincristine lower blood sugar levels through free radical scavenging action (Bharti et al., 2018). On the other hand, p-synephrine increased the glucose output concentration and ameliorated glycolysis and glycogenolysis (Suntar et al., 2018). N-trans-pcoumaroyloctopamine, N-trans-p-feruloyl-octopamine, N-transp-coumaroyltyramine, and N-trans-p-feruloyltyramine, amide alkaloids, showed alpha-glucosidase effect and free radicals inhibitions .

Amino Acids, Amines, and Carboxylic Acid Derivatives
Alliin offered protection against glucose or methylglyoxalinduced glycation of superoxide dismutase (Anwar and Younus, 2017). S-allyl cystein sulfoxide (SACS), allicin, and garlic oil precursor stimulated in vitro insulin secretion from beta cells isolated from normal rats (Kodera et al., 2017). It restored erectile function in diabetic rats . Unique and repeated intraperitoneal administrations of a protein (Mo-LPI) decreased blood glucose concentration at different times in rats. 2S, 3R,4S hydroxy isoleucine, an amino acid considered an insulinotropic agent, possesses antidiabetic potential by several mechanisms, including regulating glucose metabolism, lipid profile, and uric acid (Rangari et al., 2014).

Carbohydrates and Sucrose Esters
Peruvioses A,B,C,D,E, and F possess antidiabetic potential by alpha-amylase inhibition activity (Bernal et al., 2018). In the Streptozotocin-induced diabetic mice group, rhamnogalacturonan (a polysaccharide) decreased blood glucose level and glucose tolerance and slightly improved blood glucose within 30 min (Liu et al., 2017). Polysaccharides repaired the pancreatic β cells damages in a high-fat diet STZinduced type 2 diabetic mice by improvement of SOD concentration and the reduction of MDA level and restoration of kidney and pancreas tissues (Wang et al., 2019). Furthermore, a water-soluble polysaccharide significantly lowered fasting blood glucose level and improved glucose tolerance and weight loss in alloxan-induced diabetic mice compared to the diabetic control group (Xu et al., 2015).
Supplementation of Naringin improved glucose intolerance and insulin resistance in a model of high-fat-diet-fed mice (Pu et al., 2012). Naringin (together with Neohesperidin, hesperidin, and nobiletin) significantly inhibited amylase-catalyzed starch digestion and played roles in hyperglycemia management by increasing hepatic glycolysis and glycogen concentration and lowering hepatic gluconeogenesis. Furthermore, hesperidin, naringin, and nobiletin reduced hepatic gluconeogenesis and improved insulin sensitivity in animal models (Lv et al., 2015).
Neohesperidin significantly decreased fasting glucose, serum glucose, and glycosylated serum protein in mice. In addition, this compound significantly reduced serum triglycerides, total cholesterol, leptin level, and liver index; it inhibited lipid accumulation in the liver and decreased the size of epididymal adipocytes in the KK-Ay mice (Osfor et al., 2013;Jia et al., 2015).
Galactomannan showed significant dose-related hypoglycaemic and antihyperglycaemic effects; the obtained results were better than glibenclamide used as reference (Anwar et al., 2011).
Aloe-emodin-8-O-glycoside enhanced glucose transport through proximal and distal marker modulation involved in glucose uptake and its transformation into glycogen (Salehi et al., 2018).
Syringin, a phenylpropanoid glucoside, indicated a significant reduction of blood glucose and HbA1c levels and improved transaminase enzymes, plasma protein, blood urea, serum creatinine, and uric acid levels. Inversely, it increased plasma insulin and hemoglobin levels in diabetic rats (Sundaram et al., 2014).
Rutin (a flavonol glycoside) significantly increased in vivo glucose-induced insulin secretion and acted as an insulin secretagogue in the management of glucose homeostasis (Kappel et al., 2013). Hirsutrin was suggested to prevent osmotic stress in hyperglycemia conditions by inhibiting RLAR activity and galactitol formation in rat lenses .
According to Fernando et al. (2019), three flavone C-glycosides, vicenin-1, isoschaftoside, and schaftoside, respectively, inhibited 60.3, 33.8, and 95.5% of pancreatic lipase enzyme, which plays a vital role in obesity (as a crucial factor in the occurrence of DMT2). Phenolic-C-glycosides enhanced and stimulated the glucose update process in mouse skeletal muscle cells (Mishra et al., 2013).
Cinnamic acid and its derivatives (caffeic acid, ferulic acid, isoferulic acid, and p-hydroxycinnamic acid) are associated with a beneficial influence on Diabetes and its complications through many mechanisms. The most well-known are: stimulation of insulin secretion, improvement of pancreatic β-cell functionality, inhibition of hepatic gluconeogenesis, enhanced glucose uptake, increased insulin signaling pathway, delay of carbohydrate digestion and glucose absorption, and inhibition of protein glycation (Adisakwattana, 2017). Ferulic acid regenerated pancreatic beta-cells, reduced the risk of high-fat diet-induced hyperglycemia via insulin secretion and hepatic glucoseregulating enzyme activities, and regulated blood glucose levels by elevating glucokinase activity and production of glycogen (Silva and Batista, 2017). Caffeic acid produced a significant alpha-glucosidase inhibition comparing with acarbose (Olennikov and Kashchenko, 2014). In addition, together with chlorogenic acid and chicoric acid, it increased glucose uptake in muscle cells and stimulated insulin secretion from an insulin-sensitizing and insulin-secreting cell line and islets (Tousch et al., 2008;Ferrare et al., 2017). Caffeoylquinic acid derived from caffeic acid showed high inhibitory activity against digestive enzymes, exceptionally higher against alpha-amylase and alpha-glucosidase (Olennikov et al., 2018).
Compared with the reference compound acarbose, Aesculetin and isorhamnetin demonstrated significantly higher inhibitory activity (Olennikov and Kashchenko, 2014). Polyphenols Frontiers in Pharmacology | www.frontiersin.org October 2021 | Volume 12 | Article 757090 compounds such as proanthocyanidins and anthocyanins showed as potential natural alpha-glucosidase inhibitors (Dey and Mitra, 2013). Anthocyanins efficiently protected pancreatic beta-cells from cell death in HIT-T15 cell culture and db/db mice (Hong et al., 2013). Johnson et al. (2015) demonstrated that prenylated anthranols possess an alpha-glucosidase inhibitory potential. According to their findings, the most antidiabetic activity was found with harunganol compared to acarbose.
Chicoric acid lowered the glycaemic levels of diabetic mice (Casanova et al., 2014) significantly. Valoneic acid dilactone, a hydrolyzable tannin, showed a potential antidiabetic effect alphaamylase enzyme activity compared to the value obtained by acarbose. In the same way, it significantly inhibited aldose reductase enzyme activity and PTP1B enzyme activity. However, in vivo evaluation, it reduced the BGL considerably in acute evaluation for 4 h. Furthermore, oral administration of the compound for 21 days significantly decreased BGL and improved the tolerance to glucose compared to control groups (Jain et al., 2012). Gingerols demonstrated antidiabetic potential by enhancing glucose uptake. Primarily, (S)-[8]-Gingerol was found to be the most potent on glucose uptake and increase in the surface distribution of GLUT4 protein on the L6 myotube plasma membrane (Noipha and Ninla-aesong, 2018).

Saponins
Pseudoprototinosaponin AIII and prototinosaponins AIII produced a hypoglycaemic effect on glucose uptake and insulin release due to their actions on hepatic gluconeogenesis or glycogenolysis (Patel et al., 2012). Furostanol saponin showed significant antidiabetic potential in vitro by reducing the fasting plasma glucose level by 46.14% and increasing insulin and C-peptide levels . [3β,7β,momordicine I,momordicine II,and kuguaglycoside G] were potent in the β-cell insulin secretion evaluation. Momordicine II and kuguaglycoside have stimulated insulin secretion 7.3 and 7.1 times and 8.1 and 7.8 times more, respectively, than the control group (Keller et al., 2011).

Terpenoids
Oleanolic acid, a plant-derived triterpenoid, boosted insulin secretion in vitro and stimulated insulin secretion at both basal and stimulatory glucose concentrations in INS-1 832/13 cells, and enhanced acute glucose-stimulated insulin secretion cultured β-cells (Teodoro et al., 2008). Furthermore, it decreased serum glucose and insulin concentrations in mice fed with a high-fat diet and enhanced glucose tolerance (Sato et al., 2007). Oleanolic and ursolic acids showed potent alphaglucosidase and alpha-amylase inhibition. Ursolic acid showed uncompetitive inhibition of alpha-glucosidase compared to acarbose as a competitive inhibitor (Ali et al., 2006;Salah El Dine et al., 2014).
Thujone, a monoterpene existing as two stereoisomers (α-and β-Thujone), is an ingredient of essential oils of many great different herbs; it can increase free insulin-stimulated glucose transporter by activation of adenosine monophosphate-activated protein kinase (Daradka et al., 2014).
α-amyrin acetate (a pentacyclic triterpenoid) lowered the blood glucose profile in STZ-induced diabetic rats and db/db mice at 50 mg kg1 dose level (Singh et al., 2009). Pahlavani et al. (2019) showed that some compounds like charantin (a triterpenoid phytoconstituent), possess antidiabetic potential by several mechanisms, including insulin secretion increase, insulin resistance decrease, skeletal muscle cell glucose utilization increase, and inhibition of intestinal enzymes.

CONCLUSION
Multiple investigations have been carried out on natural products, mainly plants used to treat Diabetes Mellitus worldwide. In DRC, a country with a high ecological, cultural and human diversity, traditional medicine through plants occupies an important place in the health system. Several ethnopharmacological and ethnobotanical studies have been conducted previously in this perspective, and various plant species have been identified. Contrary to the previous review, the present review assessed the quality of studies carried inside DRC and resorted similarities/discrepancies with studies conducted outside. The findings confirm the high diversity of the flora and the various ethnic groups in DRC. Most of the plants claimed as antidiabetic and used by traditional healers in the DRC are not specifically native to DRC. One hundred thirty-four native and introduced species have been experimentally validated by various pharmacological, toxicological, and phytochemical researches. Many plants are safe at doses < 500 mg/kg, but long-term use may trigger sub-chronic toxicity. Exclusively conducted in DRC, preclinical and clinical studies of some plant species demonstrated poor protocol quality. Locally specific species deserve in-depth investigations to meet scientific requirements for their introduction studies into the national pharmacopeia. Although a few plants reduced blood sugar levels, clinical data and antidiabetic studies of the isolated compounds remain limited to allow the availability and accessibility of standardized phytomedicines to Congolese. This review constitutes a primary database for further experimental studies, especially for unstudied species in the perspective of safe and efficient use of easily accessible natural resources.

AUTHOR CONTRIBUTIONS
FK conceived the manuscript, wrote the first draft, and analyzed data. JK analyzed data and rewrote the manuscript draft. All authors conducted the literature search. All authors read, corrected, and approved the final manuscript.