Diabetes is one of the chronic metabolic diseases and it is characterized by increased blood glucose level. Diabetes is noted as one of the leading causes of mortality in the world (Kishore et al., 2016). In Malaysia, 17.5% of the total population are affected by this disease (Tee and Yap, 2017). There are various factors which can contribute to development of this disease such as genetic factor or environmental factors (Skyler et al., 2016). As blood glucose increase in our body, it will lead to various diabetic complications such as microvascular disease, macrovascular disease as well as neurological diseases (Duh, Sun, and Stitt, 2017). There are macro-vessels and micro-vessels found in our body which has different function (Chawla et al., 2016). The macro-vessels mainly supply blood to organs while micro-vessels function as supply nutrient and maintaining the blood pressure (Chawla et al., 2016). In diabetic patient, high blood glucose level will affect the vessels’ function and cause various complication such as cardiovascular disease, diabetic retinopathy, diabetic nephropathy, Alzheimer’s disease, and more (Huang et al., 2019).

Due to long term of hyperglycemia, the glucose found in blood tend to form covalent adduct with A protein through glycation while the end-products are known as Advanced Glycation End-product (GE) (Singh, Bali, Singh, and Jaggi, 2014). These abnormal proteins will lose its function and will lead to development of diabetic complications (Borg and Forbes, 2016). Besides, it also will react with other components such as lipid and nucleic acid (Singh et al., 2014). In the glycation process, it can be divided into three stages (Singh et al., 2014). The formation of Schiff base is the first step of glycation followed by development of Amadori product (Ho et al., 2024). Finally, AGEs are formed which is the last stage and it is an irreversible process.

The pathogenesis of the diabetic complications is mainly due to interaction between Advanced Glycation End-product (AGE) and Receptor of AGE known as (RAGE) (Yong et al., 2024). RAGE is a multiligand family, when AGE binds to the receptor, it will trigger various pathways such as Janus Kinase-Signal Transduced and Activator of Transcription protein (JAK/STAT), Nicotinamide Adenine Dinucleotide Phosphate (NADPH) oxidase and Mitogen Activated Protein Kinase (MAPK) pathway which lead to release of various components such as Nuclear Factor-kb (NF-ĸB), Interferon-Stimulated Response Elements (IFN-SRE) that lead to increased oxidative stress and release of inflammatory cytokines and lead to apoptosis (Singh et al., 2014).

There are a lot of drugs developed to inhibit the Advanced Glycation End-product (AGE) which plays an important role in diabetic complication. The example of drugs is aminoguanidine and metformin. The aminoguanidine are the first AGE inhibitor and it can inhibit the dicarbonyl compound formation in the intermediate stage of glycation process and eventually inhibit the formation of AGE (Borg and Forbes, 2016; Ho et al., 2024; Yong, Shirley, Azzani, Anbazhagan, and Ng, 2024; Yong, Wong, Azzani, Mac Guad, and Ng, 2024; Ho, Yong, Lim, and Ng, 2024). For metformin, this drug can activate the 5’AMP-Activated Protein Kinase (AMPK) pathway which can help to reduce the glucose level by reducing the glucose production in liver (Rodrigues et al., 2017; Rhee and Kim, 2018; Ramasamy, Yan, and Schmidt, 2011; Rena, Hardie, and Pearson, 2017). However, these drugs possess different side effects such as increase liver enzyme, formation of autoantibodies, dizziness, muscle pain, and allergic reaction (Nasri and Rafieian-Kopaei, 2014). Hence, an alternative treatment for diabetic complication is needed. Recently, there are many researchers focusing on natural products, as they contain potential Advanced Glycation End-product (AGE) inhibitor (Yang, Li, Yin, Chen, and Gao, 2016). For an example, Coptis chinensis, which also known as Huang Lian has the potential to inhibit the formation of AGE (Yang, Li, Yin, Chen, and Gao, 2016; Jiang et al., 2015). Other medicinal plants such as Eucommia ulmoides, and Bacopa monnieri are shown to be effective in inhibition of Advanced Glycation End-product (AGE) (Kishore et al., 2016; Hussein and Mahfouz, 2016). Furthermore, these medicinal plants can also inhibit the formation of AGEs, the receptor of AGE as well as increase serum insulin level through regeneration of beta pancreatic cells (Kishore, Kaur, and Singh, 2017).

The aim of this systematic review was to provide insights of in vitro studies on natural products which inhibited the advanced glycation end-product that causes the diabetic complications.

This systematic review was performed by using three different search engines and databases which were PubMed, ScienceDirect as well as Scopus. Besides, manual searches based on the references found in the reviewed articles was also carried out.

The search strategy was carried out based on the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (Moher, Liberati, Tetzlaff, Altman, and PRISMA Group, 2009). The flowchart of the searching process, searching criteria as well as selecting of articles was shown in Figure 1. The search strategy included the research papers which evaluated the efficiency of natural product to inhibit the formation of advanced glycation end-product. The search was done by using catalogue descriptors in Medical Subject Heading (MeSH) in English which contained in the title, abstract as well as in text of the studies.

Figure 1:
Flowchart of search criteria and study selection.

The selection of articles was screened and identified by two authors independently. The full texts that fulfilled the inclusion criteria were obtained. In addition, disagreement between two authors based on the selection was resolved by discussion.

Studies that fulfilled the inclusion criteria were included. All the research papers were searched for studies were published between January 2015 and December 2019. Besides, the studies only limited to publications in English. All the in vivo and in vitro studies on advanced glycation end-product were eligible for inclusion criteria.

For exclusion criteria, review articles, letters, editorials, conference proceeding as well as case reports studies were excluded in the study. In the first screening, those studies that were not relevant were rejected. Besides, those studies that contain only abstracts or incomplete data were also excluded. For the second screening, all the full text articles were screened and those articles that do not fulfilled the inclusion criteria were removed.

After performing selection of articles based on inclusion and exclusion criteria, extraction of data was performed by the authors. Last name of the first author, publication year as well as country of the study were extracted. Besides, all the important information from the papers such as test model, sample modification, intervention, control vintage, measurement of advanced glycation end-product, assay parameter as well as the outcomes of the studies were extracted. Multiple papers which contained same measurement will be grouped together and considered as one unique study paper.

Thirteen articles were selected and the important information from each article was summarized in Table 1. Among the 13 articles, different methods were used for the measurement of Advanced Glycation End-product (AGE). Among them, there were eight articles which focused on percentage inhibition of AGE in bovine serum albumin or human serum albumin while three articles were measured based on IC₅₀ value of the AGE. There were two articles measuring the different stages of glycation.

As mentioned above, the formation of Advanced Glycation End-product (AGE) can be divided into three stages and these articles were measuring the formation of Amadori product, dicarbonyl product as well as AGE. From Hung et al., (2017) it measured Amadori and dicarbonyl product while in Yang et al., (2016) it measured all three stages (Yang et al., 2016; Hung et al., 2017). The measurement of Amadori and dicarbonyl product from these two articles were based on NBT assay and Girard-T assay. Among 13 articles, there were four articles in which the exact value of the inhibition or IC₅₀ were unknown. In summary, there were 13 articles measuring the glycation inhibition assay while one article measured both glycation inhibition assay and different stages of AGE. There is only one article that measured based on stages of AGE (Hung et al., 2017). Among these 13 articles reviewed, 12 articles used bovine serum albumin as experimental sample while another one article used human serum (Poornima et al., 2015). For the control vintage, the range were from 24 hr to 42 days while the temperature was kept ranged between 37ºC to 80ºC.

As mentioned above, there were eight articles measuring the percentage inhibition of Advanced Glycation End-product (AGE). In Baseni et al., (2017) which used Seriphium plumosum leaves as model showed the most potent glycation inhibition found in acetone crude extract which was 2.22% AGE formation followed by hexane crude extract and methanol crude extract having value of 4.9% and 7.3% AGE formation (Beseni et al., 2017). In another article from Baseni et al., (2019) which used Tonna siliata and Schkuhria pinnata leaves as model also showed positive effect on inhibition (Beseni et al., 2019). Among the different crude extracts, methanolic crude extract of Tonna siliata showed highest inhibition which gave result of 2.49% followed by acetone extract and hexane crude extract which gave 2.56% and 2.79%, respectively.

In another article from Kishore et al., (2016), the ethanol and hydro-alcohol crude extract from Bacopa monnieri with different concentration ranging from 50-500 ug/mL gave glycation inhibition ranging from 36.34-92.91% and 31.88-93.37%, respectively (Kishore et al., 2016). In this article, Aminoguanidine was used as control and gave 93.37% inhibition which have similar effect with 500 ug/mL concentration of Bacopa Monnieri. Another article, which used Canarium album fruit with the core removed, showed that the water-ethanolic extract had the highest inhibition, followed by methanolic crude extract, water extract, ethanolic extract, acetone extract, and ethyl acetate extract (Kuo et al., 2015). The acetone extract and ethyl acetate crude extract gave similar effects.

Besides, three articles from Do et al., (2017, 2018a, 2018b) which used 70% of ethanolic crude extract from medicinal plants such as Lespedeza bicolor, Eucomnia ulmoides and Spatholobus suberectus showed dose-dependent manner in inhibition of Advanced Glycation End-product (AGE) (Do et al., 2017, 2018a, 2018b).

For the measurement of IC₅₀, three articles were selected. First, in Ahmad et al., (2016) which used 80% methanolic crude extract of both Ziziphus oxyphylla and Cedrela serrata leaves were tested (Ahmad et al., 2016). From the crude extract, its sub-fractions- n-hexane, chloroform, ethyl acetate and n-butanol were used. However, only the ethyl acetate fraction was sub-fractionated, yielding M1, M2, M3, and M4. In this sub-fraction, there were seven compounds isolated by High Pressure Liquid Chromatography (HPLC). Among the sub-fraction, M3 gave most potent IC~50~ values which was 541±19 followed by M1, M2 and M4 which were 586±21, 589±21 and 593±21, respectively. In this experiment, aminoguanidine was used as positive control which gave value of 510±18. Among the seven compounds, compound 2 had the lowest IC~50~ value (530±19) while compound 7 had the largest IC₅₀ value (818±29).

In Deo et al., (2016), five different plants were used as models. There were Petalostigma pubescens, Petalostigma banksia, Memecylon pauciflorum, Millettia pinnata and Grewa nesomischa (Deo et al., 2016). Different parts of the plant such as leaf, fruit, root, inner bark and bark were used in the experiment. Among them, leaves from Petalostigma banksia was significant higher with IC~50~ value of 56.06±6.10. In Dzib-Guerrea et al., (2016), nine different types of medicinal plants were used (Dzib-Guerra et al., 2016). There were Brosimum alicastrum (BAL) Swartz, Bunchosia swartziana Griseb, Ehretia tinifolia (L.), Manilkara zapota (L.) P. royen, Cassia fistula (L.), Cocos nucifera (L.), Ocimum campechianum Willdenow, Piper auritum Kunth, Rhizophora mangle (L.). The leaves, stems, and roots of each plant were tested in the form of ethanol and ethyl acetate extracts, except for Cocos nucifera, which was tested as an aqueous extract. However, the results measured vesperlysine-like AGE and pentosidine-like AGE.

In Yang et al., (2016), Coptis chinensis polysaccharide was tested in the form of a water extract (Yang et al., 2016). The glycation inhibition assay and NBT assay showed a dose-dependent effect, while the Girard-T assay demonstrated a comparable inhibitory effect. However, the exact inhibition values were not reported in the results.

Lastly, in Hung et al., (2017), the aerial parts of Siegesbeckia orientalis were tested (Hung et al., 2017). The 95% ethanol crude extract was fractionated into n-hexane, ethyl acetate, and methanol. In the experiment, this plant exhibited a high inhibitory effect on dicarbonyl compounds and a low inhibitory effect on Amadori products. In the NBT assay, the ethanol extract showed the highest inhibitory effect (24.9%), followed by n-hexane (18.8%), methanol (17.2%), and ethyl acetate (15.6%). In the Girard-T assay, the ethyl acetate extract showed the highest inhibition (61.9%), followed by n-hexane (47.3%), ethanol (46.5%), and methanol (28.2%).

Hyperglycemia is the principal cause of diabetic complications. It leads to increased production of Reactive Oxygen Species (ROS), breakdown of starch by α-amylase, α-glucosidase as well as development of Advanced Glycation End-product (AGE) (Deo et al., 2016). As mentioned above, AGEs are the key component in development of diabetic complications. Glycation, a spontaneous reaction of sugars with proteins is one of the sources of developing oxidative stress in our body (Beseni et al., 2019). The reaction forms a stable product known as Amadori product before it turns into dicarbonyl compound such as methylglyoxal and glyoxal. Eventually, AGE is formed from the reaction which then circulate in our body to develop complications (Hung et al., 2017). Nephropathy is one of the diabetic complications developed due to AGEs (Do et al., 2018a). Kidney renal proximal tubule cells are known to absorb the circulating AGEs from glomerular filtrate and detoxify it (Do et al., 2018a). Hence, elevation of AGEs in body can result in accumulation of AGEs, triggering various pathway which lead to increase in ROS and eventually causing permanent damage to renal tubule (Do et al., 2018b). Besides, AGEs also affect the function of the liver. In the study by Yang et al., (2016), diabetes causes liver destruction and decrease the function of the hepatocyte through generation of AGEs (Yang et al., 2016).

Recently, various studies have discussed the pathological pathways of AGEs. The apoptosis mechanism mediated by the p53 protein plays an important role in diabetic complications (Hori, Kuno, Hosoda, and Horio, 2013). When AGE levels increase, oxidative stress in the cell rises, triggering the activation of the B-cell lymphoma 2 (Bcl-2) family and Bcl-2-associated X protein (Bax). This leads to increased mitochondrial permeability, ultimately resulting in cell death (Brunelle and Letai, 2009). Another pathway that induces diabetic complications is the activation of the Receptor for Advanced Glycation End products (RAGE) by AGEs (RAGE-AGE activation) (Do et al., 2018a). RAGE is a multiligand receptor of the immunoglobulin family (Ramasamy et al., 2008). This receptor is normally expressed at low levels but increases in expression when AGE accumulates. When AGE binds to RAGE, it activates various intracellular signaling pathways, such as Mitogen-Activated Protein Kinase (MAPK), which subsequently activates Nuclear Factor-kB (NF-kB). This leads to the release of pro-inflammatory cytokines, including interleukin-6 and tumor necrosis factor-α, ultimately causing cell death (Do et al., 2018a).

Another mechanism by which AGE contributes to diabetic complications is through the polyol pathway (Kishore et al., 2016). Aldose reductase, the first and rate-limiting enzyme in this pathway, is responsible for converting glucose into sorbitol (Reddy et al., 2014). Under euglycemic conditions, glucose is metabolized into pyruvate. However, when in hyperglycemia condition, glucose will then enter polyol pathway and metabolized into sorbitol (Reddy et al., 2014). As a result, increase oxidative stress and over production of sorbitol which increases the osmotic stress in the cell will lead to cell death and causes diabetic complications (Kishore et al., 2016).

To reduce the diabetic complication, compound that can inhibit the AGE could be useful. To date, there are various types of drugs which can inhibit the formation of AGE such as metformin as well as Aminoguanidine. However, the poor development prospects of these drugs and adverse effects are not safe to human (Ramkissoon, Mahomoodally, Subratty, and Ahmed, 2016). Hence, an alternative method needed to be found to reduce the diabetic complication. Recently, many researchers focused on medicinal plants as an alternative way to treat diabetic complications (Kishore et al., 2016). Many studies had shown that there are potential of medicinal plant in inhibiting the formation of AGE (Beseni et al., 2017).

Studies by Ahmad et al., (2016), Deo et al., (2016) and Dzib-Guerra et al., (2016), which measures the IC~50~ values of medicinal plants extracts on fluorescence AGE, it is found that several medicinal plants have similar effect compared to aminoguanidine which is a type of anti-AGE drug (Ahmad et al., (2016), Deo et al., (2016) and Dzib-Guerra et al., 2016). In the study by Ahmad et al., (2016), compound 2 exerted a similar effect to aminoguanidine. Also, compound 1, 2, 3, 5, 6 and mixture 3 also showed similar effect (Ahmadet al., 2016). Besides, mixture of compound showed greater effects, and this might be due to synergistic effect of flavonoid glycoside (Ahmad et al., 2016). In Deo et al., (2016), Petalostigma banksia showed greatest effect as it possess lowest IC~50~ values (Deo et al., 2016). The antiglycation effect on medicinal plant are closely related to antioxidant properties (Mahomoodally, Subratty, Gurib-Fakim, and Choudhary, 2012; Sadowska-Bartosz and Bartosz, 2015). However, the studies do not show any correlation between antiglycation and antioxidant activities (Deo et al., 2016). This suggested that different medicinal plants may have their independent pathway in inhibition of protein glycation (Deo et al., 2016). In a study by Dzib-Guerra et al., (2016), the root extract of C. fistula had greater effect on vesperlysine and pentosidine like AGE compared to aminoguanidine (Dzib-Guerra et al., 2016). It is also shown that there is no correlation between antioxidant and antiglycation in this study (Dzib-Guerra et al., 2016).

Besides, in Baseni et al., (2017 and 2019), methanolic, hexane and acetone extract of Toona ciliata, Seriphium plumosum and methanolic extract of Schkuhria pinnata showed greater AGE inhibition compared to positive control which is Arbutin (Beseni et al., 2017, 2019). It is shown that more polar solvents have better yield compared to less polar solvents as plant contain more polar compounds (Beseni et al., 2019). The lower effect of Schkuhria pinnata compared to positive control may be due to highly fibrous nature of its leaves which contain a lot of non-soluble component that reduce the therapeutic function (Kumar, Sinha, Makkar, de Boeck, and Becker, 2012). In studies by Do et al., (2017, 2018a, 2018b), which used 70% ethanolic extracts of medicinal plants, showed a dose-dependent results (Do et al., 2017, 2018a, 2018b). They showed a similar effect to aminoguanidine as a positive control at higher concentrations (Do et al., 2017, 2018a, 2018b). A similar result was observed in a study (Kishore et al., 2017) that also used aminoguanidine as a positive control. Free radicals were shown to increase the formation of AGE (Rains and Jain, 2011). As a result, phenolic antioxidants found in the medicinal plants gained attention as potent AGE inhibitors (Ramkissoon et al., 2013). As water-ethanolic extract of Canarium album contain highest yield of phenolic compounds, it showed greatest AGE inhibition (Kuo et al., 2015). According to Xie and Chen (2013), different flavonoids will have different inhibitory activity against AGE due to structural differences (Xie and Chen, 2013). In High Pressure Liquid Chromatography (HPLC) analysis, it was shown that three of the flavonoid compounds including genistein, quercetin and naringenin significantly inhibit the formation of AGE (Do et al., 2018b). As quercetin and genistein have hydroxyl group at carbon number 5, it can increase the free amines which the latter can breakdown the AGE (Do et al., 2018b). Besides, quercetin and genistein were found to have inhibitory effects in AGE formation (Do et al., 2018b). This is because these compounds have Thiazole ring derivative which can break the Maillard reaction crosslink via thiazolium structure (Gkogkolou and Böhm, 2012). Additionally, these compounds can attach to the pyrrole carbon ring in AGEs, making it susceptible to nucleophilic attacks, which eventually lead to the breakdown of AGEs (Kim, Kim, Moon, and Kim, 2015). In studies by Do et al., (2018a and 2018b), Spatholobus suberectus and Eucommia ulmoides were able to increase the expression and activation of Glo1 protein which play a significant role in suppressing the formation of AGE (Doet al., 2018a, 2018b). This pathway is important in detoxifying reactive dicarbonyl compound which is converted into D-lactate (Do et al., 2018a). In addition, increased expression of NQO1 and HO-1 by S. suberectus also can inhibit formation of AGE (Do et al., 2018b). These molecules are expressed by Nrf2 transcription factor which regulate the Antioxidant Response Elements (AREs) (Spoto, Pisano, and Zoccali, 2016). By upregulating these molecules expression via Nrf2 pathway, it can inhibit the accumulation and formation of AGEs as well as reduce the expression of RAGE (Do et al., 2018a). Moreover, Bacopa monnieri extract which contain triterpenoid is able to stimulate release of insulin and reduce oxidative stress (Dewanjee, Das, Sahu, and Gangopadhyay, 2009). Also, this medicinal plant contains various saponins such as bacopasaponin A, B, C, D and pseudojujubogenin which are potent radical scavenger and are found to have renoprotective effect as well as reduce fasting blood glucose level (Kishore et al., 2016). Also, phytosterols found in this medicinal plant having similar effect as stigmasterol in preventing diabetic complications by decreasing the oxidative stress and increase antioxidant levels (Nualkaew, Padee, and Chusri, 2015).

In a nutshell, AGE is one of the key components that induce various diabetic complications. The active compounds found in medicinal plants can reduce AGE formation, making them a potential treatment for diabetic complications, as they have fewer side effects compared to synthetic drugs.