An Overview of the Pharmacogenetics of Sulfonylurea in Type 2 Diabetes Mellitus

Background and Objective: Diabetes mellitus (DM) is a prevalent disease, with its prevalence increasing over the past few decades, posing a significant public health challenge. Type 2 diabetes mellitus (T2DM) is a chronic condition characterized by abnormal blood glucose levels due to insulin deficiency or resistance. This review delves into the pharmacogenetic implications of sulfonylurea (SU) therapy, elucidating the impact of genetic variations on SUs response, SU-induced hypoglycemia, and the development of secondary failure to this drug among T2DM patients. Methods: The data was obtained through a search on Pubmed and Google Scholar using the following keywords: ‘Sulfonylurea’, ‘Type 2 diabetes mellitus’, ‘genetic’, ‘polymorphism’, ‘SNP’, ‘drug response’, ‘pharmacogenomics’, and “precision medicine”. Results: Our analysis suggests that genetic variations could significantly influence the response to SU therapy, the risk of hypoglycemia associated with SUs, and the occurrence of secondary failure. However, this review reveals conflicting outcomes for different genes/variants that may be due to the heterogeneity among previous studies. Conclusions: Translating these findings into clinical practice remains a major challenge, underscoring the critical need for more extensive and standardized research to generate precise data. Such data can then be used to develop precision medicine for T2DM and improving patient outcomes.


INTRODUCTION
Diabetes mellitus (DM) is a metabolic disorder that occurs as a result of decreased insulin activity 1 .It is a prevalent disease in the modern world 2 , with its prevalence increasing over the past few decades, posing a significant public health challenge 3 .According to the latest edition of the authoritative resource on global impact of diabetes (IDF Diabetes Atlas), more than 537 million people worldwide suffer from diabetes, and this number is projected to reach 643 million by 2030 4 .
DM is categorized into type 1 diabetes, type 2 diabetes, other types of diabetes mellitus, and gestational diabetes mellitus. 5Type 2 diabetes mellitus (T2DM) is a chronic condition characterized by abnormal blood glucose levels due to insulin deficiency or resistance 6 The pathogenesis of T2DM involves various factors, including environmental factors, unhealthy eating habits, (KATP) channels are potassium channels regulated by adenosine triphosphate (ATP) and adenosine diphosphate (ADP), control membrane potentialdependent processes to meet metabolic needs 15 .These channels are composed of octameric protein complexes, with major subunit of the ATP-sensitive K+ channel KIR6.1 or major subunit of the ATP-sensitive K+ channel (KIR6.2) protein assembly encoded by KCNJ8 and KCNJ11 genes, respectively, surrounded by sulfonylurea receptor 1 (SUR1) or SUR2A/B proteins encoded by ABCC8 and ABCC9 genes, respectively 16 .Structurally kir6.2 and SUR1 consists of four poreforming subunits surrounding the pore of the KATP channel on the plasma membrane of pancreatic ß-cell.The closure of these channels initiates insulin secretion, while their opening inhibits it 17 .
Loss-of-function mutations in KCNJ11 and ABCC8 genes have been associated with congenital forms of hyperinsulinemia and hypoglycemia, indicating the crucial role of the pancreatic β-cell KATP channel in regulation of insulin secretion 15 .Effective SU therapy involves inhibiting KATP in the membranes of pancreatic β-cell through direct and indirect interactions with the SUR subunits.This enhances the ATP sensitivity of the KIR6.2 subunits, leading to channel closure at lower intracellular ATP levels 16 .
Consequently, intracellular potassium levels rise, causing depolarization and subsequent calcium influx through voltage-gated calcium channels 18,19 .This influx of calcium triggers the controlled release of insulin from beta cell 20 , and thus reducing blood glucose levels 13 .

Pharmacogenetics of sulfonylurea
Pharmacogenetics explores how variations in the human genome can impact individual responses to drugs, including their efficacy and potential side effects.Identifying genetic factors that influence glycemic response could offer insights into the treatment mechanisms of T2DM and pave the way for personalized treatment approaches 21 .

The effect of genetic variants on the response to SUs therapy
Various medications for T2DM may not yield the same results for all patients or may lead to diverse side effects that restrict their use.Factors such as age, gender, and genetic makeup contribute to the variability in treatment responses.Pharmacogenomics aims to address why oral antidiabetic drugs exhibit varying effectiveness in treating T2DM among different individuals 22 .Genetic factors account for 20%-95% of the differences in drug responses between individuals 23 , as genetic variations can influence drug absorption, distribution, metabolism, targeting, and efficacy 24 .

Figure 1. Schematic representation of the mechanism of action of SU drugs
Genomic differences arise from genetic variations like single nucleotide polymorphisms (SNPs), insertions and deletions, or copy number variations 25 .
In the field of pharmacogenomics, studies on SU have highlighted several gene variants associated with treatment outcomes, such as CYP2C9, KCNQ1, KCNJ11, TCF7L2, IRS-1, CDKAL1, and SLCO1B3, with variable results among the different ethnic population.Genes associated with therapeutic responses to SUs are shown in Table 1.

Cytochrome P450 (CYP2C9):
CYP2C9 plays a pivotal role in the metabolism of SUs 42 .It is the predominant isoform of CYP2C in the liver, constituting approximately 20% of hepatic CYP proteins 43 .
The CYP2C9 gene, located on chromosomal region 10q23.33,spans about 55 kb with nine exons encoding a protein comprising 490 amino acids 44 .The distribution of polymorphic alleles of CYP2C9 varies significantly across populations, with many allelic variants showing altered drug metabolic activities compared to the wild type protein.To date, pharmacogenetic studies has identified 85 allelic variants of the CYP2C9 gene 42 .
The interaction between variants in CYP2C9 and P450 oxidoreductase genes is crucial in determining the efficacy of SU treatment 45 .Notably, CYP2C9*2 (Arg144Cys) and CYP2C9*3 (Ile359Leu) are key genetic variants of the CYP2C9 gene 46 .
The association between CYP2C9 genetic polymorphisms and SU treatment outcomes remains inconclusive.In Iranian patients, no significant correlation was observed between the therapeutic response to SUs and the CYP2C9*3 (rs1057910) variant 22 .Similarly, in Khyber Pakhtunkhwa Pakistan, a modest non-significant impact of this polymorphism on T2DM susceptibility was reported 46 .Conversely, a study in Mexican patients suggested that the CYP2C9*3 genetic variant independently contributes to good glycemic control in T2DM patients treated with glibenclamide 47 .Another study in Chinese patients revealed that the CYP2C9*3 rs1057910 polymorphism significantly influenced the therapeutic response to gliclazide in T2DM patients 48 .Furthermore, Lebanese individuals with the CYP2C9*3 variant exhibited maximum glycemic control when treated with a combination of metformin and SU 49 .
Regarding the CYP2C9*2 (rs1799853) variant, Egyptian T2DM patients with the CYP2C9*2/*3 genotype demonstrated improved glycemic control with glibenclamide treatment 50 .Conversely, a study in Poland did not find any association between the CYP2C9*2 variant and the therapeutic response to SUs in T2DM patients 51 .Similarly, a study in the Netherlands indicated that genotyping for CYP2C9*2 and CYP2C9*3 alleles did not have clinical implications for dosing SUs in primary care T2DM patients 52 .The human gene KCNQ1, located on chromosome 11p15.5,spans 404 kb and comprises 16 exons, encoding the pore-forming subunit of a voltagegated potassium (K+) channel (KVLQT1), known as Kv7.1 53 .

ISSN
KCNQ1 is predominantly expressed in cardiac tissues and pancreatic islets, which plays a crucial role in regulating insulin secretion 28 .The KCNJ11 gene has been linked to the development of type 2 diabetes mellitus (T2DM) and its vascular complications 54 .Genetic variations in KCNQ1 have been associated with fasting glucose levels and β-cell function 55 .
Recent studies have highlighted a significant correlation between polymorphisms in the KCNQ1 gene and the therapeutic response to SUs, including variants such as rs2237897, rs2237895, rs2237892, and rs163184.Variations in the KCNQ1 gene have been shown to impact the response to SU treatment in addition to metformin in T2DM patients 55 .
The rs2237895 polymorphism in KCNQ1 gene was found to influence the therapeutic response to SUs in Iranian and Chinese patients 24, 56 .Whereas KCNQ1 rs2237892 polymorphism was associated with SU response in Chinese patients 56 , but not in Iranian patients 24 .A common variant of KCNQ1, rs2237897, showed association with the efficacy of gliclazide in newly diagnosed Chinese T2DM patients 57 .

ATP-binding cassette transporter sub-family C member 8 (ABCC8), Potassium Inwardly Rectifying Channel Subfamily J Member 11 (KCNJ11)
The KCNJ11 and ABCC8 genes are located on chromosome 11p15.1,and encode the Kir6.2 subunit and the sulfonylurea receptor 1 (SUR1) regulatory subunit of the KATP channel, respectively 58 .ABCC8 comprises 39 exons encode for the 1,582 amino acids of SUR 59 , which is crucial for insulin secretion regulation 60 .KCNJ11, located 4.5 Kb away from ABCC8, has a single exon encodes for the 390 amino acids of Kir6.2 protein 59 .SUR-1 and Kir6.2 proteins are important for KATP channel function, and mutations in ABCC8 and KCNJ11 genes can disrupt their activity 17 .Mutations in these genes impact K-ATP channel dynamics in beta cells' membranes, leading to impaired insulin secretion, and affecting response to SUs through the SU binding region in SUR 23 .
However, a previous study in Iran found no association between the ABCC8 rs757110 variant and response to SU 61 .Another study indicated that the rs757110 variant did not influence the response to metformin and glimepiride combination therapy in Egyptian T2DM patients 62 .The ABBC8 rs1799854 variant also did not significantly impact the response to SU treatment in Iranian and Indonesian T2DM patients 61, 63 .
Various SNPs of the KCNJ11 gene have been identified, with the rs5219 polymorphism being particularly noteworthy for glycemia regulation 17 .This rs5219 variant was identified as a key SNP associated with an increased risk of developing T2DM in the Kinh Vietnamese population 64 .The rs5219 variant in the KCNJ11 gene was linked to therapeutic response to SU in Slovakian patients 65 , while another study found no association between this polymorphism and response to SU in Indonesian patients 66 .

Transcription factor 7-like 2 (TCF7L2):
The human TCF7L2 gene, located on chromosome 10q25.3,consists of 18 exons with a complex splicing pattern across various tissues 67 .It plays a role in regulating of biosynthesis, the secretion of insulin in pancreatic beta cells 68 .
TCF7L2 is considered the most significant genetic locus associated with the risk of developing T2DM, and has been consistently identified in diverse populations 67 .Numerous studies have shown that polymorphisms in the TCF7L2 gene are associated with increased susceptibility to T2DM 68 .Specifically, the intronic single nucleotide polymorphisms (SNPs) rs7903146 (C/T) and rs12255372 (G/T) within the TCF7L2 gene are strongly associated with T2DM risk 68 .Furthermore, variants of TCF7L2 have been shown to impact the initial response to SUs 67 .
The TCF7L2 rs12255372 SNP was linked to poor response to SU in Egyptian patients 69 , and also correlated with therapeutic success with SUs in Indian T2DM patients 70 .Additionally, the TCF7L2 rs7903146 polymorphism influenced response to SU in German 71 and Slovakian patients 72 , but not in Indian patients 70 .Moreover, genotype may influence the response to SU. TT homozygotes of rs4506565 showed association with increased treatment failure in Indian patients receiving SUs 70 .

Insulin receptor substrate-1 (IRS1)
The IRS1 is located on the chromosome 2p36.3 31.It encodes a protein, that is phosphorylated by the insulin receptor tyrosine kinase 73 .
IRS plays a pivotal role in insulin signaling, and is essential for maintaining fundamental cellular functions such as, survival, development, and digestion system 73 .
Dysfunction of IRS-1 can lead to impaired insulin signaling.Genetic variations in IRS-1, such as the glycine to arginine change at codon 972 (rs1801278), may contribute to the development of insulin resistance 74 .However, no significant association was found between this variant and the response of Egyptian patients to SUs 75 .

Cdk5 regulatory associated protein 1-like 1 (CDKAL1):
The human Cdkal1 gene is located on chromosome 6p22.3 38.The CDKAL1 gene encodes cyclin-dependent kinase 5 regulatory subunit-associated protein 1 (CDK5RAP1)-like 1. CDK5 is involved in the glucose-dependent regulation of insulin secretion 76 .CDKAL1 has been associated with the development of T2DM, and may be targeted for therapeutic purposes 38 .It plays an important role in regulation of insulin secretion by pancreatic beta cells 77 .
Research from Slovakia showed association between the CDKAL1 rs7756992 polymorphism, and the response of Slovakian patients with T2DM to SU treatment, showing a correlation with the reduction in fasting plasma glucose levels after six months of SU treatment 78 .Another study from Iran found a significant association between the CDKAL1 rs7754840 variant and the response to SU therapy 79 .
These proteins function as cyclin-dependent kinase inhibitors involved in various cellular processes such as inflammation, cell cycle regulation, apoptosis, senescence, aging, DNA damage response, and extracellular matrix remodeling 81 .
Specific gene polymorphisms within the CDKN2A/B genes have been associated with an increased predisposition to T2DM.For instance, the CDKN2A/B rs10811661 was shown to be associated with the pathogenesis of T2DM in the Iraqi population.It also affected insulin level in those patients 35 .Furthermore, CDKN2A has been identified as a critical regulator of glucose homeostasis in humans 82 .
Previous studies have suggested that the CDKN2A/CDKN2B genes may be linked to the efficacy of glibenclamide.Participants carrying the minor allele C of rs10811661 in CDKN2A/CDKN2B exhibited a significantly greater reduction in fasting blood glucose levels.Additionally, a significant difference in ß-cell function has been observed among carriers of different genotypes of rs10811661

Solute carrier organic anion transporter family member 1B1 (SLCO1B1)/ 1B3 (SLCO1B3):
The SLCO1B3 gene, also known as organic anion transporting polypeptide (OATP) 1B3 39 , is located on human chromosome 12p12-31.7 to 12p12-37.2,and encodes a transmembrane protein composed of 702 residues 39 .It is mainly expressed in the liver cells' basement membrane around the central vein 40 .It is also expressed in pancreas, the SU target organ, and it enhances the insulinotropic effect of SU 41 .
Similarly, the SLCO1B1 gene, located on the short arm of chromosome 12, encodes the OATP1B1 protein comprising 691 amino acids 84.The hepatic transporters, OATP1B1 and OATP1B3, play a crucial role in drug disposition by facilitating the uptake of various drugs from blood into hepatocytes 85 .Genetic variations affecting transport activity may impact the efficacy of SU 86 .Research has indicated a potential interaction between SU and rosuvastatin, a common substrate of OATP1B1 and OATP1B3 often used in combination with SUs, mediated by these transporters 85 .
Previous studies have shown that glibenclamide and glipizide are substrates of OATP1B3, while gliclazide and glimepiride are substrates of OATP1B1 87 .The OATP1B3 variant (699G > A) significantly influences the transport and metabolism of glibenclamide and glipizide 87 .
Recent research highlighted SLCO1B3 as a key determinant of the insulinotropic effect of glibenclamide at the tissue level 88 .However, a study from China found no association between the SLCO1B3 variant rs4149117 and SU effectiveness 88 .
Polymorphisms in the SLCO1B1 gene can lead to complete or partial loss of OATP1B1 function, altering the pharmacokinetic profile of substrates 89 .The C allele of rs10770791 in an intronic region of SLCO1B1 was linked to a 0.11% greater reduction in HbA1c following glipizide treatment 90 .
In (Table 2), we compiled several studies conducted to detect the association between the gene polymorphism, and SU response in T2DM patients.Inconsistent results for different variants may be attributed to factors such as insufficient sample size, and differences in study design, gender, age, lifestyle, ethnicity, concomitant use of other medications, etc 91 .

Pharmacogenetics of sulfonylurea-induced hypoglycemia in T2DM:
Hypoglycemia is a common complication of antidiabetic medications, such as SUs 91 .The United Kingdom Prospective Diabetes Study reported that 17% of patients taking SUs experienced at least one hypoglycemic event annually 93 .Some individuals metabolize the drug slowly, leading to higher levels of the drug in their bloodstream over time, which can result in prolonged hypoglycemic effects 94 .SU acts by lowering the level of blood glucose by increasing insulin secretion in the pancreas, and by blocking the ATP-sensitive potassium channels 92 .Consequently, patients with T2DM receiving SU therapy are at risk for hypoglycemia 95 .
Several patient characteristics, including sex, age, food interactions, and comorbidities, have been reported to influence hypoglycemia risk.Other established risk factors for SU-induced hypoglycemia are low hemoglobin level, polypharmacy, and the use of long-acting SU 93 .Since the increased risk of hypoglycemia with SU therapy increases with higher drug concentrations, genetic variations impacting drug clearance and effectiveness, can lead to interindividual variability in risk 95 .
Specifically, common variants such as CYP2C9*2 (Arg144Cys, rs1799853) and CYP2C9*3 (Ile359Leu, rs1057910) are known to impair the catalytic function of the CYP2C9 enzyme, affecting the metabolism of SUs, and potentially elevating the likelihood of SU-induced hypoglycemia 93 .A study conducted in Pakistan further supported the association between the CYP2C9*2 variant and hypoglycemia induced by SUs 94 .
However, findings regarding the association between CYP2C9*2 genotypes and SU-induced hypoglycemia are inconsistent.A Greek study found no association between the CYP2C9*2 variant and SUinduced hypoglycemia in T2DM patients treated with SUs 96 .
Similarly, a study in European American T2DM patients did not detect association between reduced-function CYP2C9 alleles and SU-related hypoglycemia 95 .These discrepancies in results may be attributed to differences in study design, including variations in hypoglycemia definitions, the age of participants, specific SUs analyzed, and limited statistical power due to small sample sizes 94 .
The SLCO1B1 c.521C variant was shown to have a protective effect on SU-related hypoglycemia risk independently and in interaction with CYP2C9 phenotypes 41 .
On the other hand, variants like TCF7L2 rs7903146 and KCNJ11 E23K were shown to be not associated with SU-induced hypoglycemia in T2DM( 97 , 98 ).

The effect of genetic polymorphisms on the development of secondary failure to SUs
Treatment with SUs is initially successful in T2DM 99 .However, it has been observed that each year 5-7% of diabetic patients undergoing SU therapy convert to insulin treatment progressively as SU fails.This clinical phenomenon is known as "secondary failure to SU", posing a significant challenge in the management The deterioration of beta-cell function due to prolonged overstimulation is believed to be a contributing factor to secondary SU failure 101 .
Cyb5r3, involved in regulating glucose utilization in β-cells by enhancing the stability of glucokinase, the key enzyme in glycolysis, has been implicated in the mechanism of secondary SU failure.Studies have shown that the functional loss of oxidoreductase Cyb5r3 affects SU failure through its interactions with glucokinase 102 .Genetic variants have also been associated with an increased risk of secondary failure to SUs.For instance, the common polymorphism in the pore-forming KATP channel subunit (E23K) variant of the Kir6.2 gene and the Arg972 IRS-1 variants have been linked to increased risk of secondary failure to SUs 99 .
The Arg972 IRS-1 variant is shown to be associated with increased risk for secondary failure to SU 100 .Additionally, the Kir6.2E23K polymorphism has been suggested to accelerate secondary SU failure in non-obese Japanese T2DM patients 101 .
A previous study showed that the TCF7L2 rs7903146 variant is associated with hypoglycemic response to SUs, resulting in earlier secondary failure 71 .Furthermore, the rs757110 ABCC8 gene polymorphism has been identified as an independent predictor of secondary SU failure 103 .

CONCLUSION
In the current review, we explore recent advancements in research on the pharmacogenetics of SUs.Our analysis suggests that genetic variations could significantly influence the response to SU therapy, the risk of hypoglycemia associated with SUs, and the occurrence of secondary failure.However, the review reveals conflicting outcomes for different genes/variants that may be due to the heterogeneity among previous studies.Consequently, translating these findings into clinical practice presents a substantial challenge, underscoring the critical need for more extensive and standardized investigations to generate precise data.Such data can then be leveraged to advance precision medicine for T2DM, ultimately improving patient outcomes.