A review on SGLT-2 inhibitors
HAMDANI KULSUM SHAKEEL AHMED⃰ 1, TAJ USMAN KHAN2, ABDUL RAHMAN MOHAMMAD MAAJID3
1. Assistant Professor, KBHSS Trust’s Institute of Pharmacy, Malegaon, India
2. Assistant Professor, Oriental College of Pharmacy, Sanpada, New Mumbai
3. Drug Safety Associate, Cognizant Technology Solutions, Mumbai+
After metabolism of carbohydrate, glucose is filtered in the glomerular and then it is almost completely reabsorbed into circulation from the proximal renal tubules. S1 segment of the proximal tubule contains sodium-glucose co-transporter-2 (SGLT-2), which is responsible for the majority of glucose reabsorption. Some molecules which have property of inhibiting SGLT-2 have been reported in literature few of them reached in phase III of clinical trial. These agents reduce glucose reabsorption and increase urinary excretion of glucose and could be a novel alternative for the existing anti-diabetic agents in the market.
These compounds showed reduction in fasting and postprandial blood glucose levels, and reduced haemoglobin A1C in animal models and humans with type-2 diabetes. Animal studies have suggested that lowering of blood sugar level with SGLT-2 inhibitors may also improve insulin sensitivity and preserve β-cell function. SGLT-2 inhibitors cause urinary excretion of excess calories is associated with reduction in body weight, slight reductions in blood pressure and mild diuretic action. Moreover, some SGLT-2 inhibitors having C-glucoside, have pharmacokinetic properties that make them suitable to once-daily dosing.
Key words: Aerobic respiration, anaerobic respiration, glucose sensing, membrane symporters, and mitochondrial oxidative flux.
Glucose is a vital fuel for all organisms, from bacteria to mammals, and is a metabolic substrate in aerobic or anaerobic respiration, or fermentation. It is fundamental for mammals to maintain a constant blood glucose level for physiological processes such as cerebral metabolism. Glucose homeostasis is maintained in mammals by glucose absorption from dietary carbohydrates in the small intestine and glucose filtration and reabsorption by the kidney. Glucose is stored in the kidney, liver and adipocytes and regulated by pancreatic hormones, and utilized throughout the body, especially skeletal and cardiac muscle.
Glucose sensing, which is an important survival mechanism, enables mammals to mobilize tissue energy stores in response to the metabolic needs of the body. Intracellular glucose is sensed and the resulting signal is relayed by metabolic messengers in tissues; for example, glucokinase and mitochondrial oxidative fluxes (ADP: ATP ratio) are involved in regulation of ATP-sensitive potassium channels controlling insulin secretion and thus are well-established glucose sensors in pancreatic β-cells. The inhibition of glucose sensing, by metabolic modulators, would likely be fatal.
Certain membrane symporters, such as the sodium-glucose transporter (SGLT), have long been known to transport water along with their substrates. Although the main job of SGLT is to uptake sugar molecules by utilizing the Na+-K+ gradient as the energy source1.
More than 25 years ago, it was hypothesized that a specific enzyme, glucokinase, acted as a “glucose sensor” in pancreatic-cells. However, after the discovery of different glucose transporter proteins in the mid- to late-1980s, the hypothesis proposed by several groups evolved to include a transporter molecule, GLUT2, as part of a glucose sensing mechanism2.
When the blood glucose concentration rises, the concentration gradient across the cell membrane drives the entry of glucose into the cell via diffusion. Diffusion reaches equilibrium and stops as the inner and outer concentrations equalize. If the cell’s energy demand requires additional glucose, glucose transporter units (GLUT) are mobilized. At present, there are 13 types of known GLUTs. Some GLUTs, such as GLUT4 in muscle, require insulin to mediate trans-membrane glucose transport. Others, such as GLUT1 in brain, accomplish glucose transport without the aid of insulin. As for intestinal cells and renal cells, particularly the proximal tubule cells of the kidney, glucose is transported via sodium-glucose transporters (SGLT).
At present, there are six types of known SGLTs. SGLT-1 and SGLT-2 has been more extensively studied. SGLT-1 is particularly abundant in the cells and membranes of the renal proximal tubule at the S2 site. SGLT-1 has a stronger affinity for glucose but less transporting capacity than SGLT-2. It is therefore unlikely to become a target for new drug development. SGLT-2 is found in the proximal tubule membranes at the S1 site. It has lower affinity but greater capacity for transporting glucose. About 90% of glucose reabsorbed in the proximal tubule is mediated by SGLT-2, and the remaining 10% is by SGLT-1. As blood peruses renal glomerulus, Bowman’s capsule, and the basement membrane, glucose molecules are driven by osmotic pressure into the lumen of the proximal tubule. Because glucose is the body’s main source of energy, almost all of it is reabsorbed by the cells of the renal tubules. However, this re-absorption is limited by a certain threshold. When blood glucose level exceeds 180 mg/dl, the excess glucose cannot be reabsorbed and is excreted in the urine, resulting in glucosuria. Whether this constitutes a protective mechanism to prevent the development of “acute glucotoxicity” deserves further study.
Mechanism of action of SGLT-2 inhibitors:
- Hyperglycemia causes the protein kinase C (PKC) in the membrane of renal distal tubules to become easily activated. PKC induces the formation of reactive oxygen species (ROS). ROS subsequently activate nuclear transcription factor Κappa B (NFΚB), which induces the expression of the SGLT gene. The activated SGLT gene inhibits the action of the SGLT and thus reduces the reabsorption of glucose, which is then excreted in the urine.
- PKC may also participate in the activation of calcium cytosolic phospholipase protease A2 (CPLA2), which can also inhibit sodium-glucose transporter to decrease the reabsorption of glucose from the urine.
- Besides glucose, angiotensin II (ANG II) also acts on SGLTs. After binding with AT1R receptor, ANG II activates PKC through the action of tyrosine kinase. This is accompanied by the action of mitogen-activated protein kinase (MAPK), whose P44 and P42 proteins enhance CPLA2 action. MAPK P44/P42 and CPLA2 all inhibit SGLT action. In addition, CPLA2 activates prostaglandin E2 and epoxy-eicosatrienoic acids (EET) through arachidonic acid (AA) to alter the transcription of the SGLT gene in the nucleus, and thus further inhibit the sodium-glucose transporter.
- Epidermal growth factor (EGF) receptors are also present in the proximal renal tubule. The binding of EGF to its receptor induces tyrosine kinase to form a complex with phospholipase C, which catalyzes the conversion of phosphatidylinositol 4, 5-bis-phosphate (PIP2) into diacylglycerol (DAG). DAG again reactivates PKC and inhibits sodium-glucose transporter through the same pathway as ANG II3.
- The kidneys play an important role in the control of blood glucose levels. When the capacity of glucose reabsorption has been exceeded, the surplus glucose is excreted in the urine and a state known as glucosuria develops. Although substances that suppress renal glucose reabsorption have been long known, the mechanism of action at the molecular level was not described until the early 1990’s. The mechanism of action of SGLT-2 inhibitors is to interfere with sodium-glucose co-transporters in the S1 segment in the proximal tubule of the kidneys. However, there is still little information regarding the molecular mechanism of action of SGLT-2 inhibitors. According to Pajor et al., the inhibition involves competitive binding of the glucose moiety of the SGLT-2 inhibitor to the glucose moiety binding site on the transport protein and the aglycone moiety probably interacts with hydrophobic or aromatic residues on SGLT. Their studies also suggested that a conserved residue at Cys 615 in human SGLT-2 appears to participate in maintaining the structure of the inhibitor binding site, but this cysteine probably does not bind directly to the inhibitors. Because the SGLT-2 are responsible for reabsorption of most of the glucose filtered in glomeruli, the SGLT-2 inhibitors induce glucosuria by suppressing 90% of glucose reabsorption. In patients with diabetes, SGLT-2 inhibition may bring paradoxical benefits since excessive glucose excretion lowers the plasma glucose levels, extinguishes glucose toxicity and results in a loss of energy, which leads to better control of diabetes and improvements in the disorders correlated with it9.
The concentration of ANG II in the renal tubules is 100-1000 times its concentration in the blood. In many kidney diseases, particularly diabetic nephropathy, the concentration of ANG II in the kidney is abnormally high. RENAAL10and IRMA11 studies have both shown that ANG II receptor antagonists can retard the progression of diabetes nephropathy. This may be due in part to the alteration in SGLT regulation. In hyperglycemic rats, the increased ANG II concentration in the renal interstitium enhances PKC and MAPK actions. Hyperglycemia also increases the binding of EGF to its receptor, which contributes to the development of diabetic nephropathy. Therefore, SGLT may be the common factor by which hyperglycemia, ANG II, and EGF cause disease. Together with known pathogenesis of diabetes, high blood glucose level exceeding the reabsorption threshold of the kidney causes the appearance of glucosuria. At present, a novel sodium-glucose transporter inhibitor has been developed to increase the excretion of glucose in the urine. It may lower fasting blood glucose value, improve glycated hemoglobin level, enhance glucose tolerance, reduce glucotoxicity, and further preserve pancreatic islet cell by retarding their apoptosis. Additionally, since the excretion of one gram of glucose means the loss of 4 calories of energy, and prior studies have shown such inhibitors may result in daily excretions of up to 90 grams of glucose, weight reduction may be achieved. Eventually, glucose utilization by peripheral tissues is increased and insulin resistance is reduced. Such multiple benefits point to a new direction of research for alternative glucose-lowering drugs.
Table 1: Sglt-2 Inhibitors and Phase of Development6
|Dapagliflozin||Approved by European Medicines Agency||Bristol-Myers Squibb, AstraZeneca|
|Canagliflozin||Approved by U.S. FDA||Johnson & Johnson, Mitsubishi Tanabe|
|Empagliflozin||Phase III||Boehringer Ingelheim, Eli Lilly|
|Ipragliflozin||Phase III||Astellas, Kotobuki|
|LX 4211||Phase II||Lexicon|
|GW 869682||Phase II||GlaxoSmithKline|
|ISIS 388626||Phase I||Isis|
Table 2: Clinical Studies on Sglt-2 Inhibitors
|SGLT-2 inhibitor||Clinical development
|Blood glucose reduction
|Dapagliflozin||III phase||12-weeks prospective, randomized
placebo- controlled (study from the II
|2.5, 5, 10, 20, 50||16–31|
|Remogliflozin||II phase||randomized, double-blind, parallel
assignment, safety/efficacy study
|no publication to date||no publication to date|
|Sergliflozin||II phase||double-blind, randomized,
placebo-controlled study (evaluation
weight loss, safety, tolerability and
pharmacokinetics in obese subjects
following 12-week dosing
|500, 1000 *||–|
|AVE – 2268||phase IIb||–||–||–|
|JNJ-28431754||II phase||double-blind, randomized,
parallel group, multicenter,
|50, 100, 200, 300||no publication up to date|
SGLT-2 inhibitors, benefits and side effects:
Depending on the dose, SGLT-2 inhibitors exhibit a variety of effects ranging from lowering plasma glucose levels to possible reductions of plasma insulin and glycated hemoglobin levels. Another therapeutic effect of SGLT-2 inhibitors includes a reduction in hepatic gluconeogenesis and decreased glucotoxicity. Thus, by lowering the plasma glucose, the liver sensitivity is improved, which leads to a suppression of hepatic glucose production as a result of glucose-6-phosphatase inhibition. In addition, these inhibitors increase sodium excretion, which can theoretically lead to a mild reduction in arterial blood pressure.
However, this effect is not common for all drugs of this category. Contrary to some of the currently used anti-diabetic medications, SGLT-2 inhibitors act only on the transporters in the kidneys. Therefore, they do not stimulate insulin secretion and the risk of hypoglycemia has been predicted to be low. Furthermore, high patient compliance can be expected, because there are no common gastrointestinal disturbances associated with these inhibitors. Additionally, a well-controlled serum glycaemia lowers glomerular hyper-filtration, which results in a reduction of the nephropathy caused by longstanding diabetes. The convenience of oral administration is another advantage of these potential anti-diabetic drugs.
Importantly, concerns regarding the safety of this group of inhibitors do not appear to be relevant. This point is demonstrated by patients with familial renal glucosuria who have inherited defects in SGLT-2; yet, they have normal kidney function. Furthermore, polyuria and increased thirst, as well as bacterial and fungal infections commonly associated with glucosuria do not seem to be a significant problem while using SGLT-2 inhibitors9.
Chemistry of SGLT-2 inhibitors:
Representatives of SGLT-2 inhibitors are Phlorizin and phlorethin. Phlorizin was one of the first natural phenol glycosides identified and was isolated from fruit tree bark in 1835. It was used as a research tool in “phlorizin-induced” glucosuria and as its aglycone form, phlorethin. It is a-D-glucoside consisting of a glucose moiety and an aglycone (with two aromatic car-bocycles joined by an alkyl spacer). They have not been used for the treatment of diabetes because their oral bioavailability is very low and they inhibit other transporters in addition to SGLT-2, which leads to concerns about their undesirable side effects. All of the SGLT-2 inhibitors are glycosides derived from phlorizin.
There are carbocyclic and heterocyclic O-glycosides, carbocyclic and heterocyclic C-glycosides, as well as N-glycosides and O-glycosides with modified glucose moieties . T-1095, T-1095A and TA-7284 One of the next generation of SGLT-2 inhibitors is T-1095A, which is the active form of T-1095. This inhibitor has a better bioavailability and can be orally administered. In vitro, it inhibits SGLT-2 four times better then SGLT-1 (IC50 value of 50 nM for human SGLT-2 and 200 nM for SGLT-1).
Table 3: Status of Each Sglt-2 Inhibitors in Late 201310
|Name of Drug||Development Stage||Manufacturer|
|Dapagliflozin||Authorized in EU, Under review in US; Phase III in Japan||BMS/Astra Zeneca|
|Canagliflozin||Authorized in US; Under review in EU; Tanabe
Phase III in Japan
|Johnson & Johnson, Mitsubishi
|Ipragliflozin||Filed in Japan; Phase III in
|Empagliflozin||Phase III in US/EU/Japan||Boehringer Ingelheim|
|Tofogliflozin||Phase III in Japan||Chugai|
|Luseogliflozin||Phase III in Japan||Taisho|
Adverse Effects of SGLT-2 Inhibitors:
The adverse effects of SGLT-2 inhibitors may include fatigue, hypoglycemia, increased urine output, increased hematocrit and mycotic genital or urinary tract infections. Constipation, diarrhea and nausea are other possible side effects. Small changes in serum uric acid, magnesium and phosphate may also occur. Concerns related to SGLT-2 inhibition include glucose elevation in the urine that can theoretically lead to urinary tract and genital infections, electrolyte imbalances and increased urinary frequency. Although short-term studies have shown the safety and efficacy of SGLT-2 inhibitors, long-term studies are lacking. The observation that individuals with familial renal glycosuria, in which there is a mutation in the genes encoding for SGLT-2 proteins, maintain normal long-term kidney function provides some reassurance that this mode of action will not adversely affect renal function. Similarly, comparisons between SGLT-2 inhibitors and other treatment options for diabetes type- 2 have also not yet been conducted11.
Limitations of the SGLT-2 Inhibitors:
Although the mechanism of action of SGLT-2 inhibitors may resides potential benefits in it, nevertheless they are associated with an assortment of precincts as well such as polyuria and polydipsia which results in hypovolemia particularly in the dehydrated patients. Urinary excretion of glucose also results in an increased risk of Urinary Tract Infections (UTIs) and genitourinary infections. Furthermore, most of the Oral Antidiabetic Drugs (OADs) either improve insulin resistance or insulin secretion whereas SGLT-2 inhibitors are thought to increase the glycemic control through urinary excretion of the same, irrespective of how the glucose level increased. Consequently diminution of glucotoxicity might improve the insulin resistance as well as the insulin secretion.
Currently overabundances of therapies are available to target the diabetes mellitus. Nevertheless, targeting the glucose level remain on top priority in sizeable percentage of type-2 diabetes mellitus patients. Increased blood glucose level contributes to the succession and development of type-2 diabetes mellitus. Reducing the blood glucose level through increased urinary excretion of sugar put forward impending advantages of better glycemic control, minimum risk of hypoglycemia and sympathetic effect on the body weight. At present many companies are developing potential SGLT-2 inhibitors with different selectivity, pharmacokinetic profiles, potency and efficacy. SGLT-2 inhibitors are expected to improve the glycemic control, insulin resistance as well as the conservation of pancreatic β-cells. The insulin independent mechanism of action of SGLT-2 inhibitors might provide better treatment for the type-2 diabetes mellitus patients. These are expected to provide better synergistic results when used as combinatorial therapy with other antidiabetic drugs despite predictable adverse effects for instance polyuria, urinogenital infections, and UTIs12.
Structures of some SGLT-2 inhibitors are as follows:
Acknowledgement: Author thanks to all authors and co-authors cited in references.
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