*2.5.2. SGLT2 inhibitors – A new concept for the treatment of Type 2 diabetes*

T2DM is characterized by hyperglycemia that results both from peripheral resistance to the action of insulin and from progressive failure of the pancreatic β-cell to compensate for the increasing demand for insulin. Chronic hyperglycemia triggers glucotoxicity, a term summarizing the vicious cycle between hyperglycemia inducing β-cell dysfunction and insulin resistance that aggravates disease progression leading to micro-and macrovascular complications.

Current treatments for T2DM come with significant limitations regarding their potential to induce adverse effects. Metformin can cause gastrointestinal effects such as diarrhea and nausea, while sulfonylureas and insulin can induce hypoglycemia and are associated with weight gain. Thiazolidinediones that act as insulin sensitizers can induce weight gain, and are associated with edema and are potentially associated with an increased cardiovascular risk. GLP-1 analogues which are incretin mimicking agents can cause nausea and diarrhea. New therapeutic strategies are needed that are not only effective in terms of glucose control, but provide excellent safety and potential add-on effects such as weight loss, lipid lowering or reductions in blood pressure.

Functional characterization of human SGLT3 demonstrated a lack of sugar transport activity. Instead, human SGLT3 was found to be a glucose-sensitive ion channel where sugar binding induces plasma membrane depolarization in a saturable, sodium-dependent and phlorizinsensitive manner [14]. Interestingly, this is in sharp contrast to pig and mouse SGLT3 which are able to transport glucose. The sodium-to-substrate stoichiometry is 2:1, which is similar to SGLT1, while substrate specificity appears closer to SGLT2 with no acceptance of galactose as a substrate. More in depth characterization of SGLT3 substrate specifities found that human SGLT3, similarly to SGLT1, interacts with various glucosides, while pig SGLT3 was found to

Mammalian Sugar Transporters http://dx.doi.org/10.5772/58325 15

The lack of transport activity by human SGLT3 has been shown to involve a specific amino acid: residue 457. This residue has been shown to be important for the function of human SGLT1, since mutations of that particular amino acid cause GGM. Structural information from vSGLT [15] revealed that the corresponding residue mediates direct interaction with the sugar. Accordingly, mutation of glutamate 457 in human SGLT3 to glutamine conferred transport activity on the transporter displaying SGLT1-like transport characteristics with respect to

Physiologically, SGLT3 is hypothesized to act as a glucose sensor which, at the site of its expression in cholinergic neurons and the neuromuscular junction, might modulate action potentials of neurons/skeletal muscle cells glucose dependently. This hypothesis is supported by the observation that upon expression of human SGLT3 in sensory neurons of *C. elegans* glucose sensing *in vivo* can be monitored [16]. More recent observations indicated that SGLT3 is expressed in the proximal tubule of the human kidney and might be responsible for sodium reabsorption based on uptake studies in SGLT3 overexpressing kidney cell lines which demonstrated sodium transport that was blocked by the classical inhibitor phlorizin [17].

SGLT4 was cloned from human small intestinal cDNA libraries. The mRNA encoding SGLT4 is almost exclusively found in the small intestine and kidney. SGLT4 exhibits Na+-dependent AMG transport with a Km of 2.6 mM. Inhibition studies of AMG mediated transport indicated that SGLT4 appears to transport naturally occurring sugars with a rank order of mannose, glucose, fructose, and galactose. Transport studies using radiolabeled mannose indicated that SGLT4 might be physiologically relevant for intestinal absorption as well as renal reabsorption

SGLT5 was recently cloned from human kidney cDNA and characterized as a kidney specific sodium-dependent mannose transporter which is also able to transport glucose and fructose [19]. While specifically expressed in human kidney its precise localization and physiological role remains unknown. Based on amino acid sequence homology SGLT5 represents the closest homologue to SGLT4. In a manner reminiscent of the relationship between SGLT1 and SGLT2,

substrate-to-sodium stoichiometry, sugar specificities as well as affinities [15].

transport imino sugars with high affinity.

**2.7. SGLT4 (***SLC5A9***)**

of mannose [18].

**2.8. SGLT5 (***SLC5A10***)**

The kidney has an important role in controlling blood glucose levels by mediating glucose reabsorption into the bloodstream. In patients with T2DM increased renal absorptive capacity has been observed, indicating that blocking the process of glucose reuptake by the kidney might be an attractive new strategy for treatment of T2DM. However, glucosuria has histori‐ cally been perceived as a manifestation of the disease, which appears to make this therapeutic concept seem rather counter-intuitive.

The phenotype of subjects identified with FRG, as well as studies performed with phlorizin, indicated that correcting hyperglycemia via specific inhibition of SGLT2 might provide a new option for a safe and effective treatment of T2DM.

Phlorizin, a potent SGLT inhibitor, proved to be an important tool for investigating the mechanism and consequences of blocking renal sugar reabsorption. Its use established the concept of glucotoxicity: blocking of renal glucose reabsorption with phlorizin in diabetic rats normalized insulin levels and restored insulin sensitivity [11].

Disadvantages of phlorizin include the non-selective inhibition of SGLT2, poor bioavailability, short half-life and potential for side effects caused e.g by blocking GLUT via its major metab‐ olite phloretin. These disadvantages that are inherited to the molecular phlorizin led into research for new compounds in order to achieve proof of concept for selective SGLT2 inhibition for the treatment of T2DM.

Although non-selective for SGLT2, T-1095 was the first orally available phlorizin derivative that was metabolically stable. When administered to diabetic animals, T-1095 corrected hyperglycemia and reduced hyperinsulinemia, and hypertriglyceridemia [12]. These findings indicated that SGLT2 inhibition might be a viable approach to treatment of T2DM.

In the following years, the selective SGLT2 inhibitors sergliflozin and remogliflozin progressed to clinical trials. While many selective SGLT2 inhibitors went into clinical testing, development of O-glycosidic SGLT2 inhibitors was halted, presumably due to their unfavorable pharma‐ kokinetic profile. In contrast, a number of C-glycosidic compounds which differ from Oglycosides in structure and stability are in clinical development [12] or have achieved approval and represent an innovative insulin-independent treatment option for controlling blood glucose in Type 2 diabetic patients [13].

#### **2.6. SGLT3 (***SLC5A4***)**

The human SGLT3 cDNA was cloned from colon carcinoma and was found to encode a 659 amino acid protein with 70% identity to human SGLT1. SGLT3 mRNA is detected in the intestine, testes, uterus, lung, brain and thyroid, while the protein is predominantly found in intestine and skeletal muscle. Immunohistochemical analysis of the intestine identified cholinergic neurons in submucosal and myenteric plexuses as the site of SGLT3 expression. In skeletal muscle, SGLT3 co-localized with the nicotinic acetylcholine receptor indicating expression at the neuromuscular junction.

Functional characterization of human SGLT3 demonstrated a lack of sugar transport activity. Instead, human SGLT3 was found to be a glucose-sensitive ion channel where sugar binding induces plasma membrane depolarization in a saturable, sodium-dependent and phlorizinsensitive manner [14]. Interestingly, this is in sharp contrast to pig and mouse SGLT3 which are able to transport glucose. The sodium-to-substrate stoichiometry is 2:1, which is similar to SGLT1, while substrate specificity appears closer to SGLT2 with no acceptance of galactose as a substrate. More in depth characterization of SGLT3 substrate specifities found that human SGLT3, similarly to SGLT1, interacts with various glucosides, while pig SGLT3 was found to transport imino sugars with high affinity.

The lack of transport activity by human SGLT3 has been shown to involve a specific amino acid: residue 457. This residue has been shown to be important for the function of human SGLT1, since mutations of that particular amino acid cause GGM. Structural information from vSGLT [15] revealed that the corresponding residue mediates direct interaction with the sugar. Accordingly, mutation of glutamate 457 in human SGLT3 to glutamine conferred transport activity on the transporter displaying SGLT1-like transport characteristics with respect to substrate-to-sodium stoichiometry, sugar specificities as well as affinities [15].

Physiologically, SGLT3 is hypothesized to act as a glucose sensor which, at the site of its expression in cholinergic neurons and the neuromuscular junction, might modulate action potentials of neurons/skeletal muscle cells glucose dependently. This hypothesis is supported by the observation that upon expression of human SGLT3 in sensory neurons of *C. elegans* glucose sensing *in vivo* can be monitored [16]. More recent observations indicated that SGLT3 is expressed in the proximal tubule of the human kidney and might be responsible for sodium reabsorption based on uptake studies in SGLT3 overexpressing kidney cell lines which demonstrated sodium transport that was blocked by the classical inhibitor phlorizin [17].
