Influencing the body s regulatory mechanisms in order to treat type 2 diabetes

Transcription

1 Influencing the body s regulatory mechanisms in order to treat type 2 diabetes In their quest to develop new oral drugs for the treatment of type 2 diabetes, researchers are focusing their efforts on stimulation and inhibition of enzymes and receptors that play a role in glucose homeostasis.

2 How type 2 diabetes The onset of manifest type 2 diabetes (T2D) with develops all the signs and symptoms listed on p. 13 is generally preceded by a long period of years in which the affected person may have no overt signs of diabetes and shows impaired glucose tolerance (IGT). Metabolic syndrome as a precursor of diabetes How and why does a metabolic syndrome develop? The term metabolic syndrome refers to a constellation of signs and symptoms that arise mostly as a result of a progressive insulin resistance on liver, skeletal muscle and fatty tissues. What goes on at the cellular level in insulin resistance? So far, we know that in insulin resistance insulin (see box) binds normally to the insulin receptor on cell surfaces but that the signal that normally arises as a result of this binding is no longer generated. In other words, the lock-and-key binding of insulin to the insulin receptor fails to initiate the intracellular signalling cascade that results both in glycolysis, i.e. metabolic breakdown of glucose to form the energy-storing and supplying molecule adenosine triphosphate (ATP), and in synthesis of the carbohydrate reserve molecule glycogen from glucose. The body attempts to compensate for this downregulation of the signalling pathways initiated by binding of insulin to the insulin receptor by secreting greater amounts of insulin (hyperinsulinemia). But insulin resistance is not the only sign of a metabolic syndrome. A metabolic syndrome is also characterised by high blood pressure, dyslipidemia, triglyceridemia and overweight/obesity (see below). Clinical manifest type 2 diabetes As a result of this constant overburdening, the insulin-producing -cells of the islets of Langerhans in the pancreas eventually become exhausted and cease to function. The combination of -cell dysfunction and insulin resistance on the major target tissues of insulin action, namely skeletal muscle, the liver and fat tissue, gives rise to the increased blood glucose level (hyperglycemia) that characterises untreated T2D (Fig. 2). The relative contribution of disturbed insulin secretion and insulin resistance to the condition varies between patients, and each of these factors can influence the other [1]. 46

3 Insulin The hormone insulin exerts its short-term metabolic actions and its long-term growth-promoting action by binding to the insulin receptor on cell surfaces. In doing so it stimulates the uptake of simple sugars (monosaccharides), amino acids and fatty acids into cells and promotes the synthesis and storage of carbohydrates, fats and proteins while at the same time inhibiting enzymatic breakdown and release of these substances into the blood circulation. Insulin increases the transport of glucose into fat and muscle cells by promoting movement of the glucose transporter GLUT4 from the interior of the cell to the cell membrane [4]. Insulin is synthesised at blood glucose concentrations as low as 2 4 mmol/l, but released only at blood glucose concentrations above 4 6 mmol/l. The concentration of insulin in human blood is normally about 1 ng/ml. This hormone, the only one that reduces blood glucose level, is rapidly cleared from the blood circulation and inactivated by specific insulin-metabolising enzymes in the cells of the liver and other tissues [5]. FIGURE 1: Three dimensional structure of the insulin molecule. This peptide hormone consists of a 30-amino-acid A chain (shown red) and a 21- amino-acid B chain (green). Two disulfide bridges (yellow) hold the two chains together. The figure was kindly provided by Dr Bernd Kuhn, Molecular Design, F. Hoffmann-La Roche Ltd, using data from [6]. In addition, an increased concentration of free fatty acids 1 exacerbates hyperinsulinemia and insulin resistance. This leads to a further reduction of glucose uptake into muscle cells and in turn to increased gluconeogenesis, i.e. glucose synthesis from non-carbohydrate precursors, in the liver % of patients with T2D are overweight at the time of diagnosis [1]. And unfortunately, bodyweight and the proportion of bodyweight accounted for by fat tissue often increase further with increasing duration of the disease. Fat tissue acts as an endocrine organ, promoting lipolysis (breakdown of fat) via release of mediator substances. This leads to a rise in the concentration of free fatty acids in the blood. This, in turn, not only exacerbates hyperinsulinemia and insulin resistance, but has other undesirable effects such as stimulation of the synthesis of triglyceride-rich very-low-density lipoproteins (VLDL) 2.Cholesterol is likewise esterified by these free fatty acids and bound to high-density lipoprotein (HDL) to form HDL-cholesterol or to low-density lipoprotein (LDL) to form LDL-cholesterol. 1 Free fatty acids are nonesterified fatty acids. Fats are mono-, di- and triesters of glycerol with fatty acids. Triglycerides are triesters of glycerol. 2 Fatty acids are esterified to form fats and in the body are also bound to apolipoproteins, since only in this way can they be transported in an aqueous environment (blood plasma). Type 2 diabetes: development of oral medication 47

4 -cell dysfunction pancreas islet -cell degranulation reduced insulin content low plasma insulin liver increased glucose output insulin resistence elevated plasma FFA level + - muscle increased lipolysis elevated adipocytokines levels adipose tissue hyperglycemia decreased glucose transport, decreased activity of GLUT4 FIGURE 2: How insulin resistance and -cell dysfunction lead to hyperglycemia in type 2 diabetes: In addition to reducing blood glucose level, insulin reduces glucose production in the liver (gluconeogenesis) and increases glucose uptake, utilisation and storage in muscle and fat. If less insulin is available, more gluconeogenesis occurs. Fat cells play an important role in metabolism. They release free fatty acids (FFAs) and various adipocytokines such as tumour necrosis factor alpha (TNF- ) and leptin. Adipocytokines regulate food intake, energy consumption and insulin sensitivity.if more fat tissue is present, more TNF- is released, resulting in increased breakdown of fat (lipolysis) and increased FFA levels. FFAs reduce glucose uptake by muscle cells, reduce insulin release by -cells and increase gluconeogenesis. (Source of figure: J. Mizrahi, F. Hoffmann-La Roche Ltd) Reduced levels of HDL-cholesterol and increased levels of LDLcholesterol particles have been found in T2D. LDL-cholesterol particles play an important role in the formation of atherosclerotic plaques on the internal lining of blood vessels. If these plaques become dislodged and carried away in the blood, they can obstruct small blood vessels and thus cause myocardial infarction and strokes. Hyperinsulinemia also causes increased reabsorption of Na + ions in the kidneys, leading to increased blood volume and thus to higher blood pressure. Type 2 diabetics in industrialised countries are far more commonly affected by high blood pressure than nondiabetic subjects of the same age (53 % versus 17.3 %) [3]. 48

5 Early treatment can In 1998 the results of the United Kingdom prevent complications Prospective Diabetes Study (UKPDS), the largest long-term medical study ever conducted, were published. A total of 5102 people with newly diagnosed T2D were observed over a mean period of 11.1 years. These results demonstrated unequivocally that maintenance of patients blood glucose and blood pressure within the respective normal range by means of appropriate measures significantly reduced the likelihood that complications would develop [7]. According to the recommendations of the International Diabetes Foundation, patients HbA 1c level 3 and fasting and postprandial blood glucose levels should be maintained within the ranges indicated in Tab. 1 [8]. In many patients this can be achieved simply by observance of a low-fat diet containing a high proportion of complex carbohydrates in combination with increased physical activity, since muscular activity not only improves insulin resistance [1], but also makes it possible for glucose to enter muscle cells in the absence of insulin. For patients in whom the values given in Tab. 1 cannot be achieved by dietary measures and exercise, insulin and other types of drug are available. In most patients a combination of oral antidiabetic agents is used. Combination therapy of this type has the advantage that the doses of the individual agents used are lower than with monotherapy. Since the various classes of oral antidiabetics have different mechanisms of action, use of them in combination maximises their effectiveness while minimising their side effects [9]. TABLE 1: Control parameters at preventing complications of diabetes (T1D and T2D) [7]. parameter risk for macroangiopathy risk for microangiopathy (myocardial infarct, stroke) (nephropathy, retinopathy, neuropathy) HbA 1c less than 7.5 % less than 6.5 % fasting blood glucose less than 7.0 mmol/l less than 6.0 mmol/l (126 mg/dl) (108 mg/dl) postprandial blood glucose less than 9.0 mmol/l less than 7.5 mmol/l (162 mg/dl) (135 mg/dl) 3 For a definition of HbA 1c level, see explanation on p. 13. Type 2 diabetes: development of oral medication 49

6 TABLE 2: Currently available drugs for reducing blood glucose level. drug category chemical class mode of action problems (sample substances) insulins biotechnologically manufactured like endogenous insulins dosing difficult without modified human insulins with delayed onset of action continuous monitoring substances that sulfonylureas (e.g. glibenclamide), stimulation of pancreatic risk of hypoglycemia with stimulate new generation of meglitinides cells to secrete insulin overdosage, exhaustion of secretion of insulin cells by overstimulation insulin sensitisers thiazolidinediones (glitazones) reduction of the peripheral increase in bodyweight (e.g. rosiglitazone, pioglitazone, insulin resistance present troglitazone) in 85 % of type 2 diabetics by reduction of free fatty acids in blood and consequent stimulation of glucose metabolism and reduction of endogenous glucose production in the liver (gluconeogenesis and reduction of blood glucose level) biguanides metformin inhibition of gluconeogenesis gastrointestinal symptoms and, and increased effectiveness very rarely, lactic acidosis of AMP kinase, which promotes glucose uptake into muscle and inhibits synthesis of cholesterol, lipids and triglycerides in the liver inhibitors of acarbose, miglitol inhibition of -glucosidase, digestive disturbance, carbohydrate which releases glucose from flatulence absorption starch, delayed release of glucose from dietary carbohydrates, reduction of postprandial hyperglycemia inhibitors of Xenical reduction of lipolysis in digestive disturbance, fat absorption gastrointestinal tract with flatulence consequent 30 % reduction in fat absorption The oral antidiabetics in use at present have various side effects, some of which are listed in Tab. 2, such as increase in bodyweight, risk of hypoglycemia, macrovascular complications and diarrhea. Moreover, they are effective only in the early stages of T2D, when the fasting blood glucose level in the untreated patient is no higher than 180 mg/dl. In 40 % of people with T2D the oral antidiabetics available at present fail to achieve adequate 50

7 control of blood glucose and have to be supplemented with insulin. There is thus a need for new, more effective, pharmacological treatments against T2D. Present research in this direction is focused on earlier detection and more aggressive treatment. Concretely, this means early diagnosis of T2D before it becomes clinically manifest and gives rise to cardiovascular or non-cardiovascular (i. e. ophtalmic, renal) complications, prevention of macrovascular damage by intensive treatment of the risk factors for metabolic syndrome, improvement of dyslipoproteinemia as evidenced by elevated triglyceride and reduced HDL-cholesterol levels and control of blood pressure [10]. An ideal oral antidiabetic An ideal oral antidiabetic treatment (stand alone or combination) would influence almost all the physiological processes that are disturbed in T2D. Specifically, it would restore insulin sensitivity, improve the function of the insulin-producing -cells of the pancreas, reduce the concentration of free fatty acids in the blood, have beneficial effects on lipids and other substances involved in the metabolic syndrome, have beneficial effects on vascular endothelial functions, suppress glucose synthesis in the liver and achieve long-term control of blood glucose level. In addition, it would of course also be well tolerated [10]. How can this be achieved? At present, researchers are directing their efforts towards finding ways of influencing the endogenous mechanisms, receptors and enzymes that play a role in glucose metabolism and that are known to be disturbed in T2D. Figure 3 shows various possible approaches. These are discussed in detail below. Influencing important Glucokinase modulators: metabolic enzymes promoting glucose breakdown The enzyme glucokinase is a transferase. This means that its physiological function is to transfer a phosphate group from ATP to an -OH group of a glucose molecule. This Type 2 diabetes: development of oral medication 51

8 impaired insulin secretion pancreas GLP-1, DPP-IV adipose tissue glucose free fatty acids glucose uptake glucokinase hepatic clucose output hyperglycemia glucose uptake gut AMPK glycogen phosphorylase liver insulin sensitizer PPAR Co-agonists muscle FIGURE 3: Potential targets, i.e. sites of action, at which drugs (highlighted in orange) can influence the physiological mechanisms that are disturbed or out of control in type 2 diabetics. See text for abbreviations and detailed explanation. (Source of figure: J. Mizrahi, F. Hoffmann-La Roche Ltd) phosphorylation is the first step in the metabolism of glucose. Glucokinases are found in the liver and the pancreas. In the liver the enzyme controls the breakdown of glucose and the storage of glucose in the form of the reserve carbohydrate glycogen. In the pancreas glucokinase acts as a glucose sensor and helps maintain blood glucose levels in the safe range by monitoring changes in blood glucose levels and increasing or decreasing insulin secretion depending on the blood glucose levels. Activation of this enzyme can therefore reduce blood glucose levels via two mechanisms: by enhancement of glucose-stimulated insulin release and by increased heaptic glucose uptake. Researchers at F.Hoffmann-La Roche Ltd identified small molecules that increase glucokinase s ability to metabolise glucose and identified the drugs-binding site on this enzyme that is responsible for its activation. In a variety of animal models of diabetes glucokinase modulators were found to reduce blood glucose level [13]. 52

9 Inhibition of glycogen phosphorylase: inhibiting the breakdown of reserve carbohydrate The enzyme glycogen phosphorylase cleaves the bond between two glucose units in glycogen to form a glucose-1-phosphate 4 residue. Glycogen phosphorylases are found in muscle tissue and in the liver. When blood glucose level is low, glycogen phosphorylase in the liver breaks down glycogen in order to provide glucose for transport to other organs. High blood glucose levels can inhibit the enzyme. Pharmacological inhibition of this enzyme could reduce the excessive release of glucose from the liver that is found in type 2 diabetics. Activation of AMP kinase: simulating physical activity Muscular work has an insulin-like effect in that it promotes the uptake of glucose into cells. Sustained muscular activity, as for example in endurance sports, also has positive effects on insulin sensitivity. This is partly attributed to the increase in the concentration of the glucose transporter GLUT4 in muscle tissue that occurs during sustained muscular activity. The increase in the glucose transporter function depends in turn on activation of a protein kinase present in muscle, namely AMPK (by adenosine monophosphate activated protein kinase). With increasing duration of muscular contraction, energy-rich ATP is split into ADP (adenosine diphosphate) and AMP in order to provide the energy required for the muscular contraction. At the same time, the ATP is replenished by transfer of a phosphate residue of creatine phosphate to ADP. Therefore, when a muscle contracts, the concentration of AMP increases and that of creatine phosphate decreases to an extent that depends on the intensity of the muscular activity. AMP kinase is as suggested by its name activated by the increased concentration of AMP that occurs during muscular contraction and is inactivated by increased concentrations of creatine phosphate [14]. In general, protein kinases phosphorylate (transfer phosphate groups to) and thereby regulate the activity of proteins. Activated AMPK is assumed to phosphorylate and thereby activate proteins that initiate processes that provide energy, i. e. that result in the formation of ATP. These processes include combustion of fatty acids and uptake of glucose into cells. At the same time, AMP 4 A glucose molecule bearing a phosphate group on its carbon atom 1. Type 2 diabetes: development of oral medication 53

10 kinase ensures that energy-consuming processes such as the synthesis of fatty acids, triglycerides and cholesterol are temporarily switched off. In this way it controls the energetic state of the cell. The extent of its activation depends on the energetic output of the muscle and the concentration of the long-term glucose storage molecule glycogen. A drug that could influence the activity of this enzyme would therefore have outstanding potential for the treatment of T2D, since activation of AMPK is believed to initiate the physiological processes in muscle cells that are normally initiated by physical exertion. In other words, such a drug would simulate physical activity. A step further: activation PPARs: action at the level of the cell nucleus of receptors Important metabolic enzymes could potentially be influenced not just directly, but also, for example, by activation of receptors that influence the extent to which they are produced in the body. For example, liver, muscle, fat tissue and macrophages contain receptors known as peroxisomeproliferation-activated receptors (PPARs). These are nuclear receptors that act as transcription factors after being activated. This means that they attach to segments of the genetic information within the cell nucleus that code for the construction of proteins that play a role in the transport and metabolism of glucose, lipids, and lipoproteins. In this way they regulate the extent to which these proteins are formed and thereby influence metabolism. Activation of -PPA receptors in liver and muscle tissue results in increased combustion of lipids, a reduction in the concentration of triglycerides in the blood and an improvement in HDL-cholesterol levels. The -PPA receptors present in macrophages and fat cells play a role in, among other things, regulation of glucose metabolism and energy storage in fat cells. Activation of them can reduce insulin resistance. For example, the substance troglitazone (Tab. 2) can increase glucose uptake into cells by increasing formation of the glucose transporters GLUT1 and GLUT4 [9]. Researchers from F. Hoffmann-La Roche Ltd are currently looking for other substances that can activate either -PPA receptors or both types of PPA receptors (PPAR co-agonists) which might have a stronger impact on insulin sensitisation and metabolic syndrome. 54

11 Influencing insulin resistance by interfering with intracellular signalling pathways Another interesting way of combating insulin resistance is to activate the intracellular domain of the insulin receptor, i. e. the domain which, after binding of insulin to the insulin receptor at the cell surface, activates various intracellular signalling pathways via a series of reactions. Disturbances of these reactions are believed (see above) to influence the extent of insulin resistance in T2D. Influencing enzymes and GLP-1: a hormone with many functions receptors From where do the insulin-producing -cells of the pancreas obtain information as to when and how much insulin they must secrete after a meal? A key substance in this regard is glucagon-like peptide (GLP-1). This is a peptide hormone consisting of only a few amino acid residues. Together with other peptides, it is formed in the intestine from proglucagon, which in turn is formed in the -cells of the islets of Langerhans in the pancreas. Crucially, formation of GLP-1 depends upon the presence of food in the intestine. GLP-1 delays gastric emptying and thereby delays the arrival of food in the small intestine. Depending on the concentration of glucose in the intestine, GLP-1 also inhibits the release of glucagon 5 from the pancreas and, together with glucose, promotes the release of insulin by binding to specific receptors on the surface of the insulin-producing -cells of the pancreatic islets. This initiates intracellular signalling cascades that lead to insulin release [5]. This regulation of insulin release by GLP-1 and glucose ensures that the amount of insulin released is not excessive and therefore that hypoglycemia will not occur. GLP-1 is quickly inactivated by the enzyme dipeptidyl-peptidase-iv (DPP-IV). DPP-IV is a protease that splits and thereby inactivates proteins or peptides circulating in the blood. This interesting physiological regulatory mechanism provides a number of other potential methods of treating type 2 diabetics in whom insulin is still being produced: 5 Glucagon is a natural antagonist of insulin. It stimulates glucose synthesis and glycogen breakdown and thereby increases blood glucose level. Type 2 diabetes: development of oral medication 55

12 Administration of GLP-1 delays the rise in blood glucose level that follows meals [1] and increases release of insulin. The concentration of GLP-1 in the blood can also be increased by inhibiting the enzyme DPP-IV, which breaks down this hormone. A substance which, like GLP-1, could bind to the GLP-1 receptor on the surface of -cells, i.e. a GLP-1 receptor agonist, could increase the release of insulin without eventually exhausting the insulin-producing cells, since insulin release would occur only in the presence of glucose. The presently used substances that stimulate insulin secretion, e.g. sulfonylureas (Table 2), can cause exhaustion of -cells in the long term, since they promote release of insulin independently of the presence of glucose or of any other insulin-stimulating food components [1]. As we have seen, there are now a number of promising approaches to the problem of how to influence the physiological processes that are disordered in T2D in such a way that the elevated blood glucose level is reduced. Nevertheless, the best method of combating T2D is undoubtedly to prevent it from developing in the first place by not allowing oneself to become significantly overweight and by taking a sufficient amount of exercise. 56

13 References 1. Mehnert, H., Standl, E., Usadel, K.-H.: Diabetologie in Klinik und Praxis. Thieme Verlag, Stuttgart, Fiedler, H.: Das metabolische Syndrom, Pathogenese, Diagnostik, Therapie. mta 12 (5): , Simon, A., Giral, P., Lebenson, J.: Extracoronary atherosclerotic plaque at multiple sites and total coronary calcification deposit in asymptomatic men: Association with coronary risk profile. Circulation 92: , Saltiel, A.R., Kahn, C.R.: Insulin signalling and the regulation of glucose and lipid metabolism. Nature 414: , Lexikon of Biochemistry, Spektrum Academic press, Heidelberg, Berlin, Smith, G.D., Pangborn, W.A., Blessing, R.H.: Phase changes in T3R3f human insulin: temperature or pressure induced? Acta Crystallographica, Section D: Biological Crystallography. D57(8): , Turner, R.C., Cull, C.A., Frighi, V., Holmann, R.: Glycemic control with diet, sulfonylurea, metformin, or insulin in patients with type 2 diabetes mellitus: progressive requirement for multiple therapies (UKPDS 49). UK Prospective Diabetes Study (UKPDS) Group. J Am Med Assoc 281: , Ganz, M.: Presentation, Roche Media Roundtable New Approaches in Diabetes Care, Mannheim Mehnert, H.: Typ-2-Diabetes. Pathogenese, Diagnostik, Therapie, Folgeschäden. Medikon Verlag, München, Mizrahi, J.: Presentation Roche Media Roundtable New Approaches in Diabetes Care, Mannheim Mizrahi, J.: What s new in the field of oral antidiabetics: Insulin sensitizers as novel agents for the treatment of type 2 diabetes. Abstract series Roche Media Roundtable New Approaches in Diabetes Care, Mannheim, Wagman, A.S., Nuss, J.M.: Current therapies and emerging targets for the treatment of diabetes. Current Pharmaceutical Design 7: , Die erstaunlichen Fähigkeiten der Glukokinase. Roche Nachrichten 7: 8, Winder, W. W.: Energy-sensing and signalling by AMP-activated protein kinase in skeletal muscle. J Appl Physiol 91: , 2001 Type 2 diabetes: development of oral medication 57