Recently introduced and emerging classes of glucose-lowering drugs
1. Drugs acting on the incretin system
Insulin secretagogues such as sulphonylureas cause insulin release irre-spective of the prevailing glucose concentration. Enhancing insulin secre-tion through a glucose-dependent mechanism is an attractive, novel therapeutic approach that can circumvent some of the unwanted effects of sulphonylureas principally weight gain and hypoglycaemia. This ap-proach would be expected to restore the defectivebcell insulin secretion pathway without causing excessive hyperinsulinaemia and carry an intrinsically lower risk of iatrogenic hypoglycaemia. In recent years, novel drugs have become available that are designed to fulfill these aims.
2. Pathophysiology of the incretin system in type 2 diabetes Gastrointestinal polypeptide hormones secreted in response to the inges-tion of a meal augment insulin secretion; this is known as the incretin effect and is held to account for up to 70% of postprandial insulin secre-tion (Figure 3.1). The most important incretins are glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP). These hormones are secreted by the L cells of the distal ileum and colon, and the K cells of the duodenum and upper jejunum, respectively. Circulating levels rise within minutes of eating, implying likely stimulation by neuroendocrine pathways. Incretin hormones act by means of specific b-cell G-protein-coupled receptors to enhance glucose-stimulated insulin secretion. The acute effect of GLP-1 on b-cells is the stimulation of glucose-dependent insulin release; this is followed by enhancement of insulin biosynthesis and stimulation of transcription of the insulin gene.
The incretin effect is deficient in patients with type 2 diabetes, mainly because of reduced postprandial GLP-1 secretion (see Figure 3.1). Re-duced GLP-1 levels are accompanied by a reduced insulinotropic action of GIP. The cause of these defects is presently unclear. GLP-1, but not GIP,slows gastric emptying and also suppresses appetite, effects that are im-paired by deficiency of the hormone in type 2 diabetes.
There is abundant experimental evidence that inappropriate glucagon secretion plays a role in the development of hyperglycaemia in type 2 diabetes, sustaining hepatic glucose production rates in the presence of relative insulin deficiency; this defect in islet a-cell function is thought to reflect impaired cellular glucose sensing. GLP-1 suppresses glucagon secretion during hyperglycaemia. The counter regulatory secretion of glucagon in response to hypoglycaemia, however, is preserved even in the presence of pharmacological concentrations of GLP-1. Inappropriate glucagon secretion is reduced by the incretin system
Studies in rodents and human islets indicate that GLP-1 can promote
the expansion of b-cell mass. To date, however, there is no convincing evidence that this effect is reproduced in humans with type 2 diabetes. In addition to isletbandacells the tissue distribution of the GLP-1 receptor includes the central and peripheral nervous system, kidney, lung and gastrointestinal tract. GLP-1 receptors are also present in the myocardium where their stimulation reduces ischaemic injury in animal models.
GLP-1 and GIP are subject to rapid degradation mainly by a ubiqui-tous cell surface enzyme dipeptidyl peptidase (DPP)-4, which cleaves two N-terminal amino acids thereby removing insulinotropic activity. The half-life of GLP-1 in the circulation is less than 2min, and ~7min for GIP.
Numerous gastrointestinal hormones, cytokines and chemopeptides are also substrates for DPP-4. The enzyme, which is a member of a wider family, is also the CD26 T-cell-activating antigen found in nearly all human tissues.Novel therapies that exploit the incretin effect of GLP-1 include the orally activeDPP-4 inhibitors and the injectableGLPmimetics. The latter fallinto two categories:
(1) derivatives of GLP-1modified to resist proteolysis and
(2) novel peptides that have metabolic actions similar to GLP-1 and are intrinsically resistant to proteolysis. The GLP-1 receptor agonists licensed in the UK are exenatide (twice-daily and once-weekly formulations) and liraglutide. The place of these agents in treatment algorithms is being explored.
3. Dipeptidyl peptidase 4 inhibitors
DPP-4 inhibitors, also known as gliptins, enhance the levels of the intestinally secreted incretins GLP-1 andGIP through selective inhibition of the enzyme DPP-4 (Figure 3.2). Drugs in this class are now being incor-porated into management pathways for type 2 diabetes. Sitagliptin and vildagliptin were introduced in the UK in 2007 and 2008, respectively.Saxagliptin is available in the USA, and received marketing authorization in the European Union in 2009. The drugs differ in their metabolism and safety profiles (Table 3.2). Sitagliptin is currently available in three fixed-dose combinations withmetformin: 50/500mg, 50/850mg and 50/1000mg.
Vildagliptin is offered as fixed-dose combinations with metformin:
50/850mg and 50/1000mg. In the USA, the Food and Drug Adminis-tration (FDA) demanded additional clinical safety data before it could approve vildagliptin. This followed evidence of skin lesions in a primate model and issues of safety in patients with renal impairment.
In 2011, the FDA and EMA approved linagliptin, which has a primarily non-renal route of elimination. Linagliptin has been approved for combination with insulin as well as in combination with certain oral glucose-lowering agents. Other DPP-4 inhibitors are in development, including alo-gliptin, for which the results of further safety and efficacy studies are awaited.
Sitagliptin, vildagliptin and saxagliptin are selective, competitive, reversible inhibitors of DPP-4. Sitagliptin is non-covalently bound to DPP-4,where as vildagliptin and saxagliptin bind covalently. Selectivity of action has been an important consideration in the development of these drugs.
For sitagliptin, no inhibition of DPP-8 and DPP-9, gene members of the S9b family of dipeptidyl peptidases, is anticipated at exposures required for glucose lowering in humans.Sitagliptin–This has high bioavailability (~90%) and a plasma half-life of 8–14h; Tmaxis 1–4h. Plasma protein binding is relatively low at approxi-mately 40%. A small proportion of the drug is hepatically metabolised by CYP3A4 and CYP2C6. Approximately 80% of sitagliptin is eliminated unchanged in the urine through renal tubular secretion. A single dose of 100mg sitagliptin achieves near complete inhibition of DPP-4 activity for approximately 12h, with>95% inhibition up to 24h Vildagliptin–This has a shorter plasma half-life: 1.5–4.5h. Tmaxis less than 2h. Bioavailability is approximately 85%. The majority of vildagliptin (~70%) undergoes predominantly renal metabolism to inactive metabolites with a negligible contribution from CYP450 isoforms; most (~85%) is eliminated in the urine. A dose of 50–100mg vildagliptin will provide al-most complete inhibition of DPP-4 for approximately 12h and approxi-mately 40% inhibition at 24h. Plasma protein binding is less than 10%.
Saxagliptin–Maximal inhibition of DPP-4 occurs 2–3h after oral ad-ministration; the suppression of DPP-4 activity extends to 24h. Saxagliptin is eliminated by both renal and hepatic pathways. Hepatic meta-bolism creates a hydroxylated metabolite that has approximately 50% of the activity of the parent compound. There is evidence of active renal excretion of the parent compound. Circulating levels of saxagliptin and its metabolite are increased by renal impairment.In vitro serum protein binding is 30% or less in humans.
3.2 Clinical applications
In principle, DPP-4 inhibitors can be used as monotherapy in patients with type 2 diabetes who have responded inadequately to lifestyle measures. This is not, however, permitted in all countries. At present, DPP-4 inhibitors tend to be preferred as second or third-line therapy in patients inadequately con-trolled by metformin, a sulphonylurea, or a thiazolidinedione. Theoretically,they could be used with any other class of oral agent or insulin, as their mode of action on the b-cell is different to that of sulphonylureas and meglitinides; moreover, their ability to reduce glucagon levels may provide a useful adjunct to insulin therapy even when there is major b-cell dysfunction. In the UK sitaglipitin is approved for addition to a stable dose of insulin, with or without metformin. Improvements in b-cell function reflect the mode of action of the drugs and are dependent onb-cell reserves. The absence of weight-promoting actions makes the DPP-4 inhibitors especially suitable for overweight and obese patients. The low intrinsic risk of hypoglycaemia when used in conjunction with non-insulin-releasing agents makes them suitable for higher-risk groups. These include individuals who are already approach-ing glycaemic targets, or have unpredictable meal patterns.Sitagliptin 100mg is required once a day and can be taken with or without food; vildagliptin 50mg is usually taken twice a day. Lower do-sages are recommended if combined with a sulphonylurea. In the UK the licence for sitagliptin includes monotherapy if metformin cannot be used; in combination with a sulphonylurea; in combination with a thiazolidinedione (pioglitazone); in combination with metformin plus sulphonylurea; and in combination with metformin plus thiazolidinedione.Sitagliptin is also approved in the UK as add-on to insulin, or insulin and metformin.Recently introduced glucose-lowering drugs Saxagliptin is usually taken once a day in a dose of 5mg. The dose of saxagliptin should be reduced to 2.5 mg once daily and used with caution in patients with moderate or severe renal impairment (i.e. creatinine clearance ‡30 to<50 ml/min). Saxagliptin is not recommended for pa-tients with end-stage renal disease requiring haemodialysis. The UK licence includes use with metformin, a sulphonylurea if metformin is in-appropriate, and with a thiazolidinedione; in all these situations diet and exercise will have been deemed to be providing inadequate glycaemic control Vildagliptin is licensed in the UK for use as dual oral therapy in com-bination with metformin, in patients with insufficient glycaemic control despite the maximal tolerated dose of monotherapy with metformin; with a sulphonylurea, in patients with insufficient glycaemic control despite the maximal tolerated dose of a sulphonylurea and for whom metformin is in-appropriate due to contraindications or intolerance; and a thiazolidinedione,in patients with insufficient glycaemic control. In 2012 approval was rec-ommended for use of vildagliptin in combination with insulin by the EMA.
The glucose-dependent action of these agents reduces the risk of ex-cessive reductions in blood glucose, unless combined with a sulphonylurea.There is thus no dose titration. Monitoring of fasting and postprandial glycaemia aids assessment of response. If hypoglycaemia occurs when a DPP-4 inhibitor is combined with a sulphonylurea, reducing the dose of the sulphonylurea or withdrawal of the DPP-4 inhibitor is recommended.
3.3 Cautions and contraindications
Sitagliptin is mainly eliminated unchanged in the urine. In the USA,a reduced dose is recommended for patients with moderate renal in-sufficiency, i.e. creatinine clearance of 30ml/min or more to less than 50ml/min; for patients with severe renal insufficiency or with end-stage renal disease requiring haemodialysis or peritoneal dialysis, a dose of 25mg once a day should be considered. Sitagliptin does not appear to affect P450 isoforms; this allows its use in patients with minor to moderate impair-ment of liver function, provided renal function is adequate. The levels of saxagliptin and its active metabolite are reduced in patients with impaired hepatic function. The dose of saxagliptin should be reduced to 2.5 mg once daily in patients with moderate or severe renal impairment. In view of the renal metabolism and elimination of vildagliptin, its use is not re-commended in patients with moderate or severe renal impairment. The primarily non-renal route of elimination of linagliptin means that no dose reduction is required in patients with renal impairment. Some cases of reversible elevations in alanine aminotransferase or aspartate aminotransferase have been observed in patients receiving vildagliptin. Clinical chemistry assessment of liver function is recommended before starting treatment, at 3-month intervals in the first year, and periodically there-after. A marked rise in liver enzymes, e.g. alanine aminotransferase or aspartate aminotransferase more than three times the upper limit of the normal range, or other signs of hepatic impairment contraindicate con-tinued treatment. No significant drug interactions have been noted with sitagliptin, vildagliptin, or saxagliptin. DPP-4 inhibitors should be avoided in women planning conception and during pregnancy.Saxagliptin is being evaluated in a cardiovascular outcomes study to fulfill a US FDA post marketing requirement. No evidence of cardiovascular risk has emerged to date. A pooled analysis of phase IIb and III trials suggested a trend towards a reduction in cardiovascular events compared with placebo or active comparators.
Sitagliptin–In clinical trials, the administration of 100mg/day sitagliptin as monotherapy or add-on therapy to other oral agents typically reduced glycated haemoglobin (HbA1c) from a baseline of approximately 8% by approximately 0.7 percentage points after 24–52 weeks. Individuals with higher baseline HbA1c levels have shown reductions in HbA1c greater than 1%; this observation, i.e. greater glucose lowering with higher baselineHbA1c levels, is not unique to this class of agents. Fasting plasma glucose concentrations were reduced by approximately 1.0–1.5mmol/l, and postprandial glucose levels measured 2h after a standard mixed meal were usually reduced by approximately 3mmol/l. Sitagliptin did not cause a clinically significant increase in the incidence of hypoglycaemia; the combination of sitagliptin plus metformin was associated with fewer espisodes of hypoglycaemia than a combination of glipizide plus metformin at similar levels of HbA1creduction. Sitagliptin did not increase body weight compared with placebo in clinical trials.
Vildagliptin–In trials, a single daily dose of 50–100mg/day vildagliptin showed similar efficacy and tolerability to sitagliptin when used as monotherapy or as add-on therapy to metformin or a thiazolidinedione.
The dose of vildagliptin should be 50mg once a day when combined with a sulphonylurea. Several trials with vildagliptin produced slightly greater reductions in HbA1c but these tended to be associated with slightly higher average baseline HbA1c levels, i.e. greater than 8.5%. There was no in-crease in hypoglycaemia or changes in mean body weight with vildagliptin.
Saxaglipitin–In clinical trials, a dose of 5mg a day either as monotherapy,or in combination with metformin, a sulphonylurea, or a thiazolidinedione produced mean placebo-subtracted reductions in HbA1c in the range of approximately 0.60–0.65%. No excess of hypoglycaemia or weight gain was observed in phase III trials compared with placebo.
3.5 Adverse effects
To date no serious adverse effects have been reported. Unlike GLP-1 agonists, upper gastrointestinal side-effects are not a feature of DPP-4 therapy. In clinical trials generally of 6–12 months’ duration, tolerability and adverse events were broadly similar to placebo or the comparator drug,athough there have been reports of higher rates of nasopharyngitis and upper respiratory tract infections. The a fore mentioned increases in liver enzymes observed with the 100mg dose of vildagliptin does not appear to be relevant to the 50mg dose that is marketed