Diabetes is a progressive disease characterised by impaired β-cell function, and reduced insulin sensitivity and secretion. Over time, glycaemic control deteriorates and exacerbates the risk of patients experiencing micro- and macrovascular complications [1].

Although there is an abundance of treatment options and guidelines available for the management of type 2 diabetes, they are unable to avert the natural progression of the disease and sustain glycaemic control in the long term. Furthermore, currently available treatment options (such as sulphonylureas [SUs], thiazolidinediones [TZDs] and most insulins) are often associated with hypoglycaemia and weight gain [1,2] (Figures 1 and 2). Such adverse effects can have detrimental health implications for the patient. For example, hypoglycaemia can be an unpleasant side effect of antidiabetes therapy that can compromise patient adherence to treatment, and serious hypoglycaemic events, left untreated, can lead to a loss of consciousness, brain damage or even death. Moreover, obesity is a common comorbidity in subjects with type 2 diabetes (affecting 60–90% of patients diagnosed). Additional weight gain as a consequence of treatment with SUs or insulin, for example, can reduce patients’ quality of life and hinder adherence to treatment. Obesity is also an independent risk factor for cardiovascular (CV) disease, further compromising patient outcomes [3,4,5].

The greatest challenge in treating patients with type 2 diabetes is optimising therapy to address the current unmet needs, which include:

• improving glycaemic control without compromising safety, e.g., hypoglycaemia

• preserving β-cell function

• providing clinically meaningful weight loss

• addressing cardiovascular risk factors accompanying diabetes

• offering a simple and flexible regimen

New and effective treatment options are being sought to address the unmet clinical needs of patients with type 2 diabetes. Indeed, therapies are emerging that target the incretin system by exploiting the physiological actions of glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). These therapies may provide solutions to all of these challenges.

GLP-1 and GIP are gut-derived hormones known as ‘incretins’. The incretin hormones have multiple physiological actions; most importantly, they play a crucial role in glucose homeostasis. The action of the incretin hormones accounts for 50–70% of insulin secretion after oral glucose intake [6]. This ‘incretin effect’ is characterised by the more pronounced plasma insulin secretion observed with oral versus intravenous glucose administration despite matching glucose profiles [6] (Figure 3a and3b).

IV, intravenous; T2DM, type 2 diabetes mellitus.

The incretin effect is blunted in subjects with type 2 diabetes [7] (Figure 3c). In such patients, GLP-1 infusion (but not GIP) markedly improves both the early and late phases of insulin secretion in response to glucose [8,9]. As such, it is GLP-1 rather than GIP that has been the focus of research as a promising treatment for type 2 diabetes.

The action of GLP-1 on β-cell receptors enhances insulin secretion in a glucose-dependant manner [10], which, in turn, minimises the risk of hypoglycaemia. In addition, animal studies have indicated that GLP-1 has the ability to preserve β-cell function by suppressing β-cell apoptosis and stimulating neogenesis and proliferation [11,12].

Other clinical advantages associated with GLP-1 may be explained by its hormonal influences on the gastrointestinal, central nervous system and CV systems (Figure 4). GLP-1 has demonstrated the ability to slow gastric emptying and suppress appetite, resulting in satiety and weight loss [10,13].

GLP-1 also appears to exert a protective effect on the myocardium, particularly in ischaemic conditions [14,15]. For example, Bose and colleagues reported that an intravenous infusion of GLP-1 in rats prior to induced ischaemia significantly reduced myocardial infarction compared with saline [16]. Studies have also reported improved myocardial function in patients with type 2 diabetes and congestive heart failure [17], and infusion of recombinant GLP-1 has been shown to improve left ventricular function in patients with acute myocardial infarction after primary angioplasty [18]. Improvement in endothelial function has also been reported with GLP-1: a significant increase in brachial artery diameter was observed in patients with type 2 diabetes [19].

Furthermore, GLP-1 has been found to reduce systolic blood pressure (SBP). This may be, in part, due to its ability to increase diuresis and natriuresis, thus reducing blood volume and central venous pressure. This is an important observation since CV disease is a common comorbidity of type 2 diabetes, and a mean reduction in SBP of 5.6 mmHg has been shown to reduce mortality from CV disease by 18% [20].

As active GLP-1 is secreted from L-cells in the distal small intestine, it is rapidly degraded by the enzyme dipeptidyl peptidase IV (DPP-4). An intravenous bolus of GLP-1 has a half-life of 1.5–2.1 min [21], meaning that a constant infusion of GLP-1 is required to provide any therapeutic value.

In order to overcome the short half-life of native GLP-1, longer-acting GLP-1 agents such as liraglutide have been developed. Liraglutide is an analogue of native human GLP-1, in which Lys34 has been substituted with Arg34 at the N-terminal and a fatty acid chain added to Lys26 [21] (Figure 5). These modifications mean that liraglutide shares 97% amino acid identity with native human GLP-1, whereas exenatide, a GLP-1 mimetic, shares just 53% sequence identity at the N-terminal 30-aminoacid peptide, with no mammalian homology for the remaining nine amino acids.

Liraglutide’s fatty acid side chain allows it to self-associate and form heptamers. This allows liraglutide to be absorbed slowly via the subcutaneous route [21]. Maximum plasma concentration levels are achieved between 9–12 hours after dosing [22].The fatty acid moiety, which is internally orientated when liraglutide is heptameric, also allows liraglutide in monomeric form to bind to serum albumin in the bloodstream [23] and resist DPP-4 degradation, resulting in a half-life of approximately 13 hours [24]. These properties make liraglutide suitable for once-daily dosing [24].

The phase 3 clinical development programme for liraglutide, LEAD (Liraglutide Effect and Action in Diabetes) is the largest clinical development programme ever conducted by Novo Nordisk in diabetes; a total of 4,456 subjects were included, recruited at more than 600 sites across 40 countries, of which 2,739 patients were treated with liraglutide. LEAD included six trials and was designed to investigate the efficacy and safety of patients treated with liraglutide across the continuum of care of type 2 diabetes versus placebo.

In the LEAD trials, liraglutide was also compared to some commonly used antidiabetic therapies. In addition to the registration studies (LEAD-1 to LEAD-5), a head-to-head trial against exenatide (LEAD-6) was also completed (Figure 6).

This review focuses primarily on clinical trial research as the most stringent source of current data concerning the efficacy and safety of liraglutide; however, following its recent approval by both the FDA and the EMEA, it is to be anticipated that further research and material concerning liraglutide use in clinical practice will become available. This section considers liraglutide use in the clinic in the light of current advised practice.

The LEAD programme has demonstrated that liraglutide is effective and well tolerated as a starting therapy (monotherapy) or as an add-on therapy for subjects at a later stage of disease progression. Based on results from the LEAD programme, it is possible to recommend potential applications for liraglutide use in clinical practice (Figure 13), and in these studies liraglutide has been demonstrated as an effective therapeutic intervention regardless of when, or with what, it has been used.

Although the trial data suggest a broad application, experience of liraglutide in clinical practice is limited. The best possible stage of treatment intervention is still speculative. Given a potential effect, if as yet unsubstantiated, on β-cell preservation [11,12,31], it seems possible that earlier use will extend the endogenous insulin function of patients, and perhaps slow progression of the disease. A meta-analysis of clinical data showed that reductions in HbA_1c and weight were achieved within eight weeks of liraglutide therapy, from which the authors concluded that adding liraglutide to an existing OAD regimen early in therapy may benefit patients [37]. Substantial reductions in SBP and FPG were also observed [37]. In terms of durability of use, that is, how long liraglutide can maintain good glycaemic control before an additional and/or alternative agent is needed, the LEAD studies generally showed that HbA_1c levels were sustained for the 26-week trial period [25,26,28,29,30]. The LEAD-3 study, exploring liraglutide as monotherapy, lasted for 52 weeks [27], suggesting that liraglutide’s effects may be sustained in longer-term treatment. However, given the individuality of each patient, it seems likely that the continued monitoring and assessment characteristic of treatment management in type 2 diabetes will be necessary to evaluate the durability of this new drug.

One question of particular importance concerns the relationship of liraglutide to insulin use; patients with type 2 diabetes will usually require exogenous insulin as their endogenous response fails, but barriers to insulin use are substantial. Concerns regarding weight gain and hypoglycaemia, two commonly feared side effects to insulin use [38], may mean that insulin is introduced late into the regimen once glycaemic control has started to fail. A comparison of liraglutide and insulin glargine in the LEAD-5 study demonstrated superior efficacy of liraglutide, suggesting that the drug could also be used as an alternative OAD to insulin [29]. The progressive nature of type 2 diabetes may make, insulin use inevitable for many patients, however,liraglutide may enable physicians to delay this treatment evolution, while maintaining good glycaemic control. Liraglutide may be a more helpful adjunct to metformin than SUs, given that the side effects of weight gain and hypoglycaemia are significantly lower; this, in turn, may support patient adherence and acceptance of therapy. Equally, adding liraglutide to an existing SU-based regimen may ameliorate some of the negative side effects.

Liraglutide has been approved by the European Medicines Agency (EMEA) and is currently implicated as a second- and third-line treatment option for type 2 diabetes [34].

Second-line treatment: Liraglutide in combination with metformin or SU, in patients with insufficient glycaemic control despite maximal tolerated dose of monotherapy with metformin or an SU.

Third-line treatment: Liraglutide in combination with metformin and an SU or metformin and a TZD in patients with insufficient glycaemic control despite dual therapy.

Most recently, the FDA has approved liraglutide use as second- or third-line adjunct therapy, but also, significantly, as a monotherapy.

There are typically three steps in the current treatment guidelines to adhere to. According to the ADA/European Association for the Study of Diabetes (EASD) guidelines, at the onset of type 2 diabetes (step 1), lifestyle intervention and metformin is initiated [39]. Traditionally, ‘well-validated’ basal insulin is considered when these patients fail to reach or sustain glycaemic targets (step 2), and further intensification of insulin is required (step 3). Currently, GLP-1 receptor agonists, including liraglutide, are recommended as second-line therapy by the ADA/EASD guidelines as a ‘less well-validated’ treatment [39].

In a statement, the American Association of Clinical Endocrinologists (AACE) guidelines recommend the use of incretin mimetics (GLP-1 receptor agonists) as second-line therapy, followed by DPP-4 inhibitors, glinides, or sulphonylureas (SUs) with metformin remaining the cornerstone of dual therapy.  Indeed, dual therapy is recommended if the HbA_1c ranged between 7.6% and 9.0% [40]. The panel recognised the lower risk of hypoglycaemia associated with GLP-1 receptor agonists and DPP-4 inhibitors compared with glinides and SUs, and were favoured over DPP-4 inhibitors as they demonstrated greater postprandial glucose lowering and substantial weight loss. For patients requiring triple therapy (e.g., HbA_1c > 9.0%), a GLP-1 receptor agonist is the second preferred agent [40]. The National Institute for Clinical Excellence (NICE) also recommends GLP-1 receptor agonists use in combination with metformin and an SU as a third-line treatment option [41].

The role of GLP-1 receptor agonists as a treatment option for type 2 diabetes is rapidly evolving. Liraglutide, the first human GLP-1 analogue, has the potential to overcome the shortcomings of conventional therapies and address the current unmet needs of patients with type 2 diabetes.

With its once-daily administration, liraglutide has demonstrated superior clinical efficacy and a favourable safety profile in recent clinical trials versus standard therapies. Indeed, glycaemic control improved substantially and was maintained throughout the trial period with liraglutide. Furthermore, reports of hypoglycaemia were rare. Liraglutide has also been found to promote and maintain weight loss and reduce SBP, which, in turn, may reduce the risk of CV disease. Moreover, reductions in SBP appear not to be influenced by body weight change. Interestingly, there is an indication that liraglutide may have a positive effect not only on β-cell function but also on β-cell mass, which implies potential long-term benefits. Compared with exenatide, liraglutide offers similar efficacy, but with once-daily dosing, which may be of importance to patients. Similarly, given the greater sequence identity between liraglutide and native GLP-1, fewer antibodies are formed, and this may account for improved tolerance in terms of side effects. For patients with type 2 diabetes, liraglutide offers a new and flexible treatment alternative to standard therapies that may help overcome the unmet needs of current therapies.