Background
The hyperglycemia in type 2 diabetes mellitus (T2DM) results from an imbalance between insulin secretion and insulin sensitivity [
1] with impaired insulin action and an insufficient and delayed insulin response during meals as well as an inappropriate glucagon secretion [
2]. Fasting as well as postprandial glucagon secretion increase progressively through the spectrum of impaired glucose tolerance to manifest T2DM [
3]. The hyperglucagonemia is associated with hepatic insulin resistance [
4] and an increased hepatic glucose production [
5]. Elevated levels of non-esterified fatty acids (NEFA) resulting from adipose tissue insulin resistance may play a role in the development of peripheral as well as hepatic insulin resistance and may also impair beta-cell function in T2DM and in obese prediabetic individuals [
6].
Metformin, which is recommended as first line therapy in patients with T2DM [
7], has beneficial effects on HbA1c, body weight, cardiovascular mortality [
7,
8] and insulin sensitivity [
9]. However, metformin treatment has no effect on glucagon levels [
10], whereas GLP-1 receptor agonists (GLP-1RA) have been suggested to inhibit glucagon secretion from alpha-cells [
11,
12]. Furthermore, therapy with GLP1-RA is associated with a potentiation of glucose induced insulin secretion and a modest weight loss [
13] and as a result it effectively reduces hyperglycemia in patients with T2DM [
14]. This antihyperglycemic action of the GLP1-RA liraglutide is well-established in patients with longstanding not well-controlled diabetes [
15‐
19]. The effect of GLP-1 receptor agonist on insulin sensitivity is still discussed [
20‐
22], and its effect on insulin clearance is sparsely examined [
23]. Newly diagnosed T2DM in patients with coronary artery disease (CAD) is associated with excess mortality [
24]. Accordingly, it would be of interest how liraglutide may improve postprandial glycaemia and insulinemia in a population of newly diagnosed well-controlled T2DM subjects with CAD, particularly considering that liraglutide is insulinotropic.
The aims of the present study, therefore, were to evaluate effects of the GLP-1 RA liraglutide in combination with metformin on indices of alpha- and beta-cell function, insulin sensitivity and insulin clearance. This was evaluated using a mixed meal test in obese patients with newly diagnosed, well-controlled T2DM and high cardiovascular risk, a population in which efficacy of antidiabetic medication is essential and where several recent guidelines recommend GLP-1 RA, e.g. liraglutide as drug number 2 after metformin [
7,
25].
Discussion
This meal test study of newly diagnosed well-controlled patients with T2DM and established CAD showed that liraglutide combined with metformin versus metformin (+placebo) significantly improved beta-cell function with a trend towards improved insulin sensitivity. Of note, the combination of the insulinotropic liraglutide and metformin reduced post-prandial insulin levels in face of a reduced glucose excursion, which is a new finding in this setting.
In this population we have already shown by use of an non-physiologic intravenous glucose tolerance test a significant improvement in hyperglycemia and a significant increase in insulin levels [
39], which was expected as liraglutide is an insulin secretagogue [
12].
However, in the present study we report data from a mixed meal test where we observed a reduction of insulin as well as glucose levels during this physiological test. The effect of liraglutide on beta-cell responsiveness (B
total) was neutral. These finding might in part be explained by the very well-controlled hyperglycemia with a mean HbA1c of 47 mmol/mol and a baseline fasting plasma glucose on 5.3 mmol/l. Despite these findings the improvement in disposition index is substantial in the present study, just as seen in the non-physiological setting using intravenous glucose as beta-cell stimulation in combination with the so called “minimal model” in the same patients [
39]. The novelty of the present physiologic study is that despite reduction of insulin levels, liraglutide (combined to metformin) retains its strong effect on beta-cell function (i.e. disposition index) in patients with well-controlled T2D and established CAD.
Furthermore, the combination therapy improved insulin clearance and insulin sensitivity, both hepatic and peripheral. In our setup we evaluated basal level of hepatic insulin extraction but did not find effect of liraglutide on this, however basal level was reduced compared to which was previously reported on glucose intolerant subjects [
42]. Reduced hepatic extraction is the primary cause of high levels of circulating insulin after a glucose load [
42] and in normoglycemic subjects hepatic extraction is suppressed up to 30% for ≤ 150 min following a glucose load [
38] and 40–50% of secreted insulin is extracted during a standard meal [
38]. A body weight loss of > 10 kg increased hepatic extraction of insulin [
43] and the weight loss in our study was only < 3 kg.
A recent Japanese study used a mixed meal test and compared metformin and liraglutide as monotherapy in patients with T2DM, and was able to demonstrate improvements in beta-cell function (measured as disposition index) by liraglutide compared to no effect following metformin therapy [
44]. An earlier acute study of 11 subjects with type 2 diabetes and with a HbA1c of 6.5 ± 0.6% a single dose of liraglutide revealed an increase in fasting ISR but post meal AUC
ISR was unaltered in contrast to AUC
glucose which was markedly reduced, indicating an improved beta-cell function [
45].
The effect on the alpha-cell function was unexpected; fasting glucagon levels increased during liraglutide and placebo treatments but neither treatment led to lower glucagon responses during meal test.
Data from the
liraglutide effect and action in diabetes (LEAD) studies are conflicting with respect to the effect on fasting glucagon levels; in LEAD-3 a decrease of fasting glucagon was found but no effect was observed in LEAD-4 [
15,
17]. The absent suppression of glucagon in the present study may indicate a somewhat different action of liraglutide on alpha-cell function compared to native GLP-1, which may relate to the duration of treatment, since a short term period of GLP-1-infusion inhibits glucagon secretion [
46], whereas a long term infusion does not retain the same glucagon suppressive effect [
20]. However, a previous study on patients with early stage T2DM revealed somewhat ambiguous results [
47,
48], and it is suggested that attention must be paid to the performance of the assay used to measure glucagon, in order to obtain valid results [
29]. Additionally, the observed differences in glucagon response can to some extend be caused by differences in the composition of the test meals [
49] and whether a the oral challenge is a mixed meal or glucose [
50]. It is indicated that postprandial glucagon levels are increased after at mixed meal as compared to glucose alone [
50]. We therefore speculate that the present setting may implicate a somewhat different glucagon response compared to an oral glucose challenge or, alternatively that subjects with more advanced disease might reveal a different picture.
In T2DM the suppression of NEFA by insulin is diminished and increased NEFA levels impair insulin action and insulin secretion [
1]. In the present study liraglutide did not change suppression of NEFA compared with metformin but less insulin was needed to suppress postprandial NEFA in the liraglutide arm.
The present results suggest that in well-controlled subjects with type 2 diabetes the positive effect of liraglutide treatment on beta-cell function is clinically relevant but the effect on alpha cell function is subtle. Additionally, the combination of liraglutide with metformin improves insulin sensitivity and clearance and no effect on post-meal hepatic extraction of insulin. However, it is emphasized that liraglutide may produce another metabolic response in patients with less well-controlled and more advanced diabetes patients, which limits the generalization of this study.
A limitation of the study was the relatively few samples and a short duration of the meal test, since longer protocols of longer duration including a higher number of samples will reveal a more detailed picture of early as well as late insulin secretion and alpha-cell function as well as NEFA metabolism in response to a meal test [
51]. The disposition index calculated from the mixed meal test remains to be validated. The assumption is that the beta-cell adaption to ambient insulin resistance follows a hyperbolic law (y = 1/x), and that the product AIR glucose × Si (the disposition index), therefore, is constant in people with normal beta-cell function. The disposition index in subjects with type 2 diabetes is used to obtain a correct estimate of the beta-cell function it relation to the prevailing degree of insulin resistance of the individual. In the present study we tested for the hyperbolic law in the basal state and found a R
2 = 0.81 suggesting that the disposition index may be valid. However, post hoc power analysis indicates that changes in some of the indices reported, especially regarding ISI
Composite could flawed due to the study being underpowered. Nevertheless, this explorative study could indicate a more metabolic flexibility of liraglutide treatment than previously acknowledged. The data presented her warrants further experiments in a greater population.
The data presented here were obtained during a relatively short-term treatment period and demonstrated the effects of liraglutide in newly diagnosed patients. Data from real-world clinical practice indicates that the treatments effects we observed are similar to what was observed in clinical trials [
52]. The present study aimed to reveal treatment effects on pathophysiological features of early T2DM and CAD, yet we speculate that effects in more advanced disease might reveal a different picture, which might add to our knowledge behind the long term positive cardiovascular profile of liraglutide [
53]. In accordance with clinical trails the side-effect profile of liraglutide was relatively low and comprised mainly of gastrointestinal events, which often resolves within 4 weeks of therapy [
41,
54]. The present study did not indicate pathophysiological pathways, which could indicate possible adverse long-term effects. Adverse out-come in our trial does not differ substantially from clinical trials [
41,
54]. Additionally, recent data indicates that liraglutide treatment is not associated with an increased risk for pancreatitis or pancreatic cancer [
55].
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