The Incretins

Incretins are hormones that are released in the gut in response to a meal. 

The incretins promote insulin secretion, inhibit glucagon, and slow emptying of the stomach. The result of these effects is to lower sugar levels in the morning and after eating a meal. 

The insulin-secreting effects of the incretins depend on blood sugar levels. They have their greatest effects at high sugar levels, when increased insulin secretion is needed.

When sugar levels are normal, they have minimal effects, thus avoiding hypoglycemia (low sugar).

Although these benefits were first noted over 50 years ago, incretins were not previously available as therapy since the hormones lasted less than 10 minutes in your circulation. 

Researchers however recently identified technologies that allowed the incretins to last for longer periods of time. 

This allowed physicians to use the incretins to treat people with Type 2 Diabetes, in whom incretin related actions are typically reduced.

The first class of incretin related drugs developed were the "incretin mimetics" or glucagon-like receptor agonists (the GLP-1's). 

Exenatide (Byetta) and liraglutide (Victoza) were the first drugs available in this class.
Exenatide was approved by the FDA in 2005, liraglutide in 2010. Exenatide extended release (Bydureon) was introduced in 2012. All are delivered by injection.

Exenatide is a synthetic formulation of exendin 4, a hormone found in the Gila monster. It is very similar to the human incretin (53% homology), but much more resistant to breakdown. It thus lasts for more than 2 hours (compared to 10 minutes!).

The initial formulation required two injections per day. 

Bydureon is an extended release form of exenatide. It is injected once weekly.

All of the GLP-1 agonists are available in easy to use pens.

Other once weekly formulations include dulaglutide (Trulicity) and albiglutide (Tanzeum). Among the weekly pens, Trulicity is the easiest pen to use.

Several other incretin mimetics, such as lixisenatide, are currently being studied in clincal trials.

Given its relatively short duration of action, and twice daily meal-time dosing, the short-acting formulation of exenatide is most effective at lowering after-meal blood sugars.

Liraglutide has been chemically modified to resist breakdown. It lasts for 11-13 hours, and is injected once per day. 

Given its longer duration of action, liraglutide is most effective at lowering morning sugars.

Exenatide extended release is likewise most effective for reduction of morning sugars.

On average, all GLP-1's will lower after meal sugars by ~50 mg/dl, and lower fasting sugars by ~25 mg/dl. HA1c reductions average 1.5%.

All are associated with reduced appetite and an average weight loss of 6 pounds after 6 months. At one year, the average weight loss is 10 pounds.

The weight-loss benefits associated with this class of drugs are typically described as an early sense of "fullness" while eating a meal.

Liraglutide appears to be slightly superior to exenatide with respect to glucose lowering efficacy and weight loss.

GLP-1's have also been associated with small reductions in blood pressure.

Incretin related side effects include nausea, and rarely, vomiting. 

Gastrointestinal complaints are typically dose-related, and lessen with time.

Exenatide extended release has the lowest rate of stomach upset. Regular release exenatide has the highest.

Recently, cases of acute pancreatitis have been described in people treated with exenatide and liraglutide. 

Risk assessment is complicated since pancreatitis risk is already increased in people who are obese and who have Type 2 Diabetes.

A 2013 statement from the FDA and the European Medicines Association stated, "available data do not confirm recent concerns over an increased risk for pancreatic side effects with glucagonlike peptide-1 (GLP-1)-based diabetes therapies".

This is a link to the article:

Thyroid tumors were seen in mice and rats treated with liraglutide in pre-clinical trials. 

These findings were not observed in monkeys. There were no reported cases of medullary thyroid cancer in human clinical trials.

Incretins also appear to promote the growth and survival of beta cells (the cells that release insulin). 

The LIBRA Trial (Diabetes Care 2014; 37:3270-3278) reported preservation of beta cell function at 48 weeks in 51 people with Type 2 diabetes of 2.6 +/- 1.9 years duration treated with liraglutide. Benefits were lost when therapy was discontinued. The clinical relevance of this is unknown pending further long-term trials.

The second class of incretin related drugs are the "DPP-4 inhibitors" (also known as the DPP-4's). 

The DPP-4's inhibit the breakdown of the incretins, thereby increasing circulating levels of these hormones. 

The most commonly used drugs in this class are sitagliptin (Januvia), saxagliptin (Onglyza) and linagliptin (Tradjenta). All are once daily oral medications.

The DPP-4 inhibitors reduce HA1C levels by an average of 0.8%.

Fasting and after-meal blood sugar levels are reduced, albeit to a lesser degree than with the GLP-1's. The DPP-4's 
are "weight-neutral" without an effect on appetite.

The DPP-4 inhibitors are very well tolerated due to simple once daily dosing and rare side effects.

Linagliptin does not require dose alteration in the setting of kidney disease.

Two studies published in 2013 noted an increased incidence of congestive heart failure among users of saxagliptin and alogliptin.

Although results from subsequent trials have been inconsistent, with some showing no risk (EXAMINE) and others reporting risk in combination with metformin, caution should be exercised in people with, or at high risk for, congestive heart failure.

We have several ongoing trials studying the incretins at our research center.

If interested in more information, please contact our study coordinator at 301-770-7373.