Classification and Incretin Physiology
Incretins act as natural stimulators of endogenous insulin incretion. They are released by small intestinal cells immediately following a meal and enhance glucose-stimulated insulin secretion.
Pharmacologically, this group comprises two main approaches:
- Glucagon-like peptide-1 (GLP-1) receptor agonists — include agents such as exenatide.
- Dipeptidyl peptidase-4 (DPP-4) inhibitors — include agents such as saxagliptin.
Incretins themselves are represented by GLP-1 (consisting of 30 amino acids) and GIP (glucose-dependent insulinotropic polypeptide). In the body, they are rapidly degraded by DPP-4, a serine peptidase that cleaves an N-terminal alanine residue.
Molecular Mechanism of Action
At the cellular level in pancreatic $\beta$-cells, a complex signaling cascade occurs:
- Molecules bind to GLP-1 receptors on the cell membrane.
- Adenylyl cyclase is activated, which stimulates protein kinase A (PKA), alongside phospholipase C activating protein kinase C (PKC).
- These kinases block ATP-sensitive $K^+$ channels.
- The electrophysiological effect leads to depolarization of the cell membrane.
- Voltage-gated calcium channels open, allowing $Ca^{2+}$ ions to rush into the cell, triggering insulin synthesis and exocytosis.
Exenatide: Properties of the First GLP-1 Agonist
Exenatide is derived from the salivary glands of the Gila monster (Heloderma suspectum), sharing 52% amino acid sequence homology with human GLP-1.
- Pharmacodynamics: It predominantly activates the glucose-dependent phase of insulin secretion. Because insulin secretion tapers off as blood glucose levels drop, the risk of hypoglycemia remains low.
- Administration: Administered subcutaneously at a dose of 5–10 mcg twice daily, typically combined with metformin or sulfonylureas.
- Pharmacokinetics: Peak concentration is reached in 2 hours, the half-life ($T_{1/2}$) is 2.4 hours, and elimination occurs primarily via glomerular filtration.
DPP-4 Inhibitors and Saxagliptin Pharmacokinetics
Drugs in this subclass inhibit the serine peptidase DPP-4, preventing the degradation of endogenous incretins (GLP-1 and GIP). This increases their plasma concentration 2- to 3-fold and potentiates the hypoglycemic effect.
Using saxagliptin as an example, the pharmacokinetic profile is as follows:
- Oral bioavailability is approximately 75%.
- Hepatic metabolism yields an active metabolite.
- Elimination is predominantly renal.
- The half-life of the parent compound is 2.5 hours, and that of the active metabolite is 3.1 hours. It is administered once daily.