Exenatide

Exenatide is an antidiabetic drug, injected subcutaneously. It is a synthetic version of exendin-4, a metabolic hormone originally isolated from the salivary secretions of the Gila monster (Heloderma suspectum). Exendin-4 is a 39-amino acid long peptide analog that shares a 53% sequence homology with human GLP-1 (Parkes, 2001).

Table 1. Basic Profile of Exenatide.

Figure 1. 2D and 3D structures of Exenatide. a. Amino acid sequence of Exenatide, the synthetic form of Exendin-4 (PubChem). Note: amino acid modifications (from the native sequence) are highlighted in yellow and the DPP4 cleave site is indicated with the scissors. b. 3D structure of the Exendin-4 showing amino acids 1-28 (in beige, from PDB ID 7lll, Deganutti et al., 2022) and amino acids 9-35 (in light blue, from PDB ID 3c59, Runge et al., 2008). Note: the receptor and G-protein chains are hidden here for clarity. Click on the links provided here to view the full structures interactively.

Drug Information

Table 2. Chemical and physical properties of Exenatide

Drug Target

Endogenous incretins, including GLP-1 and GIP, facilitate glucose-dependent insulin secretion in response to food intake. Incretin mimetics, such as exenatide, are functional analogs of glucagon-like peptide-1 (GLP-1), a naturally occurring peptide that has the potential to significantly improve glycemic control in patients with diabetes. Exendin-4 mimics the incretin hormone GLP-1, so it competitively binds and activates the GLP-1 receptor (GLP-1R). Learn more about GLP-1 receptors.

Structurally identical to native exendin-4, exenatide is a chemically synthesized peptide with clinical applications in the management of type 2 diabetes. Compared to GLP-1, Exendin is more resistant to DPP-4 degradation, thus making it a more potent insulinotropic agent. The truncated Exendin-4, with amino acids 9-39 (Ex4(9-39)) is also homologous to GLP-1, but it has a C-terminal extension of nine amino acid residues called the Trp cage (absent in GLP-1). The truncated hormone has a higher affinity and potency for the N terminal domain of GLP-1R and can displace both Ex4 and GLP-1. The IC50 value of truncated Ex4 is 0.6 nM compared to 1.0 nM for Ex4 and 1.6 nM for GLP-1 (Runge et al., 2008). While some researchers suggest that Trp cage explains the superior binding affinity of Ex4 relative to GLP-1, studies have shown that specific divergent residues in the C-terminal part of Ex4 contribute to greater interactions with the receptor (Runge et al. 2008).

Amongst other GLP-1R agonists, Exenatide uses a combination of mechanisms which may include glucose-dependent stimulation of insulin secretion, suppression of glucagon secretion, enhancement of β-cell mass, slowing of gastric emptying, inhibition of food intake, and modulation of glucose trafficking in peripheral tissues (Nielsen, 2004). Due to several amino acid modifications, exenatide is a poor substrate of DPP-4, resulting in a relatively longer plasma half‐life (2.4 h) in comparison with the native GLP‐1 peptide (<2 min).

Drug-Target Complex

Belonging to the class B G-protein-coupled receptor, GLP-1R is a seven transmembrane protein receptor (Figure 3). It consists of:

• An extracellular N-terminus that binds the C-terminal part of the incretin analog
•  A seven transmembrane helical domain (TMD) with the a-helices separated by three intracellular loops and three extracellular loops
• An intracellular C-terminus that is responsible for intracellular signaling

A crystal structure of the N-terminal domain of GLP-1R and Exendin 4 (residues 9-39) shows the peptide as a well-defined α-helix (FIgure 2, PDB ID 3c59, Runge et al., 2008). Only residues within the Glu15 to Ser32 engage with the N-terminal domain of GLP-1R, showing several hydrophobic interactions and a few polar interactions. The amino acids Glu16, Glu17, and Arg20 form hydrogen bonds and ionic interactions with each other and with the GLP-1R amino acid Glu128 (Figure 2, inset 1). Another cluster of hydrogen bonding and ionic bonds are formed between the Exendin-4 amino acids Glu24 and Lys27, interacting with the GLP-1R Glu127. Thus residues Glu16, Glu17, Arg20, Glu24, and Lys27 define the hydrophilic face of the peptidic drug. In addition, the side chain of Ser32 forms hydrogen bonds with Glu68. The hydrophobic face of Exendin-4(9-39) is formed by residues such as Val19, Phe22, Ile23, Leu26, Ala18, Trp25, and Pro31 which form hydrophobic interactions with the GLP-1R.

Figure 2: X-ray crystal structure of the N terminal domain of GLP-1R (pink), with bound Exendin 4(9-39) (gold). Hydrogen bonding interactions between the Ex4(9-36) and the receptor are shown as cyan lines (PDB ID 3c59; Runge et al., 2008). Insets 1 and 2 show close up views of some of the interactions.

Recently, the EM structure of Exendin-4 bound to the complete GLP-1R and G-proteins (PDB ID 7lll, Deganutti et al., 2022) shows the interaction of the N-terminal residues of the peptide. In addition to several hydrophobic interactions, the polar amino acid side chains (Ser8, Ser11, Glu3, and Glu15) form hydrogen bonds with specific amino acids in the receptor. These interactions are similar to the ones between GLP-1 and its receptor.

Figure 3. EM structure of GLP-1R (red) with bound Exendin 4 (gold) (PDB ID 7lll; Deganutti et al., 2022). Location of the membrane, and G-proteins associated with the receptor are also shown. The inset shows a close up of specific interactions between the N-terminal region of Exendin 4 and the GLP-1R.

Pharmacologic Properties and Safety

Table 3. Pharmacokinetics: ADMET of Exenatide

The mechanism behind the significant increase in cardiovascular event rates is unknown and requires further study. However, analysis of trial data from five long-acting GLP-1 agonists (exenatide once weekly, taspoglutide, albiglutide, LY2189265 and CJC-1134-PC) revealed that they were more likely than shorter-acting formulations to raise the heart rate (Robinson et al., 2013).

Drug Interactions and Side Effects

Unlike sulfonylureas and other hypoglycemic agents, exenatide regulates blood glucose levels in a glucose-dependent manner (i.e., it is self-regulating). Therefore, it acts to increase insulin release when blood glucose levels are elevated but does not continue to release insulin when blood glucose levels have returned to a homeostatic range.

Table 4. Drug interactions and side effects of Exenatide

Regulatory Approvals/Commerical

Initially developed by Amylin Pharmaceuticals, Exenatide was approved by the US FDA in April 2005 with the commercial name Byetta. It is prescribed as a subcutaneous pen-injector. In early 2012, an extended-release formulation of exenatide, Bydureon was created, as a once a week injection administered in clinical settings. Note that Bydureon is not recommended as first-line therapy for patients who have inadequate glycemic control on diet and exercise because of the uncertain relevance of the rat thyroid C-cell tumor findings to humans (Drugs.com). This formulation is prescribed only to patients for whom the potential benefits outweigh the potential risk.

Since Byetta and Bydureon injection both contain exenatide as the active ingredient, they should not be administered conjunctively. Byetta can be used in an adjunctive therapy for glycemic control. When used in combination with a sulfonylurea, metformin, or both, exenatide produces a decrease in A1C of less than 1 percentage point. In addition, Byetta can be taken with Lantus (insulin glargine, a long-acting insulin), each reducing A1C levels by an average of 1.1 percentage points (Ezzo et al., 2006). However, Byetta should not be taken with rapid-acting/short-acting insulin analogs.

In late 2022, AstraZeneca decided to discontinue Bydureon, not for safety or health but due to business reasons.

Links

Table 5. Links to Relevant Resources

References

Bray, G.M. (2006) Exenatide. Am J Health Syst Pharm. 63, 411-418. https://doi.org/10.2146/ajhp050459

Cirincione, B., Mager, D.E. (2017) Population pharmacokinetics of exenatide. Br J Clin Pharmacol. 83, 517-526. https://doi.org/10.1111/bcp.13135

Deganutti, G., Liang, Y.L., Zhang, X., Khoshouei, M., Clydesdale, L., Belousoff, M.J., Venugopal, H., Truong, T.T., Glukhova, A., Keller, A.N., Gregory, K.J., Leach, K., Christopoulos, A., Danev, R., Reynolds, C.A., Zhao, P., Sexton, P.M., Wootten,D. (2022) Dynamics of GLP-1R peptide agonist engagement are correlated with kinetics of G protein activation. Nat Commun. 13, 92. https://doi.org/10.1038/s41467-021-27760-0

Ezzo. D.C., Ambizas, E.M. (2006). Exenatide Injection (Byetta): Adjunctive Therapy for Glycemic Control. American Family Physician. 73, 2213-14. http://www.aafp.org/afp/2006/0615/p2213.html

May, M., Schindler, C. (2016) Clinically and pharmacologically relevant interactions of antidiabetic drugs. Ther Adv Endocrinol Metab. 7, 69-83. https://doi.org/10.1177/2042018816638050

Nielsen, L.L., Young, A.A., Parkes, D.G. (2004) Pharmacology of exenatide (synthetic exendin-4): a potential therapeutic for improved glycemic control of type 2 diabetes. Regul Pept. 117, 77-88. https://doi.org/10.1016/j.regpep.2003.10.028

Parkes, D., Jodka, C., Smith, P., Nayak, S., Rinehart, L., Gingerich, R., Chen, K., Young, A. (2001). Pharmacokinetic Actions of Exendin-4 in the Rat: Comparison with Glucagon-Like Peptide-1. Drug Development Research, 53, 260-267. https://doi.org/10.1002/ddr.1195

Robinson, L.E., Holt, T.A., Rees, K., Randeva, H.S., O'Hare, J.P. (2013) Effects of exenatide and liraglutide on heart rate, blood pressure and body weight: systematic review and meta-analysis. BMJ Open. 3, e001986. https://doi.org/10.1136/bmjopen-2012-001986

Runge, S., Thøgersen, H., Madsen, K., Lau, J., Rudolph, R. (20028) Crystal structure of the ligand-bound glucagon-like peptide-1 receptor extracellular domain. J Biol Chem. 283, 11340-11347. https://doi.org/10.1074/jbc.m708740200

August 2023 Jennifer Jiang and Dr. Shuchismita Dutta; Reviewed by Dr. Joseph D. Ho
http://dx.doi.org/10.2210/rcsb_pdb/GH/DM/drugs/in/Exenatide