Learn the fascinating details of the discovery of incretin-active agents, including DPP4 inhibitors, GLP-1 agonists, and GLP-2 agonists.

 
Every day, pharmacists dispense prescription medications for type 2 diabetes and counsel patients on the proper use of these medications, but pharmacists are rarely privy to the details of drug development and the people behind these therapies. One such innovator is Dr. Daniel J. Drucker, MD. Drucker’s work has led to the development of 3 novel classes of antidiabetic agents, including DPP4 inhibitors, GLP-1 agonists, and GLP-2 agonists.
 
In an award acceptance speech at the American Diabetes Association meeting in 2014, Drucker described the fascinating path that led to his drug discovery career, which is among the most productive translational research careers in history.
 
Drucker’s research path began in 1984 when Drucker was assigned to work with Joel Habener, researching the hormone glucagon. At the time, thyroid hormone research was thought to be a more promising avenue of research than gut hormone research. Drucker stated, “I worried that I had lost my chance to work on exciting research,” but Drucker soon recovered from the disappointment and began work in earnest.
 
The work involved cloning genes that encoded peptide hormones, such as glucagon, and other peptides with unknown function by, as Drucker stated, “dumping these peptides on as many cell cultures as possible.” The result of this investigation was the discovery that 2 fragments of GLP-1, the 7-37 and 7-36 fragments, were potent regulators of insulin gene expression and insulin secretion.
 
Sensing that these hormone fragments might be a fertile lead for drug development, Habener issued a patent for these 2 peptide fragments, and Drucker published what would become a seminal paper in the Proceeds of the National Academy of Science entitled, “Glucagon-like peptide I stimulates insulin gene expression and increases cyclic AMP levels in a rat islet cell line.”1,2
 
These achievements were only the beginning of Drucker’s success with gut hormones. The GLP-1 agonist and DPP4 inhibitors became an area of interest for researchers. Research led to the discovery that the venom of Heloderma suspectum (better known as the Gila monster) shared 53% of the amino acid sequence of GLP-1. Drucker and Chen cloned the DNA of exendin-4 from the salivary gland of the lizard.3
 
Having identified the genetic sequence of the GLP-1 receptor, scientists removed GLP-1 from mice, forming GLP-1 knockout mice, and learned that, even when fasting, the mice had hyperglycemia, suggesting that GLP-1 agonists have a role in homeostatic glucose regulation beyond the effect of modulating the effects of insulin.4
 
Further research revealed that GLP-1 plays a role in preserving the function of beta-cells. Li et al tested the GLP-1 agonist exendin-4 in mice with beta-cells that had been destroyed by the drug streptozotocin.5 In the mice receiving exendin-4, the negative effects of streptozotocin were less profound. A mechanism for the prevention of beta-cell destruction was later elucidated. The protective effect of GLP-1 agonists seems to be mediated through a reduction in endoplasmic reticular stress.6,7
 
Together, these discoveries suggest that GLP-1 agonists have an effect beyond stimulating insulin secretion, may have a role in inhibiting apoptosis of beta-cells, and perform a complex role in regulating glucose homeostasis. In fact, GLP-1 agonists have broad effects through multiple organ systems and tissues including cardiomyocytes, neurons, and systems that modulate body weight and energy expenditure.8
 
The effect of GLP-1 agonists on the brain was surprising given the high molecular weight of the GLP-1–active compounds. Although the brain is very important in regulating glucose homeostasis, GLP-1 agonists do not have to penetrate the blood-brain barrier to exert effects on the brain. Lamont et al showed that the GLP-1 agonists can indirectly affect brain signaling despite the high molecular weight of these agents.9
 
DPP4 Inhibitors
 
Study of GLP-1 agonists led to the characterization of a protein involved in the breakdown of GLP-1: DPP4. This enzyme was originally known as CD26 due to the initial study of it as a target in lymphoma treatment, without compelling results.10,11 Later on, these proteins became the targets of several effective medications for the treatment of diabetes—the DPP4 inhibitors, which include sitagliptin, saxagliptin, and alogliptin.
 
Effects on the Gastrointestinal Tract
 
In mice implanted with glucagon-secreting tumors, mice developed enlargements of the small bowel, “doubling and sometimes almost tripling cell mass,” —according to Drucker.12 Similar results were identified in humans through a literature search that yielded a 1971 case report on a 44-year-old woman who had an enlarged gastrointestinal tract as a result of a tumor secreting glucagon and other peptides.13-15
 
In a minor hiccup, the GLP-2 agonist was found to be ineffective in rats. However, this turned out to be a result of rat DPP4 enzymes degrading GLP-2 rapidly. When the GLP-2 was altered to resist destruction by rat DPP4 enzymes, small bowel growth was observed in rats as well.
 
Eventually, the development of GLP-2 led to a treatment for short bowel syndrome.16 In 2012, teduglutide was approved by the FDA under the brand name Gattex for the treatment of short bowel syndrome.17 With this treatment, according to Drucker, “Between 1 in 6 and 1 in 8 patients can completely come off of [total parenteral nutrition]—they no longer need nutritional support.”
 
Cardiovascular Concerns
 
The cardiovascular effects of GLP-1 agonists have recently become a popular concern.18 It is known that GLP-1 can affect the heart indirectly. However, in mice that had experienced a myocardial infarction (MI), use of the GLP-1 agonist liraglutide induced improved survival and reduced the size of infarcts.19 After an MI, a total of 77% versus 20% of mice survived 28 days, favoring the group receiving liraglutide (P = .0001). Other studies in mice also suggest a cardiomyocyte-independent effect of GLP-1 agonists.
 
Summary
 
Drucker’s illustrious and fecund career has led to the development of 3 classes of incretin-active agents for the treatment of diabetes and short bowel syndrome. The American Diabetes Association celebrates the accomplishments of scientific innovators like Drucker, and continues to fund the research of scientists who innovate in the field of diabetes research.
 
References:
1. United States Patent Office. Insulinotropic hormones and uses thereof. www.lens.org/images/patent/US/5614492/A/US_5614492_A. Accessed July 2014.
2. Drucker DJ, Philippe J, Mojsov S, Chick WL, Habener JF. Glucagon-like peptide I stimulates insulin gene expression and increases cyclic AMP levels in a rat islet cell line. Proc Natl Acad Sci U S A. 1987;84(10):3434-3438.
3. Chen YE, Drucker DJ. Tissue-specific expression of unique mRNAs that encode proglucagon-derived peptides or exendin 4 in the lizard. J Biol Chem. 1997;272(7):4108-4115.
4. Scrocchi LA, Brown TJ, MaClusky N, et al. Glucose intolerance but normal satiety in mice with a mutation in the glucagon-like peptide 1 receptor gene. Nat Med. 1996;2(11):1254-1258.
5. Li Y, Hansotia T, Yusta B, Ris F, Halban PA, Drucker DJ. Glucagon-like peptide-1 receptor signaling modulates beta cell apoptosis. J Biol Chem. 2003;278(1):471-478.
6. Yusta B, Baggio LL, Estall JL, et al. GLP-1 receptor activation improves beta cell function and survival following induction of endoplasmic reticulum stress. Cell Metab. 2006;4(5):391-406.
7. Baggio LL, Huang Q, Cao X, Drucker DJ. An albumin-exendin-4 conjugate engages central and peripheral circuits regulating murine energy and glucose homeostasis. Gastroenterology. 2008;134(4):1137-1147.
8. Hansotia T, Maida A, Flock G, et al. Extrapancreatic incretin receptors modulate glucose homeostasis, body weight, and energy expenditure. J Clin Invest. 2007;117(1):143-152.
9. Lamont BJ, Li Y, Kwan E, Brown TJ, Gaisano H, Drucker DJ. Pancreatic GLP-1 receptor activation is sufficient for incretin control of glucose metabolism in mice. J Clin Invest. 2012;122(1):388-402.
10. Rasmussen HB, Branner S, Wiberg FC, Wagtmann N. Crystal structure of human dipeptidyl peptidase IV/CD26 in complex with a substrate analog. Nat Struct Biol. 2003;10(1):19-25.
11. Drucker DJ. Dipeptidyl peptidase-4 inhibition and the treatment of type 2 diabetes: preclinical biology and mechanisms of action. Diabetes Care. 2007;30(6):1335-1343.
12. Ehrlich P, Tucker D, Asa SL, Brubaker PL, Drucker DJ. Inhibition of pancreatic proglucagon gene expression in mice bearing subcutaneous endocrine tumors. Am J Physiol. 1994;267(5, pt 1):E662-E671.
13. Gleeson MH, Bloom SR, Polak JM, Henry K, Dowling RH. Endocrine tumour in kidney affecting small bowel structure, motility, and absorptive function. Gut. 1971;12(10):773-782.
14. Drucker DJ, Erlich P, Asa SL, Brubaker PL. Induction of intestinal epithelial proliferation by glucagon-like peptide 2. Proc Natl Acad Sci U S A. 1996;93(15):7911-7916.
15. Drucker DJ, Shi Q, Crivici A, et al. Regulation of the biological activity of glucagon-like peptide 2 in vivo by dipeptidyl peptidase IV. Nat Biotechnol. 1997;15(7):673-677.
16. Buchman AL, Scolapio J, Fryer J. AGA technical review on short bowel syndrome and intestinal transplantation. Gastroenterology. 2003;124(4):1111-1134.
17. US Food and Drug Administration. FDA approves Gattex to treat short bowel syndrome. www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm333171.htm. Accessed July 2014.
18. Ussher JR, Drucker DJ. Cardiovascular actions of incretin-based therapies. Circ Res. 2014;114(11):1788-1803.
19. Noyan-Ashraf MH, Momen MA, Ban K, et al. GLP-1R agonist liraglutide activates cytoprotective pathways and improves outcomes after experimental myocardial infarction in mice. Diabetes. 2009;58(4):975-983.