A Century of Insulin

For a century now, the use of insulin has advanced the treatment of diabetes beyond what was ever thought possible in the early 1920s. This article walks through the evolution and ongoing development of progressively more physiologic formulations of insulin. The discovery of insulin was the most important landmark in treatment of diabetes by far. The discovery continues to in-spire medical scientists to develop insulins mimicking basal as well as prandial profiles. Similarly, progress has been ongoing in commercial production of insulins from animal sources to advanced recombinant DNA technologies to formulate molecules almost identical to human insulin. Finally, over the years, insulins have become progressively more purified to almost 100% without contamination leading to almost total elimination of allergic reactions. In con-clusion, what was just an idea based on previous knowledge of the role of dis-order of pancreatic islets in causing diabetes to discovery of insulin 100 years ago to what is now well established as a life-saving treatment of diabetes.

sulin and improved prognosis for patients with diabetes. Beyond the original treatment as above, interventions including hyper-caloric diets to counteract urinary loss of calories and Elliott Proctor Joslin's prolonged fasting and "starvation diet" [5] [6] [7] [8] were utilized in the 19th century.

The Idea of Insulin
It wasn't until 1921 when insulin therapy was discovered, based on the hypothesis that the pancreas secreted a substance that controlled carbohydrate metabolism [5]. The events leading up to the discovery date back to 1884, when Louis Vaillard and Charles Louis Xavier Arnozan discovered pancreatic duct occlusion resulted in pancreatic atrophy without hyperglycemia [9]. Interestingly, the islets of Langerhans were spared from the necrosis of pancreatic atrophy. German physicians Joseph von Mering and Oskar Minkowski attempted to cure diabetes in dogs with pancreatectomy with pancreatic powder extracts [8]. Similar experiments years later replicated these findings, but failed due to toxicity of pancreatic enzymes [10]. There was also difficulty in finding ways to separate the anti-diabetic extract produced by the islet of Langerhans from the rest of the pancreas.

The Discovery of Insulin
On October 30, 1920, Frederick Banting, an orthopedic surgeon, reviewed an article by Moses Barron, in which four case reports regarding the pancreas were published, relating to the work of Vaillard and Arnozan in 1884. Banting noted that in one case report, a stone blocked the pancreatic duct, leading to atrophy of the exocrine pancreas but preserved the islets of Langerhans-this patient did not develop diabetes [11]. Barron deduced that the islets secreted a hormone controlling glucose metabolism, and Banting became interested in experimenting how to isolate this substance from the islets apart from the rest of the pancreas. His idea included ligating the pancreatic ducts of dogs while keeping them alive until the acini degenerate leaving the isets for isolation of a substance that might relieve glycosuria [12].
Banting approached John James Macleod in November 1920. Macleod was a professor of physiology and department head at the University of Toronto and was agreeable to lend Banting laboratory space to work on his experiments in order to isolate the hormone secreted from the islets of Langerhans. Work began in 1921 after Macleod granted laboratory space to Banting, as well as ten dogs for experiments and a student research assistant Charles Best. [13] In August 1921, Banting and Best began an experiment involving closure of the pancreatic ducts to degenerate the pancreatic exocrine tissue and to obtain a pancreatic islet from the pure state [4]. This extract, when given four times over four days, resulted in reduced glucose and improved the status of a depancreatized dog [12]. They called this extract "isletin", which was later renamed insulin, resulting now in a century of insulin therapy.
Understanding concerns with previously known impurities in pancreas ex-

Attuning Insulin
To

Modern Insulin
Through this time, improvements in insulin purification techniques were being Journal of Diabetes Mellitus trialed, with human insulin being produced using recombinant DNA technology based on genetic modifications of bacteria in the 1970s. It wasn't until 1978 that the first recombinant DNA was prepared using the combination of insulin A and B chains expressed from E. coli bacteria. Thereafter, Genentech and Lilly signed an agreement to commercialize recombinant DNA insulin [4]. In 1982, with the advent of synthetic insulins which created fewer allergenic concerns in patients compared to animal insulin, Humulin R and Humulin N were marketed en masse by Eli Lilly [4]. During the 1980-90s, analog insulins using genetically modified insulins resulting from alterations in amino acid sequences were developed. In 1996, Eli Lilly produced the first short-acting insulin lispro under the brand Humalog, which was followed by the manufacture of insulin aspart (Novolog) in 2000 by Novo Nordisk and insulin glulisine (Apidra) in 2004 by Sanofi [4]. In recent years, several bioavailable faster acting insulins have been approved. Basal insulins were also developed around this time. Insulin glargine, manufactured by Hoechst then (presently Sanofi) and approved for use in 2000 forms micro precipitate at the site of injection; this along with alterations in amino acid sequence results in prolonged absorption with minimal peaks [23] [24]. Insulin detemir, produced by Novo Nordisk and approved in 2005 binds readily to circulating albumin and demonstrates a prolonged duration of action due to gradual release from the bound product as well as structural amino acid changes [25]. Since the early 2000s, additional basal insulins, including another glargine (Basaglar), 3 times concentrated insulin glargine (Toujeo), insulin degludec (Tresiba) have been approved. Most recently, interchangeable biosimilar glargine (Semglee) was approved by FDA and is marketed in USA as well as a few other countries. The presently available basal insulins are injected daily. New insulin formulations with once weekly administration are being studied.
Understandably, discovering needle-free insulin would be a breakthrough in itself. The concept of breath-administered insulin was documented initially in 1925; a relaunch occurred in 1971. In 2006, Pfizer and Sanofi-Aventis developed Exubera, the first marketed inhaled insulin [26] [27] [28]. This dry powder insulin is formulated with appropriate particulate characteristics for deposition in the alveoli. While inhaled insulin has low bioavailability (about 9% of the amount inhaled), it ultimately results in adequate serum insulin levels. Critics of its use noted a bulky inhaler device and lack of additional benefit over fast-acting analogs. In 2007, Pfizer ceased sales of Exubera due to lack of acceptance from providers and patients [4]. In 2014, Afreeza inhaled insulin also absorbed by alveoli was approved, however it is not commonly used. Finally, ongoing studies assessing transdermal insulin patch as well as oral insulin formulations are underway.

Summary
While Banting was involved in the discovery of insulin a century ago, he ultimately dreamt of transplanting a pancreas into humans [29]. Looking ahead, cur- rent work with artificial pancreases, creating a closed-loop system controlling blood sugar may be the future of diabetes. The advent of insulin pumps and the potential of weekly insulin injections are also advanced in achieving glucose control. The discovery of insulin one hundred years ago created a milestone in medicine and provides hope for diabetes cure in the future.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.