INTRODUCTION
The discovery of insulin and its current evolution is a fascinating story of how brilliant scientific minds figured out its existence and role in diabetes management and completely transformed the lives of millions of people! This chapter attempts to trace the history of insulin discovery documenting the entire journey to its current stage as it continues to evolve into the future.
For many people with diabetes, especially type 1 diabetes, the diagnosis was almost a death sentence in the pre-insulin era as survival was impossible. Way back in 150 AD, the Greek physician Aretaeus of Cappadocia, practicing in Rome and Alexandria, described “the melting down of flesh and limbs into urine” noting the excessive passage of urine through the kidneys. He coined the word diabetes (meaning “siphon” in Greek) dramatically mentioning that “no essential part of the drink is absorbed by the body while great masses of the flesh are liquefied into urine.” Though he did attempt to treat it, he documented its poor prognosis, describing that “life (with diabetes) is short, disgusting and painful”. However, the discovery of insulin, often described as a miracle in the world of diabetes absolutely changed the way diabetes could be treated and bestowed people diagnosed with it with a new life.
THE ROAD TO INSULIN DISCOVERY
Way before the discovery of insulin, the diagnosis of diabetes especially type 1, meant a reduced life span and there was not much that doctors could do to manage these patients. Few physicians did manage to add a few extra years by prescribing very strict diets even devoid of carbohydrates or restricting calorie intake to as less as 450 calories/day, but sadly could not really save lives and in fact resulted in starvation deaths.1 So there was a constant search to find a cure to help these patients.
The late 19th century was a turning point in the history of diabetes in many ways. Paul Langerhans, a 22-year-old German medical student, while working on his medical doctorate, identified islands of cells in the pancreas naming them “islets of Langerhans” in 1869, though he could not explain their exact function. In 1889, two German researchers from the University of Strasbourg, Oskar Minkowski and Joseph von Mering, while experimenting on dogs found that removing pancreas gland lead to these dogs developing symptoms of diabetes and eventually death, thus demonstrating the role of pancreas in diabetes for the first time.2
Later experimenters narrowed this search to the islets of Langerhans (a fancy name for clusters of specialized cells in the pancreas). In 1910, Sir Edward Albert Sharpey-Shafer hypothesized that deficiency of a single chemical produced from pancreas lead to development of diabetes, and he called it “insulin”, derived from the Latin word insula meaning island, referring to the islet cells of Langerhans.3
Year 1921 turned out be a great milestone in the journey of diabetes when Banting, Best and 4Collip while working on different extracts from pancreatectomised dogs were able to establish the endocrine function of pancreas in metabolism and the existence of insulin. This fascinating story started when Sir Frederick Grant Banting and Charles Herbert Best, a medical student training with him, while working in Prof Mcleod's lab in the summer of 1921, discovered insulin. The extract called isletin by them, was a “thick brown murky concoction” that they administered to the dog. It showed miraculous results and the dog went on to live for 70 days till the extract was finished. On November 14, 1921, a historical date in the diabetes calendar, which also happened to be Frederick Banting's birthday, they presented a preliminary report of their work to the Journal Club of the department of physiology at the University of Toronto, their workplace. Three days later, on the 17th of November, they further discovered that the extract from cattle fetal pancreas when injected into depancreatised dogs caused lowering of blood sugar levels,4 further stimulating them to look for long-term efficacy of this strategy. Dr James Bertram Collip, a biochemist from the University of Alberta, joined them in December assisting them in refining the quality of extracts to make them safe enough for human use.
The red-letter day in the history of diabetes dawned on January 11, 1922 when for the first time insulin was injected to Leonard Thompson, a 14 year-old type 1 diabetic boy at the Toronto General Hospital to treat his diabetes. His symptoms improved dramatically, he gained weight and went on to live another 13 years before finally dying of pneumonia at the age of 27 years. This was indeed a miracle in the medical world where type 1 diabetes used to be a death verdict for young children! The word “insulin” for the first time was publicly used on May 3, 1922 by McLeod while presenting his paper ‘The Effect Produced on Diabetes by the Extracts of Pancreas’ at the Association of American Physicians annual meeting at Washington DC. These results were documented as one of the greatest achievements of modern medicine.5
On October 25, 1923 Banting and McLeod were awarded the prestigious Nobel Prize in Physiology or Medicine which they then shared with Best and Collip (Fig. 1).
Meanwhile, Eli Lilly & Co., a pharmaceutical firm from Indianapolis entered into an agreement with University of Toronto in May 1922 for mass production and distribution of insulin which for the first time became commercially available for human use in the US and Canada from October 1923.
HAGEDORN
The discovery of insulin was a turning stone in the way diabetes was treated, triggering global research for further advancements. The earlier insulins were crude and impure extracts sourced from pancreases of slaughtered animals, and often highly antigenic as well as often caused local tissue inflammation at the site of subcutaneous injection. The duration of action of these animal insulins was about 6 hours, making frequent injections necessary. Gradually, as technology improved, more pure forms of insulin were developed. To alleviate the pain and inconvenience caused by these frequent injections, a need was felt for developing insulin preparations that worked for a longer time. This milestone was achieved in Denmark by Hans Christian Hagedorn who discovered that combining insulin with protamine resulted in an insulin preparation that was not only stable but also had a longer duration of action. This preparation came to be known as Neutral Protamine Hagedorn or NPH insulin. Addition of zinc to the insulin was another technique to prolong the duration of action that came into practice. Experimentally various other insulin preparations, such as histone and globin 5insulin were also developed with the same intent, but they failed to make it into clinical practice.
BIOSYNTHESIS OF INSULIN
The better survival rates of diabetes patients brought with it an increased prevalence of chronic complications of diabetes and also an improved understanding of the importance of better glycemic control in preventing these complications. Many studies advocated intensive insulin therapy with multiple injections to achieve intensive glycemic control in diabetes patients. However, there were numerous shortcomings and challenges with these older generation animal zinc-insulins. Their absorption proved to be erratic, and they were frequently associated with considerable immunogenicity. Also, in view of increasing demand for insulin, cadaveric animal insulin had limited availability and was expected to fall short of rapidly rising global insulin needs. Thus, with development of biosynthetic technology, human insulin came to be the first protein to be produced on a huge scale. Genetically engineered “synthetic” human insulin was for the first time produced in laboratory on a large scale using recombinant DNA technology in 1978 using E. coli bacteria and soon became to be sold globally as “Humulin” by Eli Lilly Company from 1982 onwards. Over the next few decades, various modifications to the structure or site of amino acids in the insulin molecule polypeptide chains were designed to change the pharmacokinetics of the human insulin molecule to aid faster absorption and earlier peak of action, and prolonged or shorter duration of action as needed.6
INSULIN ANALOGS
Human insulin initially was available in a few basic variants: regular short-acting, neutral protamine hagedorn (NPH) insulin, zinc-insulin and mixtures of short-acting and NPH insulin. However, need was felt for as near physiological insulin substitution as possible. There was a need for rapid-acting insulin with a quicker and shorter action mimicking the rapid insulin secretion following a meal as well as a longer-acting insulin than NPH insulin, preferably with a lesser peak to mimic the physiologically low rate of basal insulin secretion occurring between meals and overnight. Building on the work of Jens Brange and his team,7 the insulin molecules were bioengineered with different modifications and molecular designs to satisfy these requirements. This paved the way for development of analog insulins or designer insulins that are commonly used today.
Lispro was the first rapid-acting insulin analog approved in 19968 followed soon by aspart in 20009 and then glulisine in 2004.10 Of the current basal insulin analogs, glargine was approved in 200011 followed by detemir in 200512 and most recently degludec in 2015. Glargine has amino acid glycine substituted in place of asparagine at position A21 along with an extra two arginine residues added at position B30, maintained in a clear soluble solution at a pH of 4.0 in the vial. Once injected subcutaneously into the body, at the physiological pH, there occurs formation of micro precipitates at the site of injection, stabilizing hexamers resulting in their prolonged dissociation thus prolonging their absorption with little peak activity.11,13 Insulin detemir has a 14-carbon fatty acid (myristic acid) attached to lysine at position B29 to enable the insulin to bind to albumin in blood and slowly dissociate, prolonging its duration of action.14 Insulin degludec, the latest ultra-long acting basal analog has been designed by omitting the amino acid threonine from position B30 and adding a C-16 fatty diacid side chain through a glutamic acid spacer to lysine at B29. This enables the insulin to form depots in subcutaneous tissue after injection and slow dissociation of monomers, resulting in ultra-long duration of action.15
To have an alternative delivery method for insulin, exubera, the first inhaled insulin, was developed by Sanofi-Aventis and Pfizer and marketed by Pfizer in 2006.16 The inhaler device was bulky to use and did not add physiologic benefit over rapid-short acting insulin analogs17 and was taken off market after two years when it failed to gain acceptance from patients and providers.18 This was followed by Afrezza by Sanofi/MannKind but that also has not really replaced injectable insulins, with its own set of limitations. Research for oral insulin has also been ongoing for many decades now, though oral insulin still remains the “Holy Grail” of diabetes, elusive, yet not impossible!6
CONCLUSION
The discovery of insulin revolutionized the treatment of diabetes, giving new hope and a promise of better survival and quality of life to millions of diabetics globally. The discovery of this miracle hormone, its availability as human recombinant insulin and then the subsequent development of precisely engineered designer insulin analogs has made insulin administration convenient and more physiological, reduced hypoglycemia and improved diabetes control. Insulin continues to be the cornerstone of diabetes management with further ongoing research to design better and superior molecules with novel action profiles and convenient administration requirements!
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- VonMehring J, Minkowski O. Diabetes mellitus nach pankreas extirpation. Arch Exp Pathol Pharmakol. 1890;26(5-6):371–87.
- Polonsky KS. The past 200 years in diabetes. NEJM. 2012;367(14):1332–40.
- Lakhtakia R. The history of diabetes mellitus. Sultan Qaboos Univ Med J. 2013;13(3):368–70.
- Banting FG, Best CH, Collip JB, et al. Pancreatic extracts in the treatment of diabetes mellitus: a preliminary report 1922. CMAJ. 1991;145(10):1281–6.
- Hirsch I. Insulin Analogues. N Engl J Med. 2005;352(2): 174–83.
- Brange J, Ribel U, Hansen JF, et al. Monomeric insulins obtained by protein engineering and their medical implications. Nature. 1988;333(6174):679–82.
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- Keegan A. Weak sales lead to Exubera's market withdrawal. Doc News. 2007;4:5.