Insulin analog

An insulin analog is an altered form of insulin, different from any occurring in nature, but still available to the human body for performing the same action as human insulin in terms of glycemic control. Through genetic engineering of the underlying DNA, the amino acid sequence of insulin can be changed to alter its ADME (absorption, distribution, metabolism, and excretion) characteristics. Officially, the U.S. Food and Drug Administration (FDA) refers to these as "insulin receptor ligands", although they are more commonly referred to as insulin analogs.

These modifications have been used to create two types of insulin analogs: those that are more readily absorbed from the injection site and therefore act faster than natural insulin injected subcutaneously, intended to supply the bolus level of insulin needed after a meal; and those that are released slowly over a period of between 8 and 24 hours, intended to supply the basal level of insulin for the day. Insulin analog was first manufactured by Eli Lilly and Company.

Animal insulin
The amino acid sequence for insulin is almost the same in different mammals. Porcine insulin has only a single amino acid variation from the human variety, and bovine insulin varies by three amino acids. Both are active on the human receptor with approximately the same strength. Prior to the introduction of biosynthetic human insulin, insulin derived from sharks was widely used in Japan. Even insulin from some species of fish may be effective in humans. Non-human insulins can cause allergic reactions in a tiny number of people, as can genetically engineered "human" insulin. This is usually caused by the preservatives used in the insulin rather than the insulin molecule itself. Biosynthetic "human" insulin has largely replaced animal insulin. With the advent of high-pressure liquid chromatography (HPLC) equipment, the level of purification of animal-sourced insulins has reached as high as 99%, whereas the purity level of synthetic human insulins made via recombinant DNA has only attained a maximum purity level of 97%, which raises questions about the claim of synthetic insulin's purity relative to animal-sourced insulin varieties.

Chemically and enzymatically modified insulins
Before biosynthetic human recombinant analogues were available, porcine insulin was chemically converted into human insulin. Chemical modifications of the amino acid side chains at the N-terminus and/or the C-terminus were made in order to alter the ADME characteristics of the analogue. Novo Nordisk was able to enzymatically convert porcine insulin into 'human' insulin by removing the single amino acid that varies from the human variety, and chemically adding the correct one.

Non hexameric analogs
Unmodified human and porcine insulins tend to complex with zinc in the blood, forming hexamers. Insulin in the form of a hexamer will not bind to its receptors, so the hexamer has to slowly equilibrate back into its monomers to be biologically useful. Hexameric insulin delivered subcutaneously is not readily available for the body when insulin is needed in larger doses, such as after a meal (although this is more a function of subcutaneously administered insulin, as intravenously dosed insulin is distributed rapidly to the cell receptors, and therefore, avoids this problem). Zinc combinations of insulin are used for slow release of basal insulin. Basal insulin is the amount the body needs through the day excluding the amount needed after meals. Non hexameric insulins were developed to be faster acting and to replace the injection of normal unmodified insulin before a meal.

NPH
NPH insulin; also known as Humulin N, Novolin N, Novolin NPH, NPH Lletin II, and isophane insulin, marketed by Eli Lilly and Company under the name Humulin N, is an intermediate-acting insulin given to help manage the blood sugar level of those with diabetes. Many people reported problems following being switched to these insulins in the 80s, from porcine/bovine insulins. Problems included mood/character changes, memory problems,and hypo-unawareness.

Lispro
Eli Lilly and Company had the first insulin analogue with "lispro" as a rapid acting insulin analogue. It is marketed under the trade name Humalog. It was engineered through recombinant DNA technology so that the penultimate lysine and proline residues on the C-terminal end of the B-chain were reversed. This modification did not alter the insulin receptor binding, but blocked the formation of insulin dimers and hexamers. This allowed larger amounts of active monomeric insulin to be available for postprandial (after meal) injections.

Aspart
Novo Nordisk created "aspart" and marketed it as NovoLog/NovoRapid (UK-CAN) as a rapid acting insulin analogue. It was created through recombinant DNA technology so that the amino acid, B28, which is normally proline, is substituted with an aspartic acid residue. The sequence was inserted into the yeast genome, and the yeast expressed the insulin analogue, which was then harvested from a bioreactor. This analogue also prevents the formation of hexamers, to create a faster acting insulin. It is approved for use in CSII pumps and Flexpen, Novopen delivery devices for subcutaneous injection.

Glulisine
Glulisine is a newer rapid acting insulin analog from Sanofi-Aventis, approved for use with a regular syringe, in an insulin pump or the Opticlik Pen. Standard syringe delivery is also an option. It is sold under the name Apidra. The FDA-approved label states that it differs from regular human insulin by its rapid onset and shorter duration of action.

Shifted isoelectric point insulins
Normal unmodified insulin is soluble at physiological pH. Analogues have been created that have a shifted isoelectric point so that they exist in a solubility equilibrium in which most precipitates out but slowly dissolves in the bloodstream and is eventually excreted by the kidneys. These insulin analogues are used to replace the basal level of insulin, and may be effective over a period of up to 24 hours. However, some insulin analogues, such as insulin detemir, bind to albumin rather than fat like earlier insulin varieties, and results from long-term usage (e.g. more than 10 years) have never been released.

Glargine insulin
Sanofi-Aventis developed glargine as a longer lasting insulin analogue, and markets it under the trade name Lantus. It was created by modifying three amino acids. Two positively charged arginine molecules were added to the C-terminus of the B-chain, and they shift the isoelectric point from 5.4 to 6.7, making glargine more soluble at a slightly acidic pH and less soluble at a physiological pH. Replacing the acid-sensitive asparagine at position 21 in the A-chain by glycine is needed to avoid deamination and dimerization of the arginine residue. These three structural changes and formulation with zinc result in a prolonged action when compared with biosynthetic human insulin. When the pH 4.0 solution is injected, most of the material precipitates and is not bioavailable. A small amount is immediately available for use, and the remainder is sequestered in subcutaneous tissue. As the glargine is used, small amounts of the precipitated material will move into solution in the bloodstream, and the basal level of insulin will be maintained up to 24 hours. The onset of action of subcutaneous insulin glargine is somewhat slower than NPH human insulin. It is clear solution as there is no zinc in formula.

Detemir insulin
Novo Nordisk created insulin detemir and markets it under the trade name Levemir as a long-lasting insulin analogue for maintaining the basal level of insulin. The basal level of insulin may be maintained for up to 20 hours, but the time is clearly affected by the size of the injected dose. This insulin has a high affinty for serum albumin, increasing its duration of action.

Carcinogenicity
All insulin analogs must be tested for carcinogenicity, as insulin engages in cross-talk with IGF pathways, which can cause abnormal cell growth and tumorigenesis. Modifications to insulin always carry the risk of unintentionally enhancing IGF signalling in addition to the desired pharmacological properties.

Criticism
A meta-analysis of numerous randomized controlled trials by the international Cochrane Collaboration found "only a minor clinical benefit of treatment with long-acting insulin analogues (including two studies of insulin detemir) for patients with diabetes mellitus type 2" while others have examined the same issue in type 1 diabetes. Subsequent meta-analyses undertaken in a number of countries and continents have confirmed Cochrane's findings.

In July 2007, Germany's Institute for Quality and Cost Effectiveness in the Health Care Sector (IQWiG) reached a strikingly similar conclusion. In its report, IQWiG concluded that there is currently "no evidence" available of the superiority of rapid-acting insulin analogs over synthetic human insulins in the treatment of adult patients with type 1 diabetes. Many of the studies reviewed by IQWiG were either too small to be considered statistically reliable and, perhaps most significantly, none of the studies included in their widespread review were blinded, the gold-standard methodology for conducting clinical research. However, IQWiG's terms of reference explicitly disregard any issues which cannot be tested in double-blind studies, for example a comparison of radically different treatment regimes. IQWiG is regarded with skepticism by some doctors in Germany, being seen merely as a mechanism to reduce costs. But the lack of study blinding does increase the risk of bias in these studies. The reason this is important is because patients, if they know they are using a different type of insulin, might behave differently (such as testing blood glucose levels more frequently, for example), which leads to bias in the study results, rendering the results inapplicable to the diabetes population at large. Numerous studies have concluded that any increase in testing of blood glucose levels is likely to yield improvements in glycemic control, which raises questions as to whether any improvements observed in the clinical trials for insulin analogues were the result of more frequent testing or due the drug undergoing trials.

More recently, the Canadian Agency for Drugs and Technology in Health (CADTH) found in its 2008 comparison of the effects of insulin analogues and biosynthetic human insulin that insulin analogues failed to show any clinically relevant differences, both in terms of glycemic control and adverse reaction profile.

Timeline

 * 1922 Banting and Best use bovine insulin extract on human
 * 1923 Eli Lilly and Company (Lilly) produces commercial quantities of bovine insulin
 * 1923 Hagedorn founds the Nordisk Insulinlaboratorium in Denmark forerunner of Novo Nordisk
 * 1926 Nordisk receives Danish charter to produce insulin as a non-profit
 * 1936 Canadians D.M. Scott and A.M. Fisher formulate zinc insulin mixture and license to Novo
 * 1936 Hagedorn discovers that adding protamine to insulin prolongs the effect of insulin
 * 1946 Nordisk formulates Isophane porcine insulin a.k.a. Neutral Protamine Hagedorn or NPH insulin
 * 1946 Nordisk crystallizes a protamine and insulin mixture
 * 1950 Nordisk markets NPH insulin
 * 1953 Novo formulates Lente porcine and bovine insulins by adding zinc for longer-lasting insulin
 * 1978 Genentech produces synthetic 'human' insulin in Escheria coli bacteria using recombinant DNA technology
 * 1981 Novo Nordisk chemically and enzymatically converts porcine insulin to 'human' insulin
 * 1982 Genentech synthetic 'human' insulin approved, largely thanks to its partnership with Eli Lilly and Company, who shepherded the product through the U.S. Food and Drug Administration (FDA) approval process
 * 1983 Lilly produces synthetic, recombinant 'human' insulin, branded Humulin
 * 1985 Axel Ullrich sequences the human insulin receptor
 * 1988 Novo Nordisk produces synthetic, recombinant 'human' insulin
 * 1996 Lilly Humalog "lispro" insulin analogue approved by the U.S. Food and Drug Administration
 * 2003 Aventis Lantus "glargine" insulin analogue approved in USA
 * 2004 Sanofi Aventis Apidra insulin "glulisine" analogue approved for clinical use in the USA.
 * 2006 Novo Nordisk's Levemir "insulin detemir" analogue approved in USA