Description: Insulin is a pancreatic hormone. It is secreted by the pancreatic beta-cells of the islets of Langerhans and is essential for the metabolism of glucose and for the homeostasis of blood glucose. Direct evidence of a pancreatic contribution to blood glucose regulation was first discovered in 1889. Some time later, an extract from the pancreatic beta-cells of the islets of Langerhans revealed effectiveness in lowering elevated blood glucose concentrations. This active extract (insulin) was administered to a young diabetic patient in 1922, and daily injections of the hormonal replacement therapy were found to reverse the otherwise fatal metabolic disorder. FDA approval of insulin products began in 1939. The precise amino acid sequence of insulin was established in 1960, leading to complete synthesis of the hormone by 1963. The first biosynthetic human insulin was given FDA approval in 1982. Bovine, porcine, and recombinant human insulin preparations are currently available for use in diabetic patients worldwide; however, human insulin is the preferred preparation as it is less antigenic than both porcine and bovine insulin preparations. Bovine-tissue derived insulin is no longer on the US market as of 1999, due to FDA concerns over the transmission of bovine spongiform encephalopathy (i.e., 'mad-cow disease'). In addition, the use of porcine formulations has diminished greatly, and, as a result, manufacturers of porcine formulations in the US (Iletin® brand) have discontinued their production. A variety of insulin preparations are available; all have different times to onset of activity and durations of action. The pharmacokinetic profile of regular insulin can be prolonged by combination with protamine (e.g., NPH insulin) or by the production of an insulin analog (e.g., insulin glargine or insulin detemir). In addition, rapid-acting insulin analogs have been produced that have a quicker onset and shorter duration of action when compared to regular insulin (i.e., insulin aspart, insulin glulisine, or insulin lispro). The majority of formulations that add zinc ions to modify the pharmacokinetic profile of regular insulin are no longer available in the US and include lente insulin, protamine zinc insulin, semilente insulin, and ultralente insulin.
Neutral Protamine Hagedorn (NPH) or isophane insulin is an intermediate-acting insulin that is produced by adding zinc and protamine to regular insulin causing a delay in absorption and a prolongation of the duration of action after subcutaneous administration. Combination with protamine and low concentrations of zinc enhances the aggregation of insulin into dimers and hexamers after subcutaneous injection; a depot is formed after injection and the insulin is released slowly. NPH insulin is commonly administered twice daily in combination with a quick-acting insulin (regular insulin, insulin aspart, insulin glulisine, or insulin lispro) to patients with type 2 diabetes mellitus and during the honeymoon phase of type 1 diabetes mellitus. The duration of action of NPH insulin is variable; some patients may require only one injection while others require 3 or more injections daily. Patients with established type 1 diabetes mellitus will often require > 2 injections of NPH insulin daily when used as a basal insulin. NPH insulin is considered to be equipotent to other basal insulins including lente insulin, ultralente insulin, insulin detemir, and insulin glargine. NPH insulin is not the ideal basal insulin as it has variable absorption, unwanted peaks leading to hypoglycemia including nocturnal hypoglycemia, and often an inadequate duration of action (even when administered twice daily) causing fasting hyperglycemia.[30644] When compared to newer, longer-acting insulins such as insulin glargine, however, NPH insulin has a long history of clinical experience. In 2006, joint consensus guidelines by the ADA and European Association for the Study of Diabetes for the management of type 2 diabetes were published. After initial metformin therapy, basal or intermediate-acting insulin is suggested as one of 3 second line choices or as a third line agent when 2 oral drug therapies do not achieve optimal glycemic control. The guidelines also suggest that in those patients taking a sulfonylurea, it be discontinued prior to initiating insulin therapy. Once insulin is added, glycemic control can be achieved by intensifying the insulin regimen (e.g. adding prandial insulin).[33174]
Alternative routes for administering insulin are also being evaluated. Exubera™ (a collaborative effort of Pfizer, Inc., Inhaled Therapeutics Systems, Inc., and Aventis), an insulin powder for inhalation, is used in patients with type 1 and type 2 diabetes mellitus. Exubera™ uses a device similar to an asthma inhaler for fixed insulin dosage prior to meals; patients in studies to date supplemented inhalations of insulin with a long-acting insulin injection (e.g., Ultralente) administered at bedtime. An NDA was filed with the FDA for Exubera™ in March of 2005 after a 3 year delay due to pulmonary safety concerns; the Endocrinologic and Metabolic Drugs Advisory Committee reviewed Exubera™ for use in patients with type 1 and type 2 diabetes mellitus on September 8, 2005. The Committee recommended approval of Exubera™ for the intensive treatment of type 1 diabetes and the treatment of type 2 diabetes, and the drug received FDA approval for both indications on January 27, 2006; in October 2007, Pfizer decided to discontinue Exubera® in the US secondary to limited use by patients and health care professionals. The development of AERx® iDMS™ (Novo Nordisk and Aradigm), a soluble insulin for inhalation, has also been discontinued as it will most likely not offer additional benefit over injectable insulin. The FDA is considering an IND for a sublingual/buccal insulin dosage form called Oralin™; this product is currently in phase III trials in Canada.
Mechanism of Action: Endogenous insulin regulates carbohydrate, fat, and protein metabolism by several mechanisms; in general, insulin promotes the storage and inhibits the breakdown of glucose, fat, and amino acids. Insulin lowers glucose concentrations by facilitating the uptake of glucose in muscle and adipose tissue and by inhibiting hepatic glucose production (glycogenolysis and gluconeogenesis). Insulin also regulates fat metabolism by enhancing the storage of fat (lipogenesis) and inhibiting the mobilization of fat for energy in adipose tissues (lipolysis and free fatty acid oxidation). Finally, insulin is involved in the regulation of protein metabolism by increasing protein synthesis and inhibiting proteolysis in muscle tissue.
Diabetes mellitus type 1 is caused by insulin deficiency while diabetes mellitus type 2 is caused by a combination of insulin deficiency and resistance. Biosynthetic insulin is used as replacement therapy in patients with diabetes mellitus to temporarily restore their ability to use fats, carbohydrates, and proteins, and to convert glycogen to fat. Insulin administration also enables these patients to replete their liver glycogen stores. Commercially available insulin is prepared using recombinant DNA technology (E. coli bacteria) or enzymatic modification of beef or pork insulin to create a product identical in structure and function to endogenous human insulin.
Pharmacokinetics:
Isophane insulin (NPH) is administered via the subcutaneous route by intermittent injections only. Endogenous insulin distributes widely throughout the body. A small portion is inactivated by peripheral tissues, but the majority is metabolized by the liver and kidneys. Insulin is filtered and reabsorbed by the kidneys; the plasma half-life of human endogenous insulin is approximately 5—6 minutes.
•Route-Specific Pharmacokinetics
Subcutaneous Route
After subcutaneous administration, the onset of glucose lowering activity begins approximately 1—4 hours after injection with maximal activity occurring between 4 and 14 hours (mean 5.5 hours). Roughly 14% of the total activity occurs in the first 4 hours (range 3—48%). The duration of activity ranges from 10—24 hours or longer.