Insulin therapy: Past, present and future (Proceedings)

Insulin was first discovered in 1921 by Frederick Banting and colleagues. The original preparations were bovine and porcine extracts. These formulations were short-acting, relatively dilute and impure. As a result, frequent administration and large quantities were required. Local reactions were common and many people developed insulin antibodies necessitating different insulin formulations and, in some cases, immunosuppressive therapy. Over time more concentrated insulins were produced and purity was improved. In the 1930's – 1950's it was discovered that protamine and zinc would cause micro crystallization of insulin and slow absorption. In the 1980's recombinant DNA technology led to the development of human recombinant insulin. This led to a dramatic decrease in the use, and production of, animal-origin insulin. The 1990's led to the discovery of synthetic insulin analogues in an attempt to achieve better glycemic control in man.

Ultra Short and Short–Acting Insulin

In the 1990's insulin analogues were developed to better mimic the basal-bolus requirements of man as well as to improve consistency. The premise is that a basal amount of insulin is produced by the body between meals as a result of gluconeogenesis in the liver. There is also a larger, more rapid increase in insulin after meals due to post-prandial hyperglycemia. Human recombinant regular insulin was the traditional 'bolus' insulin used separately or in mixed preparations. Using regular insulin, peak insulin levels occurred at 60 minutes with duration of up to 6 hours in man. This is because regular insulin exists as a hexamer that dissociates into dimers and monomers once injected subcutaneously. Monomers are the active form and act rapidly but dimers must dissociate to monomers, slowing absorption and effect. When given intravenously, regular insulin rapidly dissociates to monomers so there is a more rapid onset and shorter duration. Because of the pharmacokinetics of subcutaneously administered human recombinant regular insulin in man, it should be taken 30 to 45 minutes prior to eating. For this reason ultra short-acting analogues were developed that could be administered at the time or 15 minutes prior to eating. These insulins achieved a more rapid onset, higher peaks and shorter duration. Insulin lispro (Humalog® - Eli Lilly & Co.) was created by switching proline-lysine to lysine-proline at the 28th and 29th positions on the beta chain. Insulin aspart (Novolog® - Novo Nordisk Laboratories) was created by the substitution of aspartic acid for proline at the 28th position on the beta chain. These substitutions prevent self-aggregation of the insulin molecules that can delay absorption. These ultra short-acting analogues are not of any clinically significant benefit over human recombinant regular insulin in dogs and cats. Human recombinant regular insulin is still available and used for management of hospitalized dogs and cats requiring short acting insulin.

Intermediate-acting Insulin

In 1946 Krayenbuhl and Rosenberg of Novo-Hagedorn Laboratories, following the work of Scott and Fisher (see PZI below), discovered that the addition of equal amounts of a protein derived from fish sperm, protamine, and regular insulin with a small amount of zinc prolonged the duration of insulin. This led to the development of Neutral Protamine Hagedorn (NPH). Neutral Protamine Hagedorn insulin is also referred to as isophane insulin due to its symmetrical structure and equal composition of regular insulin and protamine with a small amount of zinc. This is one of the few insulins that can be mixed with regular insulin because the regular insulin is not bound by the NPH so mixtures were also introduced. Neutral Protamine Hagedorn must be given to cats and dogs twice daily. The peak is often too rapid and duration too short in both species, even with twice daily administration. This is particularly problematic in cats so this insulin is not routinely used in this species. Recombinant NPH insulin is available and the initial dose in dogs is 0.25 to 0.5 U/kg twice daily. Cats are initially given 1 to 2 units per cat or 0.25 U/kg twice daily.

In 1952 Hallas-Moller developed lente insulin when they discovered that the addition of zinc and an acetate buffer crystallized insulin and prolonged its duration. Lente insulin is actually 30% of short-acting semilente and 70% long-acting ultralente. Initial preparations were beef/pork products because the pork insulin tends to be amorphous whereas the beef insulin crystallizes to a greater extent. The greater amount of zinc in lente insulin can lead to micro crystallization of any added regular insulin so they are not mixed. Currently a pork lente insulin (Vetsulin® - Intervet) is available in the US as a U-40 preparation. Pork insulin is considered ideal for the dog because porcine and canine insulin are identical. The initial dose of lente insulin for the dog using Vetsulin® at 1 U/kg and if < 10 kg add 1 unit, if 10 – 11 kg add 2 units, if 12 – 20 kg add 3 units and if > 20 kg add 4 units. If giving twice daily the above dose should be decreased by 50% and given twice daily. The initial dose in cats is 0.25 U/kg or 1 to 2 units per cat twice daily.

Long – Acting Insulin

In 1936 the work of Hagedorn, Scott and Fisher led to the development of protamine zinc (PZI) insulin. Hagedorn discovered that the addition of various proteins, including protamine, could prolong insulin activity. Scott and Fisher also added the zinc and PZI was born. Eli Lilly & Co. manufactured PZI until 1991 at which time this insulin was unavailable for several years. It is now commercially available as a beef/pork (PZI VET® - IDEXX) and a pure beef formulation. There are concerns that dogs will develop antibodies against the beef component of this insulin so it is not routinely recommended in this species. The majority of cats can not be regulated on once daily PZI so the initial recommended dose is 0.22 – 0.66 U/kg BID or 1 to 2 units per cat. This is a U-40 insulin so special syringes are required.

Insulin analogues have also been developed to more closely mimic the basal insulin secretion in man. Traditionally, human recombinant intermediate insulin had to be given to man twice daily. They also showed patient-to-patient, interpatient and site variability. Ultralente was a long-acting insulin that occasionally could be administered to humans once daily, particularly the beef formulation, but also showed variability between and within patients. For this reason the long-acting analogues such as insulin glargine (Lantus® - Aventis Pharmaceuticals) and insulin detemir (Levemir® - Novo Nordisk) were developed.

Insulin glargine was created utilizing recombinant technology and by substituting a glycine amino acid for an asparagine at the 21st position of the alpha chain. Two arginines were also added to the beta chain. This results in an insoluble insulin molecule at a neutral pH. It is solubilized at an acidic pH and once administered into the neutral subcutis becomes insoluble to slow absorption. Glargine is relatively peakless, exhibits little patient-to-patient variability and can be given once daily in man. Because it is formulated in an acidic pH it can not be diluted or mixed. Dogs have showed an unpredictable and poor response to glargine. Glargine has become increasingly popular in cats with diabetes due to increased rates of diabetic remission compared to lente and PZI insulin in newly diagnosed diabetic cats. Glargine has a longer duration than lente or PZI and can be used once daily in cats. A study comparing once vs. twice daily administration of glargine showed twice daily administration prolonged duration. Although many cats can be controlled with once daily administration, twice daily administration is believed to provide superior glycemic control and improve remission rates, particularly of newly diagnosed diabetic cats. The initial dose recommendations in cats are 0.5 U/kg if glucose is ≥ 360 mg/dL and 0.25 U/kg if glucose is < 360 mg/dL. Because hypoglycemia is fairly common when instituting therapy, glucose curves (pre insulin then every 4 hours) are recommended for the first 72 hours when instituting therapy. If hypoglycemia is measured but clinical signs not observed, the dose is reduced by 0.5 to 1 U. If clinical signs of hypoglycemia are observed (extremely rare) the dose is reduced by 50%. The recommended dose in dogs is 0.25 U/kg twice daily. Glargine can be stored in the refrigerator for 6 months. Additional information on insulin glargine can be found at www.uq.edu.au/vetschool/centrecah.

Insulin detemir is another long-acting analogue created by the addition of a fatty acid moiety to position 29 of the beta chain. This fatty acid binds albumin in circulation and slows activity. There is limited experience with this analogue in dogs and cats and no dose has been established for insulin detemir in dogs and cats. An empiric dose of 0.5 U/kg once daily in the cat and 0.125 to 0.25 U/kg once daily in the dog has been proposed.

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