Insulin has become increasingly important in the treatment of dogs and cats with diabetes mellitus. Almost all dogs are classified
with insulin-dependent or type 1 diabetes mellitus. Although up to 60% of cats are non insulin-dependent or type 2 diabetics
at the time of diagnosis1 , early intervention with exogenous insulin is an important component to achieving remission and preserving the function
of insulin-secreting beta cells. For this reason almost all of our diabetic dogs and cats will be treated with insulin so
it is important to understand which insulin preparations are available and their utility in our canine and feline patients.
Insulin was first discovered in 1921. The original preparations were bovine and porcine pancreatic extracts. These formulations
were short-acting, relatively dilute and impure. As a result, frequent administration and large volumes of insulin were required.
Due to impurities, local reactions were common. Because beef and pork insulin differ from human insulin by 3 and 1 amino acids,
respectively, many people developed insulin antibodies which resulted in variable and poor responses to these insulins in
some diabetic people. Over time more concentrated beef and pork insulins were produced and purity improved. In the 1930's
– 1950's it was discovered that the addition of protamine and zinc would cause crystallization of insulin and slow absorption.
In the 1980's recombinant DNA technology led to the development of human recombinant insulin. The production of human recombinant
insulin led to a dramatic decrease in the use of animal-origin insulin in man. The 1990's lead to the discovery of synthetic
insulin analogues in an attempt to achieve better glycemic control.2
Ultra Short and Short Duration Insulins (Lispro, Aspart, Human Recombinant Regular)
Insulin analogues were developed to better mimic the basal-bolus requirements of man as well as improve consistency. The premise
is that a more constant (basal) amount of insulin is produced by the pancreas as a result of glucose produced by the liver
in between meals, i.e. in the fasted state. There is also a larger, more rapid increase (bolus) in insulin after meals as
a result of post-prandial hyperglycemia.
Human recombinant regular insulin was the traditional 'bolus' insulin used separately or in mixed preparations with an intermediate
insulin. Using regular insulin, peak insulin levels occurred at 60 minutes with a duration of up to 6 hours. This is because
regular insulin exists as a hexamer (6 insulin molecules) that dissociates into dimers and monomers once injected subcutaneously.
Monomers are the active form and required for absorption into the bloodstream. 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 is administered 30 to 45 minutes prior to eating. This was inconvenient
so ultra short-acting analogues were developed that could be administered at the time of or 15 minutes prior to eating. Insulin
analogues manipulate the amino acids sequences to alter absorption and dissociation. The ultra-short analogues 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 of the insulin molecule. 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.2
Ultra short acting analogues are not of any clinically significant benefit over human recombinant regular insulin in dogs
and cats but may become important if we lose human recombinant regular insulin formulations. A single study in dogs reported
that insulin lispro was comparable to regular insulin when given by continuous rate infusion to dogs with diabetic ketoacidosis.3