2. Insulin preparations
- Four principal types of insulins are available: (1) rapid-acting insulin analogs with more rapid onset and a shorter duration of action than regular insulin after subcutaneous injection; (2) short-acting regular insulin; (3) intermediate-acting; and (4) long-acting, with slow onset of action. Rapid-acting insulin analogs and regular insulin are dispensed as clear solutions at neutral pH and contain small amounts of zinc to improve their stability and shelf life. The long-acting insulin analogs are also dispensed as clear solutions; insulin glargine is at acidic pH and insulin detemir is at neutral pH. NPH insulin is dispensed as a turbid suspension at neutral pH with protamine in phosphate buffer. The Lente series of insulin (ultralente and lente) are no longer available in the United States. The rapid-acting insulin analogs, intermediate-acting, and long-acting insulins are designed for subcutaneous administration, while regular insulin can also be given intravenously. Insulin aspart has been approved for intravenous use, but there is no advantage in using this insulin over regular for this purpose.
a. Rapid-acting insulin analogs - Insulin lispro (Humalog) is an insulin analog produced by recombinant technology, wherein two amino acids near the carboxyl terminal of the B chain have been reversed in position: Proline at position B28 has been moved to B29 and lysine has been moved from B29 to B28. Insulin aspart (Novolog) is a single substitution of proline by aspartic acid at position B28. Insulin glulisine (Apidra) differs from human insulin in that the amino acid asparagine at position B3 is replaced by lysine and the lysine in position B29 by glutamic acid. These changes result in these three analogs having less tendency to form hexamers, in contrast to human insulin. When injected subcutaneously, the analogs quickly dissociate into monomers and are absorbed very rapidly, reaching peak serum values in as soon as 1 hour - in contrast to regular human insulin, whose hexamers require considerably more time to dissociate and become absorbed. The amino acid changes in these analogs do not interfere with their binding to the insulin receptor, with the circulating half-life, or with their immunogenicity, which are all identical with those of human regular insulin.
Clinical trials have demonstrated that the optimal times of preprandial subcutaneous injection of comparable doses of the rapid-acting insulin analogs and of regular human insulin are 20 minutes and 60 minutes, respectively, before the meal. While this more rapid onset of action has been welcomed as a great convenience by diabetic patients who object to waiting as long as 60 minutes after injecting regular human insulin before they can begin their meal, patients must be taught to ingest adequate absorbable carbohydrate early in the meal to avoid hypoglycemia during the meal. Another desirable feature of insulin lispro is that its duration of action remains at about 4 hours irrespective of dosage. This contrasts with regular insulin, whose duration of action is prolonged when larger doses are used.
The rapid-acting analogs are also commonly used in pumps. In a double-blind crossover study comparing insulin lispro with regular insulin in insulin pumps, persons using insulin lispro had lower HbA1c values and improved postprandial glucose control with the same frequency of hypoglycemia. The concern remains that in the event of pump failure, users of the rapid-acting insulin analogs will have more rapid onset of hyperglycemia and ketosis.
b. Short-acting regular insulin - Regular insulin is a short-acting soluble crystalline zinc insulin whose effect appears within 30 minutes after subcutaneous injection and lasts 5-7 hours when usual quantities are administered. Intravenous infusions of regular insulin are particularly useful in the treatment of diabetic ketoacidosis and during the perioperative management of insulin-requiring diabetics. When intravenous insulin is needed for hyperglycemic emergencies, the rapid-acting insulin analogs have no advantage over regular human insulin, which is instantly converted to the monomeric form when given intravenously. Regular insulin is indicated when the subcutaneous insulin requirement is changing rapidly, such as after surgery or during acute infections - although the rapid-acting insulin analogs may be preferable in these situations.
For markedly insulin-resistant persons who would otherwise require large volumes of insulin solution, a U500 preparation of human regular insulin is available. Since a U500 syringe is not available, a U100 insulin syringe or tuberculin syringe is used to measure doses. The physician should carefully note dosages in both units and volume to avoid overdosage.
c. Intermediate-acting neutral protamine Hagedorn (NPH) insulin - NPH (neutral protamine Hagedorn or isophane) insulin is an intermediate-acting insulin whose onset of action is delayed by combining 2 parts soluble crystalline zinc insulin with 1 part protamine zinc insulin. This produces equivalent amounts of insulin and protamine, so that neither is present in an uncomplexed form (“isophane”).
Its onset of action is delayed to 2-4 hours, and its peak response is generally reached in about 8-10 hours. Because its duration of action is often less than 24 hours (with a range of 10-20 hours), most patients require at least two injections daily to maintain a sustained insulin effect. Occasional vials of NPH insulin have tended to show unusual clumping of their contents or “frosting” of the container, with considerable loss of bioactivity. This instability is rare and occurs less frequently if NPH human insulin is refrigerated when not in use and if bottles are discarded after 1 month of use.