Types of Insulins

Insulin is formed by two peptide chains, the A and B chains (con- sisting of 21 and 30 amino acids, respectively), joined by disulfide bridges. Different kinds of insulin are available for clinical use, which differ accord- ing to the species of origin, the degree of purity and the duration of action.

Table 1. Indications for insulin treatment Type 1 diabetes

Diabetic vascular complications Ketoacidosis or ketoacidotic coma

Liver diseases in diabetic patients Hyperosmolar coma

Renal failure in diabetic patients Diabetic pregnancy

Secondary failure with oral hypogly- Acute diabetes decompensation for:

cemic drugs in type 2 diabetes Severe illness or fever Infections or sepsis Stress or steroid treatment Injury or surgery

Species: Bovine insulin differs from human insulin because it contains alanine, valine and alanine at the sites A-8, A-10 and B-30, respectively,

whereas human insulin contains threonine, isoleucine and threonine at the same sites. Porcine insulin is more similar to human insulin inasmuch as it has the same amino acids as bovine insulin at positions A-8 and B-30 but the same amino acid as human insulin at position A-10. Human insulin can be obtained by modification of animal (pork) insulin (human semisyn- thetic insulin) or can be synthesized by the recombinant DNA technique. Human insulin is today the type of insulin most used in many countries. Human insulin (especially the long-acting preparations – see below) is more rapidly absorbed and has a quicker and shorter action than the porcine insulins, perhaps because it is more soluble in subcutaneous tissues. Some patients may have less awareness of hypoglycemia with human insulins than with animal insulins.

Purity: In the past years, conventional insulin preparations contained a significant amount (10,000 parts per million (ppm)) of impurities (proinsulin and proinsulin-like compounds, insulin dimers, glucagon, somatostatin, VIP, PP, etc). Impurity content was much less in ‘single peak’ (monocomponent) insulins (=50 ppm) and even lower in purified insulins (1–10 ppm). Today, in most countries, only very pure human insulin prepared by the recombinant DNA technique is used.

Duration of Action: Three types of insulin are available which differ in the duration of action: (a) the rapid-acting, (b) the intermediate-acting and

(c) the long-acting insulin. (a) The rapid-acting insulin preparations include the regular insulin as well as the semilente insulin which is a suspension of insulin and zinc in acetate buffer (with formation of zinc-insulin crystals). (b) The intermediate-acting insulins comprise the lente insulin, which is a 30:70% mixture of semilente and ultralente (see later) insulin as well as the

Neutral Protamine Hagedorn (NPH) insulin, obtained by adding the protein protamine to insulin and adjusting the pH. (c) The long-acting insulin preparations include the ultralente insulin, obtained by modifying, during the preparation, the pH of a mixture of zinc and insulin to produce larger zinc-insulin crystals (the larger the crystals, the slower the release of the injected insulin) as well as the protamine-zinc insulin obtained by adding also protamine and adjusting the pH.

After subcutaneous injection, regular insulin presents a rapid onset of action (0.5–1 h), an early peak of activity (2–4 h) and a duration of action of 4–6 h. Thus, rapid-acting insulin, beginning to act in about 30 min, should

be given 20–30 min (perhaps 45 min) before a meal to optimize synchronization of postprandial glycemia and circulating insulin levels. It is effective in blunting

elevations in glucose following meals and for rapid adjustments in insulin dosage, but the pharmacokinetics of rapid-acting insulins entails that a definite time interval is observed between insulin injection and eating. A better synchrony between insulin peaks and meal absorption after injection of rapid-acting insulin is observed with human insulin, which acts more rapidly after injection and exerts shorter effects compared to previously used animal insulins.

Intermediate-acting NPH insulin presents a delayed onset of action (3–4 h), a delayed peak of activity (8–12 h) and a duration of action of 20–24 h; similar activity is possessed by the lente insulin. The NPH and lente intermediate-acting insulins have the same, long time-course of action, which is useful to provide the basal level of insulin through the 24 h when given twice per day. Intermediate-acting human insulin produces earlier peaks, that may cause hypoglycemic events during sleep and fails to maintain an adequate effect for a full 24-hour period.

Ultralente (long-acting) insulins present a slow onset of action (6–8 h), a much more delayed peak of activity (14–24 h) and a duration of action of about 32 h. The ultralente human insulin has a shorter duration of action, compared to the animal preparations, and requires also twice-daily injections. Table 2 summarizes the most common insulin preparations and their onset, peak and duration of action.

Insulin Analogues Recently, to improve the outcome of insulin therapy and to use human

insulin products with more physiological effect, a short-acting monomeric insulin analogue, insulin lispro (Lys[B28], Pro[B29]), was developed which

was approved for clinical use and is already commercially available. It has been used extensively in clinical practice. Reversal of the amino acids proline and lysine at position B28 and B29 of human insulin produces an analogue

Table 2. Types of insulin Type

Preparation Onset of action Peak action Duration of action h h h

Rapid-acting Regular

NPH or lente

Ultralente Bovine

24–28 There is great variability in the onset, peak and duration of insulin action from patient

to patient, as indicated by the time intervals given. Human insulin tends to show somewhat earlier onset and peak and shorter duration of action, compared to nonhuman insulins. This is especially true for long-acting preparations. For this reason, different figures for the time intervals are reported in the table for this insulin preparation.

with less tendency to self-association. Conventional insulin preparations are prevailingly in hexameric form, which delays the absorption from subcutaneous injection sites, requiring the dissociation of hexamers into monomers. The monomeric insulin lispro is rapidly absorbed from subcutaneous tissues (so reducing the postprandial hyperglycemia) and shows a shorter duration of action that should decrease the risk of hypoglycemia between meals and at nighttime. Indeed, it shows early peak (1 h) (which allows a much shorter interval between injection and eating) and a shorter duration of action (3–4 h). The insulin lispro in appropriate dosage may result in a profile of insulin close to the physiological one and is suitable for treating both type

1 and type 2 diabetic patients under intensive insulin therapy. Another rapid- acting insulin analogue is currently under evaluation (B28 Asp). Longer- acting ‘basal’ analogues are also under development such as HOE 901 that is an insulin analogue with a lower peak of activity than NPH and a duration of action very long (about 24 h), especially appropriate for type 1 diabetics.

Insulin Concentration The insulin preparation available on the market today is a concentration

of 100 U/ml (U-100). In the past (and still today in some countries) a concentration of 40 U/ml (U-40) was in use. However, preparations with more concentrated insulin also exist (500 U/ml or U-500).

Factors Influencing Insulin Concentration or Bioavailability

Pharmacokinetics of Injected Insulin An optimal therapeutic use of insulin requires the knowledge of the factors

affecting its absorption, disposal and action. Within 5–7 min, insulin given intravenously is concentrated in the heart, liver and kidneys and after 15 min

mainly in the latter two organs. It has been shown that, in the range of physiological concentrations, liver extracts as much as 70% of insulin on a single passage, and that the kidney also removes a significant percentage of the insulin from the blood. The importance of liver and kidney in the insulin disposal is apparent as well as the need to adjust the insulin dosage in patients with hepatic or renal diseases.

Insulin Concentration and Dose Insulin bioavailability is unaffected by insulin concentrations between

40 and 100 U/ml, while more diluted insulin is more rapidly absorbed. A more concentrated regular insulin (which has a more prolonged action) can be used for insulin-resistant patients. Increasing the dose of regular insulin delays the time of peak serum level and prolongs the duration of action, while increasing the dose of NPH insulin can reduce insulin absorption. It is noteworthy that, when the dose of insulin lispro is increased, the duration of action is not prolonged.

Insulin Mixtures Manufactured insulin mixtures exist on the market. Biphasic premixed

insulins have been developed in various ratios of rapid-acting to NPH (30/70, 40/60, 50/50, etc.). The effect of 30/70 mixtures of regular and NPH insulins

is the same as if the components were injected separately and simultaneously, because regular insulin retains its pharmacokinetic characteristics. When a mixture of lente and regular insulins is used, the excess of zinc tends to bind to regular insulin and may cause precipitation of regular insulin out of solution, delaying its absorption and blunting its quick-acting effect. Thus, there are some advantages in using NPH for insulin mixtures. When two types of insulin are mixed, it is important to consider (to assure accuracy of the dose) the variable amount of ‘dead space’ between the hypodermic syringe and the needle. For this reason, it can be useful to always use syringes from the same manufacturer.

Type of Administration and Site of Insulin Injection Subcutaneous administration (with all its disadvantages) remains the only

practical method for the delivery of insulin. The peak concentration of insulin practical method for the delivery of insulin. The peak concentration of insulin

Pen injectors have been introduced for clinical use, loaded with insulin cartridges of 1.5 ml (containing 150 U) or 3 ml (300 U). They are more practical than syringes, especially for the traveler patients, and give more accurate dosage. Only short-acting insulin and NPH insulins can be used with pens (lente insulins are in crystal form and crystals would be broken by the glass marble present in the pen cartridges to help insulin stirring).

Intravenous administration (continuous or pulse) is not practical but permits much more physiological insulin profiles. Other routes of insulin delivery have been proposed, including the intraperitoneal route (which allows insulin to enter, at least in part, the portal vein, similarly to the endogenously secreted insulin), as well as the subcutaneous insulin pellets, skin iontophoresis, oral administration (insulin is degraded by gastrointestinal enzymes and therefore the absorption is extremely variable), nasal or pulmonary spray of insulin (mixed with 1% deoxycholate capable to increase the mucosal insulin absorption) and rectal suppositories (unable to induce a physiological profile of insulinemia).

It should be underlined that the physiologically secreted insulin enters the portal vein and is taken up in substantial amount by the liver, so that only the remaining amount reaches the general circulation and is distributed to the peripheral tissues. In contrast, insulin given for therapeutic purposes through the commonly used routes (subcutaneous or intravenous) enters the general circulation and is distributed to all tissues (liver and peripheral tissues) in approximately similar amount. Therefore, during insulin treatment, liver is relatively hypoinsulinized and the peripheral tissues relatively hyper- insulinized.

Depth of Injection and Massage The depth of insulin injection is an important variable. In fact, the deeper

insulin is injected, the quicker the onset of action and the higher the peak.

Usually, it is recommended to place insulin consistently into deep subcutaneous tissue by means of a lifted flap with the injection device at a 45º angle. Absorption is much faster when insulin is injected intramuscularly rather than subcutaneously, and erratic intramuscular injections in thin individuals may occur. Massage of the injection site increases insulin absorption.

Exercise and Stress Exercise of a leg may increase absorption of the insulin injected in that

extremity; moreover, stress (by increasing epinephrine) may affect local blood flow and absorption of insulin.

Insulin Antibodies Insulin antibodies bind insulin and can delay the onset of action and the

duration of its effect. Among patients, the time course of a given insulin preparation is highly variable, probably for differences in circulating insulin antibodies. Human insulin generates lower titers of insulin antibodies and therefore is most useful in patients who are initiating insulin therapy (and who have not yet produced antibodies) and in patients requiring insulin inter- mittently (intermittent use of insulin increases its antigenic effects).

Destruction of Insulin Insulin is destroyed variably at the site of injection by some insulin-

degrading enzymes. An unusual cause of altered insulin pharmacokinetics may be the local degradation of insulin by proteases, at the site of injection. This mechanism, however, has not been established in patients with poorly controlled diabetes either at the injection site or by in vitro studies with patient’s fat incubated with insulin.

Storage of Insulin Insulin can be kept at room temperature (=25 ºC) for 1–2 months without

losing activity, whereas for longer conservation it should be stored in the refrigerator (between 4 and 8 ºC). Insulin does not withstand temperatures = 2 ºC and freezing must therefore be avoided.

Types of Insulins