In the basal-bolus insulin therapy or the combination therapy of insulin and oral hypoglycemic agents, the short-acting insulin is used to control the post-prandial glucose and the intermediate- or long-acting insulin is used as basal insulin. However, the time-action profile of the short-acting insulin cannot mimic the physiological rapid insulin secretion responding to food uptake, due to the delay in onset of action, nor can it mimic the fast drop of insulin after a meal, because of the duration of action.
(Hirsch, 2005) For the intermediate- or long-acting insulin intended for basal insulin requirement, a pronounced peak of action is unavoidable, consequently resulting in hypoglycemia in diabetic patients. Furthermore, the duration of action of these insulin preparations varies within and between individuals dramatically. Overall, the drawbacks in the pharmacokinetics and pharmacodynamics of the insulin preparations make tight glycemic control in those diabetic patients who are on insulin therapy difficult to achieve.
The interest in producing safer insulin preparations that better duplicate the physiological insulin action is the driving force for development of insulin formulations. (Binder and Brange, 2003) There are five insulin formulations approved by the US Food and Drug Administration so far, as listed in Table 2. All these formulations are produced by recombinant DNA technology, with slight modification in the primary sequence that results in change in the properties compared to native human insulin.
Insulin lispro (Humulog®, Eli Lilly) is the first insulin formulation approved by the FDA. It is a rapid-acting formulation, with an onset of 5 to 15 minutes. Its glucose lowering effect peaks at 30 to 90 minutes and lasts for 4 to 6 hours (Hirsch, 2005). Lispro insulin is an insulin analogue in which the natural amino acid sequence of the B-chain at positions 28-29 is inverted. Thus, B-28 praline and B-29 lysine are switched to B-28 lysine and B-29 proline. Positions B-28 and B-29 are association sites for insulin aggregation.
(Heinemann, 1998) The changes reduce the capacity of lispro insulin to self-association. Structural study has shown that the change in the sequence results in disruption of two critical hydrophobic interactions involving ProB28 and weakening of two hydrogen bonds that stabilize the insulin dimer. Therefore, insulin lispro monomers have a less tendency to self-associate into dimers (Ciszak et al. , 1995). The commercial insulin lispro preparation is a neutral clear aqueous solution for injection, containing both zinc ions and m-cresol.
X-crystallographic studies showed that insulin lispro forms hexameric structure in the presence of both Zn2+ and phenol. However, this hexameric structure is a relatively loose one compared to that of the native insulin. With the diffusion of phenolic ligands from the site of injection, the hexamers dissociate directly into monomers, thus insulin lispro preparation maintains the rapid onset and short duration of action of the monomers in spite of the hexameric structure (Ciszak et al. , 1995).
Conventional regular insulin exists in solution as hexamers. Following subcutaneous injections, hexamers break down into dimers and monomers and only the dimers and monomers can leave the subcutaneous tissue to enter the blood stream. The speed of dissociation of hexameric insulin varies among the different insulin preparations. Thus, lispro insulin is more likely to remain in the monomeric state and is less likely to bind zinc in solution. These conditions favor the absorption and distribution of lispro insulin so that its onset of action is faster.
This also diminishes the depot-effect in the subcutaneous tissue so that the duration of action is shortened. The addition of zinc into the solution increases the long-term stability of Lispro insulin. (Heinemann, 1998) Following the subcutaneous injection of lispro insulin, the time to peak concentration (Tmax) is achieved in about 45 minutes, compared to human regular insulin which is about 2 hours. Lispro insulin achieves much higher maximum concentration (Cmax) than human regular insulin, 698 ± 227 pM compared to 308 ± 132 pM.
The half life (t1/2) of subcutaneous lispro insulin (46 minutes) is also much faster than subcutaneous human regular insulin (82. 5 minutes), but mimics the t1/2 of intravenous human regular insulin (46. 3 minutes) more closely. (Heinemann, 1998) However, the overall area under curve (AUC) of the two insulins is equal. Mixing lispro with zinc will significantly change the kinetics of lispro but it still has a significantly shorter onset Tmax, higher Cmax and faster t1/2 than human regular insulin (Torlone et al., 1994).
The pharmacokinetic properties of lispro suggest that this insulin may be injected immediately prior to the meal. Also, the short duration action of lispro insulin reduces the possibility of iatrogenic hypoglycemia during exercise later after the meal. The potential benefits of lispro may improve the compliance of diabetics pursuing intensive insulin therapy because of the greater flexibility of lispro insulin to be fitted into the diabetic lifestyle (Torlone et al. , 1994).
After intravenous injection, insulin lispro and regular insulin showed almost identical biological potency (Heinemann, 1998). Subcutaneously injected insulin lispro gives twice the maximal concentration and takes around half the time to reach the maximal concentration, as do equivalent doses of regular insulin. For those diabetic patients who are on multiple insulin injections regimen, insulin lispro works better in the reduction of post-prandial hyperglycemia than regular insulin. However, it has not been demonstrated that insulin lispro improves the control of HbAlc level.
On the other hand, insulin lispro is superior to regular insulin in the control of HbAlc level when used in continuous subcutaneous insulin infusion therapy (Heinemann, 1998). Insulin aspart (Novolog®, Novo Nordisk) is another rapid-acting insulin formulation. Insulin aspart is generated by substituting the proline at position 28 in the B-chain with aspartic acid (Binder and Brange, 2003). It was designed to introduce a negatively-charged side-chain carbonyl group to generate charge repulsion between monomers, therefore the propensity to self-associate of insulin monomers is weakened (Hirsch, 2005).
Obviously the design was successful in terms of producing an insulin formulation with faster onset and shorter duration of action, the mechanism of this change turned out to be different from the original hypothesis. Although the charge repulsion may play a role, it is more likely that the loss of some critical intermolecular van der waals contacts due to the replacement of the ProB28 hinders the self-association of monomers (Raslova et al. , 2004). Insulin aspart has pharmacokinetic and pharmacodynamic profiles that are similar to those of insulin lispro.
Insulin glulisine (Apidra®, Aventis) is the most recently approved rapid-acting insulin formulation. Compared to native human insulin, the asparagine at position B-3 is replaced by lysine and the lysine at position B-29 is replaced by glutamic acid. Chemically, it is 3B-lysine-29 B-glutamic acid-human insulin (Garg et al. , 2005). Insulin glulisine has a rapid onset, peak effect at 1 hour, and a shorter duration of action (approximately 4 hours) after subcutaneous injection (Garg et al. , 2005). The most probableexplanation of this property is the reduced tendency for monomers to self-associate.
However, a survey of literature showed that to date, no study investigating the reason for fast acting of insulin glulisine could be found. Insulin glulisine should be injected either within 15 minutes before a meal or within 20 minutes after starting a meal. Garg et al. (2005) reported that insulin glulisine injected 0 to 15 minutes before meals gave slightly better HbAlc reduction than when it is injected immediately after meals. Insulin glargine (Lantus®, Aventis) is the first approved long-acting insulin formulation.
The structural modification is the substitution of glycine at position 21 in the A-chain for the asparagine and the addition of two arginine residues to the B-chain at position 30 (Hirsch, 2005). The modification results in the shift of isoelectric point of the molecule from 5. 4 to 7. 0, rendering it soluble at pH 4. 0 and insoluble at physiological pH (pH 7. 4). Insulin glargine is formulated into a clear solution at pH 4. 0, which will precipitate at the injection site after subcutaneous injection. (Garg et al. , 2005)
Furthermore, the zinc added to the formulation as well as the substitution of glycine makes the hexamers more stable. Overall, the absorption of the molecules into circulation is slowed down significantly, providing longer duration of action without prominent peak action. Insulin glargine has a reported onset of action of 2 hours, and it provides consistent insulin concentration for 24 hours. No pronounced peak action has been observed (Garg et al. , 2005). Insulin glargine is used to provide basal insulin, injected once a day in the morning, at dinner time or bed time.
Various clinical studies demonstrated that once-daily insulin glargine is at least as effective in reducing HbAlc level or lowering fasting plasma glucose as NPH insulin injected once or twice daily in both type 1 and type 2 diabetic patients (Hirsch, 2005). Because there is no pronounced peak in the time action profile, insulin glargine is associated with less hypoglycemia (especially nocturnal hypoglycemia). Less intra- and inter-patients variability is believed to be another advantage of insulin glargine over the NPH and Ultralente insulin preparations. (Garg et al. , 2005)
Insulin detemir is another soluble long-acting insulin formulation recently approved by the US FDA. It is different from native human insulin by the addition of a 14-carbon fatty acid side chain on the lysine in position 29 in the B-chain and the removal of the threonine at position 30. The added fatty acid moiety is also referred to as myristic acid or myristoyl fatty acid (Hirsch, 2005). Insulin detemir forms hexamer and dissociates into dimers and monomers after subcutaneous injection. The slow dissociation of hexamers is part of the reason why it exerts prolonged action.
The addition of the fatty acid moiety and the removal of the threonine allow insulin detemir to bind reversibly to serum albumin. This unique binding property further delays the availability of the molecules and results in longer duration of action. (Raslova et al. , 2004). Insulin detemir may have a dose-dependent duration of action. The duration of action was 20 hours when dosed at 0. 4 units/kg. Insulin detemir should be injected once or twice daily. When given twice daily, morning and evening injections are recommended, the evening injection at dinner time or bed time.
The two injections being 12 hours apart (morning and dinner time) or more (morning and bed time) could affect the glycemic control, although the difference was not statistically significant (Raslova et al. , 2004). When compared to NPH insulin, insulin detemir provides similar or slightly better in glycemic control and causes less variability. Less hypoglycemia and weight gain has been observed in the patients receiving insulin detemir in the clinical trials. Insulin detemir and NPH insulin have similar safety profiles when dosed appropriately (Raslova et al. , 2004).