Short-acting insulin analogues
The short-acting insulin analogues were developed to overcome the perceived flaws in the pharmacokinetics of human regular insulin. Compared to human physiology, the insulin peak after s.c. injection of regular insulin came too late and was not high enough, whereas the duration of action and the residual activity after a few hours was considered too high. This was thought to necessitate injection half an hour before the meal, and to be the cause of late post-prandial hypoglycaemia. These flaws were attributed to the fact that human insulin self-associates to dimers, and in the presence of zinc to hexamers, that need to dissociate first before they can be absorbed into the capillaries. Thus, soon after bioengineering became a possibility, the insulin molecule was modified in an attempt to rectify these flaws. While the resulting short-acting insulin analogues indeed result in plasma insulin profiles that more closely resemble the physiological peak, the clinical consequences of this improvement have been rather underwhelming.
In comparison to regular insulin used at meals, post-prandial glycaemia improves, but overall glycaemia does not improve unless the short-acting analogue is used in CSII pumps. The use of short-acting insulin analogues is associated with about 10% reduction in hypoglycemia rates in type 1 diabetes but this is less clearly demonstrable in type 2 diabetes, probably because insulin absorption in the very obese is prolonged anyway. The introduction of mixes of the short-acting analogues with NPH-like basal insulins has no clear benefit at all, although as a result of cynical marketing the analogue-mixtures have now all but replaced the conventional insulin mixtures.
Figure 1. Hexameric vs monomeric insulins and the resulting differences in absorptionHuman insulin in the presence of zinc atoms self-associates to hexamers[a]. Physiologically, this is useful because the relatively small size of the insulin hexamer compared to 6 insulin monomers allows for closely packed insulin within the secretory granulae of the pancreatic beta-cells. However, since the hexamers are too large for diffusion across the cell membrane, dissociation of the hexamers is a prerequisite for absorption into the circulation. The common denominator of all currently available short-acting insulin analogues is their reduced tendency to form hexamers: in solution they are mainly present as monomers and dimers. Thus, by eliminating the dissociation step, absorption into the circulation is more rapidly achieved (see figure 1). When looking at the amino-acid composition of the short-acting insulins (figure 2), it is also noteworthy that all three have a modification at the B28 position of the insulin molecule. This is in the tail of the molecule that forms a so-called beta pleated sheet. When two monomers associate to dimers they align head-to-tail along these beta sheets. The substitutions made in the insulin analogues disturb this process by either changing the conformation of the sheet (lispro) or by introducing a charged amino-acid that induces charge repulsion between the monomers.
Figure 2. Amino acid compositions of the short-acting insulin analogues
Carcinogenicity: the Asp B10 story
Brange et al. were the first to use the technology of biosynthesis to modify the amino-acid structure of human insulin, which initially resulted in a whole range analogues with a reduced tendency to self-association. However, most of the changes in amino-acid structure also changed the biological properties. Thus, one of the tested analogues, AspB10, was shown to have a dose-dependent carcinogenic effect on the mammary glands of female rats and clinical studies were interrupted.
Lispro, aspart, glulisine
Ultimately, three short-acting insulin analogues were developed for clinical use. All have about the same pharmacokinetic and clinical characteristics. They all have a rapid onset and relatively short duration of action. Clinically, they slightly reduce (nocturnal) hypoglycaemia rates.No safety issues have arisen despite widespread use.
^ A group of 6 identical insulin molecules (from the Greek 'hexa'=6)