Insulin efficacy and safety
Insulin drugs can be classified into animal insulins, human insulins and insulin analogues according to their sources and chemical structures. With the advent of genetic recombination technology, semi-synthetic and biosynthetic human insulins have started to be used in clinical practice. These insulins have the same molecular structure as human insulin, so their immunogenicity is weak, impurity-free and high potency, but it still has certain defects in clinical application, namely, it is difficult to simulate the human physiological insulin secretion pattern well. Therefore, in order to make exogenous insulin better meet the physiological needs of patients, insulin analogues have emerged.
Since 1922, when Canadian scholar Frederick G. Banting and his assistant Charles Best successfully isolated insulin and used it in diabetes treatment, insulin has become an indispensable drug in diabetes treatment. With the widespread use of insulin, new preparations have been introduced, from animal insulin to human insulin to insulin analogues. So far, insulin analogues have been used in diabetic patients for several years and have become an important member of the insulin family.
Human insulin cannot mimic the physiological insulin secretion pattern of human
Insulin secretion in normal humans includes continuous secretion in small doses at basal state and rapid secretion at mealtime. To treat diabetes with insulin, it is necessary to finely simulate this basal and mealtime insulin secretion pattern of normal human in order to control blood glucose well. Human insulin has some individual differences due to the metabolic characteristics and the limitation of dosage form, etc. The subcutaneous injection time-of-action curve cannot match the physiological needs and cannot ideally meet the basal and mealtime insulin requirements.
Duration of action curve is different from physiological insulin secretion
Short-acting recombinant human insulin takes effect in 30 minutes of subcutaneous injection, peaks in 2 to 4 hours, and lasts for 6 to 8 hours. This requires patients to inject subcutaneously 30 minutes before mealtime. If patients have an earlier mealtime after injection, it will easily lead to poor glycemic control, and if mealtime is delayed, it is prone to hypoglycemia, which shows that it cannot meet physiological mealtime insulin needs.
Medium-acting human insulin is recombinant human insulin mixed with zinc and fisetin, thus delaying its action time. Although the duration of action of NPH insulin is extended, it is still less than 24 hours, and there is obvious peak effect and unstable absorption, which is not conducive to the smooth control of blood glucose and has a small gap with the physiological basal insulin secretion mode. .
Structural characteristics and dosage form lead to obvious individual differences
The molecule of human insulin is a hexameric structure, which must be depolymerized into dimer or monomer after subcutaneous injection in order to enter the circulation through capillaries and exert hypoglycemic effects. Because of the large differences in the depolymerization and absorption of insulin in different individuals, the final amount of insulin entering the circulation may also vary significantly. Different injection sites will also affect the effect of human insulin. In addition, in order to prolong the duration of action, human insulin is mixed with zinc and fisetin and prepared as a suspension. This suspension needs to be mixed before injection, and if it is not mixed adequately, it can also lead to unstable insulin concentration and variation in absorption rate.
Insulin analogues lowering glucose more in line with physiological needs
The defects of human insulin are inseparable from its molecular structural characteristics, so researchers have developed long-acting and fast-acting insulin analogue formulations that better meet the basal and mealtime insulin requirements of the human body by altering the molecular structure of human insulin through genetic recombination technology. Long-acting insulin analogues (e.g., dextran insulin, glargine insulin) have less absorption variability and longer duration of action, which can better mimic basal insulin secretion pattern; fast-acting insulin analogues (e.g., menthol insulin, lysergic acid insulin) have shorter onset, peak and duration of action than human insulin, which can better meet the mealtime insulin demand. To meet both basal and mealtime insulin requirements, researchers have also developed premixed insulin analogs, such as diphasic mentored insulin 30 (containing 30% mentored insulin and 70% arginine).
Long-acting insulin analogs to meet basal insulin requirements
The ideal basal insulin should have the following conditions: the duration of action can be maintained for 24 hours; the pharmacokinetic curve is smooth, the absorption is stable after subcutaneous injection, and there is no obvious absorption and peak effect; the preparation is a clarified solution and does not need to be re-mixed before injection.
As a new generation long-acting insulin analogue, dettaglin is formed by removing the threonine at position 30 of the B chain of human insulin and then attaching a 14-carbon fatty acid (myristic acid) to the lysine residue at position 29 of the B chain of human insulin. On the one hand, the 14-carbon fatty acid chain can enable the formation of a double hexamer of ditrins after subcutaneous injection, which significantly enhances its own polymerization at the injection site and thus delays the absorption. At the same time, 14-carbon fatty acid chains can reversibly bind to albumin in subcutaneous tissues, which further slows down the absorption of insulin into the blood circulation. In the peripheral blood circulation, more than 98% of the insulin is reversibly bound to plasma albumin, which buffers the distribution of insulin to the surrounding target tissues and further prolongs its duration of action, thus effectively reducing blood glucose fluctuations. In addition, the insulin is a colorless and clear solution that does not require shaking for use. These features enable it to act for up to 24 hours, bringing a long-lasting and stable basal blood glucose control effect.
The PREDICTIVE study (Dornhorst A. Diabetes Obes Metab. 2008, 10:75-81.) has shown that after 3 months of conversion to insulin treatment with NPH insulin or glargine, patients with It is suggested that compared with glargine insulin, which is also a long-acting insulin analogue, the same number of injections of diste insulin also has the advantages of good glucose-lowering effect and less variability.
Fast-acting insulin analogues meet mealtime needs
Soluble short-acting insulin must be depolymerized into monomers or dimers after subcutaneous injection in order to be absorbed into the bloodstream. To meet the physiological demand for insulin at mealtime, insulin needs to be depolymerized rapidly after injection. Menthol insulin is a fast-acting insulin analogue synthesized by replacing proline at position 28 of the B chain of human insulin with menthol. The change of its molecular structure reduces the interaction force between insulin monomers and makes it less likely to form stable hexamers, which can be quickly dissociated into monomers after subcutaneous injection and act rapidly. Its absorption is fast, with an onset of effect in 10-20 minutes, a short peak time of 1-3 hours and a higher peak, and a duration of action of 3-5 hours, which is significantly better than human insulin and can better simulate the mode of action of insulin in the human physiological state, and is convenient to use.
Studies have shown that compared with human insulin, menthol insulin has a faster onset of action and a more significant effect on lowering blood glucose, and can better control postprandial blood glucose. In addition, the indications for menthol insulin are wider. In addition to being used as mealtime insulin in the conventional treatment of diabetes, it can also be used in the treatment of insulin pump, the treatment of diabetes in children over 2 years old and the treatment of diabetes in pregnancy, and is the most widely used fast-acting insulin analogue.
Premixed insulin analogues meet both basal and mealtime needs
As the upgraded product of premixed human insulin, dual-phase mentored insulin 30 can better simulate physiological insulin secretion and meet both basal and mealtime insulin needs. Unlike premixed human insulin, the fast-acting component of biphasic mentored insulin 30 has a faster time to peak and a higher peak after injection. Its component of 30% of total menthol insulin can meet the mealtime insulin demand and better control postprandial blood glucose.
Since glycemic control after lunch is not satisfactory in some cases after twice-daily premixed insulin injections, recent studies have investigated the feasibility of three daily injections. 1-2-3 study [Garber AJ et al. Diabetes Obes Metab. 2006,8(1):58-66] showed that for type 2 diabetic patients with poor glycemic control, one, two or three daily injections , 2 or 3 injections of diphasic portal insulin 30 resulted in glycemic compliance (HbA1c <7.0%) in 41%, 70% and 77% of patients, respectively. Patients with two or three daily injections of diphasic phase menthol insulin 30 had significantly lower HbA1c levels after 24 weeks, and the HbA1c reduction was greater in the group with three daily injections, with a higher proportion of patients achieving blood glucose standards.
Figure 1 Diat insulin significantly reduced blood glucose
Figure 2 Compared with human insulin, menthol insulin better controlled postprandial blood glucose
Figure 3 The diphasic menthol insulin 30 group was comparable to basal-meal insulin treatment
Figure 4 Analysis of insulin molecular safety
Table 1 Comparison of the safety of different insulin analogue molecules
Molecular safety of insulin analogues
These issues related to the safety of insulin therapy are called metabolic safety. Metabolic safety is only one part of insulin safety that clinicians are already very familiar with and often talk about. The molecular safety is the part that pharmacologists focus on from the beginning of drug development. Molecular safety, which includes pro-mitotic and genotoxic effects, is directly related to the molecular structure of insulin analogs. Its effects on patients often take decades of clinical use to become clear. Here, let us clinicians also get to know molecular safety together.
As shown in Figure 4, insulin mainly binds to insulin receptors in human body to exert metabolic effects such as promoting glucose uptake, but at the same time, insulin also binds to insulin-like growth factor 1 (IGF-1) receptor, and although its affinity is only 1/500 of the affinity of IGF-1 and IGF-1 receptor, once insulin binds to IGF-1 receptor, it also causes downstream mitogenic effects .
Studies have shown that the mitogenic effects of insulin are enhanced under the following 3 conditions.
(i) High concentrations of insulin stimulate IGF-1 receptors and cause downstream mitogenic effects;
(ii) Structural changes in insulin molecules lead to increased affinity for IGF-1 receptors, which leads to enhanced mitogenic effects;
(iii) The insulin analogue molecule binds to the insulin receptor for a long time, i.e., the dissociation rate is slow, and the long-term stimulation causes enhanced mitogenic effects.
Mitogenesis is a downstream effect of the IGF-1 receptor pathway, and if the affinity of insulin analogs to IGF-1 receptors is strong, their mitogenic effects are enhanced accordingly. So what does the mitogenic effect entail from a cellular perspective? Analysis by Kohn (Kohn et al.) showed that in an in vitro setting, as the affinity of insulin to IGF-1 receptor increased, its pro-proliferative capacity also increased, and the two were linearly correlated. The factor that lies between the two is mitogenic.
Lessons from insulin B10
Insulin B10, an insulin analogue obtained by amino acid substitution, was ultimately not used clinically because it showed a dose-dependent tumorigenic effect with 52 weeks of treatment in rats. A careful analysis of the molecular safety profile of insulin B10 revealed that it has a 6-fold increase in affinity for IGF-1 receptors and a 10-fold increase in mitogenic effects compared to human insulin.
How to consider insulin mitogenesis-related studies
First, there is no difference in the pro-proliferative effects of different insulins (or insulin analogs) at high doses, so studies need to provide dose-response curves rather than single concentration comparisons, especially at high concentrations.
Second, the cell lines selected for the study need to be able to express the IGF-1 receptor in order to observe its downstream effects.
In addition, the cell lines selected for the study must have a proliferative capacity of 2-fold or more to show differences between the different insulin analogs.
Finally, it is important to emphasize that more than one cell line was preferably used in the study for analysis to avoid misleading differences in the selection of cell lines.
Mitogenic effects of different insulin analogues
A study published in Diabetes in 2000 by Kurtzhals et al. showed that the IGF-1 receptor affinity and mitogenic capacity of the fast-acting insulin analogs, menadione and lysergic insulin, were comparable to those of human insulin, and the paper showed that they had the same in vitro safety profile as human insulin. Among the long-acting insulin analogues, the IGF-1 receptor affinity and mitogenic capacity of glargine insulin were higher than those of human insulin, whereas the IGF-1 receptor affinity and mitogenic capacity of dextran insulin were lower than those of human insulin (Table 1).
In an in vitro study, German scholar Shukla et al. explored the proliferative effects of insulin and insulin analogues on normal breast epithelial cell lines and breast cancer cell lines, and showed that for normal breast epithelial cell lines (MCF10A), whether it was bovine insulin, human insulin or insulin analogues (dextran, glargine insulin, lysergic acid insulin and menadione insulin) The pro-proliferative effects were consistent at high doses. In the case of breast cancer cell line (MCF7), the pro-proliferative effect of glargine insulin was significantly enhanced among the above insulins and insulin analogues; glargine insulin showed strong pro-proliferative effect in MCF7 cells at all concentration gradients. This study suggests that the pro-proliferative effect of insulin differs in different cell lines (with different expression of insulin receptors and IGF-1 receptors).
It also showed that in various cell lines (SaoS/B10, CHO-K1, MCF-7 and L6-hIR), the affinity of digitonin for IGF-1 receptor was equal to or slightly lower than that for insulin receptor compared to human insulin, while its mitogenic capacity was only 9%-25% of that of human insulin.
Summary
The available in vitro safety studies suggest that there is no need to be concerned about the molecular safety of digitonin, because it has similar or lower affinity to human insulin than IGF-1 receptor. There is a lack of strong evidence as to whether glargine insulin is associated with malignant tumorigenesis, and the information on glargine insulin and tumorigenesis provided in the latest report of the “Four European Studies” in 2009 is also inaccurate and inconsistent. Despite a new round of discussion around the world, no authority believes that a negative clinical decision regarding the use of insulin can be made until more reasonably designed and convincing findings are available. In the meantime, mitosis is necessary for life to exist, and how high is an excessive elevation of IGF-1 receptor affinity before there is a real risk? How much risk is associated with this for diabetic patients? are still questions that one has to continue to ponder.
As clinicians, we are aware that blood insulin levels in non-diabetic patients weighing more than 100 kg may be 6 to 10 times higher than normal. Blood insulin levels in diabetic patients with multiple daily insulin injections may even reach more than 10 times the normal level. Therefore if it is suspected that the high affinity of insulin with IGF-1 receptor may genuinely induce malignancy, then one should be concerned not only with insulin analogues but with the whole population with endogenous or exogenous hyperinsulinemia.
No one can change the fact that advanced or severe diabetes must be treated with insulin. “If you can’t change it, you have to adapt to it”. In this sense, what we can do in our clinical work is not to restrict the use of insulin, but not to forget that while supplementing insulin, severe medically derived hyperinsulinemia should be avoided as much as possible. The combined use of drugs that increase insulin sensitivity and weight reduction are effective ways to solve this dilemma.