This chapter is related to Section U2(ii) and U2(vi) from the 2017 CICM Primary Syllabus, which expect the exam candidate to "understand the pharmacology of insulin preparations" and "outline the pharmacology of glucagon". The difference between "understand" and "outline", in the mind of this author, is the permission to completely lose his thread among the smallest pointless details of insulin pharmacology; whereas for glucagon only a sober summary is permitted. The reader is reminded that only Question 6 from the second paper of 2013 has ever asked about the pharmacology of insulin (actrapid was the specific subject), which means this chapter is superfluous to the needs of the fatigued exam candidate and can be ignored completely.
- Chemistry and presentation of pharmacological insulin products
- 51-amino-acid peptide, with several variants modified to adjust self-association behaviour
- Usually presented as a concentrated solution for injection
- In concentrated solution, associates into hexamers, and only soluble at a pH of 2-3
- As infusion, or more commonly subcutaneously
- Zero oral bioavailability
- Absorption from subcutaneous depot depends on rate of dissociation from oligomers
- Rapid acting insulins (eg. insulin aspart, Novorapid) have weak self-association and absorb more rapidly, whereas long-acting insulins remain in hexameric form for longer
- Distributed to extracellular fluid; apart from detemir, most species are minimally protein bound; VOD is between 0.1 and 0.44 L/kg
- Half-life of IV insulin is 2-5 minutes mainly because of this distribution
- Receptor/ligand complex is endocytosed and degraded, preserving the receptor which is recycled
- This happens in most insulin-sensitive tissues, but mainly in the liver (50%) and kidney (30%)
- Elimination half life of insulin is 50-120 minutes
- Duration of action
- Short-acting: 1-3 hrs (aspart, lispro, glulisine)
- Intermediate-acting: 6-12 hrs (isophane), 6-10 hrs (regular human insulin)
- Long-acting: 24-26 hours (glargine, detemir, degludec)
- Pharmacodynamic effects
- Binds to transmembrane receptors with intracellular tyrosine kinase domain
- Activates mechanisms to translocate GLUT4 glucose transporter proteins to the cell membrane, to increase cellular glucose uptake
- Also multiple metabolic effects:
- decreased hepatic glycogenolysis
- increased hepatic glycogen synthesis
- decreased free fatty acid mobilisation by fatty tissue
- increased lipoprotein lipase activity
- decreased protein catabolism
- positive inotropic effects
- decreased release of glucagon
For the casual reader and the trainee preparing for CICM exams, insulin pharmacology is abundantly available in many different forms. Good professional peer reviewed resources include "Donner & Sarkar (2015), Bushra (2004), Joshi et al (2007) or Heinemann & Richter (1993). What these may lack, you do not need; i.e. there are minute details (eg. the pKa of hexameric insulin) that might be difficult to track down without dumpster-diving out the back of the medical library, but nobody could possibly need that level of detail for exams, and outside of exams you could simply Google it.
All insulins are synthetic polypeptide hormones, of 51 amino acids in length, which means their "chemical relatives" would have to include all the other peptide hormones (eg. those secreted by the hypothalamus and pituitary) as well as peptide cytokines and paracrine mediators. Within the family of insulins all members are roughly the same molecular weight and the variation is rather minor, usually differing by only one or two amino acids. For example, porcine insulin has an alanine instead of threonine at the end of the B-chain, and bovine insulin has two different amino acids on the A-chain (instead of threonine and isoleucine at positions A8 and A10, cows get alanine and valine). These days we have phased out animal extracts, and the insulin available to human patients is human insulin, also known as "regular insulin" or "neutral insulin". It is generally marketed as Actrapid (a registered trade name), and it is identical to the molecule which is made in your pancreas, except these days it is mass-produced by an enslaved yeast species.
It would be doubly pointless to report all the minor molecular differences between the various insulin formulations, firstly because this could never be included in any sort of exam question, and secondly because the structural variations do not follow any kind of predictable pattern and are therefore without value from an educational perspective, as there is no high overarching concept to be explained by discussing them. Fortunately, pointlessness is no barrier for Deranged Physiology.
|Insulin preparation||Structural variation|
|Neutral human insulin
|Two chains, B (30 amino acids) and A (21 amino acids), with amino acids labelled by chain letter and number.|
|Bovine insulin||Two amino acid substitutions at A8 and A10: alanine and valine instead of threonine and isoleucine|
|Porcine insulin||Alanine instead of threonine at B30|
|Aspartic acid substituted for proline at B28|
|Lysine replaces proline at B28 and proline replaces lysine at B29 Koivisto (2009)|
|Asparagine is replaced by lysine at position B3, and lysine is replaced by glutamic acid at B29.|
|Isophane insulin, or NPH*
|A complex of regular human insulin and protamine|
(Optisulin, Lantus, Toujeo)
|Asparagine is replaced with glycine in position A21, and the B-chain is extended by 2 arginine residues, which makes this formulation less soluble at physiological pH.|
|At position B29, myristic acid (a fatty acid) is attached to the lysine, which makes this insulin bind to albumin with greater affinity|
|At position B29, hexadecanedioic acid is added via an amide linkage to the lysine. With this, the insulin can form multi-hexamers, greatly slowing its release ("ultra-long-acting" is sometimes used to describe this formulation)|
Contradicting some of the comments above, there are a few general statements to be made about some molecular rules important for the structure-activity relationship of insulin. What there is to say on this subject is said in the most intelligent way by Mayer et al (2007), and what follows is a series of dot-point oversimplifications.
In the modern era, one international unit of insulin is defined as 38.5 µg (0.03846 mg) of dry insulin crystals. This was decided by the WHO Committee on Biological Standardisation in 1987, which was the last such revision, in a long string of progressively improving efforts to define and standardise the dosing of insulin worldwide.
Like units of vasopressin and units of heparin, this measurement standard is bioefficacy based, i.e. it corresponds to the minimum amount of insulin required to do some physiological thing. In this case the physiological thing is "make a 2kg rabbit so hypoglycaemic that they develop seizures". Specifically, this was the definition used by Banting et al in 1922, when they were injecting experimental animals with pancreatic extract, and trying to somehow rate the potency of their concoction. They settled on using rabbits (mice being the other option) and the measurement they used was "the number of cubic centimetres which lowers the percentage of blood sugar in normal rabbits to 0.045 in from two to four hours", that percentage being a level of hypoglycaemia necessary to cause convulsions.
Unfortunately, after generating whole piles of comatose rabbits, the investigators were forced to acknowledge that there was some considerable variability in the seizure threshold of individual animals. Moreover, reasonable people would agree that convulsions are not the correct clinical endpoint for therapeutic insulin administration, and therefore most doses would have to be less than one "rabbit seizure unit" in magnitude. This consideration had led to a confusing proliferation of units and standards, lovingly detailed by Lacey (1967). In short, when dry pancreatic extract finally became available (around 1923), several independent laboratories tested its potency (again, by killing rabbits) and finally declared 0.125mg as the standard unit. This value has decreased over the years mainly because the purity and therefore the potency of dry insulin has improved, with modern insulin capable of taking out a healthy bunny with only 0.03846 mg.
Being a rather large peptide hormone, insulin would be regarded by the digestive system in much the same way as a pork chop and would have zero oral bioavailability as the result. This means one could consume any amount of fried pancreas without the risk of insulin overdose. Valiant efforts to help it withstand enzymatic degradation have so far failed to yield a commercially available product. As the result, the regular insulin user needs to become comfortable and familiar with needles, as their only means of accessing the benefits of this drug would be to inject it into themselves.
From this, it follows that any discussion of insulin bioavailability is a discussion of how it performs when it is absorbed from a subcutaneous depot. True, some insulins can be given intravenously, but for the majority of people regular intravenous injections would be too much of an imposition, and compliance with regular IV drugs would be rather poor (unless they possess heroin-like addictiveness). Moreover, the half-life of insulin in the circulation is rather short (4-6 minutes), which is why the pancreas secretes it at a constant rate, and why a constant infusion of insulin would be required instead of intermittent doses. Thus, subcutaneous administration is the most common and convenient mode, and the most important modifications made to insulin molecules in the course of their commercial development have focused on delaying its absorption from the depot at the injection site, to make it more infusion-like.
Thus, the "absorption" of insulin is really its diffusion away from the site where it was injected. Hildebrandt (1991) and Søeborg et al (2009) offer great overviews of these kinetics and the factors that influence them. Fortunately, aside from the quality of the microcirculation at the site of injection, there are only two main influences:
Given that the range of available insulin concentrations is limited (most present as 100 IU/ml), and considering we never dilute it before subcutaneous injection, the onset of clinical activity is mostly dependent on the species of insulin you have injected. A quick Google Image search immediately yields numerous similar-looking concentration/time curves where different insulins are compared to one another. On closer inspection, the vast majority of these are totally made up - or, to insult them more precisely, they are either rough sketches made to illustrate the concept (without much attention to pharmacological accuracy), or they are created using pharmacokinetic modelling software (with all the caveats inherent therein). Valuable though these are, a regular reader of this site will probably expect real pharmacokinetic data from some kind of cruel human experiments, such this one taken directly from Heinemann & Anderson (2004).
Those are not insulin concentration measurements, of course, as that assay is difficult and expensive - instead, the authors used a euglycaemic glucose clamp technique to indirectly assess the effect of the drug over time. The jaggedness of their data is a reflection of this fact, as the automated glucose controlled struggled to maintain a stable BSL, but you don't need smooth curves here - in fact, the reader will have realised by this point that the actual shape of the curves is rather meaningless, and what is probably more important is the time of onset, timing of the peak, and total duration of the effect, which would work better as a table. Here's a representative one, compiled mainly from Arshag et al (2006) and Freeman (2009):
|Insulin species||Onset||Peak||Duration of action|
|Neutral human insulin
|30-60 min||2-3 hrs||6-10 hrs|
|10-20 min||1-3 hrs||3-5 hrs|
|15-30 min||0.5-2.5 hrs||3-6.5 hrs|
|10-15 min||1.0-1.5 hrs||3-5 hrs|
|Isophane insulin, or NPH*
|1.5-4 hrs||6-14 hrs||16-24 hrs|
(Optisulin, Lantus, Toujeo)
|1-3 hrs||N/A||~24 hrs|
|50-120 min||N/A||~24 hrs|
|30-90 min||N/A||24-36 hrs|
The volume of distribution for normal regular insulin is approximately the same as the extracellular fluid space. According to Turnheim & Waldhäusl (1988), there is a rapid early elimination (2-5min) from the intravascular space by distribution to the extracellular fluid, and then a slower elimination which is accounted for by metabolism. Insulin does not enter the intracellular waters, as its molecular size concentration and lipid solubility would not permit this, which means its VOD should be something like 0.2 L/kg. To tell whether this approximation is true or not is remarkably difficult, as hardly anybody seems to have ever published a straight answer for the different species; but fortunately Potocka et al (2011) at least has the VOD for regular human insulin, which they reported as 30.7L, or about 0.44 L.kg.
For something that is supposed to play nice with the bloodstream into which is is naturally secreted, on paper insulin is remarkably insoluble in water at normal physiological pH. Regular insulin preparations are said to have a pKa of 5.4, and are rendered soluble only by titrating the solution to a facemelting pH of 2-3, or by adding various solubility-enhancing excipients like glycerol and phosphate. At least this is the series of steps taken to create a concentrated commercial product, or to homecook an aqueous solution of lyophilized dry insulin crystals which have been delivered to your lab. As Brange & Langkjœr (1993) patiently point out, this is because at high concentrations insulin molecules naturally associate into hexamers - this is why they develop these weird solubility problems. On their own, insulin monomers are easily soluble in water, which is how insulin tends to traffic in the bloodstream, where the concentration is nanomolar.
One other important feature needs to be mentioned, which does not really belong in any other section. Endogenously secreted insulin goes directly to the liver via the portal circulation, whereas IV insulin is delivered to the systemic bloodstream. This has all sorts of implications, discussed by Arbit (2004). In summary, the result is a delay in the magnitude, the onset and the offset of effect:
Protein binding is only really a feature of insulin detemir, which has a fatty acid hanging off it. This trailing acid causes it to catch on passing albumin molecules, enhancing its slow release characteristics and limiting its bioavailability in the bloodstream. There is little published material out there to describe the protein binding of other insulin species, but we can generally assume that it is probably low.
Insulin is a rather large molecule, and meets a fate similar to other peptides and protein fragments, where it is ultimately captured by endocytosis and degraded. About 50-60% is degraded by the liver, around 30% by the kidneys, and the rest by skeletal muscle, adipocytes, CNS and the myocardium.
Insulin is a pleiotropic hormone that exerts its effect by binding to a large transmembrane receptor with an intracellular tyrosine kinase domain. These receptors are present in all mammalian cells (though they are expressed with different density). The intracellular signalling pathway is mainly the PI3K secondary messenger pathway. Detailed discussions on the physiology of insulin are offered elsewhere, and here only the main effects are:
The pancreas secretes insulin continuously, and then increases production when stimulated by a meal. If it stops doing this because of diabetes, the diabetic needs some constant baseline insulin source as well as pre-meal doses. The vast majority of the time this is achieved by prescribing some of the total daily insulin as a long-acting formulation, and then splitting the short-acting doses for the three meals of the day. An example approach would be to split the total daily dose into halves, with 50% of the total dose given as basal insulin (usually 0.1-0.5 units/kg/day, i.e. 7-30 units/day), and the rest split evenly into three prandial bolus doses. There are all manner of algorithms to guide this more specifically and to protocolise the titration of insulin; interested readers are redirected to the diabetes section of the RACGP website and the ADEA guidelines.
For the intensivist, the more relevant topic would be a discussion of how to commence regular subcutaneous insulin after an insulin infusion is no longer required. Intravenous insulin has a relatively short half life due to its distribution, and will disappear from the circulation very quickly. For this reason, an insulin infusion needs to be overlapped with a subcutaneous dose by at least a couple of hours. Moreover, for a variety of reasons the total 24-hr dose of insulin is not really reflective of what the subcutaneous requirements are going to be, and some suggest only using 80% of the 24-hour dose.
As promised, here is a perfunctory and oversimplified detour into glucagon pharmacology. The use of this substance has diminished, and its significance for the critical care trainee along with it. These days the CICM exam candidate needs to be aware of this drug only in the context of its historical use as an antidote for β-blocker toxicity, and that awareness need only be dim and limited, as in this application glucagon is rapidly being surpassed by high dose insulin euglycaemic therapy. For the reader with a deeper need, Müller et al (2017) do glucagon over 46 exhausting pages.
|Class||Pancreatic hormone analog|
|Chemistry||Peptide hormone (29 amino acids)|
|Routes of administration||IV, subcutaneous, IM|
|Absorption||Essenially zero oral bioavailability. Absorbed well from subcutaneous depot, with onset of effect within about 20 minutes|
|Solubility||pKa of around 7.1; poor water solubility at high concentrations (because it self-associates into a trimer)|
|Distribution||VOD=0.25 L/kg; minimally protein bound|
|Target receptor||Glucagon receptors - G-protein (Gs and Gq) coupled receptors which activate adenylyl cyclase and therefore produce increased cAMP.
Mainly found in the liver
|Metabolism||30% metabolised in the liver, 30% metabolised in the kidney, the rest degraded by (probably) the reticuloendothelial system|
|Elimination||Minimal free drug is eliminated in the urine|
|Time course of action||Half life is about 20-30 minutes|
|Mechanism of action||By increasing intracellular cAMP, glycogen stimulates cAMP-dependent protein kinases and therefore activates pathways of glycogen breakdown and glucose release (among many metabolic pathways)|
|Clinical effects||Mainly hepatic effects: Increased glycogenolysis, decreased glycogen synthesis, increased gluconeogenesis, decreased synthesis of VLDLs and increased β-oxidation of fatty acids, leading to ketosis. Also decreased release of insulin, decreased appetite, increased basal energy expenditure. In high doses, increased cardiac contractility and heart rate|
|Single best reference for further information||Müller et al, 2017|