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European Review for Medical and Pharmacological Sciences
2001; 5: 73-83
Insulin formulations – a review
A.M. GUALANDI-SIGNORINI, G. GIORGI
Department of Pharmacology “G. Segre”, University of Siena - Siena (Italy)
Abstract. – Although the improvement
on insulin therapy since it was first conceived,
it is still far from mimicking physiological secretion of pancreatic b-cells and research to
find new insulin formulations and new routes
of administration continues. Human biosynthetic insulin (rapid-acting, intermediate-acting and
long-acting), produced by recombinant DNA
technique, is currently available. The pharmacokinetic profile of rapid-acting insulin (regular) does not adequately reproduce the physiological post-prandial insulin response. This has
led to the development of molecular analogues
with slight modifications that prevent the spontaneous polymerisation underlying delayed absorption. Fast-acting analogues such as
Lyspro and Aspart can be injected immediately
before the meal, inducing a very fast and substantial peak of insulin, similar to that produced by b-cells, but have the disadvantage of
short duration of action. For this reason, and
because of the difficulty of obtaining sufficient
basal insulin concentrations to control preprandial blood glucose levels with current
long-acting insulins, analogues known as
Glargine and Detemir have been synthesized.
They have virtually no plasma peak and acts
for about 24 h. These characteristics make it
ideal to cover basal insulin requirement. With
insulin analogues, it also seems possible to
overcome the problem of intra- and inter-individual variability in absorption after subcutaneous injection. This variability is directly proportional to the duration of insulin action.
Research into new routes of administration has
led to production of inhaled insulin powder,
soon to become commercially available. Insulin
is absorbed through the lung alveoli. Trials to
evaluate efficacy and toleration have shown
that inhaled insulin has a similar kinetic profile
to the fast-acting injected analogue and can
therefore be used for mealtime requirement,
combined with a single daily injection of longacting insulin. Oral insulin is currently being
studied in type 1 diabetes prevention with
promising results.
Key Words:
Human insulin, Lyspro, Aspart, Glargine, Inhaled insulin, Oral insulin.
Introduction
In the history of insulin therapy some milestones are present. In 1921, the young physician
Frederick Banting (1881-1941) and the forth
year medical student Charles Best (1899-1978),
working in the laboratory of Prof. McLeod in
the Toronto University, found the final link in a
series of studies begun in 1916 by other researchers that guessed the pancreas secretetion
of substance, already named “insulin”, capable
to decrease blood glucose concentrations. The
young researchers isolated this pancreas extract
that “cured” hyperglycemia in diabetic dogs1-2
and in 1922 they successfully administered it
for the first time to a 14-year-old diabetic patient, Leonard Thompson3.
In 1923, the company Lilly began marketing
animal insulin. In 1928, the hormone was identified to be a protein. Since the insulin of
Banting and Best could not function for more
than 6 h, much research went into finding ways
of prolonging action. In 1936, Hagedorn noted
that addition of a basic protein, such as protamine, to the insulin preparation, kept the hormone in suspension at the injection site, delaying absorption and prolonging its action4. In
1946, the first insulin “Isophane NPH”
(Neutral Protamine Hagedorn), obtained combining insulin and protamine in stoichiometric
quantities (hence the term isophane from the
Greek equal and manifest) at neutral pH, was
marketed5. In 1952, the first Lente insulin, retarded with zinc and without protamine, was
produced in Denmark6. In 1955, Sanger determined the exact formula of bovine insulin7. The
sixties saw the development of a radioimmunoassay for insulin8. In the seventies, production of mono-component insulins, obtained
by ion exchange chromatography and of monopeak ones, separated by Sephadex G50 column, began. High performance chromatography made it possible to isolate an insulin 99%
pure from proinsulin and other islet hormones9.
73
A.M. Gualandi-Signorini, G. Giorgi
Human Insulin
Human insulin is a globular protein with a
molecular weight of about 5,800 kd, consisting of 51 aminoacid residues organised in two
polypeptide chains (A and B), linked by two
disulphide bonds. Chain A consists of 21
residues with an extra disulphide bond between A6 and A11; chain B consists of 30
aminoacids. Complete synthesis of the human
insulin molecule was achieved in 196610.
Insulin exists as a monomer only at low concentrations while it shows propensity to aggregate into stable dimers at higher concentrations, in aqueous solution at pH 2-8 and into
hexamers in the presence of zinc ions. The
hexamer, in which chain A constitutes much of
the polar surface, is almost spherical in structure, with a diameter of 5 nm and a height of
3.5 nm. Polymerisation of the hormone has
major pharmacological implications.
Before the eighties, insulin was extracted
from bovine or pig pancreas. Commercial
production of human insulin began in 1982.
This milestone in the history of diabetes is of
similar scientific relevance to the discovery of
insulin itself. Once it became clear that it was
impossible to obtain sufficient human insulin
for world requirements from cadavers, the
techniques for synthesising insulin were developed. Semisynthetic insulin was obtained
by an enzymatic method, in which trypsin
catalyses the substitution of alanine, in position B30 of pig insulin, with threonine. The
insulin thus produced has the same
aminoacid sequence that the human one and
it was completely free of other pancreatic and
gastroenteric hormones11-12.
Genetic engineering has enabled industrial
synthesis of human insulin by the technique of
recombinant DNA. Fragments of DNA coding for chains A and B are inserted separately
in plasmids and then in a special strain of
Escherichia coli which acts as a microlaboratory for in vitro synthesis. The insulin chains released into the medium are harvested (together with other bacterial proteins) and reassembled with the appropriate disulphide bonds1315
. To avoid contamination during the various
steps of the procedure, plasmids were produced with genetic material coding for the
proinsulin molecule, which is isolated from the
culture medium and deprived of its connecting
peptide16-18. Biosynthetic insulin can be also
74
produced by another technique. The procedure exploits the yeast Saccharomyces cerevisiae as a living laboratory by inserting purified
DNA that dictates the synthesis of a single
chain precursor different from proinsulin in
one of the yeast’s plasmids. The protein, produced by yeast continuously during fermentation, is harvested from the culture medium,
isolated by centrifugation and crystallisation,
converted to insulin ester by trypsin transpeptidisation and finally hydrolysed. This method
guarantees high purity12,19-20.
Human insulins currently on the market include rapid-acting (Regular), intermediateacting (NPH and Lente) and long-acting
(Ultralente) formulations. The former is a
clear, colourless, aqueous solution buffered at
neutral pH (7-7.8). Meta-cresol is added as
preservative, glycerol as tonic stabiliser, as
well as zinc chloride. Hexamers, made stable
by zinc ions, are the predominant quaternary
structure of pharmacological insulin; other
structures include dimers and tetramers 21.
The molecule tends to aggregate in the vial
and in the tissue where it is injected. The
hexamers need to be split for the insulin absorption from the subcutaneous injection site.
For this reason, Regular insulin enters the
general circulation after a lag phase about 30
min after injection. It achieves the plasmatic
peak at 2 to 4 h and lasts for 6 h.
Absorption of NPH is delayed by protamine, a protein extracted from the nucleus of
fish sperm, where its role is to stabilise DNA.
The commercial form is Isophane-NPH insulin, a white suspension of orthorhombic
crystals containing 0.9 molecules of protamine and two atoms of zinc per hexamer5. In
the crystal, protamine regulates interactions
between dimers and hexamers. The vehicle is
water buffered at pH 6.9-7.5. Phenol or metacresol are added as preservatives21. Insulin
crystals are insoluble in water and tend to
precipitate to the bottom of the vial, which
has to be tipped various times to resuspend
them before use22. NPH is longer-acting: in
fact its blood absorption begins 1.5 h after
subcutaneous injection; it has a peak plasma
concentration at 4 to 12 h and disappears
within 24 h. The “tail”, however, is relatively
ineffective. Although NPH has the most regular absorption of all intermediate- and longacting insulins, inter- and intra-individual
variability is high.
Insulin formulations – a review
Lente, delayed without addition of protamine, is obtained by precipitating the hormone in the presence of zinc salts. When its
molar ratio with respect to insulin is greater
than one, the zinc ion reduces the solubility
of insulin in neutral solvent6. When an acid
solution of insulin is brought to pH 7.4 with
an excess of zinc ions, the resulting precipitate is amorphous and has moderately delayed absorption after subcutaneous injection. This kind of insulin formulation is
known as Semilente, and was produced until
several years ago. If insulin is allowed to crystallise at pH 5.5 before zinc is added and pH
corrected, the obtained formulation is known
as Ultralente. This insulin consists of crystals
insoluble in water which remain in suspension and are absorbed very slowly. A 3:7 mixture of amorphous and crystalline insulin,
known as Lente, has an intermediate absorption profile. In commercial preparations,
methyl-parahydroxybenzoate is used as
preservative and the suspension is buffered at
pH 7-7.821. It appears in the circulation 2.5 h
after injection, has the peak plasma concentration at 7 to 15 h and disappears by 20 h.
Both NPH and Lente insulins can be mixed
with Regular in the syringe. However, while
NPH and Regular conserve their pharmacokinetic characteristics, Lente/Regular mixture
changes in time because the excess of zinc
ions binds part of the rapid-acting insulin,
transforming it into a form similar to
Semilente.
Long-acting insulin (Ultralente) is an aqueous suspension of zinc-insulin crystals at neutral pH, of milky appearance21. It has an onset of action after 4 h, achieves a slight but
undesirable peak plasma concentration at 7 h
and sustains blood insulin levels approximately 8 to 20 h; it inconsistently mimics endogenous basal secretion. Moreover its absorption is completely irregular23 and it cannot be mixed with Regular insulin in the syringe because the action of the latter would
be excessively delayed.
Insulin Analogues
Since the nineteen-eighties, pharmacological research has been concentrated on analogues of human insulin, seeking to develop
molecules with minimal modifications and
appropriate kinetics. In 1996, an analogue in
which the aminoacids in positions B28 and 29
were inverted, was marketed24-26. The inversion conferred the property of prompt absorption and hence fast action. The analogue
is known as Lyspro, from lysine and proline,
the aminoacids transposed. The handling of
the molecule prevents its natural tendency to
hexamers aggregation, since position B28 is
crucial for its spatial configuration27. Because
the interaction between B28 and B23, two
molecules of insulin align antiparallel28 forming a non polar dimer which in turn aggregates into hexamers by linking to zinc in
pharmacological preparations29. Since modification of aminoacid residue B28 counteracts
polymerisation of insulin, the analogue may
be injected immediately before the meal, as it
acts in a few minutes. Lyspro maintains a
plateau for 1 to 2 h and disappears by 3 to 4
h. Need for availability of insulin analogue
was dictated by three pharmacokinetic defects typical of Regular insulin:
1. Its relatively slow absorption causes
lacking reproduction of physiological
profile of the b-pancreatic secretion.
2. The comparison of the insulin curve in a
normal subject to that in a type 1 diabetic patient (injected with Regular insulin)
shows the following figures: (a) the absence of the first peak, (b) the flat overall profile in the first two hours after the
meal (leading to early post-prandial hyperglycemia) and (c) the persisting
raised insulin plasma levels during postabsorptive period.
3. This pattern may lead to pre-prandial
hypoglycemic events and conditions the
prescription of snacks for type 1 diabetic
patients.
A lot of Italian, European and American
trials carried out after experimentation and
consolidated use of insulin Lyspro30-37 indicate that the post-prandial insulin profile after the analogue administration is very close
to the physiological one. Comparison with
the insulin profile after an injection of
Regular insulin (even when the patient waits
the prescribed 30 min before eating) confirms the higher validity of Lyspro to mimic
physiological glucose-stimulated insulin se75
A.M. Gualandi-Signorini, G. Giorgi
cretion. In clinical practice, this phenomenon
should lead to more physiologic glycemia 90120 min after the meal. However, while the
short duration of action allows for better
control over post-prandial glucose levels, it
does not satisfy, alone, the basal insulin requirement in diabetic patients with low endogenous insulin reserve (type 1). In order to
use the fast-acting analogue in the daily therapeutic protocol of the type 1 diabetic subject, a combination of Lyspro and intermediate-acting insulin is therefore necessary. On
the other hand, this imposes the use of the
syringe to mix insulins or the practice of a simultaneous double injection by the “pen”,
which is the simplest and more pleasant actual method for self-administration of insulin.
Until Lyspro becomes available in “penfills”
with a premixed preparation of the analogue
(25, 50 and 75%) and intermediate-acting insulin (neutral protamine Lyspro, NPL, 75, 50
and 25%) (expected soon), it cannot completely replace Regular insulin, but it can only coexist for a more a versatile use. For example, it can be administered to type 2 diabetic patients requiring insulin due to secondary failure of oral hypoglycemic agents.
In this case it can be used at mealtimes, in association to intermediate- or long-acting insulin at bed-time, if necessary. The pharmacokinetic of Lyspro suggests that type 2 diabetics, characterised by lacking first peak of
insulin secretion, can mostly benefit from the
analogue. It may also be successfully used in
those patients with iatrogenic diabetes due to
steroids, in whom hyperglycemia occurs selectively after meals, or in those patients
with diabetes associated to liver disease having the same feature. It may be useful in type
1 diabetic subjects, educated to manage
“free” variations in diet, for spot correction
of occasionally glucose peaks, detected during self-monitoring (well illustrated in the
“pizza, coca-cola and tiramisù” study) 36 .
Another application is in the first period after diagnosis, when a residual basal insulin
secretion is consistent. An interesting indication is provided by patients who have an uncertain food intake (intercurrent pathologies,
gastroparesis diabeticorum…) In these subjects, Lyspro can be injected immediately after the meal at a dose calculated on the basis
of the quantity of food eaten, without deteriorating post-prandial glucose levels.
76
In 2000, another analogue of human insulin, Aspart, became available. This analogue is obtained by substitution of the proline residue in position B28 with an aspartic
acid residue38. This substitution eliminates
the interaction with glicine B23 that triggers
polymerisation. Monomer status is also aided
by repulsion between the aspartic acid
residue and the nearby glutamic acid B21,
both positively charged. Since the variation
introduced in Aspart is not close to the receptor site, there is no change in receptor binding affinity39 or in the rate of intracellular dissociation40-41 with respect to native insulin.
This manipulation has also solved the problem of affinity for IGF-1 receptors. The delay
in making Aspart available was due to the intense mitogenic activity (due to excessive
affinity for the IGF-1 receptor) of the originally proposed analogue with aspartic acid in
B10 position39.
There are many advantages of therapy using fast-acting insulin analogues. The need to
limit post-prandial hyperglycemia, both in
type 1 and 2 diabetics, emerges from recent
studies with large cohorts, the results of
which unequivocally correlate poor metabolic
control with the development and progression of complications. Specifically, post-prandial hyperglycemia seems a major factor for
the cardiovascular manifestations so frequent
in type 2 diabetics42-44. This is the rationale for
another indication of insulin therapy in type 2
patients, using analogues. Restoration of the
early insulin peak seems to be particularly
important. In fact, correcting hyperglycemia
90-120 min after the meal, it is possible to reduce late hyperinsulinemia and, as a consequence, the insulin resistance highly related
to the development of cardiovascular complications45-46. Pre-prandial and nocturnal hypoglycemia can also be removed by fast-acting
insulin37. Finally, the possibility of injecting
the hormone immediately before the meal,
without having to wait 30 min, greatly improves the quality of life, especially for young
patients who are dependent on injections
throughout their life. However, type 1 diabetic subjects still require simultaneous administration of intermediate- or long-acting insulin.
The pharmaceutical formulation of fastacting insulin is the same as that of regular insulin.
Insulin formulations – a review
Long-Acting Analogues
All available intermediate- and long-acting
insulins (NPH, Lente, Ultralente) have two
major handling problems. The first is that all
the classical formulations have wide variations
in absorption between different individuals,
and even in different injection sites and at different moments in the same subject, in a manner which is directly proportional to the duration of their action 23. This phenomenon is
small, but present and significant with Regular
insulin; it is greater with NPH, Lente and
greatest of all with Ultralente, which is rarely
used for this reason. The second problem is
their time-action profile. After a delay of a few
hours, their absorption gives rise to an insulin
peak and a lowering effect on blood glucose,
the major cause of nocturnal hypoglycemia,
often unawareness, which afflicts many insulin-treated patients47. The peak is followed
at an interval of 7-8 h by a significant drop in
plasma insulin concentrations, not sufficient to
prevent morning hyperglycemia. An attempt
was made to correct this by giving long-acting
insulin at bedtime, between 10 and 12 pm, instead of at mealtime, before dinner, together
Regular or analogue48. This strategy has improved basal hyperglycemia in many subjects,
enabling a reduction in dose, and consequently a reduced risk of nocturnal hypoglycemia,
but is still far from successful in all cases.
The need for an ideal long-acting insulin, that
gives rise to a square wave of plasma concentration, or in other words, constant basal insulin
levels, has prompted research into long-acting
analogues. The aim of the research is to obtain
an insulin formulation, which can be combined
with a fast-acting analogue to ensure a closer
parallel to physiological b-cell secretion.
Candidate analogues for daily use are: (i)
soluble analogues, created by varying the
electrical charge of the insulin molecule and
hence its isoelectric point, which affects solubility in vivo49 (insulin Glargine) and (ii) acyl
analogues in which a fatty acid molecule is
linked to an aminoacid residue so as to bind
with albumin in the blood, forming a “depot”
product that gradually releases insulin into
the tissues50 (insulin Detemir).
Glargine
In 2001, insulin Glargine has become commercially available. This long-acting analogue
is produced from human DNA recombinant
insulin, modified by the addition of two arginine residues, conferring a positive charge, to
the C-terminal (B30a and B30b) of the molecule. This addition shifts the isoelectric point
(pI) from 5.4 to 6.7, making the molecule
more soluble at the acid pH of the vial and
less soluble at the neutral pH of subcutaneous site. The acidity of the medium in
which insulin is conserved dictated another
modification of the molecule, namely substitution of asparagine in position A21 with
glycine. This prevents deamidation and confers stability 51. It has been demonstrated 52
that Glargine interacts with insulin receptors
in a similar way to human regular insulin and
also that its dissociation is similar; it does not
have mitogenic effects due to activation of
IGF-1 receptors53.
Pharmacological studies54-55 have been carried out with the iso- and euglycemic clamp
techniques to evaluate the action profile of
Glargine in normal and diabetic subjects. Its
absorption rate has been measured in type 2
diabetics and compared with that of insulin
NPH: the mean 25% disappearance times of
125
I-insulin from the injection site were 15 and
6.5 h for Glargine and NPH, respectively, and
residual radioactivity at 24 h were 54.4 and
27.9% 56. The area under the blood insulin
curve between 0 and 6 h after administration
of Glargine was significantly less than that of
NPH57. Maximum plasma concentration in
the first 4 h was 5.11 IU/l for Glargine and
10.8 IU/l for NPH58. A recent study57 in type
1 diabetic patients without endogenous insulin reserve (C-peptide negative) confirmed
that the action of Glargine lasts 24 h and that
it gives no plasmatic insulin peak, whereas
NPH and Ultralente achieve significantly high
plasma levels a few hours after injection.
Moreover, NPH lasts only 12 h and the disadvantage of Ultralente is its enormous variability in absorption. Compared to NPH,
Glargine therefore shows a constant increase,
followed by a stable plateau, which, in some
patients, lasts almost 24 h. All these features
indicate that the pharmacokinetical characteristics of Glargine meet the criteria necessary to ensure basal insulin requirement in
type 1 diabetics with negligible plasma Cpeptide concentrations. Many clinical trials
show that Glargine provides good control of
basal glycemic levels59-64.
77
A.M. Gualandi-Signorini, G. Giorgi
Glargine insulin is a clear liquid (like fastacting insulin) with a pH of 4. After injection, it
forms a microprecipitate in the tissues, ensuring slow and steady absorption. Unfortunately,
it cannot be mixed with fast-acting formulations and this is a problem since the number of
daily injections increases when therapy protocol is based on the introduction of Glargine.
Stability of Insulin in
Commercial Preparations
Agents as heat, repeated and strong shaking, exposure to hydrophobic surfaces (such
as drip attachments) cause changes in insulin
conformation leading to linear aggregation
and formation of insoluble fibrils65. This phenomenon mainly affects intravenous infusions (drips required in acute situations) and
continuous subcutaneous infusions by
minipump used for intensive insulin therapy.
Many measures have been devised to avoid
it, such as addition of glycerol66, albumin67,
patient’s serum68 or blood69, but none of them
have proved practical, physiological and safe.
At temperatures above 25° C, chemical
destabilisation due to deamidation or polymerisation may occur70. Current insulin formulations ensure stability of the hormone below 25° C for a month. Vials for daily use can
therefore be kept at room temperature and
supplies must be kept at 2-8° C, where their
physical and chemical characteristics are
maintained for a period of about 30 months.
Absorption
Insulin must be administered parenterally
because digestive enzymes destroy it. When
injected s.c., insulin must be split into
monomers before it enters circulation. The
lymphatic system only plays a secondary role
in absorption of the hormone. Less than 5%, a
clinically insignificant amount, is broken down
at the injection site. As already mentioned, the
problem of intra- and inter-individual variability in insulin absorption is very significant and
can cause a variation of 10-52% in the amount
of insulin absorbed daily, responsible for excursions in glycemia by as much as 80%71. This
problem has not yet been solved.
78
Many factors influence the absorption rate
of insulin.
Injection site
The abdominal region offers faster and
more reproducible absorption (T/2=87 min)
than the arm (140 min), buttocks (155 min)
and thighs (165 min)72. This difference is of
clinical significance and is linked to regional
differences in blood flow. Absorption of insulin monomers is correlated with available
capillary area, explaining why muscle contraction (which causes vasodilation) around
the site of a recent injection, significantly
speeds onset of the glucose lowering effect of
the hormone. Similarly, high room temperature or sauna facilitate Regular but not longacting insulin to be absorbed; the mechanism
involved may not be related to changes in
blood flow but rather to accelerated diffusion
in the tissues. The fast action of analogues is
maintained irrespective of injection site25.
Injection depth
If the injection is too deep and does not
distribute the insulin solution or suspension
in the subcutaneous tissue but rather in the
muscle, absorption is faster due to the greater
vascularisation of muscle with respect to subcutaneous fat. If the injection is too superficial (in the derma), the process is slower and
incomplete.
Concentration
Injection of smaller volumes of insulin solution, corresponding to higher concentrations, improves absorption. The influence of
concentration has been eliminated by standardising all commercial preparations to 100
IU/ml.
Massage of injection site
Massage with cotton wool soaked with disinfectant should be avoided because it increases the absorption rate by a mechanism
apparently not related to blood flow73.
Distribution and Elimination
Intravenous insulin has a half-life of 4 minutes. If administered subcutaneously, it is distributed in the plasma phase in the free form,
Insulin formulations – a review
that is, not bound to proteins, as does endogenous insulin, unless the organism has
produced a significant quantity of antibodies
against it. These antibodies act as a buffer
system, binding insulin with a capriciously reversible bond. In normal subjects, insulin has
a distribution volume of 85 ml/kg74. It reaches
specific receptors in target tissues. It is eliminated by break down in the liver (60-80%),
kidney (10-20%), muscle and adipose tissue
(10-20%). A small amount is excreted in the
urine. Paradoxically, significant impairment
of renal function reduces its clearance more
than severe liver disease 75. The residence
time in subcutaneous tissue is brief for fastacting analogues, making their action profile
short. Subcutaneous deposit of Regular and
long-acting insulin is significant and is
achieved after repeated administrations. The
size of deposit is related to the number of injections/die. For example, a dose of 40 IU/die
in two injections, is calculated to deposit 2260 IU; the same dose in multiple doses deposits 12-20 IU and continuous infusion deposits 1-14 IU76. This explains why patients
taking few doses/die are protected for longer
against rises in blood glucose levels, whereas
patients undergoing continuous infusion cannot even tolerate brief interruptions without
developing ketosis.
Alternative Routes of
Insulin Administration
Since 1924 researches about alternative
routes of insulin administration (oral, nasal,
rectal, transdermal) were reported77-78, but
until recently no realistically useful preparations had been obtained. Of all these routes,
inhalation is the one most tested greatly. The
rationale behind inhalation is the passage of
protein molecules, including insulin, across
the mucosa of the lung alveoli, which have a
surface area of 100 m2. Insulin has shown a
particular aptitude for this route79. A pharmacological formulation of the hormone for
inhalation will soon become commercially
available. It is a dry powder dosed in mg, in
blisters containing 1 or 3 mg of insulin in 5
mg of excipient (mannitol, glycine and sodium citrate powder). The blisters are opened
and emptied directly into the inhalation ap-
paratus. One or two inhalations (2-6 mg) are
generally the appropriate dose for counteracting post-prandial hyperglycemia in a type
1 diabetic patient with poor endogenous insulin reserve. Pharmacokinetical studies in
normal and diabetic subjects80 show that its
absorption rate is similar to that of injected
analogue. A plasma peak is achieved 5-60
min after inhalation, compared to 60-80 min
for Regular insulin. It is therefore administered immediately before the three meals. Its
duration of action is intermediate between
analogue and Regular. A mean daily requirement of 12 mg is equivalent to 350 IU of soluble insulin. The divergence between these
doses and those usually administered by injection of soluble or suspended insulin is due
to a series of factors that cause dispersion.
1) The first loss of hormone is in the inhalation apparatus, but 80-95% reaches
the oral cavity.
2) The second loss is the quantity that deposits on the mucosa of the mouth and
pharynx on the way to the alveoli.
3) Lung tissue metabolises part of the insulin, which is not absorbed. The efficiency of absorption is calculated at 2040%, in other words, 6-8 out of 10 insulin molecules do not enter circulation80-81.
Many trials have been conducted with type
1 and 2 diabetic patients, comparing traditional therapy with inhaled insulin at mealtimes associated to a subcutaneous Ultralente
injection at bedtime. All unequivocally found
the similarity in the hypoglycaemic effect of
the inhaled insulin in respect to subcutaneous
route82-85. In a 3-month study, inhalation was
so well tolerated and accepted that 80% of
patients chose to continue inhalation therapy
for a year 80. However there are still some
doubts as to whether this route is suitable for
all patients, that it is actually more practical
than subcutaneous injection and that it is not
subject to substantial interference by factors
such as smoking and intercurrent pathologies
of the airways81,84.
The oral route has been neglected for some
time because of the difficulty to keep insulin
intact in the gastric environment and to get it
across the barrier of the intestinal mucosa. It
is currently used in experimental studies for
79
A.M. Gualandi-Signorini, G. Giorgi
the prevention of type 1 diabetes mellitus, exploiting its property of modifying the immune
response underlying insulitis by contact with
the tolerant mucosa of the digestive system86.
Moreover, at the annual meeting of the
American Society for Clinical Pharmacology
and Therapy, Still recently presented a hexylinsulin monoconjugate 2 (HIM2) in which an
alkyl-polyethylene-glycol group attached to
lysine B29 confers stability in the gastric environment. The phase II trials of oral insulin
have been presented at American Diabetes
Association annual meeting82. Oral administration of insulin would be a break-through
for obvious reasons of patient convenience
and also because it restores the primary role
of the liver as manipulator of insulin, a role
lost with the introduction of subcutaneous
administration. Under physiological conditions, b-cell hormone is captured by the liver
which metabolises about 40% and then releases the remaining 60% into the peripheral
circulation, whereas injected insulin is absorbed directly into the general circulation,
causing excessive impregnation of the peripheral tissues. This is a major aspect in which
insulin replacement therapy diverges from
what happens in nature.
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