Commercial interest in oral delivery of insulin
Several companies have attempted drug delivery systems for proteins, including insulin. Although a commercial product is not
yet available on the market, the current state of research spans from proof-of-concept studies to late-stage clinical studies
(61).
The Orasome technology (Endorex Corp., Chicago, IL) involves encapsulation of proteins in liposomes. These lipo-somes are
rendered resistant to harsh conditions of the gastrointestinal tract such as exposure to acidic pH, bile salts, and detergents
by polymerization (62,63). Drugs are absorbed by the uptake of intact liposomes and are released in the tissues of the body.
Researchers at Emisphere Technology (New York, NY) are working with non-Acylated α-amino acids as carriers for the oral delivery
of macromolecules (64, 65). They claim that upon oral administration of the carrier with the protein, the carrier forms a
noncovalent association with the conformation of the protein that has a higher transport rate when compared with the physiological
conformation. This complex dissociates after crossing the cell membrane.
The M2 system (Nobex Corp., Research Triangle Park, NC) is based on the attachment of low molecular weight polymers to specific
sites in the protein. These polymer conjugates have been reported to improve stability and absorption when compared with performance
of native protein (66).
Limitations of oral delivery of insulin
There are several barriers associated with the oral delivery of insulin:
Enzymatic degradation of insulin.
Upon ingestion, insulin is subjected to acid catalyzed degradation in the stomach, luminal degradation in intestine, and intracellular
degradation. The pancreatic enzymes that degrade insulin are trypsin and α-chymotrypsin (17). The cytosolic enzyme that degrades
insulin is insulin-degrading enzyme (IDE) (67). Insulin is not subjected to enzymatic degradation by brush-border enzymes.
The rate of degradation of insulin also depends on its associated state in solution. Insulin is a monomer at lower concentrations
(<0.1 μM) and dimerizes in a pH range of 4-8 at higher concentrations. At concentrations >2 mM, the hexamer is formed at neutral
pH. The associated state affects the rate of degradation of insulin. In the presence of bile salts, the rate of degradation
may increase close to six times (68).
Intestinal transport of insulin.
Evidence of active transport for insulin was negative (69). Morpho-cytochemical and biochemical evidence for insulin absorption
was demonstrated in rat gastrointestinal tract (70, 71). This result was achieved by direct instillation of a solution of
insulin into various parts of the gastrointestinal tract, followed by visualization with gold markers and immunoassay of the
insulin in blood. No evidence exists for the transport of insulin by the paracellular route. Researchers found that insulin
is absorbed to the apical plasma membrane and is internalized by endocytosis. The presence of insulin receptors has been demonstrated
in enterocytes on both the apical and baso-lateral sides (72–74). Permeability studies of insulin across isolated segments
of the gastrointestinal tract have been performed with an aim to evaluate the apparent permeability coefficient of insulin.
The in vitro permeability studies also serve as screening tools to test the efficacy of absorption modifiers. Insulin permeability
across the gastrointestinal tract has been studied by using isolated segments of various regions of the intestine. Table I
lists the apparent permae-abilities coefficients of various regions of the gastrointestinal tract. These differences are attributed
to the histological differences between various sites (75).
Dosage-form stability issues.
The activity of proteins depends on the three-dimensional molecular structure of the protein. The dosage-form development
of proteins may expose them to harsh conditions that may alter their structure. This will have implications in the efficacy
and immunogenic response to the proteins. During dosage-form development, proteins might be subjected to physical and chemical
degradation. The stability of insulin preparations has been documented in detail (76), and research data on solid-state stability
of proteins in dosage forms have been reviewed recently (77). Proteins must be characterized for change in conformation, size,
shape, surface properties, and bioactivity upon formulation processing. Changes in conformation, size, shape can be observed
by use of spectrophotometric techniques, X-ray diffraction, differential scanning calorimetery, light scattering, electrophoresis,
and gel filtration (78). Changes in surface properties can be detected with electrophoretic and chromatographic techniques,
and changes in the bioactivity of proteins can be observed with bioavailability studies. The interference by formulation excipients
also may be a factor when selecting the characterization technique (79). Size-exclusion chromatography with reverse-phase
high-performance liquid chromatography was used to determine the formation of covalent insulin dimmers with trace amounts
of high molecular weight transformation products after microencapsulating insulin in a mixture of poly(DL-lactide-co-glycolide)
and poly(l-lactide) (80). Differential scanning calorimetry was used to differentiate denaturation endotherms of amorphous
and crystalline insulin (81). X-ray diffractograms of insulin have been obtained with mixtures of lactose and mannitol to
evaluate the effect of spray-drying on the crystalline changes of insulin (82).
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