Protein delivery limitations

The use of therapeutic proteins is growing rapidly and it has been suggested that this class of drugs may soon represent a significant fraction of the pharmaceutical market [45]. There is an urgent need for the development of delivery systems for proteins because, so far, the use of therapeutic proteins is limited to their low oral and transdermal bioavailability. 

The latter studies on pDNA delivery of a therapeutic gene to treat metabolic diseases, autoimmune diseases, viral infections or cancer suggest that naked pDNA could be used for gene delivery. The advantages of this type of therapy include the simplicity of i.m. injection, the requirement for only a limited number of i.m. injections to achieve measurable serum levels of the therapeutic protein, the maintenance of stable serum levels of the protein thereby avoiding side-effects associated with bolus protein delivery and the avoidance of the use of viral vectors which pose safety concerns and which can induce anti-vector antibodies. With improvements in pDNA design and delivery, this type of gene therapy may someday prove useful for therapy of a variety of human diseases. 

Although routine oral delivery of proteins has not been realized, some protein formulations have been developed for pulmonary delivery. Pulmonary delivery can result in either parenteral or local administration of the drug and, like oral delivery, is considered non-invasive. As with other routes of delivery, the size of the protein may limit its ability to be delivered systemi-cally via the pulmonary route of administration. Pulmozyme , a DNase-based formulation approved for the treatment of cystic fibrosis (CF), is delivered to the lungs by a nebulizer to clear blockage of the airways in the CF patient.Formulations for insulin to be administered by inhalation for systemic delivery of... 

It is well known that the oral delivery of many drugs, particularly peptides and proteins, is limited due to... 

The relatively non-invasive nature of transdermal drug delivery, and the fact that this route can simultaneously avoid problems associated with presystemic metabolism and mimic (at least, to some extent) parenteral input profiles, are significant advantages. There have been, therefore, diverse attempts to exploit the skin for peptide and protein delivery. As we have noted before, transdermal administration, with or without one or more enhancement technologies, will always be limited to potent drugs and this accounts, once more, for the effort devoted to peptide and protein (i.e., typically very active substances) administration via this route. 

Stream due to the inter-particle collisions. Although jet milling can typically produce particles in the size-range 1-20 pm, lack of control over parameters such as size, shape, morphology and surface properties of jet milled particles coupled with the high energy input, which can facilitate chemical degradation of proteins constitute limitations to the use of this technique in pulmonary delivery of proteins. 

Because furosemide, bumetanide, ethacrynic acid, and torsemide are bound extensively to plasma proteins, delivery of these drugs to the tubules by filtration is limited. 

Intracellular protein delivery is considered to be the most direct, fastest and safest approach for curing gene-deficiency diseases or disorders affecting primarily cellular processes, such as cystic fibrosis, coagulation disorders, otl-antitrypsin deficiency, immunoglobulin deficiencies, endocrine disorders and lysosomal storage diseases, where the expression of required proteins in the host cells is limited due to the defects in the required cellular (endogenous transcriptional) machinery/ ... 

One particular hydrophobic polymer, EVAc, has been investigated extensively as a matrix system for protein delivery. This polymer is biocompatible, a major consideration because of the interest in developing systems for human health. Other classes of hydrophobic polymers, like silicone elastomers and polyurethanes, may also be useful for controlled protein delivery, although there are fewer examples available in the literature. Nondegradable, hydrophilic polymers, such as poly(2-hydroxyethyl methacrylate) [p(HEMA)], are also biocompatible but usually release proteins over a relatively short period. However, a few examples oflong-term release of peptides and proteins from hydrophilic polymers are available. Longterm release of peptides from devices that employ cross-linked p(HEMA) as rate-limiting barriers has been reported (Davidson et al, 1988). The use of hydrophilic polymers for protein release is discussed in more detail elsewhere in this volume. 

Multiple emulsions are unique in that a true liquid phase is maintained separate from an external aqueous phase. This may be especially important for bioactive molecules that cannot be appropriately stabilized in the solid state. In addition, the separation of aqueous phases enables highly specialized environments, conducive to protein activity, to be prepared. The physical instability of conventional systems remains a major factor limiting their wider application. Attempts to improve the physical stability of the aqueous dispersions through interfacial complexation and the use of microemulsions are improving the short-term stability. As an alternative approach, solid-state emulsions attempt to store the multiple emulsion as a solid. Although solid-state emulsions appear to have the potential to be useful protein delivery systems, a substantial experimental data base has yet to be generated. 

Multiple emulsions have been widely studied as means of delivering drugs via oral, topical, and parenteral routes. The applications include protein delivery (Cournarie et al., 2004), delivery of antibiotics to the vagina (Tedajo et al., 2005), sustained delivery (Vaziri and Warburton, 1994), and vaccine delivery (Bozkir and Hayta, 2004). The immunological response to a vaccine also depends on the route of administration. Most current vaccines are administered intramuscularly, which induces immunization as a systemic immunity. However, the live polio vaccine and the live typhoid vaccine are administered orally. Local immunization (oral, intranasal, or intravagina) may be preferred, since mucosal surfaces are the common entrance to many pathogens. Moreover local immunization induces both mucosal and systemic immunity. Ease of administration and avoidance of systemic side effects are additional advantages of local immunization (Walker, 1994 Shalaby, 1995). Nevertheless, successful local immunization has only been achieved with a limited number of oral vaccines. Also there are very few studies on multiple emulsions used in the immunization process, especially on parenteral and oral administration. 

Physically crosslinked HA hydrogel containing vasoactive intestinal peptide-loaded liposomes was developed for the treatment of endotoxin-induced uveitis. Interactions between HA chains and liposomes increased the viscosity of the gel and resulted in a delayed release of the vasoactive intestinal peptide [87]. However, the low stability of HA microparticles and physically crosslinked HA hydrogel is a limitation for protein delivery applications. 

Testiculat androgens are synthesized in the interstitial tissue by the Leydig cells. The immediate precursor of the gonadal steroids, as for the adrenal steroids, is cholesterol. The rate-limiting step, as in the adrenal, is delivery of cholesterol to the inner membrane of the mitochondria by the transport protein StAR. Once in the proper location, cholesterol is acted upon by the side chain cleavage enzyme P450scc. The conversion of cholesterol to pregnenolone is identical in adrenal, ovary, and testis. In the latter two tissues, however, the reaction is promoted by LH rather than ACTH. 

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