1.3.2.1 Percutaneous Penetration Enhancers
The most widely researched approach to enhance transdermal delivery of drugs is formulation with chemical penetration enhancers (sorption promoters or accelerants).
These enhancers increase the diffusivity of the drugs by stratum corneum lipid fluidisation thus decreasing barrier function. This approach has been successful for enhancing the permeation of a wide range of small molecules but many of these
chemical penetration enhancers are of limited clinical application due to irritation.
Magnusson and Runn97 reported enhanced transdermal penetration of M-TRH, an analogue of the endogenous tripeptide, thyrotrophin releasing hormone (TRH), in the presence of a terpene and ethanol vehicle. Steady state flux values for M-TRH across human epidermis from PBS were 0.34 mg/cm2/h. In the presence of 50% ethanol the flux increased threefold and a combination of 47% ethanol and 3% cineole resulted in a flux of 1.60 mg/cm2/h. Hence substantial enhancement was achieved for this small peptide but this approach is of limited value for larger peptides and care needs to be taken that enhancer chemicals do not denature the peptide. Karande et al reported the skin permeation enhancing properties of an anionic surfactant, sodium lauroylsarcosinate (NLS) and a non-ionic surfactant, sorbitan monolaureate (S20) in 1:1 phosphate buffered saline (PBS):ethanol solvent. It was observed from the results that combinations of NLS and S20 exhibited significantly higher enhancement of skin permeability compared to that induced by individual surfactants. It was thus concluded based on the data generated from nuclear magnetic resonance (NMR) and fourier transform infra red (FTIR) studies that NLS acts as a strong extractor of stratum corneum lipids and S20 behaves as a weak fluidizer.98 The author also undertook a study to uncover the fundamental mechanisms that define the potency and irritation of chemical penetration enhancers and reported that the irritation behavior of chemical penetration enhancers is related to the ratio of hydrogen bonding to polar interactions.99
1.3.2.2 Encapsulation
Encapsulation involves the entrapment of a peptide drug within a polymeric, phospholipid or carbohydrate particulate delivery system such as microspheres, liposomes, or nanoparticles. Liposomes are phospholipid based vesicles which have been extensively investigated in transdermal delivery. It is generally acknowledged that whilst they are useful for local delivery to superficial skin layers they do not penetrate the epidermis intact. Encapsulation of interferon (IFN)-α into liposomes
with a number of small molecules and with insulin. When the effect of skin application of an ethosomal insulin formulation on blood glucose levels (BGL) in vivo in normal and diabetic rats was investigated, it was found that insulin delivered from the ethosomal patch significantly decreased (up to 60%) blood glucose level in both normal and diabetic rats. A prolonged plateau effect of at least 8h was also observed using the ethosomal insulin patch.101 Niosomes are vesicles composed of non-ionic surfactants that have been evaluated as carriers for a number of cosmetic and drug applications. Gupta et al studied the relative potential of transfersomes, niosomes and liposomes in non-invasive delivery of tetanus toxoid (TT). They observed that the entrapment efficiency, elasticity and topical effect with these encapsulated forms were higher than intramuscular administration of alum-adsorbed tetanus toxoid.102 In another study, conducted on trans-retinoic acid or tretinoin, the potential of niosomes as topical delivery systems capable of improving the cutaneous delivery of tretinoin was assessed. Tretinoin cutaneous delivery was strongly affected by vesicle composition and thermodynamic activity of the drug with negatively charged niosomal formulations showing higher cutaneous drug retention than both liposomes and commercial formulations.103 Transfersomes are composed of phospholipids such as phosphatidylcholine, but also contain surfactant which acts as an ‘edge activator’ conferring deformability on the vesicle.104, 105 However, in all cases scale up of these delivery systems to provide therapeutic peptide levels in humans is yet to be demonstrated.
1.3.2.3 Chemical modification
Lipophilic derivatives of peptides can be made either to increase the encapsulation efficiency into liposomes or directly increase their delivery through skin. The approach has been used successfully to enhance peptide permeability across intestinal and rectal barriers.106 Recently this approach has been applied to skin delivery. Foldvari et al100 reported cutaneous absorption of IFN-α with fatty acids with increasing chain length (C12, C14, C16, C18 and C18:1). The skin absorption of acylated IFNa derivatives was tested in a gel-type pharmaceutical formulation.
The cutaneous and percutaneous absorption of acylated IFN-α was, overall, 2.5–5 times greater than IFN-α. Increasing fatty acid chain length from 12 to 16 resulted in increased palmitoyl substitutions to lysine residues in the protein compared with the
parent IFN-α while further fatty acid chain increase lead to a decrease in absorption.
In the early 1970’s it was shown that conjugating fatty acids to bovine serum albumin changed the immune response from humoral to a mainly delayed type hypersensitivity response. Subsequently, it was shown that dipalmitoyl-peptide conjugates in combination with Freund’s complete adjuvant were able to induce antibodies and cytolytic T cells (CTL) and lipopeptides were used.107 The major priority to improve the transmembrane absorption of poorly-absorbed compounds was to impart them with increased membrane-like character, by conjugation to lipidic amino acids and their oligomers. However, the resultant underivatised hydrophobic conjugates were often poorly soluble in water. The problem of maintaining the balance between the lipophilic and hydrophilic properties of the conjugates was addressed by modifying the lipidic amino acids themselves and secondly by conjugating the lipidic amino acids with hydrophilic molecules.108 Lipoamino acid conjugated peptides and lipopeptides have been discussed in detail in section 1.5.
1.3.2.4 Effect of hydration
The state of hydration of the stratum corneum may influence the percutaneous absorption of a drug. The stratum corneum water content at normal relative humidity is between 15 and 20% of dry weight. Increased water content results in increased elasticity and permeability of the stratum corneum, whereas reducing water content will lead to opposite effect.110 One of the proposed mechanisms for the increase in transport of the drug is by water being absorbed into the stratum corneum where it acts as a plasticiser in its bound state (Figure 1.5). Other studies suggest that the stratum corneum not only swells, but also develops multiple folds, resulting in a 37%
increase in surface area.110 Increase in water content of the stratum corneum is associated with a decrease in lipid/protein phase transition temperature indicating lipid disruption and protein denaturation.111 Hydration does not always increase permeation and in case of molecules with strong lipophilic character there was no effect on the absorption rate.