**4. Conditions that modify the barrier function**

During hydration the greater part of the water is associated with intracellular keratin; the natural factor of hydration or natural moisturizing factor (NMF) absorbs a noticeable amount of water (10% of the weight of the corneocyte). Corneocytes swell and the barrier properties of the stratum corneum are deeply altered. In the intercellular space the small

Topical Delivery of Haptens: Methods of Modulation of the

coefficient.

preparations [**28**].

way of penetration [**30**].

**5.2 Carrier vesicular systems** 

**5.3 Scratch-patch test** 

Cutaneous Permeability to Increase the Diagnosis of Allergic Contact Dermatitis 65

stratum corneum. Chemical enhancers may: a) increase the diffusibility of the substance inside the barrier, b) increase the solubility in the vehicle or both, or c) improve the partition

These substances may frequently have a not specific action. Enhancers of this type, that are not widely used, are Azone, Dermac SR-38, and oleic acid [**27**]. In some cases, however, these have an irritating effect and must be carefully evaluated in the various

Excipients like ethanol, propylene-glycol, and dimethylsulfoxide (DMSO) may increase the diffusion by altering the organization of lipids of the horny layer [**29**]. The interference with the biosynthesis of some lipids may alter the structure of the barrier and increase the penetration. Methods have also been studied that interfere with secretion and organization of lipids (e.g., brefeldine, monetine, and cloroquine). In addition, enhancers that alter the supramolecolar organization of the bistratified lamellae (synthetic analogs of fats, inducing abnormalities of the organization of the membranes; complex precursors that can not be metabolized, etc.) have been studied. These methods produce an alteration of the critical molar ratio among ceramides, cholesterol, and fatty acids; if there is decrease or excess of one of these 3 key lipids, the lamellar organization cannot be maintained. There may be separation of the phases with more permeable interestitial spaces and formation of a new

The efficacy of the enhancers may be increased by inhibition of the metabolic reaction of repair once the alteration of the barrier has been obtained. This would involve inhibiting metabolic sequences that can rebuild and maintain the barrier function. Inhibitors of enzymes with relevant functions (e.g., lovastatin) or specific inhibitors of enzymes synthesizing ceramides or fatty acids induces alteration of the molar ratio of the three critical lipids and leads to discontinuity in the lamellar layer system [**31**]. Other

The number of drugs for which transdermic methods for systemic use has been possible is very small and restricted to lipophilic and low molecular weight substances (e.g. nicotinic

Liposome formulations can be very effective. However, they probably increase penetration only through the transappendigeal avenue [**34**]. Niosomes and transferosomes, formed by modified liposomes (phosphatidilcoline, sodium cholate, ethanol), are systems based on the ability of vesicles to cross the unaltered horny layer because of the osmotic gradient between external and internal layers of the barrier. These are "flexible" vesicles able to transport their

Although closed patch tests are the mainstay for the evaluation of allergic contact dermatitis, occasionally, even when appropriate concentrations of allergens are used and contact allergy is strongly suspected, positive reactions are not always obtained. As in the cases that will be described patch test with high molecular weight substances as heparin,

enhancements may be obtained by modifying the polarity [**32**].

acid, nitroglycerin, clonidine, steroid hormones, and scopolamine) [**33**].

contents through the intercellular tortuous route of the corneous layer.

amount of water linked to polar groups by hydration does not alter the organization of lipids and does not reduce of permeability [**17**]. The effect of the hydration however has a discontinuous effect; the increase in permeability may be by ten times for some substances and very limited for others [**18**]. Occlusion partially hinders the loss of humidity of the skin, increasing the content of water of the horny layer. However the NMF level in the horny layer is almost zero. It seems therefore that there is a homeostatic mechanism that prevents hyperhydration of the skin [**9**]. Occlusion may increase the absorption by several times, especially for hydrophilic compounds. However, in some conditions it may promote the formation of a reservoir effect. The acidity of the cutaneous surface, controlling homeostasis and enzymatic activities, influences permeability [**19**]; the metabolic activity of the skin (enzymatic oxidoreductive processes) may modify the substances applied, influencing permeability and effects.

Absorption is also influenced by other skin properties that vary at different cutaneous anatomical sites. For instance, the absorption diminishes greatly as one moves from the palpebral skin to the plantar surfaces [**20**].

Age influences skin absorption. Various biological activities are lower in the skin of the aged individual. Great variation is also noted for the premature infant and neonate, who have greater cutaneous permeability [**21**]. There are no experimental data confirming the validity of friction on transcutaneous absorption [**6**]. Alterations of the barrier induce modifications of TEWL [**9**]. In addition, the horny layer may be defined as a biosensor; alterations of external humidity regulate proteolysis of filaggrin, synthesis of lipids, DNA, and proteins within keratinocytes, which can lead also to inflammatory phenomena [**22**].

The cutaneous bioavailability of most commercial dermatological formulations IS about 1-5% of applied dose [**23**].

The active substances of topical formulations are generally absorbed in small quantities; only a reduced fraction passes from the vehicle into the stratum corneum. The greater part remains on the surface of the skin, subject to loss in several ways such as by sweating, chemical degradation, and removal. Future standards would therefore aim to make formulations not merely high in concentration, but pharmaceutically optimized to have an elevated (50-100%) bioavailability. On the other hand, one must consider the marked variations of the different cutaneous areas and skin conditions that make uncertain the therapeutic equivalence when compared with other ways of administration in clinical conditions [**24**].
