THE RELATIONSHIP OF THE STRUCTURE OF THE EPIDERMIS TO PERCUTANEOUS ABSORPTION

Effects of magnesium deficiency--more than skin deep

Dead Sea and magnesium salt therapy are two of the oldest forms of treatment for skin disease and several other disorders, supported by a body of largely anecdotal evidence. In this paper we review possible pathways for penetration of magnesium ions through the epidermis to reach the circulation, in turn replenishing cellular magnesium levels. We also discuss mechanisms for intercellular movement of magnesium ions and possible mechanisms for the interaction between magnesium ions and inflammatory mediators. Upon addition of magnesium ions in vitro, the expression of inflammatory mediators such as tumour necrosis factor α (TNFα) and nuclear factor κβ (NFκβ) is down regulated. Dysregulation of these and other inflammatory mediators has been linked to several inflammatory disorders, including asthma, arthritis, atherosclerosis and neuroinflammation.

The permeability of oral mucosa

In discussing permeability, we are describing one of the fundamental barrier functions of oral mucosa. Despite assumptions to the contrary, the oral mucosa is not a uniformly, highly permeable tissue like gut, but shows regional variation. The keratinized areas, such as gingiva and hard palate, are least permeable and nonkeratinized lining areas are most permeable. This variation appears to reflect differences in the types of lipid making up the intercellular permeability barrier in the superficial layers of the epithelium. Differences in permeability may be related to regional differences in the prevalence of certain mucosal diseases and can be utilized to advantage for local and systemic drug delivery.

Estradiol distribution and penetration in rat skin after topical application, studied by high resolution autoradiography

Transdermal pathways and targets in the skin for estradiol were investigated using dry-mount autoradiography. 3H-estradiol-17 beta was applied at doses of 30.1 pmol, 120.4 pmol and 301 pmol/cm2 to shaved rat skin in the dorsal neck region. Vehicles were DMSO, ethylene glycol or sesame oil. After 2 h of topical treatment with 30.1 pmol 3H-estradiol x cm-2 dissolved in DMSO a distinct cellular distribution was apparent. Target cells with concentrations of radioactivity were found in epidermis, sebaceous glands, dermal papillae of hair and fibroblasts. After treatment with 120.4 and 301 pmol/cm2, a penetration gradient of radioactivity was recognizable however it masked specific cellular and subcellular uptake. The stratum corneum accumulated and retained radioactivity, apparently forming a depot for the hormone. Strong concentration and retention of the hormone was conspicuous in sebaceous glands for more than 24 h, suggesting that sebaceous glands serve as a second storage site for the hormone. In all autoradiograms two penetration pathways to the dermis were visible: one through the stratum corneum and epidermis, the other through the hair canals and hair sheaths.

The permeability of human oral mucosa and skin to water

Specimens from four regions of oral mucosa (palate, buccal mucosa, lateral border of the tongue, and the floor of the mouth) and of abdominal skin were taken from 58 individuals at autopsy, for determination of permeability constants (Kp) to tritium-labeled water. Comparisons between fresh specimens and those stored at -80 degrees C revealed no significant effect on Kp as a result of freezing; similar results were found with use of specimens from corresponding regions of the pig. Values for Kp were significantly different for all of the tissue regions examined and ranged from 44 +/- 4 x 10(-7) cm/min for skin to 973 +/- 33 x 10(-7) cm/min for the floor of the mouth, which was the most permeable region. Similar differences were evident among corresponding regions of porcine oral mucosa and skin. Moreover, the Kp values obtained for human tissues were not significantly different from those of the pig, except for the floor of the mouth, which was more permeable in human than in pig tissue. The results reveal interesting differences in the permeability of human oral mucosa that might be related to susceptibility to mucosal disease in those conditions where local extrinsic etiological agents are implicated.

Measurement of the stratum corneum drug reservoir to predict the therapeutic efficacy of topical iododeoxyuridine for herpes simplex virus infection

A rapid, in vivo measurement of the penetration of antiviral compounds into the skin would improve our ability to predict the therapeutic efficacy of topical treatments for herpes simplex virus (HSV) infection. We have studied the concentration of iododeoxyuridine (IDU) in the stratum corneum of guinea pig skin by tape stripping at different time points after single and multiple topical doses of the drug. These results were correlated with the efficacy of topical IDU against an experimental cutaneous HSV infection. Ten adhesive tape strippings were performed on depilated guinea pig dorsum in vivo at serial intervals after a single topical dose of [3H]IDU. Iododeoxyuridine levels in the stratum corneum peaked at 1-3 h (67-70 mg/g of tissue) and then gradually declined over the next 3-24 h. We hypothesized that the peak IDU stratum corneum concentration would correlate with therapeutic efficacy. Accordingly, we determined the quantity of IDU in guinea pig stratum corneum 2 h after a topical application of seven different concentrations of IDU in dimethylsulfoxide (DMSO) and examined the in vivo efficacy of these formulations in an experimental dorsal cutaneous HSV-1 infection in guinea pigs. The results showed an excellent correlation between the quantity of IDU in the stratum corneum and reduction in lesion severity (r = 0.95-0.97). Fifteen percent IDU in DMSO provided the highest therapeutic efficacy (90-94%). We also studied the relationship between the clinical efficacy of different dosing frequencies and the amount of IDU in the stratum corneum. Serial IDU stratum corneum concentrations were measured over 24 h following 1, 2, 3, or 4 applications per day of 1, 3, and 15% IDU in DMSO treatments and parallel efficacy studies of the different regimens were conducted in the animal model. Within each dosing frequency, the cumulative amount of drug in the stratum corneum correlated with the strength of the test formulation and with efficacy in the animal model. For each of the three formulations, increasing the number of daily doses from one up to three led to progressive increases in cumulative stratum corneum IDU levels and clinical efficacy. An increase in the number of daily applications to four had little effect on drug efficacy and was associated with a plateau in stratum corneum IDU levels. Stratum corneum IDU concentrations were rapid and easy to determine and correlated well with clinical events.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo percutaneous penetration of some organic compounds related to anatomic site in humans: predictive assessment by the stripping method

The effect of anatomic site on the in vivo relationship between the total penetration of four compounds and the amount of the compounds present in the stratum corneum at the end of application was studied in humans. For each anatomic site, 1,000 nmol of 14C-radiolabeled benzoic acid, benzoic acid sodium salt, caffeine, or acetylsalicylic acid was applied to 1-cm2 area of skin of male Caucasian patients aged 28 +/- 2 years (groups of 6-8). For each molecule and each site, a first application on the right-hand side of the body allowed total absorption to be determined by measuring the amount excreted in the urine. A second application, performed 48 h later on the contralateral site, enabled the total amount of substance present in the stratum corneum at the end of application (30 min) to be assessed after cellophane-tape stripping of the treated area. The results showed that skin permeability varied substantially, depending both on the physicochemical nature of the molecule and on the anatomical location. In general, the rank order in skin permeability of the studied areas appears to be as follows: arm less than or equal to abdomen less than postauricular less than forehead. Whatever the compound applied, the forehead was approximately 2 times as permeable as the arm or abdomen. Independent of the origin of the differences in permeability observed among sites, there exists a linear correlation (r = 0.97, p less than 0.001) between the amounts of substance present in the stratum corneum at the end of application (30 min) and the total amounts which penetrated within a 4-d period.(ABSTRACT TRUNCATED AT 250 WORDS)

An approach to reduce the number of skin samples in testing the transdermal permeation of drugs

Although glyceryl trinitrate (GT) is a drug that easily permeates through skin, the variations in its transepidermal fluxes were high. The arithmetic mean of the GT flux (n = 31 skin samples from different individuals) was 16.5 micrograms cm-2 h-1 with a standard deviation of 42%. The extreme values were 4.1 and 36.9 micrograms cm-2 h-1, i.e. they differed by a factor of 9. Wide variations were also found for ephedrine, frusemide, caffeine, ethacrynic and benzoic acids and especially trospium chloride. All these fluxes were determined in an in-vitro permeation model at 32 degrees C using human epidermis. With the aim of standardizing epidermal preparations on their permeability, the extent to which the in-vitro GT fluxes through a human epidermal preparation correlate with those of other compounds was evaluated. The resulting standardization procedure consisted of two interactive parts: the correlation of the flux of a test-substance with that of GT using epidermal samples from three donors and estimating the minimum, mean and maximum flux of the test compound and quantitation of the transepidermal permeation of the test compound with those standardized epidermal preparations by calculating the GT standard coefficient defined by the slope of the line derived from the relation between GT flux units and the corresponding flux from the test compound.

The permeability of skin and oral mucosa to water and horseradish peroxidase as related to the thickness of the permeability barrier

The permeability of porcine skin and keratinized and nonkeratinized oral mucosa to tritium-labeled water and horseradish peroxidase (HRPO) was determined using perfusion chambers. Small blocks from each tissue were also incubated with HRPO and the extent of penetration visualized microscopically; this enabled measurements to be made of the thickness of the permeability barrier to this water-soluble tracer. Results obtained after inverting the oral mucosa in the chambers or adding metabolic inhibitors indicated that both compounds diffuse across the tissue. The permeability constants derived directly in the study showed that skin was less permeable than oral mucosa and that the floor of the mouth was significantly more permeable than all other regions. When these constants were normalized in terms of a standard permeability barrier thickness and the different tissues compared, the values obtained for skin were again less than those of the oral regions but, of these, the buccal mucosa was significantly higher. The difference in permeability between epidermis and keratinized oral epithelium may be due to differences in the volume density of membrane-coating granules known to exist between the tissues; differences between the oral mucosal regions may reflect differences in the nature of the intercellular barrier material.

The measurement of the stratum corneum reservoir. A predictive method for in vivo percutaneous absorption studies: influence of application time

The relation existing in vivo between the reservoir function of the stratum corneum following a 30-min application time and the total percutaneous absorption of molecules applied for a longer time was studied on hairless rats. One thousand nanomoles of 4 radiolabeled molecules (theophylline, nicotinic acid, acetyl salicylic acid, benzoic acid) were applied on 1 cm2 of back skin for 0.5, 2, 4, and 6 h. The total recovery in excreta and the whole animal body were measured 4 days later in order to determine the level of absorption. A direct proportionality (r = 0.99) between the total percutaneous absorption of these compounds and the duration of their application was observed. This suggests that a constant flux of penetration does exist in vivo. The reservoir function of the stratum corneum was measured by successive stripping of the treated area. Following a 30-min application, a strict correlation (r = 0.99) between the total amounts of molecules penetrated in 4 days and the amounts found in the reservoir of the horny layer was demonstrated. All together these findings show that the simple knowledge of the reservoir effect of the stratum corneum for a molecule applied for 30 min allows the predictive assessment of its total penetration resulting from various times of application.

Effect of enzyme digestion on the permeability barrier in keratinizing and non-keratinizing epithelia

In an attempt to characterize the permeability barrier of the oral mucosa and skin, small pieces of keratinized and non-keratinized oral epithelia and epidermis were digested with specific enzymes. These enzymes were selected for their effect on carbohydrate-protein, or carbohydrate-lipid compounds and phospholipids. The effect of the enzyme treatment was monitored by exposing the digested tissue to horseradish peroxidase. Electron microscopic examination of tissue treated with phospholipases revealed considerable damage to membrane structures but not to the integrity of the permeability barrier. Hyaluronidase and neuraminidase caused less structural damage but did not impair barrier function; this was only seen after treatment with chondroitinase ABC. This enzyme may degrade certain of the polar molecules thought to be necessary to stabilize the neutral lipid bilayers of the intercellular barrier and thus disrupt its barrier properties.

Prediction of drug disposition kinetics in skin and plasma following topical administration

The development of a physically based pharmacokinetic model for percutaneous absorption is described. The simulation includes four first-order rate constants assigned the following significance: (a) absorption across the stratum corneum; (b) diffusion through the viable tissue; (c) a retardation process which retains penetrant in the stratum corneum (and hence provides a means to mathematically produce a "reservoir" effect, for example); and (d) uptake from the skin into the systemic circulation and subsequent elimination from the body. The kinetic equations of the model are solved and expressions are obtained for the concentration of penetrant within the stratum corneum (and available to subsequently partition into the viable epidermis) and the plasma concentration of the administered substance, as a function of time. Using example values for the four rate parameters, disposition profiles for the penetrant in skin and plasma were derived. The cases considered cover slow and fast stratum corneum penetrants, substances which are excreted rapidly or slowly from the body, and absorbing molecules with a variety of relative stratum corneum-viable tissue affinities. The results suggest a framework for the prediction of pharmaceutically and clinically relevant information following the topical administration of therapeutic agents for local or systemic effect.

The permeability of mammalian nonkeratinized oral epithelia to horseradish peroxidase applied in vivo and in vitro

Horseradish peroxidase, an intercellular tracer, was injected sub-epithelially beneath keratinized gingiva and a variety of nonkeratinized oral epithelia. Fresh tissue biopsies from the same regions were also incubated with this tracer for 1 h. In separate experiments, horseradish peroxidase was applied topically to biopsies of oral mucosa maintained in tissue culture for periods up to 12 h. All specimens were treated so as to visualize the site of peroxidase penetration and the tissues were examined with the light and electron microscopes. In all the epithelia, a surface barrier was evident from which the tracer had been excluded. This was often narrow in the thin, nonkeratinized epithelia and could only be identified unambiguously with the electron microscope. Membrane-coating granules of the so-called nonkeratinized type were invariably associated with superficial plasma membrane of epithelial cells at the level where the barrier was first seen. The results suggest that a permeability barrier exists to horseradish peroxidase in all nonkeratinized oral epithelia with a similar location to that seen in the keratinized oral tissues. However, results obtained with a water-soluble tracer such as horseradish peroxidase may not apply to many drugs, the ready absorption of which may reflect their high lipid solubility.

In vivo correlation between stratum corneum reservoir function and percutaneous absorption

A relationship between stratum corneum reservoir function and percutaneous absorption has been established in the hairless rat. Two hundred nanomoles of 10 substances that have a wide range of chemical structures were topically applied for 30 min and the total body distribution was measured after 96 h. The quantity of substance present in the stratum corneum reservoir after 30-min application was measured by liquid scintillation counting after tape-stripping the treated area. A linear relationship exists between the quantity of substance in this reservoir x(nmol X cm-2) and the total amount of radioactivity distributed in the body and excreta y(nmol X cm-2) after 96 h. The relationship is given by: y = 1.644 X x - 0.536 (r = 0.998, p less than 0.001). Apart from the steroids, 80-95% of the compounds were excreted in the urine; and with the exception of thiourea, this elimination was rapid, especially for mannitol and benzoic acid. We confirmed that in terms of penetration there is a factor of 50 between benzoic acid (best) and dexamethasone (worst). Thus the quantity of substance penetrating through intact rat skin can be predicted by measuring the horny layer concentration. The animal data reported here should be verified in humans.