Forearm site variation with the corticosteroid vasoconstrictor assay

The skin-blanching assay

The skin-blanching assay is used for the determination and bioequivalence of dermatologic glucocorticoids (GCs). The exact mechanism of the production of blanching is not fully understood, but it is considered that local vasoconstriction of the skin microvasculature and the consequent blood-flow reduction cause this phenomenon. Several factors influence skin blanching, including drug concentration, duration of application, nature of vehicle, occlusion, posture and location. The intensity of vasoconstriction can be measured in several ways: visual or quantitative methods, such as reflectance spectroscopy, thermography, laser Doppler velocimetry and chromametry. In literature, contradicting results in the correlation of the skin-blanching assay with different tests to determine GC sensitivity have been reported, limiting its clinical usefulness.

Effect of barrier perturbation on cutaneous salicylic acid penetration in human skin: in vivo pharmacokinetics using microdialysis and non-invasive quantification of barrier function

We have used microdialysis in the dermis for assessing penetration kinetics of salicylic acid (SA) in healthy volunteers (n = 18), following application on the volar aspect of the left forearm. Penetration was monitored at four locations: in normal (unmodified) skin and in skin with perturbed barrier function from (i) repeated tape stripping (ii) irritant dermatitis from 1 or 2% sodium lauryl sulphate (SLS) for 24 h and (iii) delipidization by acetone. The order of the treatments was randomized according to a latin square design. Epidermal barrier function and skin irritation were assessed in each location using evaporimetry and colorimetry. Transepidermal water loss (TEWL) values confirmed that both mild (acetone), moderate (1% SLS) and severe barrier damage (tape stripping and 2% SLS) had occurred. Microdialysis sampling with two parallel probes in the dermis was performed in each of the four treatment areas for every subject. SA (5% in ethanol) was applied in a chamber glued to the skin overlying the microdialysis probes and sampling was continued for 4 h. SA was detectable in all samples and measurable in all samples from penetration through perturbed skin. Comparing the SA penetration in barrier-perturbed skin with the penetration in unmodified skin in the same subject, the mean SA penetration increase was 2.2-fold in acetone-treated skin (P = 0.012), 46-fold in mild dermatitis and 146- and 157-fold in severe dermatitis and tape stripped skin, respectively (P < 0.001). The penetration of SA significantly correlated with the measurements of barrier perturbation by TEWL (P = 0.01) and erythema (P = 0.02) for each individual. Microdialysis sampling of SA penetration was more sensitive than non-invasive measuring techniques in detecting significant barrier perturbation in acetone-treated skin. A positive dose-response relationship for the percutaneous penetration of SA in response to increasing SLS pretreatment concentrations and thus the degree of irritant dermatitis was found. When analysing data by location on the forearm, a tendency towards an intraregional variation in the reactivity to barrier damage was found, with the most proximal location displaying higher reactivity scores than the most distal location in response to the same barrier perturbation procedures. The penetration of SA was not significantly different between locations. In conclusion, using microdialysis in the dermis to obtain real-time dermal pharmacokinetics in the target organ, this study demonstrates highly increased and differentiated cutaneous penetration of SA in barrier-perturbed skin. The measured drug penetration was demonstrated to correlate with non-invasive quantification of barrier damage.

A single parameter, oxygenated hemoglobin, can be used to quantify experimental irritant-induced inflammation

To quantify the dose-response relation of irritant-induced erythema, we examined inflammation in human skin after application of an irritant, using perpendicular polarized photography and diffuse reflectance spectroscopy as compared to clinical visual scoring. The ventral forearms of 11 healthy subjects were patch-tested for 24 h under occlusion in finn chambers with five concentrations of the irritant sodium lauryl sulfate. The tested sites and three control sites were evaluated clinically for erythema at 24, 48, and 72 h after occlusion, photographed using standard and perpendicular polarized photography, and measured by diffuse reflectance spectroscopy. All photographs were evaluated for erythema by three investigators. Diffuse reflectance spectra were analyzed, and changes in apparent oxyhemoglobin and deoxyhemoglobin concentrations were estimated. Clinical and photographic assessments of erythema yielded similar linear dose-response relations. A linear dose-response relation, with no minimum threshold, also was obtained for changes in the apparent oxyhemoglobin concentration with increasing irritant dose, whereas the apparent deoxyhemoglobin concentrations were unchanged with increasing dose. These results show that diffuse reflectance spectroscopy permits the characterization of irritant-induced inflammation in terms of a single parameter, the apparent concentration of oxyhemoglobin, and that irritant-induced inflammation primarily involves the capillaries and the superficial arterial plexus.

Sensitivity of different areas of the flexor aspect of the human forearm to corticosteroid-induced skin blanching

The intensity of corticosteroid-induced blanching has been found to vary at different areas of the flexor aspect of the human forearm. A retrospective analysis of 38,880 observations of skin blanching in 56 volunteers was conducted to assess the sensitivity of forearm skin to betamethasone 17-valerate. The mid-forearm appears to be more sensitive to the blanching response than do the areas close to the wrist or elbow. These results indicate that each preparation under evaluation should be applied to several sites along the forearm when using the human skin blanching assay in order to obtain an accurate comparative assessment of corticosteroid release from topical delivery vehicles.

Cutaneous reactivity to allergens at 24-h increases from the antecubital fossa to the wrist: an echographic evaluation by means of a new image analysis system

To establish the existence of regional variations in response to allergens, we performed patch tests with nickel sulfate 5% pet. at 4 different sites on the volar forearm skin in 17 nickel-sensitive women, aged from 18 to 45 years. After a 24-h application, patch tests were removed and evaluated both by clinical scoring and by means of a 20 MHz ultrasound B scanner. Recorded echographic images referring to uninvolved skin and to positive test sites were evaluated by a traditional image analysis method for the determination of skin thickness. Furthermore, they were processed by software which ascribes arbitrary values to the amplitude levels of the echoes and enables quantification of the pixels, (the smallest elements in a visual display with controllable colour and brightness) forming objects reflecting within a selected amplitude band. Thus, we demonstrated an increase in thickness of uninvolved skin at distal sites and a greater reactivity at 24 h to patch tests in skin areas near to the wrist crease, as evidenced by a greater increase in pixel and in thickness values at distal sites. From our data, we have a clear indication for the use of rotating patterns or symmetrical sites in quantitative and comparative investigations concerning the evaluation of contact allergy on volar forearm skin.

Potential for irritation increases from the wrist to the cubital fossa

The effect of site on the irritant dermatitis potential was studied in 10 subjects using the volar surface of the forearm. Erythema scoring and transepidermal water loss (TEWL) measurements were used to assess irritation at different levels on the forearm. A sodium lauryl sulphate (SLS 2 g/v%) solution was used as a standard irritant in a patch test procedure and on open tests. Both the erythema scores and measurements of the TEWL of exposed skin showed significant localization differences, and demonstrated that the same sites on the forearm should be used for irritancy tests and possibly physiological studies.

Bioequivalence (bioavailability) of generic topical corticosteroids

A new, quantitative modification of the vasoconstrictor assay was used to measure bioavailability of generic topical corticosteroid products in two categories: betamethasone valerate 0.1% cream and triamcinolone 0.1% cream. Betamethasone valerate products could be separated into two distinct categories, and triamcinolone acetonide products could be grouped into three categories of activity. A survey of local pharmacies showed that generic substitution would be made for the prescribed brand of betamethasone valerate in 58% of cases and for the prescribed triamcinolone acetonide 75% of the time. Because of the prevalence of generic substitution of products that may not have equivalent bioavailability, some standardization of the bioavailability of topical products is needed.

Spatial variability of vasodilatation in human forearm skin

Vasodilatation elicited by topical application of methyl nicotinate was measured by photoplethysmography at various positions on human ventral forearm skin. The time-to-peak response, the magnitude of the peak response, the area under the response-time curve and the time for the response to decay to 75% of the maximum value, were recorded at six positions on the left and right forearms of eight subjects. There was no significant difference between response on the right and left forearm sites and no difference in response between the lateral and medial sites on the forearms. In a second experiment, the vasodilatation was measured at proximal and distal positions on the forearm. The magnitude of the peak response at the proximal position was significantly higher than that found distally (P less than 0.01) as was the area under the response-time curve (P less than 0.05). These results suggest a possible cause for the variation observed in vasoconstrictor assays of corticosteroids.