Mechanisms of Tape Stripping and Protein Quantification

By: Ali Alikhan, MD, and Howard I. Maibach, MD

Posted: February 26, 2010, from the March 2010 issue of Cosmetics & Toiletries.

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Finally, Lademann et al. tested a corneocyte density analyzer, a reportedly inexpensive, reproducible optical device based on a slide projector, which also measures corneocyte pseudo-absorption at 430 nm.¹ When compared with standard UV-visible spectrometric measurements, a correlation factor of R2 = 0.95 was demonstrated.¹ This device may simplify the calculation of removed SC without the need for chemistry or a spectrometer. Additionally, the device includes a mechanical autofeed system that is well-suited for the handling of tape strips.¹

Colorimetric Assays for Keratolytic Classification Besides the assessment of proteins collected via tape stripping, colorimetry has successfully been employed to determine the desquamating effects of keratolytics. Table 3 presents the results obtained for three keratolytics from colorimetric protein assays, as described by Dreher et al.¹⁵ The process begins with cutaneous application of the agent. The agent is placed on a patch and taped onto the subject’s skin for a predetermined number of hours. After this period, the placement and removal of tape strips (number varies by study) onto the site of topical treatment is performed.

The assay involves the immersion and shaking of SC adhering tapes in sodium hydroxide solution, resulting in an extraction of the soluble SC protein fraction. The solution, now containing SC protein, is neutralized with hydrogen chloride, since the assay is ineffective under strong alkaline conditions. The assay is then performed using a protein assay kit^a and following a prescribed procedure. This assay is similar to the Lowry assay and is based on the reaction of protein with an alkaline copper tartrate solution and Folin reagent. Finally, absorbance at 750 nm is measured using a spectrophotometer.^b This method allows for the quantification of microgram amounts of SC, diminishing confounding factors, namely vehicle and water uptake by the SC.

The protein measured using the assay described can be compared amongst groups, with statistical analyses allowing the determination of strong and weak keratolytics. SC removal via tape stripping in treatment and control groups is attributable to keratolytic mechanisms, which loosen SC cohesion. The disintegrated SC is subsequently collected by the adhesive. In the first keratolytic bioassay using this technique, salicylic acid (SA) was examined as a function of pH.²¹ The test preparations were: an aqueous vehicle control of pH 7.4; 2% SA aqueous solution of pH 3.3; 2% SA aqueous solution of pH 3.3 with menthol; and 2% SA aqueous solution of pH 6.95.21 A statistically significant mass of SC was removed after 6 hr, and 20 tape strips in all three experimental groups were compared to vehicle, untreated and untreated but occluded groups.²¹ However, after 10 strips, the SA pH 3.3 solution with menthol and the SA pH 6.95 solution removed significantly more SC than any other group, including the SA pH 3.3 solution.²¹

This data suggests that a neutral preparation of SA resulted in a pronounced keratolytic effect. Moreover, the neutral preparation was associated with the least skin irritation among treatment groups.²¹ This finding differs from that of a previous study, which demonstrated superior SA skin penetration in an acidic solution compared to neutral solution.²²