Flux of ionic dyes across microneedle-treated skin: effect of molecular characteristics

Abstract

Drug flux across microneedle (MN)-treated skin is influenced by the characteristics of the MN array, formed microconduits and physicochemical properties of the drug molecules in addition to the overall diffusional resistance of microconduits and viable tissue. Relative implication of these factors has not been fully explored. In the present study, the in vitro permeation of a series of six structurally related ionic xanthene dyes with different molecular weights (MW) and chemical substituents, across polymer MN-pretreated porcine skin was investigated in relation of their molecular characteristics. Dyes equilibrium solubility, partition coefficient in both n-octanol or porcine skin/aqueous system, and dissociation constants were determined. Results indicated that for rhodamine dyes, skin permeation of the zwitterionic form which predominates at physiological pH, was significantly reduced by an increase in MW, the skin thickness and by the presence of the chemically reactive isothiocyanate substituent. These factors were generally shown to override the aqueous solubility, an important determinant of drug diffusion in an aqueous milieu. The data obtained provided more insight into the mechanism of drug permeation across MN-treated skin, which is of importance to both the design of MN-based transdermal drug delivery systems and of relevance to skin permeation research.

Figure 1. Chemical structures of the six xanthene dyes.

Figure 2. Relation between Log K_{O/PBS and} Log K_S/PBS of the test dyes.

Error bars represent SD values; with n ≥ 3.

Figure 3. SEM images showing top (a) and side (b) views of solid MN arrays (600 μm-long and 121 MN/array dense).

Figure 4. Permeation profiles of the test dyes through (a) untreated porcine skin and (b) MN-treated porcine skin.

Error bars represent SD values; with n ≥ 3.

Figure 5. Light microscopy images showing vertical sections of porcine skin, control (a), MN-pretreated skin (b) treated with Rh B dye solution.

Bar scales represent 100 μm.

Figure 6. Relative contribution of microchannel pathway flux (J_MCP) and intact skin pathway flux (J_ISP) to the total transdermal flux (J_TOT) of dyes through full thickness porcine skin.

Error bars represent SD values; with n ≥ 3.

Figure 7. The permeation profile of RITC, RITC-D and TRIC-D through MN-pre-treated DHS.

Error bars represent SD values; with n = 3.