Iontophoretic transport of charged macromolecules across human sclera

Abstract

The mechanisms of transscleral iontophoresis have been investigated previously with small molecules in rabbit sclera. The objective of the present study was to examine transscleral iontophoretic transport of charged macromolecules across excised human sclera. Passive and 2mA iontophoretic transport experiments were conducted in side-by-side diffusion cells with human sclera. The effects of iontophoresis upon transscleral transport of model permeants bovine serum albumin (BSA) and polystyrene sulfonic acid (PSS) as well as a model drug bevacizumab (BEV) were determined. Passive and iontophoretic transport experiments of tetraethylammonium (TEA) and salicylic acid (SA) and passive transport experiments of the macromolecules served as the controls. The results of iontophoresis enhanced transport of TEA and SA across human sclera were consistent with those in a previous rabbit sclera study. For the iontophoretic transport of macromolecules BSA and BEV, higher iontophoretic fluxes were observed in anodal iontophoresis as compared to passive and cathodal iontophoresis. This suggests the importance of electroosmosis. For the polyelectrolyte PSS, higher iontophoretic flux was observed in cathodal iontophoresis compared to anodal iontophoresis. Both electroosmosis and electrophoresis affected iontophoretic fluxes of the macromolecules; the relative contributions of electroosmosis and electrophoresis were a function of molecular size and charge of the macromolecules.

Figure 1

Comparison of the relationships between passive permeability coefficients and permeant MW for human sclera in the present study and those for human, rabbit, and porcine sclera in the literature. Symbols: closed diamonds, experimental passive permeability with human sclera in the present study (mean and standard deviation, n ≥ 4); open squares, previous human passive permeability data (Olsen et al., 1995); open triangles, rabbit passive permeability data (Ambati et al., 2000); open circles, porcine passive permeability data (Nicoli et al., 2009).

Figure 2

Permeability coefficient ratios of permeant vs. permeant MW. Symbols: experimental permeability coefficient ratios of TEA to SA, TEA to BSA, TEA to PSS, TEA to BEV, and TEA to DEXs with MW 4 and 20 kDa (DEX 4k and DEX 20k). The lines represent the theoretical calculations of the permeant permeability ratio vs. permeant MW at different effective membrane pore radius using Eqs. 4–6.

Figure 3

Effective permeability coefficients of TEA and SA obtained in the passive (open bars), anodal iontophoretic (black bars), and cathodal iontophoretic (gray bars) transport experiments with human sclera. Data represent the mean and standard deviation, n ≥ 4. The Student’s t-test analyses show significant differences between the iontophoresis and passive data (p < 0.05) and between the cathodal iontophoresis and anodal iontophoresis data (p < 0.05).

Figure 4

Representative plots of cumulative amount transported across human sclera vs. time for (a) BSA, (b) PSS, and (c) BEV in passive transport (squares), anodal iontophoresis (diamonds), and cathodal iontophoresis (triangles). Data represent the mean and standard deviation, n ≥ 4 for each condition.

Figure 4

Representative plots of cumulative amount transported across human sclera vs. time for (a) BSA, (b) PSS, and (c) BEV in passive transport (squares), anodal iontophoresis (diamonds), and cathodal iontophoresis (triangles). Data represent the mean and standard deviation, n ≥ 4 for each condition.

Figure 5

Effective permeability coefficients of BSA, PSS, and BEV in the passive (open bars), anodal iontophoretic (black bars), and cathodal iontophoretic (gray bars) transport experiments with human sclera. Data represent the mean and standard deviation, n ≥ 4. The Student’s t-test analyses show significant differences between the iontophoresis and passive data (p < 0.05) and between the cathodal iontophoresis and anodal iontophoresis data (p < 0.05).