TRANSPORT CHARACTERISTICS OF EPITHELIAL TISSUES

III. TRANSPORT CHARACTERISTICS OF EPITHELIAL TISSUES

The normal expected mechanisms of corneal penetration is shown in Table 1 (8). Transport across epithelia occurs via two pathways: transcellular and paracellular. The former involves cell/tissue partitioning/diffusion, channel

284 Lee and Robinson Table 1 Expected Mechanisms of Corneal Penetration Apparent rate-limiting

Drug type

Mechanisms Water soluble

membrane

Epithelium Low o/w partition into epithelium Slow diffusion through epithelium High partition rate + rapid

diffusion through stroma/ endothelium

Via leaky channels Solute movement may be

intercellular and/or transcellular Water and oil

Epithelium-stroma Both mechanisms operate soluble Oil soluble

Stroma High o/w partition into epithelium Rapid diffusion through epithelium

Ionizable Epithelium + stroma or Mechanism not solely dependent leaky channel

upon partition coefficient Source : Adapted from Ref. 8.

diffusion, and carrier-mediated transport. In contrast, the latter represents diffusive and convective transport occurring through intercellular spaces and tight junctions. Due to its aqueous nature, hydrophilic solutes would preferably adopt the paracellular pathway. However, there are three forms of junctional complexes that form between cells which hinder transport of hydrophilic molecules, namely, tight junctions (zonula occludens), inter- mediate junctions (belt desmosome or zonula adherens), and spot desmo- somes (macula adherens) (Fig. 3) (9). Among them, the tight junction is the uppermost and tightest, and it gives the greatest resistance for hydrophilic molecules to go between cells. The barrier property of the tight junction can

be reflected by the transepithelial electrical resistance (TEER). The higher the TEER, the tighter the junctions that give a higher resistance for trans- port of molecules. Generally, epithelia with resistances in the range of 10–

100 cm 2 are considered leaky, whereas those with resistance ranging from 300 to 10,000 cm 2 are ‘‘tight.’’ The cornea is generally classified as a moderately tight or moderately leaky tissue (400–1000 cm 2 ). A compar- ison of the electrophysiology and permeability of the cornea with other tissues is shown in Table 2 and 3, respectively (10).

The cornea also shows permselectivity (11). It has an isoelectric point (pI) of 3.2. At pHs above the pI, it carries a negative charge and is selective to positively charged molecules. On the other hand, at pHs below the pI, it

Ocular Penetration Enhancers 285

Figure 3 Summary of the various cell junctions found in animal cell epithelia. (From Ref. 9.)

Table 2 Comparative Permeability Coefficients of Several Drugs Between Cornea and Other Tissues

Permeability coefficient (cm/s) Permanent

MW Rabbit/dog buccal Rabbit cornea Human skin Water

18 (rabbit) (dog)

Glycerol 92 Benzylamine

107 Octanol

130 Amphetamine

135 salicylic acid

584 Source : Adapted from Ref. 10.

286 Lee and Robinson Table 3 Electrical Resistances and Permselective of Various

Epithelia

Resistances

Tissue Species

( cm 2 )

P NA P Cl

Proximal renal tube Dog

1.10 1.00 0.72 Gall bladder

Rabbit 20 2.30 1.00 0.23 Small intestine Duodenum

Rat

Jejunum Rat 51 1.60 1.00 0.20 Ileum

1.00 1.00 Gastric mucosa Antrum

Urinary bladder Toad

1.40 1.00 0.72 Amphibian skin

0.12 1.00 0.1 Free solution

Rabbit

1.47 1.00 1.52 Source : Ref. 10.

carries a net positive charge. As a result, a positively charged molecule can pass across the cornea more effectively at physiological pH (7.4).

The transcellular pathway is a path that a solute uses to diffuse through the apical lipid matrix of the epithelial membrane continuing through the cytoplasm and across the basolateral membrane. The ability of a solute to pass across the cell using this pathway depends on the inter- action of the solute with plasma membrane components, e.g., lipids, cell surface receptors. For a molecule that adopts a passive transport mechan- ism, partitioning is a crucial step since this is a prerequisite for a molecule to enter the cell. As a result, the partition coefficient becomes a key factor in determining its transport across the epithelium. The optimum partition coefficients for corneal absorption has been reported to be in the range of 10–100 (12), indicating lipophilic molecules are preferred. However, other factors such as size, charge, etc. may also play a role. Theoretically, a small and lipophilic molecule can pass across the cornea effectively. However, this may not always be the case, as can be easily understood when the histology of the cornea is taken into account. The cornea is composed of three layers (Fig. 4) (8). The outermost layer is the epithelium, which is lipophilic in nature and has tight junctions. The middle layer is an acellular matrix, which contains about 85% water and is therefore hydrophilic in nature.

Ocular Penetration Enhancers 287

Figure 4 A simplified diagram of histology of the cornea. (Modified from Ref. 8.)

The innermost layer is the endothelium. Although the endothelium is lipo- philic, it is leaky and does not give any significant resistance to the transport of molecules. It is believed that the epithelium provides the major resistance for hydrophilic/charged molecules and gives minimal resistance to small lipophilic molecules. However, after passing across the epithelium, further movement of these lipophilic molecules is limited by the matrix, which is hydrophilic in nature. As a result, in order to pass across the whole cornea, the molecule has to have a balance between its lipophilic and hydrophilic character.

Other transport mechanisms such as carrier-mediated transport, endo- cytosis, etc. may also be involved in transcellular transport but they are poorly understood.