Ocular Drug Transport And Delivery Modes of Transport

Passive transport or simple diffusion of molecules is a transport process dependent on water and lipid solubility, size of the molecule, and concentration gradient across the cellular membrane. No energy is expended in the process, and transport will cease when the concentrations of the molecules on both sides of the membrane are equal. Passive transport is not inhibited by metabolic inhibitors (inhibiting ATP production or utilization) or by competitive substrates. In general, hydrophilic molecules pass through proteinaceous pores in the cellular membrane, and lipophilic molecules diffuse through the lipid portion of the membrane. Transport through the pores is limited by the pore size that is specific to each tissue. The low lipid solubility of ionized molecules may be increased by altering the degree of ionization with changes in solution pH. Passive transport is important in diffusion of drugs across the cornea and in nutrient uptake across the corneal endothelium.

Active transport is an energy-dependent process requiring ATP, is carrier mediated, and is capable of transporting substrates against a concentration gradient. Macromolecular carriers are membrane bound and have varying degrees of substrate specificity. The carrier reversibly binds to the substrate, transports and releases the molecule on the other side of the membrane and returns to the original state. These characteristics also make active transport subject to metabolic inhibitors, competitive inhibition from other similar substrates, and saturation at high substrate concentrations. Active transport in the corneal endothelium is essential to maintain proper stromal hydration.

Facilitated transport combines some properties of both mechanisms discussed above. This type of transport is carrier mediated so that there is substrate specificity, a transport maximum, and competitive inhibition. However, facilitated transport is not energy dependent and is unable to transport a substrate against a concentration gradient.

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