Phytochemicals in ocular health: Therapeutic potential and delivery challenges

Table 1 Route of administration dependent physiological barriers encountered in the diffusion path to the neural retina

Physiological barriers	Route of administration
	Topical	Peri-ocular	Systemic	Oral
Precorneal loss	√
Corneal ultrastructure	√
Corneal efflux proteins	√
Corneal tight junctions	√
Conjunctival efflux proteins	√
Conjunctival tight-junctions	√
Conjunctival vasculature	√
Aqueous humor outflow	√
Anterior chamber metabolism	√
Scleral vasculature	√	√	√	√
Choroidal vasculature	√	√	√	√
Bruch’s membrane	√	√	√	√
Outer BRB
RPE tight junctions	√	√	√	√
RPE efflux pumps	√	√	√	√
Inner BRB
Tight-junctions			√	√
Efflux proteins			√	√
Low concentration gradient			√	√
First pass effect				√
Absorption limitations				√
Metabolism in the GIT				√
Systemic metabolism				√

RPE: Retinal pigment epithelium; BRB: Blood retinal barrier; GIT: Gastrointestinal tract.