Table 1: The main challenges in ocular drug delivery and key considerations.

Challenges

Absorption: only 3 to 4% ocular bioavailability after topical administration with traditional eye drops
Poorly-soluble drugs: conventional aqueous eye drops not suitable for lipophilic drugs (40–60% of new chemical entities)
Patient compliance: multiple instillations are often needed with eye drops to reach therapeutic levels
High tolerability/comfort requirements limit the formulation options
Excipient choice: few excipients listed in ophthalmology (oils, surfactants, polymers…)
Posterior segment drug delivery: no topical system for the posterior segment; invasive treatments are used due to lack of alternatives

Considerations

Anatomy & physiology of the eye: mucus layer, eyelids, metabolism, blink wash-out…
Tear composition: lipid outer layer, stability of the tear film, enzymes…
Disease state: impact of keratitis or inflammation on absorption and clearance…
Ocular comfort: tolerability of the formulation, pH, osmolality, viscosity, drop size…
Patient expectations: type of packaging and squeeze ability impacting compliance…
Drug loading: impact on absorption, efficacy, dosing regimen, compliance…