Why is the eye difficult to deliver drugs to?

The eye is protected by rapid tear turnover and the corneal barrier, so only a small fraction of a topically applied drug is absorbed, and reaching the back of the eye often requires delivery directly into the eye rather than onto its surface.

What is nose-to-brain delivery?

It refers to the proposed pathway by which drugs given into the nose may reach the central nervous system along olfactory and trigeminal nerve routes, partly bypassing the bloodstream; its practical extent, particularly for large molecules, is still being studied.

Specialized Delivery (Nasal, Rectal, Ocular)

Several routes deliver drugs across mucosal or specialized surfaces for purposes that the major routes serve less well. Nasal delivery uses the well-perfused nasal mucosa for local effect, rapid systemic absorption, or potential nose-to-brain transport; rectal delivery uses the rectal mucosa for local or systemic effect when the oral route is unsuitable; and ocular delivery applies drugs to the eye, which is largely a self-contained compartment protected by its own barriers.

Definition

Specialized delivery groups the nasal, rectal, and ocular routes, in which a drug is applied to a mucosal or ocular surface for local action, systemic absorption, or access to a compartment that other routes reach poorly.

Scope

This topic surveys the nasal, rectal, and ocular routes as a group: the mucosal or ocular surfaces involved, what each route is suited to, and the barriers (mucociliary clearance, tear turnover, limited residence time) that constrain them. It is a pharmaceutical reference and does not provide application or treatment guidance.

Core questions

What makes the nasal mucosa attractive for rapid systemic and potential central nervous system delivery?
When is the rectal route used as an alternative to oral administration?
Why is the eye treated as a largely self-contained delivery compartment?
What barriers limit residence time and absorption across these surfaces?

Key concepts

Nasal mucosal absorption
Nose-to-brain delivery
Mucociliary clearance
Rectal absorption
Ocular surface barriers
Local versus systemic mucosal delivery
Residence time

Mechanisms

The nasal mucosa is thin and well-perfused, allowing rapid systemic absorption of suitable drugs and offering a possible direct pathway to the central nervous system along olfactory and trigeminal routes, though mucociliary clearance limits how long a formulation remains in place (Lochhead & Thorne, 2012). The rectal mucosa absorbs drugs into the circulation and can be used when the oral route is impractical, with the degree of first-pass avoidance depending on where in the rectum absorption occurs. The eye is largely a self-contained compartment: topically applied drugs face rapid tear turnover and the corneal barrier, so only a small fraction is absorbed, and reaching the back of the eye often requires intraocular delivery, as in intravitreal injection of biologic agents (Mehta et al., 2018). Across these routes, short residence time and protective clearance mechanisms are recurring constraints addressed through formulation.

Clinical relevance

These routes provide options when the major routes are unsuitable, for example rapid nasal absorption, rectal delivery when swallowing is not possible, or local ocular treatment, and understanding their barriers supports appraisal of why such products are formulated as they are. This entry describes delivery principles for reference and is not a basis for treatment or administration decisions in any individual.

Evidence & guidelines

Reviews of intranasal delivery describe nasal absorption and proposed nose-to-brain transport (Lochhead & Thorne, 2012). Real-world studies of intravitreal therapy illustrate the ocular compartment and the need for intraocular delivery to the posterior eye (Mehta et al., 2018). Standard pharmaceutics texts codify nasal, rectal, and ocular dosage-form design (Aulton & Taylor, 2018).

History

Nasal, rectal, and ocular routes have long been used for local treatment, but interest in their wider potential grew through the late twentieth and early twenty-first centuries, including nasal delivery for rapid systemic action and as a proposed route to the brain, and intraocular injection as a means of reaching the posterior eye for retinal disease (Lochhead & Thorne, 2012; Mehta et al., 2018).

Debates

How real and clinically useful is nose-to-brain delivery?: Whether intranasal administration delivers therapeutically meaningful amounts of drug, especially large biologics, directly to the central nervous system rather than via systemic absorption remains a matter of active investigation and debate.

Key figures

Robert Thorne
Jeffrey Lochhead

Seminal works

lochhead-thorne-2012

Frequently asked questions

Why is the eye difficult to deliver drugs to?: The eye is protected by rapid tear turnover and the corneal barrier, so only a small fraction of a topically applied drug is absorbed, and reaching the back of the eye often requires delivery directly into the eye rather than onto its surface.
What is nose-to-brain delivery?: It refers to the proposed pathway by which drugs given into the nose may reach the central nervous system along olfactory and trigeminal nerve routes, partly bypassing the bloodstream; its practical extent, particularly for large molecules, is still being studied.