Each eye is said to have conjunctiva measuring about 15 square cm. The fact that this is anatomically close to the brain and is well supplied by blood vessels raises the above-mentioned question. Are there studies discussing this possibility? Animal studies could give us clues as to whether antibiotics and antimalarial drugs could be introduced into the brain by administration through eye drops.
This is not an answer, but information relevant to my question. Visit
University of Helsinki. "Unraveling the link between brain, lymphatic system." ScienceDaily. ScienceDaily, 15 June 2015.
One issue with eye drops is the relatively short time where they remain in contact with the eye. Tearing and blinking can wash away the compound very rapidly (few tens of seconds), unless some additives that improve residence time (e.g. viscosity increasing agents) are used. In any case, you'd need a pretty permeable compound. And possibly a pretty soluble one too: concentrations of eyedrop formulations can be in the milimolar range.
Also, if you need to achieve an effective concentration in other organs than the eye (e.g. brain), it means that your eye tissues (cornea, conjunctiva) should be able to tolerate way higher concentrations of your compound, without too much side effect.
Fully agree with Sandro's answer.
Several aspects should be considered: i) compatibility of the substance intended to be delivered with the conjunctiva ii) the amount of the substance that can be loaded (again the amount can influence local tolerability) iii) pH and osmolarity of the solution iv) Physicochemical characteristics and potency.
Said this, I never read about this route for brain delivery.
Other routes e.g. nasal, are better suited to reach this goal.
Article Formulation and In-vivo Pharmacokinetic Consideration of Int...
Sardina's comments are useful and interesting. I have two questions about the nasal route.
a) Will not the liquid runoff problem be worse with the nasal route than with the eye socket route?
b) What is the area of permeable surface available in the nostrils as compared to the conjunctiva area available in the eye sockets?
The so called "Nose-to-brain" deivery is a field of active investigation. The principle is to administer drugs in the upper part of the nasal cavity where olfactory non myelinated terminations and vessels are located and can facilitate transport directly delivering drug to the brain without the need to cross blood brain barrier in spite of the small dimensions (about 2.5 cm2).To achive this goal there are several means. FIrast the device should be optimized for delivery in the upper nasal cavity (normal sprays cannot work properly). Second different formulations using polymers, nanocarriers, nano particles etc should be used.
An example is described in the attached paper.
Many attempts have been made to deliver drugs to the retina (the part of the CNS closest to the front of the eye) by topical ocular delivery, and all have failed in the clinic. This augurs poorly for the success of delivering small molecules further back into the CNS.
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