The role of cannabis
Cannabinoids and, in particular, tetrahydrocannabinol (THC, the main active ingredient in recreational marijuana) is of great interest in the treatment of neuropathic pain. Cannabinoid receptors are known to modulate pain and inflammation and are located throughout the eye, including the corneal epithelium and the retina; they are present on immune cells and may be involved in wound healing, as well.5 So, from a theoretical perspective there are a number of reasons to pursue the use of cannabinoids to treat dry eye.
And the time may be right to conduct such research now. Regulatory restrictions around the use of cannabinoids are being loosened, with more than half of U.S. states and several European countries now permitting the use of THC for medical purposes.
In Germany, where I practice, a special permit is required and the drug must be used in a controlled research setting as part of a clinical trial, which seems to be a reasonable way to open the doors to research on cannabinoid compounds while still limiting the potential for abuse.
In my laboratory, we have been actively investigating what the best targets for treating neuropathic pain might be and how THC could be safely and effectively delivered to those targets.
There are a number of challenges in using THC therapeutically. Smoking marijuana or consuming THC in edible form is impractical for the treatment of ocular disease; these systemic applications deliver an unpredictable dose with unwanted systemic side effects.
Sublingual and dermal administration have similar effects and are probably not appropriate for ocular conditions. For these reasons, and due to a lack of evidence of predictable, long-lasting therapeutic effect, a number of professional organisations, including the Canadian Ophthalmological Society and the American Academy of Ophthalmology, have taken positions against the use of systemic THC treatment for ocular conditions.
Local application of THC is considerably more compelling. Cannabinoid eye drops have been investigated and are available for glaucoma from at least one small manufacturer. But noone yet has been particularly successful with this approach due to the challenges of formulating the drops appropriately.
THC and most other pain-relieving or anaesthetic compounds are quite lipophilic. Like oil with water, they do not mix well with the aqueous solutions in most eye drops, making it difficult to get THC into the target ocular tissues.