|Posted on November 20, 2017 at 8:15 PM|
Cannabinoids could have potential in a novel topical drug delivery vehicle to treat neuropathic dry eye pain.
The disconnect between signs and symptoms in dry eye disease is an intriguing clinical problem. Although we know quite a lot about the etiology of dry eye, the ideal structure of the tear film and how to diagnose and treat dry eye disease, there are still some patients who do not respond to treatment or who present complaining of severe symptoms that seem to be poorly correlated with clinical signs. Complaints may include a burning or stabbing sensation or a feeling of pressure in the eyes.
Many clinicians find these patients—and the chair time they consume—frustrating. But before dismissing them as “crazy,” it may help to consider whether they could be suffering from a different form of dry eye that is difficult to detect by traditional methods.
In 2009, Perry Rosenthal and colleagues observed what they presumed to be corneal neuralgia, which they termed “pain without stain” because of the paucity of corneal staining or other clinical signs.1 They speculated that the pain symptoms could be related to a neuropathic disorder, rather than to qualitative or quantitative tear film factors.
Later work by Rosenthal further elucidated how the nociceptive system—critical to vision when functioning properly, because it monitors and restores the optical tear film—could become dysfunctional.2 When this happens, the corneal nerves, the central trigeminal sensory network, and/or the pain organising centres in the brain escalate the eye’s natural ‘alarm system’, sending pain signals that are out of proportion to the physical insult (if indeed there is any injury at all).
Neuropathic dry eye pain may be triggered or exacerbated by ocular surgery, systemic conditions or even psychological distress.
The dry eye community has been increasingly interested in neuropathic dry eye. In fact, the new DEWS II definition of dry eye explicitly includes “neurosensory abnormalities” in the list of etiological factors in dry eye.3
A DEWS II subcommittee report on pain and sensation notes that long-term inflammation can alter neuron excitability, connectivity and impulse firing and that disturbances in ocular sensory pathways may ultimately lead to neuropathic pain.4 However, as yet, there are no dry eye therapies that specifically target neuropathic pain.
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.
Enhanced topical drug delivery
Recently, we have been investigating the use of a novel semi-fluorinated alkane (SFA) drug delivery technology (EyeSol, Novaliq) as a vehicle to deliver cannabinoids to the ocular surface. This class of molecules has a number of advantages as an eye drop.
EyeSol-based drops contain no water, so they mix well with lipophilic substances, including cyclosporine and THC, and allow them to remain solubilised instead of separating out in the bottle. They have a very low surface tension so a drop on the eye rapidly forms a very thin layer, giving it spreading properties that are far superior to an aqueous eye drop (Figure 1).
These properties should help to distribute a drug across the uneven ocular surface without the drug being immediately drained or spilling out of the eye as excess aqueous. SFA molecules are metabolically inert and do not interact with the immune system. Finally, they have a refractive index similar to water, so they do not disturb vision the way oil emulsions of lipophilic ingredients can.
In several observational trials we conducted, q.i.d. treatment with an EyeSol delivery vehicle alone resulted in a highly significant reduction in corneal staining and a 20-point improvement in OSDI scores after just six weeks.6 We thought that if this vehicle could be combined with a compound like THC for pain and inflammation, it could be very beneficial in dry eye treatment.
We began to conduct experiments using an established mouse model for dry eye disease that relies on behavioural testing methods to determine pain levels, as well as other testing methods (Figure 2). We compared a group treated with EyeSol THC eye drops to a control group treated with topical cyclosporine.
One-year results of these preclinical studies suggest that the drop is at least as effective as cyclosporine and may have dual or even triple mechanisms of action, including lubrication, anti-inflammation and pain reduction. Although we continue to evaluate various parameters, the pharmacodynamics thus far are very promising.
We are fairly confident that topical application of THC dissolved in EyeSol will not result in any measurable systemic effects. The first human clinical trials in dry eye disorder are expected to begin in the second half of 2018.
Understanding neuropathic pain
Even as research into THC delivery to the ocular surface continues, we have also been seeking to gain a better understanding of neuropathic pain and the characteristics of patients who may suffer from this form of dry eye. To do this, we in the dry eye clinic have been working closely with experts from the University of Cologne’s pain clinic.
We retrospectively evaluated 52 patients seen in our dry eye clinic who fell into the ‘pain without stain’ category. These subjects had normal Schirmer’s, no corneal staining and no signs of blepharitis, but they had OSDI scores >40, in the ‘severe’ symptom range (median score 77).
In addition to a very thorough history and examination in our clinic, many of the patients also underwent a full pain inventory and Hospital Anxiety and Depression Score (HADS) questionnaire. Comorbidities included depression (n=9), chronic pain syndrome (9), anxiety disorders (4), and prior eye surgery (17).7 Other researchers have also recently shown that severe dry eye pain is correlated with antidepressant use but not with corneal staining.8
For ophthalmologists, these are exactly the sort of patients who are mystifying at best, and often get dismissed as people whose symptoms are “just” psychological. To the pain experts we consulted, however, it was very clear that these were typical pain patients, with the same types of concomitant psychosomatic conditions they often see among patients who suffer from chronic headache or back pain.
A typical patient in our study, for example, might have fibromyalgia and rheumatoid arthritis, with an OSDI score of 65 but no clinical signs of dry eye. Upon questioning, she might have first noticed the dry eye pain following LASIK or around the time of a particularly stressful life event, such as the death of a close family member.
In some cases, the patients had no response to topical anaesthesia or to systemic pain medications, which was quite interesting, since it indicates there is something very unusual going on with regards to their pain response. It seems that these individuals may be predisposed to systemic neuropathic pain, of which ocular pain is but one manifestation.
These insights have changed the way I practice in a number of ways. For example, I have started using the HADS questionnaire more frequently for patients with severe symptoms. I ask broader questions about fatigue, anxiety and depression as part of my history taking.
Recognising that ocular surgery can trigger an underlying pain syndrome, I am more thoughtful about evaluating postoperative patients with unexplained symptoms to determine whether they might be most appropriately managed as a pain patient. As medical doctors, it is our responsibility to see the whole patient and, when appropriate, help them to seek treatment for systemic autoimmune disorders, psychosomatic illnesses and other pain syndromes.
‘Pain without stain’ patients provide us with a model to understand the process of neuropathic dry eye. Because they do not have significant meibomian gland dysfunction or aqueous deficiency, we are able to isolate the neuropathic disease and hopefully identify new therapies to relieve their symptoms.
While these patients represent what is probably a very rare subtype of dry eye, it is likely that many more dry eye patients who do have clinical signs also have neuropathic pain as a component of their dry eye. In fact, I believe that most of the patients who fail, even partially, to respond to stepwise, appropriate dry eye therapy or for whom resolution of signs does not lead to significant improvement in symptoms, might also have an additional neuropathic disorder.
What we learn from treating the extreme form, therefore, has the potential to benefit a much broader range of patients whose pain is inadequately addressed with current therapy.
It has become clear to me that there is a strong unmet need for treatments that specifically address ocular surface pain. Although much research remains to be undertaken, I am optimistic about the potential for cannabinoid therapies, particularly if we are able to confirm that use of an SFA drug delivery technology is effective in reaching the target tissues without systemic side effects.
Rosenthal P, Baran I, Jacobs DS. Corneal pain without stain: Is it real? Ocul Surf. 2009;7(1):28-40.
Rosenthal P, Borsook D. Ocular neuropathic pain. Br J Ophthalmol. 2016;100(1):128-134.
Nelson JD, Craig JP, Akpek EK, et al. TFOS DEWS II introduction. Ocul Surf. 2017;15:269-275.
Belmonte C, Nichols JJ, Cox SM, et al. TFOS DEWS II pain and sensation report. Ocul Surf. 2017;15(3):404-437.
Toguri JT, Caldwell M, Kelly MEM. Turning down the thermostat: modulating the endocannabinoid system in ocular inflammation and pain. Front Pharmacol. 2016;7:304.
Steven P, Scherer D, Krösser S, et al. Semifluorinated alkane eye drops for treatment of dry eye disease—a prospective, multicenter noninterventional study. J Ocul Pharmacol. Ther 2015;31(8):498-503.
Steven P, Schneider T, Ramesh I, et al. Pain in dry eye patients without corresponding clinical signs—a retrospective analysis. Association for Research in Vision and Ophthalmology. 2016, Abstract 2848-A0057.
Satitpitakul V, Kheirkhah A, Cmej A, et al. Determinants of ocular pain severity in patients with dry eye disease. Am J Ophthalmol. 2017;179:198-204.
Professor Philipp Steven, MD
Prof. Steven is principle investigator for the Ocular Surface Group in the Department of Ophthalmology at the University of Cologne, in Germany. He consults for and receives research funding from Novaliq and has a patent pending related to the work described within this article.