A client recently asked for my opinion about the value of some over-the-counter eye drops purported to treat cataracts in dogs and cats. Not having read much about this product, I did a little research.
What Is It?
There are a number of eye drops marketed for prevention or treatment of cataracts in humans and pets. Most commonly, they contain a chemical called n-acetylcarnosine, which is made up of a couple amino acids and an acetyl group. In vitro research suggests this chemical has anti-oxidant effects, so it has been hypothesized to prevent or reduce the gradual opacification of the lens of the eye through this mechansism. As I have pointed out before, the role of oxidative damage in disease, and the value of anti-oxidant therapies, is a developing area of research. Many of the exuberant and enthusiastic hopes in this area have proven false, so while it is an area deserving further investigation, claiming something is an anti-oxidant is not automatic validation of its safety or benefits (e.g. 1, 2, 3).
Does It Work?
There have been a number of studies of n-acetylcarnosine, both in vitro studies and clinical trials in humans. Interestingly, almost all of these appear to have been published by the same researcher and his team, Dr. Mark Babizhayev. A clinical trial was published in 2001 (and it appears the same data was published again in a different journal in 2002, which if true is quite a significant science no-no). The trial was randomized and controlled in a small number of patients, and it appeared to show significant changes in a number of objective measures of cataract severity.
I am not familiar with the specific techniques used to measure the disease or response to treatment, so I will presume they are standard and appropriate measures for this kind of study. One thing that is not clear from the published report is whether the individuals making these measurements were blinded to the treatment status. This obviously has a significant bearing on the reliability of the results, especially when they stem from only one researcher, as such debacles as the Benveniste affair show.
A number of subsequent papers have been published by Dr Babizhayev and his team, mostly in vitro or lab studies investigating properties of n-acetylcarnosine, not clinical trials. Dr. Babizhayev has also become the leader of a commercial firm marketing n-acetylcarnosine for cataract treatment, and many other uses (including skin care, wound care, respiratory disease, and neurologic disease).
This commercial effort, and the spreading of claims for n-acetylcarnosine to a wide range of apparently unrelated applications, does raise some questions about the reliability of Dr. Babizhayev as the sole source of scientific validation for this compound. Overall, the status of the evidence for use of n-acetycarnosine for cataracts in humans is best summarized by the Royal College of Opthalmology:
The evidence for the effectiveness of N-acetyl carnosine eye drops is based on experience on a small number of cases carried out by a Russian researcher team. To date, the research has not been corroborated and the results replicated by others. The long-term effect is unknown.
Unfortunately, the evidence to date does not support the ‘promising potential’ of this drug in cataract reversal. More robust data from well conducted clinical trials on adequate sample sizes will be required to support these claims of efficacy.
Furthermore, we do not feel the evidence base for the safety is in any way sufficient
Unusually, there is actually some clinical trial evidence in veterinary species as well. An uncontrolled, unblended pilot trial has been published on an n-acetylcarnosine product (not the one Dr. Babizhayev sells) in dogs with cataracts.
David L Williams, Patricia Munday. The effect of a topical antioxidant formulation including N-acetyl carnosine on canine cataract: a preliminary study. Vet Ophthalmol. 2006 Sep-Oct;9(5):311-6. The results showed marginal improvement in all groups, though it was only significant in patients with 2 out of 5 types of cataract treated. However, subjectively owners reported improvement in 80% of the subjects.
Dr. Williams has apparently performed a blinded, placebo-controlled follow-up trial on this product which did not show any benefits (in fact improvements were greater in the placebo group than in the treatment group), showing once again the importance of proper controls for bias, confounding, and other sources of error in clinical trials. Unfortunately, it appears unlikely this trial will be published for reasons which are not completely clear, though one implied issue is that journals are often reluctant to accept papers that show negative results, which are less exciting for readers than positive studies. All of this is, of course, through word-of-mouth among veterinarians, so I cannot confirm it is true.
Is It Safe?
I have not found any reports of adverse effects from ophthalmic application of N-acetylcarnosine itself, and given its chemical makeup it seems unlikely to be hazardous. As usual, products containing this agent are not regulated as licensed medicines are, and there is no way to ensure label accuracy, proper manufacturing quality standards, or the safety of other ingredients that may be included with the N-acetylcarnosine.
The theoretical arguments for why this drug might be useful I the treatment of canine cataracts are plausible but largely unproven. There is limited clinical trial evidence in humans suggesting a benefit, but this has not been replicated and is at high risk of bias. The limited clinical trial evidence available in dogs does not suggest a benefit. There are minimal safety concerns with products containing N-acetylcarnosine.