Nacuity is focused on developing a treatment for retinitis pigmentosa (RP), while screening our diverse library of novel antioxidant molecules to assess opportunities for other ocular diseases involving oxidative stress. Oxidative stress has been implicated in a variety of ocular conditions and diseases, including retinitis pigmentosa, cataract, age-related macular degeneration, diabetic retinopathy, glaucoma, presbyopia, retinal detachment, and vitreous degeneration.
Retinitis pigmentosa (RP) refers to a group of inherited diseases causing retinal degeneration and vision loss that worsens over time. More than 190 genes have been implicated in inherited retinal disease and RP accounts for approximately one-half of those cases. RP itself is highly heterogeneous: mutations in more than 50 genes are known to cause non-syndromic RP and nearly 3100 mutations have been reported in these genes.1 Forms of RP and related diseases include Usher syndrome,2 Leber’s congenital amaurosis, rod-cone disease, Bardet-Biedl syndrome, and Refsum disease, among others.
The retina’s photoreceptors (rods & cones) convert light into neural signals and send them to the brain for visual recognition.
Rods are responsible for peripheral, night, and black and white vision.
Cones, centrally concentrated, provide color and central vision.
People with RP experience a gradual decline in their vision because photoreceptor cells (rods and cones) die. In most forms of RP, rods are affected first. Because rods are concentrated in the outer portions of the retina and are triggered by dim light, their degeneration affects peripheral and night vision. When the more centrally located cones - responsible for color and sharp central vision - become involved, people lose color perception and central vision. Night blindness is one of the earliest and most frequent symptoms of RP. RP is typically diagnosed in adolescents and young adults and is a progressive disorder. The rate of progression and degree of visual loss varies from person to person. Most people with RP are legally blind by age 40.
The National Eye Institute (NEI) estimates that approximately 100,000 people in the United States have RP. About 1.5 million people are afflicted worldwide.
There is only one approved drug for RP; LUXTURNA® is indicated for the treatment of patients with RPE65 mutations,3 about 1% of RP patients. While gene therapy has the potential to address pathogenic mutations, with thousands of mutations already associated with RP, a relatively small number of patients are likely to benefit from any single gene therapy. An effective small molecule antioxidant has the potential to work alone or in concert with gene therapies to improve patient outcomes.
Role of Antioxidants
- An immunohistochemical study in a RP pig model suggested that the death of rods results in decreased oxygen consumption and hyperoxia in the outer retina resulting in gradual cone cell death from oxidative damage. 4
- In a study using the rd1 mouse RP model, a mixture of antioxidants including alpha-tocopherol, ascorbic acid, Mn(III)tetrakis(4-benzoic acid)porphyrin and alpha-lipoic acid improved biomarkers of oxidative stress (protein carbonyl adducts and acrolein staining) and partially preserved cone function.5
- Antioxidant-treated rd10+/+ mice showed preservation of cone function, including a significant increase in photopic ERG b-wave amplitudes 6
Mutations that cause RP initially lead to rod cell death. After rod photoreceptors die, cone photoreceptors gradually die. 10
Glutathione is the most effective endogenous antioxidant and the body’s first line of defense against oxidative stress.
NPI-001 increases cysteine levels, providing the key substrate for glutathione synthesis and protection from oxidative stress.
In a study of RP patients, Campochiaro et al. (2015)11 reported a significant reduction in the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) in aqueous humor and a significant increase in aqueous protein carbonyl content, compared to control subjects. In contrast, there was no significant decrease in the serum GSH/GSSG ratio or increase in carbonyl content of serum proteins. These data suggest that patients with RP may exhibit ocular oxidative stress, ultimately leading to cone death. Potent antioxidants, therefore, should promote cone survival and function in patients with RP and aqueous GSH/GSSG ratio and protein carbonyl content may provide useful biomarkers.
The Amide Advantage
N-acetyl-L-cysteine (NAC) is a well-known, endogenous antioxidant moiety that is capable of facilitating GSH biosynthesis, replenishing GSH within cells that are undergoing oxidative stress. It has been approved by numerous regulatory agencies for the treatment of hepatotoxicity caused by acetaminophen overdose, and as a mucolytic.
Nacuity’s GMP-grade N-acetylcysteine amide (NPI-001) is the amide form of NAC manufactured using a patented process. This refined form is more lipophilic and more easily permeates cell membranes than NAC. In animal studies, it has exhibited the ability to cross blood-brain12 and blood-retinal barriers. NPI-001, if demonstrated safe and effective, has the potential to slow or halt disease progression in RP patients with functioning cones, regardless of their disease-causing genetic mutation.
NPI-001 for the Treatment of Retinitis Pigmentosa
The use of NAC and NACA (NPI-001) for the treatment of RP was initially investigated by Dr. Peter Campochiaro, George S. & Dolores Doré Eccles Professor of Ophthalmology & Neuroscience, Wilmer Eye Institute, The Johns Hopkins School of Medicine. Dr. Campochiaro, a renowned expert in retinal degeneration, has advised Nacuity on our clinical approach to a treatment for RP.
Campochiaro’s Group13 demonstrated that orally administered NAC reduced cone cell death and preserved cone function by reducing oxidative damage in two models of RP, rd1+/+ and rd10+/+ mice. In addition, they compared NAC with NPI-001 in an animal model of RP14 and reported mean photopic ERG b-wave amplitude was 50% higher (p=0.001) at all 3 stimulus intensities in NPI-001 treated versus NAC treated mice and more than 4-fold greater than controls. While mean peak scotopic ERG b-wave amplitude was higher in both NAC- or NPI-001-treated mice compared with controls, mean b-wave amplitudes were significantly greater in NPI-001 treated mice compared with NAC treated mice at 10 of 11 stimulus intensities. Furthermore, cone cell density was significantly greater in 3 of 4 quadrants measured for NPI-001- treated compared to NAC- treated mice. Even with a substantially lower oral dose, NPI-001 exhibited significantly greater preservation of cone cell function and survival compared with NAC in rd10+/+ mice.14, 10
In a study of RP patients, Dr. Campochiaro’s Group examined the safety, tolerability and visual function effects of oral NAC.15 Subjects (n = 10 per cohort) received 600 mg (cohort 1), 1200 mg (cohort 2), or 1800 mg (cohort 3) NAC bid for 12 weeks and then tid for 12 weeks. During the 24-week treatment period, mean BCVA significantly improved at 0.4 (95% CI: 0.2–0.6, P < 0.001), 0.5 (95% CI: 0.3–0.7, P < 0.001), and 0.2 (95% CI: 0.02–0.4, P = 0.03) letters/month in cohorts 1, 2, and 3, respectively. There was also a significant improvement in mean retinal sensitivity over time for cohort 3 (0.15 dB/month, 95% CI: 0.04–0.26). The results of this 24-week study of 30 subjects showed that oral NAC was relatively well-tolerated, and suggested slight improvements in suboptimally functioning cones in patients with moderately advanced RP.
With its greater cell permeability,16 a lower dose of NPI-001 should prove more effective while minimizing potential side effects observed with high dose NAC.
- Exhibited the ability to cross blood-brain17 and blood-retinal barriers14, 10 in animal models
- Was approximately 3-fold more effective than oral NAC in the rd10+/+ mouse model of RP 14
Oral NPI-001, if demonstrated safe and effective, has the potential to slow or halt disease progression in RP patients with functioning cones, regardless of their disease-causing genetic mutation.
Opacity or clouding of the lens, cataract, is a direct result of ocular oxidative stress. While cataracts occur primarily due to age, they can also develop as a result of an injury or disease. In fact, RP patients are at a greater risk for cataract, as are patients undergoing vitrectomy surgery. Symptoms of cataracts include blurry vision, double vision, glare and faded colors. Currently, the only treatment is surgical removal of the affected lens and replacement with an artificial, intraocular lens (IOL). Cataracts are the leading cause of blindness worldwide.
NPI-002 as an Anticataract Agent
Nacuity is developing a novel small molecule antioxidant, NPI-002, as an anticataract agent. In a rat lens ex-vivo model, NPI-002 inhibited oxidative-induced cataracts. NPI-002 is currently undergoing additional preclinical testing.