This protocol will investigate whether topical application of N-acetyl cysteine (NAC) eye drops decreases oxidative stress and confers cytoprotection in patients with FECD.

Fuchs Endothelial Corneal Dystrophy (FECD) is the most common corneal endothelial disorder and has been estimated to affect approximately 4% of the US population over the age of 40. Corneal transplantation is currently the primary treatment modality available to treat FECD, and there are no medical therapies that treat the disease or stall its progression. Therefore, there is an unmet need to identify pharmacotherapeutic interventions that would prevent the endothelial cell loss in early through late stages of FECD, as well as after corneal transplantation, aiding in the survival of the corneal grafts.

The investigators know that cell loss in FECD is caused by increased oxidative stress in the CEC and aqueous humor of FECD. The investigators have in vitro and in vivo data showing that the processes involved in the development of FECD can be arrested with administration of N-acetyl cysteine (NAC). Currently topical solutions of NAC are already used in ophthalmology for application in keratoconjunctivitis sicca (using 20% weight/volume solution or 10%), meibomian gland dysfunction (using 5%) and Sjögren's dry eye syndrome (also using 5%).

This is a single center, double-blind, placebo-controlled, randomized trial. Adult participants with advanced FECD and cataracts who qualify for the standard treatment (combined Descemet Membrane Endothelial Keratoplasty (DMEK) and cataract surgery) will be enrolled at Massachusetts Eye and Ear.

After meeting eligibility criteria and completing informed consent, 30 eyes from up to 30 participants with advanced FECD and cataracts with indication for combined DMEK and cataract surgery will be initially enrolled. (NOTE: Some participants may be enrolled twice if they elect to have both eyes treated within the study.) Eyes will be randomized 1:1 to NAC 10% and the placebo, Visine Tears Dry Eye Relief artificial tears ophthalmic solution. Each participant will self-administer 1 drop of study drug four times a day in the planned operative eye for 28 days prior to planned surgery.

After interim analysis of the study with 30 eyes, if a significant difference in the primary endpoint between NAC 10% and placebo is not found, an additional 15 eyes from up to 15 participants will be enrolled and assigned to 20% NAC.

Prior to and after using the study medication, participants will complete patient-reported visual disability questionnaires and have corneal measurements and images taken. Participants additionally will complete a tolerability and medication adherence questionnaire after using the study medication.

In the operating room, a standard incision will be made to collect the fluid from the anterior part of the eye and replaced with a salt solution or viscous agent routinely used during the surgery. This procedure is routinely performed during this type of surgery, however, some of the aqueous fluid will be collected and analyzed rather than being discarded. Additionally, corneal tissue which is normally removed during corneal transplantation surgery and discarded will be kept for further research on FECD.

Inclusion Criteria:

1. Male or female ≥21 years of age at time of surgical evaluation.
2. Diagnosis of advanced FECD and visually significant cataract
3. Indication for DMEK (Descemet Membrane Endothelial Keratoplasty) with concurrent cataract surgery
4. Written informed consent obtained from subject and ability for subject to comply with the requirements of the study.
5. Willingness and ability to adhere to medication regimen

Exclusion Criteria:

1. Pregnant, breastfeeding, or unwilling to practice birth control during participation in the study.

2. History of prior intraocular surgery in study eye including cataract, glaucoma and/or retina surgery

3. History of other corneal diseases, such as severe dry eye, corneal scars, pseudophakic bullous keratopathy, corneal degenerations, corneal infections

4. Use of ocular prescription medications except for lubricants, hyperosmotic agents, or ocular hypotensive agents

5. History of ocular surface infection within the past 30 days

6. Use of systemic, inhalational, or topical N-Acetylcysteine within the past 30 days

7. History of intolerance to topical N-Acetylcysteine

8. Presence of a condition or abnormality that in the opinion of the Investigator would compromise the safety of the patient or the quality of the data.

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This is a double-blind trial, where neither the study participant nor the investigators will know to which treatment group the study participant has been assigned.