Intervention: Diphenhydramine
Trial phase: Phase 1 | Phase 2
ATC code: D04AA32 | D04AA33 | R06AA02 | R06AA52
SMILES: CN(C)CCOC(c1ccccc1)c1ccccc1
Sex: All
Age groups: Adult | Older Adult
Healthy volunteers: No
Participants at risk: 7
Group description: Dextromethorphan hydrobromide: Participants instructed to take 60 mg dextromethorphan capsules orally two times a day for 24 months.
Eligibility criteria:
Inclusion Criteria

Participant is 18 years of age or older.
Participant must understand and sign the protocol's informed consent document.
Female participants of childbearing potential must not be pregnant or breast-feeding, must have a negative urine pregnancy test within 24 hours prior to initiation of study medication and must be willing to undergo urine pregnancy tests throughout the study.

Female participants of childbearing potential and male participants able to father children must have (or have a partner who has) had a hysterectomy or vasectomy, be completely abstinent from intercourse or must agree to practice two acceptable methods of contraception throughout the course of the study and for one week after study medication discontinuation. Acceptable methods of contraception include:

hormonal contraception (i.e., birth control pills, injected hormones, dermal patch or vaginal ring),
intrauterine device,
barrier methods (diaphragm, condom) with spermicide, or
surgical sterilization (hysterectomy or tubal ligation).
Participant must agree to notify the study investigator or coordinator if any of his/her doctors initiate a new medication during the course of this study.
Participant must agree not to take medications containing dextromethorphan during the course of this study.
Participant must have normal renal function and liver function, or have mild abnormalities no greater than grade 1 as defined by the Common Terminology Criteria for Adverse Events v4.0 (CTCAE).

Participant has a diagnosis of diabetic mellitus (type 1 or type 2). Any one of the following will be considered to be sufficient evidence that diabetes is present:

Current regular use of insulin for the treatment of diabetes;
Current regular use of oral anti-hyperglycemia agents for the treatment of diabetes;
Documented diabetes by American Diabetes Association (ADA) and/or World Health Organization (WHO) criteria.

Participant has documented hemoglobin A1C 12% or less within one month of baseline.

Participants with elevated hemoglobin A1C but within the 12% or less cutoff will undergo appropriate evaluation, and unstable patients will be excluded according to the investigator's best medical judgment.
Participant agrees to refrain from consuming grapefruit juice, grapefruits and Seville oranges at any time while s/he is enrolled in this study.
Participant has at least one eye that meets the study eye criteria listed below.

Exclusion Criteria

Participant is in another investigational study and actively receiving another study medication for diabetic macular edema (DME).
Participant is unable to comply with study procedures or follow-up visits.
Participant has a known hypersensitivity to sodium fluorescein dye.

Participant has a condition that, in the opinion of the investigator, would preclude participation in the study (e.g., unstable medical status including blood pressure and glycemic control).

• Patients in poor glycemic control who, within the last four months, initiated intensive insulin treatment (a pump or multiple daily injections) or plan to do so in the next four months should not be enrolled.

Participant has a history of chronic renal failure requiring dialysis or kidney transplant.
Participant has a history of hepatitis or liver failure.
Participant has an allergy or hypersensitivity to dextromethorphan or levorphanol.
Participant is taking or has taken within the last 14 days any medication that could adversely interact with dextromethorphan such as selective serotonin reuptake inhibitors (SSRIs) or monoamine oxidase inhibitors (MAOIs) including but not limited to the following: almotriptan; amitriptyline; amoxapine; bromocriptine; buspirone; cabergoline; citalopram; clomipramine; desipramine; desvenlafaxine; dihydroergotamine; doxepin; duloxetine; eletriptan; ergoloid mesylates; ergotamine; escitalopram; fluoxetine; fluvoxamine; frovatriptan; imipramine; isocarboxazid; linezolid; lithium; maprotiline; meperidine; methylergonovine; milnacipran; mirtazapine; moclobemide; naratriptan; nefazodone; nortriptyline; paroxetine; phenelzine; procarbazine; promethazine; protriptyline; rasagiline; rizatriptan; SAMe (S-adenosylmethionine); selegiline; sertraline; sibutramine; St. Johns wort; sumatriptan; tapentadol; tramadol; tranylcypromine; trazodone; trimipramine; tryptophan; venlafaxine; vilazodone; zolmitriptan.

Participant has a blood pressure of > 180/110 (systolic above 180 OR diastolic above 110).

• If blood pressure is brought below 180/110 by anti-hypertensive treatment, a patient can become eligible.

Participant has a history of treatment with systemic anti-vascular endothelial growth factor (VEGF) agents or steroids within three months prior to study entry.

Study Eye Inclusion Criteria

Best-corrected visual acuity (BCVA) Early Treatment Diabetic Retinopathy Study (ETDRS) score of 34 letters or better (i.e., 20/200 or better).
Definite retinal thickening due to diabetic macular edema, based on clinical examination, that is not refractory to further therapy as based on the investigator's clinical judgment.
Retinal thickening due to DME within 3000 μm of the center of the macula, as measured by Spectral optical coherence tomography (OCT).
Media clarity, pupillary dilation and patient cooperation sufficient for adequate fundus photographs.

Study Eye Exclusion Criteria

Macular edema is considered to be due to a cause other than diabetic macular edema.

An eye should not be considered eligible if:

The macular edema is considered to be related to cataract extraction; or
Clinical exam and/or OCT suggest that vitreoretinal interface disease (e.g., a taut posterior hyaloid or epiretinal membrane) is the primary cause of the macular edema.
An ocular condition is present such that, in the opinion of the investigator, visual acuity would not improve from resolution of macular edema (e.g., foveal atrophy, pigmentary changes, dense subfoveal hard exudates, non-retinal condition).
An ocular condition is present (other than diabetic retinopathy (DR) that, in the opinion of the investigator, might affect macular edema or alter visual acuity during the course of the study (e.g., vein occlusion, uveitis or other ocular inflammatory disease, neovascular glaucoma, Irvine-Gass Syndrome, etc.).
Substantial cataract that, in the opinion of the investigator, is likely to be decreasing visual acuity by three lines or more (i.e., cataract would be reducing acuity to 20/40 or worse if eye was otherwise normal).
History of panretinal scatter photocoagulation (PRP) within four months prior to study entry.
History of prior pars plana vitrectomy within six months prior to study entry.
History of major ocular surgery (including cataract extraction, scleral buckle, any intraocular surgery, etc.) within three months prior to study entry.
History of Yttrium aluminium garnet (YAG) capsulotomy performed within two months prior to study entry.
History of treatment within three months prior to enrollment with any drug that has not received regulatory approval at the time of study entry, such as intravitreal or periocular steroids or intravitreal anti-VEGF agents.

Choice of Study Eye in Cases of Bilateral Disease

If both eyes of a participant meet the criteria described above, the following will be used to determine the study eye:

If one eye is treatment-naïve and the other is not, the treatment-naïve eye will be chosen as the study eye.
If both eyes are treatment-naïve, the eye with the better visual acuity will be chosen as the study eye.
If both eyes are treatment-naïve and are equivalent, the choice of study eye will be determined at the investigator's discretion after consultation with the participant.
If both eyes have been previously treated, the choice of study eye will be determined at the investigator's discretion after consultation with the participant.