A Study to Evaluate the Safety and Tolerability of EXN407

Inclusion Criteria: 1. Subject is at least 18 years of age inclusive, at the time of signing the informed consent. 2. BCVA better than or equal to 69 ETDRS score (approximate Snellen equivalent 20/40 or 6/12) in the study eye using the ETDRS visual acuity scale at Screening or BCVA less than 69 ETDRS score (approximate Snellen equivalent 20/50 or 6/15) but who, in the Investigator's opinion, is unsuitable for treatment with anti-VEGF by intravitreal injection or refuses it. Subjects should have no more than a 7-letter difference in BCVA at Screening and baseline visit. 3. Ocular media is consistent with SD-OCT imaging and cataracts are not expected in the subject for the duration of the study. 4. The subject has no other retinal disease. 5. Subject or the subject's partner successfully demonstrates their ability to self-administer/administer eye drops at Screening, with multiple attempts allowed at the discretion of the Investigator. Exclusion Criteria: 1. Any other retinal disease in the study eye, other than centre involved DMO or diabetic retinopathy. 2. Poor vision (VA 6/60 or worse) in the contralateral eye. 3. Intraocular inflammation (including trace or greater) in the study eye. History of idiopathic or autoimmune uveitis in either eye. 4. Use of intravitreal anti-VEGF drugs including ranibizumab, bevacizumab, aflibercept in the study eye within 6 months of the Screening Visit, or in the fellow (non study) eye within 3 months of the Screening Visit. Use of topical corticosteroids or topical non-steroidal anti-inflammatory agents in the study eye within 28 days of the Screening Visit. Use of intravitreal corticosteroids in either eye or systemic steroids within 12 months of the Screening Visit. Prior use of Iluvien (without time limitation). 5. Within 180 days prior to the Screening visit, use of medications known to be toxic to the retina, lens or optic nerve (e.g. desferoximine, chloroquine/hydrochloroquine, chlorpromazine, phenothiazines, tamoxifen, and ethambutol). 6. History of (within 90 days of Screening date) cerebral vascular accident (stroke) or MI. 7. Significant renal impairment including subjects on chronic renal dialysis and subjects with a history of nephrectomy or kidney transplant (regardless of renal function). 8. History of anaphylaxis, anaphylactoid (resembling anaphylaxis) reactions, or severe allergic responses. 9. Positive pregnancy test (all female subjects of childbearing potential must have a urine β-human chorionic gonadotropin \[hCG\] pregnancy test performed at Screening and within 7 days prior to randomisation) or is known to be pregnant or lactating. 10. Known to have, or history of a positive test result for, hepatitis B or C, HIV, syphilis, tuberculosis, or COVID-19.