The study is designed to evaluate the Safety, Tolerability and Pharmacokinetics of ATB1651 in participants with mild to moderate onychomycosis

This Phase 2, multicenter, randomized, double-blind, placebo-controlled study is designed to assess the efficacy, safety, and tolerability of ATB1651 when administered topically to the great (hallux) toenail of participants with mild to moderate onychomycosis who are otherwise healthy.

This study will enroll upto 120 participants in 3+1 (optional) sequential cohorts.

• Participants in Cohorts 1 to 3 will be randomized within each cohort to receive either ATB1651 3% or placebo at a ratio of 5:1.
• An optional cohort (Cohort 4) may enroll up to 30 participants who will be randomized to receive either ATB1651 5% or placebo at a ratio of 5:1.

Dosing will start with Cohort 1 followed at least 4 weeks later by Cohort 2. The decision to continue dosing in Cohort 2 beyond 12 weeks and/or to commence Cohort 3, at the planned dosage regimen or a modified dosage regimen, will be determined by the Safety Review Committee (SRC).

Inclusion Criteria:

1. Confirmation of onychomycosis by positive mycological (KOH) staining and positive culture from affected great toenail(s).
2. Appearance of onychomycosis involving 20% to 60% of 1 (or both) affected great toenail(s), as determined by visual inspection after the nail has been trimmed. If the percentage of infection is outside this range but is still considered appropriate for this study, based on the overall impression of the Principal Investigator (PI), participation can be considered in consultation with the Medical Monitor (MM). The visual inspection results of the appearance of onychomycosis involving 20% to 60% of 1 (or both) affected great toenail(s) will be reviewed by the Sponsor before enrollment.
3. The combined thickness of the distal nail plate at the associated hyperkeratotic nail bed is < 2 mm.
4. Adult males and females, 18 to 70 years of age (inclusive) at the time of Screening.
5. In good general health, with no significant medical history, and no clinically significant abnormalities on physical examination or ECG at Screening and/or before the first administration of IP at the discretion of the PI or designee. Participants with mild stable disease may be considered eligible at the discretion of the PI or designee.
6. Body mass index (BMI) between 17.5 and 35.0 kg/m2, inclusive, at Screening.

Exclusion Criteria:

1. History of allergy to any of the excipients in ATB1651.
2. Positive test for hepatitis C antibody (HCV), hepatitis B surface antigen (HBsAg), or human immunodeficiency virus (HIV) antibody at Screening or Day 1.
3. Underlying physical or psychological medical conditions that, in the opinion of the PI, would make it unlikely for the participant to comply with the protocol or complete the study per protocol.
4. Unwilling to refrain from the use of nail cosmetics such as clear and/ or colored nail lacquers from the Screening visit until the end of the study.
5. Use of any IP or investigational medical device within 30 days prior to Screening, or 5 half-lives of the product (whichever is the longest) or participation in more than 4 investigational drug studies within 1 year prior to Screening.
6. Diabetes mellitus requiring treatment other than diet and exercise.