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

Onychomycosis (also known as tinea unguium) is a contagious infection of toe nails by fungal organisms including dermatophytes, yeast, and molds.

This is phase 1, first in human, randomized, double-blind, placebo-controlled, MAD study designed to assess the safety, tolerability, and PK of ATB1651 when administered in participants with mild to moderate onychomycosis.

The study consists of 2 parts. In both parts, participants will receive multiple doses of ATB1651 applied to 1 affected great toenail and the remaining toenails (affected or not)

Part A: Participants will be enrolled into 1 of 3 cohorts and randomized to receive either ATB1651 or placebo at a ratio of 2:1.

Up to 2 additional cohorts may be added at the discretion of the Sponsor and Safety Monitoring Committee, if deemed necessary

Part B: Participants will be randomized within a single cohort to receive either ATB1651 or placebo at a ratio of 4:1

There will be 18 participants enrolled in part A, 30 participants in part B

Inclusion Criteria:

1. Confirmation of onychomycosis by mycological staining and/or culture from affected great toenail(s).
2. Appearance of onychomycosis involving 20% to 70% 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 Investigator, participation can be considered in consultation with the Medical Monitor.
3. The combined thickness of the distal nail plate at the associated hyperkeratotic nail bed is less than 3 mm.
4. Medically healthy with clinically insignificant Screening results (eg, laboratory profiles, medical history, ECGs, physical exam), as judged by the PI.
5. Negative urine drug screen and alcohol breath test at Screening and Day 1.
6. Body Mass Index (BMI) between 17.5 and 35.0, inclusive.
7. Agree to adhere to the current state and national advice regarding minimizing exposure to coronavirus disease of 2019 (COVID-19) from the Screening visit until the EOS visit.

Exclusion Criteria:

1. History of allergy to any of the excipients in ATB1651.
2. Positive COVID-19 test at Screening or any symptoms consistent with COVID-19 prior to initial dosing.
3. Positive test for hepatitis C antibody (HCV), hepatitis B surface antigen (HBsAg), or human immunodeficiency virus (HIV) antibody at Screening.
4. Have any underlying physical or psychological medical conditions that, in the opinion of the Investigator, would make it unlikely that the participant will comply with the study.
5. Pregnant or lactating at Screening or planning to become pregnant (self or partner) at any time during the study, including the follow-up period.
6. 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.
7. 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.