Inclusion Criteria:  Confirmation of onychomycosis by mycological staining and/or culture from affected great toenail(s). Appearance of onychomycosis involving at least 20% but not more than 70% of 1 (or both) affected great toenail(s) as determined by visual inspection after the nail has been trimmed. The combined thickness of the distal nail plate at the associated hyperkeratotic nail bed is less than 3 mm. Medically healthy with clinically insignificant Screening results (eg, laboratory profiles, medical history, ECGs, physical exam), as judged by the PI. Negative urine drug screen/alcohol breath test at Screening and Day 1. Body Mass Index (BMI) between 17.5 and 35.0, inclusive. 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:  History of allergy to any of the excipients in ATB1651. Positive COVID-19 test at Screening and/or at admission to the CRU. Positive test for hepatitis C antibody (HCV), hepatitis B surface antigen (HBsAg), or human immunodeficiency virus (HIV) antibody at Screening. 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. Pregnant or lactating at Screening or planning to become pregnant (self or partner) at any time during the study, including the follow-up period. 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. 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.