This is a randomized, double-blind, placebo-controlled, dose escalation study to evaluate the safety, tolerability, and pharmacokinetics (PK) of ziresovir following a single ascending oral dose administration in healthy adult subjects under fasted conditions.

Up to 3 dose cohorts are planned. The ziresovir dose level of each cohort is determined based on the collective clinical and nonclinical data of ziresovir.

The proposed dose levels of Cohorts 1, 2 and 3 are 300 mg and up to 600 mg and up to 900 mg, respectively.

A total of up to 24 subjects will be randomized with 18 subjects to receive active drug and 6 subjects to receive placebo in a double-blind fashion. Eight subjects will be randomized in each dose cohort, with 6 subjects to receive active drug and 2 subjects o receive placebo.

Inclusion Criteria:

1. Capable of giving written informed consent and complying with study procedures;
2. Between the ages of 18 and 55 years, inclusive;
3. Body mass index (BMI) of 18.0 to 32.0 kg/m2 inclusive and body weight not less than 50 kg;
4. Female subjects must have a negative pregnancy test result at screening;
5. Considered healthy by the Investigator, based on subject's reported medical history, full physical examination, 12-lead ECG, and vital signs;
6. Willing and being able to adhere to study restrictions and to be confined at the Clinical Research Unit.

Exclusion Criteria:

1. Clinically significant reported history of gastrointestinal, cardiovascular, musculoskeletal, endocrine, hematologic, psychiatric, renal, hepatic, bronchopulmonary, neurologic, immunologic, lipid metabolism disorders, or drug hypersensitivity as determined by the Investigator;
2. Poor venous access;
3. Taken an investigational drug or participated in a clinical trial evaluating an investigational drug or device within 30 days (or 5 half-lives) prior to the study drug dose, whichever is longer;
4. Taken any prescription medications within 14 days or 5 half-lives (whichever is longer) of the study drug dose;
5. Major surgery or hospitalization within 6 months prior to screening that in the Investigator's opinion would put the subject or study conduct at risk, or have any scheduled surgery or hospitalization during the study period;
6. Any condition or finding that in the Investigator's opinion would put the subject or study conduct at risk if the subject were to participate in the study.