This study with ALTB-268 will determine the safety, tolerability, pharmacokinetics and pharmacodynamics of single ascending doses of intravenously administrated ALTB-268 in healthy participants.

This is a Phase I, randomized, double-blind, single ascending dose (SAD) study to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of intravenously administered ALTB-268 in healthy participants. Approximately 24 healthy participants will be recruited.

The primary objective is to evalute the safety and tolerability of intravenous infusion of SAD in healthy participants. The secondary objectives are (1) to characterize the PK profile of ALTB-268 in plasma following single IV doses in healthy participants, and (2) to assess the PD of ALTB-268 following single IV doses in healthy participants.

Inclusion Criteria:

1. Male or female, non-smoker (no use of tobacco or nicotine products within 3 months prior to screening), ≥18 and ≤55 years of age, with body mass index (BMI) >18.5 and <32.0 kg/m2 and body weight ≥50.0 kg for males and ≥45.0 kg for females.

2. Healthy as defined by:

   1. the absence of clinically significant illness and surgery within 4 weeks prior to dosing.
   2. the absence of clinically significant history of neurological, endocrine, cardiovascular, respiratory, hematological, immunological, psychiatric, gastrointestinal, renal, hepatic, and metabolic diseases.

3. Female participants of non-childbearing potential must be:

   1. post-menopausal (spontaneous amenorrhea for at least 12 consecutive months prior to dosing) with confirmation by documented follicle- stimulating hormone (FSH) levels ≥40 mIU/mL; or
   2. surgically sterile (bilateral oophorectomy, bilateral salpingectomy, hysterectomy, or bilateral tubal ligation) at least 3 months prior to dosing.

4. Able to understand the study procedures, agree to comply with all study visits, procedures, and restrictions, agree to comply with the prescribed dosage regimens and communicate to study personnel about AEs and concomitant medication use, and provide signed informed consent to participate in the study.

Exclusion Criteria:

1. Any clinically significant abnormal finding at physical examination at screening and/or Day -1.
2. Clinically significant abnormal laboratory test results at screening and/or Day -1; or positive serology test results for hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibody, or human immunodeficiency virus (HIV) antigen and antibody, or QuantiFERON®-TB test at screening.
3. Any history of suicidal ideation as evidenced by answering "yes" to questions 4 or 5 on the suicidal ideation portion of the C-SSRS completed at screening, or any history of suicide attempts.
4. Any history of clinical depression.
5. C-SSRS score at Day -1 (baseline) above Type 1 ideation.
6. PHQ-8 total score ≥5 at screening and/or Day -1 (baseline).