This is a Phase 1, randomized, double-blind, placebo-controlled, single-ascending dose study in healthy volunteers to investigate the safety, tolerability, and pharmacokinetics (PK) of MHAB5553A.

Inclusion Criteria:

• Signed informed consent form (ICF)
• Body mass index (BMI) between 18 and 32 kg/m2, inclusive
• Weight 40-100 kg
• In good health, determined by no clinically significant findings from medical history, 12-lead ECG, and vital signs
• Clinical laboratory evaluations should be within reference range for the test, unless deemed not clinically significant by the investigator and sponsor at screening
• Willing to abstain from using drugs of abuse while enrolled in the study
• Willing and able to comply with protocol-specified criteria in regard to contraceptive protection
• Able to comply with the study protocol, in the investigator's judgment

Exclusion Criteria:

• History or clinically significant manifestations of metabolic, hepatic, renal, hematologic, immunodeficiency, pulmonary, cardiovascular, gastrointestinal, urologic, neurologic, or psychiatric disorders
• History of anaphylaxis, hypersensitivity or drug allergies, unless approved by the investigator and sponsor
• History or presence of an abnormal ECG, which, in the investigator's or sponsor's opinion, is clinically significant (including evidence of previous acute myocardial infarction, complete left bundle branch block, second-degree heart block, or complete heart block)
• History of significant alcohol abuse within 1 year prior to screening or regular use of alcohol within 6 months prior to the screening visit
• History of significant drug abuse within 1 year prior to screening or use of soft drugs (such as marijuana) within 3 months prior to screening visit or hard drugs (such as cocaine, phencyclidine [PCP], and crack) within 1 year prior to screening
• Current tobacco smokers (positive history within 3 months before initiation of dosing on Day 1), or those with positive cotinine test at check-in
• Positive drug screen at screening or at check-in
• Positive pregnancy test result at screening or Day -1 or breast feeding during the study
• Any serious medical condition or abnormality in clinical laboratory tests that, in the investigator's judgment, precludes the participant's safe enrollment in and completion of the study
• Unwillingness to comply or other conditions that, in the opinion of the investigator, would interfere with the ability to comply with the study protocol