Inclusion Criteria:
* Healthy Korean adult volunteers aged 19 to 50 years at the time of screening (single dose study)
* Healthy Korean or Caucasian adult volunteers aged 19 to 50 years at the time of screening (multiple dose study dose groups 1-4)
* Healthy elderly Korean volunteers aged 65 to 85 years at the time of screening (multiple dose study dose group 5)
* Individuals with a body weight of ≥ 50.0 kg and ≤ 90.0 kg and a body mass index of ≥ 18.5 kg/m2 and ≤ 29.9 kg/m2 at the time of screening
* Individuals who have given written consent on a voluntary decision to participate and agree to adhere to the precautions after being fully informed of and completely understanding this clinical trial
Exclusion Criteria:
* Individuals with a clinically significant disease or history in hepatobiliary, renal, nervous, immune, respiratory, endocrine, hemato-oncology, cardiovascular, urinary system, or psychiatric disorder.
* Individuals with a history of a gastrointestinal disorder or surgery that may affect the safety, pharmacokinetic and pharmacodynamic evaluation of the investigational product
* Individuals who answered 'yes' to any question on the Columbia Suicide Severity Rating Scale administered at the time of screening
* Individuals with hypersensitivity or a history of clinically significant hypersensitivity to AS-S603 and the same class or other drugs (aspirin, antibiotics, etc.)
* Individuals with a positive result in the serology test (hepatitis B, hepatitis C, human immunodeficiency virus test, syphilis test)
* Individuals with a history of drug abuse or a positive urine screening for any drug of concern for abuse
* Individuals who exhibited the following results on vital signs measured at a sitting position after resting, during the screening tests: Systolic blood pressure \< 80 mmHg or ≥ 140 mmHg, Diastolic blood pressure \< 45 mmHg or ≥ 90 mmHg
* Individuals who exhibited QTcB interval \> 450 msec (male), 470 msec (female), or clinically significant abnormal findings in rhythm on an electrocardiogram during the screening tests
* Individuals who exhibited any of the following results on the clinical laboratory test during the screening tests, including additional tests: Aspartate transaminase or alanine transaminase \> 60 IU/L, Estimated glomerular filtration rate \< 90 mL/min/1.73m2
* Individuals who had taken any prescription drug or herbal medicine within 2 weeks, or any Over-The-Counter (OTC) drugs, dietary supplements including liver supplements, or vitamin supplements within 1 week prior to the scheduled first administration of investigational product, or are expected to take them
* Individuals who had taken an inducer or inhibitor of any drug metabolic enzyme, such as barbiturates or clarithromycin, within 1 month prior to the scheduled first administration of investigational product
* Individuals who have participated and administered investigational product in another clinical trial within 6 months prior to the scheduled first administration of investigational product
* Individuals who donated whole blood within 2 months, donated blood components within 1 month, or received a blood transfusion within 1 month prior to the scheduled first administration of investigational product
* Current smokers
* Individuals who engage in persistent alcohol consumption or are not able to avoid alcohol consumption from 3 days prior to the scheduled first administration of investigational product until the end of the clinical trial.
* Individuals who have consumed excessive caffeine or are not able to avoid consuming caffeine-containing foods from 3 days prior to the scheduled first administration of investigational product until the end of the clinical trial.
* Individuals who have consumed grapefruit, grapefruit juice, or grapefruit-containing foods or cannot avoid consuming grapefruit-containing foods from 3 days prior to the scheduled first administration of investigational product until the end of the clinical trial.
* Individuals with unusual dietary habits or who cannot adhere to the standardized diet provided by the institution during the confinement period.
* Women of childbearing potential, individuals whose menstrual period was not confirmed at screening, with a positive urine pregnancy test, or who do not agree to a highly effective contraceptive method for at least 30 days after the last investigational product administration.
* Men who do not agree to use condoms and ensure that their female partners use a highly effective contraceptive method throughout the entire clinical trial period and for at least 90 days after the last administration of investigational product.
* Individuals who do not agree to refrain from donating sperm or egg throughout the entire clinical trial period and for at least 90 days after the last administration of investigational product.
* Individuals who cannot undergo cerebrospinal fluid (CSF) tapping (for groups requiring CSF tapping)
* Individuals determined to be ineligible for participation in the clinical trial by the investigators due to other reasons
