A Study to Investigate the Safety, Tolerability, and Pharmacokinetics of KYN-5356 in Healthy Subjects Aged 18 to 55 Years

Inclusion Criteria: * Subject understands the study procedures and agrees to participate by providing written informed consent prior to any study procedures. * Subject is between 18 and 55 years of age (inclusive), on the date of signing the informed consent form. * Subject has a body mass index between 18 and 30 kg/m2, inclusive. * A female subject is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies: Not a woman of childbearing potential or a woman of childbearing potential who agrees to follow the contraceptive guidance (highly effective birth control method) from Screening until end of the study. Note: Males should use appropriate contraceptive method from Screening until end of the study. * Subject is judged to be in good health by the Principal Investigator based upon the results of a medical history, physical examination, vital signs, 12-lead ECG, and clinical laboratory safety tests. * Subject is willing to adhere to the study requirements and restrictions. Exclusion Criteria: * Subject has positive serology for HBsAg, HCV, or HIV, or history of hepatitis from any cause with the exception of hepatitis A that was resolved at least 3 months prior to first dosing of the Investigational Medicinal Product. * Subject has any illness judged by the Principal Investigator as clinically significant, in the 3 months prior to first dosing of the Investigational Medicinal Product. * Subject has a suicidality score of "yes" on item 4 or item 5 of the suicidal ideation section of the C-SSRS, if this ideation occurred in the past 6 months, or "yes" on any item of the suicidal behavior section, except for the "Non-Suicidal Self-Injurious Behavior" (item also included in the suicidal behavior section), if this behavior occurred in the past 2 years. * Subject has a history or current sign or symptom of psychiatric or neurologic disease. * Subject has a history of head trauma in the past year and/or current seizures. * Subject is a smoker (use of tobacco products in the previous 3 months from Screening) or uses nicotine or nicotine-containing products and has a positive urine cotinine test at Screening or Admission. * Subject has a history of alcohol and/or illicit drug abuse within 2 years of entry or has a positive screening test for alcohol and/or drugs of abuse at Screening or Admission. * Subject consumes caffeine containing beverages exceeding 500 mg caffeine per day (5 cups of coffee) during the off-site days during the study. * Subject has regular excessive consumption of alcohol within 6 months prior to Screening (7 drinks/week for females, 14 drinks/week for males where 1 drink = 150 mL of wine or 360 mL of beer or 45 mL of hard liquor). * Subject has a history or current sign or symptom of a cardiovascular, renal, or metabolic bone disease or disease of bone remodeling, or any history of endocrine disease, including an abnormal laboratory result for pre-specified clinical laboratory safety parameters related to these conditions. * Subject has a history or presence of clinically significant abnormalities detected on 12-lead ECG of either rhythm or conduction. A first-degree atrioventricular block will not be considered as a significant abnormality. Subject has QTcF \> 450 ms (mean values per parameter will be considered) detected on the 12-lead ECG at Screening or Admission. * Presence or sequelae of gastrointestinal, liver, kidney (creatinine clearance \<85 mL/min, using the Cockcroft-Gault formula), or other conditions known to interfere with the absorption, distribution, metabolism, or excretion of drugs at Screening or Admission. * Subject has a history of malignancy within the past 5 years prior to Screening with the exception of excised and curatively treated non-metastatic cell carcinoma of the skin which is considered cured with minimal risk of recurrence. * Subject tests positive for SARS-CoV-2 at Admission. * Additional exclusion criteria for subjects in the MAD Part in view of lumbar puncture for cerebrospinal fluid (CSF) sampling and the ERP assessment. * Subject uses any prescription drugs, herbal supplements, within 4 weeks prior to initial dosing, and/or over-the-counter medication, dietary supplements (vitamins included) within 2 weeks prior to initial dosing, other than what is allowed per protocol. * Subject received vaccination 1 month before admission or plans to receive vaccination during the study. * Use of other investigational drugs/devices within 5 half-lives of the drug from enrollment (time of the subject signing the informed consent form), or within 30 days, whichever is longer. * Donation or loss of 400 mL or more of blood within 8 weeks prior to initial dosing. * Plasma donation (\> 200 mL) within 7 days prior to first dosing. * Resting vital signs at Screening (blood pressure \> 140/90 mmHg, heart rate \> 90 beats per minute, respiratory rate \> 20 breaths per minute, and clinically significant elevated body temperature), or any clinically significant abnormalities in vital signs requiring intervention in the opinion of the Investigator. Up to 2 repeat measurements are permitted to confirm eligibility.