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 providingwritten informed consent prior to any study procedures.

• Subject is between 18 and 55 years of age (inclusive), on the date of signing theinformed 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, notbreastfeeding, and at least one of the following conditions applies: Not a woman ofchildbearing potential or a woman of childbearing potential who agrees to follow thecontraceptive guidance (highly effective birth control method) from Screening untilend of the study. Note: Males should use appropriate contraceptive method fromScreening until end of the study.

• Subject is judged to be in good health by the Principal Investigator based upon theresults of a medical history, physical examination, vital signs, 12-lead ECG, andclinical 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 fromany cause with the exception of hepatitis A that was resolved at least 3 monthsprior to first dosing of the Investigational Medicinal Product.

• Subject has any illness judged by the Principal Investigator as clinicallysignificant, in the 3 months prior to first dosing of the Investigational MedicinalProduct.

• Subject has a suicidality score of "yes" on item 4 or item 5 of the suicidalideation 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-SuicidalSelf-Injurious Behavior" (item also included in the suicidal behavior section), ifthis behavior occurred in the past 2 years.

• Subject has a history or current sign or symptom of psychiatric or neurologicdisease.

• 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 fromScreening) or uses nicotine or nicotine-containing products and has a positive urinecotinine test at Screening or Admission.

• Subject has a history of alcohol and/or illicit drug abuse within 2 years of entryor has a positive screening test for alcohol and/or drugs of abuse at Screening orAdmission.

• Subject consumes caffeine containing beverages exceeding 500 mg caffeine per day (5cups of coffee) during the off-site days during the study.

• Subject has regular excessive consumption of alcohol within 6 months prior toScreening (7 drinks/week for females, 14 drinks/week for males where 1 drink = 150mL 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, ormetabolic bone disease or disease of bone remodeling, or any history of endocrinedisease, including an abnormal laboratory result for pre-specified clinicallaboratory safety parameters related to these conditions.

• Subject has a history or presence of clinically significant abnormalities detectedon 12-lead ECG of either rhythm or conduction. A first-degree atrioventricular blockwill not be considered as a significant abnormality. Subject has QTcF > 450 ms (meanvalues per parameter will be considered) detected on the 12-lead ECG at Screening orAdmission.

• Presence or sequelae of gastrointestinal, liver, kidney (creatinine clearance <85mL/min, using the Cockcroft-Gault formula), or other conditions known to interferewith the absorption, distribution, metabolism, or excretion of drugs at Screening orAdmission.

• Subject has a history of malignancy within the past 5 years prior to Screening withthe exception of excised and curatively treated non-metastatic cell carcinoma of theskin 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 lumbarpuncture for cerebrospinal fluid (CSF) sampling and the ERP assessment.

• Subject uses any prescription drugs, herbal supplements, within 4 weeks prior toinitial dosing, and/or over-the-counter medication, dietary supplements (vitaminsincluded) within 2 weeks prior to initial dosing, other than what is allowed perprotocol.

• Subject received vaccination 1 month before admission or plans to receivevaccination during the study.

• Use of other investigational drugs/devices within 5 half-lives of the drug fromenrollment (time of the subject signing the informed consent form), or within 30days, 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 > 90beats per minute, respiratory rate > 20 breaths per minute, and clinicallysignificant elevated body temperature), or any clinically significant abnormalitiesin vital signs requiring intervention in the opinion of the Investigator. Up to 2repeat measurements are permitted to confirm eligibility.