Inclusion Criteria:
(Parts A and B)
* 1)Healthy Japanese male adults aged \>=18 and \<45 years at the time of informed consent.
* 2\) Subjects with a body mass index (BMI) of \>=18.5 and \<25.0 kg/m2 at screening.
* 3\) Subjects who have received sufficient explanation regarding the study from the principal investigator or subinvestigator, have understood the objectives of the study, voluntarily agreed to participate, and have provided written informed consent of their own free will.
(Part C)
* 1\) Patients diagnosed with idiopathic Parkinson's disease according to the International Parkinson and Movement Disorder Society (MDS) Clinical Diagnostic Criteria (2015).
* 2\) Patients with Parkinson's disease classified as Stage 3 or below according to the modified Hoehn and Yahr staging scale.
* 3\) Patients who are either untreated or have been receiving one of the following treatments at a stable dosage regimen for at least 8 weeks prior to screening, with no planned changes during the study period: selegiline up to 5 mg twice daily, rasagiline up to 1 mg once daily, or immediate-release carbidopa/levodopa up to 25/100 mg three times daily.
* 4\) Patients with an average Bristol Stool Scale score of \<=3 from the date of informed consent to eligibility assessment, or patients with fewer than two bowel movements per week.
If the period between informed consent and eligibility assessment is less than one week, information prior to informed consent will also be collected to assess bowel conditions for at least one week in total.
* 5\) Male or female patients aged \>=40 and \<85 years at the time of informed consent.
* 6\) Patients with a BMI of \>=18.5 and \<32.0 kg/m2 at screening.
* 7\) Female patients who are postmenopausal for at least one year at the time of informed consent, including menopause resulting from hysterectomy or oophorectomy.
* 8\) Patients who have received sufficient explanation regarding the study from the principal investigator or subinvestigator, have understood the objectives of the study, voluntarily agreed to participate, and have provided written informed consent of their own free will.
Exclusion Criteria:
(Parts A and B)
* 1\) Subjects with clinically significant cardiovascular, neurological, pulmonary, hepatic, renal, metabolic, gastrointestinal, urological, immunological, endocrine, or psychiatric disorders, or any other abnormalities that may affect safety, increase seizure risk, lower seizure threshold, or confound study results.
* 2\) Subjects with current or past diseases or surgical histories involving the gastrointestinal tract, liver, kidneys, or other organs that may affect drug absorption, metabolism, or excretion.
* 3\) Subjects who used any medication, including over-the-counter drugs, within 7 days prior to the day before the first administration of the investigational product.
* 4\) Subjects with seizure disorders such as epilepsy, or a history thereof.
* 5\) Subjects with allergies or a history of allergies to drugs or foods.
* 6\) Subjects with allergic predisposition who are considered unsuitable for participation by the principal investigator or subinvestigator.
* 7\) Subjects with current or past alcohol or drug dependence.
* 8\) Subjects who donated \>=400 mL of whole blood within 12 weeks, \>=200 mL of whole blood within 4 weeks, or blood components within 2 weeks prior to investigational product administration.
* 9\) Subjects who tested positive at screening for HBs antigen, HCV antibody, HIV antigen/antibody, or syphilis serology (TP antibody test or RPR test).
* 10\) Subjects unwilling to use appropriate contraception from the time of informed consent until the final study visit.
* 11\) Subjects who answered "Yes" to Question 4 or 5 regarding suicidal ideation on the Columbia-Suicide Severity Rating Scale (C-SSRS) at screening, or who had a history of suicidal behavior within 6 months prior to screening.
* 12\) Subjects who received investigational treatment in another clinical trial within 4 months prior to investigational product administration.
* 13\) Subjects judged unsuitable for study participation by the principal investigator or subinvestigator based on findings from screening or admission assessments, observations, or examinations.
(Part C)
* 1\) Patients with drug-induced parkinsonism, metabolic neurogenetic disorders, encephalitis, Parkinson-plus syndromes, or other atypical parkinsonian syndromes.
* 2\) Patients with freezing of gait.
* 3\) Patients with a history of stereotactic brain surgery for Parkinson's disease (e.g., pallidotomy, deep brain stimulation, or fetal tissue transplantation).
* 4\) Patients with clinically significant cardiovascular, neurological, pulmonary, hepatic, renal, metabolic, gastrointestinal, urological, immunological, endocrine, or psychiatric disorders other than Parkinson's disease, or any other abnormalities that may affect safety, increase seizure risk, lower seizure threshold, or confound study results.
* 5\) Patients with current or past diseases or surgical histories involving the gastrointestinal tract, liver, kidneys, or other organs that may affect drug absorption, metabolism, or excretion.
* 6\) Patients with seizure disorders such as epilepsy, or a history thereof.
* 7\) Patients currently receiving antiplatelet agents or anticoagulants.
* 8\) Patients with allergies or a history of allergies to drugs or foods.
* 9\) Patients with allergic predisposition who are considered unsuitable for participation by the principal investigator or subinvestigator.
* 10\) Patients with current or past alcohol or drug dependence.
* 11\) Patients who donated \>=400 mL of whole blood within 16 weeks, \>=200 mL of whole blood within 4 weeks, or blood components within 2 weeks prior to investigational product administration.
* 12\) Patients who tested positive at screening for HBs antigen, HCV antibody, HIV antigen/antibody, or syphilis serology (TP antibody test or RPR test).
* 13\) Patients unwilling to use appropriate contraception from the time of informed consent until the final study visit.
* 14\) Patients who answered "Yes" to Question 4 or 5 regarding suicidal ideation on the Columbia-Suicide Severity Rating Scale (C-SSRS) at screening, or who had a history of suicidal behavior within 6 months prior to screening.
* 15\) Patients who received investigational treatment in another clinical trial within 4 months prior to investigational product administration.
* 16\) Patients judged unsuitable for study participation by the principal investigator or subinvestigator based on findings from screening or admission assessments, observations, or examinations.
