A Study of TAK-385 in Hormone Treatment-naïve Participants With Prostate Cancer

Inclusion Criteria:

1. Participants judged by the investigator to have the capacity to understand the study and follow the study rules.
2. Participants whose written consent (signature or printed name and personal seal on informed consent form) can be obtained before any study procedures are performed.
3. Japanese male participants 20 or more years of age at the time of informed consent.
4. Participants who, if they have a female partner who could become pregnant, agree to practice appropriate means of contraception from the time of informed consent throughout the entire study treatment period and for 4 months after the last dose of study drug.
5. Participants in stable medical condition, including the absence of acute exacerbations of chronic illnesses, serious infections, or major surgery within 4 weeks (28 days) prior to study treatment initiation.
6. Participants with histologically or cytologically confirmed prostate cancer.
7. Participants whose clinical diagnosis is T1-4 N0 M0, or Tx N0 M0 for participants who have undergone radical prostatectomy.
8. Participants who are considered eligible for hormone therapy for prostate cancer.
9. Participants who have not received hormone therapy (example, gonadotropin-releasing hormone \[GnRH\] agonist, GnRH antagonist, steroidal antiandrogen, non-steroidal androgen) for prostate cancer.
10. Participants who have not undergone surgical castration.
11. Participants with serum testosterone at screening greater than (\>) 150 nanogram per deciliter (ng/dL).
12. Participants meeting either of the following criteria for prostate-specific antigen (PSA) at screening. Untreated prostate cancer: PSA at screening \> 4.0 nanogram per milliliter (ng/mL) Treated\* prostate cancer: PSA at screening \> 0.2 ng/mL.

    * Participants who have undergone prostatectomy or either or both of high intensity focused ultrasound therapy or radiotherapy (excluding 125I-brachytherapy) prior to the start of this study.
13. Eastern Cooperative Oncology Group (ECOG) Performance Status \[17\] of 0 or 1.
14. Body mass index (BMI) at screening greater than or equal to (\>=) 18.0 kilogram per square meter (kg/m\^2).

Exclusion Criteria:

1. Participants exhibiting symptoms judged related to prostate cancer by the investigator (example, bone pain, pelvic pain, ureteral obstruction) who urgently require hormone therapy such as GnRH agonist, GnRH antagonist, or CAB/MAB therapy, chemotherapy, or radiotherapy.
2. Participants who have received 5-alpha reductase inhibitors (except for participants who have been treated for male-pattern alopecias).
3. Participants who have received chemotherapy for prostate cancer (including estramustine).
4. Participants who have received 125I-brachytherapy.
5. Participants who received radiotherapy (except for 125I-brachytherapy) within 16 weeks (112 days) before study treatment initiation.
6. Participants who underwent prostatectomy within 16 weeks (112 days before study treatment initiation.
7. Treatment with any investigational compound within the 4 weeks (28 days) prior to the first dose of study drug or ongoing participation in another experimental trial related to the treatment of prostate cancer.
8. Diagnosis or treatment for another systemic malignancy within 2 years before study treatment initiation, or who had received a diagnosis of another malignancy before that and have evidence of residual disease. Participants with non-melanoma skin cancer or carcinoma in situ who have undergone complete resection will not be excluded from the study.
9. Participants taking drugs with moderate to strong cytochrome P450 3A4 (CYP3A4) inhibitory or inducing effects, or any medications, supplements, or food products with P-glycoprotein (P-gp) inhibitory effects, in the 2 weeks (14 days) prior to study treatment initiation.
10. Participants who have received TAK-385 in a past clinical study.
11. Participants for whom it would be difficult to collect blood from a peripheral vein.
12. Participants with uncontrolled and clinically significant nervous, circulatory, pulmonary, hepatic, renal, metabolic, gastrointestinal, urogenital, or endocrine disorders, or other abnormalities (except for the targeted disease) that could affect study participation or the study results. Also, participants meeting any of criteria a through c below.

    A. Participants with uncontrolled diabetes (Hemoglobin A1c \[HbA1C\] \> 8 percent \[%\] at screening). However, participants whose HbA1c is brought under control with diabetes medications may be rescreened.

    B. Participants with uncontrolled hypertension (systolic blood pressure \> 150 millimeter of mercury (mmHg) and diastolic blood pressure \> 90 mmHg at 2 separate measurements taken no more than 60 minutes apart at screening). Participants whose blood pressure is brought under control by antihypertensive medication may be rescreened.

    C. Participants with myocardial infarction, unstable symptomatic ischemic heart disease, arrhythmias of common terminology criteria for adverse events (CTCAE) Grade \> 2, thromboembolism (deep vein thrombosis, pulmonary embolism, or symptomatic cerebrovascular events), or other heart diseases (example, pericardial effusion, restrictive cardiomyopathy). However, chronic stable atrial fibrillation controlled by stable anticoagulant therapy will be allowed.
13. Participants with bilateral hydronephrosis or bladder neck outlet obstruction.
14. Known hypersensitivity to TAK-385, TAK-385 excipients, or gonadotropin-releasing hormone (GnRH) antagonists.
15. Participants with a past history of gastrointestinal tract treatments (including gastrectomy) or gastrointestinal disease that could affect the drug absorption or tolerability (malabsorption, esophageal reflux, peptic ulcer, erosive esophagitis).
16. Participants positive for hepatitis B surface antigens (HBsAg), hepatitis C antibodies (HCV), human immunodeficiency virus (HIV) antibodies, or serologic test for syphilis, or with life-threatening disease other than cancer, at screening.
17. Clinically relevant electrocardiogram (ECG) abnormalities, or the following ECG abnormalities, at screening.

    * Q-wave infarction, unless identified 6 or more months prior to TAK-385 treatment initiation.
    * QTcF interval \> 450 millisecond (msec) (when calculating the QTc interval, Fridericia's equation \[QT/RR0.33\] will be used).
18. Participants with congenital QT prolongation.
19. Current use of Class 1A or Class 3 antiarrhythmic medications.
20. New York Heart Association Class III or IV heart failure.
21. Participants with clinical laboratory abnormalities suggesting clinically relevant underlying disease, or with any of the following abnormal results, at screening.

    * Serum creatinine \>= 2.0 mg/dL.
    * Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \>= 1.5\*upper limit of normal (ULN) for the study site.
    * Total bilirubin \>= 2\*ULN for the study site.
    * Neutrophil count less than (\<) 1,500 per cubic millimeter (/mm\^3), platelet count \< 100,000 per microliter (/mcL), hemoglobin \< 10.0 g/dL.
    * Results of heart-related tests (creatine kinase MB \[CK-MB\] and cardiac troponin T) exceeding the study sites reference value.
22. Participants found to have clinical problems on the basis of examination findings, ECG findings, or chest X-ray findings at screening.
23. Participants considered unlikely by investigators to be able to follow the study protocol or considered ineligible for the study by investigators for other reasons.