This FIH study is designed to assess the safety, tolerability, and preliminary efficacy signals of DS9051b in participants with ACC and mCRPC.

The purpose of the dose escalation part is to determine the maximum tolerated dose (MTD) and recommended dose for expansion (RDE), as well as to establish the safety profile of DS9051b in participants with advanced or metastatic ACC and mCRPC.

General Inclusion Criteria for All Participants

1. Sign and date the main ICF, prior to the start of any trial-specific procedures.

2. Adults ≥18 years of age at the time the ICF is signed (Follow local regulatory requirements if the legal age of consent for trial participation is >18 years old)

3. Histologically confirmed diagnosis of ACC or adenocarcinoma of the prostate

4. ECOG PS of 0 or 1 assessed no more than 14 days prior to C1D1. (ECOG PS of 2 due to cancer pain is acceptable)

5. Has adequate organ and bone marrow function as assessed by local laboratory within 14 days prior to C1D1 as defined below. Organ and bone marrow function criteria must also be met when laboratory tests are repeated within 3 days of initiation of trial intervention as appropriate.

   Additional Inclusion Criteria for Participants with mCRPC

6. Treatment with at least 1 line (or more) of ARPI therapy for CSPC or in CRPC for a minimum of 12 weeks (eg, abiraterone, enzalutamide, darolutamide, apalutamide).

7. Treatment with at least 1 line (or more) of chemotherapy, having ineligibility for chemotherapy or refusal of chemotherapy.

8. Documented disease progression by one or more of the following criteria:

   • PSA progression as defined by PCWG3 criteria.
   • Soft tissue disease progression as defined by RECIST 1.1 criteria.
   • Bone disease progression as defined by Prostate Cancer Working Group 3 criteria.

An individual who meets any of the following criteria will be excluded from participation in this trial:

1. History of pituitary gland dysfunction.

2. Active or uncontrolled autoimmune disease requiring systemic treatment.

3. Any medical condition (other than the cancer disease) requiring a dose of concurrent systemic corticosteroid treatment greater than 5 mg of prednisone and/or 100 μg of fludrocortisone per day (or equivalent).

4. Active infection or other medical conditions that would make corticosteroids contraindicated.

5. Has spinal cord compression or clinically active central nervous system metastases, defined as untreated, clinically progressive, symptomatic, or requiring systemic therapy with corticosteroids or anticonvulsants or local therapy (ie, radiotherapy) to control associated symptoms.

6. Uncontrolled or significant cardiovascular disease.

7. History of another primary malignancy (beyond the indication[s] under study) except for:

   • Malignancy treated with curative intent and with no known active disease ≥3 years before the first dose of trial intervention and of low potential risk for recurrence.
   • Non-melanoma skin cancer, lentigo maligna, or lentigo maligna melanoma treated with curative intent and without evidence of disease.
   • Carcinoma in situ treated with curative intent and without evidence of disease.
   • Participants with a history of prostate cancer (tumor/node/metastasis stage) of Stage ≤T2cN0M0 without biochemical recurrence or progression and who, in the opinion of the investigator, are not deemed to require active intervention.

8. Unresolved toxicities from previous anticancer therapy, defined as toxicities (other than alopecia) not yet resolved to NCI-CTCAE version 5.0, Grade ≤1 or baseline.

9. Participants who cannot swallow whole DS9051b tablets and/or follow DS9051b administration schedule at protocol-defined frequency due to some reasons (eg, dysphagia etc).

10. Known gastrointestinal disease or gastrointestinal procedure that may interfere with absorption of trial intervention including proton pump inhibitors and antacids.

De-identified individual participant data (IPD) on completed studies and applicable supporting clinical trial documents may be available upon request at https://vivli.org/. In cases where clinical trial data and supporting documents are provided pursuant to our company policies and procedures, Daiichi Sankyo will continue to protect the privacy of our clinical trial participants. Details on data sharing criteria and the procedure for requesting access can be found at this web address: https://vivli.org/ourmember/daiichi-sankyo/

Completed studies that has reached a global end or completion with all data set collected and analyzed, and for which the medicine and indication have received European Union (EU) and United States (US), and/or Japan (JP) marketing approval on or after 01 January 2014 or by the US or EU or JP Health Authorities when regulatory submissions in all regions are not planned and after the primary study results have been accepted for publication.

Formal request from qualified scientific and medical researchers on IPD and clinical study documents on completed clinical trials supporting products submitted and licensed in the United States, the European Union and/or Japan from 01 January 2014 and beyond for the purpose of conducting legitimate research. This must be consistent with the principle of safeguarding study participants' privacy and consistent with provision of informed consent.