First-in-Human Study of DS-3939a in Participants With Advanced Solid Tumors

Inclusion Criteria:

• Sign and date the main Informed Consent Form (ICF).

• Has a left ventricular ejection fraction ≥50% by either an echocardiogram ormultigated acquisition within 28 days of enrollment.

• Has adequate organ function.

• Measurable disease based on RECIST V1.1.

• Eastern Cooperative Oncology Group performance status score of 0 or 1.

Additional inclusion criteria for Part 1

• Has a histologically or cytologically documented locally advanced, metastatic, orunresectable solid malignant tumors.

Additional inclusion criteria for Part 2

• Has a histologically or cytologically documented locally advanced, metastatic, orunresectable cancer meeting the protocol criteria and documented radiographicdisease progression during or after the most recent anticancer therapy.

• Is able to provide either of the following baseline tumor samples:

• Fresh tumor biopsy samples meeting either of the following requirements thatwere obtained during the Main Screening or Tissue Screening Period, or

• Fresh core needle biopsy sample

• Biopsy samples obtained with forceps or cryobiopsy, such as bronchoscopic ortransbronchial lung biopsy (if the sample amount is equivalent to core needlebiopsy and processing after sample collection follows the procedure describedin the Study Laboratory Manual)

• FFPE tumor tissue samples obtained by biopsy or surgery performed within 6months before signing the main ICF. If samples were obtained prior to the startof the most recent anticancer therapy, the Sponsor Medical Monitor should beconsulted regarding the adequacy of the sample.

Exclusion Criteria:

• Has had prior treatment targeting mucin 1 (MUC1) or TA-MUC1.

• Has spinal cord compression or clinically active central nervous system metastases.

• Has multiple primary malignancies, except adequately resected non-melanoma skincancer, curatively treated in situ disease, or other solid tumors curativelytreated, with no evidence of disease for ≥3 years.

• Has a history of noninfectious interstitial lung disease (ILD)/pneumonitis (including suspected one), has current ILD/pneumonitis, or where suspectedILD/pneumonitis cannot be ruled out by imaging at Screening.

• Has active or uncontrolled human immunodeficiency virus (HIV) infection.

• Has evidence of active or uncontrolled hepatitis B virus or hepatitis C virusinfection.

• Any of the following within the past 6 months: cerebrovascular accident, transientischemic attack, or other arterial thromboembolic event.

• Has an active, known, or suspected autoimmune disease.

• Current participation in other therapeutic investigational procedures, except forparticipation in Long Term Follow-Up without any investigational treatment.