This study will evaluate the safety, tolerability, and efficacy of DS-3939a in participants with advanced solid tumors.

DS-3939a is an antibody drug conjugate (ADC) being developed for the treatment of malignant tumors. This is a first-in-human, dose-escalating clinical study divided into 2 parts: the Dose Escalation Part (Part 1) and the Dose Expansion Part (Part 2).

Inclusion Criteria:

• Sign and date the main Informed Consent Form (ICF).
• Has a left ventricular ejection fraction ≥50% by either an echocardiogram or multigated 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, or unresectable solid malignant tumors.

Additional inclusion criteria for Part 2

• Has a histologically or cytologically documented locally advanced, metastatic, or unresectable cancer meeting the protocol criteria and documented radiographic disease 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 that were 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 or transbronchial lung biopsy (if the sample amount is equivalent to core needle biopsy and processing after sample collection follows the procedure described in the Study Laboratory Manual)
  • FFPE tumor tissue samples obtained by biopsy or surgery performed within 6 months before signing the main ICF. If samples were obtained prior to the start of the most recent anticancer therapy, the Sponsor Medical Monitor should be consulted 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 skin cancer, curatively treated in situ disease, or other solid tumors curatively treated, 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 suspected ILD/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 virus infection.
• Any of the following within the past 6 months: cerebrovascular accident, transient ischemic attack, or other arterial thromboembolic event.
• Has an active, known, or suspected autoimmune disease.
• Current participation in other therapeutic investigational procedures, except for participation in Long Term Follow-Up without any investigational treatment.

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.