A Study of DS-1471a In Subjects With Advanced Solid Tumors

Key Inclusion Criteria:

The clinical site will screen for the full inclusion criteria per protocol.

* Sign and date the informed consent form (ICF)
* Adults ≥18 years at the time the ICF is signed
* Has a histologically or cytologically documented, locally advanced, metastatic, or unresectable solid tumor that is refractory to or intolerable with standard treatment, or for which no standard treatment is available
* Has at least 1 measurable lesion according to RECIST v1.1 1 on computed tomography (CT) or magnetic resonance imaging (MRI)
* Is willing and able to provide tumor tissue (newly obtained or archived)
* Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1
* Life expectancy ≥3 months
* Has a left ventricular ejection fraction (LVEF) ≥50% within 28 days prior to Cycle 1 Day 1
* Required baseline local laboratory data (within 7 days prior to Cycle 1 Day 1) as prespecified in the protocol
* A female participant of childbearing potential is eligible to participate if the following conditions are met:

  * Not pregnant as confirmed by highly sensitive pregnancy test within 7 days prior to study drug administration (Cycle 1 Day 1)
  * Agrees to adhere to a highly effective contraceptive method and agrees not to donate eggs or freeze/store eggs, during the intervention period, and for 7 months following the last dose of study drug
* A male participant is eligible to participate if he agrees to the following during the intervention period and for 4 months following the last dose of study drug:

  * Avoid donating sperm
  * Adhere to either abstinence or use of a condom during intercourse with a nonparticipant of childbearing potential PLUS partner use of an additional contraceptive method
* Is willing and able to comply with scheduled visits, study drug administration plan, laboratory tests, other study procedures, and study restrictions
* Patients with liver cirrhosis and liver cancer may be eligible to participate if they meet additional protocol specified criteria

Key Exclusion Criteria:

The clinical site will screen for the full exclusion criteria per protocol.

* Has an inadequate treatment washout period prior to start of study treatment (Cycle 1 Day 1) as prespecified in the protocol
* Has history of or current presence of untreated central nervous system (CNS) metastases
* Has a history of leptomeningeal carcinomatosis
* Has a history of (non-infectious) interstitial lung disease (ILD) other than radiation pneumonitis, currently has ILD, or when suspected ILD cannot be ruled out by imaging at screening
* Clinically severe pulmonary compromise resulting from intercurrent pulmonary illnesses
* Has any of the following within the past 6 months: cerebrovascular accident, transient ischemic attack, arterial thromboembolic event, or pulmonary embolism
* Has uncontrolled or clinically significant cardiovascular disease
* Is requiring chronic steroid treatment (\>10 mg daily prednisone equivalents)
* Has multiple primary malignancies, except adequately resected non-melanoma skin cancer, curatively treated in situ disease, superficial cancer in the gastrointestinal tract curatively resected by endoscopic surgery, or any other solid tumors curatively treated with no evidence of recurrent disease for ≥3 years
* Has unresolved toxicities from previous anticancer treatment
* Exposure to another investigational medical product within 4 weeks prior to Cycle 1 Day 1 or current participation in other therapeutic investigational procedures
* Has an active, known, or suspected autoimmune disease
* Has evidence of ongoing uncontrolled systemic bacterial, fungal, or viral infection.
* Has an active hepatitis or uncontrolled hepatitis B or C infection, except for participants with hepatitis B infection that is controlled by antiviral therapy
* For the Dose Escalation phase, has human immunodeficiency virus (HIV) infection. For the Dose Expansion phase, has active or uncontrolled HIV infection with exceptions per protocol.
* Has received a live, attenuated vaccine (messenger RNA \[mRNA\] and replication-deficient adenoviral vaccines are not considered live, attenuated vaccines) within 30 days prior to first exposure to study drug (Cycle 1 Day 1)
* Female who is pregnant or breastfeeding or intends to become pregnant during the study
* Has psychological, social, familial, or geographical factors that would prevent regular follow-up
* Has prior or ongoing clinically relevant illness, medical condition, surgical history, physical finding, or laboratory abnormality that, in the investigator's opinion, could affect the safety of the subject; alter the absorption, distribution, metabolism, or excretion of the study drug; or confound the assessment of study results