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, orunresectable solid tumor that is refractory to or intolerable with standardtreatment, 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 1Day 1

• Required baseline local laboratory data (within 7 days prior to Cycle 1 Day 1) asprespecified in the protocol

• A female participant of childbearing potential is eligible to participate if thefollowing conditions are met:

• Not pregnant as confirmed by highly sensitive pregnancy test within 7 daysprior to study drug administration (Cycle 1 Day 1)

• Agrees to adhere to a highly effective contraceptive method and agrees not todonate eggs or freeze/store eggs, during the intervention period, and for 7months following the last dose of study drug

• A male participant is eligible to participate if he agrees to the following duringthe 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 anonparticipant of childbearing potential PLUS partner use of an additionalcontraceptive 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 ifthey 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 thanradiation pneumonitis, currently has ILD, or when suspected ILD cannot be ruled outby imaging at screening

• Clinically severe pulmonary compromise resulting from intercurrent pulmonaryillnesses

• 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 skincancer, curatively treated in situ disease, superficial cancer in thegastrointestinal tract curatively resected by endoscopic surgery, or any other solidtumors 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 1Day 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 forparticipants with hepatitis B infection that is controlled by antiviral therapy

• For the Dose Escalation phase, has human immunodeficiency virus (HIV) infection. Forthe Dose Expansion phase, has active or uncontrolled HIV infection with exceptionsper protocol.

• Has received a live, attenuated vaccine (messenger RNA [mRNA] andreplication-deficient adenoviral vaccines are not considered live, attenuatedvaccines) 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 thestudy

• Has psychological, social, familial, or geographical factors that would preventregular follow-up

• Has prior or ongoing clinically relevant illness, medical condition, surgicalhistory, physical finding, or laboratory abnormality that, in the investigator'sopinion, could affect the safety of the subject; alter the absorption, distribution,metabolism, or excretion of the study drug; or confound the assessment of studyresults