A Study of Tobemstomig + Nab-Paclitaxel Compared With Pembrolizumab + Nab-Paclitaxel in Participants With Previously Untreated, PD-L1-Positive, Locally-Advanced Unresectable or Metastatic Triple-Negative Breast Cancer

Inclusion Criteria:

• Metastatic or locally advanced unresectable, histologically documentedtriple-negative breast cancer (TNBC) (absence of HER2-over-expression, ER, and PgRexpression by local assessment)

• HER2-low-status

• Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1

• If metastatic disease (Stage IV), measurable disease outside of the bone

• No prior systemic therapy for metastatic or locally advanced unresectable TNBC

• Tumor PD-L1 expression as documented through central testing of a representativetumor tissue specimen

• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1

• Adequate hematologic and end-organ function

• Negative HIV test at screening, with the following exception: individuals with apositive HIV test at screening are eligible provided they are stable onanti-retroviral therapy, have a CD4 count ≥ 200/uL, and have an undetectable viralload

• Negative hepatitis B surface antigen (HBsAg) test at screening

• Positive hepatitis B surface antibody (HBsAb) test at screening, or a negative HBsAbat screening accompanied by either of the following: negative hepatitis B coreantibody (HBcAb); positive HBcAb test followed by quantitative hepatitis B virus (HBV) DNA < 500 IU/mL

• Negative hepatitis C virus (HCV) antibody test at screening, or a positive HCVantibody test followed by a negative HCV RNA test at screening

• Adequate cardiovascular function

Exclusion Criteria:

• Pregnancy or breastfeeding, or intention of becoming pregnant during the study orwithin 4 months after the final dose of tobemstomig or pembrolizumab, and 6 monthsafter the final dose of nab-paclitaxel

• Poor venous access

• History of malignancy within 5 years prior to consent, except for the cancer underinvestigation in this study and malignancies with a negligible risk of metastasis ordeath (e.g., 5-year OS rate >90%), such as adequately treated carcinoma in situ ofthe cervix, nonmelanoma skin carcinoma, localized prostate cancer, ductal carcinomain situ, or Stage I uterine cancer

• Symptomatic, untreated, or actively progressing central nervous system (CNS)metastases

• History of leptomeningeal disease

• Pleural effusion, pericardial effusion, or ascites requiring recurrent drainageprocedures (once monthly or more frequently)

• Hypercalcemia or hypercalcemia that is symptomatic

• Active or history of autoimmune disease or immune deficiency, including, but notlimited to, myasthenia gravis, myositis, autoimmune hepatitis, systemic lupuserythematosus, rheumatoid arthritis, inflammatory bowel disease, antiphospholipidantibody syndrome, Wegener granulomatosis (granulomatosis with polyangiitis),Sjögren syndrome, Guillain-Barré syndrome, or multiple sclerosis

• History of idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitisobliterans), drug-induced pneumonitis, or idiopathic pneumonitis, or evidence ofactive pneumonitis on screening chest computed tomography (CT) scan. History ofradiation pneumonitis in the radiation field (fibrosis) is permitted

• Active tuberculosis (TB)

• Significant cardiovascular/cerebrovascular disease within 3 months prior to consent

• History or presence of an abnormal ECG that is deemed clinically significant

• History of ventricular dysrhythmias or risk factors for ventricular dysrhythmiassuch as structural heart disease (e.g., severe left ventricular systolicdysfunction, left ventricular hypertrophy), coronary heart disease (symptomatic orwith ischemia demonstrated by diagnostic testing), clinically significantelectrolyte abnormalities (e.g., hypokalemia, hypomagnesemia, hypocalcemia), orfamily history of sudden unexplained death or long QT syndrome

• Major surgical procedure within 4 weeks prior to initiation of study treatment

• Treatment with therapeutic oral or IV antimicrobials (anti-bacterial, anti-fungal,antiviral, anti-parasitic) within 1 week prior to initiation of study treatment

• Prior allogeneic stem cell or solid organ transplantation

• Any other disease, metabolic dysfunction, physical examination finding, or clinicallaboratory finding that contraindicates the use of an investigational drug, mayaffect the interpretation of the results, or may render the participant at high riskfrom treatment complications

• Treatment with a live, attenuated vaccine within 28 days prior to initiation ofstudy treatment

• Treatment with investigational therapy within 28 days prior to initiation of studytreatment

• Prior treatment with CD137 agonists or anti-CTLA therapeutic antibodies or ananti-LAG3 agent

• Treatment with systemic immunostimulatory agents (including, but not limited to,interferon and IL-2) within 4 weeks or 5 drug-elimination half-lives (whichever islonger) prior to initiation of study treatment

• Treatment with systemic corticosteroids or other systemic immunosuppressivemedications (including, but not limited to, prednisone, dexamethasone,cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-TNF agents)within 2 weeks prior to initiation of study treatment

• History of severe allergic anaphylactic reactions to chimeric or humanizedantibodies or fusion proteins

• Known hypersensitivity to Chinese hamster ovary cell products or to any component ofthe tobemstomig or pembrolizumab formulation

• Known allergy or hypersensitivity to any component of the to nab-paclitaxelformulation