A First in Human Study of BAY2701439 to Look at Safety, How the Body Absorbs, Distributes and Excretes the Drug, and How Well the Drug Works in Participants With Advanced Cancer Expressing the HER2 Protein

Inclusion Criteria: * Male or female participants at least 18 years of age on the day of signing informed consent. * Participants must meet the study phase-specific disease requirements: Dose escalation: Pathologically documented, HER2-expressing (IHC3+, 2+, or 1+ and/or ISH+), unresectable locally advanced or metastatic gastric, gastroesophageal, or breast cancer that has relapsed after standard treatment options, or for which no standard treatment is available. Participants with gastric or gastroesophageal cancer must not have had prior definitive radiotherapy. Participants in the dose escalation cohorts must have evaluable disease by RECIST 1.1, assessed by local imaging. \- Dose expansion: Group A: Pathologically documented unresectable, locally advanced or metastatic breast cancer with HER2 overexpression or amplification (IHC3+ or IHC2+/ISH+) that has relapsed that has relapsed after standard treatment options, or for which no standard treatment is available. Group B: Pathologically documented unresectable locally advanced or metastatic breast cancer with HER2 low expression (IHC2+/ISH-, IHC1+/ISH-, or IHC1+/ISH untested) that has relapsed after standard treatment options, or for which no standard treatment is available. Group C: Pathologically documented, unresectable locally advanced or metastatic carcinomas other than breast cancer with HER2 overexpression or amplification/mutation (IHC3+ or IHC2+/ISH+), that has relapsed after standard treatment options or for which no standard treatment is available. Participants in the dose expansion cohorts must have measurable disease by RECIST 1.1, assessed by local imaging. * Availability of fresh or archival tumor samples - archival tumor samples obtained after disease progression on the most recent anti-cancer treatment may be accepted; those obtained prior to the last anti-cancer treatment may be accepted, upon agreement between the Sponsor and the Investigator. * Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0 or 1. * Life expectancy of at least 6 months, as estimated by the Investigator. * Adequate bone marrow, hepatic, and renal function, as assessed by the following laboratory requirements, to be conducted within 28 days before start of BAY2701439 administration: * Hemoglobin ≥ 9.0 g/dL * Absolute neutrophil count (ANC) ≥ 1500/mm\*3 * Platelet count ≥ 100,000/mm\*3 * Total bilirubin ≤ 1.5 X the upper limit of normal (ULN), except if confirmed history of Gilbert's disease * Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST) ˂ 2.5 x ULN (≤ 5 x ULN for participants with liver involvement) * Participants on a stable dose of anti-coagulation therapy will be allowed to participate if they have no sign of bleeding or clotting, and Prothrombin time/International normalized ratio (PT/INR) and aPTT test results are compatible with the acceptable benefit-risk ratio at the Investigator's discretion * Serum creatinine ≤ 1.5 x ULN and glomerular filtration rate (GFR)≥ 45 mL/min/1.73 m\*2, according to the Modified Diet in Renal Disease (MDRD)abbreviated formula. * A negative serum pregnancy test in women of childbearing potential (WOCBP) performed within 7 days before the start of BAY2701439 administration. Women and men of reproductive potential must agree to use highly effective methods of contraception, when sexually active, during the time period between signing the informed consent form until at least 6 months after the last administration of BAY2701439. * Male and/or female who meet the requirements for contraception and breastfeeding as follows: Male participants: A male participant must agree to use highly effective contraception during the intervention period and for at least 6 months after intervention and refrain from donating sperm during this period. Female participants: A female participant is eligible to participate if she is not pregnant (confirmed by a negative serum pregnancy test within 7 days of first study treatment), not breastfeeding, or is not a woman of childbearing potential. Women of childbearing potential (WOCBP) must agree to use highly effective contraception during the intervention period and for at least 6 months after the last dose of study treatment. Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. \- Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol Exclusion Criteria: * Impaired cardiac function or clinically significant cardiac disease (i.e. congestive heart failure (CHF) New York Heart Association (NYHA) Class II, III or IV). * Left ventricular ejection fraction (LVEF) \< 50% (as measured at screening by echocardiogram). * History or concurrent condition of interstitial lung disease/pneumonitis or severely impaired pulmonary function. * Participants known to be affected by genetic defects linked to radiation hypersensitivity, such as ataxia-telangiectasia (A-T; Online Mendelian Inheritance in Man \[OMIM\] #208900) and A-T-like disorder (meiotic recombination 11 homolog \[MRE11\]), Nijmegen breakage syndrome (OMIM #251260) and Nijmegen breakage Syndrome-like disorder (RAD50), Fanconi anemia (OMIM #227650), DNA ligase IV deficiency (OMIM #606593), RIDDLE syndrome (RNF168), radiosensitive severe combined immunodeficiency (RS-SCID), DNA-PK radiosensitive combined immunodeficiency (DNA-PK-RS-SCID), Cornelia de Lange syndrome. * History of Myelodysplastic syndrome (MDS)/treatment-related acute myeloid leukemia (t-AML) or with features suggestive of MDS/AML. * Infections of Common terminology criteria for adverse events (CTCAE) version 5.0 Grade 2 not responding to therapy or active clinically serious infections of CTCAE Grade \>2. * History of hypersensitivity or severe infusion related reaction to any Trastuzumab-containing drug (e.g. trastuzumab, T-DM1) or any other ingredients contained in BAY2701439. * Chemotherapy, experimental cancer therapy, biologic therapy or immunotherapy within 4 weeks before start of BAY2701439 administration. Start of study treatment is allowed in shorter timeframes provided 5 half-lives of the prior drug(s) have elapsed before the start of BAY2701439 administration. Previous high-dose chemotherapy needing hematopoietic-stem-cell-rescue is prohibited.