• Signed the informed consent form

• Age ≥18 years.

• Weighs more than 40 kg. For doses \<0.3 mg/kg, subject must weigh ≥70 kg.

• Has a life expectancy of ≥3 months.

• Has documented locally advanced or metastatic HER2-positive solid tumor(s) (IHC 1+ to 3+ or in situ hybridization \[ISH\] positive) or HER2-mutant tumor specimen not amenable to curative surgery or radiation and has received at least 1 line of standard therapy in the advanced/metastatic setting, or for which no standard treatment is available, including:

∙ Cohort 1: HER2-positive breast cancer (BC);

‣ Cohort 2: HER2-positive gastric/gastroesophageal junction cancer (GC/GEJ);

‣ Cohort 3: HER2-positive or HER2-mutant non-small cell lung cancer (NSCLC);

‣ Cohort 4: HER2-positive endometrial cancer (EC);

‣ Cohort 5: HER2-positive ovarian cancer (OC), including fallopian tube cancer and primary peritoneal cancer;

‣ Cohort 6: HER2-positive urothelial cancers (UC);

‣ Cohort 7: Other HER2-positive solid tumors as approved by the medical monitor. Note: For indications in which a HER2-directed therapy is approved, the approved treatment is recommended although not mandated, at the discretion of the investigator.

• Agree to provide most recent existing tumor samples (FFPE tissue block or slides) from primary or metastatic sites for tissue-based IHC staining to centrally determine HER2 expression:

∙ In dose escalation and dose finding: archival tissue or fresh biopsy. If no archival tissue is available or it is not possible to obtain a fresh tissue biopsy, medical monitor approval is required;

‣ In dose expansion: an FFPE block or slides from fresh biopsy or the most recent archival tissue is required.

• Has at least one measurable lesion based on RECIST (Response Evaluation Criteria in Solid Tumors) V1.1.

• Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 to 1.

• Toxicity of previous antitumor therapy has returned to Grade ≤1 as defined by NCI CTCAE V5.0, except for alopecia and endocrinopathies controlled by replacement therapy that must be Grade ≤2.

⁃ No serious cardiac dysfunction and left ventricular ejection fraction ≥50%.

⁃ Has adequate organ function before enrollment, defined as:

• Marrow function: Absolute neutrophil count (ANC) ≥1.5×109/L, platelet count ≥100×109/L, hemoglobin (Hb) ≥9.0 g/dL (blood transfusion, platelet transfusion, erythropoietin, platelet-stimulating agents, and G-CSF use are not allowed 1 week prior to screening);

∙ Hepatic function: Total bilirubin (TBIL) ≤1.5×ULN (≤3×ULN for subjects with Gilbert's syndrome or liver metastasis at baseline), AST and ALT without liver metastasis ≤3.0×ULN, AST and ALT with liver metastasis ≤5.0×ULN;

∙ Renal function: Creatinine (Cr) clearance ≥60 mL/minute (Cockcroft-Gault equation).

⁃ Coagulation parameters: International normalized ratio (INR) ≤1.5×ULN, and activated partial thromboplastin time (aPTT) ≤1.5×ULN, unless receiving anticoagulation therapy with PT and aPTT levels within the intended therapeutic range.

⁃ Urine protein ≤2+ or ≤1000 mg/24 hours.

⁃ Sexually active fertile subjects and their partners must agree to use highly effective methods of contraception during the course of the study and for 7 months after the last dose of study treatment. An additional contraceptive method, such as a barrier method (eg, condom), is recommended.

⁃ Women of childbearing potential (WOCBP) must have a negative serum pregnancy test at screening and must be nonlactating. Female subjects are considered WOCBP unless one of the following criteria are met: documented permanent sterilization (hysterectomy, bilateral salpingectomy, or bilateral oophorectomy) or documented postmenopausal status (defined as 12 months of amenorrhea in a woman \>45 years old in the absence of other biological or physiological causes. In addition, females \<55 years old must have a serum follicle stimulating hormone (FSH) level \>40 mIU/mL to confirm menopause.

⁃ Note: Documentation may include review of medical records, medical examination, or medical history interview by study site staff.