This study is a Phase 1, first-in-human, open-label, dose-escalation and cohort expansion study designed to characterize the safety, tolerability, pharmacokinetics, preliminary antitumor activity and immunogenicity of 9MW2821 administered by intravenous (IV) infusion.

Inclusion Criteria:

• Competent to comprehend, sign, and date an independent ethics committee/institutional review board/research ethics board (IEC/IRB/REB) approved informed consent form.
• Male or female subjects aged 18 to 80 years (including 18 and 80 years).
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
• Phase Ia：Histologically or cytologically confirmed advanced malignant solid tumors (except sarcoma). Phase Ib：Only local advanced or metastatic UC tumors.
• Subjects must have received ICIs or GC/GP therapies in the previous treatment.
• Subjects must submit tumor tissues for test.
• Life expectancy of ≥ 3 months.
• Subjects must have measurable disease according to RECIST (version 1.1).
• Adequate organ functions.
• Sexually active fertile subjects, and their partners, must agree to use methods of contraception during the study and at least 6 months after termination of study therapy.
• Subjects are willing to follow study procedures.

Exclusion Criteria:

• Chemotherapy、radiotherapy or immunotherpy within 14 days prior to the first dose of study drug.
• Preexisting treatment related toxicity Grade ≥ 2 (except alopecia).
• Major surgery within 28 days prior to first dose of study drug.
• History of uncontrolled diabetes mellitus.
• Preexisting peripheral neuropathy Grade ≥ 2.
• Received treatment of ADCs with MMAE payload.
• Any live vaccines within 4 weeks before first dose of study drug or during the study.
• Documented history of clinically significant cardiac or cerebrovascular diseases within 6 months prior to the first dose of study drug.
• Other severe or uncontrolled disease, i.e. severe respiratory system disease, thromboembolic events, active bleeding or active infection.
• Uncontrolled central nervous system metastases.
• History of another malignancy within 3 years before the first dose of study drug. Subjects with curable malignancies are allowed.
• History of autoimmune disease requiring systemic treatment within 2 years before the first dose of study drug.
• Has ocular conditions that may increase the risk of corneal epithelium damage.
• Known sensitivity to any of the ingredients of the investigational product; History of drug abuse or mental illness.
• Any P-glycoprotein (P-gp) inducers/inhibitors or CYP3A4 inducers/inhibitors for high and medium effect within 14 days prior to the first dose of study drug.
• Use of any investigational drug or device within 30 days prior to the first dose of study drug.
• Conditions or situations which may put the subject at significant risk.