Inclusion Criteria:

          1. Willing and able to provide signed and dated informed consent

          2. Patients with histologically or cytologically confirmed, locally advanced or
             metastatic solid tumors.

          3. Patients must be: a) progressed after standard therapies, b) intolerant of standard
             therapies, or c) with a tumor type without standard therapy. d) Patients with HER2+
             GC/GEJ cancer must have progressed after HER2-targeted therapy.

          4. CLDN18.2 expression in tumor (Part B only): patients with locally advanced or
             metastatic unresectable GC, GEJ cancer, or other solid tumor including but not limited
             to pancreatic cancer, cholangiocarcinoma, ovarian cancer, and lung cancer.

          5. Eastern Cooperative Oncology Group Performance Status (ECOG PS): Part A 0~1, Part B
             0~2.

          6. Life expectancy ≥ 3 months.

          7. At least one measurable lesion per RECIST 1.1 (Part B only).

          8. Provide archived tumor tissue samples

          9. Adequate organ function

         10. Recover to Grade 0-1 from adverse events related to prior anti-cancer therapy except
             alopecia

        Exclusion Criteria:

          1. Symptomatic central nervous system metastases.

          2. Prior anticancer therapy:

               1. Prior systemic anti-cancer treatment (chemotherapy, immunotherapy, biologic
                  therapy, or targeted therapy) within 4 weeks prior to Cycle 1 Day 1, or
                  chemotherapy without delayed toxicity within the last 2 weeks preceding the first
                  dose of study treatment with ≤ grade 1 treatment related AEs.

               2. Radiation therapy within 4 weeks prior to Cycle 1 Day 1;

               3. Prior treatment with an anti-CLDN18.2 antibody.

          3. Major surgery within 8 weeks prior to study entry; Minor surgery within 2 weeks prior
             to study entry.

          4. Gastrointestinal abnormalities including:

               1. Documented unresolved gastric outlet obstruction or persistent vomiting defined
                  as ≥3 episodes within 24 hours

               2. Active peptic ulcer disease required treatment in the past 3 months

               3. Gastrointestinal bleeding as evidenced by hematemesis, hematochezia, or melena in
                  the past 3 months without evidence of resolution documented by endoscopy or
                  colonoscopy

               4. Documented active colitis within 4 weeks prior to study entry, including
                  infectious colitis, radiation colitis and ischemic colitis

               5. History of ulcerative colitis or Crohn's disease

          5. Allergy or sensitivity to TST001 or known allergies to antibodies produced from
             Chinese hamster ovary cells, which in the opinion of the investigator suggests an
             increased potential for hypersensitivity to TST001.

          6. History of a Grade 3-4 allergic reaction to treatment with another monoclonal
             antibody.

          7. Severe cardiovascular disease.

          8. QTc ≥470ms at baseline.

          9. Concurrent malignancy within 5 years prior to entry other than adequately treated
             cervical carcinoma-in-situ, localized squamous cell cancer of the skin, basal cell
             carcinoma, prostate cancer not requiring treatment (with or without resection), ductal
             carcinoma in situ of the breast, or ≤ T1 urothelial carcinoma.

         10. Prior stem cell, bone marrow or solid organ transplant.

         11. Active infection requiring intravenous therapy within 2 weeks prior to entry.

         12. Women of childbearing potential, unless they are using highly effective methods of
             contraception during the intervention period and for 90 days after the last dose of
             study intervention.

         13. Men with a partner of childbearing potential who do not consent to use two highly
             effective methods of birth control during treatment and for an additional 90 days
             after the last administration of investigational drug.

         14. Any condition that the investigator or primary physician believes may not be
             appropriate for participating the study.