This phase II study is a clinical study to explore the efficacy and safety of SI-B003 Monotherapy, BL-B01D1+SI-B003 Combination Therapy and BL-B01D1+PD-1 Monoclonal Antibody in patients with locally advanced or metastatic esophageal cancer, gastric cancer, colorectal cancer and other gastrointestinal tumors.

To explore the efficacy, safety and tolerability of SI-B003 Monotherapy, BL-B01D1+SI-B003 Combination Therapy and BL-B01D1+PD-1 Monoclonal Antibody in patients with locally advanced or metastatic esophageal cancer, gastric cancer, colorectal cancer and other gastrointestinal tumors, and to further explore the optimal dose and combination way.

Inclusion Criteria:

1. Sign the informed consent form voluntarily and follow the protocol requirements;
2. Gender is not limited;
3. Age: ≥18 years old and ≤75 years old;
4. Expected survival time ≥3 months;
5. Patients with locally advanced or metastatic esophageal cancer, gastric cancer, colorectal cancer and other gastrointestinal tumors;
6. Agreed to provide primary tumors or metastases 2 years archive of tumor tissue samples or fresh tissue samples;
7. At least one measurable lesion meeting the RECIST v1.1 definition was required;
8. ECOG 0 or 1;
9. The toxicity of previous antineoplastic therapy has returned to ≤ grade 1 as defined by NCI-CTCAE v5.0;
10. No severe cardiac dysfunction, left ventricular ejection fraction ≥50%;
11. No blood transfusion, no use of cell growth factors and/or platelet-raising drugs within 14 days before screening, and organ function levels must meet the criteria;
12. Blood coagulation function: international standardization ratio of 1.5 or less, and the part activated clotting time live enzymes ULN 1.5 or less;
13. Urinary protein ≤2+ or ≤1000mg/24h;
14. For premenopausal women with childbearing potential, a pregnancy test must be performed within 7 days before starting treatment, serum or urine must be negative for pregnancy, and must be non-lactating; All enrolled patients (male or female) were advised to use adequate barrier contraception throughout the treatment cycle and for 6 months after the end of treatment.

Exclusion Criteria:

1. Antitumor therapy such as chemotherapy or biological therapy was used within 4 weeks or 5 half-lives before the first dose in this study; Mitomycin and nitrosoureas were administered within 6 weeks before the first dose; Oral drugs such as fluorouracil;
2. Cohort using BL-B01D1, previously treated with an ADC drug with topoisomerase I inhibitor as toxin; Immunomodulatory drugs were administered within 2 weeks before the first dose in this study;
3. Systemic corticosteroids were required within 2 weeks before the first dose of the study;
4. Had received immunotherapy and developed grade ≥3 irAE or grade ≥2 immune-related myocarditis according to the CSCO guidelines;
5. History of severe heart disease;
6. QT prolongation, complete left bundle branch block, III degree atrioventricular block;
7. Active autoimmune and inflammatory diseases;
8. Other malignant tumors were diagnosed within 5 years before the first dose in this study;
9. Presence of: a) poorly controlled diabetes mellitus before starting study treatment; b) poorly controlled hypertension; c) history of hypertensive crisis or hypertensive encephalopathy;
10. Pulmonary disease defined as grade ≥3 according to CTCAE v5.0; The patient was diagnosed with grade ≥1 radiation pneumonitis according to the RTOG/EORTC definition. Patients with existing or a history of interstitial lung disease;
11. Unstable thrombotic events requiring therapeutic intervention within 6 months before screening; Infusion-related thrombosis was excluded;
12. Patients with unstable pericardial effusion, pleural effusion, ascites and other serous cavity effusion;
13. Patients with active central nervous system metastasis;
14. Patients with a history of allergy to recombinant humanized antibody or human-mouse chimeric antibody or to any ingredient of BL-B01D1 or SI-B003;
15. Prior organ transplantation or allogeneic hematopoietic stem cell transplantation (Allo-HSCT);
16. Human immunodeficiency virus antibody positive, active tuberculosis, active hepatitis B virus infection or hepatitis C virus infection;
17. Active infections requiring systemic therapy, such as severe pneumonia, bacteremia, sepsis, etc;
18. Enrolled in another clinical trial within 4 weeks before the first dose of this study;
19. Patients who received live vaccine within 4 weeks before the first dose;
20. Patients with a history of mental illness or drug abuse who are unable to cooperate with clinical trial requirements;
21. The investigator did not consider it appropriate to apply other criteria for participation in the trial.