This study is an open-label, multicenter, non-randomized Phase Ib/II clinical study to evaluate the safety, tolerability, and pharmacokinetic characteristics of BL-M11D1 for injection in patients with relapsed/refractory myelodysplastic syndromes.

The study consists of two phases: a dose-exploration phase (Phase Ib) and a dose-expansion phase (Phase II).

Inclusion Criteria:

1. Voluntarily sign the informed consent form and comply with the protocol requirements;
2. No gender restrictions;
3. Age: ≥18 years and ≤75 years;
4. Expected survival time ≥3 months;
5. Relapsed/refractory CD33+ MDS;
6. Morphological assessment showing blasts in bone marrow ≥5% and <20%;
7. Eastern Cooperative Oncology Group (ECOG) performance status ≤2;
8. Toxicities from prior anti-tumor therapy must have recovered to ≤ Grade 1 as defined by NCI-CTCAE v6.0;
9. Meet the required organ function levels;
10. For premenopausal women of childbearing potential, a pregnancy test (serum/urine) must be negative within 7 days before starting treatment, and they must not be breastfeeding; all enrolled trial participants (regardless of gender) must practice adequate barrier contraception throughout the entire treatment period and for 6 months after treatment completion.

Exclusion Criteria:

1. Use of chemotherapy, biotherapy, immunotherapy, etc., within 4 weeks or 5 half-lives prior to the first dose;
2. Presence of uncorrected folate deficiency or vitamin B12 deficiency, etc.;
3. History of severe cardiovascular or cerebrovascular disease;
4. Thromboembolic events requiring therapeutic intervention within 6 months prior to screening;
5. Active autoimmune diseases and inflammatory diseases;
6. History of extensive bowel resection or presence of Crohn's disease, ulcerative colitis, chronic diarrhea, or intestinal obstruction;
7. Diagnosis of another malignancy within 5 years prior to the first dose;
8. Poorly controlled hypertension;
9. Poorly controlled hyperglycemia or diabetes mellitus;
10. Pulmonary diseases classified as Grade ≥3 according to CTCAE v6.0, etc.;
11. Trial participants with central nervous system involvement;
12. Trial participants with extramedullary involvement;
13. Trial participants with a history of allergy to recombinant humanized antibodies or chimeric human-mouse antibodies, or hypersensitivity to any excipient component of BL-M11D1;
14. Prior organ transplantation or hematopoietic stem cell transplantation;
15. Positive for human immunodeficiency virus antibody, active tuberculosis, active hepatitis B virus infection, or active hepatitis C virus infection;
16. Active fungal, bacterial, or viral infections;
17. History of severe neurological or psychiatric disorders;
18. Trial participants with clinically significant bleeding or obvious bleeding tendency within 4 weeks prior to signing informed consent;
19. Presence of clinically symptomatic pleural, peritoneal, or pericardial effusion requiring repeated drainage;
20. Participation in another clinical trial within 4 weeks or 5 half-lives prior to the first dose;
21. Pregnant or breastfeeding women;
22. Other conditions deemed by the investigator to make the participant unsuitable for participation in this clinical trial.