This study is an open, multicenter, non-randomized Phase I clinical trial to evaluate the safety, tolerability, pharmacokinetic characteristics, and preliminary efficacy of BL-M24D1 in patients with locally advanced or metastatic head and neck squamous cell carcinoma and other solid tumors.

The study is divided into two phases: a dose escalation phase (Phase Ia) and a dose expansion phase (Phase Ib).

Inclusion Criteria:

1. Voluntarily sign the informed consent form and comply with the protocol requirements;
2. Gender unrestricted;
3. Age: ≥18 years and ≤75 years (Phase Ia); ≥18 years (Phase Ib);
4. Expected survival time ≥3 months;
5. Patients with locally advanced or metastatic head and neck squamous cell carcinoma and other solid tumors;
6. Agree to provide archived tumor tissue specimens or fresh tissue samples from the primary or metastatic foci within the past 3 years;
7. Must have at least one measurable lesion meeting the RECIST v1.1 criteria;
8. ECOG performance status score of 0 or 1;
9. Toxicities from prior antitumor therapy have recovered to ≤ Grade 1 as defined by NCI-CTCAE v5.0;
10. No severe cardiac dysfunction, left ventricular ejection fraction ≥50%;
11. Organ function levels must meet the requirements;
12. Coagulation function: International Normalized Ratio ≤1.5, and activated partial thromboplastin time ≤1.5 × ULN;
13. Urine protein ≤2+ or ≤1000mg/24h;
14. For premenopausal women with childbearing potential, a serum pregnancy test must be performed within 7 days before starting treatment, and the result must be negative. Patients must not be lactating. All enrolled patients (regardless of gender) should adopt adequate barrier contraception throughout the treatment cycle and for 6 months after treatment ends.

Exclusion Criteria:

1. Use of chemotherapy, biotherapy, or immunotherapy within 4 weeks or 5 half-lives prior to the first dose;
2. History of severe heart disease;
3. QT interval prolongation, complete left bundle branch block, or third-degree atrioventricular block;
4. Active autoimmune or inflammatory diseases;
5. Diagnosis of other malignancies within 5 years prior to the first dose;
6. Hypertension poorly controlled by two antihypertensive medications;
7. Poorly controlled blood glucose levels;
8. Unstable thrombotic events requiring therapeutic intervention within 6 months prior to the first dose;
9. Lung diseases graded ≥3 according to CTCAE v5.0;
10. Symptoms of active central nervous system metastasis;
11. History of allergy to recombinant humanized antibodies or human-mouse chimeric antibodies, or allergy to any excipient component of BL-M24D1;
12. Previous organ transplantation or allogeneic hematopoietic stem cell transplantation (Allo-HSCT);
13. Cumulative anthracycline dose >360 mg/m² during prior (neo)adjuvant anthracycline therapy;
14. Positive human immunodeficiency virus antibody, active tuberculosis, active hepatitis B virus infection, or active hepatitis C virus infection;
15. History of interstitial lung disease (ILD) requiring steroid treatment, or current ILD;
16. Active infection requiring systemic treatment within 4 weeks prior to the first dose; lung infection within 2 weeks prior to the first dose;
17. Pleural, abdominal, pelvic, or pericardial effusion requiring drainage and/or accompanied by symptoms within 4 weeks prior to the first dose;
18. Subjects with clinically significant bleeding or obvious bleeding tendency within 4 weeks prior to the first dose;
19. Participation in another clinical trial within 4 weeks prior to the first dose;
20. Pregnant or lactating women;
21. Other conditions deemed by the investigator as unsuitable for participation in this clinical trial.