Study to Evaluate the Safety, Tolerability, Pharmacokinetics (PK) and Anti-tumor Activity of SYHA1801 Monotherapy in Patients With Advanced Solid Tumors

Inclusion Criteria: 1. Age ≥18, ≤75 years, no gender limitation. 2. Histologically or cytologically confirmed diagnosis of advanced or metastatic solid tumors, for which standard therapy either does not exist or has proven ineffective, intolerable or inacceptable for the patient. 3. At least one measurable lesion as per RECIST version 1.1. 4. Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) ≤1. 5. Life expectancy ≥3 months. 6. Adequate bone marrow reserve, renal and liver function. 7. Women of childbearing potential should agree to use contraceptive measures (such as IUD, contraceptive or condom) during the study and within 6 months after the end of the study; the serum pregnancy test should be negative within 7 days before enrollment, and must be non-lactating subjects; men should agree to use contraceptive measures during the study and within 6 months after the end of the study. 8. Signed informed consent form. Exclusion Criteria: 1. Administration of chemotherapy, radiotherapy, biotherapy, endocrine therapy, targeted therapy, immunotherapy and other anti-tumor treatment within 4 weeks before the first dose of the study drug, except the following: using nitrosourea and mitomycin C within 6 weeks, using fluorouracil and small molecule targeted drugs within 2 weeks (or within 5 half time period), using traditional Chinese medicine with anti-tumor indications within 2 weeks. 2. Administration of other unlisted clinical research drugs within 4 weeks before the first dose of SYHA1801. 3. Major organ surgery (excluding biopsy) or significant trauma within 4 weeks before the first dose of SYHA1801. 4. Administration of glucocorticoids or other immunosuppressants within 14 days prior to the first dose of SYHA1801, except the following: local, ocular, intraarticular, intranasal and inhaled glucocorticoids; short-term use of glucocorticoids for preventive treatment. 5. Concomitant therapy with strong CYP3A4 inhibitors or inducers within 14 days. 6. Prior treatment with BET inhibitors. 7. Persistent grade \>1 clinically significant toxicity related to prior antineoplastic therapies (except alopecia). 8. Central nervous system metastasis or meningeal metastasis with clinical symptoms, or other evidence that the patient's central nervous system metastasis or meningeal metastasis has not been controlled, that is not suitable for the group according to the judgment of the investigator. 9. Uncontrollable active infection. 10. History of autoimmune diseases, immunodeficiency, including HIV positive, or other acquired, congenital immunodeficiency, or organ transplant history. 11. Active hepatitis B; positive for hepatitis C antibody. 12. History of serious cardiovascular disease. 13. Inability to swallow oral medications or presence of a gastrointestinal disorder deemed to jeopardize intestinal absorption of SYHA1801. 14. Other serious illness or medical conditions. 15. Alcohol or drug dependence. 16. A clear history of neurological or psychiatric disorders. 17. Pregnant or breast-feeding female. 18. In the opinion of the investigator, not suitable for enrollment due to other reasons.