• Histologically or cytologically confirmed solid tumor.

• Phase 1b dose-escalation stage: patients with advanced solid tumors, who have received standard treatment, for who no standard treatment exists, who are not suitable for standard treatment at the current situation, or who could not tolerate standard treatment.

• Phase 1b dose-expansion stage and phase 2: patients with ES-SCLC who have received first-line platinum-based regimen chemotherapy with or without immunotherapy or intolerance to such therapy.

• Measurable lesions according to RECIST version 1.1 (only applicable for phase 1b dose-expansion and Phase 2).

• Male or non-pregnant, non-lactating female patients age ≥18 years on day of signing the informed consent.

• Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 1.

• Life expectancy ≥ 3 months.

• Adequate organ function .

• Females of child-bearing potential (nonlactating) must have a negative blood pregnancy test within 7 days before enrollment, and must agree to use a medically effective contraception from the time they provided the informed consent until at least 6 months (or at least 180 days) after the last dose of study drug, unless surgical sterilization or menopause for more than 1 year. Patients who are sexually active men with a female partner of child-bearing potential must agree to use adequate contraception from the time they provided informed consent until at least 6 months after the last dose.

• Subjects voluntarily participate in this study and sign the informed consent form.