This is a phase 1, multicenter, nonrandomized, open-label, first-in-human study of YL201 conducted in China and the United States. The study will include 2 parts: a dose escalation part (Part 1) followed by a dose expansion part (Part 2).

Part 1 will estimate the MTD/RED(s) in dose escalation cohorts of patients with advanced solid tumors unresponsive to currently available therapies or for whom no standard therapy is available.

Part 2 will include patients with selected advanced solid tumor types enrolled at the MTD/RED(s), to better define the safety profile and evaluate the efficacy of YL201.

Inclusion Criteria:

• Informed of the trial before the start of the trial and voluntarily sign their name and date on the ICF
• Aged ≥18 years
• Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1
• Adequate organ and bone marrow function
• Female patients of childbearing potential must agree to use a highly effective form of contraception and not donate, or retrieve for their own use, ova from the time of screening and throughout the study period, and for at least 5 months after the last dose of atezolizumab or 6 months after the last dose of YL201, whichever is later. Male patients must agree to use a highly effective form of contraception and not freeze or donate sperm from the time of screening and throughout the study period, and for at least 6 months after the last dose of YL201.
• Life expectancy of ≥3 months
• Able and willing to comply with protocol visits and procedures
• Have at least 1 evaluable tumor lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1.
• Pathologically confirmed diagnosis of an advanced solid tumor (SCLC, mCRPC, ESCC and NSCLC are preferred) for which standard treatment had proven to be ineffective or intolerable, or no standard treatment is available. For ES-SCLC patients in Arm C: no prior anti-cancer treatment

Exclusion Criteria:

• Concurrent enrollment in another clinical study, unless it is an observational (noninterventional) clinical study or during the follow-up period of an interventional study
• Prior systemic anticancer treatment including chemotherapy, molecular -targeted therapy, hormonal therapy, immunotherapy, or biological therapy within 3 weeks before the first dose of study drug (use of oral fluorouracil [eg, tegafur and capecitabine] or small molecular-targeted therapy within 2 weeks or 5 half-life periods [whichever is shorter]before the first dose; use of mitomycin or nitrosoureas within 6 weeks before the first dose; use of herbal medicine with antitumor indications or nonspecific immunomodulators [eg, thymosin, interferon, and interleukin] within 2 weeks before the first dose).
• Prior radiation therapy, including palliative stereotactic radiation with abdominal, within 4 weeks before the first dose of study drug (if palliative stereotactic radiation therapy without abdominal, within 2 weeks)
• Undergone major surgery (not including diagnostic surgery) within 4 weeks before the first dose of study drug or expect major surgery during the study
• Undergone allogeneic hematopoietic stem cell transplantation (HSCT) before the first dose of study drug, or autologous HSCT within 3 months before the first dose of study drug
• Received systemic steroids (>10 mg/day of prednisone or its equivalent) or other immunosuppressive therapy within 2 weeks before the first dose of study drug. Received any live vaccine within 4 weeks before the first dose of study drug or intend to receive a live vaccine during the study
• Known human immunodeficiency virus (HIV) infection
• Active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection. Active HBV is defined as hepatitis B core antibody (HBcAb) or hepatitis B surface antigen (HBsAg) positive, and HBV DNA level above ULN at the study site; active HCV is defined as positive hepatitis C antibody and HCV RNA level above ULN at the study site
• Unresolved toxicities from previous anticancer therapy, defined as toxicities (other than alopecia and pigmentation) not yet resolved to NCI CTCAE Grade ≤1, baseline, or the level specified in the inclusion/exclusion criteria. Patients with chronic Grade 2 toxicities who are asymptomatic or adequately managed with stable medication may be enrolled after discussion with the sponsor
• A history of severe hypersensitivity reactions to the drug substances, inactive ingredients in the drug product, or other mAbs
• Women who are breastfeeding or pregnant as confirmed by pregnancy tests performed within 7 days before the first dose