A Study of Atezolizumab With or Without Tiragolumab Consolidation in Limited Stage Small Cell Lung Cancer

Inclusion Criteria:

* Signed Informed Consent Form
* ECOG performance status of 0 or 1
* Histologically confirmed limited-stage SCLC.
* Patients who have not progressed during/after chemoradiotherapy.
* Concurrent or sequential chemoradiotherapy per local clinical practice must have been completed within 6 weeks prior to the first study treatment. If concurrent CRT is used, at least two cycles of chemotherapy should have been conducted during radiotherapy. If sequential radiotherapy is used, induction chemotherapy should be given 2 cycles of chemotherapy before thoracic radiotherapy.
* Adequate hematologic and end organ function.
* For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods that result in a failure rate of \< 1% per year during the treatment period and for at least 5 months after the final dose of atezolizumab or placebo, and 90 days after the final dose of tiragolumab or placebo, and 6 months for chemotherapy after the last dose of chemotherapy treatment, whichever is later.
* For men: agreement to remain abstinent or use contraceptive measures and agreement to refrain from donating sperm.
* Patients must have recovered from all acute toxicities from previous therapy, excluding alopecia and toxicities related to prior therapy.
* Patients must submit a pre-treatment tumor tissue sample.

Exclusion Criteria:

* Histology mixtured or Extensive-stage SCLC (per the Veterans Administration Lung Study Group (VALG) staging system).
* Uncontrolled pleural effusion or pericardial effusion requiring recurrent drainage procedures
* Evidence of significant uncontrolled concomitant disease that could affect compliance with the protocol, including significant liver disease
* Malignancies other than SCLC within 5 years prior to study treatment initiation, with the exception of those with a negligible risk of metastasis or death treated with expected curative outcome
* Pregnancy or breastfeeding, or intention of becoming pregnant during study treatment or within 5 months after the final dose of atezolizumab and 90 days after the final dose of tiragolumab, and 6 months for chemotherapy after the final dose of the chemotherapy treatment.
* Active or history of autoimmune disease or immune deficiency
* Uncontrolled or symptomatic hypercalcemia
* History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced pneumonitis, idiopathic pneumonitis, or evidence of active pneumonitis on screening chest CT scan.
* Positive test result for HIV
* Patients with active hepatitis B or hepatitis C virus
* Active tuberculosis
* Severe infections within 4 weeks prior to study treatment initiation, including but not limited to hospitalization for complications of infection, bacteremia, or severe pneumonia
* Significant cardiovascular disease
* Prior treatment with CD137 agonists or immune checkpoint blockade therapies, anti-CTLA4, anti-tigit, anti-PD-1, and anti-PD-L1 therapeutic antibodies
* Unresolved toxic effects of grade 2 or higher (CTCAE 5.0), including grade ≥ 2 pneumonitis from previous therapy
* Active EBV infection or known or suspected chronic active EBV infection at screening.