• Provision of signed and dated written informed consent form prior to any mandatory study specific procedures, sampling, and analyses. For inclusion in i) the optional exploratory genetic research and ii) the optional biomarker research, patients must fulfill the following criteria:

‣ Provision of informed consent for genetic research prior to collection of sample;

⁃ Provision of informed consent for biomarker research prior to collection of sample;

⁃ If a patient declines to participate in the optional exploratory genetic research or the optional biomarker research, there will be no penalty or loss of benefit to the patient. The patient will not be excluded from other aspects of the study

• At least one lesion (measurable and/or non-measurable) that can be accurately assessed at baseline by computed tomography (CT) and is suitable for repeated assessment

• Patients with previously treated brain metastases that are asymptomatic for at least 14 days and only require prednisone equivalent of 10 mg daily or less prior to study treatment

• Histological or cytological documented ES-SCLC: American Joint Committee on Cancer (AJCC) stage IV SCLC (T any, N any, M1 a/b), including patients with T3-4 due to multiple lung nodules that are too extensive or have tumor/nodal volume that is too large to be encompassed in a tolerable radiation plan

• No prior systemic therapy for ES-SCLC, including, but not limited to, chemotherapy, PARP inhibitor, and PD-1/PD-L1 checkpoint inhibitors. Palliative radiation is allowed if completed a minimum of three days prior to beginning of study treatment

• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1 at enrollment

• Body weight \> 30 kg

• Hemoglobin \>=10.0 g/dL with no blood transfusion in the past 28 days (measured within 28 days prior to administration of study treatment)

• Absolute neutrophil count \>= 1.5 x 10\^9 /L (measured within 28 days prior to administration of study treatment)

• Platelet count \>= 100 x 10\^9/L (measured within 28 days prior to administration of study treatment)

• Serum bilirubin =\<1.5 x institutional upper limit of normal (ULN) (measured within 28 days prior to administration of study treatment)

• Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) =\< 2.5 x institutional ULN, unless liver metastases are present in which case they must be =\< 5 x ULN (measured within 28 days prior to administration of study treatment)

• Calculated estimated creatinine clearance \>= 51 mL/min using the Cockcroft- Gault equation or based on a 24-hour urine test (measured within 28 days prior to administration of study treatment)

• Evidence of post-menopausal status or evidence of non-childbearing status for women of childbearing potential: negative urine or serum pregnancy test within 28 days of study treatment and confirmed prior to treatment on day 1. Postmenopausal status is defined as:

‣ Amenorrheic for 1 year or more following cessation of exogenous hormonal treatments

⁃ Luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels in the post-menopausal range for women under 50

⁃ Radiation-induced oophorectomy with last menses \> 1 year ago

⁃ Chemotherapy-induced menopause with \> 1-year interval since last menses

⁃ Surgical sterilization (bilateral oophorectomy or hysterectomy)

⁃ Women of childbearing potential and their partners, who are sexually active, must agree to the use of TWO highly effective forms of contraception in combination. This should be started from the signing of the informed consent and continue throughout the period of taking study treatment and for at least 3 months after last dose of study drug(s)

⁃ Male patients must use a condom during treatment and for 3 months after the last dose of study treatment when having sexual intercourse with a pregnant woman or with a woman of childbearing potential. Female partners of male patients should also use a highly effective form of contraception if they are of childbearing potential. Male patients should not donate sperm throughout the period of taking study treatment and for 3 months following the last dose of study treatment

• Subject is willing and able to comply with the protocol for the duration of the study including undergoing treatment and scheduled visits and examinations including follow-up