• Histologically or cytologically confirmed extensive stage small cell lung cancer (ES-SCLC)

• Considered suitable to receive a platinum-based chemotherapy regimen with durvalumab as 1st line treatment for ES-SCLC.

• Measurable disease defined as lesions that can be accurately measured in at least one dimension (longest diameter to be recorded) as ≥ 10 mm with CT scan, as ≥ 20 mm by chest x-ray, or ≥ 10 mm with calipers by clinical exam.

• At least one lesion must be able to be biopsied at the time of enrollment. The site utilized for biopsy cannot be utilized as a target lesion for efficacy measurement.

• At least 18 years of age.

• ECOG performance status ≤ 1

• Life expectancy of at least 12 weeks.

• Body weight \> 30 kg.

• Adequate normal organ and marrow function as defined below:

‣ Hemoglobin ≥ 9.0 g/dL

⁃ Absolute neutrophil count ≥ 1.0 K/cumm

⁃ Platelet count ≥ 75 K/cumm

⁃ Serum bilirubin ≤ 1.5 x institutional upper limit of normal (IULN)

⁃ AST (SGOT)/ALT (SGPT) ≤ 2.5 x IULN unless liver metastases are present, in which case it must be ≤ 5 x IULN

⁃ Measured creatinine clearance \> 40 mL/min or calculated creatinine clearance \> 40 mL/min by the Cockcroft-Gault formula or by 24-hour urine collection for determination of creatinine clearance

• Adequate cardiac function defined as QTcF \< 470 ms on 12-lead ECG.

• Baseline pulse oximetry must be \> 92% on room air.

• Evidence of post-menopausal status or negative urinary or serum pregnancy test for female pre-menopausal patients. Women will be considered post-menopausal if they have been amenorrheic for 12 months without an alternative medical cause. The following age-specific requirements apply:

‣ Women \< 50 years of age would be considered post-menopausal if they have been amenorrheic for 12 months or more following cessation of exogenous hormonal treatments and if they have luteinizing hormone and follicle-stimulating hormone levels in the post-menopausal range for the institution or underwent surgical sterilization (bilateral oophorectomy or hysterectomy).

⁃ Women ≥ 50 years of age would be considered post-menopausal if they have been amenorrheic for 12 months or more following cessation of all exogenous hormonal treatments, had radiation-induced menopause with last menses \> 1 year ago, had chemotherapy-induced menopause with last menses \> 1 year ago, or underwent surgical sterilization (bilateral oophorectomy, bilateral salpingectomy, or hysterectomy):

• Male and female patients of reproductive potential must be willing to employ effective birth control from screening to 90 days after the last dose of durvalumab

• Ability to understand and willingness to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable). This includes consent for tumor/normal exome sequencing and dbGaP-based data sharing.

• Eligible to receive durvalumab

• ECOG performance status ≤ 1

• Body weight \> 30 kg

• Baseline pulse oximetry must be \> 92% on room air