Tarlatamab for Advanced Extrapulmonary Small Cell Carcinoma and Neuroendocrine Carcinoma (TAURUS)

Inclusion Criteria: 1. Written informed consent. 2. Failed prior platinum-based chemotherapy 3. Histologically proven extrapulmonary small cell carcinoma or neuroendocrine carcinoma from gastrointestinal tract, genitourinary tract, gynecologic origin, and head and neck or unknown primary. When mixed histology (ex, adenocarcinoma, squamous cell carcinoma or transitional carcinoma) is found, small cell carcinoma or neuroendocrine carcinoma should be the predominant part. 4. Measurable lesions by RECIST 1.1 within 28 days prior to the first dose of tarlatamab. 5. ECOG Performance Status (PS) of 0 or 1. 6. Age greater or equal to 18 years old. 7. Subjects willing to provide archived tumor tissue samples (formalin fixed, paraffin embedded \[FFPE\] sample) or willing to undergo pretreatment tumor biopsy. 8. Subjects with treated brain metastases are eligible provided they meet the following criteria: 1. Definitive therapy was completed at least 2 weeks prior to the first dose of tarlatamab. 2. There is no evidence of radiographic central nervous system (CNS) progression following definitive therapy and by the time of study screening. 3. Any CNS disease is asymptomatic for at least 7 days (unless symptoms are deemed irreversible by the investigator), the patient is off steroids for at least 7 days (physiologic doses of steroids are permitted), and the subject is off or on stable doses of anti-epileptic drugs for malignant CNS disease for at least 7 days. 9. Adequate organ function, defined as follows: 1. hematological function: 1. absolute neutrophil count ≥ 1.5 x 109/L 2. platelet count ≥ 100 x 109/L 3. hemoglobin \> 9 g/dL (90 g/L) 2. coagulation function: (1) prothrombin time (PT)/international normalized ratio (INR) and partial thromboplastin time (PTT) or activated partial thromboplastin time (APTT) ≤ 1.5 x institutional upper limit of normal (ULN). Subjects on chronic anticoagulation therapy who do not meet the criteria above may be eligible to enroll after discussion with the chief investigator. 3. renal function: (1) estimated glomerular filtration rate (eGFR) based on Modification of Diet in Renal Disease (MDRD) calculation \> 30 mL/min/1.73 m2. 4. hepatic function: 1. aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (ALP) \< 3 x ULN (or \< 5 x ULN for subjects with liver involvement). 2. total bilirubin \< 1.5 x ULN (or \< 2 x ULN for subjects with liver metastases). 5. pulmonary function: 1. no clinically significant pleural effusion 2. baseline oxygen saturation \> 90% on room air 6. cardiac function: (1) cardiac ejection fraction ≥ 50%, no clinically significant pericardial effusion as determined by an echocardiogram (ECHO) and no clinically significant electrocardiogram (ECG) findings Exclusion Criteria: 1. Uncontrolled brain metastasis and leptomeningeal disease. 2. Has evidence of interstitial lung disease or active, non-infectious pneumonitis. 3. Prior exposure to DLL3 targeting agent. 4. Subjects who experienced recurrent pneumonitis (grade 2 or higher) or severe, life-threatening immune-mediated adverse events or infusion-related reactions including those that lead to permanent discontinuation while on treatment with immuno-oncology agents. 5. Unresolved toxicity from prior anti-tumor therapy, defined as not having resolved to Common Terminology Criteria for Adverse Events (CTCAE) version 5.0 grade 1, or to levels dictated in the eligibility criteria with the exception of alopecia or toxicities from prior anti-tumor therapy that are considered irreversible (defined as having been present and stable for \> 21 day) which may be allowed. 6. History of other malignancy within the past 2 years, with the following exceptions: 1. malignancy treated with curative intent and with no known active disease present for \> 2 years before enrollment and felt to be at low risk for recurrence by the treating physician. 2. adequately treated non-melanoma skin cancer or lentigo maligna without evidence of disease. 3. adequately treated cervical carcinoma in situ without evidence of disease. 4. adequately treated breast ductal carcinoma in situ without evidence of disease. 5. prostatic intraepithelial neoplasia without evidence of prostate cancer. 6. adequately treated urothelial papillary non-invasive carcinoma or carcinoma in situ. 7. Myocardial infarction and/or symptomatic congestive heart failure (New York Heart Association \> class II) within 12 months of first dose of tarlatamab. 8. History of arterial thrombosis (e.g., stroke or transient ischemic attack) within 12 months of first dose of tarlatamab. 9. Presence of fungal, bacterial, viral, or other infection requiring oral or IV antimicrobials for management within 7 days of first dose of tarlatamab with the exception: a. Those who have an active infection requiring parenteral antibiotic treatment: simple urinary tract infection and uncomplicated bacterial pharyngitis are permitted if responding to active treatment. Subjects requiring oral antibiotics who have been afebrile for \>24 hours, have no leukocytosis, nor clinical signs of infection are eligible. 10. History of hypophysitis or pituitary dysfunction. 11. Exclusion of hepatitis infection based on the following results and/or criteria: 1. Active hepatitis C infection (subjects with detectable hepatitis C antibody \[HCV Ab\] and HCV RNA viral load above the limit of quantification) (1) Subjects with presence of HCV Ab and HCV RNA viral load below the limit of quantification (HCV RNA negative) with or without prior treatment are allowed. 2. Active hepatitis B infection (presence of hepatitis B surface antigen \[HBsAg\] and hepatitis B virus \[HBV\] DNA viral load above the limit of quantification \[HBV DNA positive\]) 1. Subjects with resolved HBV infection defined as absence of HBsAg and presence of HBV