Patient description: A 57-year-old man was admitted to the clinic because of weight loss and persistent dry cough 4 months ago. Chest computed topography showed bilateral multiple infiltrates in the upper lobes and thickened bronchial walls. There is a documented positive serum MPO-ANCA in his medical record. Transbronchial biopsy revealed necrotic granulomas with multinucleated giant cells and the Wegener's granulomatosis was diagnosed for him. He is treating with corticosteroid and cyclophosphamides since 4 months ago. His Birmingham Vasculitis Activity Score (BVAS) is above 4 since the beginning of his disease. His last physical exam and lab study was performed yesterday and showed the results bellow: A wellbeing, well-nourished man, non-icteric, cooperative and alert Weight: 73 kg Height: 177 BP: 120/80 HR: 90/min RR: 22/min Hgb: 13 g/dl WBC: 8000 /mm3 (Neutrophil: 2700/mm3) Plt: 300000 /ml AST: 40 U/L ALT: 56 U/L Alk P: 147 U/L Bill total: 1.2 mg/dl ESR: 120 mm/hr MPO-ANCA: 153 EU

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  RET-fusion Non Small Cell Lung Cancer Lung Neoplasm Carcinoma, Non-Small-Cell Lung Respiratory Tract Neoplasms Thoracic Neoplasms Neoplasms by Site Neoplasms Lung Diseases Respiratory Tract Disease Carcinoma, Bronchogenic Bronchial Diseases Head and Neck Neoplasms Adenocarcinoma Carcinoma Neoplasms by Histologic Type Neoplasms, Germ Cell and Embryonal Neoplasms, Nerve Tissue Main Patient is ≥18 years of age Patient has pathologically confirmed, definitively diagnosed, advanced (not able to be treated with surgery or radiotherapy) or metastatic NSCLC and has not been treated with systemic anticancer therapy for metastatic disease Patient must have a documented RET-fusion Patient has measurable disease based on 1.1 as determined by the local site Investigator/radiology assessment Patient has an ECOG PS of 0-1 Patient should not have received any prior anticancer therapy for metastatic disease Patients can have received previous anticancer therapy (except a selective RET inhibitor) in the neoadjuvant or adjuvant setting but must have experienced an interval of at least ≥ 6 months from completion of therapy to recurrence Patients that received previous immune checkpoint inhibitors in the adjuvant or consolidation following chemoradiation are not allowed to receive pembrolizumab if randomized in Arm B Patient is an appropriate candidate for and agrees to receive 1 of the Investigator choice platinum-based chemotherapy regimens if randomized to Arm B  Patient's tumor has any additional known primary driver alterations other than RET, such as targetable mutations of EGFR, ALK, ROS1, MET, and BRAF. Investigators should discuss enrollment with Sponsor designee regarding co-mutations Patient previously received treatment with a selective RET inhibitor Patient received radiotherapy or radiosurgery to any site within 14 days before randomization or more than 30 Gy of radiotherapy to the lung in the 6 months before randomization Patient has a presence of Grade 2 or worse interstitial lung disease or interstitial pneumonitis, including radiation pneumonitis within 28 days before randomization Patient has CNS metastases or a primary CNS tumor that is associated with progressive neurological symptoms or requires increasing doses of corticosteroids to control the CNS disease. If a patient requires corticosteroids for management of CNS disease, the dose must have been stable for the 2 weeks before Cycle 1 Day 1