A Pilot Study of Dacomitinib With or Without Osimertinib for Patients With Metastatic EGFR Mutant Lung Cancers With Disease Progression on Osimertinib.

Inclusion Criteria: * Written informed consent * Advanced biopsy-proven metastatic non-small cell lung cancer * Somatic activating mutation in EGFR in a tumor biopsy or plasma cfDNA liquid biopsy * Prior treatment with osimertinib with response followed by disease progression * No prior first or second generation EGFR inhibitor treatment (gefitinib, afatinib, erlotinib) * Archival tissue available from a tumor biopsy done after disease progression on osimertinib or willing to undergo a tumor biopsy or liquid biopsy after disease progression on osimertinib prior to study initiation * Measurable (RECIST 1.1) indicator lesion not previously irradiated * Karnofsky performance status (KPS) \>/= 70% * Age \>/= 18 years old * Ability to swallow oral medication * Adequate organ function * AST, ALT \</= 3 x ULN * Total bilirubin \</= 1.5x ULN * Creatinine \</= 1.5x ULN OR calculated creatinine clearance \>/= 60ml/min * Absolute neutrophil count (ANC) \>/= 1000 cells/mm3 * Hemoglobin\>/=8.0 g/dL * Platelets \>/=75,000/mm3 Exclusion Criteria: * Pregnant or lactating women * Any radiotherapy within 1 week of starting treatment on protocol. * Any major surgery within 1 weeks of starting treatment on protocol. * Any evidence of active clinically significant interstitial lung disease * Continue to have unresolved \> grade 1 toxicity from any previous treatment Treatment * Patients with a known mechanism of resistance to osimertinib that will clearly not respond to dacomitinib therapy (i.e. known MET amplification, ALK fusion, RET fusion). * Symptomatic brain metastases or leptomeningeal disease requiring escalating doses of steroids