A Study of ASP8273 in Epidermal Growth Factor Receptor (EGFR) Tyrosine Kinase Inhibitor-Naïve Patients With Non-Small Cell Lung Cancer Harboring EGFR Mutations

Inclusion Criteria: * Patients with Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 1. * Patients with a histologically or cytologically confirmed diagnosis of Stage IIIB or IV NSCLC. * Patients confirmed to have the deletion of exon 19 (del ex19), L858R, G719X, or L861Q mutation among the EGFR activating mutations (patients at the study site who are documented to have any of the above-stated EGFR activating mutations can be enrolled in the study). * Patients with a life expectancy ≥ 12 weeks based on the principal investigator's/subinvestigator's judgment. * Patients who meet all of the following requirements for laboratory tests within 7 days before enrollment. When 2 or more test results for a single parameter are found within the specified period, the last data before enrollment should be used for assessment. * Neutrophil count: ≥ 1,500/mm3 * Platelet count: ≥ 75,000/mm3 * Hemoglobin: ≥ 9 g/dL * Serum creatinine: \< 1.5 mg/dL * Total bilirubin (TBL): \< 1.5 × the upper limit of normal (ULN) at the site (this does not apply to patients with Gilbert syndrome) * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT): \< 2.5 ×the ULN at the site * Patients who meet all the following requirements for prior treatment for NSCLC: * Patients who have not received previous treatment with EGFR-TKIs\*1 \*1: Erlotinib, gefitinib, afatinib, and EGFR-TKIs under clinical investigation (e.g.,neratinib, dacomitinib). EGFR-TKIs that can inhibit EGFR with the T790Mmutation (e.g., ASP8273, CO-1686, AZD9291) are also included. * Patients who have not received more than one regimen of previous drug treatment (however, this does not include preoperative or postoperative therapies used within at least a 6-month interval after the last dose of the treatment). * Patients who have at least 1 measurable lesion based on Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1. Exclusion Criteria: * Patients with persistent clinical evidence of previous antitumor treatment-related toxicity ≥ Grade 2 using the Japan Clinical Oncology Group (JCOG) Japanese translation of the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 4.0 (NCI CTCAE v4.0 - JCOG) (except alopecia). * Patients with a history of or concurrent interstitial lung disease. * Patients who have received previous treatment with intended antitumor effects or treatment with another investigational drug/medical device within 14 days before the start of the study treatment. * Patients who have received transfusion or hematopoietic growth factor therapy within 14 days prior to the start of the study treatment. * Patients who have received oral or intravenous corticosteroids within 7 days prior to the start of the study treatment (except to treat or prevent an allergic reaction). * Patients who are scheduled to undergo a surgical procedure during the course of the study or the patient still has an unhealed wound after previous surgery. * Patients with a positive test for hepatitis B surface antigen (HBsAg) or hepatitis C virus antibody (anti-HCV). * Patients with a known history of a positive test for human immunodeficiency virus (HIV) infection. * Patients with symptomatic central nervous system (CNS) lesions. * Patients with a known history of serious drug hypersensitivity. * Patients with evidence of active infection requiring systemic drug therapy within 14 days prior to the start of the study treatment. * Patients who have received strong CYP3A inhibitors within 9 days prior to the start of the study treatment (for itraconazole, within 14 days prior to the start of the study treatment). * Patients who have received moderate CYP3A inhibitors within 9 days prior to the start of the study treatment (only for subjects included in BA evaluation). * Patients who have received strong or moderate CYP3A inducers within 14 days prior to the start of the study treatment (only for subjects included in BA evaluation). * Patients with prolongation of the QTc interval (male: ≥ 451 ms, female: ≥ 471 ms) on the 12-lead electrocardiogram (ECG) in the screening period. * Patients with cardiac arrhythmias requiring treatment. * Patients with Class 3 or 4 New York Heart Association (NYHA) congestive heart failure. * Patients with a history of acute coronary syndrome, myocardial infarction, or cerebrovascular accident within 6 months prior to enrollment. * Patients with a history of or concurrent active peptic ulcer disease or gastrointestinal bleeding within 3 months prior to enrollment. * Patients with corneal disease ≥ Grade 2. * Patients with difficulty to take oral medication, or any gastrointestinal malfunction or inflammatory bowel disease that is considered to affect drug absorption. * Patients with active multiple cancers (simultaneous multiple cancers). * Patients with other conditions ineligible for participation in the study based on the principal investigator's/sub-investigator's judgment.