• Stage IV (American Joint Committee on Cancer \[AJCC\] 8th edition) non-small cell lung cancer with an oncogenic ALK fusion

• Histologies include adenocarcinoma, squamous cell carcinoma, adenosquamous adenocarcinoma, and NSCLC NOS (not otherwise specified)

• The presence of an oncogenic ALK fusion established from any Clinical Laboratory Improvement Act (CLIA) certified laboratory

• The patient must belong to one of the following treatment cohorts.

‣ Cohort 1: Prior 1st generation ALK tyrosine kinase inhibitor (TKI) (crizotinib) and/or prior 2nd generation TKI (ceritinib, brigatinib, alectinib) and/or lorlatinib

⁃ Cohort 2: Prior 1st generation ALK TKI (crizotinib) and/or prior 2nd generation TKI (ceritinib, brigatinib, alectinib) and/or lorlatinib, and platinum-doublet chemotherapy

⁃ Cohort 3: Prior 1st generation ALK TK (crizotinib) and/or prior 2nd generation TKI (ceritinib, brigatinib, alectinib) and/or lorlatinib, platinum-doublet chemotherapy, and any other number of antineoplastic agents (including immunotherapy, standard or investigational)

• Age ≥ 18

• Eastern Cooperative Oncology Group (ECOG) performance status 0-2

• Absolute neutrophil count (ANC) ≥ 1500/mcL

• Platelets ≥ 100,000/mcL

• Hemoglobin ≥ 9 g/dL without transfusion or erythropoietin (EPO) dependency (within 7 days of assessment)

• Measured or calculated creatinine clearance (CrCl) ≥ 50mL/min (calculated per Cockcroft-Gault formula)

• Serum total bilirubin ≤ 1.5 X upper limit of normal (ULN) (per institutional guidelines) OR direct bilirubin ≤ ULN for subjects with total bilirubin levels \> 1.5 ULN

• Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase \[SGOT\]) ≤ 2.5 X ULN OR ≤ 5 X ULN for subjects with liver metastases

• Alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase \[SGPT\]) ≤ 2.5 X ULN OR ≤ 5 X ULN for subjects with liver metastases

• Albumin ≥ 2.5g/dL

• Female subject of childbearing potential should have a negative serum pregnancy test within 21 days of enrollment prior to receiving the first dose of study medication

• Female subjects of childbearing potential must be willing to use a highly effective method of contraception for the course of the study, through 180 days after the last dose of study medication. Note: Abstinence is acceptable, if patient documents that this is their usual lifestyle or preferred contraception method

• Male subjects of childbearing potential must agree to use an adequate method of contraception starting with the first dose of study therapy through 4 months after the last dose of study therapy. Note: Abstinence is acceptable, if patient documents that this is their usual lifestyle or preferred contraception method

• Ability to swallow pills orally and per investigator's assessment, do not have any significant issues limiting absorption of drug

• Ability to understand and the willingness to sign a written informed consent

• Measurable disease per RECIST v1.1 criteria assessed per screening imaging

• If a cancerous lesion is easily and safely accessible, a pre-treatment biopsy of this lesion is strongly encouraged but NOT required prior to first dose of gilteritinib. Archival or fresh tissue biopsy may be used as long as it was obtained prior to cycle 1 day 1 (C1D1)

• At least 7 days must have elapsed since last anti-neoplastic TKI, chemotherapy, immunotherapy, or investigational agent prior to the first dose of gilteritinib