Inclusion Criteria:

1. Male or female patients aged 20 years or older

2. Pathologically confirmed stage IIIB (unresectable) or IV lung adenocarinoma

3. Patients who have 1 or more than 1 measurable or evaluable but unmeasurable lesions
according to RECIST ver1.1

4. Patients who received prior 1st generation EGFR TKIs (gefitinib or erlotinib)
monotherapy and meet the following criteria:

1. Patients with EGFR mutation (e.g., G719X, exon 19 deletion, L858R, L861Q, etc)
known to be associated with sensitivity to TKIs

2. Patients who showed objective clinical benefit from treatment with an EGFR TKI
as defined by either:

- Patients who showed complete (CR) or partial response (PR), or

- Patients who maintained stable disease (SD) status ≥ 6 months

3. Patients who showed progressive disease (PD, RECIST ver1.1) while on continuous
treatment with gefitinib or erlotinib within the last 30 days (However, patients
whose progressive disease is limited in the brain cannot participate in this
trial.)

4. No intervening systemic chemotherapy between cessation of the EGFR TKI and
participation of this study

5. Patients who agree to the collection of tumor tissue specimen

6. ECOG performance status ≤ 2

7. Life expectancy of ≥ 12 weeks

8. Adequate hematological, hepatic and renal functions:

WBC ≥ 4,000/mm3, Platelet ≥ 100,000/mm3, Serum creatinine ≤ 1.5 X ULN, AST and ALT ≤
2.5 X ULN, Total bilirubin ≤ 1.5 X ULN

9. Patients who give written informed consent voluntarily

Exclusion Criteria:

1. Patients who receive IP within 3 days from prior treatment with gefitinib or
erlotinib

2. NCI-CTCAE grade > 1 adverse events due to treatment with gefitinib or erlotinib

3. Prior systemic chemo, immuno, hormonal and/or biological therapy except gefitinib or
erlotinib within 4 weeks before IP administration

4. Acquired resistance to EGFR TKI due to conversion of adenocarcinoma into small cell
lung cancer

5. Patients who received major surgery within 4 weeks before IP administration

6. Symptomatic CNS metastases (patients with radiologically and neurologically stable
metastases and being off corticosteroids for at least 4 weeks are able to participate
in this trial.)

7. History of other malignancies except effectively treated non-melanoma skin cancers,
carcinoma in situ of the cervix, ductal carcinoma in situ or effectively treated
malignancy that has been in remission for ≥ 3 years and considered to be cured by
investigator's judgment

8. Known pre-existing interstitial lung disease (ILD)

9. NYHA class III or IV heart failure, uncontrolled hypertension, unstable angina or
myocardial infarction within 6 months, poorly controlled arrhythmia or other
clinically significant cardiovascular abnormalities at investigator's discretion

10. Patients whose left ventricle ejection fraction (LVEF) is below the institutional
lower limit of normal (if no lower limit of normal is defined in the site, the lower
limit is 50%.)

11. Patients with known active hepatitis B, HIV infection, or other uncontrolled
infectious disease

12. Clinically significant or recent acute gastrointestinal disorders with diarrhea as a
major symptom (e.g., Crohn's disease, malabsorption disorders, CTCAE grade ≥ 2
diarrhea due to any etiology)

13. Patients who cannot receive IP by mouth and be diagnosed with clinically significant
gastrointestinal disorders which can prevent administration, transit or absorption of
the IP

14. Pregnancy or breast-feeding

15. Women of childbearing potential (WOCBP) or men who are unwilling to use adequate
contraception or be abstinent during the trial and for at least 2 months after the
end of treatment

16. Patients who received other investigational products except gefitinib and erlotinib
within 4 weeks before participation

17. Patients who cannot participate in this trial by investigator's judgment