A Trial to Evaluate the Efficacy of Poziotinib, Pan HER Inhibitor in Recurrent/Metastatic Esophageal Cancer (R/M ESCC)

Inclusion Criteria: 1. Histologically confirmed esophageal squamous cell carcinoma (Includes cases with pathologic findings with clinical course comparable to squamous cell carcinoma. For example, undifferentiated carcinoma) 2. Diseases that can not achieve curative aim with radical surgery, radiation, or chemo-radiotherapy. Previous palliative chemotherapy including platinum-based agents for advanced or recurrent esophageal squamous cell carcinoma (If the disease progresses within 6 months after the curative platinum-based chemotherapy or disease progresses within 6 months after last adjuvant chemotherapy, it is considered to be the previous palliative chemotherapy. 3. Have measurable disease based on RECIST 1.1. 4. Be 20 years of age on day of signing informed consent. 5. Performance status of 0 or 1 on the Eastern Cooperative Oncology Group Performance Scale. 6. Demonstrate adequate organ function 7. Absolute Neutrophil Count ≥ 1500/μL 8. Platelet Count ≥100,000/μL 9. Hemoglobin ≥ 9.0 g/dL 10. Serum creatinine ≤ 11. Serum bilirubin ≤ 1.5 x ULN 12. AST, ALT ≤ 1.5 x ULN (with liver metastasis ≤ 5.0 x ULN) 13. Be willing and able to provide written informed consent/assent for the trial. Exclusion Criteria: 1. Without measurable lesion based on RECIST 1.1 2. Has received prior therapy with EGFR/HER2 directed therapy. 3. Has history of intestinal obstruction or recent bleeding (subjects with clinically significant hemorrhagic or clotting events within the past 6 months) 4. Pregnant or lactating women 5. Women who have not received a baseline pregnancy test or have a positive result. (A woman who has not undergone a previous hysterectomy, or history of amenorrhea of more than 12 months, or undergone bilateral oophorectomy is considered to be a non-pregnant menopausal woman.) 6. Male or reproductive women who are not willing to use contraception during the trial 7. Has a known additional malignancy that is progressing or requires active treatment within 3 years. (Except skin basal cell carcinoma, cervix epithelial carcinoma in situ, Superficial bladder cancer) 8. Subjective who is likely to interfere with the understanding of the written consent or influence the compliance of the trial medication. Patients with a history of uncontrolled seizures, central nervous system disorders or psychiatric disorders that are considered clinically significant by the investigator 9. Has stable LVEF of less than 50% 10. Received organ transplants that require immunosuppressive therapy 11. Has un-controlled active infectious disease. 12. Has symptomatic central nerve system metastasis (Except in case of discontinuation of corticosteroids at least 2 weeks before clinical trials and radiologic and neurologically stable cases over 4 weeks) 13. Has known history of, or any evidence of active, non-infectious pneumonitis. 14. Has New York Heart Association, NYHA class III or IV Heart failure, un-controlled hypertension 15. Experienced unstable angina or myocardial infarction within the last 6 months 16. Has un-controlled arrhythmia or judged to have clinically significant cardiovascular abnormalities by investigators 17. Incapability of Oral ingestion and digestion.