All-trans Retinoic Acid (ATRA) in the Treatment of Recurrent/Metastatic Adenoid Cystic Carcinoma of the Head and Neck

Inclusion Criteria: 1\. Age ≥ 18 years, male or female; 2. ECOG PS (performance status) score: 0-1; 3. Pathologically or histologically confirmed advanced, recurrent/metastatic ACC, with measurable disease (≥10 mm by spiral CT scan, meeting RECIST 1.1 criteria); 4. Patients with therapeutic indications; 5. Main organ functions normal, i.e., meeting the criteria below: 1. Criteria for routine blood test: (no blood transfusion within 14 days) 1. HB ≥ 90 g/L; 2. WBC ≥ 3.5 × 109/L and \< 10 × 109/L; 3. ANC ≥ 1.5 × 109/L; 4. PLT ≥ 80 × 109/L 2. Criteria for biochemical tests: 1. BIL \< 1.25 × upper limit of normal (ULN) 2. ALT and AST \< 2.5 × ULN; in the presence of metastases to liver, ALT and AST \< 5 × ULN; 3. Serum Cr ≤ 1 × ULN, endogenous creatinine clearance \> 50 mL/min (Cockcroft-Gault equation); 5. Subjects who volunteer to participate in this study, sign the informed consent, have good compliance and cooperate in follow-up; 6. Patients who, in the doctor's opinion, can benefit from the treatment. Exclusion Criteria: 1. Previous or existing concomitant malignancies except cured skin basal cell carcinoma or cervical carcinoma in situ; 2. Coagulation abnormal (INR\>1.5, APTT\>1.5×ULN), history of gastrointestinal hemorrhage in the past 6 months or bleeding tendency \[e.g., presence of active ulcer focus in the stomach, stool occult blood (++), melena and/or hematemesis, hemoptysis in the past 3 months\]; 3. Confirmed hypersensitivity to ATRA; 4. Grade I and above coronary artery diseases, arrhythmias \[including QTc prolongation (males: \> 450 ms, females: \> 470 ms)\] and cardiac dysfunction; 5. Presence of multiple factors affecting oral administration (e.g. dysphagia, nausea, vomiting, chronic diarrhea and intestinal obstruction, etc.); 6. Pregnant or lactating women; 7. History of psychotropic abuse with abstinence failure, or existing mental disorder; 8. Participation in other drug clinical trials within 4 weeks; 9. Other concomitant diseases which seriously jeopardize the patient's safety or prevent the patient from completing the study, as judged by the investigator.