This is a prospective, open-label, randomized controlled clinical intervention study to evaluate the efficacy and safety of all-trans retinoic acid (ATRA) in treating patients with 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.