Inclusion Criteria:
1. Understanding and voluntarily signing the Informed Consent Form (ICF) for this study;
2. Age 18-75 years (inclusive), male or female;
3. Diagnosed as BD based on the ICBD-2013;
4. At least 2 oral ulcers present at V1 (screening), and:
1. at least 2 oral ulcers present at V2 (the day of randomization) when V2 occurs 14-56 days after V1; OR
2. at least 3 oral ulcers present at V2 (the day of randomization) when V2 occurs 0-13 days after V1;
5. Applicability of systemic treatment for oral ulcers: Based on the severity of the disease and the involved area, the investigator determines that the patient's oral ulceration is not suitable for topical treatment or that the patient's oral ulceration cannot be effectively controlled by topical treatment, so that systemic treatment is to be used;
6. Throughout the study period from signing of ICF through 3 months after the last study dose, women of childbearing potential and male subjects who have not undergone vasoligation should use effective contraceptive measures, including vasoligation, abstinence, intrauterine device (IUD), hormones (oral, patches, rings, injections, implants) and barrier methods (diaphragms, cervical caps, sponges, condoms);
7. Being able to comply with the follow-up schedule and other protocol requirements.
Exclusion Criteria:
1. Active lesions associated with BD in major organs requiring immunosuppressive treatment, e.g., those in lungs (e.g., pulmonary aneurysm), blood vessels (e.g., thrombophlebitis, recurrent malignant aneurysms), gastrointestinal tract (e.g., gastrointestinal ulcers), and central nervous system (e.g., meningoencephalitis); Note: Patients with refractory BD who experienced gastrointestinal perforation, active bleeding, or obstruction, etc. within 3 months prior to randomization are to be excluded.
2. Any clinically significant heart disease (including but not limited to: unstable ischemic heart disease, NYHA III/IV left ventricular failure, or myocardial infarction) or clinically significant 12-lead ECG abnormalities detected during the 6 months prior to screening, which, at the investigator's discretion, may put the subject at safety risk or may interfere with the study assessments;
3. Use of the following immunomodulatory therapies:
* Colchicine within 7 days prior to randomization;
* Perazathioprine, mycophenolate, baritinib, or tofacitinib within 10 days prior to randomization;
* Cyclosporine, methotrexate, cyclophosphamide, thalidomide, or dapsone within 4 weeks (28 days) prior to randomization;
* Biologics within 5 half-lives prior to randomization, e.g.:
* Etanercept within 4 weeks prior to randomization;
* Infliximab or leflunomide within 8 weeks prior to randomization;
* Adalimumab, golimumab, abatacept, or tolizumab within 10 weeks prior to randomization;
* Secukinumab within 6 months prior to randomization;
4. Intraarticular or systemic corticosteroid treatment prior to randomization and within 5 pharmacokinetic/pharmacodynamic half-lives; Note: For subjects with eye symptoms, glucocorticoid eye drops are allowed throughout the trial (except for within 24 hours prior to a trial visit).
5. Chinese patent medicines with immunomodulatory effect within 2 weeks prior to randomization; any Chinese pate nt medicines or decoctions within 2 weeks prior to randomization that might affect efficacy evaluation, or containing sinomenine, total glucoside of paeony, or tripterygium wilfordii, etc.;
6. Laboratory tests:
* Hemoglobin ≤85g/L;
* White blood cell count \<3.0×10\^9/L or \>14×10\^9/L;
* Platelets \<100×10\^9/L;
* Serum creatinine \>1.5 mg/dL (\>132.6 μmol/L);
* Total bilirubin of \>2.0 mg/dL (\>34.2 μmol/L);
* Both aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≥1.5×ULN; Note: The above tests can be repeated at most once during the screening period. If the result within 2 weeks prior to randomization falls into the specified range, the subject is eligible for the study;
7. Use of potent inducers of cytochrome P450 enzymes (e.g., rifampicin, phenobarbital, carbamazepine, phenytoin sodium) within 4 weeks prior to randomization;
8. Other autoimmune diseases or chronic inflammatory diseases associated with immunity, e.g., rheumatic fever, rheumatoid arthritis, systemic lupus erythematosus, dermatomyositis, multiple sclerosis, Sjögren's syndrome, and inflammatory bowel disease;
9. Currently active infections or recurrent bacterial, fungal, viral, mycobacterial or other infectious diseases (including but not limited to tuberculosis, atypical mycobacteriosis, hepatitis B, hepatitis C, herpes zoster, histoplasmosis, and coccidiosis; however, onychomycosis is excluded), which, at the investigator's discretion, may put the subject at safety risk; Note: Subjects positive for hepatitis B surface antigen (HBsAg) or hepatitis C virus (HCV) antibody, or with a history of active mycobacterial infection of any species (including Mycobacterium tuberculosis) within 3 years prior to screening visit should be excluded. Screening is permitted if the subject has been cured for at least 3 years prior to randomization with documentation available for verification;
10. Clinically significant chest X-ray or CT abnormalities, which, at the investigator's discretion, may put the subject at safety risk; Note: If a chest X-ray or CT was performed within 3 months prior to V1, the examination may be omitted for V1;
11. History of transplantation or immunodeficiency;
12. Positive for human immunodeficiency virus (HIV) antibody or treponema pallidum antibody test;
13. Currently having a malignant tumor, or a history of any malignant tumor within 5 years prior to screening (except for treatment-experienced squamous cell carcinoma in situ of the skin, basal cell carcinoma or cervical carcinoma in situ with no evidence of relapse within the past 12 months);
14. Use of any clinical investigational product within 4 weeks prior to randomization or 5 pharmacokinetic/pharmacodynamic half-lives, whichever is longer; Note: Subjects who have participated in HM005BD2S01 study are not eligible to participate in this trial;
15. Known allergy to the study drug or any of its components or allergic constitution;
16. A history of alcohol or drug abuse or dependence, or psychiatric disorder;
17. Any conditions that may interfere with oral drug absorption, e.g., subtotal gastrectomy, clinically significant diabetic gastrointestinal disease, or certain types of bariatric surgery such as gastric bypass surgery, not including procedures that simply separate the stomach into separate chambers such as gastric banding surgery;
18. Prior use of apremilast;
19. Female subjects who are pregnant or breast feeding;
20. Concomitant serious, progressive, or uncontrolled diseases, with which participation in the study may, at the investigator's discretion, put the subject at potential risk or affect the interpretation of study results.
