TDM-based Infliximab Treatment for Active Perianal Fistulizing Crohn's Disease

* Inclusion Criteria: 1. Age: 19-80 years 2. Subjects diagnosed with perianal fistulizing Crohn's disease based on clinical, endoscopic, histological, and radiologic findings, etc. 3. Subjects naive to both biological drugs (anti-TNFs, anti-integrin, anti-IL12/23, etc.) and investigational new drugs 4. Subjects with at least one draining perianal fistula 5. Subjects not responding to two or more conventional treatments (antibiotics, drainage, immunosuppressants, etc.) 6. Women with a childbearing potential: Those who agree to follow contraception during study drug administration and for at least 6 months from the last dosing of the study medication * Exclusion Criteria: 1. In cases where written informed consents cannot be provided by the study subjects or the subjects' legally acceptable representative 2. Subject with a probability of receiving bowel surgery within 12 weeks after baseline, decided by investigators 3. Subjects with temporary or permanent stoma 4. Subjects with short bowel syndrome 5. Subjects not eligible due to significant bowel strictures or intra-abdominal abscesses 6. Subjects who received bowel surgery within 6 months of baseline or subjects who were admitted due to complications associated with bowel strictures or intra-abdominal abscesses within 3 months of baseline 7. Subjects with enterovaginal fistula, enterocutaneous fistula, or enteroenteric fistula 8. Subjects previously exposed to biologics (anti-TNFs, anti-integrin, anti-IL12/23, etc.) or investigational new drugs 9. Subjects with a history of hypersensitivity to monoclonal antibody 10. Subjects requiring corticosteroid therapy. However, if oral corticosteroid dose lower or equivalent to prednisolone 20 mg/day before baseline is given and tapering of oral corticoseroid from baseline is planned, that subjects can be included in the study. Oral corticoseroid is tapered at a schedule of prednisolone 5 mg/7 days (example: if the subject was on oral prednisolone 20 mg/day before baseline, oral prednisolone is tapered as follows: 15 mg/day x 7 days -\> 10 mg/day x 7 days -\> 5 mg/day x 7 days -\> stopping of prednisolone) 11. Subjects with active tuberculosis. However, if the subject has a history of tuberculosis, which was cured with standard anti-tuberculosis therapy according to the standard anti-tuberculosis treatment guidelines, that subject can be included 12. Subjects with latent tuberculosis: Subjects determined to be positive for latent tuberculosis by the pulmonology specialist after history taking, physical examination, chest X-ray, and interferon gamma release assay during the screening period. However, even if positive for latent tuberculosis, if 4 week-treatment for latent tuberculosis is completed and if further treatment for latent tuberculosis is planned to be completed, that subject can be included 13. Subjects positive for HBsAg. In cases of HBsAg (-), but with IgG Anti-HBc (+), real time quantitative PCR for HBV DNA is required. If HBV DNA is 10 IU/mL or over, that subject should be excluded 14. Subjects positive for anti-HCV antibody 15. Subjects with a history of infection with HIV or subject positive for HIV Ag 16. Subjects positive for Clostridioides difficile toxin assay or Clostridioides difficile culture assay 17. Subjects with a heart disease of NYHA Class III/IV 18. Subjects with current or previous demyelinating disease 19. Subject with a history of malignancy (excluding skin basal cell carcinoma, skin squamous cell carcinoma, and uterine cervix cancer) within 5 years or with a history of dysplasia of colon or small bowel within 5 years. 20. Subjects with symptoms or signs of active infection or with a history of treatment for infection within 8 weeks 21. Subjects with a history of organ transplantation 22. Pregnant or lactating women 23. Non-Korean ethnicity according to a family tree 24. Subjects decided to be not eligible for the study by investigators