The goal of this clinical trial is to assess the efficacy and safety of Pimicotinib (ABSK021) in patients with Tenosynovial Giant Cell Tumor (TGCT). The main questions it aims to answer are:

• Whether the Pimicotinib(ABSK021) works well in patients with TGCT.
• Whether the Pimicotinib(ABSK021) is safe in patients with TGCT.

Participants will be asked to complete the study procedures:

• Receive the administration of Pimicotinib(ABSK021) or placebo (a placebo is a look-alike substance that contains no active drug) about 24 weeks in study part 1.
• Receive the administration of Pimicotinib(ABSK021) about 24 weeks in study part 2.
• Receive the administration of Pimicotinib(ABSK021) till study end in study part 3.
• Complete the study procedures speficied in the protocol, which is guided by researchers.

This study consists of part 1 and part 2. Part 1 is a double-blind phase, eligible patients will be randomized to Pimicotinib(ABSK021) treatment group or matching placebo group and will receive Pimicotinib(ABSK021) or matching placebo until completion of Part 1.

Part 2 is an open-label treatment phase, and all patients entering this phase will receive open-label Pimicotinib(ABSK021) until completion of 24 weeks of dosing or withdrawal from the study.

Part 3 is an open-label extension treatment phase, and patients who completed the part 2 and continuted to be eligible, will go to the Part 3. Patients will receive the open-label Pimicotinib(ABSK021) until all patients withdraw from the study, or the sponsor decides to terminate the study, whichever occurs first.

Inclusion Criteria:

• Patients should understand the study procedures and sign the informed consent form prior to screening.
• Age ≥ 18 years.
• A histologically confirmed TGCT with unresectable.
• Measurable disease as defined by RECIST 1.1, and with at least one lesion of ≥ 2 cm.
• Stable prescription of analgesic regimen for patients with an analgesic need.
• Participants should complete stiffness and pain scales during the screening period, and symptomatic disease because of active TGCT should meet minimum requirements as outlined in study protocol.
• ECOG PS (Eastern Cooperative Oncology Group Performance Status) of 0 or 1.
• Adequate organ function and bone marrow function.

Exclusion Criteria:

• Known allergy or hypersensitivity to any components of the investigational drug product.
• Previous treatment with highly selective inhibitors targeting CSF-1/CSF-1R. Previous therapy with imatinib and nilotinib is allowed.
• Known additional malignancy that required active treatment and may affect the patient's participation in the study or affect the outcome of the study as assessed by the Investigator.
• Known metastatic TGCT.
• Significant concomitant arthropathy in the affected joint, serious disease, uncontrolled infection.
• Known MRI contraindications.
• Has factors that significantly affected the absorption of oral drug.
• Major surgery or previous anti-tumor therapy for TGCT within 4 weeks prior to randomization.
• Concomitant use of strong CYP inhibitors or inducers as outlined in study protocol.
• Impaired cardiac function or clinically significant cardiac disease.
• Known active human immunodeficiency virus, active hepatitis B, active hepatitis C, or known active tuberculosis.
• Known active liver or biliary disease, or other diseases that may lead to abnormal liver function test results during the study.
• Pregnant or lactating women.
• Childbearing potential males or non-surgically sterilized female patients must agree to use effective methods of contraception during the study.
• Any other clinically significant comorbidities, which in the judgment of the Investigator, could compromise compliance with the protocol, interfere with the interpretation of study results, or predispose the patient to safety risks.