The study intends to explore the efficacy and safety of Ivarmacitinib in therapy for polymyalgia rheumatica through a multicenter, randomized, double-blind, placebo-controlled study, and to explore the effectiveness of Ivarmacitinib as an oral glucocorticoid-sparing alternative in the treatment of polymyalgia rheumatica.

This study plans to enroll 80 patients with clinically confirmed severe active polymyalgia rheumatica. After enrollment, participants will be randomly assigned in a 1:1 ratio to either the experimental group or the placebo group. All patients will receive a single dose of long-acting glucocorticoid in Week 1. Both groups will continue their assigned treatment until Week 12, when unblinding will occur. Thereafter, the placebo group will switch to ivarmacitinib, and both groups will continue treatment until Week 48, followed by a 4-week safety follow-up period until Week 52.

Inclusion Criteria:

1. Age and Weight: 50-75 years old, body weight 40-80 kg.

2. Diagnosis: Confirmed diagnosis of polymyalgia rheumatica (PMR) according to the 2012 ACR/EULAR classification criteria .

3. Disease Activity: CRP-PMR-AS (C-reactive protein polymyalgia rheumatica activity score) ≥17 .

4. Glucocorticoid Use:

   • Newly Diagnosed Patients: No glucocorticoid use within 12 weeks prior to enrollment.
   • Relapsed Patients: No increase in glucocorticoid dosage within 2 weeks prior to enrollment, and willingness to discontinue current glucocorticoids after enrollment.

5. Compliance: Participants must understand and agree to adhere to study procedures and restrictions.

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Exclusion Criteria:

1. Allergy: Known hypersensitivity to the investigational drug or excipients (including lactose, cellulose-lactose, low-substituted hydroxypropyl cellulose (L-HPC), colloidal silicon dioxide, stearic acid).

2. Comorbidities:

   • Giant cell arteritis (GCA) .
   • Other diffuse connective tissue diseases (e.g., systemic lupus erythematosus), spondyloarthropathy, or active fibromyalgia .

3. Uncontrolled Chronic Conditions:

   • Diabetes (HbA1c ≥8.0%) or uncontrolled hypertension (resting SBP ≥140 mmHg and/or DBP ≥90 mmHg) .
   • Clinically significant ECG abnormalities (e.g., acute myocardial ischemia, myocardial infarction, severe arrhythmia, QTc >500 ms).

4. Organ Dysfunction:

   • Liver/Kidney Impairment:
   • AST/ALT ≥2× upper limit of normal (ULN).
   • Serum creatinine or total bilirubin ≥1.5× ULN .

5. Malignancy: History of malignancy within the past 5 years.

6. Infections:

   • Active uncontrolled infections (e.g., tuberculosis, hepatitis B surface antigen (HBsAg) positive with elevated HBV-DNA, HCV, HIV, or active syphilis).
   • Severe herpes zoster infection or systemic antimicrobial therapy within 2 weeks prior to randomization.

7. Reproductive Plans: Pregnancy planning within 1 year.

8. Prior Medications: Previous use of JAK inhibitors.

9. Thrombosis: History of thrombotic events.

10. Other: Any condition deemed inappropriate by the investigator for participation in this clinical study.