A Study to Assess the Efficacy and Safety of THB335 in Adults With Chronic Spontaneous Urticaria (CSU)

This study will evaluate the efficacy and safety of THB335 in adults with chronic spontaneous urticaria (CSU) who continue to have hives and itching despite treatment with a second-generation H1 antihistamine. Approximately 81 participants will be randomly assigned to receive 1 of 2 doses of THB335 or matching placebo orally once daily for 12 weeks, in addition to their stable antihistamine treatment. Neither participants nor study personnel will know the assigned treatment. Participants will then be followed for 4 weeks. The main assessment is the change in weekly Urticaria Activity Score (UAS7) from baseline to Week 12.

This study will have approximately 40 locations.

Key Inclusion Criteria:

• Males and females 18 through 85 years.
• Diagnosis of chronic spontaneous urticaria (CSU) ≥ 6 months.
• Active CSU despite treatment with a second-generation H1 antihistamine.
• Weekly Urticaria Activity Score (UAS7) ≥ 16 and ISS7 ≥ 8 during the 7 days before randomization, with sufficient electronic diary entries to calculate the baseline scores.
• Body mass index of 18 to 38 kg/m² and minimum body weight of 50 kg for male participants or 45 kg for female participants.
• Willing and able to complete the daily electronic diary, attend scheduled study visits, and comply with study procedures.
• Participants capable of becoming pregnant or producing sperm must agree to follow the protocol-defined contraception requirements.

Key Exclusion Criteria:

• Chronic urticaria with an identified cause other than chronic spontaneous urticaria.
• Another active skin condition associated with chronic itching that could interfere with the study assessments.
• Recent treatment with prohibited immunosuppressive therapy, biologic therapy, a Bruton's tyrosine kinase inhibitor, an investigational drug, or another therapy that inhibits the c-KIT receptor.
• Clinically significant hepatic, hematologic, or renal disease or laboratory abnormalities.
• Active, chronic, or recurrent infection, including human immunodeficiency virus or viral hepatitis infection.
• Clinically significant cardiovascular disease or electrocardiogram abnormality, including clinically significant arrhythmia, prolonged QT interval, familial long QT syndrome, or use of medication known to prolong the QT interval that cannot be discontinued.
• History of idiopathic anaphylaxis, a severe allergic reaction that may increase the risk of study participation, or hypersensitivity to the study drug or related compounds.
• History of malignancy within the previous 5 years, except successfully treated nonmetastatic basal cell or squamous cell skin cancer or carcinoma in situ.
• Current or planned use of prohibited medications, including specified inhibitors, inducers, or sensitive substrates of drug-metabolizing enzymes or transporters, or proton pump inhibitors.
• Positive urine drug screen at Screening.
• Pregnancy or breastfeeding.
• Any medical or psychiatric condition that, in the investigator's judgment, could place the participant at increased risk, interfere with the study assessments, or affect the participant's ability to complete the study.

Additional eligibility criteria are outlined in the full protocol. The study investigator will review all requirements with potential participants to determine whether they are eligible to participate.