This is a multicenter, randomized, double-blind, placebo-controlled, parallel-group Phase Ib clinical trial conducted in adult patients with mild to moderate seborrheic dermatitis (IGA-SD score of 2-3 points). The study aims to evaluate the safety, tolerability, and steady-state pharmacokinetic (PK) profiles of three concentrations (0.5%, 0.75%, and 1.0%) of ZYG24002 Lotion following continuous topical application once daily (QD) or twice daily (BID) for 28 days, and to conduct a preliminary exploration of the drug's efficacy.

Inclusion Criteria:

1. Aged 18 (inclusive) to 65 (inclusive) years, of either sex. Male subjects must have no childbearing potential or agree to adopt contraceptive measures until 3 months after the end of the study; female subjects of childbearing potential must be non-pregnant and non-lactating, and adopt reliable contraceptive measures during the study period and within 3 months after the last dose.
2. Diagnosed with seborrheic dermatitis by a dermatologist (diagnostic criteria refer to the Expert Consensus on Integrated Traditional Chinese and Western Medicine Diagnosis and Treatment of Seborrheic Dermatitis (2024 Edition) and the 2015 Asian Consensus on Seborrheic Dermatitis), with a disease duration of ≥ 3 months; baseline Investigator's Global Assessment for Seborrheic Dermatitis (IGA-SD) score (0-4 point scale) of 2 (mild) or 3 (moderate); at baseline, the overall scores for erythema and scaling are at least mild severity (score 1) respectively. The involved body surface area (BSA%) is ≤ 10%, and skin lesions are distributed in at least two anatomical sites such as the scalp, face, and trunk.
3. No obvious signs of infection at the test site (e.g., pustules, crusts caused by bacterial infection). The subject is in good general skin condition at enrollment, with no other active skin lesions that may interfere with the clinical assessment.
4. Has not participated in any other clinical trials or used any investigational drugs within the past 3 months.
5. Study participants provide informed consent to participate in this study, sign the informed consent form (ICF), are able to understand the study requirements, and are willing to complete all scheduled visits and examinations in accordance with the investigator's instructions.

Exclusion Criteria:

1. Severe active seborrheic dermatitis: IGA-SD score of 4 points or extensive skin lesions (BSA > 10%), accompanied by obvious exudation and infection, requiring systemic therapy. Participants whose disease activity is deemed unsuitable for study enrollment by the investigator (including but not limited to situations where 4 weeks of placebo-only treatment may lead to unacceptable disease progression).
2. Comorbidities of other skin diseases that may interfere with study assessments, such as psoriasis, severe acne, rosacea, atopic dermatitis, etc., which would confound the observation of seborrheic dermatitis lesions.
3. Administration of systemic glucocorticoids, systemic retinoids, immunosuppressants, oral/intravenous antifungals, phototherapy, PDE-4 inhibitors, or JAK inhibitors within 4 weeks; topical glucocorticoids, calcineurin inhibitors, PDE-4 inhibitors, antifungals, retinoids, keratolytics, selenium/tar-containing preparations on the study area within 2 weeks; any biologic agents within 6 months; or potent CYP3A4 inhibitors/inducers within 4 weeks.
4. Receipt of physical or chemical cosmetic treatments on the head and face within the past month (e.g., intense pulsed light, glycolic acid peels), resulting in temporary skin barrier dysfunction that has not yet recovered to normal.
5. A history of known severe hypersensitivity or severe adverse reactions to ZYG24002 or its excipients.
6. Comorbidities of other skin diseases that may increase the systemic absorption of ZYG24002, such as Netherton syndrome or erythroderma.
7. Current use of immunosuppressants or diagnosis of immune deficiency.
8. A history of malignancy within 5 years prior to screening or at screening.
9. Poorly controlled chronic diseases, including but not limited to endocrine disorders (e.g., diabetes mellitus with significantly elevated fasting blood glucose and obvious clinical symptoms, hyperthyroidism); a history of severe cardiovascular and cerebrovascular diseases (e.g., myocardial infarction, stroke); active peptic ulcers or chronic inflammatory bowel disease, etc.
10. Abnormal liver function: ALT or AST > 2 × upper limit of normal (ULN), or total bilirubin > 1.5 × ULN at screening; abnormal renal function: serum creatinine > 1.5 × ULN; or other clinically significant abnormal laboratory findings that are deemed unsuitable for enrollment by the investigator.
11. Human immunodeficiency virus (HIV) infection; active hepatitis C virus (HCV) infection (anti-HCV positive); active hepatitis B virus (HBV) infection (HBV-DNA > 2000 IU/mL or 10⁴ CPs/mL); or positive treponema pallidum antibody with evidence of active infection.
12. Diagnosis of psychiatric or neurological disorders that may affect compliance or increase study-related risks.
13. Pregnancy, lactation, or planned pregnancy; female subjects of childbearing potential who have not initiated reliable contraceptive measures at least 30 days prior to the first dose and who cannot commit to maintaining such measures for 3 months after the last dose.
14. Diagnosis of alcohol or addictive substance dependence within the past 12 months, or other conditions that the investigator believes may compromise study compliance.
15. Any other conditions that the investigator deems may interfere with study results or pose an unacceptable safety risk to the participant.