Determine the Dose-dependent Efficacy and Safety of EC-18 in Patients With Moderate to Severe Atopic Dermatitis

Inclusion Criteria: At the time of screening: * aged 19 to under 80 at the time of written consent. * Patients diagnosed with atopic dermatitis according to the Hanifin and Rajka criteria (≥3 major symptoms out of 4 and ≥3 minor symptoms out of 23) for at least 12 months prior to screening * Patients requiring systemic therapy for disease control due to either a documented inadequate response to topical drug treatment for at least 4 weeks (within 6 months prior to screening visit), or where topical therapy is medically not recommended (due to significant side effects or safety concerns) * Patients with Moderate to Severe Atopic dermatitis * BSA ≥ 10% * EASI score ≥16 * IGA score ≥3 * Pruritus NRS score ≥3 * Patients who have been fully informed about this clinical trial, have voluntarily decided to participate, and have provided written consent to faithfully comply with the trial requirements. At the time of baseline visit * Signed informed consent * Patients who have used emollients continuously for the 7 days prior to baseline visit Exclusion Criteria: * Patients with serious skin conditions other than atopic dermatitis (e.g. psoriasis, discoid lupus erythematosus, or erythema multiforme that, as judged by the investigator, would adversely affect the evaluation of atopic dermatitis) * Patients with a history of chronic immunosuppression * Patients with ongoing malignant tumors at screening or diagnosed within the last 5 years * Patients who have undergone bone marrow or organ transplantation * Patients with hypertension (e.g. systolic blood pressure \> 160 mmHg or diastolic blood pressure \> 100 mmHg) * Patients with ulcerative or erosive lesions * Patients with Netherton's Syndrome * Patients with clinically significant severe infections (e.g. infections requiring prolonged antibiotic or immunotherapy treatment) or severe trauma as judged by the investigator * Patients with positive serum test results at screening (HBs Ag, HCV Ab, HIV Ab). However, patients with a positive HCV Ab result may be eligible if they have achieved HCV RNA negativity after treatment * Patients at screening (Visit 1) with laboratory test results falling under the following criteria: * HbA1c ≥ 7%(Despite medication treatment) * ALT and AST ≥ 5 times the upper limit of normal (ULN) * eGFR (CKD-EPI\_ \< 30 ml/ min/1.73m² * Serum potassium \> 5.5 mEq/L * LDL-C \>190mg/dL or TC \> 500mg/dl (Despite medication treatment) * Patients with hypersensitivity reactions to the components of the investigational durg or its formulations * Treatment with topical corticosteroids, local calcineurin inhibitors, topical Janus kinase inhibitors, Tars, antibiotic cream, topical or oral antihistamines within the past 7 days prior to baseline * Patients who have undergone sunbathing, tanning bed use, ultraviolet B (UVB) therapy, or PUVA (psoralen + ultraviolet A (UVA)) therapy for relief of atopic dermatitis symptoms within the 14 days prior to baseline * Treatment with systemic immunosuppressants/immunomodulators and/or systemic corticosteroids within the 28 days prior to baseline (patients who have used stable doses of inhaled corticosteroids or nasal sprays for more than 3 months are eligible to participate) * Patients who have received treatment with biologics other than dupilumab within 3 months prior to baseline. * Patients who have received treatment with dupilumab within 3 months prior to baseline * Patients with a history of drug or alcohol abuse within 1 year prior to screening * Pregnant or breastfeeding women. * Women who test positive for pregnancy at screening or who plan to become pregnant during the clinical trial period and up to one month after the end of the investigational drug administration, or who do not agree to use appropriate contraception\*, and men. * Appropriate contraception: complete abstinence, hormonal contraceptives without known drug interactions \[levonorgestrel intrauterine system (IUS) (Mirena), medroxyprogesterone\], surgical sterilization (vasectomy, bilateral tubal ligation, etc.). Note that periodic abstinence (e.g., ovulation timing, symptothermal method, or post- ovulation) or withdrawal is not considered appropriate contraception. * Patients who have received any investigational drug within 6 months prior to screening (those who have not received an investigational drug or who have participated in non-interventional observational studies are eligible) * Other patients deemed unsuitable for participation in this clinical trial by the investigator.