This is a Phase 1, open-label, randomized, crossover clinical trial designed to evaluate the potential drug-drug interaction (DDI) between R1_PBK_M2301 (levodropropizine 60 mg) and R2_PBK_M2301 (Pelargonium sidoides ethanol extract 11%) in healthy adult volunteers. The study aims to assess the pharmacokinetics of each investigational drug when administered alone and in combination. Approximately [insert number] healthy subjects will participate in two treatment periods with appropriate washout intervals. Safety, tolerability, and pharmacokinetic parameters will be evaluated to support future combination development.

This Phase 1 clinical trial is designed to assess potential drug-drug interactions between R1_PBK_M2301 (levodropropizine 60 mg) and R2_PBK_M2301 (Pelargonium sidoides 11% ethanol extract) in healthy adult volunteers. The study adopts an open-label, randomized, two-period crossover design. Each participant will receive both investigational products in different periods, with a washout interval between treatments.

The primary objective is to compare the pharmacokinetic parameters (e.g., C_max, AUC) of the individual drugs when administered alone versus in combination. Secondary objectives include evaluating safety and tolerability through clinical assessments, vital signs, ECGs, and laboratory tests.

Approximately 24 healthy adults will be enrolled. Each subject will be screened for eligibility based on predefined inclusion and exclusion criteria. Eligible participants will be randomly assigned to one of two sequence groups. Blood samples will be collected at predetermined time points to analyze plasma concentrations of the study drugs.

The findings of this study are expected to support further clinical development and regulatory planning for the combination use of levodropropizine and Pelargonium sidoides extract.

<Inclusion Criteria>

1. Healthy male or female adults aged ≥19 and <65 years at the time of screening.

2. Body mass index (BMI) between 18 and 30 kg/m² (BMI = weight [kg] / height² [m²]);

   • Male participants must weigh at least 50 kg.
   • Female participants must weigh at least 45 kg.

3. No clinically significant congenital or chronic diseases, and no abnormal findings from a medical examination (e.g., electroencephalogram, electrocardiogram, chest and/or gastrointestinal endoscopy, or radiologic examinations, if deemed necessary).

4. Laboratory test results (including hematology, clinical chemistry, coagulation tests, serology, urinalysis) and ECG results deemed suitable for study participation by the investigator (or sub-investigator).

5. Willing and able to voluntarily provide written informed consent after receiving detailed information about the study, and agrees to comply with study requirements.

6. Agrees to use medically accepted contraception methods* from the first administration of the investigational product through 1 week after the final administration, and agrees not to donate sperm or ova during this period.

   • Medically accepted contraception includes intrauterine devices (IUD, IUS), sterilization (vasectomy or tubal ligation), or combination of barrier methods (e.g., male/female condoms, cervical caps, diaphragms, sponges), possibly with spermicide.

<Exclusion criteria>

1. Use of enzyme-inducing or enzyme-inhibiting drugs such as barbiturates within 30 days prior to the first dose, or use of any medication that may interfere with the study within 10 days prior to the first dose.

2. Participation in another clinical trial involving administration of an investigational drug or bioequivalence study within 6 months prior to the first dose.

3. Donation of whole blood within 8 weeks, donation of blood components within 2 weeks, or receipt of blood transfusion within 4 weeks prior to the first dose.

4. History of gastrointestinal surgery that may affect drug absorption (excluding appendectomy and hernia repair).

5. Within 1 month prior to the first dose, meets one of the following:

   • Alcohol consumption >21 drinks/week (males) or >14 drinks/week (females);

     (1 drink = 50 mL soju, 250 mL beer, or 30 mL whiskey)

   • Smoking >20 cigarettes/day on average.

6. Individuals with any of the following conditions:

   • Known hypersensitivity to the investigational product or its components
   • Increased bronchial mucus secretion
   • Mucociliary dysfunction (e.g., Kartagener syndrome, primary ciliary dyskinesia)
   • Bleeding tendency or use of anticoagulants
   • Severe hepatic impairment
   • Severe liver or kidney disease
   • Genetic disorders such as galactose intolerance, Lapp lactase deficiency, or glucose-galactose malabsorption

7. History of clinically significant psychiatric illness.

8. Any other condition deemed by the principal investigator (or sub-investigator) to render the subject unsuitable for study participation.

9. Female participants who are pregnant, suspected of being pregnant, or breastfeeding.

Individual participant data (IPD) will not be shared due to internal policy and data protection regulations. The dataset is limited to a small population of healthy volunteers and is intended solely for internal analysis and regulatory submission.

This is an open-label study. No parties, including participants, investigators, or outcomes assessors, are masked to intervention assignments.