SNB-101 for Treatment of Extensive Stage Small Cell Lung Cancer

Inclusion Criteria: 1. Male or female patients over 18 years of age. 2. Male or fertile female patients who have agreed to comply with effective contraceptive methods as required by this protocol and the specified contraceptive period. 3. Participants must have a cytologically or histologically confirmed small cell lung cancer which is locally advanced or metastatic and has progressed on or after standard therapy for advanced disease containing platinum-based therapy plus etoposide with or without atezolizumab or durvalumab immunotherapy and is not suitable for complete surgical resection. 4. Have measurable or evaluable disease consistent with RECIST (Response Evaluation Criteria in Solid Tumors) version 1.1 5. With life expectancy more than 12 weeks 6. Patients with Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1. 7. Have adequate hematological, renal, and liver function as defined by the following (lab test can be repeated during screening): 8. Have recovered to grade 1 or better to (CTCAE version 5.0) from any reversible side effects of previous treatments. Note: Grade 2 alopecia and Grade 2 sensory neuropathy are not exclusionary. 9. No major surgery, no antineoplastic or experimental therapy, and no direct radiation therapy to a hematopoietic site within 4 weeks prior to baseline, and no biologic drugs, nitrosoureas or mitomycin C administered within 6 weeks prior to baseline. 10. Fully informed regarding the investigational nature of the study protocol and is capable of signing an Institutional Review Board/Ethics Committee-approved informed consent form. Exclusion Criteria: 1. Patients in whom the homozygous variant alleles UGT1A1\*28 or UGT1A1\*6 are confirmed through UGT1A1 genotype testing at screening (or through previously conducted testing), or patients with heterozygous variant alleles UGT1A1\*6/\*28. (For Phase 1 only) 2. Female patients who are pregnant, lactating (breast feeding) or planning a pregnancy during the course of study. 3. Known or suspected intolerance or hypersensitivity to the main ingredient or any of the excipients of SNB-101. 4. Patients deemed unsuitable or ineligible for participation in this clinical trial as judged by the Principal Investigator. Medical History or Current Pathological Conditions and Diseases 5. Known uncontrolled symptomatic heart failure. 6. History of alcohol abuse or other substance abuse within 1 year prior to screening. 7. Have intestinal palsy or bowel obstruction. 8. Have chronic inflammatory bowel disease. 9. Are on dialysis. 10. Patients with any evidence of current ILD or pneumonitis or a prior history of interstitial lung disease (ILD) or non-infectious pneumonitis requiring high-dose glucocorticoids. 11. Patients with clinically significant congestive heart failure that is uncontrolled. 12. Have multiple ascites or pleural effusion. 13. Have hematologic malignancy (including lymphoma). 14. Have severe infections or other uncontrolled active infections that require the administration of antibiotics, antivirals, or other anti-infective agents that may affect safety or efficacy assessments during the clinical study, as determined by the investigator. 15. When the QTc interval exceeds 480 msec\* (same standard for both men and women). \* Fridericia's QT correction formula 16. Positive for Human Immunodeficiency Virus (HIV). 17. Patients with known active hepatitis B (e.g., HBsAg response positive and HBV DNA detected) or hepatitis C (e.g., anti-HCV positive and HCV RNA \[qualitatively\]) detected). 18. Central nervous system or brain metastases with clinically significant symptoms requiring systemic corticosteroids at least 2 weeks prior to baseline. (however, participants who have completed treatment and recovered from the acute phase of radiation therapy or surgery may participate in this study.). Patients Who Have Received, are Receiving, or Cannot Discontinue the Following Medications or Therapies. 19. Have received another investigational drug within the last 4 weeks prior to screening. 20. If additional anticancer therapy\* other than the investigational drug is required during the clinical trial participation period. (However, localized radiation therapy for palliation purposes, such as for bone pain, bronchial obstruction, or skin lesions, is allowed). 21. Surgery, radiation (chemotherapy), chemotherapy, targeted therapy (small molecule drugs, monoclonal antibodies), immunotherapy (biological agents), or hormone therapy, etc. 22. If strong CYP3A4 inducer was administered within 2 weeks before baseline : (Phenytoin, carbamazepine, rifamycin (rifampicin, rifabutin, rifapentine), phenobarbital, etc., Preparations containing St. John's wort. 23. Have received a strong CYP3A4 and/or UGT1A1 inhibitor (azole antifungal agents \[ketoconazole, fluconazole, itraconazole, miconazole, voriconazole, etc.\], macrolide antibiotics \[erythromycin, clarithromycin, etc.\], antiretroviral agents \[atazanavir, ritonavir, indinavir, lopinavir, nelfinavir, saquinavir, telaprevir\], sorafenib, diltiazem hydrochloride, nefazodone, gemfibrozil, nifedipine, etc., or grapefruit juice) within 1 week prior to baseline 24. Will require administration of neuromuscular blockers, peripheral muscle relaxants, etc. during the study : Non-depolarizing neuromuscular blockers (such as succinylcholine) and depolarizing neuromuscular blockers (such as rocuronium) 25. Will require lapatinib during the study. 26. When administration of live herbal medicine detoxified vaccine is required during the clinical trial participation period: Yellow fever vaccine, etc. 27. Are taking a drug during the study that in the judgment of the investigator will have a significant effect on liver metabolism or kidney excretion. However, taking such a drug is permitted if (a) it was taken more than 4 weeks prior to baseline. 28. If the participant is deemed to be taking medications (including over-the-counter drugs, herbs, or homeopathic remedies) that significantly affect the action, metabolism, and excretion of the IP, as well as its efficacy and safety of the IP, as determined by the investigator. (However, if the participant has been taking the same dose of medication for at least 1 week prior to baseline \[excluding drugs taken daily; if taken regularly or periodically, it is included\], they are exempt).