Phase 1/2 Multicenter, Open-label, Dose-escalation Study of IDP- 121 in Patients With Platinum-resistant Small Cell Lung Cancer (MYCrocytic)

1. Age ≥18 years
2. Performance status (ECOG) ≤ 2
3. Life expectancy ≥2 months
4. Patient is, in the investigator's opinion, willing and able to comply with the protocol requirements.
5. Patient has given voluntary written informed consent before performance of any study- related procedure not part of normal medical care, with the understanding that consent may be withdrawn by the patient at any time without prejudice to their future medical care.
6. Patients diagnosed with advanced/extensive stage SCLC that have platinum-sensitive relapse after 1st line treatment defined as a Progression Free Interval (PFI) of ≥ 3 months (platinum-based chemotherapy plus IDP-121 arm) and patients with a PFI \< 3 months (topotecan plus IDP-121 arm).

7. Adequate haematological or biochemical parameters as specified below

   a. b. Haemoglobin > 8.0 g/dl (without transfusion support within 7 days) Platelets count > 100 x109/L (without transfusion support within 7 days). c. Absolute neutrophil count (ANC) > 1.5 x109/L (without G-CSF support within 7 days) d. Aspartate transaminase (AST): \<2.5 x the upper limit range. e. Alanine transaminase (ALT): \< 2.5 x the upper limit range. f. Total bilirubin: \< 2 x the upper limit range. g. Calculated or measured creatinine clearance: ≥ 60 mL/min (calculated from the Cockcroft-Gault formula).

8. Left ventricular ejection fraction > 50% or above the Institutional Lower Limit of Normal (LLN), whichever is lower, determined by echocardiogram.

Exclusion Criteria:

1. Persistent clinically significant or grade 3 or 4 non-haematological toxicity related to previous treatments. The presence of alopecia and NCI-CTC grade \<2 symptomatic peripheral neuropathy is allowed.
2. Active CNS metastases and/or carcinomatous meningitis. Participants with previously treated brain metastases (e.g., whole brain radiation treatment [WBRT], stereotactic radiosurgery, or equivalent) may participate only if they satisfy the following:

3. 4. 5. 6. 7. 8. 9. • Completed treatment at least 14 days prior to the first dose of study intervention.

   • Are clinically stable, without requirement of steroid treatment > 10 mg or prednisone or equivalent for at least 7 days prior to first dose of study intervention. Pregnant or lactating women; men and women of reproductive potential* (as defined in the Appendix 2) who are not using effective contraceptive methods (combined hormonal contraception associated with inhibition of ovulation; progestogen-only hormonal contraception associated with inhibition of ovulation, intrauterine device, intrauterine hormone-releasing system, bilateral tubal occlusion, vasectomized partner, sexual abstinence).

     *A woman is considered of childbearing potential (WOCBP), i.e. fertile, following menarche and until becoming post-menopausal unless permanently sterile. A man is considered fertile after puberty unless permanently sterile by bilateral orchidectomy.

   History of any other neoplastic disease in the last five years (except basal cell carcinoma, skin epithelioma or carcinoma in situ of any site) History of clinically significant hypotension. History of clinically significant allergic or hyper-sensitivity reactions. History or known clinically significant vascular disease or known high risk of vascular disease (as assessed by the treating physician) including (but not limited to): - Thromboembolism - Peripheral arterial disease

   - Vasculitis

   Other relevant diseases or adverse clinical conditions:

   - Congestive heart failure or angina pectoris, myocardial infarction within 12 months before inclusion in the study.

   • Uncontrolled arterial hypertension or cardiac arrhythmias (i.e., requiring a change in medication within the last 3 months or hospital admission within the past 6 months).
   • History of significant neurological or psychiatric disorders Clinically significant or active infection.

10. Significant non-neoplastic liver disease (e.g., cirrhosis, active chronic hepatitis) 11. The patient is known to be human immunodeficiency virus (HIV) positive, unless the patient is on antiviral therapy with HIV RNA levels \<50 copies/mL; Hepatitis B surface antigen-positive or active hepatitis C infection, unless treated with undetectable hepatitis B DNA or hepatitis C RNA levels; or active CMV infection (IgM positive). 12. Concomitant anti-tumour therapy within 14 days prior to Day 1 of Cycle 1. 13. Prior allogeneic transplantation in the last 3 months or currently active GVHD with immunosuppressive treatment 14. Limitation of the patient's ability to comply with the treatment or follow-up protocol.

15. If a COVID-19 vaccine is administered, it should be done >72 hours prior to study treatment initiation or after the completion of the dose-limiting toxicity (DLT) period (if patient is participating in the dose-escalation phase").