This is a study to investigate the potential clinical benefit of roniciclib when given in combination with chemotherapy Carboplatin / Etoposide or Cisplatin / Etoposide as first line treatment in patients with extensive disease small cell lung cancer. Approximately 140 patients will be randomized (1:1) to receive treatment with either roniciclib or placebo in combination with chemotherapy.

Roniciclib is an oral (i.e. taken by mouth) protein kinase inhibitor. A kinase inhibitor targets certain key proteins that are essential for the survival of the cancer cell. The growth of the tumor may be decreased by preventing these specific proteins from functioning. By specifically targeting these proteins, roniciclib in combination with chemotherapy may stop cancer growth.

The primary endpoint (the most meaningful result to be tracked) of this study is based on the progression free survival, i.e. the time the disease is not worsening. The aim is to show that the therapy with roniciclib in combination with chemotherapy prolongs the time the disease is not worsening in this patient population compared to patients receiving placebo in combination with chemotherapy.

Inclusion Criteria:

• Male or female subjects aged ≥18 years (or country-specific legal age of maturity, if >18 years)
• Histologically or cytologically confirmed (extensive-stage disease) ED SCLC (small cell lung cancer)
• At least 1 solid tumor lesion measurable by computer tomography (CT) scan or magnetic resonance imaging (MRI) according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 - 1

Exclusion Criteria:

• Prior systemic anticancer therapy for SCLC (including previous therapy with a cyclin-dependent kinase [CDK] inhibitor)