Berzosertib + Topotecan in Relapsed Platinum-Resistant Small-Cell Lung Cancer (DDRiver SCLC 250)

Inclusion Criteria: * Dose level 1 participants with histologically proven advanced solid tumors, for which no effective standard therapy exists, or standard therapy has failed or cannot be tolerated * Dose level 1 participants with Eastern Cooperative Oncology Group Performance Status (ECOG PS) less than or equal to (\<=) 1 and Karnofsky Scale greater than or equal to (\>=) 70 percent (%) * Dose level 2 and main part participants with ECOG PS \<= 2 and Karnofsky Scale \>= 60% * Dose level 2 and main part participants with histologically confirmed SCLC * Dose level 2 and main part participants with radiologically confirmed progression after first-line or chemoradiation platinum-based treatment (carboplatin or cisplatin), with or without immunotherapy, for treatment of limited or extensive stage SCLC, with a Platinum-free interval (PFI) less than (\<) 90 days. The PFI is measured by the elapsed time from the last day of the regimen of a platinum-based treatment until the first day of documented disease progression * Dose level 2 and main part participants with measurable disease according to Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1 (RECISTv1.1) at Screening. Evidence of measurable disease must be confirmed by the IRC prior to start of treatment * Tumor tissue provision: archival (collected within 12 months before date of informed consent form \[ICF\]) signature for Screening) or fresh biopsy specimen, if medically feasible * Have adequate hematologic …