ATR Inhibitor Elimusertib (BAY1895344) Plus Niraparib Phase 1b Study in Advanced Solid Tumors and Ovarian Cancer

Inclusion Criteria: * Participant must be ≥ 18 years of age, at the time of signing the informed consent. * Participants must have histologically confirmed diagnosis of the following indications as described below: * Dose escalation (Part A): recurrent advanced solid tumors, excluding prostate cancer, who experienced disease progression after treatment with standard of care therapy for metastatic disease. * Dose expansion (Part B): recurrent EOC, fallopian tube or primary peritoneal cancer * Sub-population 1: participants PARPi naïve and with a platinum-resistant/refractory disease (recurrence with a PFI \< 6 months from last platinum-based regimen). Participants may not have had more than 3 prior therapies since the development of platinum resistance. * Sub-population 2: participants with disease progression on PARPi (including niraparib), administered as maintenance as well active line of therapy. Participants must have not received further line of therapy after disease progression on PARPi. * Participants in dose escalation (Part A) of the study will need to have tumor-associated DDR deficiency and/or CCNE1 gene amplification. \-- A homozygous deletion and/or a deleterious mutation in a gene reported to be involved in DNA repair and/or sensitive to ATRi's and/or PARPi's. * Participants in dose expansion (Part B) of the study will need to have tumor associated DDR deficiency (Sub-population 1). Participants in Part B (Sub-population 2) are not enrolled based on the presence or absence of a particular biomarker. * Participants must have disease progression and measurable disease, as defined by RECIST 1.1. * Available archival tumor tissue ≤ 12 months old, otherwise a fresh baseline tumor biopsy should be obtained. * ECOG PS of 0 to 1 * Life expectancy of at least 12 weeks * Adequate bone marrow function as assessed by the following laboratory tests to be conducted within 7 (±2) days before the first dose of study intervention: * Hemoglobin (Hb) ≥ 10 g/dL * Platelet count ≥ 150 x 10\^9/L * Absolute neutrophil count (ANC) ≥ 1.5 x 10\^9/L * Participants must have adequate organ function. * Participants must have adequate coagulation. * Adequate cardiac function per institutional normal measured by echocardiography (recommended) or cardiac MRI per institutional guidelines. * A female participant is eligible to participate if she is not pregnant (confirmed by a negative serum pregnancy test within 7 (±2) days of first study intervention), not breastfeeding, or is not a woman of childbearing potential (WOCBP). WOCBP must agree to use highly effective contraception during the intervention period and for at least 6 months (180 days) after the last dose of study intervention. Exclusion Criteria: * Inability to swallow oral medication * Known hypersensitivity to elimusertib and/or niraparib or excipients of the preparations or any agent given in association with this study * History of myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) diagnosis * Ongoing or active uncontrolled infection (bacterial, fungal, or viral; e.g. hepatitis viral) of CTCAE grade ≥ 2 that requires systemic treatment * Participants with HIV may be be ineligible depending on various parameters, but are not automatically excluded. * Immunocompromised participants (e.g. diagnosis of immunodeficiency or ongoing immunosuppressive therapy) * Pleural effusion or ascites that causes respiratory compromise (CTCAE grade ≥ 2 dyspnea) * Active HBV or HCV infection that requires treatment. * Moderate or severe hepatic impairment, i.e. Child Pugh Class B or C * Participants with significant cardiovascular disease and/or relevant findings meeting the below criteria are excluded: * History of cardiac disease: congestive heart failure NYHA class \> II, unstable angina (angina symptoms at rest), new-onset angina (within the past 6 months before study entry), myocardial infarction within the past 6 months before study entry, or cardiac arrhythmias requiring anti-arrhythmic therapy (beta-blockers, calcium channel blockers, and digoxin are permitted). * Clinically relevant findings in the ECG such as a second- or third-degree atrioventricular block, prolongation of the QRS complex ≥ 120 ms, or prolongation of the of the QTc interval (Fridericia) over 450 ms unless agreed otherwise between the investigator and the sponsor's medically responsible person. QTc \> 450 ms detected in 2 or more time points within a 24-hour period are excluded. * Clinically significant arterial hypertension despite optimal medical management (per investigator´s opinion). Clinically significant hypertension defined as systolic blood pressure above 150 mmHg and/or diastolic blood pressure above 90 mmHg, despite optimal medical management. For participants taking antihypertensive medication, blood pressure should be stable/ controlled for more than 7 days before first dose of study medication. * Previous treatment with an ATR Inhibitor * Participants in Part A and Part B (Sub-population 2): Previous treatment with known or putative PARPi, if discontinued for CTCAE grade ≥ 3 AEs or CTCAE grade ≥ 3 hypersensitivity to PARPi. Participants in Part B Sub-population 1 must not have received prior PARPi treatment.