Inclusion Criteria:

          -  Written informed consent, according to local guidelines, signed and dated by the
             patient or legal guardian prior to the performance of any study-specific procedures,
             sampling, or analyses.

          -  Male or female and ≥18 years-of-age at the time of signature of the ICF.

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 or 1.

          -  Histologically confirmed solid tumors resistant or refractory to standard treatment
             and/or patients who are intolerant to standard therapy.

          -  Measurable disease as per RECIST v1.1

          -  Existing biomarker profile (tumor tissue or plasma) reported from a local test
             obtained in a certified lab per institutional guidelines:

          -  Available tumor tissue or willingness to have a biopsy performed to obtain tissue.

          -  Ability to comply with the protocol and study procedures detailed in the Schedule of
             Assessments.

          -  Ability to swallow and retain oral medications.

          -  Acceptable organ function at screening

          -  Acceptable blood counts at screening

          -  Negative pregnancy test (serum or urine) for women of childbearing potential at
             Screening and prior to first study drug. Women who are not of childbearing potential
             is defined as 1) adequate time with absence of menses (period) or 2) documented
             infertility.

          -  Resolution of all toxicities of prior treatment or surgery.

          -  Male patients with female partners of childbearing potential and women of childbearing
             potential must follow a contraception method (oral contraceptives allowed) during
             their participation in the study and for at least 4 months following last dose of
             study drug. Male patients must also refrain from donating sperm during their
             participation in the study and for 4 months following last dose of study drug.

        Exclusion Criteria:

          -  Chemotherapy, small molecule anticancer or biologic anticancer therapy given within 14
             days prior to first dose of study drug.

          -  History or current condition (such as transfusion dependent anemia or
             thrombocytopenia), therapy, or laboratory abnormality that might confound the study
             results, or interfere with the patient's participation for the full duration of the
             study treatment.

          -  Prior therapy with an ATR or DNA-dependent protein kinase (DNA-PK) inhibitor.

          -  Known hypersensitivity to any of the ingredients of RP-3500.

          -  Life-threatening illness, medical condition, active uncontrolled infection, or organ
             system dysfunction or other reasons which, in the investigator's opinion, could
             compromise the patient's safety.

          -  Uncontrolled, symptomatic brain metastases.

          -  Uncontrolled high blood pressure

          -  Patients with active, uncontrolled bacterial, fungal, or viral infection, including
             hepatitis B virus (HBV), hepatitis C virus (HCV), known human immunodeficiency virus
             (HIV) or acquired immunodeficiency syndrome (AIDS) related illness.

          -  Moderate or severe hepatic impairment (ie, Child-Pugh class B or C).

          -  History or presence of an abnormal ECG that is clinically significant in the
             investigator's opinion.

          -  History of ventricular dysrhythmias or risk factors such as structural heart disease,
             coronary heart disease (clinically significant electrolyte abnormalities or family
             history of sudden unexplained death or long QT syndrome

          -  Current treatment with medications that are well-known to prolong the QT interval

          -  History of myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) diagnosis