Inclusion Criteria:

          -  Body weight ≥10 kilograms and body surface area (BSA) ≥0.5 meters squared.

          -  Participants with any relapsed/refractory malignant solid tumor (excluding lymphoma),
             including central nervous system tumors, that have progressed on standard therapies
             and, in the judgment of the investigator, are appropriate candidates for the
             experimental therapy combination in the study part that is currently enrolling.

               -  Participants must have at least one measurable (per Response Criteria in Solid
                  Tumors [RECIST v1.1; [Eisenhauer et al. 2009] or Response Assessment in
                  Neuro-Oncology (RANO) for central nervous system (CNS) tumors [Wen et al. 2010])
                  or evaluable lesion.

               -  Participants must have had histologic verification of malignancy at original
                  diagnosis or relapse, except:

                    -  Participants with extra-cranial germ-cell tumors who have elevations of
                       serum tumor markers including alpha-fetoprotein or beta- human chorionic
                       gonadotropin (HCG).

                    -  Participants with intrinsic brain stem tumors or participants with CNS-germ
                       cell tumors and elevations of CSF or serum tumor markers including
                       alpha-fetoprotein or beta-HCG.

          -  A Lansky score ≥50 for participants ≤16 years of age or Karnofsky score ≥50 for
             participants >16 years of age.

          -  Participants must have discontinued all previous treatments for cancer or
             investigational agents and must have recovered from the acute effects to Grade ≤1 at
             the time of enrollment.

          -  Able to swallow.

          -  Adequate hematologic and organ function ≤2 weeks (14 days) prior to first dose of
             study drug.

          -  Females of reproductive potential must have negative serum pregnancy test at baseline
             (within 7 days prior to starting treatment).

          -  Both female and male participants of reproductive potential must agree to use highly
             effective contraceptive precautions (and avoid sperm donation for males) during the
             trial. For abemaciclib, females should use contraception for at least 3 weeks
             following the last abemaciclib dose (males have no restriction for contraceptive use
             following treatment with abemaciclib). For other study drugs, highly effective
             contraceptive precautions (and avoiding sperm donation) must be used according to
             their label.

          -  Life expectancy of at least 8 weeks and able to complete at least 1 cycle of
             treatment.

          -  Caregivers and participants willing to make themselves available for the duration of
             the trial.

        Exclusion Criteria:

          -  Received allogenic bone marrow or solid organ transplant.

          -  Received live vaccination (within 4 weeks prior to starting study treatment).

          -  Have a personal history of any of the following conditions within the last 12 months:
             syncope of cardiovascular etiology, ventricular tachycardia, ventricular fibrillation,
             or sudden cardiac arrest.

          -  Intolerability or hypersensitivity to any of the study treatments or its components.

          -  Diagnosed and/or treated additional malignancy within 3 years prior to enrollment that
             may affect the interpretation of results, with the exception of curatively treated
             basal cell carcinoma of the skin, squamous cell carcinoma of the skin, and/or
             curatively resected in situ cervical and/or breast cancers.

          -  Pregnant or breastfeeding.

          -  Active systemic infections or viral load.

          -  Serious and/or uncontrolled preexisting medical condition(s) that would preclude
             participation in this study.

          -  Have a bowel obstruction (Part A of this study only).

          -  Treated with drugs known to be strong inhibitors or inducers of isoenzyme cytochrome
             P450 3A (CYP3A) or strong inhibitors of uridine diphosphate-glucuronosyl transferase
             1A1 (UGT1A1) if the treatment cannot be discontinued or switched to a different
             medication at least 5 half-lives prior to starting study drug.

          -  Received prior treatment with cyclin-dependent kinase (CDK) 4 & 6 inhibitor.

          -  Currently enrolled in any other clinical study involving an investigational product or
             non-approved use of a drug or device.

          -  Has received an experimental treatment in a clinical trial within the last 30 days or
             5 half-lives, whichever is longer.

          -  Tumor contains known somatic or germline retinoblastoma (RB) mutation.