Key Inclusion Criteria:

          -  Subject has one of the following solid tumors which has progressed on or following at
             least one systemic therapy regimen administered for advanced or metastatic disease or
             for which no approved therapy exists:

               -  NTRK-fusion-positive, locally advanced (i.e., not amenable to surgical resection)
                  or metastatic solid tumor Note: Subjects with any grade of malignant glioma
                  previously treated with systemic therapy are eligible.

        Phase 1

          -  NTRK-gene amplified, locally advanced or metastatic solid tumor

          -  EWSR1-WT1-positive DSRCTs.

          -  Subjects with NTRK-fusion-positive solid tumors other than primary brain tumors must
             have previously received treatment with a TRK inhibitor, unless the subject does not
             have access to TRK-inhibitor therapy (e.g., no TRK inhibitor is marketed and available
             to the subject in the subject's country) or the subject has declined treatment with
             available marketed TRK inhibitors.

          -  Subjects with NTRK-gene amplified solid tumors, primary brain tumors or
             EWSR1-WT1-positive DSRCTs may have received prior treatment with a TRK inhibitor but
             this is not required.

        Phase 2

          -  Has measurable disease by RECIST v1.1 for subjects with non-brain primary tumors or
             RANO criteria for subjects with primary brain tumors.

          -  Subjects with non-brain primary tumors must have previously received treatment with a
             TRK inhibitor and a documented resistance mutation(s) (e.g., solvent front, gatekeeper
             or xDFG mutation). Archival tissue from a prior biopsy taken after the subject
             completed TRK inhibitor treatment but prior to additional systemic therapy may be used
             to meet this eligibility criterion with Medical Monitor approval.

          -  Subjects with primary brain tumors may have received prior treatment with a TRK
             inhibitor but this is not required. Biopsies of brain tumors are not required for
             eligibility.

        Key Exclusion Criteria:

          -  Cytotoxic chemotherapy, biologic agent, investigational agent, or radiation therapy ≤
             3 weeks prior to the first dose of PBI-200 (6 weeks for nitrosoureas).

               -  Subjects with either primary brain tumors or brain metastasis must have completed
                  brain radiation therapy 12 weeks prior to the brain MRI obtained within 4 weeks
                  of the first dose of PBI-200.

          -  Small-molecule kinase inhibitors or hormonal agents ≤ 14 days and within 5 half-lives
             prior to the first dose of PBI-200.