Main Inclusion Criteria:

          -  Histologically- or cytologically-confirmed locally advanced or metastatic solid tumor
             with documented recurrence or disease progression from standard anticancer therapy in
             the advanced/metastatic setting

          -  No standard therapy available or standard therapy is considered unsuitable or
             intolerable according to the Investigator and consultation with the Medical Monitor

        Phase 1 Only:

          -  Solid tumor patients with alterations that may be associated with antitumor activity
             based on preclinical data for BDTX-189 such as:

               1. Allosteric HER2 or HER3 mutation(s)

               2. EGFR or HER2 exon 20 insertion mutation(s)

               3. HER2 amplified or overexpressing tumors

               4. EGFR exon 19 deletion or L858R mutation

        Phase 2 Only:

          -  Patients with a solid tumor harboring an:

               1. Allosteric HER2 mutation (including but not limited to S310F/Y, R678Q, L755S/P,
                  V777L, V842I)

               2. EGFR or HER2 exon 20 insertion mutation

        Eligible mutations must be determined by a validated next-generation sequencing (NGS) test
        routinely used by each institution and performed in a CLIA-certified or equivalent
        laboratory.

          -  Adequate archival tumor tissue or willing to undergo pretreatment biopsy

          -  Measurable disease according to RECIST version 1.1

        Main Exclusion Criteria:

          -  Clinical laboratory values meeting the following criteria within 4 weeks (28 days)
             prior to baseline:

               1. Serum creatinine ≥1.5 × upper limit of normal (ULN) or calculated creatinine
                  clearance ≤60 mL/min using Cockcroft-Gault equation

               2. Total bilirubin ≥1.5 × ULN or ≥3.0 × ULN in the presence of documented Gilbert's
                  syndrome

               3. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) ≥2.5 × ULN, or
                  AST or ALT ≥5.0 × ULN in the presence of liver metastases

               4. Hematologic function:

                    1. Absolute neutrophil count (ANC) ≤1000 cells/μL

                    2. Hemoglobin ≤8.5 g/dL or 5.28 mmol/L

                    3. Platelet count ≤75,000/μL

          -  Significant cardiovascular disease, including:

               1. Cardiac failure New York Heart Association Class III or IV, or left ventricular
                  ejection fraction (LVEF) <50% or below the lower limit of the Institution's
                  normal range

               2. Myocardial infarction, severe or unstable angina within 6 months prior to
                  baseline

               3. Significant thrombotic or embolic events within 3 months prior to baseline

               4. History or presence of any uncontrolled cardiovascular disease

               5. Personal or family history of long QT syndrome

          -  ECG findings meeting any of the following criteria:

               1. Evidence of second- or third-degree atrioventricular block

               2. Clinically significant arrhythmia (as determined by the Investigator)

               3. QTcF interval of >470 msec

          -  Leptomeningeal or untreated and/or symptomatic CNS malignancies (primary or
             metastatic)

          -  Women who are pregnant or breast-feeding

          -  Taking or unable to discontinue proton pump inhibitors within 1 week prior to baseline

          -  Known concurrent KRAS mutation

          -  Known tumor-harboring resistance mutations including EGFR T790M or C797S mutations or
             HER2 C805S mutation

        Phase 2 Only:

        - Prior documented treatment response to approved or investigational HER2 or EGFR tyrosine
        kinase inhibitor therapies