Inclusion Criteria:

          1. Male of female subjects who are at least 18 years of age (inclusive) at the time of
             signing the Informed Consent Form (ICF).

          2. Have at least one measurable lesion per RECIST v1.1.

          3. Have formalin-fixed paraffin-embedded (FFPE) tissue available from a metastatic
             lesion; a tumor block or 25 unstained slides from an archived (within 2 years) or
             fresh tumor samples (core or punch needle biopsy) are acceptable.

          4. Documented tumor progression following no more than 3 lines of systemic chemotherapy,
             PARP inhibitor therapy or immunotherapy for metastatic disease, as appropriate. Of
             note, neoadjuvant and adjuvant therapy will not count as prior lines of therapy.

          5. Histologically confirmed diagnosis of inoperable locally advanced or metastatic TNBC
             defined as ER and progesterone receptor staining <10%, and HER2 negative defined as
             IHC 0 to 1+

          6. Documented Notch activation from tumor biopsy results from within the last 2 years
             from a commercially available NGS assay, LDT or other validated IUO clinical trial
             assay.

          7. Women of childbearing potential (WOCBP) must have a negative serum or urine pregnancy
             test.

        Exclusion Criteria:

          1. A known additional malignancy that is progressing or requires active treatment that is
             considered medically active and may interfere in the ability to detect responses in
             this subject. Exceptions include basal cell carcinoma of the skin, squamous cell
             carcinoma of the skin that have undergone potentially curative therapy or in situ
             cervical cancer.

          2. BC that, in the opinion of the investigator, is considered amenable to potentially
             curative treatment.

          3. Symptomatic central nervous system (CNS) metastases.

          4. Current or recent (within 2 months of IP administration) gastrointestinal (GI) disease
             or disorders that increase the risk of diarrhea, such as inflammatory bowel disease
             and Crohn's disease.

          5. Developed immune-mediated colitis with immunotherapy unless resolved to G1 or lower
             and without requirement of steroid treatment for at least 14 days prior to first dose
             of IP.

          6. Peripheral neuropathy Grade 2 for at least 14 days prior to first dose of IP.

          7. Evidence of uncontrolled, active infection, requiring systemic anti-bacterial,
             anti-viral or anti-fungal therapy ≤7 days prior to administration of IP such as known
             active infection with hepatitis B, hepatitis C, or human immunodeficiency virus (HIV).

          8. Unstable or severe uncontrolled medical condition (e.g., unstable cardiac or pulmonary
             function or uncontrolled diabetes) or any important medical illness or abnormal
             laboratory finding that would, in the investigator's judgment, increase the risk to
             the subject associated with his or her participation in the study.

          9. Eastern Cooperative Oncology Group (ECOG) performance status ≥2.

         10. Abnormal organ and marrow function defined as:

               1. neutrophils <1000/mm3,

               2. platelet count <75,000/mm3,

               3. hemoglobin <8 g/dL,

               4. total bilirubin >1.5 upper limit of normal (ULN) (except known Gilbert's syndrome
                  whereby the total bilirubin must be < 5 mg/dL),

               5. aspartate aminotransferase (AST) and alanine aminotransferase (ALT) >2.5 ULN OR
                  >5 ULN for subjects with liver metastases,

               6. creatinine clearance (CrCl) <50 mL/min (calculation of CrCl will be based on
                  acceptable institution standard),

               7. uncontrolled triglyceride ≥Grade 2 elevations per CTCAE v5.0 (>300 mg/dL or >3.42
                  mmol/L).

         11. Myocardial infarction within 6 months prior to enrollment or has New York Heart
             Association (NYHA) Class III or IV heart failure, uncontrolled angina, severe
             uncontrolled ventricular arrhythmias, or electrocardiographic evidence of acute
             ischemia or active conduction system abnormalities.

         12. Mean QT interval corrected for heart rate using Fridericia's formula (QTcF) ≥480 msec.

         13. Completed palliative radiation therapy < 7 days prior to initiating IP.

         14. Prior treatment with gamma secretase inhibitors.

         15. Last chemotherapy, biologic, or investigational therapy agent at least 4 weeks or 5
             half-lives (whichever is shorter) prior to initiating IP; at least 6 weeks if the last
             regimen included BCNU or mitomycin C. Prior treatment with investigational monoclonal
             antibody will be reviewed case-by-case by the Sponsor.

         16. Receiving chronic systemic steroid therapy (in dosing exceeding 10 mg/day of
             prednisone or equivalent) or any other form of immunosuppressive therapy within 7 days
             prior to the first dose of IP. The use of physiologic doses of corticosteroids may be
             approved after consultation with the Sponsor.

         17. Use of strong inhibitors of CYP3A4 within 1 week or 5 half-lives (whichever is longer)
             or strong inducers of CYP3A4 within 2 weeks or 5 half-lives (whichever is longer).

         18. Life expectancy is less than 3 months.