A Study of TAS-120 in Patients With Metastatic Breast Cancer

Open to people ages 18 years and up

1. Provide written informed consent
2. Age ≥ 18 years of age
3. Histologically or cytologically confirmed recurrent locally advanced or metastatic breast cancer not amenable to treatment with curative intent, and the following cohort specific criteria:
   1. Cohort 1
   2. HR+ HER2- breast cancer harboring an FGFR2 gene amplification.
   3. Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1
   4. Has received 1-3 prior endocrine-containing therapies and up to 2 prior chemotherapy regimens for advanced/metastatic disease
   5. Has received prior treatment with a CDK4/6 inhibitor or is ineligible for such treatment
   6. Cohort 2
   7. TNBC harboring an FGFR2 gene amplification
   8. Measurable disease per RECIST 1.1
   9. Has received at least 1 prior chemotherapy or chemotherapy/immunotherapy (PD-L1/PD-1 inhibitors) regimen for advanced/metastatic disease
   10. Cohort 3
   11. TNBC or HR+ HER2- breast cancer harboring an FGFR2 gene amplification
   12. Non measurable, evaluable disease per RECIST 1.1. Patients with bone-only disease must have lytic or mixed lytic-blastic lesions
   13. Other criteria for either HR+ HER2- breast cancer or TNBC should be met as described for Cohort 1 and 2, respectively
   14. Cohort 4
   15. HR+ HER2- breast cancer harboring an FGFR1 high-level gene amplification
   16. Measurable disease per RECIST 1.1
   17. Has received 1-2 prior endocrine-containing therapies and no more than 1 prior chemotherapy regimen for advanced/metastatic disease. Prior treatment with fulvestrant is not permitted.
   18. Has received prior treatment with a CDK4/6 inhibitor or is ineligible for such treatment
   19. Pre/peri-menopausal patients must be on goserelin
4. Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1
5. Archival or (preferably) fresh tumor tissue must be available
6. Adequate organ function

1. History and/or current evidence of any of the following disorders:
   1. Non-tumor related alteration of the calcium-phosphorus homeostasis that is considered clinically significant
   2. Ectopic mineralization/calcification, including but not limited to soft tissue, kidneys, intestine, or myocardia and lung, considered clinically significant
   3. Retinal or corneal disorder confirmed by retinal/corneal examination and considered clinically significant
2. Prior treatment with an FGFR inhibitor
3. A serious illness or medical condition(s)
4. Brain metastases that are untreated or clinically or radiologically unstable
5. Pregnant or lactating female