Inclusion Criteria:

          -  Participants must have histologically confirmed breast cancer that is metastatic or
             unresectable with the following:

               -  Estrogen receptor expression by immunohistochemistry greater than or equal to 10%

               -  ESR1 mutation identified using a Clinical Laboratory Improvement Amendments
                  (CLIA) certified assay via tumor biopsy tissue or circulating free DNA (cfDNA)

               -  human epidermal growth factor receptor 2 (HER2) negative

          -  Participants must have measurable disease as defined by RECIST 1.1 or evaluable
             bone-only disease with at least one lesion measuring 10 mm or greater in size.
             Participants with liver-only disease are not eligible due to the inherent hepatic
             uptake related to the radiopharmaceutical's hepatobiliary route of elimination.

          -  Participants must have received at least 1 prior line of non-tamoxifen containing
             endocrine therapy in the metastatic setting or have had progression within 12 months
             of adjuvant non-tamoxifen endocrine therapy.

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0-1 (See Appendix A)

          -  Life expectancy of greater than 12 weeks.

          -  Ability to take oral medications.

          -  Informed consent: participant must be informed of the investigational nature of the
             study and must be able to sign a written informed consent.

          -  Participants with central nervous system (CNS) metastases must be stable after therapy
             for CNS metastases (such as surgery, radiation, or stereotactic radiosurgery) for at
             least 1 month.

          -  Participants must have adequate normal organ and bone marrow function as defined
             below:

               -  Absolute neutrophil count >/= 1,000/mcL

               -  Hemoglobin >/= 9.0 g/dL

               -  Platelets >/= 100,000/mcL

               -  Total bilirubin </= 1.5 x upper limit of normal (ULN)

               -  AST (SGOT)/ ALT (SGPT) </= 2.5 x ULN; </= 5 x ULN in the setting of metastatic
                  liver disease

               -  Creatinine </= 1.5 x ULN or creatinine clearance >/= 50 mL/min

        Exclusion Criteria:

          -  Participants must have received at least 1 prior line of non-tamoxifen containing
             endocrine therapy in the metastatic setting or have had progression either while
             taking or within 12 months of adjuvant non-tamoxifen endocrine therapy (i.e. on or
             within 12 months of an aromatase inhibitor).

          -  Prior chemotherapy, radiotherapy, targeted, immunotherapy or investigational therapy
             within 2 weeks or major surgery within 4 weeks of study enrollment or those who have
             not recovered (to grade ≤ 1 or baseline) from clinically significant adverse events
             due to agents administered more than 2 weeks earlier (alopecia and fatigue excluded).

          -  Participants must not be receiving an ER blocking endocrine therapy (includes
             fulvestrant, tamoxifen, toremifene, raloxifene) and must be off the agents for a
             minimum of 60 days prior to planned FES PET/CT to allow for adequate uptake of FES.

          -  History of allergic reactions attributed to compounds of chemical or biologic
             composition similar to those of tamoxifen or [18F]-fluoroestradiol.

          -  Peripheral neuropathy of severity greater than grade 1.

          -  Current optic nerve disorders, retinopathy, lattice degeneration, macular
             degeneration, retinal vascular disorder, or retinal tears of severity greater than
             grade 1.

          -  History of cerebellar disorders, ataxia, and uncontrolled seizures unless related to
             transient medical condition and in investigator's opinion is not an active medical
             issue.

          -  History of venous thrombosis/thromboembolic event, including pulmonary embolism and
             stroke.

          -  Have a heart-rate corrected QT interval (using Fridericia's formula) (QTcF) ≥ 470msec
             or other factors that increase the risk of QT prolongation or arrhythmic events (e.g.,
             heart failure, chronic hypokalemia, family history of long QT interval syndrome).

          -  Are taking medications that are known to prolong the QT interval, unless they can be
             transferred to other medications ≥ 5 half-lives prior to dosing or unless the
             medications can be properly monitored during the study. If equivalent medication is
             not available, QTcF should be closely monitored.

          -  Tamoxifen has demonstrated vaginal bleeding, birth defects and fetal loss in pregnant
             women. Tamoxifen use during pregnancy may have a potential long-term risk to the fetus
             of a Diethylstilbestrol syndrome (DES)-like syndrome. Women of childbearing potential
             (WOCP) must not be pregnant (confirmed by a negative urine/serum pregnancy test within
             14 days of tamoxifen treatment). In addition, a medically acceptable method of birth
             control must be used such as an intrauterine device (IUD), use of a double barrier
             method (condoms, sponge, diaphragm, or vaginal ring with spermicidal jellies or
             cream), or total abstinence during the study participation and for 3 months after last
             dose of study drug. Women who are postmenopausal for at least 1 year or surgically
             sterile (bilateral tubal ligation, bilateral oophorectomy, or hysterectomy) are not
             considered to be WOCP.

          -  Ongoing treatment with other investigational agents. Participants cannot be receiving
             concomitant chemotherapy, radiotherapy, experimental therapy or any other therapy not
             otherwise outlined by the trial for the purposes of anti-cancer treatment.

          -  History of uterine malignancy unless participant has had hysterectomy with no evidence
             recurrent disease for ≥ 3 years from definitive therapy.

          -  Concurrent malignancy except for the following:

               -  Basal cell or squamous cell skin cancer

               -  In situ cervical cancer

          -  The following medications are contraindicated or must be used with caution.

               -  Contraindicated:

                    -  CYP2D6, CYP3A4, and CYP2C9 strong inhibitors

                    -  CYP2D6, CYP3A4, and CYP2C9 strong inducers

               -  Use with caution:

                    -  CYP2C9 sensitive substrates

                    -  CYP2D6 moderate inhibitors or inducers

                    -  CYP3A4 moderate inhibitors or inducers

        Note: Transdermal products designed for systemic delivery must be assessed for interaction
        potential. Topical products not designed to provide systemic delivery (including inhaled
        products, ophthalmologic products and transvaginal preparations) do not need to be
        considered.

        Contraindicated medications are not allowed. Participants taking these concurrent
        medications are ineligible unless they can discontinue or switch to alternative medications
        prior to initiation of study drug (at least 5 half-lives).

        Use with caution agents are permitted if a) discontinuation is not feasible or b) no
        acceptable alternatives are available as determined by the treating physician; however,
        caution should be used. Consider monitoring by symptoms, labs or drug levels and dose
        adjustments of the medication.

          -  Uncontrolled intercurrent clinically significant illness including, but not limited
             to, ongoing or active infection, symptomatic congestive heart failure, unstable angina
             pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would
             limit compliance with study requirements.