RecruitingDrug trialERADICATE: A Phase Ib/II Study of Elacestrant Plus Trastuzumab Deruxtecan in Patients With CDK4/6 Inhibitor and Endocrine-resistant HR+/HER2-low or HER2-ultralow Metastatic Breast CancerThe goal of this study is to evaluate the safety and efficacy of elacestrant in combination with trastuzumab deruxtecan (T-DXd) in participants with hormone receptor-positive (HR+), HER2-low or HER2-ultralow, metastatic breast cancer that is resistant to prior CDK4/6 inhibitor and endocrine therapy. The names of the study drugs involved in this study are: * Elacestrant (a type of selective estrogen receptor degrader) * Trastuzumab deruxtecan (a type of standard of care antibody drug conjugate)