Study of 68Ga-R10602

There are three eligible disease populations for the study. Population 1 are patients with metastatic or locoregionally recurrent, endocrine-resistant, ER+ and/or PR+ breast cancer who have received at least one line of chemotherapy or ADC. Population 2 are patients with ER+ and HER2- locoregional or metastatic non-resectable breast adenocarcinoma with progression on at least one line of prior endocrine therapy in the adjuvant or metastatic setting and starting next line of therapy that will include tamoxifen, fulvestrant, aromatase inhibitor, or elacestrant with or without ovarian suppression. Population 3 are patients with ER+ and/or PR+ and HER2- breast cancer who are ineligible for Population 1 or 2. All disease populations are eligible for both cohorts. Cohort 1 will consist of three dose de-escalating cohorts with four patients each for a total of twelve patients. Once the twelve patients have completed the dose de-escalation, the three dose levels will be evaluated, and additional subjects may be enrolled into Cohort 1 at the single chosen dose. All patients will undergo imaging at a single timepoint and no dosimetry will be performed. In Cohort 2, approximately six to eight patients will receive the single chosen dose followed by imaging at five timepoints and blood dosimetry at seven timepoints.