A DLT is defined as an adverse event or abnormal laboratory value assessed as unrelated to disease, disease progression, inter-current illness/injury, or concomitant medications that occurs within the DLT period from C1D1 of treatment with \[177Lu\]Lu-NeoB and capecitabine. The National Cancer Institute (NCI) CTCAE version 5.0 will be used for all grading.

The distribution of adverse events for 177-Lu-NeoB in combination with capecitabine will be done via the analysis of frequencies for Adverse Event (AEs) and Serious Adverse Event (SAEs) through the monitoring of relevant clinical and laboratory safety parameters.

Dose modifications (dose interruptions, dose discontinuations and reductions) for \[177Lu\]Lu-NeoB in combination with capecitabine will be assessed and summarized using descriptive statistics.

Objective Response Rate (ORR) with confirmed response is defined as the proportion of participants with Best Overall Response (BOR) of Complete Response (CR) or Partial Response (PR), as per local review and according to RECIST 1.1.

Clinical Benefit Rate (CBR) with confirmed response is defined as the proportion of participants with Best Overall Response (BOR) of Complete Response (CR) or Partial Response (PR), or maintaining an overall response of Stable Disease (SD) for at least 24 weeks. CR, PR, and SD are defined as per local review according to RECIST 1.1.

Time to response is the time from the date of randomization to the first documented response (Complete Response (CR) or Partial Response (PR), which must be confirmed subsequently) as per local review and according to RECIST 1.1.

Duration of Overall Response (DoR) applies only to participants whose best overall response is confirmed CR or confirmed PR according to RECIST 1.1. The start date is the date of first documented confirmed response (CR or PR) and the end date is the date defined as first documented progression or death due to underlying cancer.

Progression Free Survival (PFS) is defined as the time from the date of first dose of \[177Lu\]Lu-NeoB to the date of confirmed progression or death due to any cause. PFS will be assessed via local review according to RECIST 1.1.

Overall Survival (OS) is defined as the time from date of first dose of \[177Lu\]Lu-NeoB to date of death due to any cause.

A DLT is defined as an adverse event or abnormal laboratory value assessed as unrelated to disease, disease progression, inter-current illness/injury, or concomitant medications that occurs within the DLT period from C1D1 of treatment with \[177Lu\]Lu-NeoB, ribociclib and fulvestrant with or without goserelin. The National Cancer Institute (NCI) CTCAE version 5.0 will be used for all grading.

The distribution of adverse events will be done via the analysis of frequencies for treatment emergent Adverse Event (TEAEs), Serious Adverse Event (TESAEs) and Deaths due to AEs, through the monitoring of relevant clinical and laboratory safety parameters.

Dose interruptions, discontinuation and dose reductions will be assessed for tolerability.