The study comprises of a dose escalation part, followed by a dose expansion part.~The dose escalation part will estimate the RD of [177Lu]Lu-NeoB in combination with ribociclib and fulvestrant; four provisional dose levels are planned to be tested: 100 millicurie (mCi) (initial dose), 150mCi, 200 mCi and 250mCi in cohorts of 3 to 6 participants. After inclusion of each cohort of 3 to 6 participants, the incidence rate of DLTs will be compared to the pre-defined toxicity rate boundaries to decide whether the next cohort will receive a lower, higher or same dose or whether the trial will be terminated.~The expansion part will assess the safety, tolerability and anti-tumor activity of the RD dose of [177Lu]Lu-NeoB, as established in the dose escalation part, in combination with ribociclib and fulvestrant in both post menopausal and pre-/peri-menopausal participants (who will additionally receive goserelin). A total of 15 participants will be enrolled in the expansion part.~During screening, study participants will receive the investigational imaging agent [68Ga]Ga-NeoB. An additional administration of the [68Ga]Ga-NeoB will be performed within 2-8 weeks from the last administration of [177Lu]Lu-NeoB for a positron emission tomography (PET)/computed tomography (CT) or PET/magnetic resonance imaging (MRI).~Study treatment will include [177Lu]Lu-NeoB on day 1 of each 28-day cycle (+≤3 days) for 6 cycles, ribociclib (once daily; days 1 to 21 in a 28-day cycle) and fulvestrant (C1D1, C1D15, C2D1 and every 28 days thereafter) until disease progression. Pre- and perimenopausal participants in the expansion part will additionally receive goserelin on day 1 of every cycle.~During the treatment period participants will be required to attend a site visit approximately every 28 days, on the first day of each cycle (as well as on C1D2, C1D3, C1D8, C1D15, C2D15, C3D3 and C5D3), to undergo study treatment administration, dosimetry and safety assessments. Tumor assessments are performed every 8 weeks until month 18, every 12 weeks until month 36 and as clinically indicated thereafter, until disease progression. After study treatment discontinuation, participants will be followed up for safety for 8 weeks after their last study treatment administration. Beyond the initial 8 weeks of safety follow-up, all participants will be followed up every 12 weeks until month 36 and every 24 weeks thereafter until month 60, for a total of 5 years from the participant's enrollment in the study, or until death, lost to follow-up, or withdrawal of consent (WoC).~The end of study is defined as the date of the last visit, scheduled procedure or follow up (or date of death, WoC or lost to follow up, whichever occurs first) of the last participant in the study globally, or at 5 years from the date of the last participant enrolled, whichever occurs earlier.
The purpose of this phase 1b, single arm, multicenter, open label, dose finding study is to estimate the RD of [177Lu]Lu-NeoB in combination with ribociclib and fulvestrant in adult female participants with ER+, HER2-, GRPR+, advanced or metastatic breast cancer who have relapsed during or within 12 months from completion of prior (neo)adjuvant ET (escalation part). Additionally this study aims to characterize the safety and tolerability of [177Lu]Lu-NeoB in combination with ribociclib and fulvestrant in post-menopausal participants and in combination with ribociclib, fulvestrant and goserelin in pre/peri-menopausal participants (expansion part) in the same indications.