After an initial screening phase (SAFIR 03 - SCREENING) to identify patients with blood circulating mutated-PIK3CA tumours persistent, patients will be enrolled in the treatment phase of SAFIR 03 (SAFIR 03 - ARRIBA) that was designed as a randomised, open-label, multicentre, phase II study, for comparison of alpelisib to ribociclib in combination with fulvestrant (as the continuation of the CDK4/6 inhibitor-fulvestrant strategy) in terms of progression-free survival (PFS).

INDICATION The population eligible to the screening phase is composed of all women or men with HR+, HER2- metastatic breast cancer who are eligible for first-line treatment with a cyclin-dependent kinases (CDK) 4/6 inhibitor combined with fulvestrant (and a luteinizing hormone realeasing hormone (LH-RH) analogue in men and premenopausal women) in the context of the standard healthcare management. The screening will identify patients with high risk of relapse on any CDK4/6 inhibitor thanks to ctDNA kinetic between baseline and 4 weeks of treatment. The purpose is to early adapt the therapeutic intervention for ctDNA no drop patient to prevent from relapse. This study will propose an intervention for PIK3CA mutated patients with alpelisib vs. ribociclib. Other therapeutic approaches might be proposed to patients with wild type PIK3CA through other protocols. The randomised study phase will include patients with persistent mutations on exons 4, 9 or 20\* of PIK3CA ctDNA after 4 weeks of treatment with any CDK4/6 inhibitor-fulvestrant in first-line setting.