PIK3CA/PTEN-altered Advanced Breast Cancer Treated With MEN1611 Monotherapy or in Combination With Eribulin

This is a multicenter, two-cohort, non-comparative, open-label, phase II clinical trial to assess: * the efficacy of MEN1611 in combination with eribulin as determined by the clinical benefit rate (CBR) in unresectable locally advanced or metastatic HR-known/HER2-negative, PIK3CA/ PTEN-altered MpBC patients (Cohort A), and * the efficacy of MEN1611 as monotherapy as determined by the objective response rate (ORR) at 6 weeks in unresectable locally advanced or metastatic HR-known/HER2-negative, PIK3CA/ PTEN-altered MpBC patients (Cohort B). Upon meeting all selection criteria, 28 patients will be enrolled as follows: \- Cohort A: A run-in phase for safety and tolerability of MEN1611 in combination with eribulin will be conducted in this patient population (N=3). A Steering Committee decision/agreement is needed, after reviewing all toxicities, to expand this cohort with 11 additional patients (up to N=14). Cohort B will be run only if positive finding in Cohort A, defined as ≥ 6 patients (42.9%) with achieved clinical benefit (CB) among 14 first recruited patients. \- Cohort B: Simon's two-stage minimax design. Stage I (N=7) will be initiated with MEN1611 monotherapy. This cohort will be stopped for futility if no responders (no complete response \[CR\] or partial response \[PR\]) are observed after 2 cycles among the first 7 patients included. Otherwise, Cohort B will be expanded with 7 additional patients for Stage II (up to N=14).