A First-in-Human Study of MEN2312 in Adults With Advanced Breast Cancer

Key Inclusion Criteria: * Participant has advanced (locoregionally recurrent or metastatic) breast cancer not amenable to curative therapy. * Participant must have received at least one prior line of endocrine therapy for advanced/metastatic disease or participant who has radiological evidence of breast cancer recurrence or progression during or within 12 months from the end of or during adjuvant treatment with endocrine therapy, as these participants are considered as first-line relapsed participants. Progression on previous cyclin-dependent kinase 4/6 inhibitor treatment in combination with fulvestrant or aromatase inhibitor is required. No other therapeutic options are considered appropriate by the investigator. * Up to 6 prior lines of systemic therapy (including up to 2 prior lines consisting of chemotherapy, cytotoxic antibody drug conjugate, or a combination of both regimens) are allowed in the advanced/metastatic setting. * Presence of genetic alterations in PIK3CA/AKT1/PTEN in participants' tumor tissue. Key Exclusion Criteria: * Active or newly diagnosed central nervous system metastases. * Participants with advanced, symptomatic visceral spread, who are at risk of life-threatening complications in the short term, including massive uncontrolled effusions (peritoneal, pleural, pericardial), pulmonary lymphangitis, or liver involvement \>50%. Note: Other inclusion/exclusion criteria may apply.