HER3-DXd in Breast Cancer and NSCLC Brain Metastases and Solid Tumor Leptomeningeal Disease

The goal of this phase II clinical trial\] is to analyze the efficacy of patritumab deruxtecan (HER3-DXd) in patients with metastatic breast cancer (MBC) or advanced non-small cell lung cancer (aNSCLC) with active brain metastases (BM) who have received at least one line of systemic therapy in the advanced setting, or patients with active leptomeningeal carcinomatosis/disease (LMD) after radiotherapy from an advanced solid tumor who do not need immediate local treatment, and have not received prior treatment with an anti-HER3 targeted drug\]. The main questions it aims to answer are: * The intracranial objective response rate (ORR-IC) per local investigator as judged by best central nervous system (CNS) response according to Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) criteria of HER3-DXd in patients with active BM from MBC (Cohort 1) and aNSCLC (Cohort 2). * The overall survival (OS) rate at 3 months of HER3-DXd in patients with advanced solid tumors with untreated LMD (Cohort 3). Participants will receive HER3-DXd on day (D1) of each 21-day cycle until disease progression, unacceptable toxicity, death, or discontinuation from the study treatment for any other reason. Researchers will compare historical groups to see if HER3-DXd positively impacts patient outcomes.