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.

In this international, multicenter, single-arm, multicohort, optimal Simon's design phase II clinical trial, patients will be treated with HER3-DXd, which is a new antibody-drug conjugate (ADC) that targets specifically the HER3 protein (which is expressed in the surface of tumor cells) and that is attached to deruxtecan.

Male or female patients ≥ 18 years of age with MBC or aNSCLC with untreated or progressing BM after local treatment, or solid tumor patients with treatment-naive LMD or patients with recurrence of LMD after radiotherapy, and no need for immediate local treatment. All patients except for patients with LMD (cohort 3) must have received one prior line of systemic therapy in the advanced setting.

Note I: prior systemic treatments for breast cancer (BC) eligible patients would be defined as follows:

• triple negative breast cancer (TNBC) patients must have received at least one line of prior systemic therapy for advanced disease.
• luminal BC patients must have received at least one line of endocrine therapy (ET) and one line of chemotherapy (CT) in the advanced setting.
• HER2-positive (HER2[+]) BC patients must have progressed on at least two previous treatments with HER2-targeted therapies in the advanced setting.

Note II: prior systemic treatments for NSCLC eligible patients would be defined as follows:

• Patients without and with epidermal growth factor receptor (EGFR) (and other) activating driver alterations are allowed.
• Patients with activating driver alterations must have received at least one prior line of an approved genotype directed therapy.
• Patients with EGFR T790M mutation following first-line treatment with erlotinib, gefitinib, afatinib, or dacomitinib must have received second-line osimertinib, and have documentation of radiological disease progression on treatment.

After confirmed eligibility, patients will be assigned to one of three study cohorts as follows:

• Cohort 1 (N=20): 10 patients in the stage I and 10 patients in the stage II. MBC with untreated or progressing BM after local treatment.
• Cohort 2 (N=20): 10 patients in the stage I and 10 patients in the stage II. aNSCLC with untreated or progressing BM after local treatment.
• Cohort 3 (N=20): 10 patients in the stage I and 10 patients in the stage II. Advanced solid tumor with treatment-naive LMD or LMD progressing after radiotherapy.

Upon meeting all selection criteria, patients enrolled in the study will receive patritumab deruxtecan (HER3-DXd), which will be dosed at 5.6 mg/kg body weight as an intravenous (IV) infusion administered on day 1 (D1) of each 21-day cycle.

Patients will receive treatment disease progression, unacceptable toxicity, death, or discontinuation from the study treatment for any other reason.

Patients discontinuing the study treatment period prematurely, will enter a post-treatment follow-up period during which survival and new anti-cancer therapy information will be collected, until end of study (EoS) or study termination, whichever occurs first.

GENERAL INCLUSION CRITERIA (Patients will be included in the study only if they meet all the following inclusion criteria):

1. Patient must be capable to understand the purpose of the study and have signed written informed consent form (ICF) prior to beginning specific protocol procedures.

2. Age ≥ 18 years at the time of signing ICF.

3. Life expectancy ≥ 6 weeks.

4. Karnofsky Performance Status (KPS) ≥70%, Eastern Cooperative Oncology Group (ECOG) performance status (PS) ≤ 2.

5. Patient must be able to tolerate therapy.

6. Availability and willingness to provide the most recently available tumor tissue sample (formalin-fixed and paraffin-embedded [FFPE], no cytology/cell block, no bone/decalcified bone sample) of primary tumor or any metastatic site from biopsy collected after last round of prior treatment and ≤ 6 months prior to HER3-DXd, if possible, at the time of inclusion for retrospective exploratory biomarker testing. If archival tissue is not available, a newly obtained baseline biopsy of an accessible tumor lesion is required prior to start of study treatment (unless not possible because of inaccessible tumor location or safety concerns). Collection and/or shipment of pre-treatment tumor tissue biopsy for retrospective biomarker testing should be at least initiated treatment at the time of inclusion.

7. No indication for immediate local therapy (neurosurgery, brain radiotherapy).

8. Patient has adequate bone marrow, liver, and renal function:

   1. Hematological (without platelet, red blood cell transfusion, and/or granulocyte colony-stimulating factor support within 14 days prior to hematological assessments during the screening period): White blood cell (WBC) count > 3.0 x 109/L, absolute neutrophil count (ANC) ≥ 1.5 x 109/L, platelet count ≥ 100.0 x109/L, and hemoglobin ≥ 10.0 g/dL (≥ 6.2 mmol/L).
   2. Hepatic: Serum albumin ≥ 2.5 g/dL; total bilirubin ≤ 1.5 times upper limit of normal (ULN) (≤ 3 in patients with liver metastases or known history of Gilbert's disease); both alkaline phosphatase (ALP) and Gamma Glutamyl Transferase (GGT) ≤ 2.5 times ULN (ALP ≤ 5 times ULN in patients with liver and/or bone metastases, and GGT increased in patients with liver metastases); aspartate transaminase (AST); alanine transaminase (ALT) ≤ 3 times ULN (≤ 5 times ULN in patients with liver metastases); international normalized ratio (INR) < 1.5. Prothrombin time (PT) or Prothrombin time-international normalized ratio (PT-INR) and activated partial thromboplastin time (aPTT)/partial thromboplastin time (PTT) ≤ 1.5 × ULN, except for subjects receiving coumarin-derivative anticoagulants, factor Xa inhibitors, or other similar anticoagulant therapy, who must have PT-INR within therapeutic range as deemed appropriate by the Investigator from product safety requirements (PSR).
   3. Renal: serum creatinine ≤ 1.5 x ULN or creatinine clearance ≥ 50 mL/min/1.73 m2 based on Cockcroft-Gault glomerular filtration rate estimation for patients with creatinine levels above institutional normal.

9. Resolution of all acute toxic effects of prior anti-cancer therapy to grade ≤ 1 as determined by the US National Cancer Institute (NCI)-Common Terminology Criteria for Adverse Events (CTCAE) version 5.0 (v.5.0).

   Note: Except for alopecia or other toxicities not considered a safety risk for the patient at investigator's discretion.

10. For women of childbearing potential: agreement to remain abstinent (must refrain from heterosexual intercourse) or use highly effective contraceptive methods, or two effective contraceptive methods, as defined in the CSP, during the treatment period and for at least 7 months after the last dose of study treatment, whichever is longer. Women of childbearing potential must have a negative serum pregnancy test within 14 days before study treatment initiation (with result available prior to dosing) and must agree to refrain from donating eggs during the entire study treatment period and for 7 months after the last administration of the study drug.

11. For male subjects: being surgically sterile or having agreed to true abstinence (must refrain from heterosexual intercourse) or having female partners willing to agree with true abstinence or use barrier contraceptive measures mentioned above during the entire study treatment period and for 4 months after the last administration of the study drug. Male patients must agree to refrain from donating sperm during the entire study treatment period and for 4 months after the last administration of the study drug.

12. Patient must be accessible for treatment and follow-up.

Specific inclusion criteria for cohorts 1 and 2

Patients from cohorts 1 and 2 must meet all the following inclusion criteria to be eligible for enrolment into the study:

1. Radiologically documented metastatic disease.
2. Newly diagnosed BM or BM progressing after local treatment. Any number of brain lesions is acceptable as long as all the eligibility criteria are fulfilled.
3. Measurable disease according to Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) criteria, with at least one measurable brain lesion of ≥ 10 mm on T1-weighted, gadolinium-enhanced magnetic resonance imaging (MRI).

Specific inclusion criteria for cohort 1

Patients from cohort 1 must meet all the following inclusion criteria to be eligible for enrolment into the study:

1. Histologically documented BC.

2. Locally determined HER2 status.

3. Patients have received at least 1 prior line of systemic treatment in the advanced setting, which is defined as follows:

   • Patients with TNBC must have received at least one line of prior systemic therapy for advanced disease.
   • Patients with luminal BC must have received at least one line of ET and one line of CT in the advanced setting.
   • Patients with HER2-positive BC must have progressed to at least two previous treatments with HER2-targeted therapies in the advanced setting.

Specific inclusion criterion for cohort 2

Patients from cohort 1 must meet ALL the following inclusion criteria to be eligible for enrolment into the study:

1. Histologically documented NSCLC of squamous or non-squamous histologic types.

2. Patients have received at least 1 prior line of systemic treatment in the advanced setting, which is defined as follows:

   • Patients without actionable driver alterations must have received at least one prior line of standard of care systemic therapy for locally advanced or metastatic disease.
   • Patients with actionable driver alterations must have received at least one prior line of an approved genotype-directed therapy.
   • Patients with EGFR T790M mutation who received first-line treatment with erlotinib, gefitinib, afatinib, or dacomitinib must have received second-line treatment with osimertinib and have documentation of radiological disease progression.

Specific inclusion criteria for cohort 3

Patients from cohort 3 must meet ALL the following inclusion criteria to be eligible for enrolment into the study:

1. Histologically documented solid tumor of any type.
2. Type I LMD, defined by positive CSF cytology or leptomeningeal biopsy, or type II LMD, defined by clinical findings and neuroimaging only, according to European Society for Molecular Oncology (ESMO) Standard Operating Procedures (SOPs) for Clinical Practice Guideline (CPG) 2017.
3. Newly diagnosed LMD or LMD progressing after radiotherapy.

Exclusion criteria

Patients will be excluded from the study if they meet any of the following criteria:

1. Current participation in another therapeutic clinical trial.

2. Treatment with approved or investigational cancer therapy within 14 days prior to initiation of study drug.

3. Patients have a concurrent malignancy or malignancy within five years of study enrollment with the exception of carcinoma in situ of the cervix, non-melanoma skin carcinoma, or stage I uterine cancer. For other cancers considered to have a low risk of recurrence, discussion with the Medical Monitor is required.

4. Previous systemic therapy with any anti-HER3 directed drug.

5. Known allergy or hypersensitivity to HER3-DXd or any of the drug components.

6. Radiotherapy or limited-field palliative radiotherapy within seven days prior to study enrolment, or patients who have not recovered from radiotherapy-related toxicities to baseline or grade ≤ 1 and/or from whom ≥ 25% of the bone marrow has been previously irradiated.

7. Patients with an active cardiac disease or a history of cardiac dysfunction or conduction abnormalities including any of the following:

   1. Unstable angina pectoris or documented myocardial infarction within 6 months prior to study entry.
   2. Symptomatic pericarditis.
   3. Documented congestive heart failure (CHF) (New York Heart Association [NYHA] Class III-IV).
   4. Left ventricular ejection fraction (LVEF) < 50% as determined by multigated acquisition (MUGA) scan or echocardiogram (ECHO).
   5. Ventricular arrhythmias except for benign premature ventricular contractions.
   6. Other cardiac arrhythmias requiring a pacemaker or not controlled with medication.
   7. Long QT syndrome (corrected QT interval by Fredericia [QTcF] > 450 ms, average of triplicate determinations at screening), or diagnosed or suspected long QT syndrome or known family history of long QT syndrome.

8. Clinically severe pulmonary compromise resulting from intercurrent pulmonary illnesses including, but not limited to, any underlying pulmonary disorder (i.e., pulmonary emboli within three months of the study enrolment, severe asthma, severe chronic obstructive pulmonary disease [COPD], restrictive lung disease, pleural effusion etc.), and any autoimmune, connective tissue or inflammatory disorders with pulmonary involvement (i.e., rheumatoid arthritis, Sjogren's syndrome, sarcoidosis etc.), or prior pneumonectomy.

9. History of non-infectious interstitial lung disease (ILD)/pneumonitis that required steroids, has current ILD/pneumonitis, or has suspected ILD/pneumonitis that cannot be ruled out by imaging at screening.

10. Pregnant or lactating women.

11. Any serious medical condition or abnormality in clinical laboratory tests that, in the investigator's judgment, precludes the patient's safe participation in and completion of the study.

12. Current known infection with hepatitis B virus (HBV), or hepatitis C virus (HCV). Patients with past HBV infection or resolved HBV infection (defined as having a negative hepatitis B surface antigen [HBsAg] test and a positive hepatitis B core antibody [HBcAb] test, accompanied by a negative HBV DNA test) are eligible. Patients positive for HCV antibody are eligible only if polymerase chain reaction (PCR) is negative for HCV RNA.

13. Known human immunodeficiency virus (HIV) infection that is not well controlled. All of the following criteria are required to define an HIV infection that is well controlled: undetectable viral RNA, CD4-positive cells' count ≥350, no history of AIDS-defining opportunistic infection within the past 12 months, and stable for at least 4 weeks on the same anti-HIV medications (meaning there are no expected further changes in that time to the number or type of antiretroviral drugs in the regimen). If an HIV infection meets the above criteria, monitoring of viral RNA load and CD4-positive cells' count is recommended.

14. History of a major surgical procedure (defined as requiring general anesthesia) or significant traumatic injury within 21 days prior to randomization, or patients who have not recovered from the side effects of any major surgery.

15. History of uncontrolled seizures, CNS disorders or serious and/or unstable pre-existing psychiatric disability judged by the investigator to be clinically significant and adversely affecting compliance to study drugs or interfering with subject safety.

16. Patients requiring concomitant use of chronic systemic (intravenously [IV] or oral) corticosteroids at doses higher than 8 mg dexamethasone per day or other immunosuppressive medications except for managing adverse events (AEs), including immune-related adverse events (irAEs) for patients that received immunotherapy in a previous line; (inhaled steroids or intra articular steroid injections are permitted in this study).

    Note: The use of stable corticosteroid therapy in patients with brain metastases can be discussed with the Medical Monitor.

17. Patients with known substance abuse or any other medical conditions such as clinically significant cardiac or psychological conditions, that may, in the opinion of the investigator, interfere with the subject's participation in the clinical study or evaluation of the clinical study results.

18. Participants who are unable or unwilling to comply with the requirements of the protocol in the opinion of the investigator.

Data collected within this study will be made available to researchers after contacting the corresponding author and upon revision and approval based on scientific merit by the trial management group (which includes a qualified statistician) of a detailed proposal for their use. The data required for the approved, specified purposes and the trial protocol will be provided after the completion of a data-sharing agreement that will be set up by the study sponsor. All data provided are anonymized to respect the privacy of patients who have participated in the trial in line with applicable laws and regulations. Data Sharing should begin 1 month after publication of study main results and ending 5 years after article publication. Estimate timeframe for response will be within 30 days.