• Patients with incurable advanced and/or metastatic cancer.

• Patients with any of the following cancers:

‣ In dose escalation: carcinoma of the breast, stomach, esophagus, pancreas, endometrium, ovary or lung.

⁃ In cohort expansion (HER2-positive): carcinoma of the breast and gastric/gastro-esophageal that express HER2 (2 cohorts) . Eligibility is based on HER2 overexpression as determined locally.

• Prior treatment with at least one prior systemic therapy in the advanced metastatic setting

‣ Dose escalation: no limit on number of prior systemic therapies and considered as failing standard therapeutic alternatives and candidate to a phase I study by a multi-disciplinary tumor board.

⁃ Cohort expansion: patients must have previously re-ceived (or be considered as non-eligible to) all authorized standard treatments

• Breast cancer: patient must have received prior (or be considered as ineligible to) trastuzumab pertuzumab, trastuzumab emtansine, trastuzumab deruxtecan and capecitabine+anti-HER2 (trastuzumab, lapatinib or trastuzumab tucatinib) according to label.

• Gastric cancer: patient must have received (or be con-sidered as ineligible to) prior treatment with platinum salts and trastuzumab.

• Presence of at least one measurable lesion by RECIST outside of the CNS.

• At least 18 years of age.

• ECOG performance status of ≤1.

• For patients included in cohort expansion, adequate echocar-diogram, with a left ventricular ejection fraction ≥55%. Patients with a history of LVEF decline (\< 50%) on anti-HER2 treatment will not be allowed to participate.

• For patients included in the cohort expansion, feasibility of obtaining tumor biopsy at study entry.

• All non-hematological AEs related to prior therapy must have completely resolved or improved to Grade 1 prior to screening for this study (except for alopecia).