Evaluation of Safety and Efficacy of Sodium Thiosulfate (BYON5667) Eye Drops to Reduce Ocular Toxicity in Cancer Patients Treated With SYD985

Main Inclusion Criteria: 1. Male or female, age ≥18 years at the time of signing first informed consent; 2. Patient with histologically-confirmed, unresectable locally advanced or metastatic cancer with the following restriction: Single arm part: patient with solid tumours of any origin (excluding gastric tumours and adenocarcinomas of the gastroesophageal junction) who has progressed on standard therapy or for whom no standard therapy exists; Randomized part: patient with breast cancer who had either progression during or after at least two human epidermal growth factor receptor 2 (HER2)-targeting treatment regimens for locally advanced or metastatic disease, or progression during or after \[ado-\]trastuzumab emtansine treatment for locally advanced or metastatic disease; 3. HER2 tumour status as determined by a local laboratory using immunohistochemistry (IHC) and/or in situ hybridization (ISH): Single arm part: at least IHC 1+; Randomized part: IHC 3+ and/or ISH positive; 4. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1; 5. Patient should refrain from wearing any kind of contact lenses during trial treatment; 6. Adequate organ function Main Exclusion Criteria: 1. Current or previous use of prohibited medication as listed in the protocol 2. History of infusion-related reactions and/or hypersensitivity to trastuzumab containing treatment or excipients of the trial treatments which led to permanent discontinuation of the treatment; 3. History or presence of keratitis; 4. Left ventricular ejection fraction (LVEF) \< 50%, or a history of clinically significant decrease in LVEF during previous trastuzumab containing treatment leading to permanent discontinuation of treatment; 5. History or presence of idiopathic pulmonary fibrosis, organizing pneumonia (e.g. bronchiolitis obliterans), drug-induced or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest CT scan; 6. History or presence of clinically significant cardiovascular disease; 7. Severe, uncontrolled systemic disease; 8. Symptomatic brain metastases, brain metastases requiring steroids to manage symptoms, or treatment for brain metastases within 8 weeks.