This study is to establish a safe and tolerable dose and to investigate pharmacokinetics and
the first clinical efficacy signals of M1231 as a single agent in participants with solid
tumors (Part 1) and with metastatic Non-small Cell Lung Cancer (NSCLC) and esophageal
squamous cell carcinoma (Part 2). Dose escalation will be followed by the dose expansion once
the maximum tolerated dose (MTD) or recommended dose for Expansion (RDE) has been defined.