A phase 1/2 open-label multicenter study will be performed with an initial dose escalation part to determine the MTD and/or the RP2D of MCLA-129 in monotherapy or in combination in patients with NSCLC, HNSCC, GC/GEJ, ESCC, or other solid tumors and who are treatment naïve or have progressed after receiving prior therapy for advanced/metastatic disease.

Part One: Patients with NSCLC, GC/GEJ, HNSCC, or ESCC who have failed prior standard first-line treatment. Patients must have progressed on or be intolerant to therapies that are known to provide clinical benefit. There is no limit to the number of prior treatment regimens.

Part Two: Patients with NSCLC, HNSCC, other solid tumors and applicable mutations as determined by the investigator.

• Availability of archival or a fresh tumor tissue sample.
• Measurable disease as defined by RECIST version 1.1 by radiologic methods.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
• Life expectancy ≥ 12 weeks, as per Investigator.
• Adequate organ function (as per protocol)

Exclusion Criteria:

• Central nervous system metastases that are untreated or symptomatic, or require radiation, surgery, or continued steroid therapy (> 10 mg prednisone or equivalent) to control symptoms within 14 days of study entry.
• Known leptomeningeal involvement.
• Participation in another clinical study or treatment with any investigational drug within 4 weeks prior to study entry.
• Systemic anticancer therapy or immunotherapy within 4 weeks or 5 half-lives, whichever is shorter, of the first dose of study drug. For cytotoxic agents that have major delayed toxicity (e.g., mitomycin C, nitrosoureas), a washout period of 6 weeks is required.
• Major surgery or radiotherapy within 3 weeks of the first dose of study drug. Patients who received prior radiotherapy to ≥25% of bone marrow at any time are not eligible.
• Persistent grade >1 clinically significant toxicities related to prior antineoplastic therapies (except for alopecia); stable sensory neuropathy ≤ grade 2 NCI-CTCAE v5.0 and hypothyroidism ≤ grade 2 which is stable on hormone replacement are allowed.
• History of hypersensitivity reaction or any toxicity attributed to human proteins or any of the excipients that warranted permanent cessation of these agents. History of hypersensitivity reaction or any toxicity attributed to chemotherapy and components.
• History of clinically significant cardiovascular disease
• Past medical history of ILD or pneumonitis, or any evidence of clinically active ILD or pneumonitis.
• Previous or concurrent malignancy, excluding non-basal cell carcinomas of skin or carcinoma in situ of the uterine cervix, unless the tumor was treated with curative or palliative intent and in the opinion of the Investigator, with Sponsor agreement, the previous or concurrent malignancy condition does not affect the assessment of safety and efficacy of the study drug.
• Current serious illness or medical conditions including, but not limited to uncontrolled active infection, clinically significant pulmonary, metabolic or psychiatric disorders
• Active Hepatitis B infection without receiving antiviral treatment.
• Positive test for Hepatitis C
• Known history of HIV (HIV 1/2 antibodies). Patients with HIV with undetectable viral load are allowed. In