This is the first in human study of KK2260. In Part 1a, the maximum tolerated dose (MTD) will be determined while evaluating the safety and tolerability of KK2260 in patients with advanced or metastatic solid tumors (any cancer type). In Part 1b and Part 2, at least two dosing regimens and two dosing regimens by cancer type, respectively, will be selected, and the safety, tolerability, and efficacy of each regimen will be evaluated.

Inclusion Criteria:

<Common Inclusion Criteria for Part 1a, Part 1b, and Part 2

1. Patients who have given informed written consent.
2. Male or female subjects ≥18 years of age, at time of signing informed consent.
3. Subjects who are refractory to standard treatment, intolerant of standard treatment, for whom standard treatment does not exist, or who have refused standard treatment.
4. Patients with measurable disease according to RECIST version 1.1
5. Patients who have had the certaion periods between the date of completion of prior therapy and the date of enrollment
6. Subjects who agree to have a tumor biopsy as part of the baseline examination. Patients who have difficulty in performing a tumour biopsy and have agreed to submit a previously collected stored specimen.
7. Patients with an ECOG PS of 0 or 1 at baseline.
8. Patients with haematopoietic, hepatic, renal, cardiac and respiratory functions that meet certain criteria in a baseline test.

<Additional Inclusion Criteria for Part 1a

1) Patients with pathologically diagnosed advanced or metastatic solid tumors.

<Additional Inclusion Criteria for Part 1b

1. Patients with pathologically diagnosed with advanced or metastatic esophageal cancer, or advanced or metastatic head and neck cancer whose primary site of origin is the oral cavity, oropharynx, hypopharynx, larynx, nasal cavity, or paranasal sinuses.
2. Patients with pathologically diagnosed squamous cell carcinoma.
3. Patients who agree to undergo tumor biopsy after administration.

<Additional Inclusion Criteria for Part 2a

1. Patients with pathologically diagnosed with advanced or metastatic esophageal cancer.
2. Patients with pathologically diagnosed squamous cell carcinoma.
3. Patients who agree to undergo tumor biopsy after administration.

<Additional Inclusion Criteria for Part 2b

1. Patients with advanced or metastatic head and neck cancer whose primary site of origin is the oral cavity, oropharynx, hypopharynx, larynx, nasal cavity, or paranasal sinuses.
2. Patients with pathologically diagnosed squamous cell carcinoma.
3. Patients who agree to undergo tumor biopsy after administration.

Exclusion Criteria:

<Common Exclusion Criteria to Part 1 and Part 2>

1. Patients with central or brain pia mater metastases that are untreated and symptomatic or that require treatment.
2. Patients with concurrent multiple or synchronous cancers, or with iatrogenic multiple or synchronous cancers with a disease-free interval of 5 years or less.
3. Patients receiving continuous systemic administration of steroids or other immunosuppressive drugs.
4. Patients who have had a Grade 3 or higher allergic reaction to an antibody agent or an additive of the study drug.
5. Patients who have not recovered to Grade 1 or below from adverse events caused by previously administered anticancer therapy.
6. Patients with active interstitial lung disease or a history of active interstitial lung disease.
7. Patients with infectious diseases requiring systemic treatment.
8. Patients with a fever of 38.0°C or higher at the time of registration.
9. Patients who test positive for Hepatitis B virus antigen or antibody, Hepatitis C virus antibody, or HIV antibody in a baseline test.

The datasets generated and/or analyzed during the study sponsored by Kyowa Kirin will be available in the Vivli repository, https://vivli.org/ourmember/kyowa-kirin/ as long as conditions of data disclosure specified in the policy section of the Vivli website are satisfied.