This study is being conducted to determine the safety, tolerability, and preliminary efficacy of INCB099280 in participants with advanced Cutaneous Squamous Cell Carcinoma.

Inclusion Criteria:

• Histopathological diagnosis of cSCC.
• Previously untreated or recurrent locally advanced (without nodal metastases) or metastatic (distant or regional metastasis) cSCC not amenable to curative surgery and/or radiotherapy.
• Measurable disease based on either radiographic imaging per RECIST 1.1 or WHO criteria.
• Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1.
• Life expectancy > 3 months.
• Willingness to avoid pregnancy or fathering children.

Exclusion Criteria:

• Known history of an additional malignancy.
• Central nervous system (CNS) metastases requiring treatment and/or leptomeningeal disease.
• Toxicity from prior therapy that has not recovered.
• Prior receipt of an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent; treatment with an immune modulator (eg, CTLA-4, GITR, LAG3, TIM3, OX40, ICOS, IL-2, 4-1BB, CAR-T cell).
• Received thoracic radiation within 6 months of the first dose of study treatment.
• Participation in another interventional clinical study while receiving INCB099280.
• Impaired cardiac function or clinically significant cardiac disease.
• History or evidence of interstitial lung disease including noninfectious pneumonitis.
• Presence of gastrointestinal conditions that may affect drug absorption.
• Any autoimmune disease requiring systemic treatment in the past 5 years.
• Diagnosis of primary immunodeficiency or receiving chronic systemic steroid therapy at a daily dose exceeding 10 mg of prednisone or equivalent.
• Active infection requiring systemic therapy.
• History of organ transplantation, including allogeneic stem cell transplantation.
• Receipt of systemic antibiotics within 28 days of first dose of study treatment.
• Probiotic usage is prohibited during screening and throughout the study treatment period.
• Received a live vaccine within 28 days of the planned start of study drug.
• Laboratory values outside the Protocol-defined ranges.
• Inadequate organ function.

Other protocol-defined Inclusion/Exclusion Criteria may apply.

Incyte shares data with qualified external researchers after a research proposal is submitted. These requests are reviewed and approved by a review panel on the basis of scientific merit. All data provided is anonymized to respect the privacy of patients who have participated in the trial in line with applicable laws and regulations. The trial data availability is according to the criteria and process described on https://www.incyte.com/our-company/compliance-and-transparency

Data will be shared after the primary publication or 2 years after the study has ended for market authorized products and indications.

Data from eligible studies will be shared with qualified researchers according to the criteria and process described in the Data Sharing section of the www.incyteclinicaltrials.com website. For approved requests, the researchers will be granted access to anonymized data under the terms of a data sharing agreement.