Background: JZP898 is a conditionally activated prodrug of interferon alpha (IFNα) that can preferentially deliver IFNα to the tumor microenvironment while potentially minimizing systemic IFNα therapy-associated toxicity. Antitumor activity of IFNα is well-established, but concerns regarding systemic toxicity restrict its use in clinical practice. JZP898, in combination with a checkpoint inhibitor such as pembrolizumab, may provide lasting and potent antitumor activity with minimal additive toxicity attributed to conventional or pegylated IFNα. This phase 1 FIH study will determine a maximum tolerated dose (MTD)/recommended dose (RD) and investigate safety, tolerability, pharmacokinetics (PK), immunogenicity, and preliminary antitumor activity of JZP898 monotherapy and JZP898 combined with pembrolizumab in adult patients with advanced or metastatic solid tumors. Methods: This phase 1, FIH, open-label, multicenter study (NCT06108050) is investigating JZP898 intravenous infusion in adult patients (≥18 years old) with histological or cytological diagnosis of advanced or metastatic solid tumors, European Cooperative Oncology Group performance status ≤1, and measurable disease per RECIST v1.1 criteria. The study consists of a dose finding phase (JZP898 as monotherapy and in combination with pembrolizumab; Part A) and a combination dose expansion phase (JZP898 in combination with pembrolizumab; Part B). In Part A (dose finding), the JZP898 MTD and/or monotherapy RD will be determined using Bayesian optimal interval (BOIN) design based on the incidence of dose-limiting toxicities (DLTs) and all available safety and PK data; the combination recommended phase 2 dose (RP2D) will be determined using BOIN criteria and all available safety, PK/PD, immunogenicity, and preliminary signal of antitumor activity data. Patients will be treated until disease progression, death, unacceptable toxicity, or withdrawal from the study. All patients will be followed for survival until 18 months after the last patient starts study intervention. Primary endpoints include: the incidence and nature of DLTs (Part A); investigator-assessed objective response rate (ORR) per RECIST v1.1 (Part B); the incidence and severity of treatment-emergent adverse events (TEAEs) and serious adverse events; changes in laboratory values or vital signs; and the incidence of dose modifications or discontinuations due to TEAEs. Secondary endpoints include: PK parameters; incidence of antidrug antibodies; investigator-assessed duration of response; progression-free survival; disease control rate; investigator-assessed ORR (Part A); and overall survival (Part B). The study is actively enrolling patients with a planned enrollment of up to 177 patients. Clinical trial information: NCT06108050.