A Phase 1, First-in-human, Open-label, Multicenter Study of JZP898 as Monotherapy and in Combination With Pembrolizumab in Participants With Advanced or Metastatic Solid Tumors

Inclusion Criteria * Adult ≥ 18 years of age * Histological or cytological diagnosis of advanced or metastatic solid tumor. 1. Previously treated participants with solid tumors that are amenable to CPI therapy (eg. NSCLC, melanoma, HNSCC, RCC, HCC, gastroesophageal carcinomas, UC, or CRC \[MSI-H\]) for whom, in the opinion of the investigator, there is no SoC available to convey clinical benefit. 2. Parts A2 and B: previously-treated (≥ 1 line of prior anticancer therapy) participants with select tumor types (NSCLC, HNSCC, melanomas, RCC, and UC) who have progressed on/after prior CPI therapy based on investigator assessment per RECIST version 1.1. * Participants in select tumor types: 1. NSCLC: eligible for platinum-based therapy and received platinum-based therapy prior to inclusion in the study. 2. HNSCC: eligible for platinum therapy and received platinum-based therapy prior to inclusion in this study. 3. Melanoma with known BRAFv600 mutation: received BRAF/MEKi therapy before this study. * ECOG score of 0 to 1. * Measurable disease per RECIST version 1.1 criteria. * Parts A1 and A2 only: willing to consent to mandatory tumor biopsies (both pretreatment and post-treatment with JZP898) unless medically infeasible * Adequate organ and bone marrow function as indicated by the following laboratory values (within 4 weeks prior to starting the study interventions) * Men and women of reproductive potential to observe highly effective birth control for the duration of treatment and for 4 months following the last dose of study drug; * Additional criteria may apply Exclusion Criteria * Unresolved toxicities from previous therapy that is \> Grade 1. * Hypersensitivity to mAb, IFNα, or study intervention components. * Primary CNS tumor or symptomatic CNS metastases. * Have a second primary malignancy treated within the previous 2 years (exceptions: non-metastatic, non-melanomatous skin cancers, carcinoma in-situ, and melanoma in-situ). * Active autoimmune disease (in the last 2 years) requiring systemic steroids or immunosuppressive agents. * Active or history of pneumonitis (noninfectious) or interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease. * Any history of suicidal behavior or any suicidal ideation * Clinically significant ischemic/hemorrhagic cerebrovascular accident/stroke and/or clinically significant active cardiovascular disease * Received any anticancer therapy within 5 half-lives or 4 weeks (whichever is shorter) prior to the first dose of study drug * Received prior radiotherapy within 2 weeks of the first dose of study drug or have had a history of radiation pneumonitis * Major surgery within 2 weeks prior to the first dose of study intervention. * Participant is pregnant, breastfeeding, or expecting to conceive or father children within the projected duration of the study * Had a stem cell/solid organ transplant. * Receipt of prior IFNα therapy