Phase 1/2 Dose Determination and Dose Expansion Study of Cobolimab in Combination with Dostarlimab in Pediatric and Young Adult Participants with Newly Diagnosed and Relapsed/Refractory Tumors (POPSTAR)

Inclusion Criteria: Participants are eligible to be included in the study only if all of the following criteria apply:
*  Participants between the age of 0 to not more than 21 years at the time of signing informed consent form (ICF).
*  Disease characteristics: Part 1: Participants with advanced or metastatic solid tumors who have had disease progression after treatment with available therapies that are known to confer clinical benefit and who have limited available treatment options as determined by the investigator. Additionally, exposure to prior immunotherapy or experimental therapies is acceptable: 1. Melanoma 2. Hodgkin Lymphoma 3. High and Low Grade Glioma: including Glioblastoma multiforme (GBM), Diffuse intrinsic pontine glioma (DIPG), and ependymoma. 4. Osteosarcoma 5. Hepatic tumors \[including Hepatoblastoma, Hepatocellular carcinoma (HCC), and Fibrolamellar carcinoma\] 6. Rhabdomyosarcoma Part 2: 1. Participants with Melanoma who have not received prior systemic therapy:
*  Participants with BRAF gene, found on chromosome 7 (BRAF) mutations who are eligible for a BRAF-targeted therapy are eligible if they qualify for immunotherapy.
*  Participants with locally treated and controlled metastatic CNS lesions without leptomeningeal spread are eligible 2. Relapsed/refractory Hodgkin lymphoma (HL) that has failed at least 2 prior lines of systemic therapy)
*  Participants must have performance status ≥60% on the Karnofsky scale for participants \>16 years of age and ≥60% on the Lansky scale for participants ≤16 years of age.
*  Adequate organ function as demonstrated by a complete blood count at screening obtained without transfusion \[platelets or red blood cells (RBC)\] or receipt of Colony stimulating factor (CSF), Granulocyte colony stimulating factor (G-CSF), Granulocyte macrophage colony stimulating factor (GMCSF) or rErythropoeitin (rEPO) within 2 weeks prior to screening.
*  Adolescent participants who have entered puberty must consent (be willing) to use of contraceptive measures, or refrain from sexual intercourse, if in line with their usual practice, as well as sperm/egg donation for the duration of treatment

Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: Medical conditions:
*  Participant has uncontrolled CNS involvement by any tumor pathology
*  Participant has a heart rate-corrected QT interval according to QTcF prolongation at screening \>470 msec or \>480 msec for participants with bundle branch block.
*  Participant has clinically significant cardiovascular disease Participant has chronic respiratory disease Prior/ Concomitant therapy
*  Has received treatment with an investigational agent within 21 days, or \<5 times the half-life of the most recent therapy prior to study Day 1, whichever is shorter.
*  Has received systemic steroid therapy within 3 days prior to the first dose of the study treatment or is receiving any other form of immunosuppressive medication. Replacement therapy is not considered a form of systemic therapy. Use of inhaled corticosteroids, local steroid injection, or steroid eye drops is allowed.
*  Has not met the following waiting/washout periods for X-ray therapy (XRT) including external beam radiation therapy/external beam irradiation including protons:
*  Participant has had major surgery within 3 weeks prior to the first dose of study treatment or has not adequately recovered from any AEs (Grade ≤1) and/or complications from any major surgery. Surgical implantation of a port catheter is not exclusionary.
*  Prior Bone Marrow Transplant \<60 days of screening. Prior/Concurrent clinical study experience
*  Is currently enrolled or has participated in any other clinical study involving an investigational study or interventional medical research within 21 days or 5 half-lives, whichever is shorter, of an investigational medicinal product before signing ICF Diagnostic assessments
*  Has documented presence of Hepatitis B surface antigen (HbsAg) at Screening or within 3 months prior to first dose of study intervention.
*  Has a positive Hepatitis C virus (HCV) antibody test result at Screening or within 3 months prior to first dose of study intervention.
*  Has a positive HCV Ribonucleic Acid (RNA) test result at Screening or within 3 months prior to first dose of study intervention.
*  Has a known history of Human immunodeficiency virus (HIV) or has a HIV-positive test result at Screening.
*  Is pregnant or breastfeeding.