Background: Intravenous irinotecan hydrochloride (IRN-IV) is approved for the treatment of adult colorectal cancer and also used off-label to treat a range of adult and pediatric tumors including recurrent Ewing sarcoma, rhabdomyosarcoma, neuroblastoma, hepatoblastoma, Wilms tumor, gynecologic cancers, lung cancer and medulloblastoma. Previously, a regimen of IRN-IV administered as a 60-min i.v. infusion daily for 5 days, bi-weekly in combination with other agents such as temozolomide has been recommended use in treating children with solid tumors (Blaney. ClinCanRes, 2001). Protracted administration schedule of IRN-IV is inconvenient for patients, so oral regimens utilizing IRN-IV have been developed and implemented with favorable tumor response (Wagner. ClinSarcRes, 2015). Unfortunately, the palatability of the IRN-IV is poor, leading to reduced compliance especially in younger pediatric patients. Development of an advanced formulation to improve tolerability and patient compliance is an important unmet need. VAL-413 is a novel formulation developed to improve palatability of oral irinotecan. This pilot trial will examine the safety and tolerability of this novel formulation and assess pharmacokinetics compared to current oral regimen. Methods: Eligibility: Up to 20 patients ≥ 1 year of age or ≤ 30 years of age with recurrent pediatric solid tumors and adequate bone marrow, renal and liver function, for whom irinotecan therapy is a treatment option will be enrolled. Trial Design: Two different dose levels of VAL-413, 90mg/m2/day or 110mg/m2/day will be studied in combination with fixed-dose temozolomide using a standard 3 + 3 phase I design. In the event the starting dose of 90 mg/m2/day is not tolerable due to toxicity, a lower dose of 75 mg/m2/day may be implemented. Treatment: During the first cycle of treatment, each patient will receive 4 daily doses of VAL-413 and one daily dose of the intravenous preparation of irinotecan taken orally (IRN-IVPO). During all subsequent cycles, only VAL-413 will be given with temozolomide in 5-day courses administered every 21 days, as tolerated. Outcome Measures:Toxicity is assessed by NCI CCTCAEv5; tumor response is assessed by RECIST 1.1. A palatability survey instrument will assess palatability of VAL-413 vs. IRN-IVPO; comparative intra-patient pharmacokinetics of irinotecan and its metabolites will be assessed. This trial is ongoing (CT.gov: NCT04337177), with no DLT observed to date. Clinical trial information: NCT04337177.