Although anti-cancer drugs are widely used in infants and young children with malignant brain tumors, the dosing of these toxic drugs is often scaled based on body size (e.g., BSA or body weight) or arbitrary age cut-offs. This dosing approach does not take into account the many developmental changes that occur in infants and young children. These changes, which include hepatic and renal maturation, may affect the disposition of anticancer drugs, leading to more severe toxicity, particularly myelosuppression, which puts these children at risk of life-threatening infections. Yet the pharmacokinetics and toxicity of many anticancer drugs have not been adequately studied in infants and young children. Thus, we propose to perform clinical pharmacokinetic (PK), pharmacogenetic, and pharmacodynamic (PD) studies of methotrexate (MTX), cyclophosphamide (CTX), and topotecan (TPT) as part of a clinical protocol that treats infants and young children with malignant brain tumors. In this grant application, we propose three hypothesis directed aims: 1.) To characterize the disposition of MTX, oral and intravenous CTX, and oral and intravenous TPT in infants and young children with malignant brain tumors, 2.) To use statistical and mechanistic models to identify PD and PKPD response relationships for anti-cancer drugs in infants and young children, and 3.) To develop dosing regimens for infants and young children to achieve more uniform systemic exposure to anticancer drugs across all pediatric age groups. Collectively, these studies narrow the gap in our understanding of the clinical pharmacology of anti-cancer drugs used to treat infants and young children. Our long-term goal is to determine rational dosing regimens for infants and young children by better understanding the developmental pharmacology of anti-cancer drugs and to apply these regimens to therapy for other childhood malignancies and chronic medical conditions.