Individualized High Dose Methotrexate to Treat Cancer in Children Who Have a Significant Risk for Side Effects to Methotrexate

Pediatric cancer patients are being asked to take part in this study who have a cancer that is treated with high doses of the drug methotrexate (MTX). In addition, these patients have either had significant side effects to methotrexate in the past or their doctor thinks that they are at high risk for side effects from receiving methotrexate. Methotrexate is a cancer-fighting drug that is very important in the treatment of leukemia. In this study, investigators are testing a new method of giving high dose methotrexate to cancer patients which may reduce the chances that the level of methotrexate in the blood is too high. When the levels are too high this is thought to lead to an increase in side effects. Side effects are unintended and unwanted results of treatment. The initial ordered amount of methotrexate and the period over which methotrexate is given will not change from the current standard of care (meaning what is usually done by doctors, and would likely be done if the patient was not on this study). This study is testing a new method of monitoring and potentially adjusting the final amount of methotrexate that the patient will end up receiving based on levels of methotrexate in the blood in the first 24 hours in order to try to prevent side effects in patients with a previous history of side effects from methotrexate or who are at high risk for having side effects. On this study the investigators will check methotrexate levels in the blood 2 hours after the patient starts receiving the drug and the investigators will lower the dose of methotrexate if needed. Investigators will do the same thing again 6-8 hours later. Investigators will also collect an optional blood sample from the patient because the investigators want to study how genetic (DNA) differences are involved in how the body processes methotrexate.