Who can participate?

Inclusion Criteria:

• Patients must be =< 12 years of age at the time of study enrollment. Patients will be stratified into 4 age groups:

• 0 to 6 months

• 6 months and 1 day to 12 months

• 12 months and 1 day to 36 months

• 36 months and 1 day to 12 years

• Newly diagnosed and relapsed cancer diagnosis that is being treated with vinCRIStine at the 1.5 mg/m^2 dose level

• Any disease status

• Patients must have a Lansky performance status of 50 or higher

• Patients must be receiving a treatment regimen that includes 1.5 mg/m^2 vinCRIStine (maximum dose 2 mg)

• Patients with a BSA < 0.6 m^2 must be dosed according to the Children's Oncology Group (COG) BSA-banded infant dosing table for the 1.5mg/m2 dose level for vinCRIStine

• Note: Patients can be studied after any dose of vinCRIStine

• Patients who are NOT enrolled on a COG clinical trial and who have a BSA < 0.6 m^2 and who are being dosed according to another infant dosing method (e.g., the 30-Rule) can receive a dose of vincristine from the infant dosing table for the pharmacokinetic study. These patients will NOT be part of the Dose Modification Assessment

• Patients with a seizure disorder may be enrolled if on allowable anticonvulsants and well controlled as evidenced by no increase in seizure frequency in the prior 7 days

• Nervous system toxicities (Common Terminology Criteria for Adverse Events [CTCAE]) version (v)5 resulting from prior therapy must be grade =< 2

• Central venous access device in place (e.g., percutaneous indwelling central catheter [PICC], port, Broviac) that can be used for pharmacokinetic (PK) sampling

• VinCRIStine may be given as an outpatient, as long as all sample time points can be collected, which will require return for hour 24 sampling

Exclusion Criteria:

• Azoles antifungals and macrolide antibiotics: Patients who are currently receiving an azole or macrolide (e.g., fluconazole, isavuconazole, itraconazole, posaconazole, voriconazole, ketoconazole, eryromycin, clarithromycin, azithromycin, roxithromycin, or telithromycin) are not eligible

• CYP3A4/5 inducers/inhibitors: Patients receiving any medications or substances that are considered moderate or strong inhibitors or inducers of CYP3A4/5 are not eligible. Moderate or strong inducers or inhibitors of CYP3A4/5 should be avoided from 14 days prior to enrollment to the end of the study. Note: dexamethasone for central nervous system (CNS) tumors or metastases, on a stable dose, is allowed

• Anticonvulsants: Patients receiving moderate or strong CYP3A4/5 enzyme inducing anticonvulsants are not eligible.

• Patients with Charcot-Marie-Tooth disease

• A baseline neurological disorder with manifestations that overlap with vinCRIStine-associated neurotoxicities

• Patients receiving a modified dose (< 1.5 mg/m^2) of vinCRIStine due to prior toxicity

• Patients who in the opinion of the investigator may not be able to comply with the sampling requirements of the study