Pharmacokinetic Study of Forodesine in Children With Relapsed or Refractory T-cell or B-cell Precursor Acute Lymphoblastic Leukaemia or T-cell Non- Hodgkin's Lymphoma.

Inclusion Criteria: * Males and females aged ≥ 2 years to ≤18 years. ≥ 13 kg * Female subjects of childbearing potential (i.e. have reached the age of menarche) must have a negative serum or urine pregnancy test recorded prior to the first dose of study medication, be non-lactating, and be willing to use adequate and highly effective method of contraception throughout the study and for one month after the last dose of study medication, if sexually active. * Sexually active male subjects must be willing and able to use a barrier form of contraception (i.e. condoms) or sexual abstinence throughout the study and for one month after the last dose of study medication * Unequivocal histological diagnosis of T-ALL, BCP-ALL or T-NHL (World Health Organisation \[WHO\] classification) at initial diagnosis * Relapse (³25% marrow blasts) or failure to respond after at least one standard regimen for their disease for subjects with a T-cell malignancy who are ineligible for other therapy of greater curative potential, or failure to respond after at least two standard regimens for subjects with a B-cell precursor malignancy * KPS or LPS (as appropriate for subject's age) scores ³60 * Anticipated life expectancy of at least 6 weeks * Adequate kidney (creatinine levels ≤ 2.0 times upper limit of normal) and liver function tests (aspartate aminotransferase \[AST\] and/or alanine aminotransferase \[ALT\] ≤3 times upper limit of normal and total bilirubin ≤5 times upper limit of normal) * Signed ICF and assent if appropriate according to local laws and regulations prior to start of any study specific procedures. Exclusion criteria: * Females who are pregnant (positive β-hCG test) or lactating * Subjects with a history of HIV and/or HTLV-1 * Subjects with known active HBV, HCV, CMV and/or EBV infection * Subjects with clinical evidence of active symptomatic CNS disease * Subjects with active serious infection * Prior treatment with any antileukemic agent, chemotherapy or leukophoresis treatment within 7 days (within 4-5 days for 6-mercaptopurine (MP) and within 2 days for low-dose methotrexate) prior to study entry * Lack of full recovery from adverse drug reactions due to prior therapy, independent of when that therapy was given * Concurrent treatment with other anticancer agents (CNS prophylaxis e.g. intrathecal methotrexate and corticosteroid use will not be excluded) * Subjects who have chronic gastrointestinal disease or conditions that may hamper compliance and/or absorption of the product; however, study drug administration via nasogastric or gastrostomy tube is allowed * Any history of hypersensitivity or intolerance to any component of the study medication. * Subjects who have received an investigational medicinal product within 30 days of study entry (defined as the start of the Screening Period). * Current participation in another clinical trial is not permitted unless the sole purpose of the trial is for long term follow up/survival data.