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.