Nucleoside analogues in the treatment of haematological malignancies.

The nucleoside analogues are a group of antimetabolite cytotoxics which generally have to be metabolised to the equivalent nucleotide before incorporation into DNA. Cytarabine is a well established component of the treatment of acute leukaemias and has its principal action on dividing cells. New formulations include a liposome encapsulated product for intrathecal use and oral cytarabine ocfosfate which may be suitable for long-term outpatient use. Pentostatin acts by causing accumulation of deoxynucleotides and, although active against hairy cell leukaemia, is associated with a poor tolerance profile. Cladribine and fludarabine have substantial activity in the treatment of chronic lymphocytic leukaemia (CLL) and low-grade non-Hodgkin's lymphoma (NHL). Fludarabine is the more thoroughly investigated of the two and is currently being developed in combination therapies for CLL and NHL and also in a combination with cytarabine for acute myeloid leukaemia. Fludarabine's immunosuppressive activity is being exploited in the conditioning of patients for non-myeloablative stem cell transplantation. Gemcitabine is an established agent in the treatment of a number of solid tumours but also has activity in haematological malignancies which might be exploited by the use of extended infusion schedules. Newer agents including nelarabine, clofarabine and troxacitabine are undergoing clinical evaluation and show promising activity.