Studies on the effect of asparaginase in paediatric leukaemia

Asparaginase, an important agent in the treatment of acute lymphoblasticleukaemia, is thought to cause cell death by depriving lymphoblasts ofasparagine. However, because asparaginase also hydrolyses glutamine,a critical fuel for normal lymphocyte function, the mechanism ofasparaginase action requires further investigation. Plasma samples fromnewly diagnosed children who had received either native E. coli orpegylated (PEG)- asparaginase were assayed to determine asparagineand glutamine levels, asparaginase activity and antibody formation toasparaginase. In parallel, in vitro studies of asparagine and glutaminedeprivation on growth inhibition, apoptosis and the cell cycle in Molt-4 andNALM-6 cells were compared to the effects of asparaginase.Asparaginase activity in patient samples up to 3 days post treatment(native-treated) and 14 days (PEG- asparaginase) was generally good (>100 U/L). Asparagine levels were depleted throughout treatment;however, there was large inter-patient variation in the extent of glutaminedepletion which was reduced up to 9% of baseline levels. Antibodieswere largely undetected in either study. When asparagine or glutaminewas depleted after 72 hours in vitro, there was little effect on growthinhibition, apoptosis or cell cycle changes compared to cells treated withasparaginase. Only when asparagine and glutamine were depletedsimultaneously to low, clinically relevant levels (3 uM and 50 uMrespectively) was there an increase in apoptosis and growth inhibition,but little effect on the cell cycle. This suggests that within these cell populations under stress, a proportion of cells undergo division, someenter apoptosis and some become quiescent, which may allow the wholepopulation to survive. In vivo, this may explain how 'resistant' populationspersist, especially if low asparagine and glutamine levels are maintainedby mesenchymal cells in the bone marrow.XI