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6580 Background: Advances in therapies have resulted in an overall complete remission rate of approximately 85% for childhood acute lymphoblastic leukemia (ALL). In contrast, the overall remission rate of adults with leukemia continues to be poor, only about 40% in cases of T cell-ALL (T-ALL). Therefore, it is imperative to generate new therapies that alone or in combination with other treatments could potentially increase the percentages of complete responders or be used to treat the refractory ALL population. Our published results show that a pegylated form of human arginase I (peg-Arg I) prevented T-ALL cell proliferation in vitro and in vivo through the induction of tumor cell apoptosis. Interestingly, the anti-leukemic effects induced by peg-Arg I did not affect the anti-tumor activity of normal T cells, suggesting an anti-tumor specific effect. Our hypothesis states that peg-Arg I has an anti-tumoral effect on B-ALL and T-ALL cells in vitro and that the sensitivity of ALL cells to peg-Arg I depends on their expression of argininosuccinate synthase (ASS) and their ability to produce L-arginine de novo from citrulline. Methods: Malignant T cell proliferation was tested using nonradioactive cell proliferation yellow tretrazolium salt kit. Apoptosis studies were based on the expression of annexin V. Western blot assays were conducted to determine enzymatic expression in different cell lines. Results: The results of our in vitro experiments showed that peg-Arg I had a pro-apoptotic and anti-proliferattive effect on B-ALL cells similar to the one previously seen on T-ALL cells. These effects can be overcome in cell lines able that express ASS and therefore to produce L-arginine de novo. Conclusions: Our results suggest the role of ASS in the ALL-apoptosis induced by peg-Arg-I. Our next steps include: _Understand why ASS-expressing ALL cells do not undergo apoptosis when cultured with peg-Arg-I_Determine the role of ASS in the anti-leukemic effect induced by peg-Arg-I in vivo. Completion of this research is expected to lead to a better understanding of how peg-Arg-I kills ALL cells and could provide the foundation for a novel therapy for ALL patients.