Rasburicase prevents tumor lysis syndrome despite extreme hyperleukocytosis.

Hyperleukocytosis [white blood cell count (WBC) >100 x 10(9)/l] occurs in 5%-22% of pediatric patients with acute leukemia. Rasburicase (recombinant urate oxidase) is very effective for the prophylaxis and treatment of hyperuricemia, but its efficacy in marked hyperleukocytosis (WBC >200 x 10(9)/l) is not well known. We describe three children with marked hyperleukocytosis (WBC of 508, 320, and 242 x 10(9)/l) at initiation of induction chemotherapy. All three received rasburicase. Minimal metabolic disturbance occurred in all patients. We conclude that WBC reduction strategies may not be required solely for the risk of tumor lysis syndrome in patients with very high WBC (>200 x 10(9)/l) who are treated with rasburicase.