Given the toxicities associated with pediatric acute lymphoblastic leukemia (ALL) induction, consolidation, and delayed intensification courses, arrival at the maintenance phase was once thought to be a time to celebrate and relax. Unfortunately, results of comparative studies of 6-thioguanine (6-TG) and 6-mercaptopurine (6-MP) in maintenance initiated a decade ago took the shine off maintenance "relaxation," as up to 11% of children treated with 6-TG developed veno-occlusive disease. A higher percentage of children developed a syndrome of periportal fibrosis and portal hypertension. This left many groups with the seemingly more benign 6-MP at the core of their maintenance regimens. However, as Schmiege-low et al show, while use of 6-MP may avoid the early toxicities of its antimetabolite relative, because of the risk of late malignancies, its use in maintenance is not without consequences.
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