Among various complications associated with L-asparaginase, an essential treatment component of virtually all protocols for childhood acute lymphoblastic leukemia (ALL), clinical hypersensitivity is one that limits continued use of this agent in reacting patients. In the
United States and many other developed countries, pegaspargase, anEscherichia coli derived asparaginase enzyme that is conjugated with polyethylene glycol (PEG), is used as the first-line treatment, whereas Erwinia asparaginase (Erwinase), isolated from Erwinia chrysanthemi, is used in patients who developed hypersensitivity to any E. coli asparaginase preparation: native E. coli asparaginase (which is no longer available in the United States) or pegaspargase. Some studies suggest that Erwinase may not be necessary for some ALL patients who developed hypersensitivity reaction to E. coli product, provided that they have already received 50% or more of the prescribed dose and/or treatment is augmented with other chemotherapeutic agents. 1 However, other studies have shown that less intensive asparaginase treatment regimens and even subclinical hypersensitivity (so-called silent inactivation) were associated with inferior outcomes overall 2 ; moreover, the optimal number of doses of asparaginase for individual patients is unknown and likely treatment regimen dependent. Hence, in most clinical trials, patients would be switched to receive Erwinase if they developed hypersensitivity to native E. coli asparaginase or pegaspargase.Because adequate Erwinase treatment requires an onerous thrice weekly schedule, and as many as one-third of the patients develop hypersensitivity or infusion reactions to it, 3 a long-acting and less immunogenic Erwinia asparaginase (pegcrisantaspase) was developed to address these issues. A Phase I study of this agent was successfully conducted in 10 adults aged 18-50 years (median, 40.6 years) without a previous history of allergic reaction to Erwinase 4 ; few if any of these patients would have been previously exposed to pegaspargase, a drug seldom used to treat adults with ALL, especially before native asparag- were responsible for the immune-mediated reactions to pegcrisantaspase in these three patients. Each of the three patients with a hypersensitivity reaction or silent inactivation was exposed to pegaspargase 1-6 weeks prior to administration of pegcrisantaspase, whereas the remaining patients, who lacked reaction or silent inactivation, had not been exposed to pegaspargase for 5.5 years, suggesting that anti-PEG-mediated immune reactions may not elicit durable immunologic memory. Regrettably, this "unexpectedly" high frequency of immune reaction resulted in permanent closure of the study to further accrual.As mentioned by Rau and colleagues, 5 PEG has been used not only in the pharmaceutical industry but also in processed foods and cosmetics. Although PEG has been considered poorly immunogenic over the years, multiple recent studies have shown that anti-PEG IgG and IgM can be detected at low levels in approximately ...