Asparaginase (ASNase) has long been considered to be an important element in the management of childhood ALL. Its antileukemic effect is thought to be related to a metabolic deficiency reflected by the blasts' incapability to synthesize asparagine (ASN) from aspartic acid. Treatment with ASNase aims therefore at depleting the blood of ASN in order to exhaust the substrate supply that became selectively essential to the malignant cells. Highly interesting findings on PEG-ASNase, which is a polyethylene glycol (PEG) conjugate of an ASNase derived from Escherichia coli, have recently been reported by Avramis et al.1 In the context of a randomized trial, the authors evaluated several important pharmacologic parameters after a single intramuscular (IM) administration of 2500 IU/m2 PEG-ASNase in children with newly diagnosed ALL.

The pharmacokinetic results show that the mean ASNase serum activity peaked on day 5 after the first IM dose at an average of 1000 U/L and was quantifiable within the therapeutic range of above 100 U/L over a period of about 28 days. Elimination from the serum was described by a single exponential function.1 Remarkably, the presented findings contrast in every aspect with results observed after the intravenous use of the identical dose of PEG-ASNase within the ALL protocols of the Berlin-Frankfurt-Münster (BFM) study group.2 Using a well-established drug monitoring program at our own institution, we have been able to identify peak ASNase serum activities ranging around 2500 U/L immediately after administration, but the time period with activity values above 100 U/L, which was usually 3 weeks, was significantly shorter than the one reported by Avramis et al.1 Comparison of these data with intravenous results after 1000 IU/m2 (Müller et al3) have also shown that a substantial increase of the PEG-ASNase dose neither translated into a prolongation of time with activities above 100 U/L nor influenced the rate of patients with early ASNase inactivation. For these reasons and with regard to the predictive use of pharmacokinetic (PK) modeling, we conclude that the elimination of PEG-ASNase from the serum cannot generally be characterized using the linear model as proposed by the authors. Due to the effects of the IM route of administration on the distribution of the ASNase activities, the suggested method might, however, in their study have served as the most appropriate approach to describe PEG-ASNase elimination.

ASN concentrations in the serum and cerebrospinal fluid (CSF) that allow a more accurate estimate of an effect caused by ASNase have also been measured in the study performed by Avramis et al.1The authors state that ASN was depleted rapidly from serum and CSF after administration of PEG-ASNase. This is somewhat confusing because, in contrast to the authors' conclusion, ASN is represented in considerable amounts in every figure illustrating the amino acid results. Independent of the applied ASNase or the activity measured at the same time, the lowest mean ASN concentrations are depicted in a range of about 1 μM. In view of in vitro findings demonstrating ongoing leukemic blasts' growth in medium containing comparable amounts of ASN,4 as well as findings from other working groups showing complete depletion at a level of 0.2 μM after administration of different schedules using native ASNase preparations,5-7 a clarification of the outlined discrepancy would be desirable for an adequate estimate of the treatment intensity achieved with the schedules presented in the paper. Under methodologic aspects, giving a clear definition of ASN depletion or a description of the preanalytic sample preparation might be helpful since hemolysis of blood after withdrawal is eg known to influence ASN concentrations leading to false high values.

PEG-asparaginase and deamination of serum asparagine in children with standard-risk lymphoblastic leukemia (CCG-1962)

We are aware of Vieira Pinheiro et al's data showing the rapid elimination of PEG-asparaginase at low serum concentrations. We have not seen that late decay with a similar formulation in our pediatric population. Our findings are consistent with those reported by Asselin et al1-1 using the same PEG-ASNase formulation as that used in the Children's Cancer Group studies. The Europeans use a different form of pegylated nativeEscherichia coli asparaginase produced in Japan and marketed by Medac than that used in the United States. Dr Boos's group reported that the native asparaginase marketed by Medac has different pharmacokinetic properties than other native E coliasparaginase preparations.1-2 The native E coliasparaginase used in the United States is produced by Merck and is pegylated by a different chemistry according to the manufacturer (Enzon, Bridgewater, NJ). This may have lead to a pegylated product with different characteristics in drug disposition and elimination. The rapid fall in asparaginase activity at late time points suggests that their enzyme preparation may contain a mixture of variously pegylated products with different half-lives.

The route of administration in our pediatric trial was intramuscular, while the Boos group gives their drug intravenously. Our population analyses estimated a long half-life of absorption from the injection site of this drug. This could have lead to a depot effect that masked any late decline in enzyme activity. Nevertheless, we also found a first-order disappearance of PEG-asparaginase in 24 adult patients treated intravenously.1-3 It should be added that neither the native E coli asparaginase nor the PEG-asparaginase marketed by Medac are licensed in the United States. We would like to conduct a randomized study between the 2 PEG-asparaginase formulations if the Medac product becomes licensed for use in the United States.

We have consistently found low residual asparagine concentrations in some patient samples with high levels of enzyme activity. We rapidly chilled the samples to prevent continued ex vivo deamination of asparagine, and we found that very few samples had visual signs of hemolysis. Asselin et al have reported that asparagine concentrations return toward normal much faster when care is taken to inhibit ex vivo enzyme activity.1-4 We sent a blinded set of our samples to Dr Asselin, and she confirmed the asparagine values we reported. Liver perfusion studies and modeling of asparaginase treatment suggest that there is a high rate of asparagine input into the circulation from diet and the tissues.1-5,1-6 An equilibrium between the rate of asparagine input and the asparaginase activity can result in a low (nonzero) steady-state asparagine concentration.

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