Intramuscular PEG-Asparaginase At 1000 U/m2 Achieves Adequate Trough Activity Levels in the Majority of Patients Treated on the UKALL 2003 Childhood Acute Lymphoblastic Leukemia (ALL) Protocol
Authors: Fong Caroline YK , Parker Catriona Anne , Hussain Adiba , Liu Jizhong , Essink Monika , Kuehnel Hans Juergen , Lancaster Donna L , Pridham Genevieve , Payne Debbie , O'Horo Charlotte , Krishnan Shekhar , Hancock Jeremy , Goulden Nicholas , Moorman Anthony V. , Richards Sue , Vora Ajay J , Saha Vaskar , Masurekar Ashish Narayan 
Published: October 2019

Abstract
Abstract 2573

Introduction:
Polyethylene glycol conjugated L-asparaginase (PEG-ASNase) is used in varying doses between 1000–2500 U/m2 in the therapy of ALL and associated with a wide variation in pharmacokinetics. UKALL 2003 used intramuscular PEG-ASNase at 1000 U/m2.

Patients & Methods:
Patients enrolled in the UKALL 2003 trial (ISRCTN: 07355119), were consented for trough asparaginase activity analysis during therapy. National Cancer Institute (NCI) standard risk (SR) patients with rapid early response and non-high risk (HR) cytogenetics (Lancet Oncol. 2010;11:429) received a 3 drug induction (Dexamethasone, Vincristine & PEG-ASNase). All other patients received additional Daunorubicin. PEG-ASNase was given on days 4 and 18 of induction, and at least once post induction. Trough plasma asparaginase activity was measured by the indooxine method (Anal. Biochem. 2002;309:117). The lower limit of assay detection was 34 U/L. Adequate asparaginase activity was defined as a trough level of > 100 U/L. IgG and IgM antibodies to PEG-ASNase and native asparaginase were measured by indirect ELISA. Asparaginase activity was correlated with defined risk factors, minimal residual disease (MRD) at day 28 of induction and development of anti-asparaginase antibodies using the chi-squared test.

Results:
Between July 2008 to July 2011, 482 patients aged 1–25 years from 27 centres were recruited. Numbers of samples assayed in induction were 335 & 371 after first and second doses respectively. Overall, 86% (n=606/706) of samples had adequate activity during induction time points. There was > 10 fold variation in activity levels (Figure 1). Three hundred and nine out of 706 samples had activity > 3 times the therapeutic threshold, while 51/100 samples with inadequate activity had no detectable drug levels (median: below detection limit, range: < 34–99 U/L). Thus, increasing the dose of PEG-ASNase in induction is unlikely to benefit patients with inadequate activity. Compared to SR patients, NCI HR patients had a higher incidence of inadequate asparaginase activity in induction (p=0.002). Inadequate asparaginase activity correlated with high MRD (≥ 10−4) in SR patients (p=0.045), especially those with good risk cytogenetics (p=0.012), and in particular the high hyperdiploid subgroup (p=0.03). Inadequate asparaginase activity during induction did not correlate with MRD in HR patients (p=0.699), possibly because these patients received in addition Daunorubicin (Table 1).
Results of serial asparaginase activity (at least one time point each in induction and post induction), measured in 282 patients are summarised in Table 2. Antibodies were detected in 18 of 81 patients tested. All had anti-PEG and 7 in addition also had anti-asparaginase antibodies. While all antibody positive patients had inadequate asparaginase activity at one time point, 17 had adequate activity prior to antibody detection, suggesting immune-mediated drug inactivation at re-exposure. Antibodies were not detected in 14/15 patients who had inadequate activity at first exposure, so the mechanism here remains unclear.
The reported incidence of asparaginase toxicity in this study was 6.6% (n=32/482). This included hypersensitivity (n=17/482) that was almost exclusively seen in HR patients (n=16/17), thrombosis (n=10/482) and pancreatitis (n=5/482).

Conclusions:
Intramuscular PEG-ASNase given fortnightly at 1000 U/m2 during induction provides adequate asparaginase activity in 86% of patients. Monitoring asparaginase activity may benefit patients who receive 3 drug induction and improve the resolution of the current prognostic classification.

Disclosures:
Off Label Use: PEG-asparaginase. Essink:medac: Employment. Kuehnel:medac: Employment.