Clinical utility and implications of asparaginase antibodies in acute lymphoblastic leukemia

Abstract

Hypersensitivity to asparaginase is common, but the differential diagnosis can be challenging and the diagnostic utility of antibody tests is unclear. We studied allergic reactions and serum antibodies to E. coli asparaginase (Elspar) in 410 children treated on St. Jude Total XV protocol for acute lymphoblastic leukemia. Of 169 patients (41.2%) with clinical allergy, 147 (87.0%) were positive for anti-Elspar antibody. Of 241 patients without allergy, 89 (36.9%) had detectable antibody. Allergies (P=0.0002) and antibodies (P=6.6 × 10(-6)) were higher among patients treated on the low-risk arm than among those treated on the standard/high-risk arm. Among those positive for antibody, the antibody titers were higher in those who developed allergy than in those who did not (P<1 × 10(-15)). Antibody measures at week 7 of continuation therapy had a sensitivity of 87-88% and a specificity of 68-69% for predicting or confirming clinical reactions. The level of antibodies was inversely associated with serum asparaginase activity (P=7.0 × 10(-6)). High antibody levels were associated with a lower risk of osteonecrosis (odds ratio=0.83; 95% confidence interval, 0.78-0.89; P=0.007). Antibodies were related to clinical allergy and to low systemic exposure to asparaginase, leading to lower risk of some adverse effects of therapy.

Figure 1. Frequency of hypersensitivity to Elspar

The frequency of allergic reactions and anti-Elspar antibody (Ab) positivity in patients enrolled on the low-risk (LR) and standard/high-risk (SHR) treatment arms. Compared with those on the SHR arm, patients on the LR arm had more reactions (P < 0.00001) and anti-Elspar antibody (P = 0.0002).

Figure 2. Antibody levels in patients with and without clinical reactions

Left: Anti-Elspar optical density (OD) measures (natural log) for 410 patients at days 5, 19, and 34 of remission induction and weeks 7 and 17 of continuation therapy. At each time point, patients are grouped based on whether they ever (or never) experienced a clinical allergic reaction (Rxn), and whether they were ever (or never) positive for anti-Elspar antibody (Ab) at anytime during therapy. Among the patients positive for anti-Elspar, those who had reactions (white bars) had significantly higher anti-Elspar levels than did those who had silent hypersensitivity (light grey) at day 34, week 7, and week 17. Among the patients negative for anti-Elspar antibodies (as defined by the threshold in the Supplement), those who had allergic reactions (hatched) exhibited slightly elevated anti-Elspar levels compared with those without any allergic reactions (dark grey) at weeks 7 (P = 0.05) and 17 (P = 0.03). Right: cumulative anti-Elspar antibody AUC over the first 35 weeks of therapy for antibody-positive patients who had allergic reactions versus those who had silent hypersensitivity. Box plots: median, quartiles, non-outlier range.

Figure 3. Correlation between serum asparaginase activity and antibody level

Serum asparaginase activity (IU/mL) measured 6–8 days after the last Elspar dose of 25000 U/m². (A) Asparaginase activity was significantly lower in the samples positive than in those negative for anti-Elspar antibodies. Box plots: median, quartiles, non-outlier range. (B) Asparaginase activity inversely correlated with anti-Elspar OD reading of serum samples. Individual data points and the regression line are shown for all cases (n = 146).

Figure 4. Cumulative incidence of osteonecrosis based on anti-asparaginase antibody AUC

Patients were stratified by low-risk (LR) versus standard/high-risk (SHR) treatment arm and age at diagnosis: (A) patients in the LR arm with age greater than 10 years; (B) patients in the SHR arm with age greater than 10 years; (C) patients in the LR arm with age under 10 years; (D) patients in the SHR arm with age under 10 years. Left: Cumulative incidence of symptomatic (grade 2 to 4) osteonecrosis (ON) in patients with anti-Elspar antibody AUC (OD × days) lower than 2.12 (blue) versus greater than 2.12 (red). Right: box plot of anti-Elspar antibody AUC in patients with (blue) versus without symptomatic ON (red). a = P value based on log rank test and b = P value based on Wilcoxon rank sum test. *Fisher’s exact test (P = 0.11) was also performed because the sample size is small.

Figure 5