Ammonia Levels Should Not Be Used as a Surrogate Marker of Levels of Asparaginase Activity in Acute Lymphoblastic Leukemia Patients

To the Editor: Asparaginase is an enzyme which hydrolyses asparagine into aspartic acid and ammonia. Levels of asparaginase activity are relatively easy to measure, and a trough level>100U/L is associated with complete asparagine depletion. [1] So, the measurement of levels of asparaginase activity is a very useful tool tomonitor asparaginase activities in clinical practice. However, these assays are not available in all countries in contrast to ammonia measurements. Recently, a Polish study group published ammonia levels of 87 children during acute lymphoblastic leukemia (ALL) induction therapy. They concluded that the ammonia concentration reflects the levels of asparaginase activity and could be useful when asparaginase activity assays are not available. [2] We evaluated if ammonia levels could be used as a surrogate marker of levels of asparaginase activity. We prospectively obtained 487 ammonia measurements in 89 patients during the first 30 weeks of intensification with asparaginase of the Dutch Childhood Oncology Group (DCOG) ALL-10 protocol. [3] At week 1 (baseline), week 3, 5, 7, 9, 15, and 25 during the intensification course and week 37 (8 weeks after the final PEGasparaginase course), ammonia levels were obtained. Blood samples were put in an ice bath and were immediately processed at the laboratory to minimize the ongoing production of ammonia by asparaginase ex vivo. Levels of asparaginase activity were assessed and processed as described earlier. [4] The individual ammonia levels and asparaginase activities in patients receiving PEGasparaginase are presented in Figure 1. Previously, we found that 90% of the allergic reactions occurred on the second PEGasparaginase dose in the intensification phase, and that the first dose was completely inactivated in these patients. [5] All patients with an allergy to PEGasparaginase or with silent inactivation of PEGasparaginase showed ammonia levels below the upper limit of normal (50mM) prior and 2 weeks after the first PEGasparaginase dose, as shown in Figure 1A. 64% of these patients showed an modest increase in ammonia level, whilst their levels of asparaginase activity remained zero. Ammonia levels after the first PEGasparaginase course increased in all patients without an allergy to and without silent inactivation of PEGasparaginase. However, only 50% of these children had increased ammonia levels above the upper limit of normal (Fig. 1A). All these patients had sufficient levels of asparaginase activity (>100U/L) independent of either normal or increased ammonia levels. The correlation between ammonia levels and PEGasparaginase activity levels showed a strong association (Spearman correlation coefficient1⁄4 0.64, P< 0.001) (Fig. 1B). Therefore, we conclude that the ammonia levels were related to the levels of asparaginase activity during therapy. In conclusion, the ammonia levels increase after the first PEGasparaginase course in all patients without an allergy to or silent inactivation of PEGasparaginase, however, 50% of these ammonia levels remain below the upper limit of normal. No increase of ammonia levels after the first PEGasparaginase course is suggestive for inactivation of PEGasparaginase, but approximately 2/3 of the patients with complete inactivation of asparaginase showed an modest increase of ammonia levels. So, ammonia levels do not predict the asparaginase activity and should not be used as a surrogate marker of levels of asparaginase activity.