Haemostasis during early treatment of childhood acute lymphoblastic leukaemia with the ALLTogether protocol

The survival rate for childhood acute lymphoblastic leukaemia (ALL) has improved over time, due to uniform treatment with risk-adapted therapy developed through international collaboration. The international ALLTogether treatment protocol (EUDRACT 2018–001795-38; ClinicalTrials.gov: NCT03911128) replaced the NOPHO ALL2008 (ALL2008; EUDRACT NCT03911128) protocol in Sweden in 2019. ALLTogether introduced a high dose of intravenous asparaginase during early induction combined with dexamethasone. Both asparaginase and glucocorticoids lead to a transient coagulopathy. Asparagine depletion reduces liver synthesis both pro- and anticoagulants, most importantly fibrinogen and antithrombin,1 leading to coagulopathy, whereas dexamethasone induces hypercoagulability.2, 3 After the initiation of ALLTogether, we encountered patients with bleeding tendency during early treatment regardless of sufficient platelet levels. Further examination revealed activated partial thromboplastin time (aPTT) without clot formation and immeasurable low fibrinogen (<0.3 g/L). Replacement of fibrinogen corrected the bleeding tendency and aPTT. To address concerns on increased risk of bleeding, we collected antithrombin and fibrinogen values and clinical data on thromboses and bleeding events during early treatment, and compared the results between the two protocols. This national study was approved by the Swedish Ethical Review Authority (2021–03146). We identified all children (n = 120) between the age of 1 and 18 years at diagnosis and treated in the six Swedish childhood cancer centres according to ALLTogether protocols between August 2019 (initiation of the pilot in Sweden) and March 2021 (the first substantial amendment) from the Swedish Childhood Cancer Registry. Patients with Down syndrome (n = 2) or transferred to high-risk block therapy during consolidation 1 (n = 1) were excluded. We selected controls matched by NCI-risk group treated with the ALL2008 protocol (n = 185). Patient characteristics between groups were balanced (Table 1). Data on coagulation parameters, thromboses and bleeding events during induction and consolidation 1 were collected from medical charts. As clinical practice in measuring fibrinogen and antithrombin values during treatment varied between the paediatric oncology centres, we could not compare the values at specific timepoints and recorded only the lowest measured abnormal values. Otherwise, fibrinogen and antithrombin were classified as normal or below reference value. Fibrinogen values were available for 76/117 (65%) of the ALLTogether and 99/185 (53%) of the ALL2008 patients. During ALLTogether induction, 46% (29/63) had fibrinogen levels below the reference (<2.0 g/L), comparable to 54% (45/84) during ALL2008 (p = 0.295). Hypofibrinogenemia was more pronounced during ALLTogether induction, 86% (25/29) had fibrinogen values <1.0 g/L, and 35% (10/29) had fibrinogen values <0.5 g/L, p < 0.001 (Table 1). During consolidation 1, 58% (28/48) of the ALLTogether patients had fibrinogen levels below the reference, compared with 75% (51/68) in the ALL2008 group (p = 0.058). Extremely low fibrinogen values (<0.5 g/L) during consolidation 1 were uncommon (1/28 and 1/51, respectively). Antithrombin was measured in 56/117 (48%) in the ALLTogether and 86/185 (46%) in the ALL2008 groups. More patients treated according to ALLTogether (65%; 28/43) than ALL2008 (10%; 7/72) had antithrombin values under the reference (<0.85 IU/mL) during induction (p < 0.001), but the level of reduced antithrombin activity did not differ between the protocols. No differences between the groups were seen under consolidation 1 (Table 1). As the initial reason to compare the two protocols was the unexpectedly low fibrinogen levels, we collected both major (>20 g/L decrease in haemoglobin level due to the bleed and/or bleeding event involving critical organs such as the central nervous system) and non-major (defined as bleeding events leading to intervention by healthcare professionals, e.g. transfusions, hospital admission or other measures that would otherwise not have been taken) bleeding events. There were no significant differences in bleeding events between the protocols: 15/117 (13%) of the ALLTogether patients had 23 bleedings compared with 34/185 (18%) patients with 47 bleeding events in ALL2008 (p = 0.202). Both groups had two major bleedings: one epidural bleeding and one secondary to cerebral sinus venous thrombosis (CSVT; ALLTogether) and one haemopneumothorax and one intestinal bleeding (ALL2008). We then compared thrombosis during early treatment: 6% (7/117) of ALLTogether patients had a thrombotic event, defined as thrombosis leading to anticoagulant treatment (three CSVTs, two upper and two lower venous system). One CSVT was associated with secondary bleeding. By comparison, 4/182 (2%) had a thrombosis (three CSVTs and one lower venous system) in the ALL2008 group. In line with previous reports,3-5 combining asparaginase with dexamethasone led to a pronounced hypofibrinogenemia during induction in ALLTogether. Despite low fibrinogen during ALLTogether, the number of bleeding events did not increase. This is likely due to simultaneous decrease in anticoagulant proteins maintaining the delicate haemostatic balance. Non-major bleeds, expected but seldom reported side effects during ALL treatment, were common and led to outpatient visits, hospital admissions or transfusions. Our cohort was not powered to compare the incidence of thrombotic events between the two protocols, but we would expect the timing of thromboses to be earlier during ALLTogether than during ALL2008. Study limitations included the relatively low number of patients, retrospective approach, and differences in routines and the laboratory methods used for measuring coagulation parameters between centres and protocols. Many patients lacked fibrinogen and antithrombin values, especially during ALL2008 induction, before the start of asparaginase treatment. Due to these limitations, the results should be interpreted with caution. Study strengths were the national, population-based design and detailed patient chart review. In conclusion, our study showed no significant differences in the frequency of thrombosis or bleeding events between treatment protocols despite significantly decreased fibrinogen levels during ALLTogether. Yet, haemostatic reserve capacity during ALLTogether may be even more reduced than during ALL2008 and should be considered with clinical bleeding tendency and prior to surgery. As some patients had immeasurable fibrinogen levels, we encourage to consider the increased potential predisposition to bleeding in ALLTogether, particularly during induction, to optimise haemostasis in high-risk scenarios when managing these patients. Future studies should try to clarify whether novel methods including global haemostasis assays can help to identify children at risk of bleeding and those who may benefit from thromboprophylaxis. SR, AH, JA, MB, MH, ASH and HV contributed to the design and implementation of the study. JF, JJ, ID, LH, AV, AW and OZ collected data from paediatric cancer centres. JF, AH and SR performed data analysis and interpretation. JF wrote the original draft. All authors reviewed, edited and approved the original draft. We thank the Ellen Bachrach Memorial Fund (AH) and the Swedish Childhood Cancer Fund (MH, SR) for financial support. The authors have no conflict of interest to report.