Anticoagulation prophylaxis reduces venous thromboembolism rate in adult acute lymphoblastic leukaemia treated with asparaginase‐based therapy

Leukemia and risk of stroke: a Mendelian randomization analysis

BACKGROUND

Observational studies suggest an association between leukemia and stroke, but causality remains unclear. Certain leukemia types may increase stroke risk, but variations exist in stroke and mortality rates across leukemia subtypes. This study employed Mendelian randomization (MR) to investigate links between leukemia subtypes and stroke.

METHODS

We conducted a two-sample Mendelian randomization (TSMR) study utilizing genetic variants linked to various subtypes of leukemia as instruments to investigate their causal effects on stroke, specifically ischemic stroke (IS) and intracerebral hemorrhage (ICH). The leukemia dataset comprised 456,276 subjects from the UK Biobank, while the stroke dataset was sourced from the FINNGEN consortium, encompassing 212,774 participants.

RESULTS

In the present study, there was suggestive evidence that genetically predicted chronic lymphocytic leukemia (CLL) is associated with ischemic stroke (odds ratio, 1.02; 95% confidence intervals, 1.01-1.05; P = 0.024), but no significant association was observed with intracerebral hemorrhage (ICH) (0.74; 0.99-1.03; P = 0.237). Additionally, chronic myeloid leukemia (CML), acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) was no significant associations between with stroke according to genetical prediction even if heterogeneity test and pleiotropic test was performed.

CONCLUSIONS

Our Mendelian randomization analysis revealed that chronic lymphocytic leukemia (CLL) was associated with an increased risk of ischemic stroke (IS) but not intracerebral hemorrhage (ICH). Conversely, there was no evidence supporting causal associations of chronic myeloid leukemia (CML), acute lymphoblastic leukemia (ALL), or acute myeloid leukemia (AML) with either type of stroke. These findings enhance our comprehension of the intricate interplay between various leukemia subtypes and the risk of stroke. Further research is essential to delve into the underlying mechanisms and potential clinical implications of these observed associations.

Thromboprophylaxis with intermediate dose enoxaparin during asparaginase containing induction for young adults with acute lymphoblastic leukemia

Thrombosis rates among young adults receiving asparaginase (ASP) for acute lymphoblastic leukemia (ALL) can reach 34%, with highest risk during induction. Our institution implemented a standard practice of 1 mg/kg/day enoxaparin administered to young adults with ALL who are treated with ASP during induction. We performed a retrospective analysis of patients who received thromboprophylaxis with enoxaparin 1 mg/kg/day during ASP-containing induction for ALL at Oregon Health & Science University from 2012 to 2023. The primary outcome was the cumulative incidence of thrombosis during induction. Bleeding events were assessed. Sixty-two patients were included in our analysis. Four patients (6.5%; 95% CI 1.8%-15.7%) experienced a thrombotic event. Three events were catheter-associated and 1 event was a distal lower extremity deep vein thrombosis related to myositis. No cerebral sinus thromboses, thrombosis-related deaths or major bleeding events occurred. Intermediate-dose enoxaparin is a promising thromboprophylaxis strategy and warrants further prospective research.

Venous thromboembolism in adolescents and young adults with acute lymphoblastic leukemia treated on a pediatric-inspired regimen

Asparaginase (ASP)-containing regimens for acute lymphoblastic leukemia (ALL) are associated with venous thromboembolism (VTE). We evaluated the prevalence, risk factors, role of prophylaxis and clinical impact of VTE among adolescents and young adult (AYA) patients (15-50 years) treated on Dana-Farber Cancer Institute (DFCI) ALL protocols. The 1- and 2-year cumulative incidence of VTE were 31.9% (95% CI: 27.0%, 36.9%) and 33.5% (95% CI: 28.5%, 38.5%) respectively, with most events occurring during ASP-based consolidation phase (68.6%). VTE was more frequent in patients with overweight/obese vs. normal BMI (39.2% vs. 29.0%, p = 0.048). In a 1-year landmark analysis, the 4-year overall survival was 91.5%, without difference between patients with vs. without VTE (93.8% vs. 90.0%, p = 0.93). Relapse and non-relapse mortality rates were also similar. Among patients treated at Dana-Farber/Harvard Cancer Center, cerebral sinus vein thrombosis occurred in 3.6% of patients (8.5% of VTE events) in comparison to pulmonary embolism (32.9%) and deep vein thromboses (58.6%, 24.4% line-associated). In a Cox regression model for VTE free-time, elevated BMI was associated with shorter VTE free-time (HR 1.94 [95% CI 1.13-3.35], p = 0.018), while low molecular weight heparin (LMWH) prophylaxis as time-varying covariate was not. In conclusion, we found that VTE was frequent in AYAs treated on DFCI ALL protocols but did not impact survival outcomes. Overweight/obese BMI increased risk for VTE.

Venous Thromboembolism Prophylaxis in Patients Treated for Acute Lymphoblastic Leukemia: A Comprehensive Systematic Review and Meta-Analysis

Acute lymphoblastic leukemia (ALL) is a common malignancy in children, often treated with intensive chemotherapy regimens. Venous thromboembolism (VTE) poses a significant risk during ALL treatment, leading to suboptimal outcomes. Thromboprophylaxis is crucial in mitigating this risk, but its efficacy and safety remain uncertain. This systematic review and meta-analysis aimed to evaluate the effectiveness of thromboprophylaxis in reducing VTE incidence during ALL treatment, focusing on antithrombin, apixaban, and enoxaparin. A systematic literature search adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines was performed. Randomized controlled trials (RCTs) investigating thromboprophylaxis in ALL were included. Data extraction and quality appraisal were performed independently by three authors. Meta-analysis was conducted using Review Manager software. Three RCTs met the inclusion criteria. Apixaban, enoxaparin, and antithrombin were assessed in these trials. Meta-analysis revealed significantly reduced odds of VTE with thromboprophylaxis compared to standard care (odds ratio (OR): 0.47, 95% confidence interval (CI) 0.29-0.75; relative risk (RR): 0.52, 95% CI 0.33-0.83). However, no significant difference in bleeding risk was observed (OR: 1.33, 95% CI 0.42-4.21; RR: 1.32, 95% CI 0.43-4.07). Heterogeneity among studies was moderate. This study showed that thromboprophylaxis with apixaban, enoxaparin, or antithrombin significantly reduces VTE incidence during ALL treatment. Despite some limitations, including heterogeneity and potential biases, these findings support the adoption of tailored thromboprophylaxis strategies to improve outcomes in ALL patients. Further research is warranted to optimize these approaches and address remaining uncertainties.

A systematic review and meta-analysis of the effectiveness of primary thromboprophylaxis in acute lymphoblastic leukemia during early-phase therapy including asparaginase or its prolonged form

Asparaginase is essential in the initial management of acute lymphoblastic leukemia (ALL) but frequently leads to venous thromboembolism (VTE). Using anticoagulants for primary VTE prevention has been studied with no consensus. We conducted a systematic literature search in PubMed, Scopus, and Web of science and performed random-effect meta-analysis using Mantel-Haenszel method in RevMan 5.4 to analyze primary pharmacological thromboprophylaxis during asparaginase treatment in early-phase (induction, consolidation, or intensification phase) therapy in patients with ALL with all ages and followed with subgroup analysis by age. Meta-analysis of 13 articles describing the effect of antithrombin supplementation in 1375 patients showed that antithrombin prophylaxis decreases the risk of VTE by 43% (RR, 0.57; 95% CI, 0.38 - 0.83; p=0.004), with mild heterogeneity (I2=35%, p=0.10) and moderate certainty by GRADE. 8 articles included for meta-analysis of low-molecular weight heparin (LMWH) treatment in 612 patients showed that it decreased the risk of VTE by nearly 40% (RR, 0.61; 95% CI, 0.45 - 0.81; p=0.00081), with minimal heterogeneity (I2=14%, p=0.31) but low certainty. Subgroup analysis showed that only prophylaxis with antithrombin supplementation significantly decreased the VTE rate in adult patients with moderate certainty. In pediatric patients, one nonrandomized prospective study showed that LMWH combined with antithrombin has a better thromboprophylaxis effect than antithrombin alone. In the PREVAPIX-ALL trial, prophylaxis with direct factor Xa inhibitor Apixaban did not benefit children younger than 18 years except for cases of obesity. We concluded that thromboprophylaxis with antithrombin is effective in ALL patients older than 18 years during the early phase of therapy, and LMWH combined with antithrombin supplementation might be effective for pediatric patients with ALL. Apixaban is effective in pediatric ALL patients with obesity and needs further study in other high-risk patients.

Cancer-associated thrombosis in hematologic malignancies

Patients with hematologic malignancies are often complicated not only by severe bleeding due to thrombocytopenia and disseminated intravascular coagulation but also by thromboembolic events, just like in patients with solid cancers, and these events can negatively impact patient outcomes. Nevertheless, the prevention and treatment of cancer-associated thrombosis (CAT) in hematologic malignancies has not been adequately investigated due to the limited size, heterogeneity, and unique pathophysiology of the patient population. This article summarizes the current understanding, risk factors, prediction models, and optimal prevention and treatment strategies of CAT in hematologic malignancies on a disease-by-disease basis, including acute leukemia, lymphoma, myeloma, and myeloproliferative neoplasms. Specific considerations of novel molecular targeted therapeutics introduced in recent years, such as immunomodulatory drugs and tyrosine kinase inhibitors, are also discussed based on the latest clinical trials.

2022 international clinical practice guidelines for the treatment and prophylaxis of venous thromboembolism in patients with cancer, including patients with COVID-19

The International Initiative on Thrombosis and Cancer is an independent academic working group of experts aimed at establishing global consensus for the treatment and prophylaxis of cancer-associated thrombosis. The 2013, 2016, and 2019 International Initiative on Thrombosis and Cancer clinical practice guidelines have been made available through a free, web-based mobile phone application. The 2022 clinical practice guidelines, which are based on a literature review up to Jan 1, 2022, include guidance for patients with cancer and with COVID-19. Key recommendations (grade 1A or 1B) include: (1) low-molecular-weight heparins (LMWHs) for the initial (first 10 days) treatment and maintenance treatment of cancer-associated thrombosis; (2) direct oral anticoagulants for the initial treatment and maintenance treatment of cancer-associated thrombosis in patients who are not at high risk of gastrointestinal or genitourinary bleeding, in the absence of strong drug-drug interactions or of gastrointestinal absorption impairment; (3) LMWHs or direct oral anticoagulants for a minimum of 6 months to treat cancer-associated thrombosis; (4) extended prophylaxis (4 weeks) with LMWHs to prevent postoperative venous thromboembolism after major abdominopelvic surgery in patients not at high risk of bleeding; and (5) primary prophylaxis of venous thromboembolism with LMWHs or direct oral anticoagulants (rivaroxaban or apixaban) in ambulatory patients with locally advanced or metastatic pancreatic cancer who are treated with anticancer therapy and have a low risk of bleeding.

D-Dimer and Risk for Thrombosis in Adults with Newly Diagnosed Acute Lymphoblastic Leukemia

D-dimer level at ALL diagnosis is associated with venous or arterial thrombosis during the first 100 days of therapy.

Future studies should include D-dimer with other known risk factors to build a risk assessment model for thrombosis in newly diagnosed ALL.

Patients with acute lymphoblastic leukemia (ALL) are at increased risk of thrombotic and/or bleeding events during early chemotherapy, especially when receiving asparaginase. D-dimer is a marker of fibrinolysis that has been associated with thrombotic risk in solid cancers and acute myeloid leukemia; however, to date, no ALL-based study has assessed D-dimer level and risk for thrombosis. We sought to examine D-dimer as a biomarker for risk of thrombosis or bleeding during ALL treatment in a retrospective cohort study at The University of Chicago. We identified 61 consecutive adult patients with ALL, gathering demographic characteristics, treatment regimens, initial biomarkers including D-dimer, and assessing occurrence of venous or arterial thrombosis and bleeding in the first 100 days after diagnosis (index). The 100-day cumulative incidence (95% confidence interval [CI]) of venous or arterial thrombosis in patients with high D-dimer (≥4 µg/mL) was 52.9% (95% CI, 26.4-73.8) compared with 13.8% (95% CI, 5.5-25.7) in patients with low to moderate D-dimer (<4 µg/mL), corresponding with a hazard ratio of 5.04 (95% CI, 1.79-14.22). When testing for potential confounders in a series of bivariate logistic regression models, the association between D-dimer and thrombosis remained after adjusting for body mass index, age, sex, asparaginase treatment, disseminated intravascular coagulation score, initial platelet level, and ALL phenotype. In conclusion, D-dimer levels at ALL diagnosis are associated with venous or arterial thrombosis at 100 days. Future studies should include D-dimer collated with other known risk factors to build a risk assessment model for thrombosis in patients with newly diagnosed ALL.

Thrombosis Complications in Pediatric Acute Lymphoblastic Leukemia: Risk Factors, Management, and Prevention: Is There Any Role for Pharmacologic Prophylaxis?

Pediatric acute lymphoblastic leukemia (ALL) has achieved close to 90% cure rates through extensive collaborative and integrative molecular research, clinical studies, and advances in supportive care. Despite this high achievement, venous thromboembolic complications (VTE) remain one of the most common and potentially preventable therapy-associated adverse events in ALL. The majority of thromboses events involve the upper central venous system which is related to the use and location of central venous catheters (CVC). The reported rates of symptomatic and asymptomatic CVC-related VTE range from 2.6 to 36.7% and 5.9 to 43%, respectively. Thrombosis can negatively impact not only disease-free survival [e.g., therapy delays and/or interruption, omission of chemotherapy agents (e.g., asparaginase therapy)] but also can result in long-term adverse effects that can impair the quality of life of ALL survivors (e.g., post-thrombotic syndrome, central nervous system (CNS)-thrombosis related complications: seizures, neurocognitive deficits). In this review, will discuss thrombosis pathophysiology in pediatric ALL, risk factors, treatment, and prevention strategies. In addition, the recently published clinical efficacy and safety of direct oral anticoagulants (DOACs) use in thrombosis treatment, and their potential role in primary/secondary thrombosis prevention in pediatric patients with ALL will be discussed. Future clinical trials involving the use of these novel oral anticoagulants should be studied in ALL not only for primary thrombosis prevention but also in the treatment of thrombosis and its secondary prevention. These future research findings could potentially extrapolate to VTE prevention strategies in other pediatric cancer diagnoses and children considered at high risk for VTE.

Prevention of venous thromboembolism in hematologic neoplasms: an expert consensus from SEHH-SETH

Venous thromboembolism (VTE) is a serious complication in hematologic neoplasms, so finding adequate prevention strategies is an urgent requirement. However, prospective studies with large enough cohorts are scarce, limiting the development of evidence-based thromboprophylaxis guidelines. The present position paper is addressed to all hematologists treating patients affected by hematologic neoplasms with the aim to provide clinicians with a useful tool for the prevention of VTE.

Cancer-Associated Thrombosis: Risk Factors, Molecular Mechanisms, Future Management

Venous thromboembolism (VTE) is a major health problem in patients with cancer. Cancer augments thrombosis and causes cancer-associated thrombosis (CAT) and vice versa thrombosis amplifies cancer progression, termed thrombosis-associated cancer (TAC). Risk factors that lead to CAT and TAC include cancer type, chemotherapy, radiotherapy, hormonal therapy, anti-angiogenesis therapy, surgery, or supportive therapy with hematopoietic growth factors. There are some other factors that have an effect on CAT and TAC such as tissue factor, neutrophil extracellular traps (NETs) released in response to cancer, cancer procoagulant, and cytokines. Oncogenes, estrogen hormone, and thyroid hormone with its integrin αvβ3 receptor promote angiogenesis. Lastly, patient-related factors can play a role in development of thrombosis in cancer. Low-molecular-weight heparin and direct oral anticoagulants (DOACs) are used in VTE prophylaxis and treatment rather than vitamin K antagonist. Now, there are new directions for potential management of VTE in patients with cancer such as euthyroid, blockade of thyroid hormone receptor on integrin αvβ3, sulfated non-anticoagulant heparin, inhibition of NETs and stratifying low and high-risk patients with significant bleeding problems with DOACs.

Sulfated non-anticoagulant low molecular weight heparin in the prevention of cancer and non-cancer associated thrombosis without compromising hemostasis

INTRODUCTION

Cancer-associated thrombosis (CAT) accounts for about 20% of all cases of Venous Thromboembolism (VTE). Tissue factor (TF) is documented to be highly expressed on cancer cells and pathological angiogenic endothelial cells. Here, we used a novel oxidized sulfated ultra-LMWH, S-NACH, which is devoid of anti-factor Xa and IIa activities with limited to no systemic anticoagulant effects. This sulfated form has enhanced binding to vascular endothelial cells (EC) and releases and potentiates the action of tissue factor pathway inhibitor (TFPI). S-NACH binds with high affinity to EC, releases and binds to EC TFPI, and promotes vascular antithrombotic effect with limited to no risk of bleeding complications.

MATERIALS AND METHODS

We investigated the effects of S-NACH on clot kinetics in vitro and in vivo. Also, we investigated the effects of S-NACH on CAT mediated by human acute leukemia cells (K562) and human pancreatic cancer cells (SUIT2).

RESULTS

S-NACH was associated with ~3-fold increase of TFPI 2 levels within 3 h. Also, S-NACH reversed the hypercoagulability state that is associated with cancer cells in vitro. In vivo, S-NACH at 20 mg/kg subcutaneously (SC) had no effect on bleeding time compared to both tinzaparin and enoxaparin at 5 mg/kg SC. S-NACH did not show any anti-IIa or anti-Xa activities in comparison to tinzaparin and enoxaparin (p < 0.001).

CONCLUSION

Data suggest the importance of S-NACH through its EC binding, EC TFPI release and its interaction with TFPI in enhancing its activity in the prevention of cancer and non-cancer associated thrombosis with limited to no bleeding complications.

Risk of Thrombosis in Adult Philadelphia-Positive ALL Treated with an Asparaginase-Free ALL Regimen

BACKGROUND

venous thromboembolism (VTE) is a well-known complication in adults with acute lymphoblastic leukemia (ALL), especially in patients treated with asparaginase (ASNase)-including regiments. However, VTE risk in adult Philadelphia-positive ALL (Ph+ve ALL) patients treated with non-hyperCVAD chemotherapy is unclear. In this study, we examined VTE incidence in adult Ph+ve ALL patients treated with imatinib plus a pediatric-inspired asparaginase (ASNase)-free regimen modified from the Dana Farber Cancer Institute (DFCI) ALL protocol.

METHODS

a single centre retrospective review of Ph+ve ALL patients treated at Princess Margaret Cancer Center (PMCC) from 2008-2019 with imatinib plus modified DFCI protocol was conducted.

RESULTS

of the 123 patients included, 30 (24.3%) had at least 1 radiology confirmed VTE event from diagnosis to the end of maintenance therapy. 86.7% (26/30) of the VTE events occurred during active treatment. Of all VTE events, the majority (53.3%) were DVT and/or PE while another significant portion were catheter-related (40.0%). Major bleeding was observed in 1 patient on VTE treatment with low molecular weight heparin (LMWH).

CONCLUSION

a high VTE incidence (24.3%) was observed in adults Ph+ve ALL patients treated with imatinib plus an ASNase-free modified DFCI pediatric ALL protocol, suggesting prophylactic anticoagulation should be considered for all adult Ph+ve ALL patients including those treated with ASNase-free regimens.

Prophylaxis of thromboembolism during therapy with asparaginase in adults with acute lymphoblastic leukaemia

BACKGROUND

The risk of venous thromboembolism is increased in adults and enhanced by asparaginase-based chemotherapy, and venous thromboembolism introduces a secondary risk of treatment delay and premature discontinuation of key anti-leukaemic agents, potentially compromising survival. Yet, the trade-off between benefits and harms of primary thromboprophylaxis in adults with acute lymphoblastic leukaemia (ALL) treated according to asparaginase-based regimens is uncertain.  OBJECTIVES: The primary objectives were to assess the benefits and harms of primary thromboprophylaxis for first-time symptomatic venous thromboembolism in adults with ALL receiving asparaginase-based therapy compared with placebo or no thromboprophylaxis. The secondary objectives were to compare the benefits and harms of different groups of primary systemic thromboprophylaxis by stratifying the main results per type of drug (heparins, vitamin K antagonists, synthetic pentasaccharides, parenteral direct thrombin inhibitors, direct oral anticoagulants, and blood-derived products for antithrombin substitution).

SEARCH METHODS

We conducted a comprehensive literature search on 02 June 2020, with no language restrictions, including (1) electronic searches of Pubmed/MEDLINE; Embase/Ovid; Scopus/Elsevier; Web of Science Core Collection/Clarivate Analytics; and Cochrane Central Register of Controlled Trials (CENTRAL) and (2) handsearches of (i) reference lists of identified studies and related reviews; (ii) clinical trials registries (ClinicalTrials.gov registry; the International Standard Randomized Controlled Trial Number (ISRCTN) registry; the World Health Organisation's International Clinical Trials Registry Platform (ICTRP); and pharmaceutical manufacturers of asparaginase including Servier, Takeda, Jazz Pharmaceuticals, Ohara Pharmaceuticals, and Kyowa Pharmaceuticals), and (iii) conference proceedings (from the annual meetings of the American Society of Hematology (ASH); the European Haematology Association (EHA); the American Society of Clinical Oncology (ASCO); and the International Society on Thrombosis and Haemostasis (ISTH)). We conducted all searches from 1970 (the time of introduction of asparaginase in ALL treatment). We contacted the authors of relevant studies to identify any unpublished material, missing data, or information regarding ongoing studies.

SELECTION CRITERIA

Randomised controlled trials (RCTs); including quasi-randomised, controlled clinical, cross-over, and cluster-randomised trial designs) comparing any parenteral/oral preemptive anticoagulant or mechanical intervention with placebo or no thromboprophylaxis, or comparing two different pre-emptive anticoagulant interventions in adults aged at least 18 years with ALL treated according to asparaginase-based chemotherapy regimens. For the description of harms, non-randomised observational studies with a control group were eligible for inclusion.  DATA COLLECTION AND ANALYSIS: Using a standardised data collection form, two review authors independently screened and selected studies, extracted data, assessed risk of bias for each outcome using standardised tools (RoB 2.0 tool for RCTs and ROBINS-I tool for non-randomised studies) and the certainty of evidence for each outcome using the GRADE approach. Primary outcomes included first-time symptomatic venous thromboembolism, all-cause mortality, and major bleeding. Secondary outcomes included asymptomatic venous thromboembolism, venous thromboembolism-related mortality, adverse events (i.e. clinically relevant non-major bleeding and heparin-induced thrombocytopenia for trials using heparins), and quality of life. Analyses were performed according to the guidelines of the Cochrane Handbook for Systematic Reviews of Interventions. For non-randomised studies, we evaluated all studies (including studies judged to be at critical risk of bias in at least one of the ROBINS-I domains) in a sensitivity analysis exploring confounding.  MAIN RESULTS: We identified 23 non-randomised studies that met the inclusion criteria of this review, of which 10 studies provided no outcome data for adults with ALL. We included the remaining 13 studies in the 'Risk of bias' assessment, in which we identified invalid control group definition in two studies and judged outcomes of nine studies to be at critical risk of bias in at least one of the ROBINS-I domains and outcomes of two studies at serious risk of bias. We did not assess the benefits of thromboprophylaxis, as no RCTs were included. In the main descriptive analysis of harms, we included two retrospective non-randomised studies with outcomes judged to be at serious risk of bias. One study evaluated antithrombin concentrates compared to no antithrombin concentrates. We are uncertain whether antithrombin concentrates have an effect on all-cause mortality (risk ratio (RR) 0.55, 95% confidence interval (CI) 0.26 to 1.19 (intention-to-treat analysis); one study, 40 participants; very low certainty of evidence). We are uncertain whether antithrombin concentrates have an effect on venous thromboembolism-related mortality (RR 0.10, 95% CI 0.01 to 1.94 (intention-to-treat analysis); one study, 40 participants; very low certainty of evidence). We do not know whether antithrombin concentrates have an effect on major bleeding, clinically relevant non-major bleeding, and quality of life in adults with ALL treated with asparaginase-based chemotherapy, as data were insufficient. The remaining study (224 participants) evaluated prophylaxis with low-molecular-weight heparin versus no prophylaxis. However, this study reported insufficient data regarding harms including all-cause mortality, major bleeding, venous thromboembolism-related mortality, clinically relevant non-major bleeding, heparin-induced thrombocytopenia, and quality of life. In the sensitivity analysis of harms, exploring the effect of confounding, we also included nine non-randomised studies with outcomes judged to be at critical risk of bias primarily due to uncontrolled confounding. Three studies (179 participants) evaluated the effect of antithrombin concentrates and six studies (1224 participants) evaluated the effect of prophylaxis with different types of heparins. When analysing all-cause mortality; venous thromboembolism-related mortality; and major bleeding (studies of heparin only) including all studies with extractable outcomes for each comparison (antithrombin and low-molecular-weight heparin), we observed small study sizes; few events; wide CIs crossing the line of no effect; and substantial heterogeneity by visual inspection of the forest plots. Although the observed heterogeneity could arise through the inclusion of a small number of studies with differences in participants; interventions; and outcome assessments, the likelihood that bias due to uncontrolled confounding was the cause of heterogeneity is inevitable. Subgroup analyses were not possible due to insufficient data.  AUTHORS' CONCLUSIONS: We do not know from the currently available evidence, if thromboprophylaxis used for adults with ALL treated according to asparaginase-based regimens is associated with clinically appreciable benefits and acceptable harms. The existing research on this question is solely of non-randomised design, seriously to critically confounded, and underpowered with substantial imprecision. Any estimates of effect based on the existing insufficient evidence is very uncertain and is likely to change with future research.