Donald L. Yee

Platelet hyperreactivity generalizes to multiple forms of stimulation

Summary.  Background: Although platelet hyperreactivity constitutes an important cardiovascular risk factor, standardized methods for its measurement are lacking. We recently reported that aggregometry using a submaximal concentration of epinephrine identifies individuals with in vitro platelet hyperreactivity; this hyperreactivity was reproducible on multiple occasions over long periods of time. Objective and methods: To better understand this aberrant reactivity, we studied in a large group of subjects (n = 386) the relationship between healthy individuals’ platelet reactivity to epinephrine and their platelet phenotype as measured by other functional assays. Results: Subjects with hyperreactivity to epinephrine were more likely to exhibit hyperfunction in each major aspect of platelet activity, including adhesion (response to low‐dose ristocetin; P < 0.001), activation (surface P‐selectin expression and PAC‐1 binding after stimulation; P ≤ 0.003) and aggregation to other agonists [no agonist, adenosine diphosphate (ADP), arachidonic acid, collagen, collagen‐related peptide and ristocetin; P ≤ 0.025] and to applied shear stress (PFA‐100 and cone‐and‐plate viscometer; P < 0.05). These differences persisted after adjusting for demographic and hematologic differences between groups. We studied candidate genes relevant to epinephrine‐mediated platelet activation and found that hyperreactivity to epinephrine was associated with a polymorphism on the gene (GNB3) encoding the beta‐3 subunit of G proteins (P = 0.03). Conclusions: Robust aggregation to a submaximal concentration of epinephrine establishes a true hyperreactive platelet phenotype that is ‘global’ as opposed to agonist specific; detection of this phenotype could be useful for studying patients at risk for arterial thrombosis. The mechanisms underlying hyperreactivity to different types of platelet stimulation may share common signaling pathways, some of which may involve specific G protein subunits.

FondaKIDS: A prospective pharmacokinetic and safety study of fondaparinux in children between 1 and 18 years of age†

The incidence of thromboembolic disease is increasing in children. New anticoagulants have been licensed in adults and need to be studied in children. This report describes the first prospective study of fondaparinux in children.

Case series of thrombotic thrombocytopenic purpura in children and adolescents.

Thrombotic thrombocytopenic purpura (TTP) is a well-described entity in adults but is rarely observed in children. The authors describe a series of seven children with suspected acquired TTP. Clinical findings included petechiae, purpura, or jaundice (6), central nervous system events (5), fever (3), diarrhea (3), renal insufficiency (2), and hematuria (2). Significant central nervous system events included cerebral vascular accidents (2), altered mental status (2), seizures (1), and hemiparesis (1). Patients were treated with daily plasma infusions (1/7) or plasma exchange (5/7). Response was prompt, although relapses were frequent. Decreased vWF-protease activity was found in four of five cases and vWF-protease inhibitors were found in three of five cases. Although rare, TTP is a life-threatening illness that does occur in children and should be considered in the differential diagnosis of thrombocytopenia with hemolytic anemia.

Pharmacokinetics and Pharmacodynamics of Anticoagulants in Paediatric Patients

Given the rising incidence of thrombotic complications in paediatric patients, understanding of the pharmacologic behaviour of anticoagulant drugs in children has gained importance. Significant developmental differences between children and adults in the haemostatic system and pharmacologic parameters for individual drugs highlight potentially unique aspects of anticoagulant pharmacology in this special and vulnerable population. This review focuses on pharmacologic information relevant to the dosing of unfractionated heparin, low molecular weight heparin, warfarin, bivalirudin, argatroban and fondaparinux in paediatric patients. The bulk of clinical experience with paediatric anticoagulation rests with the first three of these agents, each of which requires higher bodyweight-based dosing for the youngest patients, compared with adults, in order to achieve comparable pharmacodynamic effects, likely related to an inverse correlation between age and bodyweight-normalized clearance of these drugs. Whether extrapolation of therapeutic ranges targeted for adult patients prescribed these agents is valid for children, however, is unknown and a high priority for future research. Novel oral anticoagulants, such as dabigatran, rivaroxaban and apixaban, hold promise for future use in paediatrics but require further pharmacologic study in infants, children and adolescents.

Varied opinions on thrombolysis for venous thromboembolism in infants and children: findings from a survey of pediatric hematology-oncology specialists.

Recent guidelines discourage routine use of thrombolytic agents for treatment of venous thromboembolism (VTE) in pediatric patients, but actual practice patterns are unknown.

A practical approach to pediatric patients referred with an abnormal coagulation profile.

CONTEXT Workup for prolonged prothrombin time (PT) and activated partial thromboplastin time (PTT) is a frequent referral to a Hematology and Coagulation Laboratory. Although the workup should be performed in a timely and cost-effective manner, the complete laboratory assessment of the coagulation state has not been standardized. OBJECTIVE To determine which clinical and laboratory data are most predictive of a coagulopathy and to formulate the most efficient strategy to reach a diagnosis in patients referred for abnormal coagulation profiles. DESIGN Retrospective case review. Medical records of 251 patients referred for prolonged PT and/or PTT to our Hematology Service between June 1995 and December 2002 were reviewed. RESULTS The study included 135 males and 116 females with a mean age of 7.0 years. A personal history of bleeding was reported in 137 patients, and a family history of bleeding was reported in 116 patients. Fifty-one patients (20%) had a coagulopathy (ie, a bleeding risk). Factors predictive of a bleeding risk were a positive family history of bleeding (P < .001) and a positive personal history of bleeding (P = .001). Of 170 patients with findings of normal PT and PTT values on repeat testing, 14 were subsequently diagnosed with a coagulopathy. Two of these patients reported no positive personal or family history of bleeding. CONCLUSIONS Coagulopathy was identified in 20% of the children referred for abnormal PT and/or PTT. In the absence of a personal or family history of bleeding, a normal PT and/or PTT on repeat testing has a negative predictive value of more than 95%.

Clinical evaluation of glycoPEGylated recombinant FVIII: Efficacy and safety in severe haemophilia A

Turoctocog alfa pegol (N8-GP) is a novel glycoPEGylated extended half-life recombinant factor VIII (FVIII) product developed for prophylaxis and treatment of bleeds in patients with haemophilia A, to enable higher activity levels with less frequent injections compared with standard FVIII products. This phase III (NCT01480180), multinational, open-label, non-randomised trial evaluated the safety and clinical efficacy of N8-GP when administered for treatment of bleeds and for prophylaxis, in previously treated patients aged ≥12 years with severe haemophilia A. Patients were allocated to receive N8-GP for prophylaxis or on-demand treatment for up to 1.8 years. Patients on prophylaxis were administered one dose of 50 IU/kg of N8-GP every fourth day. Bleeds were treated with doses of 20-75 IU/kg. Total exposure to N8-GP in the trial was 14,114 exposure days (159 patient-years). For the prophylaxis arm (n=175), the median annualised bleeding rate (ABR) was 1.33 (interquartile range, 0.00-4.61), the mean ABR was 3.70 (95 % confidence interval 2.94-4.66) and 70 (40 %) patients had no bleeds during the trial. Across treatment arms, 83.6 % of bleeds resolved with one injection and 95.5 % with up to two injections. N8-GP had a favourable safety profile and was well tolerated. The frequency and types of adverse events reported were as expected in this population. One patient developed inhibitory antibodies against FVIII (≥0.6 Bethesda units [BU]) after 93 N8-GP exposure days. No clinically significant safety concerns were identified and N8-GP was effective for prophylaxis and treatment of bleeds in previously treated patients.

Oral Tranexamic Acid versus Combined Oral Contraceptives for Adolescent Heavy Menstrual Bleeding: A Pilot Study

STUDY OBJECTIVE To compare the efficacy of oral tranexamic acid (TA) with combined oral contraceptives (COC) in reducing menstrual blood loss (MBL) and improving quality of life in adolescent heavy menstrual bleeding (HMB). DESIGN, SETTING, AND PARTICIPANTS A prospective randomized crossover trial with 17 postmenarchal girls aged 21 years and younger with HMB who were seen at our institution. INTERVENTIONS Patients were randomized to group A (TA arm) or group B (COC arm), each for 3 cycles, with crossover to the second arm after 1-month washout. MAIN OUTCOME MEASURES The primary end points were difference in improvement in MBL and QOL, from baseline to end of each therapy. RESULTS Seventeen patients were enrolled (mean age 14.2 years, range 11.7 to 16.8 years). Nine patients completed both arms; 8 patients withdrew from the study due to adverse events or noncompliance. Compared with baseline, significant improvement (P < .05) was demonstrated by TA and COC in MBL (mean Pictorial Blood Assessment Chart score decrease: TA, 536.4; COC, 430.6) and quality of life (mean Pediatric Quality of Life Inventory(TM) version 4.0 Generic Scales score increase: TA, 15.6; COC, 16.75), but no significant difference was noted between TA and COC (P > .05). There was statistically significant reduction in the length of menstrual cycle for COC only (mean reduction 5.3 days; P = .04) and not for TA (P = .18). Ten patients (58%) experienced adverse events that were possibly drug related (TA: n = 3, 30%; COCP: n = 7, 64%). CONCLUSION In this pilot study, oral TA appeared as efficacious as COC in the management of adolescent HMB by reducing MBL and improving quality of life.

Thromboelastographic and hemostatic characteristics in pediatric patients with sickle cell disease.

CONTEXT Patients with sickle cell disease suffer from a variety of vaso-occlusive events that may be related to activation of the hemostatic system. Thromboelastography assesses the functionality of this system from a global standpoint and has demonstrated some utility in detecting hypercoagulable states in varied clinical settings, but it has not been systematically evaluated in patients with sickle cell disease. OBJECTIVE To characterize the findings of thromboelastography in patients with sickle cell disease during periods of steady state and illness, to compare these results with those of healthy controls, and to correlate these profiles with other measured hemostatic parameters. DESIGN In this cross-sectional study, we obtained thromboelastographic and other hemostatic data on specimens from 46 patients with sickle cell disease (35 with hemoglobin SS, 7 with hemoglobin SC, and 4 with hemoglobin S-beta thalassemia) and 20 healthy race-matched controls. Data were obtained from patients with sickle cell disease at baseline conditions (n = 41) and in the setting of acute illness (n = 5). RESULTS Patients with hemoglobin SS had lower reaction time and higher angle, maximum amplitude, and coagulation index values on thromboelastography than the control group. Hemoglobin SC patients had higher angle, maximum amplitude, and coagulation index values than controls. Hemoglobin S-beta thalassemia patients showed no significant differences compared with controls. Five hemoglobin SS patients with recent or current illness demonstrated increased maximum amplitude and coagulation index compared with hemoglobin SS patients at baseline conditions. CONCLUSIONS Patients with sickle cell disease demonstrated a significant hypercoagulable state in thromboelastography profiles, with the degree of abnormality dependent on the type of sickle cell disease and perhaps the presence of acute illness. Continued follow-up of this patient cohort, as well as further study of larger and more homogeneous patient groups, is required to adequately assess the utility of thromboelastography in predicting complications of sickle cell disease.

Comparison of Anti-Xa Levels in Obese and Non-Obese Pediatric Patients Receiving Treatment Doses of Enoxaparin

OBJECTIVE To determine if using actual body weight to dose enoxaparin in obese pediatric patients results in higher anti-Xa levels compared with non-obese pediatric patients. STUDY DESIGN This was a retrospective case-matched study of obese and non-obese pediatric patients receiving treatment doses of enoxaparin in a tertiary care childrens hospital. Patients were included if they were initiated on treatment doses of enoxaparin, had appropriate anti-Xa levels drawn, and were between 2 and 18 years of age. Patients with renal insufficiency, hyperbilirubinemia, goal anti-Xa level <0.5 or >1 unit/mL, or receiving mechanical circulatory support were excluded. Obese patients who met study criteria were matched on a 1:1 basis with non-obese patients. RESULTS All baseline characteristics were similar except for body mass index percentile (98.2 ± 2 vs 48.7 ± 15, P < .01). Obese patients had higher initial anti-Xa levels (0.67 ± 0.27 vs 0.53 ± 0.24 unit/mL, P = .028). Over time, obese patients required a lower mean dose to achieve therapeutic anti-Xa levels than non-obese patients (0.81 ± 0.19 vs 1.1 ± 0.4 mg/kg, P = .005). CONCLUSIONS The mean initial anti-Xa level was higher in obese pediatric patients compared with non-obese pediatric patients, but a dosage adjustment was not required. Obese patients may need closer monitoring over time to avoid supratherapeutic levels and possible bleeding events.