Platelet Refractoriness Research Articles

Increased blood plasma hydrolysis of acetylsalicylic acid in type 2 diabetic patients: A role of plasma esterases

Hydrolysis of acetylsalicylic acid (ASA, aspirin), an antiplatelet drug commonly used in the prevention of stroke and myocardial infarction, seems to play a crucial role in its pharmacological action. Thirty-eight healthy volunteers and 38 type 2 diabetic patients were enrolled to test the hypothesis that the enhanced plasma degradation and lowered bioavailability of ASA in diabetic patients is associated with the attenuation of platelet response. Aspirin esterase activities were tested at pH 7.4 and 5.5. A significantly higher overall aspirin esterase activity was noted at pH 7.4 in the diabetic patients ( P < 0.003), corresponding to faster ASA hydrolysis ( P < 0.006). This increased activity was attributable to butyrylcholinesterase and probably to albumin, because it was effectively inhibited by eserine and 4-bis-nitrophenyl phosphate ( P < 0.01). No significant differences between control and diabetic subjects were found at pH 5.5 in either enzymatic activities or ASA hydrolysis rates. The enhanced plasma ASA degradation in diabetic subjects was significantly associated with the refractoriness of blood platelets to ASA ( P < 0.05) and modulated by plasma cholesterol ( P < 0.01). No direct effects of plasma pH or albumin were observed. In conclusion, higher aspirin esterase activity contributes to the lowered response of diabetic platelets to ASA-mediated antiplatelet therapy.

Recombinant Mini-β3 Integrin Fragments for the Detection of HPA-1 Antibodies.

The αIIbβ3 integrin (GPIIbIIIa, CD61) is the platelet receptor for fibrinogen, fibronectin and vitronectin. A single nucleotide polymorphism (dbSNP Id= rs5918) in the ITGB3 gene defines the Human Platelet Antigen (HPA)-1 system encoding either a Leucine (1a) or Proline (1b) at position 33. HPA-1a immunisation is of clinical relevance in neonatal alloimmune thrombocytopenia (NAIT), post transfusion purpura (PTP) and platelet refractoriness. Currently, the detection of HPA-1 antibodies is based on a cumbersome sandwich ELISA requiring HPA genotyped platelets as source of antigen. We set out to express soluble calmodulin (CaM) tagged β3 fragments suitable for HPA-1 antibody detection. The design of fragments was informed by domain structural information, analysis of homology between β3 and integrin-like monomeric proteins and the evolutionary β3 domain phylogeny. Combined with computer modelling, this was the basis of the successful expression in Drosophila S2 cells of eight β3 fragments with either Leu33 or Pro33 (ΔSDL, ΔβA, PSI-Hybrid and PSI alone, both alleloforms for each of the four). Murine monoclonal antibodies Y2/51, SZ21 and CRC54 reacted with the β3 fragments as expected with only CRC54 reacting with the PSI fragment. Two of 3 human HPA-1a specific phage antibodies (19–7, 23–15, both derived from a PTP patient) reacted with all 4 Leu33 fragments but CamTran007 derived from a NAIT patient only reacted with the ΔSDL-Leu33 and ΔβA-Leu33 fragments, defining an epitope split. By ELISA, reactivity of both potency (NIBSC-03/152) and sensitivity (NIBSC-93/710) standards were comparable to those seen by MAIPA. Twelve polyclonal samples from NAIT patients (5 with cerebral haemorrhage [CH], 5 with platelet counts <40x109/L but no CH and 2 with counts >40x109/L) of potency ranging from 5–193 au/ml were detected by ELISA with Leu33 forms of ΔSDL and ΔβA. In conclusion soluble mini-β3-Leu33 fragments have been successfully expressed and shown to react with all polyclonal anti-HPA-1a samples tested from NAIT patients. The CaM tagged β3 fragments are now being further validated with a repository of 150 HPA-1a antibody samples from a NAIT case series with well defined treatment and clinical outcome histories to establish the relation between HPA-1a antibody potency, epitope reactivity and clinical phenotype.

Severe thrombocytopenia due to host‐derived anti‐HPA‐1a after non‐myeloablative allogeneic haematopoietic stem cell transplantation for multiple myeloma: a case report

Host- or donor-derived alloimmune thrombocytopenia can develop after non-myeloablative allogeneic haematopoietic stem cell transplantation (HSCT). We report the first case of host-derived HPA-1a antibodies. A 52-year-old male patient received HSCT from his human leucocyte antigen (HLA)-A, -B, -C, -DR identical brother after reduced intensity conditioning. Bilinear engraftment around day 12 was accompanied by a continuous decrease of platelet counts. We investigated for platelet antibodies because of a progressive decline of platelet counts and refractoriness to platelet transfusions. The patient's serum was tested by enzyme-linked immunosorbent assay (ELISA), a solid phase assay and monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay. Recipient's DNA from the time before HSCT and donor's DNA were genotyped for human platelet antigens. Serum obtained on day 15 after HSCT reacted strongly with the donor's platelets due to host-derived anti-HPA-1a- and anti-HLA I antibodies. Serum samples from days 39, 45 and 65 after HSCT contained only anti-HLA I; no antibodies were detectable on day 149. Platelet counts increased on day 20 spontaneously. The decrease of the antibodies accompanied by the increase of the platelet counts suggests progressive elimination of residual host cells. The HPA-1a antibodies affected thrombopoietic engraftment and the success of platelet transfusions.

Factors affecting posttransfusion platelet increments, platelet refractoriness, and platelet transfusion intervals in thrombocytopenic patients

AbstractA variety of patientand product-related factors influenced the outcome of 6379 transfusions given to 533 patients in the Trial to Reduce Alloimmunization to Platelets (TRAP). Responses measured were platelet increments, interval between platelet transfusions, and platelet refractoriness. Patient factors that improved platelet responses were splenectomy and increasing patient age. In contrast, at least 2 prior pregnancies, male gender, splenomegaly, bleeding, fever, infection, disseminated intravascular coagulation, increasing height and weight, lymphocytotoxic antibody positivity, an increasing number of platelet transfusions, or receiving heparin or amphotericin were associated with decreased posttransfusion platelet responses. Platelet factors that were associated with improved platelet responses were giving ABO-compatible platelets, platelets stored for 48 hours or less, and giving large doses of platelets while ultraviolet B (UV-B) or gamma irradiation decreased platelet responses. However, in alloimmunized lymphocytoxic antibody-positive patients, the immediate increment to UV-B-irradiated platelets was well maintained, whereas all other products showed substantial reductions. Refractoriness to platelet transfusions developed in 27% of the patients. Platelet refractoriness was associated with lymphocytotoxic antibody positivity, heparin administration, fever, bleeding, increasing number of platelet transfusions, increasing weight, at least 2 pregnancies, and male gender. The only factors that reduced platelet refractoriness rates were increasing the dose of platelets transfused or transfusing filtered apheresis platelets.

Platelet glycoprotein I(b)alpha and integrin alpha2 beta1 polymorphisms: gene frequencies and linkage disequilibrium in a population diversity panel.

Genetic variants in the GP1BA and ITGA2 genes have been proposed as potential modifiers for arterial vascular disease and bleeding disorders. Since ancestry may play an important role in the prevalence of these variants, we sought to determine their allele frequency and linkage disequilibrium in a collection of 1064 DNA samples from 51 ethnic groups. We studied haplotypes of ITGA2 defined by single nucleotide substitutions at positions -52, 807, and 1648, and GP1BA variants defined by sequence changes in positions -5 (Kozak), 1018 (T145M, HPA-2) and 1285 (VNTR A, B, C and D). Frequency of haplotypes of ITGA2 showed considerable variation across the different groups, with a higher prevalence of the haplotype -52C or T/807C/1648A observed in African compared with caucasian and Asian populations. The haplotypes 52C/807T/1648A and -52T/807T/1648A were not observed in caucasians or South Americans. While relative frequencies of the GP1BA Kozak alleles were comparable across groups, the methionine allele (HPA-2b) showed a higher frequency in Africa (0.26) than in the other groups. We also observed a high prevalence of the VNTR B allele in the African and Israeli populations. Haplotype analysis revealed incomplete linkage disequilibrium between the HPA-2 and VNTR alleles. Incorporation of GP1BA variants into the set of SNPs already genotyped by the HapMap project disrupted the pre-existing haplotype block. These data provide a valuable resource for optimal selection of variants best tailored for association studies of vascular disease or bleeding disorders when examining individuals of different ancestral origins.

Platelet refractoriness to classical agonists in a child with essential thrombocythemia

Background: Essential thrombocythemia is a rare disease during childhood. Platelet morphological abnormalities are frequent and defects in platelet function tests, mainly hypoaggregation, occur.Materials and methods: An incidental diagnosis of essential thrombocythemia was established in a 9-year-old boy with a platelet count of 2050 × 109/l. His platelets were studied for aggregation defects with four classical agonists employing optical aggregometry.Results: Aggregation ranged from 5% for adrenaline, 8% for collagen, 12% for ristocetin, to 25% with adenosine diphosphate, followed by complete disaggregation.Conclusion: Platelet refractoriness to classical agonists, probably compounded by platelet GPIb deficiency, was documented. The differential diagnosis is discussed.

Evaluation of different methods of leukoreduction of donor platelets to prevent alloimmune platelet refractoriness and induce tolerance in a canine transfusion model

AbstractThe effectiveness of different methods of leukoreduction in preventing alloimmune platelet refractoriness was evaluated in a canine model. Platelets from a random donor dog were administered for up to 8 weeks or until platelet refractoriness. Standard (STD; unmodified) platelets were accepted by 14% of recipients (n = 7) compared with 14% for centrifuge leukoreduced (C-LR) platelets (n = 21) and 31% for filter leukoreduced (F-LR) platelets (n = 13; no significant differences). Surprisingly, using both F-LR and C-LR platelets was highly effective (87% acceptance, n = 15). Transfusing F-LR/C-LR red blood cells (n = 4) or F-LR/C-LR plasma (n = 4), along with F-LR/C-LR platelets, did not affect platelet acceptance (100% acceptance). Overall acceptance of F-LR/C-LR platelets was 91% (n = 23; P ≤ .05 versus STD, C-LR, or F-LR platelets). F-LR/C-LR transfusions also induced tolerance to subsequent STD platelet transfusions from the same donor (82% acceptance, n = 19) as well as to donor skin grafts without recipient immunosuppression (57% acceptance, n = 7). To evaluate mechanisms of tolerance induction, F-LR/C-LR platelets were γ-irradiated. Although the γ-irradiated F-LR/C-LR platelets were uniformly accepted (n = 6), tolerance to STD platelets was lost. These data suggest that some allostimulatory white cells are filter adherent, whereas others escape filtration but can be removed by centrifugation and tolerance requires a residual functioning white cell.

Gene Frequencies of the Human Platelet Antigen-3in Different Populations

Human platelet antigen (HPA) systems consist of more than 12 biallelic antigen polymorphisms in which a base pair substitution leads to change in an amino acid of a glycoprotein expressed on the platelet. HPA-3 is a HPA that is mentioned for possible induction of neonatal alloimmune thrombocytopenia, posttransfusion purpura, and platelet refractoriness. A summary is presented of previous reports on the gene frequencies of HPA-3 among different populations. The frequency of HPA-3a and -3b ranges from 0.50 to 0.61 and 0.38 to 0.50, respectively. A significant correlation between the population ethnicity and the gene frequencies was detected in this study. However, it is quite difficult to use HPA-3 gene as a gene marker to determine the similarity of gene population in different populations. In addition, the comparison of the heterogenicity of HPA-3 frequencies to another well-known HPA gene, HPA-1 gene demonstrates that there is a greater variation in HPA-3 frequencies than in the HPA-1 gene. There was no significant correlation between the incidence of autoimmune thrombocytopenia disorder and the HPA-3 gene polymorphism pattern.

Leukoreduction to Prevent Alloimmune Platelet (Plt) Refractoriness in a Dog Transfusion Model: Types of Residual White Blood Cells (WBCs) Directly Affect Transfusion Outcomes.

Introduction: Current practice assumes that just a quantitative reduction in the number of transfused wbcs to <1 to 5 x 106/transfusion is sufficient to prevent plt alloimmunization. However, our studies indicate that different leukoreduction strategies vary in their ability to remove immunizing wbcs, and this correlates with rates of alloimmune plt refractoriness in immunocompetent recipients.Experimental Design And Methods: Pairs of donor-recipient dogs were selected either at random, as having a shared DLA DR-B epitope, or as being specifically mismatched for this DLA locus. Non-leukoreduced or leukoreduced radiochromium-labeled donor plts were transfused weekly for up to 8 weeks or until the onset of plt refractoriness defined as <5% of the donor dog's plts circulating in the recipient at 24 hours post-transfusion. Three methods of leukoreduction were evaluated: centrifugation leukoreduction (C-LR); filtration leukoreduction (F-LR) with different types of filters; or combined F-LR/C-LR. Flow cytometry was used to identify the types of residual wbcs following leukoreduction. Table 1 gives the number of residual wbcs and the transfusion outcomes based on the leukoreduction strategy used, while Table 2 gives the relative proportion of the types of residual wbcs after leukoreduction compared to the overall results for the leukoreduction method used.Results:Table 1Method Of LeukoreductionFilter (Manufacturer)Average Residual WBCsDonor-Recipient DR-B RelationshipNon-Refractory Recipients / Recipients TransfusedNone---6.7 x 106Random1/3(33%)Shared Epitope0/4(0%)C-LR---4.7 x 104Random3/21(14%)F-LR:PL1-B (Pall)5.0 x 104NDNDPLF-1 (Pall)7.9 x 104Random3/8(38%)PLF-1 x2* (Pall)<3 x 103**Random1/5(20%)PLS-5A (Fenwal)3.2 x 104Mismatched4/6(66%)F-LR/C-LR:PL1-B (Pall)<3 x 103**Mismatched1/9(11%)<3 x 103**Shared Epitope5/7(71%)PLF-1 (Pall)<3 x 103**Random15/16(94%)<3 x 103**Mismatched3/3(100%)PLS-5A (Fenwal)<3 x 103**Mismatched9/9(100%)ND-Not done. *Platelets were filtered sequentially using two PLF-1 filters. **Lower limit of detection of the assay.Table 2RESIDUAL WBCsLymphocytesMethod of LeukoreductionT CD4 dimT CD4 brightBMonocytesTotal Non-Refractory Recipients (%)None+++++++14%C-LR++++++14%F-LR:PL1-B++++++++++NDPLF-1+++++38%PLS-5A+++++66%F-LR/C-LR:PL1-B++++++11%/71%*PLF-1+++++95%PLS-5A+++100%ND-Not done. *Non-shared DR-B donor-recipient pairs/shared DR-B donor-recipient pairs (p=0.03).Conclusions: 1) a quantitative reduction in wbcs does not prevent plt refractoriness; 2) the types of residual wbcs correlate directly with transfusion outcomes; 3) even after residual monocytes are removed by F-LR using PLF-1 and PLS-5A filters, residual CD4 bright T lymphocytes are associated with a high percentage of refractory recipients; 4) following F-LR/C-LR with PLF-1 and PLS-5A filters leaving B lymphocytes and CD4 dim T lymphocytes (NKT cells?), refractoriness is prevented even to DR-B mismatched donors; and 5) DLA DR-B matching significantly improves transfusion outcomes when residual monocytes remain following F-LR/C-LR using a PL1-B filter.

Increased protein glycation in diabetes mellitus is associated with decreased aspirin-mediated protein acetylation and reduced sensitivity of blood platelets to aspirin

Reduced effectiveness of the most common antiplatelet drug, acetylsalicylic acid (ASA, aspirin), in diabetes mellitus has been associated with a lowered platelet sensitivity to ASA and related to glycemic control in diabetic patients. Our objectives were (a) to monitor the chemical background of how chronic hyperglycemia affects platelet response to ASA in diabetes and (b) to study a chemical competition between the amount of bound acetyl residues and the extent of protein glycation in blood platelets. Using whole-blood impedance aggregometry and platelet function analyzer (PFA-100) we observed a reduced platelet response to ASA in diabetic patients (14% vs. 79% for PFA-100 collagen-epinephrine occlusion time) and an association between the index of glycemic control and platelet refractoriness to ASA (r(S) = -0.378). Impaired platelet response to ASA was related to enhanced platelet protein glycation (3.6+/-0.4 in diabetes vs. 2.3+/-0.4 micromol fructosamine/microg protein in control) and reduced incorporation of acetyl residue into proteins of platelets from diabetic patients (47.4+/-2.0 in control vs. 33.1+/-0.7 micromol acetyl/microg protein in diabetic subjects). Incubation of blood platelets with increasing concentrations of glucose and ASA under in vitro conditions led to excessive modification in protein amino groups: glucose and ASA competed with each other in the course of nonenzymatic modifications, glycosylation, or acetylation, and their contributions to the occupancy of protein amino groups (R2 = 0.22 for glucose, R2 = 0.43 for ASA) were dependent upon the concentrations of glucose and ASA. Overall the effects of high glucose and high ASA on the overall occupancy of protein free amino groups are not additive. While at higher concentrations ASA overcomes the effects of hyperglycemia and retards glycation, high glucose makes acetylation less efficient, and therefore the resultant chemical modification becomes greatly reduced. In conclusion, diminished susceptibility of various platelet proteins and receptors on blood platelet membranes to acetylation and high ambient glucose might underlie the apparently differentiated sensitivity of blood platelets to ASA and determine platelet "insensitivity to aspirin" in diabetic patients.

Prophylactic and therapeutic recombinant factor VIIa administration to patients with Glanzmann's thrombasthenia: results of an international survey

Antibodies to glycoprotein (GP) IIb-IIIa and/or HLA may render platelet transfusions ineffective to stop bleeding or to cover surgery in patients with Glanzmann's thrombasthenia (GT). Anecdotal reports suggest recombinant factor (rF)VIIa might be a therapeutic alternative in these situations. An international survey was conducted to evaluate further the efficacy and safety of rFVIIa in GT patients. We analyzed the use of rFVIIa during 34 surgical/invasive procedures and 108 bleeding episodes in 59 GT patients including 29 with current or previous antiplatelet antibodies, and 23 with a history of refractoriness to platelet transfusion. rFVIIa was effective in 29 of the 31 evaluable procedures, and in 77 of the 103 evaluable bleeding episodes of which eight had a recurrence. A significantly higher success rate was observed in severe bleeding episodes when an arbitrarily defined 'optimal regimen' derived from the Canadian pilot study results (> or = 80 micro g kg(-1) rFVIIa/injection, dosing interval < or = 2.5 h, three or more doses before failure declaration) was used compared with other regimens (77%; 24/31 vs. 48%, 19/40; chi(2), P = 0.010). Patients given maintenance doses had significantly fewer recurrences within 48 h of bleed cessation compared with those not given any (Fisher's exact test, P = 0.022). One thromboembolic event and one blood clot in the ureter occurring in surgical patients following prolonged continuous infusion of high-dose rFVIIa and antifibrinolytic drug use have been previously reported. rFVIIa seems a potential alternative to platelet transfusion in GT patients, particularly in those with antiplatelet antibodies and/or platelet refractoriness.

Estudo da refratariedade plaquetária do dia 0 ao 50, em pacientes submetidos a transplante de medula óssea

Entre outubro de 1997 e julho de 1999 pesquisou-se a refratariedade plaquetaria em 15 pacientes na fase precoce do TMO alogenico e autoplastico, com idade variando de 1 a 66 anos no Hospital Sao Camilo. Para esta avaliacao, foram utilizados os seguintes parâmetros: evolucao clinica, calculo corrigido do incremento plaquetario (CCI), teste de microlinfocitotoxicidade dependente de complemento (CDC) e ensaios plaquetarios por aderencia de celulas vermelhas em fase solida (SPRCA). A refratariedade plaquetaria foi definida como falha de resposta a uma transfusao de dois concentrados de plaquetas ABO compativeis, quando o calculo corrigido do incremento plaquetario (CCI) de uma hora pos-transfusional era inferior a 7,5 ou de 24 horas < 4,5. Apenas a analise do CCI de 24 horas mostrou significância estatistica. A refratariedade plaquetaria foi detectada em 80,0 % dos casos, tendo como causa principal os fatores nao imunologicos, como: anfotericina 66,66%, doenca veno-oclusiva hepatica 53,33%, febre de origem indeterminada 40,0%, esplenomegalia epistaxe leve, febre, melena grave, hematemese grave e infeccao bacteriana 20,0%. Melena leve, enterorragia grave, epistaxe moderada e grave 13,33%, enquanto CIVD, enterorragia moderada e grave 6,66%. Os fatores imunologicos foram representados pela presenca da reacao aguda do enxerto contra hospedeiro (aGVHD) em 33,0% e infeccao por citomegalovirus (CMV) em 13,33%, embora a deteccao de auto-anticorpos tenha sido negativa. Conclui-se que a analise do CCI pos-transfusional de 24 horas mostrou ser um metodo de escolha para deteccao da refratariedade e de util aplicabilidade em pacientes trombocitopenicos refratarios, em particular naqueles submetidos ao TMO.

Universal prestorage leukoreduction in Canada decreases platelet alloimmunization and refractoriness

AbstractRandomized controlled trials have shown a reduction in platelet alloimmunization and refractoriness in patients with acute leukemia (AL) with the use of poststorage leukoreduction of blood products. Universal prestorage leukoreduction (ULR) of red cell and platelet products has been performed in Canada since August 1999. We conducted a retrospective analysis of 13 902 platelet transfusions in 617 patients undergoing chemotherapy (CT) for AL or stem cell transplantation (SCT) before (n = 315) and after (n = 302) the introduction of ULR. Alloimmunization was significantly reduced (19% to 7%, P < .001) in the post-ULR group. Alloimmune platelet refractoriness was similarly reduced (14% to 4%, P < .001). Fewer patients in the post-ULR group received HLA-matched platelets (14% vs 5%, P < .001). Alloimmunization and alloimmune refractoriness in the 318 patients who were previously pregnant and/or transfused were also reduced after ULR (P = .023 and P = .005, respectively). In a Cox regression model, the 3 independent factors that predicted for alloimmune refractoriness were nonleukoreduced blood products (relative risk [RR], 2.2 [95% CI, 1.2-4.3]), a history of pregnancy and/or transfusion (RR, 2.3 [95% CI, 1.3-4.2]), and receipt of 13 or more platelet transfusions (RR, 6.0 [95% CI, 2.4-15.3]). In conclusion, ULR reduces alloimmunization, refractoriness, and requirements for HLA-matched platelets when applied as routine transfusion practice to patients receiving CT or SCT.

A rapid and specific whole blood HPA-1 phenotyping by flow cytometry using two commercialized monoclonal antibodies directed against GP IIIa and GP IIb-IIIa complexes.

The human platelet antigen 1 (HPA-1) system has been implicated in rare but severe diseases, such as neonatal alloimmune thrombocytopenic purpura, post-transfusion purpura and immune platelet refractoriness. We developed a flow cytometry assay for HPA-1 phenotyping using two commercial monoclonal antibodies, P2 and SZ21, directed against glycoprotein (GP) IIb-IIIa and GP IIIa respectively. One hundred and twenty-seven healthy controls were studied and ratios of mean fluorescence intensity for P2 and SZ21 discriminated between HPA-1a homozygotes and heterozygotes. These two monoclonal antibodies, coupled with flow cytometry represent a rapid and reliable tool for platelet HPA-1 typing to aid diagnosis.

CT and MR imaging of central nervous system effects of therapy in patients treated for hematological malignancies

The purpose of this article is to present the imaging appearance of central nervous system effects of therapy that may occur in patients treated for hematological malignancies. Imaging in these patients relates to complications of high-dose therapy, bone marrow transplantation, infections occurring in immunocompromised patients, central nervous system dysfunction due to failure of other organ systems, or cerebral hemorrhages due to platelet refractoriness. Rapid and accurate diagnosis is essential but often difficult, as neurological manifestations are rarely disease specific. Neurological imaging, in combination with electrophysiological studies as well as blood and cerebrospinal fluid investigations, may be helpful for diagnosing most of these complications, as well as in differentiating between the manifestations of the underlying disease and complications of the treatment.

Laboratory Diagnostics of HLA Antibodies

en

IVIG therapy for platelet refractoriness due to HLA class I antibodies: a case report

IVIG therapy for platelet refractoriness due to HLA class I antibodies: a case report

Use of recombinant factor VIIa in life-threatening bleeding following autologous peripheral blood stem cell transplantation complicated by platelet refractoriness

Use of recombinant factor VIIa in life-threatening bleeding following autologous peripheral blood stem cell transplantation complicated by platelet refractoriness

Predictive Parameters for in vivo Alloreactivity

AbstractFor Abstract see ChemInform Abstract in Full Text.

Recombinant factor VIIa reduces bleeding in severely thrombocytopenic rabbits

Severe thrombocytopenia is a common complication to intensive chemotherapeutic regimens. For bleeding episodes associated with severe thrombocytopenia, the current standard treatment is platelet transfusion. However, due to several transfusion complications such as transfusion-transmitted diseases, platelet refractoriness and immunomodulation, as well as increasing problems with sufficient supply of platelet products, it is imperative to search for alternatives to platelet transfusion. To test the efficacy of recombinant activated human coagulation factor VII (rFVIIa, NovoSeven®) in thrombocytopenia, a preclinical study was conducted in thrombocytopenic rabbits. Thrombocytopenia was induced by a combination of γ-irradiation and the use of platelet antibodies, and the effect of rFVIIa on nail cuticle bleeding was determined. Administration of rFVIIa at 2 mg/kg significantly shortened the prolonged bleeding time in thrombocytopenic animals (rFVIIa vs. control, median 23 min 41 s vs. 60 min, p=0.016) as well as significantly reducing the blood loss (rFVIIa vs. control, median: 8.8 vs. 12.2 nmol hemoglobin/ml, p=0.016). This effect was also reflected by a significant reduction of the prothrombin time, activated partial thromboplastin time, as well as improvement in clotting parameters in an in vitro thromboelastography thrombocytopenia model. Histopathological evaluation of kidney biopsies for the presence of micro thrombi did not reveal evidence of prothrombotic effects of rFVIIa in this model. These data demonstrate the haemostatic efficacy of rFVIIa in a rabbit model of severe thrombocytopenia. Clinical trials will be needed to further explore the potential of NovoSeven as a haemostatic agent in thrombocytopenic patients.