Effect of factor VIII and FVIII/PC ratio on portal vein thrombosis in liver cirrhosis: a systematic review and meta­analysis.

BACKGROUND: To date, there is an ongoing debate regarding the ability to predict PVT development using markers of FVIII or FVIII/PC ratio. This study presents evidence-based medical findings on the influence of FVIII activity levels and FVIII/PC values in the formation of PVT in cirrhosis. METHODS: The search for original studies on risk factors for portal vein thrombosis (PVT) associated with cirrhosis was conducted, which primarily focused on comparing circulating FVIII activity levels or FVIII/PC ratio in cirrhotic patients with and without PVT. The quality of evidence from each study was assessed using the Newcastle-Ottawa Scale. RESULTS: The meta-analysis included a total of 10 original studies. In total, 2250 cirrhotic patients were included, with 414 having PVT and 1836 without PVT. The pooled analysis using a random-effects model showed no significant difference in standardized mean difference (SMD) for FVIII activity levels in cirrhotic patients with or without PVT (SMD = 0.12, 95% CI=-0.46 to 0.70, P = 0.68), but there was significant heterogeneity (I2 = 95.52%, P = 0.00). Meta-regression analysis indicated that differences in mean FVIII activity levels in the PVT group, the number of cases in the non-PVT group, and the study design methods partially contributed to the heterogeneity (P < 0.05). However, compared to the non-PVT group, the PVT group had higher FVIII/PC ratio with a statistically significant difference (SMD = 0.39, 95% CI: 0.15 to 0.63, P = 0.00), and there was no significant heterogeneity (I2 = 28.62%). CONCLUSION: In conclusion, the FVIII/PC ratio not only reflects the severity of liver disease, but also can be used as one of the predictors of PVT development.

Association of periodontitis with cardiometabolic and haemostatic parameters.

OBJECTIVE: To investigate the association between periodontitis and cardiometabolic and haemostatic parameters. MATERIALS AND METHODS: Between 2014 and 2019, 54 individuals needing full mouth extraction, and 50 control individuals, were recruited for a combined cross-sectional (individuals versus controls) and longitudinal (individuals before and after extraction) study. Periodontitis severity was measured using the periodontal inflamed surface area (PISA). Blood was drawn to measure the haemostatic (Factor VIII, von Willebrand factor [VWF], endogenous thrombin potential, d-dimer, clot lysis time) and cardiovascular risk (C-reactive protein [CRP], lipid profile) parameters, prior to and 12 weeks post-extraction. The results were analysed group-wise. RESULTS: The mean VWF and CRP levels were higher and the high-density lipoprotein levels were lower in the individuals prior to extraction compared to the controls. The VWF was significantly correlated with the PISA (a 21% unit increase in VWF per 1000 mm2 increase in PISA, 95%CI: 6-36%, p = 0.01). The other analyses were comparable between the individuals and controls, and did not change in the individuals after the extraction. CONCLUSION: VWF levels are associated with periodontitis severity; they do not improve after full-mouth extraction. Severe periodontitis in control individuals does not induce substantial changes in their haemostatic or inflammatory systems. CLINICAL RELEVANCE: Treatment of periodontitis has been shown to improve the cardiometabolic blood profile of patients with established cardiometabolic disease. However, whether periodontitis treatment improves cardiometabolic and haemostatic profiles in people without cardiometabolic disease is uncertain.

Pre-clinical evaluation of an enhanced-function factor VIII variant for durable hemophilia A gene therapy in male mice.

Durable factor VIII expression that normalizes hemostasis is an unrealized goal of hemophilia A adeno-associated virus-mediated gene therapy. Trials with initially normal factor VIII activity observed unexplained year-over-year declines in expression while others reported low-level, stable expression inadequate to restore normal hemostasis. Here we demonstrate that male mice recapitulate expression-level-dependent loss of factor VIII levels due to declines in vector copy number. We show that an enhanced function factor VIII variant (factor VIII-R336Q/R562Q), resistant to activated protein C-mediated inactivation, normalizes hemostasis at below-normal expression without evidence of prothrombotic risk in male hemophilia A mice. These data support that factor VIII-R336Q/R562Q may restore normal factor VIII function at low levels of expression to permit durability using low vector doses to minimize dose-dependent adeno-associated virus toxicities. This work informs the mechanism of factor VIII durability after gene transfer and supports that factor VIII-R336Q/R562Q may safely overcome current hemophilia A gene therapy limitations.

Hemoglobin concentration and body mass index are determinants of plasma von Willebrand factor and factor VIII levels.

BACKGROUND: Von Willebrand Disease (VWD) is the most common inherited bleeding disorder. VWD is characterized by an abnormal quantity or quality of von Willebrand Factor (VWF). Anemia is often found at presentation for a bleeding disorder evaluation due to chronic blood loss. OBJECTIVES/HYPOTHESIS: We hypothesized that anemia is associated with elevations in both VWF and factor VIII (FVIII) over baseline. We also hypothesized that obesity would be associated with increased levels of VWF. METHODS: We conducted a single-center review of the electronic health record for patients that had proximal von Willebrand profiles and Hb data. RESULTS: We identified 4552 unique subjects with VWF studies and a CBC within 24 h. We found that decreasing hemoglobin inversely correlated with VWF antigen, VWF ristocetin cofactor activity, and FVIII activity. We also found that obesity and Black race were independently associated with increased VWF antigen, activity, and FVIII activity. Hb, race, and body mass index (BMI) continued to be determinants of VWF and FVIII levels in multivariable analysis. CONCLUSION: Our study demonstrates that anemia, race, and BMI were found to be associated with elevation of VWF antigen, VWF activity, and FVIII levels. As many individuals with anemia present for evaluation for a bleeding disorder, these variables need to be considered. KEY POINTS: - Anemia was found to be associated with elevation of VWF antigen, VWF activity and FVIII levels. - Testing von Willebrand factor at times of anemia may mask a diagnosis of von Willebrand Disease.

Characterization of zebrafish coagulation cofactors Fviii and Fv mutants and modeling hemophilia A and factor V deficiency.

The aim of this study is to characterize zebrafish coagulation cofactors fviii and fv mutant fish and assess if they phenocopy classical hemophilia A and factor V deficiency in humans. The embryos from fviii and fv zebrafish heterozygote mutants generated by ENU mutagenesis were purchased from the ZIRC repository. They were reared to adulthood and genotyped. The heterozygote male and female were crossed to get homozygote, heterozygote, and wild-type fish. Functional kinetic coagulation assays and bleeding assays were performed on normal and mutant adult fish, and venous laser injury assays were performed on the larvae. The DNA from fviii and fv mutants were sequenced to confirm if they have a premature stop codon in exon 19, and in exon 2, respectively, and in both mutants, the amino acid glutamine is replaced with a stop codon. Homozygous and heterozygous 5 days post fertilization (dpf) larvae for fviii and fv deficient mutants exhibited prolonged time to occlusion after venous laser injury compared to wild-type controls. The homozygous and heterozygous fviii adult mutants showed modest bleeding and delayed fibrin formation in the kinetic partial thromboplastin time (kPTT) assay with their plasma. fv homozygous larvae had poor survival beyond 12 dpf. However, heterozygous fv mutants exhibited heavy bleeding and prolonged fibrin formation in the kPTT and kPT assay compared with wild-type siblings. Our characterization showed fviii and fv mutants from ZIRC phenocopied to a considerable extent classical hemophilia A and factor V deficiency in humans, respectively. These models should be useful in studying and developing novel drugs that reverse the phenotype and in generating suppressor mutations to identify novel factors that compensate for these deficiencies.

AAV mediated repression of Neat1 lncRNA combined with F8 gene augmentation mitigates pathological mediators of joint disease in haemophilia.

Haemophilic arthropathy (HA), a common comorbidity in haemophilic patients leads to joint pain, deformity and reduced quality of life. We have recently demonstrated that a long non-coding RNA, Neat1 as a primary regulator of matrix metalloproteinase (MMP) 3 and MMP13 activity, and its induction in the target joint has a deteriorating effect on articular cartilage. In the present study, we administered an Adeno-associated virus (AAV) 5 vector carrying an short hairpin (sh)RNA to Neat1 via intra-articular injection alone or in conjunction with systemic administration of a capsid-modified AAV8 (K31Q) vector carrying F8 gene (F8-BDD-V3) to study its impact on HA. AAV8K31Q-F8 vector administration at low dose, led to an increase in FVIII activity (16%-28%) in treated mice. We further observed a significant knockdown of Neat1 (~40 fold vs. untreated injured joint, p = 0.005) in joint tissue of treated mice and a downregulation of chondrodegenerative enzymes, MMP3, MMP13 and the inflammatory mediator- cPLA2, in mice receiving combination therapy. These data demonstrate that AAV mediated Neat1 knockdown in combination with F8 gene augmentation can potentially impact mediators of haemophilic joint disease.

Structural basis for inhibition of coagulation factor VIII reveals a shared antigenic hotspot on the C1 domain.

BACKGROUND: Hemophilia A arises from dysfunctional or deficient coagulation factor (F)VIII and leads to inefficient fibrin clot formation and uncontrolled bleeding events. The development of antibody inhibitors is a clinical complication in hemophilia A patients receiving FVIII replacement therapy. LE2E9 is an anti-C1 domain inhibitor previously isolated from a mild/moderate hemophilia A patient and disrupts FVIII interactions with von Willebrand factor and FIXa, though the intermolecular contacts that underpin LE2E9-mediated FVIII neutralization are undefined. OBJECTIVES: To determine the structure of the complex between FVIII and LE2E9 and characterize its mechanism of inhibition. METHODS: FVIII was bound to the antigen binding fragment (Fab) of NB2E9, a recombinant construct of LE2E9, and its structure was determined by cryogenic electron microscopy. RESULTS: This report communicates the 3.46 Å structure of FVIII bound to NB2E9, with its epitope comprising FVIII residues S2040 to Y2043, K2065 to W2070, and R2150 to H2155. Structural analysis reveals that the LE2E9 epitope overlaps with portions of the epitope for 2A9, a murine-derived inhibitor, suggesting that these residues represent a shared antigenic region on the C1 domain between FVIII-/- mice and hemophilia A patients. Furthermore, the FVIII:NB2E9 structure elucidates the orientation of the LE2E9 glycan, illustrating how the glycan sterically blocks interactions between the FVIII C1 domain and the von Willebrand factor D' domain. A putative model of the FVIIIa:FIXa complex suggests potential clashing between the NB2E9 glycan and FIXa light chain. CONCLUSION: These results describe an antigenic "hotspot" on the FVIII C1 domain and provide a structural basis for engineering FVIII replacement therapeutics with reduced antigenicity.

The combination of Asp519Val/Glu665Val and Lys1813Ala mutations in FVIII markedly increases coagulation potential.

ABSTRACT: A2 domain dissociation in activated factor VIII (FVIIIa) results in reduced activity. Previous studies demonstrated that some FVIII mutants (D519V/E665V and K1813A) with delayed A2 dissociation enhanced coagulation potential. We speculated, therefore, that FVIII encompassing a combination of these mutations might further enhance coagulant activity. The aim was to assess the D519V/E665V/K1813A-FVIII mutation as a gain of function. The FVIII mutants, D519V/E665V/K1813A, D519V/E665V, and K1813A were expressed in a baby hamster kidney cell system, and global coagulation potential of these mutants was compared with wild-type (WT) FVIII in vitro and in hemophilia A mice in vivo. Kinetic analyses indicated that the apparent Kd for FIXa on the tenase assembly with D519V/E665V and D519V/E665V/K1813A mutants were lower, and that the generated FXa for D519V/E665V/K1813A was significantly greater than WT-FVIII. WT-FVIII activity after thrombin activation increased by ∼12-fold within 5 minutes, and returned to initial levels within 30 minutes. In contrast, The FVIII-related activity of D519V/E665V/K1813A increased further with time after thrombin activation, and showed an ∼25-fold increase at 2 hours. The A2 dissociation rate of D519V/E665V/K1813A was ∼50-fold slower than the WT in a 1-stage clotting assay. Thrombin generation assays demonstrated that D519V/E665V/K1813A (0.125 nM) exhibited coagulation potential comparable with that of the WT (1 nM). In animal studies, rotational thromboelastometry and tail-clip assays showed that the coagulation potential of D519V/E665V/K1813A (0.25 µg/kg) was equal to that of the WT (2 µg/kg). FVIII-D519V/E665V/K1813A mutant could provide an approximately eightfold increase in hemostatic function of WT-FVIII because of increased FVIIIa stability and the association between FVIIIa and FIXa.

Novel mechanisms of action of emerging therapies of hereditary thrombotic thrombocytopenic purpura.

INTRODUCTION: Hereditary thrombotic thrombocytopenic purpura (hTTP) is caused by deficiency of plasma ADAMTS13 activity, resulting from ADAMTS13 mutations. ADAMTS13 cleaves ultra large von Willebrand factor (VWF), thus reducing its multimer sizes. Hereditary deficiency of plasma ADAMTS13 activity leads to the formation of excessive platelet-VWF aggregates in small arterioles and capillaries, resulting in hTTP. AREAS COVERED: PubMed search from 1956 to 2024 using thrombotic thrombocytopenic purpura and therapy identified 3,675 articles. Only the articles relevant to the topic were selected for discussion, which focuses on pathophysiology, clinical presentations, and mechanisms of action of emerging therapeutics for hTTP. Current therapies include infusion of plasma, or coagulation factor VIII, or recombinant ADAMTS13. Emerging therapies include anti-VWF A1 aptamers or nanobody and gene therapies with adeno-associated viral vector or self-inactivated lentiviral vector or a sleeping beauty transposon system for a long-term expression of a functional ADAMTS13 enzyme. EXPERT OPINION: Frequent plasma infusion remains to be the standard of care in most parts of the world, while recombinant ADAMTS13 has become the treatment of choice for hTTP in some of the Western countries. The success of gene therapies in preclinical models may hold a promise for future development of these novel approaches for a cure of hTTP.

Complete blood count, coagulation biomarkers, and lung function 6 months after critical COVID-19.

BACKGROUND: Understanding the recovery of post-COVID-19 organ dysfunction is essential. We evaluated coagulation 6 months post-COVID-19, examining its recovery and association with lung function. METHODS: Patients treated for COVID-19 at intensive care units between 3/2020 and 1/2021 were analyzed for complete blood count (CBC) and coagulation biomarkers (prothrombin time activity (%) (PT%), activated partial thromboplastin time (APTT), fibrinogen, coagulation factor VIII (FVIII), antithrombin (AT), and D-dimer) during the 6 months post-hospitalization. Results were compared with acute phase values and correlated with pulmonary function tests (PFT), including forced vital capacity (FVC) and hemoglobin-corrected diffusing capacity percentage of predicted (DLCOc%), recorded 6 months post-hospitalization. We examined the association between coagulation biomarkers and DLCOc% using linear regression with age, sex, and invasive mechanical ventilation (IMV) duration, and FVIII (correlated with DLCOc%) as covariates. RESULTS: Most CBCs and coagulation biomarkers had median values within the normal range. However, only 21% (15/70) of patients achieved full normalization of all biomarkers. Compared to acute COVID-19, hemoglobin, PT%, and AT increased, while leukocytes, fibrinogen, FVIII, and D-dimer decreased. Despite decreased levels, FVIII remained elevated in 46% (31/68), leukocytes in 26% (18/70), and D-dimer in 27% (18/67) at 6 months. A weak negative correlation (r = -0.37, p = .036) was found between DLCOc% and FVIII. Multivariable analysis revealed a weak, independent association between DLCOc% and FVIII. Excluding patients with anticoagulation therapy, FVIII no longer correlated with DLCOc%, while AT showed a moderate correlation with DLCOc%. CONCLUSION: Only a few patients had normal CBC and coagulation biomarker values 6 months after critical COVID-19. A weak negative correlation between DLCOc% and FVIII suggests that deranged coagulation activity may be associated with reduced diffusing capacity.

DNA base editing corrects common hemophilia A mutations and restores factor VIII expression in in vitro and ex vivo models.

BACKGROUND: Replacement and nonreplacement therapies effectively control bleeding in hemophilia A (HA) but imply lifelong interventions. Authorized gene addition therapy could provide a cure but still poses questions on durability. FVIIIgene correction would definitively restore factor (F)VIII production, as shown in animal models through nuclease-mediated homologous recombination (HR). However, low efficiency and potential off-target double-strand break still limit HR translatability. OBJECTIVES: To correct common model single point mutations leading to severe HA through the recently developed double-strand break/HR-independent base editing (BE) and prime editing (PE) approaches. METHODS: Screening for efficacy of BE/PE systems in HEK293T cells transiently expressing FVIII variants and validation at DNA (sequencing) and protein (enzyme-linked immunosorbent assay; activated partial thromboplastin time) level in stable clones. Evaluation of rescue in engineered blood outgrowth endothelial cells by lentiviral-mediated delivery of BE. RESULTS: Transient assays identified the best-performing BE/PE systems for each variant, with the highest rescue of FVIII expression (up to 25% of wild-type recombinant FVIII) for the p.R2166∗ and p.R2228Q mutations. In stable clones, we demonstrated that the mutation reversion on DNA (∼24%) was consistent with the rescue of FVIII secretion and activity of 20% to 30%. The lentiviral-mediated delivery of the selected BE systems was attempted in engineered blood outgrowth endothelial cells harboring the p.R2166∗ and p.R2228Q variants, which led to an appreciable and dose-dependent rescue of secreted functional FVIII. CONCLUSION: Overall data provide the first proof-of-concept for effective BE/PE-mediated correction of HA-causing mutations, which encourage studies in mouse models to develop a personalized cure for large cohorts of patients through a single intervention.

Concizumab improves clot formation in hemophilia A under flow.

BACKGROUND: Inhibition of tissue factor pathway inhibitor (TFPI) is an emerging therapeutic strategy for treatment of hemophilia. Concizumab is a monoclonal antibody that binds TFPI and blocks its inhibition of factor (F)Xa thereby extending the initiation of coagulation and compensating for lack of FVIII or FIX. OBJECTIVES: The objective of this in vitro study was to evaluate how concizumab affects clot formation in hemophilia A under flow. METHODS: Blood was collected from normal controls or people with hemophilia A. An anti-FVIII antibody was added to normal controls to simulate hemophilia A with inhibitory antibodies to FVIII. Whole blood and recombinant activated FVII (rFVIIa, 25 nM) or concizumab (200, 1000, and 4000 ng/mL) were perfused at 100 s-1 over a surface micropatterned with tissue factor (TF) and collagen-related peptide. Platelet and fibrin(ogen) accumulation were measured by confocal microscopy. Static thrombin generation in plasma was measured in response to rFVIIa and concizumab. RESULTS: Concizumab (1000 and 4000 ng/mL) and rFVIIa both rescued (93%-101%) total platelet accumulation, but only partially rescued (53%-63%) fibrin(ogen) incorporation to normal control levels in simulated hemophilia A. Results using congenital hemophilia A blood confirmed effects of rFVIIa and concizumab. While these 2 agents had similar effect on clot formation under flow, concizumab enhanced thrombin generation in plasma under static conditions to a greater extent than rFVIIa. CONCLUSION: TFPI inhibition by concizumab enhanced activation and aggregation of platelets and fibrin clot formation in hemophilia A to levels comparable with that of rFVIIa.

GP64-pseudotyped lentiviral vectors target liver endothelial cells and correct hemophilia A mice.

Lentiviral vectors (LV) are efficient vehicles for in vivo gene delivery to the liver. LV integration into the chromatin of target cells ensures their transmission upon proliferation, thus allowing potentially life-long gene therapy following a single administration, even to young individuals. The glycoprotein of the vesicular stomatitis virus (VSV.G) is widely used to pseudotype LV, as it confers broad tropism and high stability. The baculovirus-derived GP64 envelope protein has been proposed as an alternative for in vivo liver-directed gene therapy. Here, we perform a detailed comparison of VSV.G- and GP64-pseudotyped LV in vitro and in vivo. We report that VSV.G-LV transduced hepatocytes better than GP64-LV, however the latter showed improved transduction of liver sinusoidal endothelial cells (LSEC). Combining GP64-pseudotyping with the high surface content of the phagocytosis inhibitor CD47 further enhanced LSEC transduction. Coagulation factor VIII (FVIII), the gene mutated in hemophilia A, is naturally expressed by LSEC, thus we exploited GP64-LV to deliver a FVIII transgene under the control of the endogenous FVIII promoter and achieved therapeutic amounts of FVIII and correction of hemophilia A mice.

Factor VIII and Incident Hypertension in Black and White Adults: The REasons for Geographic and Racial Differences in Stroke (REGARDS) Cohort.

BACKGROUND: Nearly half of all Americans have hypertension, and Black adults experience a disproportionate burden. Hypercoagulability may relate to hypertension risk, and higher levels of factor VIII increase thrombosis risk. Black adults have higher factor VIII and more hypertension than other groups. Whether higher factor VIII associates with incident hypertension is unknown. METHODS: The Biomarkers as Mediators of Racial Disparities in Risk Factors (BioMedioR) study measured certain biomarkers in a sex-race stratified sample of 4,400 REGARDS participants who attended both visits. We included BioMedioR participants, excluding those with prevalent hypertension, missing factor VIII level, or covariates of interest. Modified Poisson regression estimated risk ratios (RR) for incident hypertension by higher log-transformed factor VIII level per SD (SD of log-transformed factor VIII, 0.33). Weighting was applied to take advantage of REGARDS sampling design. RESULTS: Among the 1,814 participants included (55% female, 24% Black race), the median follow-up was 9.5 years and 35% (2,146/6,138) developed hypertension. Black participants had a higher median (IQR) factor VIII level (105.6%; 87.1%-126.9%) than White participants (95.6%; 79.8%-115.9%; P < 0.001). The age- and sex-adjusted Black-White hypertension RR was 1.45 (95% CI 1.28, 1.63). Higher factor VIII was not associated with more hypertension (final model RR 1.01; 95% CI 0.94, 1.07). CONCLUSIONS: In a prospective study of Black and White adults without prevalent hypertension, factor VIII was not associated with greater hypertension risk.

Variant mapping using mass spectrometry-based proteotyping as a diagnostic tool in von Willebrand disease.

BACKGROUND: von Willebrand disease (VWD) is the most common inherited bleeding disorder, characterized by either partial or complete von Willebrand factor (VWF) deficiency or by the occurrence of VWF proteoforms of altered functionality. The gene encoding VWF is highly polymorphic, giving rise to a variety of proteoforms with varying plasma concentrations and clinical significance. OBJECTIVES: To address this complexity, we translated genomic variation in VWF to corresponding VWF proteoforms circulating in blood. METHODS: VWF was characterized in VWD patients (n = 64) participating in the Willebrand in the Netherlands study by conventional laboratory testing, DNA sequencing and complementary discovery, and targeted mass spectrometry-based plasma proteomic strategies. RESULTS: Unbiased plasma profiling combined with immune enrichment of VWF verified VWF and its binding partner factor VIII as key determinants of VWD and revealed a remarkable heterogeneity in VWF amino acid sequence coverage among patients. Subsequent VWF proteotyping enabled identification of both polymorphisms (eg, p.Thr789Ala, p.Gln852Arg, and p.Thr1381Ala), as well as pathogenic variants (n = 16) along with their corresponding canonical sequences. Targeted proteomics using stable isotope-labeled peptides confirmed unbiased proteotyping for 5 selected variants and suggested differential proteoform quantities in plasma. The variant-to-wild-type peptide ratio was determined in 6 type 2B patients heterozygous for p.Arg1306Trp, confirming the relatively low proteoform concentration of the pathogenic variant. The elevated VWF propeptide/VWF ratio indicated increased clearance of specific VWF proteoforms. CONCLUSION: This study highlights how VWF proteotyping from plasma could be the first step to bridge the gap between genotyping and functional testing in VWD.

In-depth structure-function profiling of the complex formation between clotting factor VIII and heme.

BACKGROUND AND AIMS: Blood disorders, such as sickle cell disease, and other clinical conditions are often accompanied by intravascular hemolytic events along with the development of severe coagulopathies. Hemolysis, in turn, leads to the accumulation of Fe(II/III)-protoporphyrin IX (heme) in the intravascular compartment, which can trigger a variety of proinflammatory and prothrombotic reactions. As such, heme binding to the blood coagulation proteins factor VIII (FVIII), fibrinogen, and activated protein C with functional consequences has been demonstrated earlier. METHODS: We herein present an in-depth characterization of the FVIII-heme interaction at the molecular level and its (patho-)physiological relevance through the application of biochemical, biophysical, structural biology, bioinformatic, and diagnostic tools. RESULTS: FVIII has a great heme-binding capacity with seven heme molecules associating with the protein. The respective binding sites were identified by investigating heme binding to FVIII-derived peptides in combination with molecular docking and dynamic simulation studies of the complex as well as cryo-electron microscopy, revealing three high-affinity and four moderate heme-binding motifs (HBMs). Furthermore, the relevance of the FVIII-heme complex formation was characterized in physiologically relevant assay systems, revealing a ~ 50 % inhibition of the FVIII cofactor activity even in the protein-rich environment of blood plasma. CONCLUSION: Our study provides not only novel molecular insights into the FVIII-heme interaction and its physiological relevance, but also strongly suggests the reduction of the intrinsic pathway and the accentuation of the final clotting step (by, for example, fibrinogen crosslinking) in hemolytic conditions as well as a future perspective in the context of FVIII substitution therapy of hemorrhagic events in hemophilia A patients.