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Introduction: Over the past 21 years, treatment options for hemophilia have evolved significantly. The objective of this study is to describe the trends observed in clinician prescribing practices for management of hemophilia A (HA) and B (HB) in the United States (US) via three surveys from 1999-2021. Methods: We administered surveys to members of the Hemostasis & Thrombosis Research Society (HTRS) via an in-person paper survey at its annual symposia in 1999 and 2015, and an online survey in 2021. The survey participants included physicians, physician assistants, and nurse practitioners who manage the care of hemophilia patients at hemophilia treatment centers in the US. The surveys collected information regarding: 1) characteristics of clinician practice, 2) prescribed clotting factor products and dosages used for routine bleeds or major life-threatening bleeding, total joint replacement, and port placement, 3) reasons for changing doses, 4) frequency of recommendation for prophylaxis and inhibitor treatment for associated factor and non-factor products, and 5) gene therapy. Results: Forty-one clinicians completed the survey in 1999 and 2021, 53 in 2015. The mean number of patients seen by respondents increased from 142 (range: 0-314) for children and 101 (0-480) for adults in 1999 to 202 (0-900) for children and 154 (0-500) for adults in 2021. The proportion of clinicians prescribing >40 units/kg of Standard Half Life (SHL) Factor IX concentrates for routine bleeding events in HB patients increased from 22.5% in 1999 to 50.9% in 2015, and 87.8% in 2021. The proportion of clinicians reported SHL Factor VIII usage for routine bleeding at a dose of >40 units/kg in HA patients increased from none in 1999 to 11.3% in 2015 and 29.3 % in 2021. The reported rates of prescribing an average >60 units/kg factor to treat major life-threatening bleeds increased from 67.5% in 1999 to 90.3% in 2021 for HB; rates were 2.5% in 1999, 17.3% in 2015 and 7.3% in 2021 for treating HA. For children 91% of clinicians reported prescribing emicizumab to treat HA inhibitors in patients of all ages, while >87% reported prescribing it to treat HA without inhibitor. Clinicians were more likely to always prescribe emicizumab to treat HA patients with inhibitors (63.2% for children and 57.1% for adults), as compared to always prescribing it for those without inhibitors (13.2% for children and 5.7% for adults). The most frequent reported method to treat a patient with a history of inhibitors on emicizumab who had break through bleeds was rFVIIa: 85.4% for children, and 75.6% for adults. The most frequently reported reasons for switching from FVIII to emicizumab were fewer injections/visits (87.8%), and improved patient quality of life (82.9%). Thirty-nine percent of clinicians reported caring for patients currently in gene therapy trials, 27.5% had patients who had completed gene therapy. When asked about potential future prescribing practices, 14.6% reported that they would prescribe gene therapy "all the time", 4.9% would prescribe it "about 3/4 of the time", 29.3% "about 1/2 the time", 29.3% "about 1/4 the time", and 22.0% "rarely or never". Conclusion: These data indicate changes in prescribing practices among hemophilia specialists in the US over the past 21 years. Prescribing of high doses of factor (>40 units/kg) increased, while ITI prescribing practices remained similar over time. To treat patients with major life-threatening bleeds, a larger proportion of clinicians prescribed high doses of factor (>60 units/kg) for patients with HB as compared to HA. Most clinicians frequently prescribed emicizumab for patients with HA inhibitors, but less frequently for those without inhibitors. At this time, there is wide diversity among clinicians in the expected uptake of gene therapy. Disclosures Curtis: Pfizer, Bayer, and Novo Nordisk: Consultancy; University of Southern California: Consultancy. Roberts: Takeda; Speakers Bureau: Novo Nordisk, Octapharma, Sanofi, Takeda.: Research Funding; Genentech, Novo Nordisk, Octapharma, Pfizer, Sanofi, Takeda, uniQure: Consultancy. Decker-Palmer: Genentech Inc. --A member of the Roche Group.: Current Employment, Current equity holder in publicly-traded company. Khairnar: Genentech Inc - A Member of The Roche Group: Current Employment; University of Maryland, Baltimore: Ended employment in the past 24 months; Roche: Current equity holder in publicly-traded company. Wu: Baxalta US Inc., Bannockburn, IL (a Takeda Company), CSL Behring L.L.C., Octapharma USA, Inc., Genentech Inc.: Research Funding. Nichol: Pfizer, Genentech Inc., Baxalta US Inc., Bannockburn, IL (a Takeda Company), Octapharma, CSL Behring, Global Blood Therapeutics, and Novo Nordisk: Research Funding.

Introduction: Persons with hemophilia suffer from recurrent bleeds, especially hemarthrosis, which results in joint damage. Hemophilia inhibitor status impacts bleeding, which is associated with acute and chronic pain. To better characterize the impact of inhibitor status, we compared patient-reported outcomes (bleed rate, pain, and joint health), work productivity and activity impairment (WPAI), and health-related quality of life (HRQoL) by inhibitor status, and investigated the correlation of patient-reported outcomes with WPAI and HRQoL. Methods: The U.S. Hematology Utilization Group Studies Part VIII prospectively collected data to examine the cost and burden of hemophilia in persons with hemophilia A (PwHA) aged ≥2 years obtaining care at four federally-supported hemophilia treatment centers. From April 2019 to May 2021 we enrolled PwHA with and without inhibitors at a 1:2 ratio. Parents/adult participants completed a survey at enrollment to collect sociodemographic and clinical characteristics, self-reported bleeds in the last month, pain, and joint stiffness (5-item scale). We also measured WPAI and HRQoL using EQ-5D-3L. Clinical chart review documented hemophilic severity, inhibitor level and treatment regimen. Participants were classified into three groups: 1) active inhibitor (inhibitor titer ≥1.0 Bethesda Units (BU) six months prior to enrollment), 2) tolerized inhibitor (history of inhibitor titer ≥1.0 BU, immune tolerance induction (ITI) and/or currently using factor VIII for prophylaxis), and 3) no inhibitor. Patient-reported data were compared across these groups using Chi-square tests for categorical variables and generalized linear models for continuous variables. Association of bleeds, pain, and joint stiffness with HRQoL and WPAI were assessed using Pearson correlation. Results: Among 80 PwHA enrolled, 9 (11%) had active inhibitors, 22 (27.5%) had tolerized inhibitors, and 49 (61.3%) had no inhibitors. Mean age was 24.9±14.3 (standard deviation) years, 66.3% were adults, 87.5% had severe hemophilia, and 87.5% used prophylaxis. Mean age of the non-inhibitor group (29.3±13.5) was older than the tolerized inhibitor group (16.3±9.5 years, p0.05). The non-inhibitor group had a lower rate of severe hemophilia (81.6%) or prophylactic treatment (81.6%) than those in the active (100%) or tolerized groups (95.5%, p=0.13). Larger proportions of participants with active inhibitors (66.7%) and no inhibitors (57.1%) reported having bleeds in the last month compared to those with tolerized inhibitors (22.7%, p=0.01). Participants without inhibitors had a greater mean number of bleeding episodes (1.09±standard error (SE) 0.26 vs. 0.23±0.38, p=0.03), specifically joint bleeds (0.58±0.16 vs. 0.08±0.24, p=0.03,) than the tolerized group. Those with active inhibitors reported significantly higher mean joint stiffness scores (35.1±2.6 vs. 27.5±1.9, p=0.006) or more joint pain (77.8% vs. 54.5%, p=0.23) than the tolerized group. Mean EQ-5D index score was significantly lower in the active inhibitor group (0.79±SE (0.07) than in the tolerized group (0.96±0.05, p=0.03). Joint bleeding, chronic pain, and joint stiffness were negatively correlated with the EQ-5D visual analogue scale, and index scores (all correlation coefficients |r|>0.43, all p Conclusions: This study is limited to a small sample skewed toward a younger age in the tolerized inhibitor group. PwHA in the active and no inhibitor groups experienced greater clinical burden as measured by bleeds compared to the tolerized group. Those with active inhibitor displayed lower HRQoL scores than the tolerized inhibitor group. Bleeds, chronic pain and joint stiffness were inversely correlated with HRQoL, resulting in lower work productivity and activity. Figure 1 Figure 1. Disclosures Roberts: Takeda; Speakers Bureau: Novo Nordisk, Octapharma, Sanofi, Takeda.: Research Funding; Genentech, Novo Nordisk, Octapharma, Pfizer, Sanofi, Takeda, uniQure: Consultancy. Khairnar: University of Maryland, Baltimore: Ended employment in the past 24 months; Roche: Current equity holder in publicly-traded company; Genentech Inc - A Member of The Roche Group: Current Employment. Decker-Palmer: Genentech Inc. --A member of the Roche Group.: Current Employment, Current equity holder in publicly-traded company. Curtis: Pfizer, Bayer, and Novo Nordisk: Consultancy; University of Southern California: Consultancy. Wu: Baxalta US Inc., Bannockburn, IL (a Takeda Company), CSL Behring L.L.C., Octapharma USA, Inc., Genentech Inc.: Research Funding. Nichol: Pfizer, Genentech Inc., Baxalta US Inc., Bannockburn, IL (a Takeda Company), Octapharma, CSL Behring, Global Blood Therapeutics, and Novo Nordisk: Research Funding.

Background: von Willebrand disease (VWD) is the most common inherited bleeding disorder. Questions remain regarding the impact of VWD related bleeding phenotype on healthcare utilization, joint health, and health-related quality of life (HRQoL) in the US. Objective: Our study investigated the impact of VWD bleeding phenotype on healthcare utilization, joint health, and HRQoL in a geographically diverse cohort of individuals with VWD who obtain care at seven US Hemophilia Treatment Centers (HTCs). Methods: Hematology Utilization Group Studies (HUGS) prospectively examined the cost and burden of illness in persons with VWD. The current study enrolled individuals age ≥12 with VWD Type 1 (VWF:Ag/RCo: ≤30%), low VWF (VWF:Ag/RCo: 30-50%), Type 2, and 3. Participants completed a standardized interview to collect sociodemographic and clinical data, self-reported healthcare utilization and bleeding in last 6 months, self-reported pain, joint health and HRQoL measured by EQ-5D-3L. Clinical chart reviews abstracted information about VWD type and treatment. Association of bleeding and VWD type with healthcare utilization, joint health, and HRQoL were assessed using Chi-square or Fisher exact tests for categorical variables and Student T-tests or one-way ANOVA for continuous variables. P value ≥0.05 indicates not statistically significant (NS). Results: We analyzed 100 participants with complete baseline information. Mean age was 31.7 (SD=18.6) years, 67.0% were adults ≥18 years, 80.0% were female, 67.7% had Type 1/low VWF, and 3.0% had Type 3 VWD. Mean age at VWD diagnosis was 13.6 (SD=13.0) years. Persons with low VWF were diagnosed and received VWD treatment at an older age (mean 19.2, SD=11.8; 19.4, SD=11.9 years for diagnosis and treatment respectively) than those with Type 1 (13.7, SD=12.7; 15.6, SD=13.7), or Types 2&3 (9.4, SD=12.9; 13.3, SD=16.6), p=0.03 and p=NS. As compared to individuals without bleeding in the previous 6 months, those reported bleeding had significantly higher rate of medical procedures related to treating bleeding events (42.5% vs. 13.3%, p=0.001), and overnight hospitalization (20.0% vs. 3.3%, p Conclusions: Our study demonstrates that persons with VWD who obtain care at US HTCs experience significant illness burden regardless of severity. Self-reported recent bleeding as expected was associated with increased healthcare utilization and negative impact on joint health and HRQoL. Bleeding phenotype was significantly associated with healthcare utilization differences. Delayed diagnosis and treatment for persons with low VWF may impact their HRQoL, and if confirmed in a larger sample size would underscore the fact that low VWF is not necessarily a mild disorder compared to other VWD subtypes. Disclosures Roberts: Octapharma: Consultancy, Speakers Bureau; Sanofi: Consultancy, Speakers Bureau; Novo Nordisk: Consultancy, Speakers Bureau; Pfizer: Consultancy; Takeda: Consultancy, Research Funding, Speakers Bureau; uniQure: Consultancy. Kulkarni:Bioverativ/Sanofi, BPL, Genentech, Kedrion, Novo Nordisk, Octapharma, Pfizer, Takeda, Catalyst Bioscience Bayer: Membership on an entity's Board of Directors or advisory committees; Sanofi/ Bioverativ, Bayer, Biomarin, Shire/Takeda, Novo Nordisk, Freeline: Other: clinical trial research grants . Sidonio:Octapharma, Grifols, Takeda and Genentech: Research Funding; Bayer, Bioverativ/Sanofi, Novo Nordisk, Takeda, Uniqure, Biomarin, Octapharma, Catalyst, Grifols, Sigilon, Tremeau, Genentech/Roche: Consultancy. Carpenter:Kedrion: Honoraria; Novo Nordisk: Honoraria; Genentech, Inc.: Honoraria; American Thrombosis and Hemostasis Network: Membership on an entity's Board of Directors or advisory committees; CSL Behring: Research Funding; Shire: Research Funding; Hemostasis & Thrombosis Research Society: Membership on an entity's Board of Directors or advisory committees; American Academy of Pediatrics: Other: PREP Heme/Onc editorial board. Konkle:Sigilon: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; BioMarin: Consultancy; Sanofi: Consultancy, Research Funding; Takeda: Research Funding; Uniquire: Research Funding; CSL Behring: Consultancy; Roche: Consultancy; Baxalta: Research Funding; Spark: Consultancy, Research Funding. Wu:Baxalta US Inc., Bannockburn, IL (a Takeda Company), CSL Behring L.L.C., and Octapharma USA, Inc.: Research Funding. Curtis:USC Hemophilia Utilization Group Study (HUGS): Consultancy; Bayer: Consultancy; Novo Nordisk: Consultancy; Patient Reported Outcomes, Burdens and Experiences: Consultancy. Carrasco:Baxalta US Inc., Bannockburn, IL (a Takeda Company), CSL Behring L.L.C., and Octapharma USA, Inc.: Research Funding. Tran:Novo Nordisk: Consultancy; Bioverativ: Consultancy; Takeda: Consultancy; Bayer: Consultancy. Nichol:Global Blood Therapeutics: Research Funding; Octapharma: Research Funding; CSL Behring: Research Funding; Pfizer: Research Funding; Baxalta US Inc., Bannockburn, IL (a Takeda Company): Research Funding; Genentech Inc.: Research Funding.

BACKGROUD Improved care and prophylactic factor replacement therapy have lengthened life expectancy for persons with hemophilia (PWH). These advancements have created a new hemophilia cohort of senior persons not seen since the previous human immunodeficiency viruses (HIV) years, and little is known about their age-related health outcomes. OBJECTIVES We describe the impact of hemophilia on comorbidities, joint problems, healthcare utilization and health related quality of life (HRQoL) from the HUGS VII baseline data. METHODS HUGS VII prospectively examines the cost and burden of hemophilia, including HRQoL, arthropathy, and economic impact in persons with hemophilia A or B age ≥ 40 years who obtained care from three US Hemophilia Treatment Centers. Participants completed a standardized interview to collect clinical and sociodemographic characteristics, hemophilia treatment regimen, pain, joint problems, comorbidities, and HRQoL using EQ-5D-3L. Clinical chart reviews documented hemophilic severity and treatment. Participants' characteristics were compared between two age groups: 40-49 years and ≥ 50 years using Chi-square tests for categorical variables and T-tests for continuous variables. RESULTS This analysis includes a total of 70 male with hemophilia, 64.3% aged ≥50 years, 75.7% with hemophilia A. Individuals ≥ 50 years had higher rates of being married or with a partner (71.11% vs. 56.0%, P=0.20), retired/not employed (66.7% vs. 28.0%, P=0.002), and having mild or moderate hemophilia (68.9% vs. 40.0%, P=0.02) than those 40-49 years old. Use of prophylaxis was similar among age groups in severe hemophilia but lower for older mild/moderate PWH. Among persons with mild/moderate hemophilia, those ≥ 50 years old reported a higher rate of joint pain (83.8% vs 70.0%, P=0.37) or range of motion limitation (73.3% vs, 60.0%, p=0.45) than the younger group, although the differences were not statistically significant. Compared to those 40-49 years old, individuals ≥ 50 years old reported fewer emergency room visits (4.6% vs. 20.8%, P=0.03), and physical therapy visits (16.0% vs. 41.7%, P=0.06). The older age group had significantly higher rates of self-reported myocardial infarction (11.6% vs. 0%, P=0.08) and diabetes (24.4% vs. 4.0%, P=0.03) than those 40-49 years. Self-reported comorbidities were measured by asking "Has a doctor ever told you that you have certain conditions?" The most frequently reported comorbidities were hepatitis C infection (75.7%, 79.3% of persons with infected hepatitis C had been treated and, 82.7% cleared the hepatitis C virus), hypertension (47.1%), depression (23.2%) and anxiety (21.7%). Mean covariates adjusted EQ-5D index score was lower in older persons (0.79 vs. 0.87, P=0.09). CONCLUSIONS Older PWH are over-represented by individuals with mild/moderate disease, potentially due to premature death among those with severe disease. Although this group included a larger proportion of mild disease than younger PWH, it presented prevalent comorbidities both of aging and of hemophilic arthropathy, despite lower rates of healthcare utilization and use of preventive therapies. Disclosures Curtis: USC Hemophilia Utilization Group Study (HUGS): Consultancy; Patient Reported Outcomes, Burdens and Experiences: Consultancy; Bayer: Consultancy; Novo Nordisk: Consultancy. Konkle:Pfizer: Consultancy, Research Funding; Roche: Consultancy; BioMarin: Consultancy; Sigilon: Consultancy, Research Funding; Sanofi: Consultancy, Research Funding; Takeda: Research Funding; Uniquire: Research Funding; CSL Behring: Consultancy; Baxalta: Research Funding; Spark: Consultancy, Research Funding. Kulkarni:Sanofi/ Bioverativ, Bayer, Biomarin, Shire/Takeda, Novo Nordisk, Freeline: Other: clinical trial research grants ; Bioverativ/Sanofi, BPL, Genentech, Kedrion, Novo Nordisk, Octapharma, Pfizer, Takeda, Catalyst Bioscience Bayer: Membership on an entity's Board of Directors or advisory committees. Wu:Baxalta US Inc., Bannockburn, IL (a Takeda Company), CSL Behring L.L.C., and Octapharma USA, Inc.: Research Funding. Tran:Takeda: Consultancy; Novo Nordisk: Consultancy; Bayer: Consultancy; Bioverativ: Consultancy. Nichol:Global Blood Therapeutics: Research Funding; CSL Behring: Research Funding; Octapharma: Research Funding; Genentech Inc.: Research Funding; Baxalta US Inc., Bannockburn, IL (a Takeda Company): Research Funding; Pfizer: Research Funding.

INTRODUCTION Depression can impact quality of life, functioning, and treatment adherence. However, depression in persons with hemophilia A (PwHA) is not uniformly examined nationwide. We report on depression and treatment-related hemophilia symptoms and utilization in a sample from six geographically representative U.S. Hemophilia Treatment Centers (HTCs). METHODS Hematology Utilization Group Studies part Va (HUGS Va, 2005-2007) was an observational study that collected data on sociodemographics and 12-Item Short Form Health Survey (SF-12) via patient survey, and clinical characteristics via medical chart review for adults (≥18 years old) with hemophilia A. Depressive disorders were assessed by the SF-12 mental component score (MCS), a validated population-based measure including depressive disorders. MCS≤45 has been identified as a depressive symptom threshold suggestive of a disorder. MCS≤36 indicate more severe psychological symptomatology and/or impairment. Demographic and clinical characteristic associations with depression were assessed using Chi-square tests. RESULTS The analysis included 147 adults with mean age 33.0±12.5 years old, 64% with severe hemophilia. Using the criteria of MCS≤45, 27.9% of sample had depressive disorder, and 10.2% had more severe depression at MCS≤36, higher than the 9.0% prevalence of current depression in the general U.S. population studied by CDC's Behavioral Risk Factor Surveillance System survey data (2006 and 2008). Compared with individuals with no depressive disorder, those with depression were less likely to complete their high school education (51.2% vs. 75.2%, P CONCLUSIONS This sample of PwHA reported higher rates of potentially depressive disorders than the general USA population. Lower educational levels, joint problems, and barriers to accessing care may be high-risk factors for depressive disorders. Disclosures Wu: Baxalta US Inc., Bannockburn, IL (a Takeda Company), CSL Behring L.L.C., and Octapharma USA, Inc.: Research Funding. Curtis:USC Hemophilia Utilization Group Study (HUGS): Consultancy; Patient Reported Outcomes, Burdens and Experiences: Consultancy; Bayer: Consultancy; Novo Nordisk: Consultancy. Tran:Bayer: Consultancy; Takeda: Consultancy; Bioverativ: Consultancy; Novo Nordisk: Consultancy. Nichol:Octapharma: Research Funding; CSL Behring: Research Funding; Baxalta US Inc., Bannockburn, IL (a Takeda Company): Research Funding; Genentech Inc.: Research Funding; Pfizer: Research Funding; Global Blood Therapeutics: Research Funding.

BACKGROUND There are limited longitudinal studies following up persons with hemophilia (PWH) in adherence to clotting factor treatment and health outcomes. The Hemophilia Utilization Group Studies part Va (HUGS Va) was a two-year observational study of persons with hemophilia A conducted from 2005-2007. Participants from HUGS Va were enrolled to the long-term follow-up study (HUGS LTS) in 2014. OBJECTIVES To compare participants' characteristics between baseline of HUGS Va and follow-up in LTS; and investigate the impacts of changes of participants' characteristics on annualized bleeding rates. METHODS We collected data on sociodemographic and clinical characteristics. Self-reported bleedings were obtained from periodic surveys for 2-years in HUGS Va, and a survey that asked bleedings in the past 6-month in HUGS LTS. Clotting factor dispensing records were collected prospectively for two years in HUGS Va, and retrospectively for six months prior to HUGS LTS enrollment. Annualized bleeding rates and factor dispensing (unit/kg body weight) were calculated. Adherence to factor treatment was determined by the ratio of dispensed clotting factor to clinical recommended factor usage. We classified age at baseline of HUGS Va into three groups: children (aged 2-11 years), adolescents (aged 12-20 years), and adults (aged ≥21 years). The characteristics of participants were compared among the three age groups using Chi-square tests for categorical variables and ANOVA for continuous variables for each study. Annualized factor dispensed and bleeding rates were compared between HUGS Va and HUGS LTS using paired T-tests. RESULTS A total of 74 persons participated in both HUGS Va and LTS with completed data to calculate annualized factor dispensed and bleeding rates were included to the analyses. The mean age was 17.8±11.4 years in HUGS Va, and 26.2±11.5 years in LTS, respectively. The sample had 43% of children, 22% adolescents, and 35% adults at baseline. All adolescents at baseline transitioned to young adults in LTS. Approximately 80% of participants were severe hemophilia. Adults (73%) were less likely to have an entire year of health insurance as compared to children (100%) or adolescents (93.8%, P0.05). The bleeding rates were not significantly different among age groups in LTS (P=0.06). CONCLUSIONS Although current literature indicated that PWH in transition from childhood to adulthood maybe at high risk of adverse health outcomes due to poor management of their condition with diminishing influence from parents, adolescents showed a significant increase in prophylactic treatment, adherence to factor treatment, and annualized factor dispensed, which may be associated with unchanged annualized bleed rates after they transitioned to adulthood in this study. Adults had a larger increase in dispensed factors than adolescents. Prophylaxis and adherence to clotting factor treatment were associated with a lower bleeding rate. Our current analyses reinforce the importance of prophylaxis and adherence to factor treatment for decreasing bleedings in persons with hemophilia A. Disclosures Nichol: Bayer: Research Funding; CSL Behring: Research Funding; Bioverativ: Research Funding; Shire/Baxter: Research Funding; Pfizer: Research Funding; Novo Nordisk: Research Funding; Genentech: Research Funding. Curtis:Gilead: Honoraria; Pfizer: Research Funding; Novo Nordisk: Honoraria, Research Funding; Shire/Baxter: Research Funding; Bioverativ: Research Funding; National Hemophilia Foundation: Honoraria; CSL Behring: Research Funding; Bayer: Research Funding, Speakers Bureau; Genentech: Honoraria, Research Funding. Ding:Novo Nordisk: Employment; Bioverativ: Research Funding. Aliyev:Genentech: Research Funding. Lou:Bioverativ: Research Funding; Novo Nordisk: Research Funding; Genentech: Research Funding; Bayer: Research Funding; Pfizer: Research Funding; CSL Behring: Research Funding; Shire/Baxter: Research Funding. Ullman:Genentech: Research Funding. Tran:Bioverativ: Honoraria; Novo Nordisk: Honoraria; Bayer: Honoraria; Genentech: Research Funding. Baker:Genentech: Research Funding. Riske:Genentech: Research Funding. Wu:Pfizer: Research Funding; Genentech: Research Funding; Bioverativ: Research Funding; CSL Behring: Research Funding; Bayer: Research Funding; Shire/Baxter: Research Funding; Novo Nordisk: Research Funding.

Thrombin-mediated proteolysis is central to hemostasis, directly controlling both platelet activation and fibrin deposition as well as positively- and negatively-regulating further thrombin generation through the activation of factors XI, VIII, V and protein C. Thrombin also appears to be important in multiple processes distinct from traditional hemostasis, but detailed genetics-based studies have been impeded by the uniform embryonic and perinatal failure of mice with constitutive prothrombin (fII) deficiency. To develop an experimental setting to explore the importance of fII in vascular biology, tissue repair, the inflammatory response and disease processes in adult animals, we have generated mice carrying a conditional prothrombin knockout allele (fIIlox). In the absence of Cre-mediated recombination, homozygous fIIlox/lox mice or compound heterozygous mice carrying one fIIlox allele and one constitutive-null allele were found to carry ~20% and ~10% the normal level of circulating prothrombin, respectively. These fIIlox/lox and fIIlox/− mice exhibited normal developmental and reproductive success, survived well into adulthood and young adults exhibited no appreciable spontaneous bleeding events or other pathologies. However, one-year-old fIIlox/− mice developed modest focal hemosiderin deposits and fibrosis within the heart, consistent with chronic low grade hemorrhage in this tissue. No gross or microscopic pathologies were observed in any other tissue examined in mice carrying an intact conditional knockout allele, regardless of age or gender. The induction of Cre recombinase in adult fIIlox mice using the poly I:C-inducible Mx1-Cre system resulted in the rapid and near-complete recombination of the fIIlox allele within the liver, loss of hepatic fII mRNA, elimination of detectable circulating prothrombin, and profound derangements in coagulation function. The life-expectancy in adults genetically-depleted of prothrombin was found to be very short (generally 5–7 days), and the loss of viability was associated with the development of severe hemorrhagic events within multiple tissues, most prominently in the heart (100% phenotypic penetrance) and brain (~50% phenotypic penetrance). Microscopic analysis of hearts from mice following deletion of the fII allele revealed widespread hemorrhage within the myocardium, particularly in the subepicardial region, focal ischemia and necrosis, neutrophil infiltrates and early granulation tissue. Gross evidence of blood was observed within the pleural cavity in nearly half of Cre-induced fIIlox/− mice at autopsy, and the lack of appreciable hemorrhage within lung tissue favored the heart as the source of this free blood. Less common hemorrhagic events were observed several other tissues, including skeletal muscle, intestines and testes. Examination of brains from mice lacking fII revealed widespread bleeding in the central nervous system, including dural-based hemorrhage and bleeding into both the brain parenchyma and ventricles. These results imply that a robust hemostatic system is essential to limit spontaneous bleeding events in tissues under repetitive mechanical or pulsatile stress. These findings also demonstrate that low levels of fII are compatible with long-term survival in adult mice, affording the ability to examine fII in a diverse spectrum of disease and physiologic processes. As evidence of this concept, mice lacking fII were challenged with S. aureus peritonitis prior to the onset of hemorrhage. Similar to mice lacking fibrinogen, these animals were found to have a profound deficit in their ability to control this infection, pointing to the need for polymerized fibrin for the effective clearance of this bacteria from the peritoneal cavity.