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In vivo gene therapy is rapidly emerging as a new therapeutic paradigm for monogenic disorders. For almost three decades, hemophilia A (HA) and hemophilia B (HB) have served as model disorders for the development of gene therapy. This effort is soon to bear fruit with completed pivotal adeno-associated viral (AAV) vector gene addition trials reporting encouraging results and regulatory approval widely anticipated in the near future for the current generation of HA and HB AAV vectors. Here we review the clinical development of AAV gene therapy for HA and HB and examine outstanding questions that have recently emerged from AAV clinical trials for hemophilia and other monogenic disorders. Expected final online publication date for the

Mechanisms thought to regulate activated factor VIII (FVIIIa) cofactor function include A2-domain dissociation and activated protein C (APC) cleavage. Unlike A2-domain dissociation, there is no known phenotype associated with altered APC cleavage of FVIII, and biochemical studies have suggested APC plays a marginal role in FVIIIa regulation. However, the in vivo contribution of FVIIIa inactivation by APC is unexplored. Here we compared wild-type B-domainless FVIII (FVIII-WT) recombinant protein with an APC-resistant FVIII variant (FVIII-R336Q/R562Q; FVIII-QQ). FVIII-QQ demonstrated expected APC resistance without other changes in procoagulant function or A2-domain dissociation. In plasma-based studies, FVIII-WT/FVIIIa-WT demonstrated dose-dependent sensitivity to APC with or without protein S, whereas FVIII-QQ/FVIIIa-QQ did not. Importantly, FVIII-QQ demonstrated approximately fivefold increased procoagulant function relative to FVIII-WT in the tail clip and ferric chloride injury models in hemophilia A (HA) mice. To minimize the contribution of FV inactivation by APC in vivo, a tail clip assay was performed in homozygous HA/FV Leiden (FVL) mice infused with FVIII-QQ or FVIII-WT in the presence or absence of monoclonal antibody 1609, an antibody that blocks murine PC/APC hemostatic function. FVIII-QQ again demonstrated enhanced hemostatic function in HA/FVL mice; however, FVIII-QQ and FVIII-WT performed analogously in the presence of the PC/APC inhibitory antibody, indicating the increased hemostatic effect of FVIII-QQ was APC specific. Our data demonstrate APC contributes to the in vivo regulation of FVIIIa, which has the potential to be exploited to develop novel HA therapeutics.

Background Limited information exists regarding the factor IX (FIX) coagulant activity (FIX:C) measured by different assays following FIX‐Padua gene therapy. Objective Assess for the first time FIX:C in five commonly used coagulation assays in plasma samples from hemophilia B subjects receiving FIX‐Padua gene transfer. Methods FIX:C was compared between central (n = 1) and local laboratories (n = 5) in the study, and across four commonly used FIX:C one‐stage assays and one FIX:C chromogenic assay. For comparison, samples of pooled congenital FIX‐deficient plasma spiked with purified recombinant human FIX (rHFIX)‐Padua protein or rHFIX (nonacog alfa) to obtain FIX:C concentrations from ~20% to ~40% were tested. Results FIX:C results at local laboratories strongly correlated with central laboratory results. However, absolute values at the central laboratory were consistently lower than those at local laboratories. Across five different FIX:C assays, a consistent pattern of FIX:C was observed for subjects receiving fidanacogene elaparvovec‐expressed gene transfer. Use of Actin FSL activated partial thromboplastin time (APTT) reagent in the central laboratory resulted in lower FIX:C values compared with other APTT reagents tested. The chromogenic assay determined lower FIX:C than any of the one‐stage assays. The rHFIX‐Padua protein–spiked samples showed similar results. In contrast, FIX:C results for rHFIX‐nonacog alfa measured within 25% of expected for all one‐stage assays and below 25% in the chromogenic assay. Conclusions Assay‐based differences in FIX:C were observed for fidanacogene elaparvovec transgene product and rHFIX‐Padua protein, suggesting the variable FIX:C determined with different assay reagents is inherent to the FIX‐Padua protein and is not specific to gene therapy–derived FIX‐Padua.

The high-specific-activity factor IX (FIX) variant Padua (R338L) is the most promising transgene for hemophilia B (HB) gene therapy. Although R338 is strongly conserved in mammalian evolution, amino acid substitutions at this position are underrepresented in HB databases. We therefore undertook a complete 20 amino acid scan and determined the specific activity of human (h) and canine (c) FIX variants with every amino acid substituted at position 338. Notably, we observe that hFIX-R338L is the most active variant and cFIX-R338L is sevenfold higher than wild-type (WT) cFIX. This is consistent with the previous identification of hFIX-R338L as a cause of a rare X-linked thrombophilia risk factor. Moreover, WT hFIX and cFIX are some of the least active variants. We confirmed the increased specific activity relative to FIX-WT in vivo of a new variant, cFIX-R338I, after gene therapy in an HB dog. Last, we screened 232 pediatric subjects with thromboembolic disease without identifying F9 R338 variants. Together these observations suggest a surprising evolutionary pressure to limit FIX activity with WT FIX rather than maximize FIX activity.

Decades of preclinical and clinical studies developing gene therapy for hemophilia are poised to bear fruit with current promising pivotal studies likely to lead to regulatory approval. However, this recent success should not obscure the multiple challenges that were overcome to reach this destination. Gene therapy for hemophilia A and B benefited from advancements in the general gene therapy field, such as the development of adeno-associated viral vectors, as well as disease-specific breakthroughs, like the identification of B-domain deleted factor VIII and hyperactive factor IX Padua. The gene therapy field has also benefited from hemophilia B clinical studies, which revealed for the first time critical safety concerns related to immune responses to the vector capsid not anticipated in preclinical models. Preclinical studies have also investigated gene transfer approaches for other rare inherited bleeding disorders, including factor VII deficiency, von Willebrand disease, and Glanzmann thrombasthenia. Here we review the successful gene therapy journey for hemophilia and pose some unanswered questions. We then discuss the current state of gene therapy for these other rare inherited bleeding disorders and how the lessons of hemophilia gene therapy may guide clinical development.

Adeno-associated virus (AAV) vector gene therapy is a promising treatment for a variety of genetic diseases, including hemophilia. Systemic administration of AAV vectors is associated with a cytotoxic immune response triggered against AAV capsid proteins, which if untreated can result in loss of transgene expression. Immunosuppression (IS) with corticosteroids has limited transgene loss in some AAV gene therapy clinical trials, but was insufficient to prevent loss in other studies. We used a nonhuman primate model to evaluate intensive T cell-directed IS combined with AAV-mediated transfer of the human factor IX (FIX) gene. Early administration of rabbit anti-thymocyte globulin (ATG) concomitant with AAV administration resulted in the development of anti-FIX antibodies, whereas delayed ATG by 5 weeks administration did not. The anti-FIX immune response was associated with increases in inflammatory cytokines, as well as a skewed Th17/regulatory T cell (Treg) ratio. We conclude that the timing of T cell-directed IS is critical in determining transgene-product immunogenicity or tolerance. These data have implications for systemically administered AAV gene therapy being evaluated for hemophilia A and B, as well as other genetic diseases., Graphical Abstract, In nonhuman primates, Samelson-Jones and Finn et al. observe that early intensive immunosuppression concomitant with AAV vector administration enhances the formation of transgene-product antibodies, whereas delaying immunosuppression by 5 weeks does not. This suggests there is a critical time period when intensive immunosuppression can shift the immune system toward immunogenicity and away from tolerance.

INTRODUCTION: The hemophilia community on Twitter is diverse, consisting of advocacy groups, patients, physicians, researchers, and other users. However, the scope of this community is uncharacterized, and limited data are available regarding effective participation in this community. AIM: To assess the types of users active in the hemophilia community on Twitter, as well as major themes present in hemophilia-related tweets. METHODS: 49,512 tweets between September 2019 and September 2021 were classified using regular expressions. A subset of the classified tweets was manually analyzed to identify prevalent discussion themes. RESULTS: Among the top 250 users by post count, the largest categories of users were support and advocacy groups, people with bleeding disorders, and healthcare providers. The largest thematic categories of tweets were gene therapy, contaminated hemophilia blood products, hemophilia research, clinical management of hemophilia, and COVID-19. While misinformation was rare, negative and incorrect perceptions of hemophilia were present among the general public. CONCLUSION: Our results demonstrate patterns of effective Twitter usage for patient care, research, and advocacy purposes among the hemophilia community.

Background The goal of gene therapy for patients with hemophilia A is to safely impart long-term stable factor VIII expression that predictably ameliorates bleeding with the use of the lowest possible vector dose. Methods In this phase 1-2 trial, we infused an investigational adeno-associated viral (AAV) vector (SPK-8011) for hepatocyte expression of factor VIII in 18 men with hemophilia A. Four dose cohorts were enrolled; the lowest-dose cohort received a dose of 5 × 1011 vector genomes (vg) per kilogram of body weight, and the highest-dose cohort received 2 × 1012 vg per kilogram. Some participants received glucocorticoids within 52 weeks after vector administration either to prevent or to treat a presumed AAV capsid immune response. Trial objectives included evaluation of the safety and preliminary efficacy of SPK-8011 and of the expression and durability of factor VIII. Results The median safety observation period was 36.6 months (range, 5.5 to 50.3). A total of 33 treatment-related adverse events occurred in 8 participants; 17 events were vector-related, including 1 serious adverse event, and 16 were glucocorticoid-related. Two participants lost all factor VIII expression because of an anti-AAV capsid cellular immune response that was not sensitive to immune suppression. In the remaining 16 participants, factor VIII expression was maintained; 12 of these participants were followed for more than 2 years, and a one-stage factor VIII assay showed no apparent decrease in factor VIII activity over time (mean [±SD] factor VIII activity, 12.9±6.9% of the normal value at 26 to 52 weeks when the participants were not receiving glucocorticoids vs. 12.0±7.1% of the normal value at >52 weeks after vector administration; 95% confidence interval [CI], -2.4 to 0.6 for the difference between matched pairs). The participants had a 91.5% reduction (95% CI, 88.8 to 94.1) in the annualized bleeding rate (median rate, 8.5 events per year [range, 0 to 43.0] before vector administration vs. 0.3 events per year [range, 0 to 6.5] after vector administration). Conclusions Sustained factor VIII expression in 16 of 18 participants who received SPK-8011 permitted discontinuation of prophylaxis and a reduction in bleeding episodes. No major safety concerns were reported. (Funded by Spark Therapeutics and the National Heart, Lung, and Blood Institute; ClinicalTrials.gov numbers, NCT03003533 and NCT03432520.).

Introduction Emicizumab is the first approved non-factor therapy for haemophilia A. It provides superior prophylactic bleeding control compared to other products in both patients with and patients without inhibitors. However, there is no real-world data about the monetary consequences of starting emicizumab. Aim To examine the estimated costs of starting emicizumab in a cohort of real-world haemophilia A patients with and without inhibitors. Methods The cost of haemostatic therapy for 6 months before and after initiating emicizumab for participants in a multicentre observational study was calculated based on the type of product and dosing that was used for prophylaxis and treating breakthrough bleeds, the number of treated bleeds and the participant weight. Results Ninety-two patients were included, 18 with an active inhibitor. The median age was 8.7 years. The median total cost for all patients decreased from $176,720 to $128,099 (p = .04) after initiating emicizumab, largely because of decrease in the total cost of high-cost outliers. The cost of prophylaxis and the total cost of bleeds also significantly decreased after starting emicizumab, both for patient with and patients without inhibitors. Conclusions Starting or switching to prophylaxis with emicizumab results in decreased costs for the treatment of patients with haemophilia A. This real-world data could inform on payer decisions as well as future cost-effective analysis.

Adeno-associated virus (AAV) vectors are a leading platform for gene-based therapies for both monogenic and complex acquired disorders. The success of AAV gene transfer highlights the need to answer outstanding clinical questions of safety, durability, and the nature of the human immune response to AAV vectors. Here, we present longitudinal follow-up data of subjects who participated in the first trial of a systemically delivered AAV vector. Adult males (n = 7) with severe hemophilia B received an AAV2 vector at doses ranging from 8 × 10(10) to 2 × 10(12) vg/kg to target hepatocyte-specific expression of coagulation factor IX; a subset (n = 4) was followed for 12–15 years post-vector administration. No major safety concerns were observed. There was no evidence of sustained hepatic toxicity or development of hepatocellular carcinoma as assessed by liver transaminase values, serum [Formula: see text]-fetoprotein, and liver ultrasound. Subjects demonstrated persistent, increased AAV neutralizing antibodies (NAbs) to the infused AAV serotype 2 (AAV2) as well as all other AAV serotypes tested (AAV5 and AAV8) for the duration of follow-up. These data represent the longest available longitudinal follow-up data of subjects who received intravascular AAV and support the preliminary safety of intravascular AAV administration at the doses tested in adults. Data demonstrate, for the first time, the persistence of high-titer, multi-serotype cross-reactive AAV NAbs for up to 15 years post- AAV vector administration. Our observations are broadly applicable to the development of AAV-mediated gene therapy.

Adeno-associated virus (AAV) vector liver-directed gene therapy (GT) for hemophilia B (HB) is limited by a vector dose–dependent hepatotoxicity. Recently, this obstacle has been partially circumvented by the use of a hyperactive factor IX (FIX) variant, R338L (Padua), which has an 8-fold increased specific activity compared with FIX-WT. FIX-R338L has emerged as the standard for HB GT. However, the underlying mechanism of its hyperactivity is undefined; as such, safety concerns of unregulated coagulation and the potential for thrombotic complications have not been fully addressed. To this end, we evaluated the enzymatic and clotting activity as well as the activation, inactivation, and cofactor dependence of FIX-R338L relative to FIX-WT. We observed that the high specific activity of FIX-R338L requires factor VIIIa (FVIIIa) cofactor. In a potentially novel system using emicizumab, an FVIII-mimicking bispecific antibody, the hyperactivity of both recombinant FIX-R338L and AAV-mediated transgene-expressed FIX-R338L from HB GT subjects was ablated without FVIIIa activity. We conclude that the molecular regulation of activation, inactivation, and cofactor dependence of FIX-R338L is similar to FIX-WT but that the FVIIIa-dependent hyperactivity of FIX-R338L is the result of a faster rate of factor X activation. This mechanism helps mitigate safety concerns of unregulated coagulation and supports the expanded use of FIX-R338L in HB therapy.

Alloimmune hemolytic disease of the fetus or newborn (HDFN) is a rare cause of neonatal cholestasis. HDFN-associated cholestasis has most often been reported secondary to anti-D alloimmunization. In utero transfusions are also an identified risk factor. A variety of diagnostic and therapeutic strategies have been described, mostly in case reports. Here, we report 2 cases of HDFN-associated cholestasis that were notable for extreme laboratory abnormalities including a peak ferritin of 24 700 ng/mL and a peak alanine aminotransferase of 1406 U/L (33.5-fold upper limit of normal). One case was due to alloimmunization other than anti-D. These cases help define the range of laboratory derangements that are consistent with HDFN-associated cholestasis, including extreme hyperferritinemia. Although in a number of cases, researchers have reported the use of iron chelation in these infants, herein, we describe successful management without iron chelation.

Introduction Abnormalities in the levels and functions of proteins that maintain hemostasis can cause thrombosis. Factor IX (FIX) R338L, i.e., Factor IX Padua, is a hyperactive clotting factor that promotes thrombosis. The R338L mutation increases the clotting rate by 8-fold despite increasing the Factor IXa enzymatic activity by only 2-fold. Protein S (PS) is a natural anticoagulant that directly inhibits FIXa. Because individuals affected by the R338L mutation have normal concentrations of PS, we speculated that the Padua hypercoagulation phenotype is due to decreased inhibition of FIXa R338L by PS. Methods We measured the ability of PS to inhibit FIX R338L, and we assessed the ability of PS to mitigate the prothrombotic effect FIX R338L. Results Plasma clotting assays demonstrated that 3-fold more PS was required to inhibit FIXa R338L compared with inhibition of wild type FIXa. Thrombin generation assays with Padua patient plasma recapitulated this biochemical consequence of the R338L mutation. Importantly, the less efficient inhibition of FIXa R338L was reversed by increasing PS concentration. Binding and co-immunoprecipitation studies revealed that the decrease in the inhibition of FIXa R338L by PS was caused by a 3- to 4-fold reduction in FIXa R338L affinity for PS. Conclusion In summary, the resistance of FIXa R338L to inhibition by PS likely contributes to the unexpectedly high clotting rate in Padua individuals. Moreover, PS-mediated reversal of the pathological properties of FIXa R338L suggests that PS administration may be a novel and effective means to mitigate thrombophilia caused by any source of elevated FIXa activity.

Key Points Skeletal muscle–directed expression of FIX-Padua resulted in complete correction of HB phenotype in an inhibitor-prone dog model. Long-term immune tolerance to FIX is sustained over years upon multiple challenges with recombinant FIX protein in 2 HB models.

The prevention of bleeding with adequately sustained levels of clotting factor, after a single therapeutic intervention and without the need for further medical intervention, represents an important goal in the treatment of hemophilia.We infused a single-stranded adeno-associated viral (AAV) vector consisting of a bioengineered capsid, liver-specific promoter and factor IX Padua (factor IX-R338L) transgene at a dose of 5×10No serious adverse events occurred during or after vector infusion. Vector-derived factor IX coagulant activity was sustained in all the participants, with a mean (±SD) steady-state factor IX coagulant activity of 33.7±18.5% (range, 14 to 81). On cumulative follow-up of 492 weeks among all the participants (range of follow-up in individual participants, 28 to 78 weeks), the annualized bleeding rate was significantly reduced (mean rate, 11.1 events per year [range, 0 to 48] before vector administration vs. 0.4 events per year [range, 0 to 4] after administration; P=0.02), as was factor use (mean dose, 2908 IU per kilogram [range, 0 to 8090] before vector administration vs. 49.3 IU per kilogram [range, 0 to 376] after administration; P=0.004). A total of 8 of 10 participants did not use factor, and 9 of 10 did not have bleeds after vector administration. An asymptomatic increase in liver-enzyme levels developed in 2 participants and resolved with short-term prednisone treatment. One participant, who had substantial, advanced arthropathy at baseline, administered factor for bleeding but overall used 91% less factor than before vector infusion.We found sustained therapeutic expression of factor IX coagulant activity after gene transfer in 10 participants with hemophilia who received the same vector dose. Transgene-derived factor IX coagulant activity enabled the termination of baseline prophylaxis and the near elimination of bleeding and factor use. (Funded by Spark Therapeutics and Pfizer; ClinicalTrials.gov number, NCT02484092 .).

New therapies for hemophilia A and hemophilia B will likely continue to change clinical practice. Ranging from extended half-life to nonfactor products and gene therapy, these innovative approaches have the potential to enhance the standard of care by decreasing infusion frequency to increase compliance, promoting prophylaxis, offering alternatives to inhibitor patients, and easing route of administration. Each category has intrinsic challenges that may limit the broader application of these promising therapies. To date, none specifically address the challenge of dispersing treatment to the developing world.

Processing by the proprotein convertase furin is believed to be critical for the biological activity of multiple proteins involved in hemostasis, including coagulation factor VIII (FVIII). This belief prompted the retention of the furin recognition motif (amino acids 1645-1648) in the design of B-domain-deleted FVIII (FVIII-BDD) products in current clinical use and in the drug development pipeline, as well as in experimental FVIII gene therapy strategies. Here, we report that processing by furin is in fact deleterious to FVIII-BDD secretion and procoagulant activity. Inhibition of furin increases the secretion and decreases the intracellular retention of FVIII-BDD protein in mammalian cells. Our new variant (FVIII-ΔF), in which this recognition motif is removed, efficiently circumvents furin. FVIII-ΔF demonstrates increased recombinant protein yields, enhanced clotting activity, and higher circulating FVIII levels after adeno-associated viral vector-based liver gene therapy in a murine model of severe hemophilia A (HA) compared with FVIII-BDD. Moreover, we observed an amelioration of the bleeding phenotype in severe HA dogs with sustained therapeutic FVIII levels after FVIII-ΔF gene therapy at a lower vector dose than previously employed in this model. The immunogenicity of FVIII-ΔF did not differ from that of FVIII-BDD as a protein or a gene therapeutic. Thus, contrary to previous suppositions, FVIII variants that can avoid furin processing are likely to have enhanced translational potential for HA therapy.

Hemophilia A (HA) is a X-linked bleeding disorder caused by deficiency of coagulation factor VIII (FVIII). Optimal clinical management centers on FVIII protein concentrate replacement. However, up to 30% of patients with severe HA develop neutralizing antibodies to FVIII (inhibitors) upon exposure to therapeutic FVIII. Inhibitors neutralize the infused FVIII and, thus, pose a significant challenge in the management of these patients. Immune tolerance induction (ITI) using high-dose FVIII infusions can eliminate inhibitors but is not effective at generating long-term eradication in all patients. Thus, we developed an immunotherapy to more effectively eliminate FVIII-specific B cells to induce lasting eradication of inhibitors. Adoptive T cell immunotherapies using chimeric antigen receptor (CAR)-modified T cells (CARTs) are showing encouraging results in the treatment of cancer. We previously reported a proof-of-concept CART approach to eliminate autoantibodies to desmoglein-3, implicated in pemphigus vulgaris, by targeting non-malignant B cells. Here, using a similar strategy, we constructed two FVIII-based CARs using isolated FVIII A2 and C2 domains, which are commonly targeted by inhibitors Both FVIII fragments were used as the CAR extracellular domain followed by CD8a-derived hinge and transmembrane domains and intracellular signaling domains derived from 4-1BB and CD3z (BBz), as has been used in a recently FDA-approved CD19-targeted CAR (CART19) that have shown efficacy in patients. A2- and C2-CARs were introduced into primary human T cells using lentivirus vectors and were found to express on the cell surface. Primary expansion following anti-CD3/CD28 activation and lentivirus transduction yielded comparable population doublings of CART19 and A2-CARTs over 9-11 days (mean, 4.3 vs. 3.7; p=0.16). However, C2-CARTs consistently achieved fewer doublings compared to CART19 (mean 4.3 vs. 2.6; p=0.01). Additionally, C2-CARTs maintained larger cell volumes following initial activation compared to A2 and CART19, reminiscent of other CARs that demonstrate a high-level of ligand-independent basal activation. Consistent with hyper-activation and activation-induced cell death, flow cytometric analysis of C2-CART cultures showed prolonged expression of CD69, increased levels of cell death, and gradual loss of CAR+ cells compared to A2 and CART19 cultures. The FVIII C2 domain contains hydrophobic surfaces involved in binding to phospholipid membranes and von Willebrand Factor. We hypothesized that these hydrophobic regions may cause unfavorable interactions that result in CAR clustering and, in-turn, ligand-independent signaling. However, since these regions are also targeted by FVIII inhibitors, mutation of hydrophobic residues to improve CAR function would likely result in loss of binding to intended targets. Thus, we tested two independent strategies to improve function of C2-CARTs, both of which maintain native FVIII domain sequences. First, to block unfavorable interactions with C2 hydrophobic surfaces, we tested whether addition of C2 domain specific antibodies to the CART culture would improve expansion. Indeed, addition of anti-C2 antibodies improved C2-CART expansion by approximately one population doubling and reduced both cell volume and CD69 expression in C2-CARTs. As an alternate and perhaps more easily translatable approach, we replaced the BBz CAR signaling domain with a killer immunoglobulin receptor (KIR)-based multi-chain signaling system, which we previously utilized to improve function of a mesothelin-targeted CAR. KIR-based signaling effectively "rescued" the C2 CAR, and showed an expansion and activation profile comparable to CART19. Lastly, KIR-based FVIII CARs, demonstrated specific lysis of and cytokine response to target cells engineered to express surface anti-FVIII, even in the presence of soluble inhibitors. These data indicate that unfavorable interactions may be ameliorated directly by blocking agents or, indirectly, by altering intracellular signal transduction domains. These strategies have allowed us to construct optimized FVIII-based CARTs to target FVIII-specific B cells that are responsible for producing FVIII inhibitors in patients with HA. Disclosures Doshi: Bayer Hemophilia Awards Program: Research Funding. Milone:Novartis: Patents & Royalties.

Background PF-06838435 (SPK-9001), a gene therapy candidate containing a high specific-activity factor IX variant (R338L, FIX-Padua), is currently in phase 3 of clinical development for the treatment of Hemophilia B. Initial data with this vector is promising with significant reductions in bleeding episodes and FIX consumption (George LA et al, NEJM 2017; 377:2215-2217). To date, little is known about the activity of the expressed transgene product as measured in FIX:C one-stage and chromogenic assay systems commonly used to monitor FIX replacement therapy in patients with Hemophilia B. Aim The goal of the study was to assess the activity of the PF-06838435 expressed transgene product in plasma samples collected from participants in the Phase 1/2 trial using four commonly used FIX:C aPTT reagents. For comparison, the activity of the expressed FIX-Padua gene product was also assessed in the ROX FACTOR IX chromogenic assay. In addition, the activity of the Padua variant in congenital FIX deficient plasma spiked with increasing concentrations of purified recombinant human FIX Padua protein (rHFIXp - Samelson-Jones Lab, CHOP/UPenn), as well as recombinant human FIX (rHFIX, BeneFIX®, Pfizer Inc.), was assessed in the same FIX:C assay procedures. Methods FIX:C, in four samples collected from two different patients who received FIX gene therapy, was tested in four in vitro diagnostic (IVD) approved FIX:C one-stage assay systems, STA®-PTT Automate and STA®-C.K. Prest® on the STA-R Evolution® (Diagnostica Stago Inc.), Dade Actin® FSL on the BCS®XP (Siemens Healthcare), and HemosIL® SynthASil® on the ACL TOP® (Instrumentation Laboratory). In addition, samples were also tested in the ROX FACTOR IX (Rossix AB) chromogenic assay on the BCS®XP (Siemens Healthcare). The aPTT reagents selected for this study correspond to 90% of the FIX:C assay reagents currently used in CAP accredited laboratories in the US(2014 CAP Proficiency Survey) and represent the three main types of activator commonly used in the FIX one-stage clot assay: silica (STA®-PTT Automate, HemosIL® SynthASil®), ellagic acid (Dade Actin® FSL) and kaolin (STA®-C.K. Prest®). For comparison, FIX:C in each of the five FIX:C assay systems was also determined in samples spiked with purified rHFIXp (provided by Dr. Samelson-Jones) or rHFIX (provided by Spark Therapeutics Inc.) in 20X buffer solutions. On the day of testing rHFIXp, and rHFIX 20X buffer solutions were diluted 1:20 in congenital FIX deficient plasma to achieve approximate final FIX:C concentrations of 40, 30 and 20%, extrapolated from an estimated 8-fold specific activity to antigen ratio. Results A consistent pattern in the measured FIX:C for the PF-06838435 transgene product was observed in the five FIX:C assay systems (Fig. 1). For the aPTT based FIX:C assays, Actin FSL, gave the lowest FIX:C values, whereas PTT Automate measured the highest FIX:C levels. In all cases, the chromogenic FIX:C assay gave the lowest activity values for the transgene product. A similar FIX:C assay dependent pattern was observed for the purified rHFIXp spiked at 40, 30 and 20% FIX:C (Fig. 2). In all FIX:C assays tested, rHFIXp was under-recovered to a varying degree from target, with recoveries for the 20% FIX:C samples ranging between -26.5% (STA®-PTT Automate) and -73.5% (Dade Actin® FSL) in the aPTT based FIX:C assays and -85% in the ROX FIX chromogenic assay. In contrast, rHFIX (BeneFIX®) was recovered within ±25% of expected values in all aPTT based FIX:C assays and, consistent with previously reported data, modestly under-recovered in the ROX FIX chromogenic assay (Fig. 3). Conclusion This study found differences in the FIX:C results obtained for a Padua FIX variant transgene product and recombinant human FIX-Padua when tested in commonly used IVD approved FIX:C assays in North America. These results suggest that FIX:C assay selection is important for measuring FIX-Padua activity, which will be particularly relevant in hemophilia B gene therapy following FIX-Padua gene transfer. Disclosures George: University of Pennsylvania: Equity Ownership; Pfizer: Consultancy. High:Spark Therapeutics: Employment, Equity Ownership, Patents & Royalties. Carr:Sparks Therapeutics Inc.: Consultancy. Tiefenbacher:Laboratory Corporation of America: Employment, Equity Ownership; Siemens Healthcare: Consultancy.

Factor IX (FIX) Padua (R338L) has been described as a game changer for hemophilia B (HB) gene therapy. The ~8-fold increased specific activity compared to wild-type FIX (FIX WT) in aPTT-based clotting assay has recently allowed for a lowering of adeno-associated virus (AAV) vector dose compared to earlier gene therapy trials using FIX WT, while still achieving sustained near-curative FIX activity levels. The lowered AAV vector dose mitigated the vector dose-dependent hepatotoxicity, which had remained a major safety and efficacy limitation for AAV gene therapy. To date, at least 2 pivotal phase III gene therapy trials for HB using AAV FIX Padua are planned. Despite this enthusiasm, the underlying molecular mechanism of the hyperactivity of FIX Padua is undefined. As such, the safety concerns of unregulated FIX activity and the potential for ensuing thrombotic complications have not been fully addressed. Indeed, recent thrombotic complications in non-gene therapy hemophilia clinical trials evaluating non-factor therapies that promote hemostasis by circumventing regulatory interactions should engender caution. Activated FVIII (FVIIIa) is essential for the biological activity of activated FIX (FIXa), which provides an important regulatory requirement. To determine the role of FVIIIa in the hyperactivity of FIX Padua, we measured the relative specific activity of FIX Padua compared to WT in HB plasma using a unique reagent, ACE910. ACE910 is a bispecific antibody that sufficiently brings together FIXa and its substrate, FX, to efficiently promote hemostasis, even in the absence of FVIIIa molecules. It has been described as a "FVIII-mimetic" and is currently approved as a bypassing agent for hemophilia A patients with FVIII inhibitors. In our assays, plasma FVIII activity is inhibited to We also measured the ability of recombinant FIX(a) Padua and WT to restore thrombin production to HB plasma in a thrombin generation assay (TGA). We observe that the EC50 of thrombin generation in HB plasma of zymogen and activated FIX Padua is ~8-fold lower than for FIX(a) WT. However, when FVIIIa is replaced with ACE910, FIX(a) Padua and WT demonstrate comparable EC50s of thrombin generation. Importantly, we observe no difference in the binding of FIXa Padua and FIXa WT with ACE910 in enzyme kinetic studies; this was expected because FIXa Padua and WT have an identical ACE910 binding site in their EGF-domain. Combined, these results demonstrate that the ~8 fold hyperactivity of FIX(a) Padua compared to FIX(a) WT in plasma assays requires FVIIIa cofactor activity. Consistent with this mechanism, we have previously observed in reconstituted systems that FIX(a) Padua and WT have comparable rates of activation by FXIa and inactivation by antithrombin. This result suggests that FIX Padua and WT are regulated similarly, supporting the safety of FIX Padua as a transgene. Enzyme kinetic studies of FIXa Padua and WT without FVIIIa demonstrate similar rates of FX activation, while FIXa Padua demonstrates increased rates of FX activation with FVIIIa compared to FIXa WT. We conclude, therefore, the mechanism of the increased specific activity of FIX(a) Padua is wholly dependent on its enhanced interaction with its cofactor FVIIIa. This enhancement is essential for the increased activity observed in both reconstituted and plasma-based assays. Moreover, the augmented interaction of FIX(a) Padua with FVIIIa is sufficient to account for the ~8-fold pro-hemostatic enhancement compared to FIX(a) WT observed in plasma assays. Combined, these results definitively address previously unanswered safety concerns regarding the potential risk of thrombotic complications with the use of FIX Padua transgene. They strongly support the ongoing use of FIX Padua in HB gene therapy. Disclosures Camire: Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer: Consultancy; Spark Therapeutics: Membership on an entity's Board of Directors or advisory committees.

Several new therapies for hemophilia have emerged in recent years. These strategies range from extended half-life factor replacement products and non-factor options with improved pharmacokinetic profiles to gene therapy aiming for phenotypic cure. While these products have the potential to change hemophilia care dramatically, several challenges and questions remain regarding broader applicability, long-term safety, and which option to pursue for each patient. Here, we review these emerging therapies with a focus on controversies and unanswered questions in each category.

MYH9-related disease is a rare cause of thrombocytopenia. We report an infant girl who presented with severe thrombocytopenia at birth and was initially diagnosed with and treated for neonatal alloimmune thrombocytopenia. However, persistent thrombocytopenia led to the suspicion of congenital thrombocytopenia and subsequent identification of a novel variant in MYH9 (E1421K). In silico analysis strongly predicts that this is a disruptive substitution. Immunofluorescent analysis of neutrophils demonstrates abnormal aggregates of MYH9 protein. This case also suggests that a very high immature platelet fraction (≥40%) may be useful for rapidly differentiating MYH9-related disease from other causes of neonatal thrombocytopenia.