Tailoring treatment of haemophilia B: accounting for the distribution and clearance of standard and extended half-life FIX concentrates

Recombinant factor IX Fc for the treatment of hemophilia B

Current hemophilia B treatment guidelines recommend routine prophylaxis with factor IX (FIX) replacement products, tailored to maintain plasma activity at levels that will prevent bleeds. However, plasma FIX activity may not be the primary determinant or best indicator of hemostatic efficacy due to its extravascular distribution. FIX replacement therapy has evolved to include extended half-life (EHL) products that provide effective bleed protection when administered at intervals of 7 days or longer. rFIXFc is a recombinant fusion protein with an extended circulation time. rFIXFc has a biodistribution profile consistent with distribution into extravascular space, where it may support hemostasis at sites of vessel injury independent of circulating plasma activity levels. The safety and efficacy of rFIXFc prophylaxis is well established in adults, adolescents and children including previously untreated patients with hemophilia B, with substantial evidence from clinical trials and real-world clinical practice. This review describes the pharmacokinetic characteristics of rFIXFc, summarizes available safety and efficacy data, and evaluates the use of rFIXFc in special populations. Current hemophilia B treatment challenges, including target FIX plasma levels, perioperative use, and management of patients with comorbidities, are discussed together with the potential role of EHL products in the future treatment landscape of hemophilia B.

The road to implementation of pharmacokinetic-guided dosing of factor replacement therapy in hemophilia and allied bleeding disorders. Identifying knowledge gaps by mapping barriers and facilitators

Clinical guidelines and expert groups recommend the use of pharmacokinetic (PK)-guided dosing of factor replacement therapy for the treatment of bleeding disorders, especially for patients with hemophilia. Although PK-guided dosing is increasingly applied, it is generally not considered standard clinical practice. The aim of this scoping review is to map barriers and facilitators for the implementation of PK-guided dosing in clinical practice and to identify knowledge gaps. A literature search was performed and 110 articles were included that describe PK-guided dosing in patients with bleeding disorders, mostly hemophilia A. We defined two overarching themes, efficacy and feasibility, and discuss five topics within each theme. For each topic, barriers, facilitators and knowledge gaps were described. Although consensus was found with regard to some topics, contradicting reports were found for others, especially with respect to the efficacy of PK-guided dosing. These contradictions highlight the need for future research to elucidate current ambiguities.

Efficacy of rFIXFc versus N9-GP Prophylaxis in Patients with Hemophilia B: Matching-Adjusted Indirect Comparison of B-LONG and PARADIGM 2 Trials

Purpose

For patients with hemophilia B, extended half-life factor IX (FIX) products are available for prophylaxis and for treating bleeds. Different methods are used to extend the half-lives of recombinant FIX Fc fusion protein (rFIXFc) and nonacog beta pegol (N9-GP). This affects their biodistribution and plasma FIX levels, although differences do not always correlate with clinical outcomes. A matching-adjusted indirect comparison (MAIC) of prophylaxis with rFIXFc and N9-GP was performed, based on licensed dosing in the European Union.

Patients and Methods

Combined rFIXFc data from the weekly and individualized interval prophylaxis arms of the B-LONG clinical trial, and N9-GP data from the 40 IU/kg once-weekly prophylaxis arm of PARADIGM 2 were used in a MAIC. Individual patient data for rFIXFc (n=87) were matched to aggregated data for N9-GP (n=29). Estimated annualized bleeding rates (ABRs) for rFIXFc were recalculated using a Poisson regression model with adjustment for over-dispersion, and compared with ABRs reported for N9-GP, using incidence rate ratios (IRRs) with 95% confidence interval (CI).

Results

There was no evidence of significant differences in estimated ABRs between prophylaxis with rFIXFc and N9-GP. Analysis of pooled rFIXFc weekly and interval-adjusted dosing compared with N9-GP 40 IU/kg once weekly produced estimated ABRs of 2.59 versus 2.51 (IRR 1.03; 95% CI 0.56-1.89), as well as 1.34 versus 1.22 (IRR 1.10; 95% CI 0.42-2.91) and 1.13 versus 1.29 (IRR 0.88; 95% CI 0.47-1.63) for overall, spontaneous, and traumatic bleeding events, respectively.

Conclusion

The study did not reveal any significant differences in the efficacy of rFIXFc and N9-GP prophylaxis. Given differences in trough levels (rFIXFc dosing was targeted to achieve a trough 1-3 IU/dL above baseline versus a reported estimated N9-GP mean trough of 27.3 IU/dL), interpreting plasma FIX levels as potential surrogate efficacy markers requires consideration of compound-specific pharmacokinetic profiles.

Primary prophylaxis in children with severe haemophilia A and B-Implementation over the last 20 years as illustrated in real-world data in the PedNet cohorts

INTRODUCTION

The prophylactic regimen in children with severe haemophilia is suggested in various publications and guidelines. Few data exist on its implementation in clinical practice.

AIM

To investigate the implementation of primary prophylaxis based on real-life data from PedNet during the last 20 years.

METHODS

All children from the PedNet cohort (n = 1260) with severe haemophilia A (SHA) or severe haemophilia B (SHB), FVIII/IX < .01 IU/mL, born between 2000 and 2009 (Cohort I; SHA n = 662; SHB n = 88) and 2010-2019 (Cohort II; SHA n = 598; SHB n = 94) were included.

RESULTS

In SHA, the median age at start of prophylaxis was 17.3 months (IQR; 12.5-26.1) in Cohort I which decreased to 13.1 months (IQR; 10.4-19.1) in Cohort II (p < .000). "Once-a-week" prophylaxis at start increased from 49% to 68% (SHA) and 38% to 70% (SHB). FVIII doses were reduced from median 43.5 (IQR; 34.6-49.0) to 30.9 IU/kg (IQR; 26.3-46.3), while dosing with FIX did not change. After 2010 approximately 60% of the patients with SHA and SHB started prophylaxis before any joint bleed. The number of CVADs needed in both cohorts was around 30%. Incidences of inhibitors were unchanged: SHA (∼31%) and SHB (∼10%). Sporadic cases were diagnosed significantly later (median 8.3 months; IQR; 3.7-11.9) and they had more joint bleeds before start of prophylaxis.

CONCLUSION

Primary prophylaxis nowadays starts at an earlier age: before any joint bleed (60% of patients with SHA and SHB). Approximately 70% started on a once-weekly schedule with significantly reduced doses in SHA but unchanged in SHB.

Revised terminal half-life of nonacog alfa as derived from extended sampling data: A real-world study involving 64 haemophilia B patients on nonacog alfa regular prophylaxis

BACKGROUND

Nonacog alfa, a standard half-life recombinant factor IX (FIX), is used as a prophylactic treatment in severe haemophilia B (SHB) patients. Its half-life determined in clinical studies involving a limited sampling (72 h) was shown to be rather short. In our clinical practice, we suspected that its half-life could have been underestimated.

OBJECTIVES

We aimed to evaluate nonacog alfa pharmacokinetics in real world clinical practice based on FIX levels in patients receiving prophylaxis.

METHODS

We retrospectively collected data on patients with SHB receiving prophylaxis from eight centres across France. The terminal half-life (THL), time to reach 5-2 IU/dl and FIX activity at 48, 72 and 96 h were derived by Bayesian estimations using NONMEM analysis.

RESULTS AND CONCLUSIONS

Infusion data (n = 455) were collected from 64 patients with SHB. The median THL measured in 92 pharmacokinetic (PK) studies was 43.4 h. In 26 patients ≤12 years of age, 51 PK studies showed a median time to reach 5 IU/dl of FIX of 70.5 h and a median time to reach 2 IU/dl of 121.5 h. In 38 patients 13-75 years of age, 41 PK studies showed a median time to reach 5 IU/dl of FIX of 92.0 h and a median time to reach 2 IU/dl of 167.5 h. Extending the sampling beyond 72 h makes it possible to observe a plateau, with FIX remaining between 2 and 5 IU/dl for several days and shows that the THL of nonacog alfa might be longer than previously described.

ESSENTIALS

Nonacog alfa terminal half-life (THL) in patients receiving regular prophylaxis was evaluated in clinical practice. The median THL was estimated to be 36.9 h for patients aged .8-12 years. The median THL was estimated to be 49.9 h for patients aged 13-75 years. For patients aged ≤12 and >12 years, the median times to reach 5 IU/dl were 70.5 and 92 h, respectively; to reach 3 IU/dl, 95.5 and 131.5 h, respectively; to reach 2 IU/dl, 121.5 and 167.5 h, respectively. We suggest that the half-life of nonacog alfa might be longer than previously described in both younger and older patients.

International consensus recommendations on the management of people with haemophilia B

Haemophilia B is a rare X-linked genetic deficiency of coagulation factor IX (FIX) that, if untreated, can cause recurrent and disabling bleeding, potentially leading to severe arthropathy and/or life-threatening haemorrhage. Recent decades have brought significant improvements in haemophilia B management, including the advent of recombinant FIX and extended half-life FIX. This therapeutic landscape continues to evolve with several non-factor replacement therapies and gene therapies under investigation. Given the rarity of haemophilia B, the evidence base and clinical experience on which to establish clinical guidelines are relatively sparse and are further challenged by features that are distinct from haemophilia A, precluding extrapolation of existing haemophilia A guidelines. Due to the paucity of formal haemophilia B-specific clinical guidance, an international Author Group was convened to develop a clinical practice framework. The group comprised 15 haematology specialists from Europe, Australia, Japan, Latin America and North America, covering adult and paediatric haematology, laboratory medicine and biomedical science. A hybrid approach combining a systematic review of haemophilia B literature with discussion of clinical experience utilized a modified Delphi format to develop a comprehensive set of clinical recommendations. This approach resulted in 29 recommendations for the clinical management of haemophilia B across five topics, including product treatment choice, therapeutic agent laboratory monitoring, pharmacokinetics considerations, inhibitor management and preparing for gene therapy. It is anticipated that this clinical practice framework will complement existing guidelines in the management of people with haemophilia B in routine clinical practice and could be adapted and applied across different regions and countries.

F9 missense mutations impairing factor IX activation are associated with pleiotropic plasma phenotypes

BACKGROUND

Circulating dysfunctional factor IX (FIX) might modulate distribution of infused FIX in hemophilia B (HB) patients. Recurrent substitutions at FIX activation sites (R191-R226, >300 patients) are associated with variable FIX activity and antigen (FIXag) levels.

OBJECTIVES

To investigate the (1) expression of a complete panel of missense mutations at FIX activation sites and (2) contribution of F9 genotypes on the FIX pharmacokinetics (PK).

METHODS

We checked FIX activity and antigen and activity assays in plasma and after recombinant expression of FIX variants and performed an analysis of infused FIX PK parameters in patients (n = 30), mostly enrolled in the F9 Genotype and PK HB Italian Study (GePKHIS; EudraCT ID2017-003902-42).

RESULTS

The variable FIXag amounts and good relation between biosynthesis and activity of multiple R191 variants results in graded moderate-to-mild severity of the R191C>L>P>H substitutions. Recombinant expression may predict the absence in the HB mutation database of the benign R191Q/W/K and R226K substitutions. Equivalent changes at R191/R226 produced higher FIXag levels for R226Q/W/P substitutions, as also observed in p.R226W female carrier plasma. Pharmacokinetics analysis in patients suggested that infused FIX Alpha distribution and Beta elimination phases positively correlated with endogenous FIXag levels. Mean residence time was particularly prolonged (79.4 h, 95% confidence interval 44.3-114.5) in patients (n = 7) with the R191/R226 substitutions, which in regression analysis were independent predictors (β coefficient 0.699, P = .004) of Beta half-life, potentially prolonged by the increasing over time ratio between endogenous and infused FIX.

CONCLUSIONS

FIX activity and antigen levels and specific features of the dysfunctional R191/R226 variants may exert pleiotropic effects both on HB patients' phenotypes and substitutive treatment.

How do we optimally utilize factor concentrates in persons with hemophilia?

The current mainstay of therapy for hemophilia is to replace the deficient clotting factor with the intravenous administration of exogenous clotting factor concentrates. Prophylaxis factor replacement therapy is now considered the standard of care in both pediatric and adult patients with hemophilia with a severe phenotype to protect musculoskeletal health and improve quality of life. Heterogeneity in bleeding presentation among patients with hemophilia due to genetic, environmental, and treatment-related factors has been well described. Accordingly, the World Federation of Hemophilia recommends an individualized prophylaxis regimen that considers the factors mentioned above to meet the clinical needs of the patient, which can vary over time. This review focuses on the practical points of choosing the type of factor concentrate, dose, and interval while evaluating appropriate target trough factor levels and bleeding triggers such as level of physical activity and joint status. We also discuss the use of a pharmacokinetics assessment and its incorporation in the clinic for a tailored approach toward individualized management. Overall, adopting an individualized prophylaxis regimen leads to an optimal utilization of factor concentrates with maximum efficacy and minimum waste.

The Clinical Genetics of Hemophilia B (Factor IX Deficiency)

Hemophilia B (HB) is a bleeding disorder caused by deficiency of or defect in blood coagulation factor IX (FIX) inherited in an X-linked manner. It results from one of over 1000 known pathogenic variants in the FIX gene, F9; missense and frameshift changes predominate. Although primarily males are affected with HB, heterozygous females may have excessive bleeding due to random or non-random X chromosome inactivation; in addition, homozygous, compound heterozygous, and hemizygous females have been reported. Somatic and germinal mosaicism for F9 variants has been observed. Development of antibodies to FIX treatment products (inhibitors) is rare and related to the type of causative variant present. Treatment is with products produced by recombinant DNA technology, and gene therapy is in clinical trials. Genetic counseling with up-to-date information is warranted for heterozygotes, potential heterozygotes, and men and women affected with HB.

Hemophilia B (Factor IX Deficiency)

The biology of factor IX deficiency leading to hemophilia B has important distinctions from factor VIII deficiency that leads to hemophilia A. In this article, the authors explore the unique biology of factor IX in hemostasis, including the importance of FIX distribution to the extravascular space and the implications on dosing of factor concentrates. The authors review basic treatment principles of hemophilia B, including extended half-life products, and highlight areas of ongoing therapeutic innovation for hemophilia B prophylaxis.

Efficacy of rFIXFc versus rIX-FP for the Treatment of Patients with Hemophilia B: Matching-Adjusted Indirect Comparison of B-LONG and PROLONG-9FP Trials

Purpose

In patients with hemophilia B, treatment with extended half-life (EHL) recombinant factor IX allows for longer dosing intervals while providing equal or superior bleeding protection compared with standard half-life products. This enables flexible, individualized treatment schedules, which reduce the burden of prophylaxis and improve patient outcomes. This analysis compared the efficacy of recombinant factor IX Fc fusion protein (rFIXFc) and recombinant factor IX albumin fusion protein (rIX-FP), two EHL therapies approved for prophylaxis and treatment of bleeding in hemophilia B.

Patients and Methods

Matching-adjusted indirect treatment comparison (MAIC) was used to adjust the between-treatment differences in baseline characteristics. Individual patient data for rFIXFc (B-LONG) were matched to aggregated data for rIX-FP (PROLONG-9FP) followed by statistical comparison for estimated annualized bleeding rate (ABR) using a Poisson regression model with adjustment for over dispersion. Data were analyzed according to treatment regimen prior to study entry: prior prophylaxis (rFIXFc, n=48; rIX-FP, n=40) or prior episodic treatment (n=43 and n=19, respectively). Relative treatment effects are presented as incidence rate ratios (IRR) with 95% confidence intervals (CI).

Results

After adjustment for baseline characteristics, estimated ABR observed for rFIXFc and rIX-FP was not significantly different in patients on prior prophylaxis (1.87 versus 1.58; IRR 1.18, 95% CI 0.67–2.10) or prior episodic (2.25 versus 2.22; IRR 1.01 95% CI 0.40–2.57) regimens.

Conclusion

This MAIC analysis shows that the estimated ABR for rFIXFc-treated patients from B-LONG was similar to that of rIX-FP-treated patients from PROLONG-9FP and, therefore, indicates that the two EHL therapies provide similar efficacy when used as prophylaxis for patients with hemophilia B. Trough levels differ between the two products (1–3% [targeted] versus 20% [observed], respectively), suggesting that trough level is not a surrogate indicator when ABR is used as a criterion for clinical efficacy when comparing these FIX products in hemophilia B.

Once-weekly prophylaxis regimen of nonacog alfa in patients with hemophilia B: an analysis of timing of bleeding event onset

In a pivotal, multicenter, open-label study, 25 patients aged 12-54 years with moderately severe/severe hemophilia B received on-demand nonacog alfa (6 months; dose at investigator's discretion) followed by once-weekly prophylaxis with nonacog alfa 100 IU/kg (12 months). During prophylaxis, patients had a median spontaneous annualized bleeding rate (sABR) of 1.0 and significant reductions in ABR (P < 0.0001). This post hoc analysis examined the time of onset of spontaneous bleeding events (sBEs) and spontaneous target joint bleeding events (sTJBEs). The postdosing day (D) of onset of sBEs observed during prophylaxis and steady-state FIX activity data (FIX:C) between 144 and 196 h postdose were collected at weeks 26 and 78. Twelve patients (48%) had no sBEs; the remaining 13 (52%) had the following onset of sBEs: less than 1 D (0%), 1 to less than 2D (5%), 2 to less than 3 D (22%), 3 to less than 4 D (9%), 4 to less than 5D (22%), 5 to less than 6D (23%), 6 to less than 7D (11%), and at least 7D (8%). Reductions in sBEs and sTJBEs during on-demand versus prophylaxis treatment were experienced by all 13 patients. Target joint sABR during prophylaxis was 0 for 5/13 patients. ABR reduction ranged from 66.1% (27.2→9.2) to 97.8% (46.2→1.0); sTJBE reductions ranged from 6.2% (2.1→2.0) to 100% (from 40.1, 19.1, 3.9, 9.0, 6.1--0). During prophylaxis, 47% (8/17) of trough FIX activity samples were more than 2%. In sBE patients, ABR and number of TJBEs were reduced with once-weekly nonacog alfa. When sBEs occurred, they followed no apparent pattern for day of occurrence. Clinicaltrials.gov identifier: NCT01335061.

Heterogeneity in Bleeding Tendency and Arthropathy Development in Individuals with Hemophilia

People with hemophilia (PWH) have an increased tendency to bleed, often into their joints, causing debilitating joint disease if left untreated. To reduce the incidence of bleeding events, PWH receive prophylactic replacement therapy with recombinant factor VIII (FVIII) or FIX. Bleeding events in PWH are typically proportional to their plasma FVIII or IX levels; however, in many PWH, bleeding tendency and the likelihood of developing arthropathy often varies independently of endogenous factor levels. Consequently, many PWH suffer repeated bleeding events before correct dosing of replacement factor can be established. Diagnostic approaches to define an individual's bleeding tendency remain limited. Multiple modulators of bleeding phenotype in PWH have been proposed, including the type of disease-causing variant, age of onset of bleeding episodes, plasma modifiers of blood coagulation or clot fibrinolysis pathway activity, interindividual differences in platelet reactivity, and endothelial anticoagulant activity. In this review, we summarize current knowledge of established factors modulating bleeding tendency and discuss emerging concepts of additional biological elements that may contribute to variable bleeding tendency in PWH. Finally, we consider how variance in responses to new gene therapies may also necessitate consideration of patient-specific tailoring of treatment. Cumulatively, these studies highlight the need to reconsider the current "one size fits all" approach to treatment regimens for PWH and consider therapies guided by the bleeding phenotype of each individual PWH at the onset of therapy. Further characterization of the biological bases of bleeding heterogeneity in PWH, combined with the development of novel diagnostic assays to identify those factors that modulate bleeding risk in PWH, will be required to meet these aspirations.

Pharmacokinetics in routine haemophilia clinical practice: rationale and modalities-a practical review

Prophylactic therapy with exogenous clotting factor concentrates in haemophilia A and B aims to achieve levels of circulating FVIII or FIX that are adequate for the prevention or reduction of spontaneous joint bleeding. Historically, a minimum trough level of at least 1% of the normal levels of circulating clotting factor has been targeted using standardised protocols. However, clearance of clotting factor varies between products and patients, and other pharmacokinetic (PK) parameters such as the frequency and magnitude of peaks may be important for ensuring optimal coverage. Thus, it is increasingly recognised that an individualised, PK-based approach to prophylaxis is necessary to achieve optimal protection.

This review focuses on the clinical implications of using PK-guided, individualised prophylaxis in haemophilia to improve patient outcomes and considers practical methods of establishing patients’ PK parameters. The most useful PK parameters will depend on the aim of the specific treatment (e.g. preventing activity-related and traumatic bleeds or addressing subclinical bleeding). In clinical practice, lengthy and frequent post-infusion sampling for PK analysis is costly and a significant burden for patients. However, a Bayesian analysis allows for the estimation of different PK parameters (e.g. half-life, factor concentrations over time, etc.) with only a minimum number of samples (e.g. 4, 24 and 48 h for haemophilia A), by using the patient’s data to adjust a relevant population PK value towards the actual value. Numerous tools are available to aid in the practical use of Bayesian PK-guided dosing in the clinic, including the Web-based Application for the Population Pharmacokinetic Service hosted by McMaster University, Canada. The PK data can be used to determine the appropriate prophylaxis regimen for the individual patient, which can be monitored by assessment of the trough level at each clinic visit.

Collection of PK data and subsequent PK-guided dosing should become standard practice when determining treatment strategies for people with haemophilia.

Clinical, pharmacokinetic and economic analysis of the first switch to an extended half-life factor IX (albutrepenonacog alfa, rFIX-FP) in Spain

Extended half-life of factor IX (FIX) demonstrated clinical benefit and lower treatment burden than standard half-life FIX products in clinical trials. We analysed the impact in efficacy, pharmacokinetics (PKs) and costs of the switch from nonacog alfa (rFIX) to albutrepenonacog alfa (rFIX-FP) in the first patient with haemophilia B (HB) treated in Spain outside clinical trials. A 7-year-old boy presented with HB with poor venous access and repetition infections using rFIX, which was switched to rFIX-FP. Prophylaxis was adjusted by PKs using WAPPS-Hemo tailoring from 100 IU/kg/week of rFIX to 80 IU/kg/3 weeks of rFIX-FP. Comparing 6 months before, rFIX-FP reduced 68.5% FIX consumption/kg and 58.3% infusion frequency, but total costs/weight showed a slight increase. Ratio of half-life between rFIX and rFIX-FP was 3.4–3.7. This case report revealed that switch to rFIX-FP decreased frequency and FIX consumption, without adverse events and bleeds.

Performance of factor IX extended half-life product measurements in external quality control assessment programs

BACKGROUND

Patients with hemophilia B are increasingly treated with extended half-life (EHL) factor IX (FIX) concentrates. For the laboratory, introduction of these EHL concentrates presents a major challenge. To understand the variation in FIX activity levels, all available diagnostic assays need to be directly compared.

METHODS

The ECAT, UKNEQAS, and RCPAQAP have collaboratively performed a global survey to evaluate the quality of FIX measurements using FIX deficient plasma samples spiked with recombinant FIX (rFIX), rFIXFP, rFIXFc, and N9-GP to levels at typical FIX trough (6 IU/dL) and peak levels (60 IU/dL). Participants were asked to use their routine protocols, using one-stage assays (OSA) or chromogenic assays (CA).

RESULTS

In samples spiked with 6 IU/dL product, median (25%-75% range) FIX activity levels (OSA), were 8.0 IU/dL (7.0-9.2) for rFIX, 6.0 IU/dL (4.0-7.1) for rFIXFP, 6.6 IU/dL (5.5-8.0) for rFIXFc, and 4.9 IU/dL (3.5-8.4) for N9-GP. In samples spiked with 60 IU/dL, FIX activity levels measured (using OSA) was 63.0 IU/dL (59.9-67.0) for rFIX, 42.5 IU/dL (28.2-47.0) for rFIXFP, 50.0 IU/dL (45.0-55.0) for rFIXFc, and 34.0 IU/dL (24.8-67.5) for N9-GP. Considerable differences were observed between reagents for all samples. With CA, there was also quite some variation, but no differences between reagents.

CONCLUSION

Large variation is observed in the measurement of FIX activity levels after administration of rFIX and EHL FIX products. For N9-GP, most silica-based assays show especially high levels. It is essential to standardize and improve reliability of measurements of these concentrates as diagnosis and treatment monitoring is based on these results.

Pharmacokinetics for haemophilia treaters: Meaning of PK parameters, interpretation pitfalls, and use in the clinic

Replacement therapy with concentrates of factor VIII or IX remains the gold standard for severe haemophilia management. The recent development of clotting factor products with extended half-life, widely available on the market since 2 years, facilitates adherence, improves considerably the patients' quality of life, and simplifies the management of breakthrough bleedings or surgery. These molecules have also brought to the limelight the concepts of optimization and personalization of anti-haemophilic prophylaxis. Pharmacokinetics (PK) is one of the tools that can help haematologists to adapt in a more objective and precise manner the prophylaxis regimen to each individual patient's specific needs. For many years, clinicians at haemophilia centres have been using some simple PK parameters, such as recovery and residual level. However, recently, they have been confronted with an important number of new PK parameters they were not familiar with, but that can be used to improve patient management. Due to the accumulation of PK data and their relative complexity, it is now necessary to analyse the relevance of the different PK parameters relative to haemophilia specificities, and also to know their limits to better use them.

Real-world assay variability between laboratories in monitoring of recombinant factor IX Fc fusion protein activity in plasma samples

Introduction

Monitoring of factor IX (FIX) replacement therapy in haemophilia B relies on accurate coagulation assays. However, considerable interlaboratory variability has been reported for one‐stage clotting (OSC) assays. This study aimed to evaluate the real‐world, interlaboratory variability of routine FIX activity assays used in clinical haemostasis laboratories for the measurement of recombinant FIX Fc fusion protein (rFIXFc) activity.

Methods

Human FIX‐depleted plasma was spiked with rFIXFc at 0.80, 0.20 or 0.05 IU/mL based on label potency. Participating laboratories tested samples using their own routine OSC or chromogenic substrate (CS) assay protocols, reagents and FIX plasma standards. Laboratories could perform more than one measurement and method, and were not fully blinded to nominal activity values.

Results

A total of 142 laboratories contributed OSC results from 175 sample kits using 11 different activated partial thromboplastin time (aPTT) reagents. The median recovered FIX activity for the 0.80, 0.20 and 0.05 IU/mL samples was 0.72 IU/mL, 0.21 IU/mL and 0.060 IU/mL, respectively. Across all OSC reagents, interlaboratory variability (% CV) per aPTT reagent ranged from 9.4% to 32.1%, 8.2% to 32.6% and 12.2% to 42.0% at the 0.80, 0.20 and 0.05 IU/mL levels, respectively. CS results showed excellent median recoveries at all nominal levels (87.5% to 115.0%; n = 11) with low interlaboratory variability (CV 3.6% to 15.4%).

Conclusion

This large, real‐world data set indicates that rFIXFc activity in plasma samples can be accurately measured with the majority of routine OSC and CS assay methods. Given the variation in FIX assay procedures between sites, it is important that individual laboratories qualify their in‐house methods for monitoring of rFIXFc activity.

Extended half-life recombinant products in haemophilia clinical practice - Expectations, opportunities and challenges

Extended half-life (EHL) products have shown robust efficacy in clinical trials, whilst allowing for less intense treatment regimens when compared with standard half-life products. Regimen optimisation with EHL products could lead to further improvements in bleeding rates, quality of life and reductions in treatment burden. Patients now expect good efficacy, a lower treatment burden and equivalent safety when compared with standard half-life products. As our knowledge base grows these expectations have evolved and targeting an annualised bleeding rate of zero has become a more realistic clinical goal. Personalised prophylaxis can help patients achieve these goals. However, a number of challenges still remain, including cost, challenges in predicting outcomes for patients and differences in patients' and clinicians' expectations. When switching a patient, comprehensive patient care can reduce the impact of these issues. This review presents in brief the protein therapeutics with an extended half-life, including key trial results, challenges of chronic care that impact on patients' outcomes and how the modified proteins might help address some of these issues. In addition, practical steps for managing the switch to EHL products are presented.

Extended Half-Life Coagulation Factors: A New Era in the Management of Hemophilia Patients

Despite effective factor replacement and various treatment schedules, there remain several challenges and unmet needs in the prophylactic treatment of hemophilia limiting its adoption and thereby posing an increased risk of spontaneous bleeding. In this regard, extended half-life (EHL) recombinant factor VIII (rFVIII) and factor IX (rFIX) products promise optimal prophylaxis by decreasing the dose frequency, increasing the compliance, and improving the quality of life without compromising safety and efficacy. EHL products might lead to higher trough levels without increasing infusion frequency, or could facilitate the ability to maintain trough levels while reducing infusion frequency. This paper aims to provide a comprehensive review of the rationale for developing EHL coagulation factors and their utility in the management of hemophilia, with special emphasis on optimal techniques for half-life extension and criteria for defining EHL coagulation factors, as well as indications, efficacy, and safety issues of the currently available EHL-rFVIII and EHL-rFIX products. Potential impacts of these factors on quality of life, health economics, and immune tolerance treatment will also be discussed alongside the challenges in pharmacokinetic-driven prophylaxis and difficulties in monitoring the EHL products with laboratory assays.

Practical aspects of extended half-life products for the treatment of haemophilia

Haemophilia A and haemophilia B are congenital X-linked bleeding disorders caused by deficiency of coagulation factor VIII (FVIII) and IX (FIX), respectively. The preferred treatment option for patients with haemophilia is replacement therapy. For patients with severe disease, prophylactic replacement of coagulation factor is the treatment of choice; this has been shown to reduce arthropathy significantly, reduce the frequency of bleeds and improve patients' quality of life. Prophylaxis with standard recombinant factor requires regular intravenous infusion at least two (FIX) to three (FVIII) times a week. Recombinant FVIII and FIX products with an extended half-life are in development, or have been recently licensed. With reported mean half-life extensions of 1.5-1.8 times that of standard products for FVIII and 3-5 times that of standard products for FIX, these products have the potential to address many of the unmet needs of patients currently treated with standard factor concentrates. For example, they may encourage patients to switch from on-demand treatment to prophylaxis and improve the quality of life of patients receiving prophylaxis. Indeed, extended half-life products have the potential to reduce the burden of frequent intravenous injections, reducing the need for central venous lines in children, promote adherence, improve outcomes, potentially allow for more active lifestyles and, depending on the dosing regimen, increase factor trough levels. Members of the Zürich Haemophilia Forum convened for their 19th meeting to discuss the practicalities of incorporating new treatments into the management of people with haemophilia. This review of extended half-life products considers their introduction in haemophilia treatment, including the appropriate dose and schedule of infusions, laboratory monitoring, patient selection, safety considerations, and the economic aspects of care.

Attempting to remedy sub-optimal medication adherence in haemophilia: The rationale for repeated ultrasound visualisations of the patient's joint status

Haemophilia is marked by joint bleeding (haemarthrosis) leading to cartilage damage (arthropathy). Lifelong prophylaxis-initiated after the first bleeding episode-leads to a dramatic decrease in arthropathy in haemophilia patients. However, adherence to continuous intravenous administrations of factor VIII (FVIII) or FIX products is challenging, and patients potentially suffer from breakthrough bleedings while on prophylaxis. Newer FVIII/FIX products with enhanced convenience attributes and/or easier infusion procedures are intended to improve adherence. However, pharmacokinetic data should be harmonised with information from individual attitudes and treatment needs, to tailor intravenous dosing and scheduling in patients who receive extended half-life products. Nor is there sound evidence as to how subcutaneous non-FVIII/FIX replacement approaches (concizumab; emicizumab; fitusiran) or single intravenous injections of adeno-associated viral vectors (when employing gene therapy) will revolutionize adherence in haemophilia. In rheumatoid arthritis, repeated ultrasound examination of a patient's major joints is a valuable tool to educate patients and parents to understand the disease and provide an objective framework for clinicians to acknowledge patient's adherence. Joint ultrasound examination in haemophilia significantly correlates with cartilage damage, effusion, and synovial hypertrophy evaluated by magnetic resonance imaging. Furthermore, in patients with haemophilia undergoing prophylaxis with an extended half-life product for a ≈ 2.8 year period, a significant continued improvement in joint health is detected at the physical examination. This provides the rationale for studies on repeated ultrasound examinations of joint status to attempt to remedy sub-optimal medication adherence and help identify which approach is most suited on which occasion and for which patient.

Performing and interpreting individual pharmacokinetic profiles in patients with Hemophilia A or B: Rationale and general considerations

OBJECTIVES

In a separate document, we have provided specific guidance on performing individual pharmacokinetic (PK) studies using limited samples in persons with hemophilia with the goal to optimize prophylaxis with clotting factor concentrates. This paper, intended for clinicians, aims to describe how to interpret and apply PK properties obtained in persons with hemophilia.

METHODS

The members of the Working Party on population PK (PopPK) of the ISTH SSC Subcommittee on Factor VIII and IX and rare bleeding disorders, together with additional hemophilia and PK experts, completed a survey and ranking exercise whereby key areas of interest in the field were identified. The group had regular web conferences to refine the manuscript's scope and structure, taking into account comments from the external feedback to the earlier document.

RESULTS

Many clinical decisions in hemophilia are based on some form of explicit or implicit PK assessment. Individual patient PK profiles can be analyzed through traditional or PopPK methods, with the latter providing the advantage of fewer samples needing to be collected on any prophylaxis regimen, and without the need the for a washout period. The most useful presentation of PK results for clinical decision making are a curve of the factor activity level over time, the time to achieve a certain activity level, or related parameters like half-life or exposure (AUC). Software platforms have been developed to deliver this information to clinicians at the point of care. Key characteristics of studies measuring average PK parameters were reviewed, outlining what makes a credible head-to-head comparison among different concentrates. Large data collections of PK and treatment outcomes currently ongoing will advance care in the future.

CONCLUSIONS

Traditionally used to compare different concentrates, PK can support tailoring of hemophilia treatment by individual profiling, which is greatly simplified by adopting a PopPK/Bayesian method and limited sampling protocol.

Using pharmacokinetics to individualize hemophilia therapy

Prevention and treatment of bleeding in hemophilia requires that plasma clotting factor activity of the replaced factor exceeds a defined target level. Most clinical decisions in hemophilia are based on implicit or explicit application of pharmacokinetic measures. The large interindividual variability in pharmacokinetics of factor concentrates suggests that relying on the average pharmacokinetic characteristics of factor concentrates would not allow optimizing the treatment of individual patients; for example, adjusting the frequency of infusions and targeting a specific clotting factor activity level on a case-by-case basis. However, individual pharmacokinetic profiles are seldom assessed as part of routine clinical care. Population pharmacokinetics provide options for precise and convenient characterization of pharmacokinetics characteristics of factor concentrates, simplified individual pharmacokinetic profiling, and individualized dosing. Population pharmacokinetics allow for the incorporation of determinants of interpatient variability and reduces the need for extensive postinfusion plasma sampling. Barriers to the implementation of population pharmacokinetics are the need for concentrate-specific pharmacokinetic models, Bayesian calculation power, and specific expertise for production, validation, and appraisal of forecasted estimates. Population pharmacokinetics provide an important theoretical and practical contribution to tailoring the treatment of hemophilia. The need remains for prospective exploration of the clinical impact of tailoring hemophilia treatment based on individual pharmacokinetics, and for the systematic validation of existing software solutions and concentrate-specific models.

In silico evaluation of limited blood sampling strategies for individualized recombinant factor IX prophylaxis in hemophilia B patients

Essentials Individual pharmacokinetic (PK) parameters can be obtained by limited sampling strategies (LSSs). Following 100 IU kg-1 rFIX, LSSs with 1 to 3 samples were evaluated in 5000 simulated subjects. For all LSSs, estimated individual PK parameters showed acceptable bias and precision. One sample between 10 min-3 h and two between 48 h-56 h showed best predictive performance.

SUMMARY

Background Patients with severe hemophilia B regularly administer prophylactic intravenous doses of clotting factor IX concentrate to maintain a trough level of at least 0.01 IU mL-1 in order to prevent joint bleeds. Assessment of individual pharmacokinetic (PK) parameters allows individualization of the recombinant factor IX (rFIX) dose. Aim To evaluate the predictive performance of limited sampling strategies (LSSs) with one to three samples to estimate individual PK parameters of rFIX. Methods Monte Carlo simulations were performed to obtain 5000 concentration-time profiles by the use of population PK parameters for rFIX from literature. Eleven LSSs were developed with one, two or three samples taken within an 80-h interval following administration of 100 IU kg-1 rFIX. Clearance (CL), half-life (t1/2 ), time to 1% and steady-state distribution volume (Vss ) were estimated for each simulated individual by the use of Bayesian analysis. Results For each LSS, average bias was small for CL (range - 1.5% to 1.4%), t1/2 (range - 4.5% to - 0.7%), time to 1% (range - 2.9% to 0%), and Vss (range - 3.7% to 0.3%). Imprecision for these parameters ranged from 6.4% to 11.9%, from 10.3% to 15.6%, from 7.3% to 10.9%, and from 9% to 20.1%, respectively. The best predictive performance was achieved with one sample taken between 10 min and 3 h and two samples taken between 48 h and 56 h after administration of rFIX. Conclusions This study demonstrates that limited sampling strategies, used for individualized dosing of rFIX in hemophilia B patients, can be developed and evaluated by in silico simulation.