Hemophilia A and B: Treatment Management

Overview

1. Classification of Severity

• Severe (<1% factor activity): spontaneous bleeds, especially hemarthroses
• Moderate (1-5% factor activity): bleeding after minor trauma
• Mild (5-40% factor activity): bleeding with surgery or significant trauma

2. Goals of Treatment

1. Avoiding inhibitor (alloantibody) formation
2. Improving treatment convenience to maximize adherence

• Henry's Clinical Diagnosis and Management by Laboratory Methods

3. Factor Replacement Products

Hemophilia A - FVIII Concentrates

• High specific activity, immunoaffinity-purified FVIII (without vWF)
• Low specific activity pd-FVIII (with vWF)

• Full-length native FVIII or B-domain deleted forms
• Characterized by presence/absence of human/animal proteins

• Pegylation of B-domain deleted construct
• Single-chain structure (covalent bonding of heavy and light chains)
• B-domain deleted FVIII fused to IgG-1 Fc domain
• EHL products reduce injection frequency from 3 to ~2 days/week

Hemophilia B - FIX Concentrates

• Plasma-derived FIX concentrates
• Recombinant FIX
• EHL FIX products (FIX-Fc fusion, albumin-fused FIX, PEGylated FIX) - reduce infusions to once weekly

All modern products are viral safe and equivalently efficacious. Cryoprecipitate (for FVIII) and FFP (for FIX) are no longer standard of care.

• Harrison's Principles of Internal Medicine, 22e

4. Dosing Formulas

FVIII Dosing (Hemophilia A)

1 unit/kg of FVIII raises plasma level by 2%

FIX Dosing (Hemophilia B)

FIX recovery after infusion is ~50% of predicted, so a larger dose is needed

Half-lives

• FVIII: 8-12 hours (requires twice-daily dosing for sustained levels)
• FIX: 18-24 hours (once-daily dosing sufficient)
• Harrison's Principles of Internal Medicine, 22e; Harriet Lane Handbook

5. Target Factor Levels by Bleed Type

• Harriet Lane Handbook, 23rd edition

6. Prophylaxis vs. On-Demand Treatment

On-Demand (Episodic)

Primary Prophylaxis (Standard of Care for Severe Disease)

• Goal: Maintain factor levels ≥1% continuously to prevent bleeds, especially hemarthroses
• Initiation: Before onset of frequent bleeding - typically in 1 to 3-year-olds with severe hemophilia
• Boys receiving regular infusions can reach puberty without detectable joint abnormalities
• FVIII: typically 3 days/week; FIX: 2 days/week (with EHL products, less frequent)

• 51% of children under 6 with severe hemophilia now receive prophylaxis (CDC data)

Surgical Prophylaxis

• Major surgery: Goal 80-100 IU/dL preoperatively through postoperative bleeding risk period
• Minor surgery/procedures: Goal 50-80 IU/dL
• Consult hematology before any invasive procedure including dental, endoscopy with biopsy, arterial blood gas
• Harrison's Principles, 22e; Harriet Lane Handbook, 23rd ed

7. Non-Factor / "Disruptive" Therapies

Emicizumab (Hemlibra) - Hemophilia A

• Mechanism: Bispecific IgG antibody - an "FVIII mimetic" that bridges FIXa and FX to reconstitute the tenase complex and generate thrombin
• Route: Subcutaneous injection
• Dosing: 1.5 mg/kg weekly, 3.0 mg/kg every 2 weeks, or 6.0 mg/kg every 4 weeks
• Indication: Prophylaxis in hemophilia A with or without inhibitors, including infants (no data <1 year)
• Advantage: Subcutaneous, infrequent dosing, can be used with inhibitors
• Caution: When combined with large repeated doses of FEIBA for breakthrough bleeds, thrombotic microangiopathy has been reported in 5 patients
• Lab interference: Artificially shortens aPTT and falsely elevates aPTT-based one-stage FVIII assay - use chromogenic (bovine reagent) assays for monitoring
• Henry's Clinical Diagnosis and Management; Goldman-Cecil Medicine; Tietz Textbook of Laboratory Medicine

Fitusiran (Qfitlia) - Hemophilia A and B

• Mechanism: siRNA therapeutic that degrades antithrombin mRNA in hepatocytes, reducing antithrombin synthesis and thereby increasing thrombin generation
• Route: Subcutaneous injection
• Dosing: Fixed 50 mg every 2 months; target antithrombin activity 15-35%
• Indication: Prophylaxis in hemophilia A or B, with or without inhibitors, ≥12 years
• Monitoring: Antithrombin activity at weeks 4, 12, 20, and 24 after initiation/dose change, then annually
• Caution: Two fatal thromboses occurred in hemophilia A patients receiving concurrent high-dose FVIII concentrate during clinical trials
• FDA approval: March 2025
• Henry's Clinical Diagnosis and Management; NBDF/MASAC 2025

Concizumab (Alhemo) - Anti-TFPI antibody

• Mechanism: High-affinity anti-TFPI monoclonal antibody, increases FXa generation and thrombin generation
• Route: Daily subcutaneous injection
• Indication: Hemophilia A or B with inhibitors (FDA-approved 2025)
• Decreased annualized bleeding rates in clinical trials

Note: A different anti-TFPI construct (Bayer) was terminated due to thrombotic complications.

8. Inhibitors - The Major Complication

Prevalence

• FVIII inhibitors: ~30% in severe hemophilia A; ~10% in non-severe hemophilia A
• FIX inhibitors: 3-5% of all hemophilia B patients (especially large gene deletions)
• Typically appear at median age 2 years, after ~10 cumulative exposure days

Risk Factors for Inhibitor Development

• Severe deficiency (>80% of cases)
• Family history of inhibitor
• African descent
• Large gene deletions or gross gene rearrangements
• Intensive replacement therapy (surgery, ICH, major trauma)

Classification

• Low responders: Inhibitor titer ≤5 Bethesda Units (BU); no anamnestic response
• High responders: Titer >5 BU or anamnestic rise to >5 BU on re-exposure

Treatment of Acute Bleeds with Inhibitors

Inhibitor Eradication - Immune Tolerance Induction (ITI)

• Method: Daily infusion of the deficient factor until inhibitor disappears
• Duration: Typically >1 year
• Success rate: ~60%
• Rituximab (anti-CD20) combined with ITI: reduces inhibitor titers in some patients but sustained eradication is uncommon
• Harrison's Principles, 22e; Henry's Clinical Diagnosis and Management

9. Gene Therapy

1. FVIII and FIX genes were cloned in the early 1980s
2. Factor levels are easily and precisely measured in small plasma volumes
3. Even small incremental increases significantly improve phenotype
4. Well-established animal models provided proof-of-principle
5. Patient population is motivated to participate

• FIX gene therapy (FIX-Padua gain-of-function mutation): Sustained FIX activity 30-40% in some patients (vs. baseline ≤2%)
• FVIII gene therapy: Clinically significant increases to normal range (≥50%) in many patients

• Pre-existing neutralizing antibodies to AAV capsid in some patients
• Durability of expression - some loss of transgene expression over time (episomal latency or immune mechanisms)
• Long-term immunologic and oncologic vigilance required
• Beqvez (fidanacogene elaparvovec-dzkt) for hemophilia B was approved then discontinued by manufacturer (2025)
• Henry's Clinical Diagnosis and Management; Thompson & Thompson Genetics and Genomics

10. Adjunctive and Special Therapies

11. Special Considerations

Perioperative Management

• Consult hematology before any procedure
• Maintain factor levels 100%+ for major surgery throughout the healing period
• Continuous infusion of factor may be preferable post-surgery for sustained levels at lower cost

Avoiding Harm

• Avoid aspirin, NSAIDs, and other antiplatelet agents
• Avoid intramuscular injections
• Avoid central venous catheters when possible (infectious and thrombotic risk)

Prenatal Diagnosis

• Chorionic villous sampling at 9-14 weeks or amniocentesis after 16 weeks
• Fetal DNA detectable in maternal circulation within 10 days of conception - non-invasive prenatal diagnosis now feasible
• In hemophilia B: 30-40% of carriers missed by FIX activity alone; mutation analysis essential

Recent Evidence Update (2024-2026)

• Fitusiran (Qfitlia) received FDA approval in March 2025 for hemophilia A/B with or without inhibitors (≥12 years)
• Concizumab (Alhemo) approved 2025 for hemophilia A or B with inhibitors
• Gene therapy products have entered and been withdrawn from the market (Beqvez discontinued 2025), highlighting durability concerns
• A 2024 network meta-analysis (PMID 38970991) evaluated first-line immunosuppression for acquired hemophilia A (autoantibody disease), distinct from congenital inhibitors
• Ongoing trials are investigating switching from emicizumab to fitusiran in hemophilia A

Clinical landscape note: Current clinical guidelines (last major update 2020) are being revised to incorporate the newer non-factor therapies. The therapeutic options have expanded substantially beyond traditional factor replacement.