Gene Therapy for Haemophilia

Overview of Haemophilia

Definition

• Haemophilia A: Caused by deficiency of factor VIII.

• Haemophilia B: Caused by deficiency of factor IX.

Pathophysiology

• Both types of haemophilia are characterized by a reduced ability to generate thrombin due to the deficiency of the respective clotting factors, leading to increased risk of bleeding.

Clotting Factors and Bleeding Rates

Clotting Cascade Importance

• Factor VIII and IX are critical in the clotting cascade, playing vital roles in forming a fibrin clot.

  • Factor VIII interacts with Factor IX to activate Factor X.

  • Factor IX is activated by Factor XI and then interacts with Factor VIII in the intrinsic pathway.

  • Thrombin Generation: Ultimately leads to fibrinogen conversion to fibrin.

Bleeding Rates

• Severe Haemophilia:

  • Factor activity < 1%: spontaneous bleeding episodes which may occur into joints, muscles, and soft tissues.

• Mild Haemophilia:

  • Factor activity between 1-5%: bleeding typically occurs after trauma, injury, dental work, or surgery.

• Endogenous Factor Activity:

  • Greater than 10% normal value correlates with a low risk of spontaneous bleeding.

• Treatment Aim:

  • Increase factor levels above 10%.

Historical Perspective on Haemostatic Therapies

Past Treatments

• Plasma-derived coagulation factors used until the 1980s.

• Shift caused by the awareness of HIV transmission; significant infection rates among haemophilia patients receiving infected blood products.

Present Treatments

• Current practice involves:

  • Recombinant purified coagulation factors: Safer alternatives to plasma-derived products.

  • Development of inhibitors observed in some patients.

  • Short half-life necessitating frequent infusions.

  • Emerging Gene Therapy: Major breakthrough for haemophilia A & B.

Why Target Haemophilia for Gene Therapy?

• Monogenic Disorder:

  • Both types are monogenic disorders, necessitating alteration in a single gene.

• Partial Restoration:

  • Restoration to less than 100% normal levels is effective for treatment.

• Liver Targeting:

  • Gene therapy vectors target liver cells, serving as a “protein factory” for distribution throughout the body.

• Success Measurement:

  • Success can be easily measured through factor activity levels.

Gene Delivery Approach

Initial Focus on Factor IX

• Factor IX gene therapy (haemophilia B) attempted as it is a smaller gene compared to factor VIII.

• Adeno-associated Virus (AAV) Vector:

  • High efficiency in transducing cells.

  • Offers long-term transgene expression.

  • Low immunogenicity potential with selective tissue tropism.

• Challenges:

  • High immunogenic response related to the viral capsid may cause hepatotoxicity in some cases.

Eligibility Criteria for Gene Therapy

• Age: 18 years or older.

• History of Inhibitors: No prior development of factors inhibiting proteins.

• Liver Health: No serious liver diseases.

• Antibody Status: No pre-existing antibodies to the AAV vector.

• Eligibility Statistics:

  • 40-50% of patients with severe haemophilia A.

  • 40-60% of patients with severe haemophilia B.

Specifics of Haemophilia B Gene Therapy Approach

Mechanism and Administration

• AAV Vector: Hepatocyte-directed, expressing factor IX.

• Administration Impact: Leads to endogenous factor IX (FIX) expression in the liver.

• Early Issues:

  • Initial therapies noted either low but durable FIX expression, or high but transient FIX expression.

Factor Expression Variability

Overview

• Variability in factor expression from no expression to above-normal levels noted.

• Typically, FVIII levels lower than FIX plasma levels.

Factor IX ‘Padua’ Variant

• Observed in a child with thrombophilia having a FIX variant.

• Mutation at position 338 (R338L) from arginine to leucine results in FIX Padua having 10-15 times higher activity compared to wild-type FIX.

• Potential Application: Exploring whether FIX Padua could boost overall FIX activity.

Current Developments in Therapy

Etranacogene Dezaparvovec (CSL Behring)

• Yielded high, durable FIX activity in trial patients.

• Associated with minimal bleeding events and a favorable safety profile.

• Now employed in many haemophilia B clinical trials (Pipe et al, NEJM 2023).

Challenges in Haemophilia A Gene Therapy

Size and Expression Issues

• Size of FVIII gene poses a significant technological challenge; exceeds AAV packaging capacity.

• Expression durability remains a critical concern and is often limited in clinical trials.

Potential Solutions

• Gene engineering efforts to remove unnecessary coding sequences (e.g., the deletion of the B domain to create BDD-FVIII).

• Result: Increased endogenous FVIII expression by approximately tenfold, though drop-off levels remain unexplained.

Clinical Trials in Haemophilia A Gene Therapy

Trial Overview

Challenges Facing Gene Therapy

Cost and Accessibility

• High treatment costs, even for a proposed ‘one-off’ treatment.

• Only individuals without existing anti-AAV immunity are eligible.

• Future dosing challenges due to immune response limiting repeat therapies.

Global Accessibility

• Queries about accessibility for patients in developing countries.

• Questions regarding the justification of costs if factor levels drop, necessitating factor replacement.

Response Uncertainty

• No current indications predicting which haemophilia patients will develop satisfactory factor levels post-gene therapy.

• Necessity for biomarkers to identify responsive individuals to optimize gene therapy effectiveness.

Durability of Treatment

• Uncertainty surrounding how long factor levels will remain sufficient to prevent bleeding episodes.

• FVIII expression durability averages 5-8 years; factor IX expression may last up to 10 years, with variability seen among individuals.

Safety Considerations

Short-Term Risks

• Potential liver inflammation upon delivery of gene therapy to liver cells.

• Steroidal treatments commonly utilized post-delivery; for approved FVIII therapies, 86% of individuals required steroids for an average duration of 7 months.

Long-Term Risks

• Concern regarding the partial integration of gene therapy vectors into patient DNA.

• Potential increased cancer risk if integration occurs in oncogenic regions of the genome.

• Noted 4 unrelated cases of cancer in haemophilia gene therapy trials.

Conclusion

• Gene therapy for haemophilia A and B is a promising new therapeutic modality.

• Current methodologies involve hepatocyte-directed AAV vectors.

• Despite significant achievements, various limitations remain, warranting ongoing research.