Haemophilia Factor Replacement Therapies

Haemophilia factor replacement therapies replace the missing clotting protein, factor VIII (haemophilia A) or factor IX (haemophilia B), to treat and prevent bleeding. Prophylaxis from early childhood prevents joint damage and is the aim of care. Newer nonfactor agents (emicizumab) and gene therapy are changing practice, but factor replacement remains the basis of treatment in most settings.

Mechanism. Recombinant or plasma-derived concentrates restore factor activity in plasma. Extended half-life products (Fc fusion, albumin fusion, PEGylation) prolong the interval. Bypassing agents (recombinant factor VIIa, activated prothrombin complex concentrate) activate coagulation independent of factors VIII and IX in patients with inhibitors. Emicizumab is a bispecific antibody that mimics factor VIII.

Pharmacokinetics. Conventional factor VIII half-life 10 to 14 hours; extended half-life products 14 to 20 hours. Conventional factor IX 18 to 24 hours; extended half-life up to 80 to 100 hours. Recovery for each IU/kg: factor VIII about 2 IU/dL; factor IX about 1 IU/dL (recombinant about 0.7 to 0.8).

Dose calculation.

• Factor VIII (IU) = body weight (kg) x desired rise (%) x 0.5.
• Factor IX (IU) = body weight (kg) x desired rise (%) x 1 (x 1.2 for some recombinant products).

Prophylaxis. Factor VIII 20 to 40 IU/kg three times weekly or on alternate days; factor IX 40 IU/kg twice weekly; extended half-life products every 3 to 7 days; low-dose regimens (10 to 15 IU/kg two to three times weekly) are used where supply is limited.

Inhibitors. Bypassing agents: recombinant factor VIIa 90 micrograms/kg every 2 to 3 hours, or activated prothrombin complex concentrate 50 to 100 units/kg every 8 to 12 hours (maximum 200 units/kg daily). Immune tolerance induction in specialist centres.

Emicizumab. 3 mg/kg subcutaneously weekly for 4 weeks, then 1.5 mg/kg weekly, 3 mg/kg every 2 weeks or 6 mg/kg every 4 weeks. Do not combine with activated prothrombin complex concentrate (thrombotic microangiopathy).

Alternatives when concentrate is unavailable. Cryoprecipitate for haemophilia A (no factor IX); fresh frozen plasma 15 to 20 mL/kg for haemophilia B (volume limits repeated dosing). Desmopressin 0.3 micrograms/kg intravenously or subcutaneously (with tranexamic acid) for mild haemophilia A.

• First-line. Prophylactic and on-demand factor replacement for haemophilia A and B; treatment of acute bleeds as early as possible without waiting for imaging.
• Alternative. Emicizumab prophylaxis (haemophilia A, with or without inhibitors); desmopressin for mild haemophilia A with proven response; antifibrinolytic drugs for mucosal bleeding.
• Specialist. Inhibitors, immune tolerance induction, surgery, gene therapy for selected adults (for example, etranacogene dezaparvovec for haemophilia B, valoctocogene roxaparvovec for haemophilia A) and other nonfactor agents.
• Emergency. Immediate factor replacement for suspected intracranial haemorrhage, airway or neck bleeding, and major trauma: 50 IU/kg factor VIII or 100 IU/kg factor IX (target about 100%) before investigation.

Selection. Treat early; match replacement level to bleed severity; use recombinant or plasma-derived products by availability; use extended half-life agents to reduce injections; test for inhibitors in any patient not responding as expected.

Contraindications. Hypersensitivity to the product or to mouse or hamster protein; heparin-induced thrombocytopenia (some factor IX preparations contain heparin); activated prothrombin complex concentrate with emicizumab; bypassing agents in thrombosis risk (caution).

Serious adverse effects.

• Inhibitor development: up to 30% of severe haemophilia A and 3 to 5% of haemophilia B, mostly in the first 50 exposure days.
• Allergic reactions and anaphylaxis (especially haemophilia B with inhibitors) and nephrotic syndrome in immune tolerance.
• Thrombosis with bypassing agents.
• Transmission of infection with plasma-derived products (low with modern viral inactivation, higher with cryoprecipitate and plasma).

Major interactions. Antifibrinolytics with prothrombin complex concentrates (additive thrombosis); aspirin and NSAIDs raise bleeding; intramuscular injections should be avoided; emicizumab with activated prothrombin complex concentrate.

Common prescribing errors.

• Delaying treatment in suspected bleeding.
• Underdosing in life-threatening bleeds.
• Giving factor VIII to a patient with haemophilia B.
• Combining emicizumab with activated prothrombin complex concentrate.
• Using intramuscular injection or NSAIDs.

Special groups.

• Pregnancy: carriers need factor monitoring and a delivery plan; avoid vacuum extraction.
• Breastfeeding: compatible.
• Children: higher doses and shorter half-life; prophylaxis from the first bleed or by age 1 to 3 years.
• Older adults: cardiovascular risk with bypassing agents.
• Kidney: no adjustment; watch nephrotic syndrome in immune tolerance.
• Liver: viral hepatitis and HIV from previous plasma products are common comorbidities.
• East Africa: access to factor concentrate is limited and cryoprecipitate and plasma are often the only options; low-dose prophylaxis, early joint treatment and physiotherapy matter; test donors and use viral-inactivated products where possible.

Monitoring. Factor activity (peak and trough), inhibitor screen (Bethesda assay) at regular intervals and after intensive treatment, joint status, viral serology, and bleeding diary.

Referral and escalation. Any suspected intracranial or neck bleed (treat first, refer second); inhibitors; surgery; poor response to replacement; haemophilia centre review at least annually.

Comparison with alternative classes. Factor replacement is the most direct and best studied approach but needs frequent injections and risks inhibitors. Emicizumab and other nonfactor agents give simpler subcutaneous prophylaxis; gene therapy offers sustained expression but is costly and limited to selected patients.