Factor Replacement Therapies (Factor VIII / Factor IX)

Mechanism

Mechanism of Action

Exogenous Factor VIII and Factor IX act by restoring the basal levels of functional proteins deficient in hemophilia syndromes of genetic origin.

Molecular Physiology of the Tenase Complex in Hemophilia

1. Factor IXa is a serine protease active in the intrinsic coagulation pathway.
2. Factor VIIIa acts as a high-affinity obligate non-enzymatic cofactor of the cell membrane that binds Factor IXa, calcium and anionic phospholipids to form the macromolecular complex Intrinsic Tenase on the platelet surface.
3. The intrinsic tenase complex is the most efficient metabolic target that catalyzes the breakdown and massive activation of Factor X to Factor Xa.
4. In Hemophilia A (congenital Factor VIII deficiency) or Hemophilia B (Factor IX deficiency), the inability to form the tenase complex prevents the propagation of the coagulation cascade, making primary peripheral hemostatic consolidation impossible and causing recurrent severe joint hemorrhages (hemarthrosis).
5. Replacement therapies directly replenish the deficient functional protein in peripheral blood, reestablishing the integrity of cellular intrinsic hemostasis.

Pharmacokinetics

Pharmacokinetics

• Rout of administration: Exclusively slow intravenous.
• Factor VIII: In vivo recovery of approximately 2% plasma increase for every 1 IU/kg administered. It is distributed primarily in the intravascular space with stable binding to endogenous von Willebrand Factor. Elimination half-life of 8 to 12 hours in adults (substantially reduced in young children). Extended half-life forms (pegylated or fusion) extend the half-life up to 19 hours.
• Factor IX: Less in vivo recovery due to more extensive extravascular diffusion cellular uptake (binds to collagen structures of the vessel wall). It increases plasma activity by approximately 1% for every 1 IU/kg administered in Hemophilia B. Prolonged elimination half-life of 18 to 24 hours. Extended formulations linked to Fc fragments extend shelf life to more than 80 hours.

Indicators and dose

Indications

• Treatment of acute bleeding episodes and surgical interventions in patients with confirmed Hemophilia A (Factor VIII) or Hemophilia B (Factor IX).
• Long-term on-demand or continuous prophylaxis to prevent recurrent hemarthroses and chronic joint damage in patients with severe hemophilia phenotype (defined by endogenous activity less than 1% of the normal factor).

Precision Dosing (Clinical Formulas)

The calculation of the dose to be infused depends strictly on the patient's body weight and the percentage of activity of the factor that is desired to be achieved based on the hemorrhagic risk of the location of the hemorrhage.

Factor VIII Dosing Equation

Each unit of Factor VIII infused per kg of body weight increases plasma activity by 2% (0.02 IU/mL).
Required Dose (IU) = Body Weight (kg) × Desired Increase in Activity (%) × 0.5

Factor IX Dosing Equation

Due to its greater extravascular distribution, each unit of Factor IX per kg of weight increases plasma activity by approximately 1% (0.01 IU/mL) in adults.
Required Dose (IU) = Body Weight (kg) × Desired Increase in Activity (%) × 1.0

Recommended plasma activity levels according to severity of bleeding:
- Mild hemarthrosis or muscle bleeding: Reach levels of 30% to 50% of plasma activity.
- Gastrointestinal bleeding or minor surgical procedures: Achieve 50% to 70% activity.
- Life-threatening intracranial hemorrhage or major emergency surgery: Reach levels of 80% to 100% of plasma activity on a mandatory basis.

Pregnancy and Breastfeeding

The clinical data of administration in pregnant women with symptomatic hemophilia carrier phenotypes are extremely limited. Use under strict indication of gynecological hematology during labor and delivery.

Security

Contraindications

• Demonstrated hypersensitivity to recombinant fusion proteins of murine origin, hamster (CHO) or human plasma proteins.
• History of acute severe anaphylactic reactions to previous factor infusions.

Adverse Effects (ADR)

Critical Complication: Development of Inhibitors (Neutralizing Antibodies)

The most serious and feared complication of replacement therapy is the development of IgG type inhibitors directed against the infused exogenous factor, identified by the patient's immune system as a foreign foreign antigen. It alarmingly affects up to 25-30% of previously unexposed patients with severe Hemophilia A (PUPs), and 3-5% in Hemophilia B. Inhibitors completely neutralize the effectiveness of the substitute factor, rendering classical prophylactic treatment useless and forcing the use of powerful, high-cost bypass bridging agents (such as Activated Prothrombin Complex Concentrate - FEIBA or Recombinant Factor VIIa - Novoseven). and cardiovascular risk.

• Systemic hypersensitivity reactions: Urticaria, hypotension, chest tightness and dyspnea during the infusion period.
• Minor thrombosis: Excessive cumulative doses of Factor IX or factor complexes may favor a peripheral peripheral prothrombotic state.

Interactions

It does not present pharmacokinetic interactions with hepatic hepatic microsomal enzymes. Concomitant use of acetylsalicylic acid or strong antiplatelet agents should be avoided due to the risk of aggravating recurrent bleeding in the event of drops in peripheral plasma factor levels.