Emicizumab

Mechanism of Action

The drug emicizumab is a humanized bispecific monoclonal antibody that restores the function of missing activated FVIII by bridging FIXa and FX, mimicking activated Factor VIIIa independently of Factor VIII levels. Consequently, emicizumab promotes effective hemostasis in patients with hemophilia A.[7][8] Emicizumab is manufactured by inserting specific genetic material into a Chinese hamster ovary (CHO) cell line, which then multiplies and produces the target antibody. The antibody is subsequently separated from the cellular component and tested for purity and potency. Neither human plasma nor blood products are used in the manufacturing of emicizumab.[9] Emicizumab is designed to substitute the hemostatic function of activated FVIII by bridging activated factor IX and factor X (FX) to activate FX, thereby facilitating the coagulation cascade and achieving hemostasis in patients with hemophilia A.[5]

PharmacokineticsAbsorption: After subcutaneous administration, the absorption half-life is 1.6±1 days. The bioavailability following subcutaneous injection of 1 mg/kg ranged between 80.4% and 93.1%. Similar pharmacokinetic profiles are noted following SC administration in the abdomen, thigh, and upper arm. Following subcutaneous administrations of a loading dose of 3 mg/kg emicizumab once weekly for the first four weeks,  trough plasma concentrations of 52.6 ± 13.6 μg/mL(mean (± SD)) are achieved at week 5. Steady-state concentrations with the recommended maintenance doses vary according to the regimen. (Ctrough = trough concentration at steady state)

• 1.5 mg/kg once every week: Ctrough 51.2+/- 15.2(µg/mL)
• 3 mg/kg once every two weeks: Ctrough 46.9 +/- 14.8(µg/mL)
• 6 mg/kg once every four weeks: Ctrough 38.5 +/- 14.2(µg/mL)

Distribution: The mean apparent volume of distribution is 10.4 L.

Elimination: The mean apparent clearance is 0.27 L/day, and the mean elimination apparent half-life is 26.9 +/- 9.1 days.

Administration

In many countries, subcutaneous emicizumab has been cleared to be used in the prophylaxis of bleeding episodes in individuals diagnosed with hemophilia A, regardless of FVIII inhibitor status.[10] In phase III clinical trials, emicizumab prophylaxis significantly reduced annualized bleeding rates in adolescents and adults with hemophilia A, including those with FVIII inhibitors. It also prevented or significantly decreased bleeding in children in the same medical situation.[11][12][13] Because of the drug's convenient subcutaneous route of administration and relatively infrequent dosing ability (dose every 1, 2, or 4 weeks), emicizumab provides an effective alternative to conventional FVIII replacement products, which tend to be more frequent.[14][15][16] The drug has a longer elimination half-life compared to other current treatments and, on average, stays in the body for approximately 4 to 5 weeks.[17]

Use in Specific Patient Population

Patients with Hepatic Impairment: No dose adjustment is necessary for mild or moderate hepatic impairment. However, emicizumab has not been studied in patients with severe hepatic impairment.

Patients with Renal Impairment: No dose adjustment is required for patients with mild or moderate renal impairment. However, emicizumab has not been studied in patients with severe renal impairment.

Pregnancy Considerations:  Animal reproductive studies have not been conducted with emicizumab. There is a lack of data on emicizumab use in pregnant women to report a drug-associated risk of congenital disabilities and miscarriage. According to product labeling, using emicizumab in pregnancy is advised only if the potential benefit to the mother outweighs the risk to the fetus. Females of childbearing age should consider using contraception while receiving emicizumab.

Breastfeeding Considerations:  No data is available on the clinical use of emicizumab during breastfeeding. Emicizumab is a large protein molecule with high molecular weight; hence the concentration in milk is probably low. Hypothetically absorption of emicizumab is unlikely because it is probably destroyed in the infant's gastrointestinal tract.[18] According to product labeling, clinicians should consider the health benefits and risks of exposure to emicizumab in breastfeeding infants and the maternal need for emicizumab.

Adverse Effects

Injection-site reactions are the most common treatment-related adverse drug reactions. Other common adverse drug reactions reported are headache (15%), arthralgia (15%), pyrexia (6%), and diarrhea (6%).[5] Rhabdomyolysis was a less common adverse reaction that occurred in less than 1% of patients taking emicizumab. Although significantly less frequent, unprovoked or traumatic bleeding may occur in patients taking emicizumab, necessitating treatment with other hemostatic agents such as recombinant FVIIa (rFVIIa) and activated prothrombin complex concentrates (aPCC). It is especially important for treating clinicians to note whether the emicizumab effect is present to anticipate consequences of concurrent administration of hemostatic agents (see Contraindications) and understand the impact on coagulation assays (see Monitoring).

Similar to FVIII replacement therapy, emicizumab has the potential for immunogenicity. Per the emicizumab prescribing information, 3.5% of patients in the HAVEN clinical trials tested positive for anti-emicizumab antibodies, though less than 1% developed antibodies with neutralizing potential.[11]

Contraindications

A boxed warning cautions that thrombotic microangiopathy (TMA) and other thrombotic complications have been reported with concurrent administration of aPCC greater than 100 U/kg within 24 hours in patients taking emicizumab.[19] Per the important safety information emicizumab, TMA occurred in 8.1% of the patients and thrombotic events in 5.4% of patients who received at least one dose of aPCC. Patients with TMA often present with: microangiopathic hemolytic anemia, low platelet levels, and/or acute kidney injury without severe ADAMTS13 deficiency. Other thrombotic events included: thrombotic microangiopathy, cavernous sinus thrombosis, and thrombophlebitis.[20] 

In emergent cases of bleeding, when the benefit of aPCC exceeds the risk of inducing a pro-coagulable state, clinicians should monitor patients for thrombotic microangiopathy and thromboembolism and discontinue aPCC if symptoms occur. Using both rFVIIa and emicizumab hypothetically should increase the risk for thrombotic complications. However, several pharmaceutical manufacturers have analyzed the data from the HAVEN study, which concluded that rFVIIa use in the context of emicizumab prophylaxis did not change the rFVIIa safety profile as described in the product information.[21]

Monitoring

Measuring plasma emicizumab levels is not required for therapy. However, a drug level may be helpful in specific medical incidences, such as confirmation of compliance. Monitoring can also be beneficial in breakthrough bleeding, situations that require an on-demand coagulation factor, or antibody monitoring.[22] Currently, the only accurate detection method is a sandwich enzyme-linked immunosorbent (ELISA) assay employing an anti-emicizumab antibody. The test is highly precise and reproducible in current trials and literature. 

A noteworthy consideration while treating patients on emicizumab is the drug’s effect on laboratory testing.[22] Coagulation tests based on the intrinsic pathway (i.e., aPTT) measure total clotting time, including the time needed to activate FVIII to FVIIIa by thrombin. Emicizumab, which mimics FVIIIa cofactor activity by bridging FIXa and FX, obviates thrombin activation, resulting in overly shortened clotting times. Adamkewicz et al. showed that emicizumab has a strong interference effect on numerous assays that are aPTT based. The drug also will affect assays for FVIII, protein C and S activity. There is a weak disturbance on tests for prothrombin time and derived fibrinogen assays. Lupus anticoagulant testing in patients taking emicizumab poses a significant problem because it utilizes both an aPTT-based assay, which may result in false-negative results, and a DRVVT-based assay, which may result in false-positive results.[23] [Level 1]

Clinicians need to be aware of assay interference when ordering FVIII activity or inhibitor titers in a patient taking emicizumab[24] [Level 5] Clotting-based assays may overestimate FVIII activity and place patients at risk for bleeding. Bethesda assays show a significant impact from emicizumab and may result in undetectable anti-FVIII titers in patients with clinically significant inhibitors.[25] Ultimately, clinicians must discuss testing modalities with laboratory directors and request FVIII activity and FVIII inhibitor testing via chromogenic assays with a bovine reagent insensitive to emicizumab.[26] Clinicians should understand that emicizumab may affect coagulation assays for up to 6 months after its last administration due to its extended half-life.

Clinicians can use the Haemophilia Quality of Life Questionnaire for Adults (Haem-A-QoL) to evaluate health-related quality of life (HRQoL) in patients with hemophilia who are on treatment with emicizumab.[27]

Toxicity

Using previous trials like HAVEN, an efficacious range for emicizumab is 30 to 70 microgram/milliliter.[22] 

Management of breakthrough bleeding is a potent concern.[28] Currently, there is no antidote to emicizumab.

Carcinogenesis, Mutagenesis, Impairment of Fertility: Studies in animals investigating the carcinogenic effects of emicizumab have not been conducted. However, emicizumab did not cause any toxicological changes in the reproductive organs in preclinical studies, with the dose of 30 mg/kg/week in subcutaneous toxicity studies of twenty-six weeks duration and a dose of 100 mg/kg/week for four-week intravenous toxicity study.

Enhancing Healthcare Team Outcomes

Researchers performed the HAVEN 2 trial on children with hemophilia with inhibitor development. Typically they develop inhibitors within 50 days of Factor VIII therapy. Children with hemophilia require high-volume infusions requiring a central venous access device (CVAD), which can lead to increased possibilities of infection/thrombosis.[11] The study revealed that once a week, subcutaneous injection decreased annual bleeding rates in children with hemophilia A with Factor VIII inhibitor by 99%.[11] [Level 1]

An open study was conducted in children with one treatment arm receiving emicizumab and the other with no inhibitors. Results supported a decrease in bleeding events throughout the year. It also supported decreased anxiety associated with possible bleed causes, including activity levels. Subjects reported increased physical activity during the trial. Patients received either 2-week dosing or 4-week dosing. Both regimens showed an appropriate decrease in bleeding and disruption in daily living.[12] [Level 2] 

Regarding adults with hemophilia A and no inhibitors, dosing once every four weeks is adequate for bleeding control and well tolerated by patients. [Level 3].[29] Overall, emicizumab is generally well-tolerated and has versatile dosing options, including once every 1, 2, or 4 weeks. This novel drug allows an alternative to conventional Factor VIII replacement products in patients with hemophilia A, with or without the presence of inhibitors.[14] [Level 5]

Typically emicizumab will be prescribed by a hematologist, but it still requires the efforts of an entire interprofessional team. Management of hemophilia requires comprehensive diagnostic and treatment services, pain control, patient education, and genetic, psychosocial, and vocational counseling. The Centers for Disease Control and Prevention(CDC) operates in collaboration with the U.S. Hemophilia Treatment Center Network (USHTCN), a federally funded, specialized health care center called hemophilia treatment centers or HTCs. HTC are specialized, multidisciplinary healthcare centers with expertise in providing care for Patients with Hemophilia(PWH). To achieve this goal, a core interprofessional team consisting of a hematologist, certified pediatric hematology-oncology nurse, physical therapist, social worker, and a comprehensive multidisciplinary team including pediatricians, internists, orthopedic surgeons, dentists, genetic counselors, pharmacists, nutritionists, vocational rehabilitation counselors, and other healthcare providers all play a vital role in the care of the patient with hemophilia. This interprofessional care provided by HTCs optimizes patient outcomes and health-related quality of life (HRQoL) in patients with hemophilia by decreasing the probability of serious adverse events.[30] The interprofessional model includes open communication between all team members, as well as meticulous documentation of all patient interactions is the responsibility of all team members, so everyone managing the case has the most accurate and up-to-date patient data.[ This interprofessional paradigm will optimize therapy with emicizumab and the overall case management. Level 5]