**3.2. Hemophilia B and its treatment**

Hemophilia B is another bleeding disorder considered to be indistinguishable from hemophilia A whereas, recent evidences elucidated that hemophilia B patients have less severe bleeding phenotype lower bleeding frequency and better long term outcomes compared to hemophilia A [42]. Hemophilia B is caused due to FIX deficiency [43]. FIX is encoded by a gene present on X chromosome, FIX is a major component of intrinsic pathway of coagulation cascade and it is activated into FIXa by FXIa or FVIIa [44–46]. Activated FIX forms a X-ase complex with FVIIIa and phosphatidylserine [47]. Tenase complex converts X to Xa [47]. The bleeding tendency depends on FIX activity in the plasma [48]. FIX deficiencies are classified based on the plasma FIX activity and they are severe (<1% FIX activity), moderate (1–5%) and

**Figure 2.** Schematic representation for FVIII biosynthesis and inactivation.


**Table 1.** Development of FVIII therapy for hemophilia.

mild (5–40%) [48]. Current therapies for hemophilia B include plasma derived FIX, recombinant FIX, recombinant FIX fused with polyethylene glycol (PEG), recombinant FIX fused with Fc portion of immunoglobulin G, FIX fused with albumin mutant FIX [48]. The disadvantages of plasma derived FIX is that it has a very short half-life in the patient plasma and plasma derived FIX has chances of viral contamination. Recombinant FIX is produced in Chinese hamster ovary cells [48]. rFIX has an increased half-life compared to plasma derived FIX, it over came the problem of viral contamination, however rFIX showed 30% less activity compared to plasma derived FIX, due to variations in the glycosylation. Conjugation of FIX with PEG is known as PEGylation. PEG serves as a shield for PEGylated FIX and protects it from proteolytic cleavage. PEGylated FIX's half-life is five times in mice compared to the half-life of rFIX [48]. FC fused FIX has a half-life of 48 h. The other FIX fusion protein in clinical trial is FIX fused with albumin. Single amino acid mutation in the catalytic domain of FIX (R338L) increased its Tenase activity by 2 fold and thrombin generation activity by 6 fold, therefore by FIX R338L usage in gene therapy is under investigation [49].

## **3.3. Hemophilia C**

K36, R336, R562, R740, R1689 and R1721 [27]. FXa mediated cleavage of FVIII at K36 and R336 leads to inactivation of FVIIIa [27]. APC inactivates FVIII by proteolytically cleaving FVIIIa at R336 and R562 which leads to destabilization of A1 and A2 domain interaction [28, 29] (**Figure 2**). In 1960s, major treatment for hemophilia A is whole blood or plasma transfusion [30, 31]. This treatment has a drawback of viral transfusion along with the coagulation factors. Treatments of mild hemophilia A include vasopressin analogs to enhance the synthesis of FVIII, 1-Desamino-8d-arginine vasopressin (DDAVP) is a vasopressin analog clinically used to enhance the plasma levels of FVIII [32, 33]. Recombinant FVIIa and FVIII are also used to prevent bleeding events in the hemophilia patients [34].These clotting factors are also supplemented with FVIIa or factor FVII inhibitor bypassing agent (FEIBA) to enhance the function of FVIIa and FVIII, whereas FEIBA enhanced the risk of thrombosis [35, 36]. Recent studies elucidated that stabilized recombinant FVIII can be used as a therapeutic for hemophilia A, this includes more stable isoforms of FVIII such as B domain deleted FVIII (BDD FVIII) [37] (**Table 1**). The ongoing research is focusing on using BDD FVIII as a gene therapy by incorpo-

Hemophilia B is another bleeding disorder considered to be indistinguishable from hemophilia A whereas, recent evidences elucidated that hemophilia B patients have less severe bleeding phenotype lower bleeding frequency and better long term outcomes compared to hemophilia A [42]. Hemophilia B is caused due to FIX deficiency [43]. FIX is encoded by a gene present on X chromosome, FIX is a major component of intrinsic pathway of coagulation cascade and it is activated into FIXa by FXIa or FVIIa [44–46]. Activated FIX forms a X-ase complex with FVIIIa and phosphatidylserine [47]. Tenase complex converts X to Xa [47]. The bleeding tendency depends on FIX activity in the plasma [48]. FIX deficiencies are classified based on the plasma FIX activity and they are severe (<1% FIX activity), moderate (1–5%) and

rating it into the viral vectors and delivering it into the patient [38].

**Figure 2.** Schematic representation for FVIII biosynthesis and inactivation.

**3.2. Hemophilia B and its treatment**

134 Hematology - Latest Research and Clinical Advances

Hemophilia C is caused by FXI deficiency where factor XI activity of 15–20 U/dL or lower. Surprisingly FXI deficiency does not show a severe bleeding phenotype [16].
