The most important and prevalent inherited bleeding disorders are hemophilia A and B, rare recessive X-linked diseases (1/5.000 and 1/30.000 male respectively). The severity of disesase is based on the levels of factor VIII (FVIII) and factor IX (FIX) into the bloodstream: <1% (severe type), 1 to 5% (moderate type), >5 to <40% (mild type). Replacement therapy represents the golden standard consisting in i.v. infusion of FVIII/FIX concentrates either plasmaderived or recombinant products. Actually prophylaxis, with administration of the specific concentrate at fixed doses and time intervals is the only regimen able to prevent bleeding and to reduce its complications. The most serious complication of replacement therapy is the onset of alloantibodies against FVIII and FIX (inhibitors) affecting at least 30% of severe hemophiliacs A and approximately 5% of severe hemophiliacs B. More rarely inhibitors develop in moderate and mild hemophiliacs. Inhibitors neutralize replacement therapy and expose patients to a high risk of life-threatening events (cerebral hemorrhage, retroperitoneal hematoma, etc.). In the last years, pharmaceutical research has employed a lot of resources in the development of long-acting FVIII/IX concentrates, as well as other coagulation factors, in order to make replacement treatment and/or prophylaxis easier and to improve patient quality of life. More recently, attention has been paid to drugs other than traditional coagulation concentrates by exploiting “no-replacement” principles of therapy.
Long-acting concentrates.
Factor VIII concentrates
Several strategies have been explored to prolong the half-life of exogenous FVIII with the aim of making either higher trough levels with a standard infusion interval or more extended the infusion interval, so that prophylaxis should be successful. Fc-fusion has been the first technology introduced for long-acting products. Fusion of the immunoglobulin G constant region (Fc) to the F.VIII molecule induces half-life extension by delaying its normal clearance. Efmoroctocog alfa is a B-domain deleted analog of human clotting factor linked to the Fc domain of human IgG1.
PEGylation represents an alternative method. It was adopted by three other manufacturers: octocog alfa, , turoctocog alfa pegol (N8-GP), and damoctocog alfa pegol (BAY94-9027). Covalent attachment of polyethylene glycol (PEG) to FVIII protects it against proteolysis, so that reduces its clearance and increases the half-life. PEG is secreted by kidneys and no long-term toxicity has been observed to date.
A third approach is able to delay the clearance of FVIII by the addition of polysialic acid (PSA), which in a sperimental animal model interferes with the receptor-mediated clearance. However all these novel concentrates, beacause of the complex structure of FVIII molecule, extend only 1.5 times a patient’s baseline half-life.
Factor IX concentrates
The new class of long-acting FIX products consistently give a great change in the hemophilia B treatment. The first extended half-life FIX concentrate available in Italy has been albutrepenonacog alfa, a recombinant FIX-albumin fusion protein (rFIX-FP), which exploits the property of the albumin’s long half-life due to its high molecular weight. The decline of FIX activity after infusion is gradual and consistent with a mean terminal half-life of 104 hours. The rFIX-FP pivotal study evaluated patients first on weekly prophylaxis giving a dosage between 35 to 50 IU/kg-1. After the initial 26 weeks, a subgroup of patients extended their infusion interval to 10-14 days at dosage of 75 IU/kg-1. Mean FIX trough levels were 20 IU/dL in patients on the rFIX-FP 40 IU/kg-1 weekly prophylaxis regimen and 12 IU/dL for those who changed dosage to 75 IU/kg-1 every 2 weeks respectively.
Recombinant FIX-Fc fusion protein (rFIX-Fc, eftrenonacog alfa) has been the second extended half-life product licensed in Italy by Italian Agency of Drugs. Patients enroled in the clinical trial received 50 IU/kg weekly achieving FIX trough levels of 1-3 IU/dL. The rFIX-Fc, unlike rFIX-FP, showed a rapid decline of FIX activity over the first 24-72 hours post-infusion, followed by an extended terminal half-life of 86.5 ± 32.2 hours.
Glycopegylated recombinant FIX (nonacog beta pegol, N9-GP) also showed to have an important mean of extended half-life around 111 hours. In the Phase III trial a weekly infusion of either 10 IU/kg-1 or 40 IU/kg-1 was given, resulting a stable trough FIX activity levels of 8.5 IU/dL and 27.3 IU/dL respectively. N9-GP, as well as rFIX-FP, demonstrated a slow and stable decline of FIX circulating activity.
Non-factor replacement strategies.
Nowadays several novel treatment options try to demonstrate to be hemostatically effective as replacement therapy with the advantage of subcutaneous administration and weekly or monthly injection intervals. These products may strongly improve the current hemophilia care.
ACE910
ACE910 (emicizumab) is a bispecific monoclonal antibody able to bind FIXa and FX on a phospholipid membrane mimicking the cofactor function of FVIII. In FVIII-neutralized plasma, emicizumab shortenes the PTT and increases thrombin generation in a dose-dependent manner. The ability of ACE 910 to normalize the PTT may help to estimate when at least low level of active FVIII is detectable into the bloodstream. The half-life of emicizumab ranges from 4 to 5 weeks making easier the treatment regimen. The phase I/II trial showed a significant dicrease of annual bleeding rates (ABR) from 15 to 0 in hemophiliacs with and without inhibitors, mainly in the highest dose cohort. Thrombotic microangiopathy and thrombosis were reported in 2 participants who received multiple infusions of activated prothrombin complex concentrate for bleeding. No antibodies against emicizumab were detected. Because of its structural difference with FVIII, except the binding sites, emicizumab is neither affected nor neutralized by FVIII inhibitors. The particular functional properties makes emicizumab useful in prophylaxis of patients with FVIII inhibitors for preventing bleeding and avoiding the by passing agents in case of hemorrhagic episodes.
Blocking Tissue Factor Pathway Inhibitor (TFPI)
TFPI is the physiological inhibitor of the initiation of coagulation via tissue factor (TF) and factor VIIa (FVIIa). Blockade of TFPI may increase thrombin generation. TFPI is a Kunitz type serin protease inhibitor with three Kunitz-type domains able to bind TF-FVIIa, FXa and protein S respectively. In animal models with hemophilia the blocade of TFPI has been shown to normalize PTT and bleeding time. This observation yielded investigators to consider the blockade of TFPI as new hemostatic approach in hemophiliacs with and without inhibitor. Humanized monoclonal antibodies and aptamers have been developed (concizumab) with high-affinity directed against the Kunitz-type protease inhibitor 2 domain. A fully humanized IgG2 monoclonal antibody, is also developing (BAY 1093884).
TFPI, BAX 499 (anti-TFPI aptamer, Shire) is a PEGylated nucleic acid aptamer that neutralizes both TF-FVIIa and FXa, even though an incomplete inhibition of the FXa was observed.
Inhibitor of antithrombin
Because some hemophiliacs who are heterozygous for prothrombotic traits, such as Factor V Leiden protein C/protein S or antithrombin deficiency, may show milder bleeding phenotypes, the hypothesis that reduction of antithrombin (AT) would be a valid hemostatic therapeutic approach has been supported. ALN-AT3 (fitusiran) is a new drug that interferes with AT mRNA production by inhibition of gene expression through destruction of specific mRNA molecules using GalNAc-siRNA conjugate technology. Fitusiran is administered subcutaneously. The reduction of AT level manifested a statistically significant increase in thrombin generation and a decrease in ABR mean. Neither adverse events, nor thromboembolic complications, nor significant increase of D-dimer were observed.
Factor Xa variant (FXaI16L)
More recently the bioengineered FXaI16L variant has been produced that makes resistent FXa to inactivation by AT and TFPI. Binding to FVa, it provides procoagulant activity with full thrombin generation. A pre-clinical study in hemophilic mice bleeding models showed that FXaI16L is effective and safe.
Conclusion
Long-acting FVIII/FIX concentrates represent a new prospective in the management of hemophilia. They potentially improve trough factor levels and/or reduce the infusion schedule. Nevertheless the frequency of FVIII infusions and patient adherence, unlike what happenes to FIX, must be considered, as well as inhibitor risk for both the products have to be kept in mind. Hemophiliacs A and their care givers hesitate to change a prophylaxis regimen so that they do not feel ready and willing to switch to a novel product. The dosage of extended half life FVIII concentrates may be overestimated or underestimated with considerable clinical and economic implications.
The risk of developing inhibitor using the new products remains under investigation. At the moment in the phase III trials of previously treated patients no inhibitor development have been reported.
Observational studies are in progress with the aim of monitoring the possible onset of inhibitor in previously treated patients, with or without a history of inhibitors,
The non-factor replacement strategies are an exciting option for treatment of patients with hemophilia, mainly in those with inhibitor and in persons affected by other bleeding disorders. Whether these products will be successful would represent a real therapeutic revolution for hemophilia patients, particularly for those with inhibitors.
Suggested Reading
- Kulkarni R, Soucie JM. Pediatric hemophilia: a review. Semin Thromb Hemost 2011;37:737–44.
- Berntorp E, Andersson NG. Prophylaxis for hemophilia in the era of extended half-life factor VIII/factor IX products. Semin Thromb Hemost 2016;42(5):518–525.
- Horava SD, Peppas NA. Recent advances in hemophilia B therapy. Drug Deliv Transl Res.2017: 7(3):359-371.
- Mannucci PM, Mancuso ME, Santagostino E, Franchini M. Innovative pharmacological therapies for the hemophilias not based on deficient factor replacement. Semin Thromb Hemost 2016;42(5): 526–532.