Platelet‐delivered factor VIII provides limited resistance to anti‐factor VIII inhibitors

Abstract

To cite this article: Gewirtz J, Thornton MA, Rauova L, Poncz M. Platelet-delivered factor VIII provides limited resistance to anti-factor VIII inhibitors. J Thromb Haemost 2008; 6: 1160-6. Summary. Background: Gene therapy strategies directed at expressing factor (F)VIII in megakaryocytes has potential advantages in the treatment of hemophilia A. Among these is that platelet (p) FVIII may be effective in the presence of circulating anti-FVIII inhibitors. Objective: We examined in a murine transgenic model whether pFVIII could correct the coagulation defect in FVIII null mouse in the presence of circulating inhibitors. Methods: FVIII null mice that were transgenic for pFVIII (pFVIII/FVIII null ) were compared with FVIII null mice receiving infused FVIII in a FeCl 3 carotid injury model in the presence of anti-FVIII inhibitors. Results: After injury, pFVIII/FVIII null mice were significantly more resistant to circulating inhibitors than after plasma FVIII correction in both an acute and chronic models of inhibitor exposure even although in the chronic model, significant amounts of inhibitor were stored within the platelets. Furthermore, bleeding in the pFVIII mice in the presence of inhibitors was not as a result of the development of thrombocytopenia. Conclusion: In FVIII null mice, pFVIII provides improved, but limited, protection in the presence of inhibitors of 6-fold greater Bethesda Units per mL relative to infused FVIII. Our findings differ from a recent report using a tail-clip exsanguination assay on the degree of efficacy of pFVIII in the presence of inhibitors. We propose that this difference in outcome is as a result of the sensitivity of the tail-vein exsanguination model to low levels of pFVIII.

Platelet-delivered ADAMTS13 inhibits arterial thrombosis and prevents thrombotic thrombocytopenic purpura in murine models

“…A similar strategy has been used to express factor VIII to treat hemophilia A with inhibitors, with limited success. 27 Our results demonstrate that platelet-delivered rADAMTS13 is efficacious for inhibiting arterial thrombosis after vascular injury and prevents the onset of Shigatoxin-2 (Stx-2)-or recombinant murine VWF (rmVWF)-induced TTP in the absence or the presence of inhibitors. The findings provide a proof of concept that platelet-derived ADAMTS13 may be explored as a novel therapeutic strategy for arterial thrombosis and TTP, even in the face of autoantibodies.…”

“…27 To determine whether platelet-delivered rADAMTS13 ameliorates the phenotype of acquired TTP with inhibitors, we administered a monoclonal antibody against human ADAMTS13 (scFv4-20) cloned from patients with acquired TTP, according to the scheme illustrated ( Figure 7A). A single bolus of scFv4-20 completely inhibited plasma Adamts13 activity in WT mice between 8 and 24 hours ( Figure 7B).…”

In vivo efficacy of platelet-delivered, high specific activity factor VIII variants

“…Our data suggest that pF8 is 100-fold more effective than plasma F8 in the presence of inhibitors in spite of the copresence of the inhibitors within the platelets. 19 However, while ectopic expression of F8 within platelets may offer a potential advantage in the presence of inhibitors, 39,44 this strategy has certain limitations. One clear disadvantage is that platelet activation needs to occur to release pF8.…”

“…19 These mice were given either 1.5 g, 3.0 g, or 7.5 g total of a 1:5 (g/g) inhibitor cocktail of monoclonal antibodies ESH8:GMA-8021 in 100 L of PBS (American Diagnostica and Green Mountain Antibodies, respectively) 19 on days 0, 3, 6, and 9 by retro-orbital injection. A control group was given control murine isotype immunoglobulin (IgG) (Innovative Research) in the same amounts over the same 9-day schedule.…”

“…17,18 Most importantly, plateletderived (p) human B-domainless (hB) F8 remains markedly more effective in the presence of circulating inhibitors than plasma hBF8. [19][20][21] This pF8 is stored in ␣-granules independent of VWF and can be effective when released at sites of injury in F8 null mice. 18,22 Despite having this advantage, we found that the efficacy of phBF8 in F8 null mice varied based on the bleeding model studied.…”