Preclinical Animal Models for Hemophilia Gene Therapy: Predictive Value and Limitations

Abstract

Hemophilia A and B are excellent candidate disorders for the application of somatic cell gene therapy. One of the major advantages in the preclinical development of hemophilia gene therapy strategies has been the availability of several animal models for both hemophilia A and B. These models recapitulate many of the phenotypic aspects of human hemophilia and have proven to be very informative in exploring the efficacy and safety of gene therapy. Considerable progress has been made in the design of gene therapy protocols, and over the last 5 years it has been shown that long-term phenotypic correction, with sustained therapeutic levels of factor VIII (FVIII) and factor IX (FIX), can be attained in FVIII- and FIX-deficient mice and dogs using various viral vector-mediated gene therapy approaches. These animal models also have elucidated potential complications of gene therapy protocols, including acute vector-associated toxicities and the induction of neutralizing antibodies to the FVIII and FIX transgene products. Nevertheless, although the preclinical paradigm of hemophilic mouse followed by hemophilic dog studies has proven to be extremely helpful in evaluating the efficacy and safety of potential clinical gene therapy protocols, several limitations to these animal models still exist. This review presents a summary of the animal models available for hemophilia gene therapy, and highlights the various strengths and weaknesses of these models.

Multiyear therapeutic benefit of AAV serotypes 2, 6, and 8 delivering factor VIII to hemophilia A mice and dogs

“…Furthermore, since prior studies have demonstrated that the hemophilia dog model, compared with the mouse model, more accurately predicts the therapeutic outcomes in humans and other primates, 32,33 we have determined the long-term efficacy and safety of AAV2-cFVIII, AAV6-cFVIII, and AAV8-cFVIII vectors in hemophilia A dogs, to assess whether dogs recapitulate the findings for different vector serotypes observed in mice. The potential application of alternative serotype AAV-FVIII vectors in humans not only may enhance the therapeutic efficacy, but also may diminish some neutralization by pre-existing anti-AAV2 antibodies that are prevalent in the human population.…”

Efficacy and safety of a new-class hemostatic drug candidate, AV513, in dogs with hemophilia A

“…The severe hemophilia A dogs in this colony have no detectable plasma levels of FVIII and have approximately 5 spontaneous bleeding episodes per year. 33 The citrated plasma from these animals fails to clot in a TEG assay when stimulated with excess calcium. As a first step, the potency of AV513 to accelerate clot initiation was compared with citrated whole blood from AAV-FVIII hemophilia dogs and the severe hemophilia A dogs in an in vitro TEG assay.…”

Correction of murine hemophilia A following nonmyeloablative transplantation of hematopoietic stem cells engineered to encode an enhanced human factor VIII variant using a safety-augmented retroviral vector

“…8 In the present study, we used BAL-fVIIIKO mice (fVIII exon 16-disrupted knockout mice bred onto the BALB/c background), which have been shown to develop 3-to 4-fold higher anti-fVIII inhibitor titers following adenovirus-mediated fVIII gene delivery or subcutaneous injection of recombinant human fVIII than those congenic with the C57BL/6 strain. [31][32][33] In the current experiments, sustained expression of efVIII as well as sfVIII⌬B in the plasma of all immunocompetent hemophilia A BM transplant recipients for up to 24 weeks after transplantation implied lack of stimulation of a potent inhibitory immune response to either fVIII protein. To , and BM cells was quantified by real-time PCR using human fVIII-specific primers at 6 months after transplantation.…”

“…7,8 The goal of the present study was to obtain comparable results by using a more clinically relevant HSC gene therapy protocol that (1) uses a less genotoxic transgene delivery system, (2) minimizes both the number of vector copies per transduced cell and the dose of transduced cells that are transplanted, (3) incorporates reduced-intensity transplant conditioning regimens, and (4) uses the BAL-fVIIIKO strain of hemophilia A mice, which mounts a more potent anti-fVIII inhibitor response to exogenous fVIII than the C57-fVIIIKO strain. [31][32][33] To minimize the number of genetically modified cells needed to produce therapeutic levels of fVIII, we constructed an enhanced fVIII variant, efVIII. Several fVIII variants have been bioengineered previously which exhibit increased secretion, altered immunogenicity, or decreased degradation.…”

“…25,30 Furthermore, we determined the effectiveness of the strategy by using a recently described hemophilia A mouse model (BAL-fVIIIKO; fVIII exon 16-disrupted knockout mice bred onto the BALB/c background) that develops a more potent inhibitory immune response to exogenous fVIII than the C57-fVIIIKO strain and therefore may more accurately predict clinical outcome. [31][32][33] We show that compared with sfVIII⌬B expressed from a conventional LTR gammaretroviral vector, the efVIII variant expressed from the RMSinOFB backbone requires transplantation of significantly fewer transduced HSCs to achieve similar curative levels of fVIII.…”