We are developing a ZFN-mediated genome editing approach to hemophilia B using our proprietary In Vivo Protein Replacement Platform (IVPRP), and a therapeutic portfolio approach to hemophilia A using our proprietary adeno-associated virus (AAV)-cDNA gene therapy and IVPRP approaches. Our hemophilia B program is in Phase 1 clinical development and our hemophila A programs are currently in preclinical development.
Hemophilia, a rare bleeding disorder in which the blood does not clot normally, is an example of a monogenic disease (a disease that is caused by a genetic defect in a single gene). There are several types of hemophilia caused by mutations in genes that encode factors which help the blood clot and stop bleeding when blood vessels are injured. Individuals with hemophilia experience bleeding episodes after injuries and spontaneous bleeding episodes that often lead to joint disease such as arthritis. The most prevalent form of the disease, hemophilia A, is caused by a defect in clotting Factor VIII, while defects in clotting Factor IX lead to hemophilia B. The most severe forms of hemophilia affect males.
According to the National Hemophilia Foundation and the World Federation of Hemophilia, hemophilia A occurs in about one in every 5,000 male births in the US with approximately 16,000 males currently affected in the US, and hemophilia B in about 1 in every 25,000 male births with approximately 4,000 males currently affected. The standard treatment for individuals with hemophilia is replacement of the defective clotting factor with regular infusion of recombinant clotting factors or plasma concentrates. These therapies are expensive, carry the risk of transmission of blood-borne diseases such as hepatitis and other viral infections and sometimes stimulate the body to produce antibodies against the factors that inhibit the benefits of treatment. In these situations, other clotting factors such as Factor VII and X may be used to treat patients. People with severe forms of the disease may need ongoing, preventive infusions.
Sangamo’s Therapeutic Approach
In Vivo Genome Editing
We are developing a therapeutic in vivo genome editing approach for hemophilia A and B, using our ZFN technology to target and integrate a functional copy of the Factor 8 or 9 gene into a safe-harbor site, the albumin gene or locus, in liver cells. Our goal is to harness the powerful albumin promoter to produce and secrete continuous, therapeutic levels of clotting factors VIII and IX for hemophilia A and B, respectively, and eliminate the need for chronic infusions of recombinant clotting factor protein.
We have published data demonstrating functional correction of the human factor 9 gene (hF9) in the liver by direct intravenous delivery of AAV encoding ZFNs in a mouse model of the disease (Nature 475, 217–221, 14 July 2011). Preclinical studies of our in vivo genome editing approach, have demonstrated that therapeutic levels of Factor IX (FIX) could be generated in a dose-dependent manner in non-human primates (NHPs). There were no significant alterations in circulating albumin levels. Studies in mice also demonstrated stable FIX production for over one year. Our in vivo genome editing approach is designed to be a broadly applicable strategy for gene replacement that will provide a permanent correction for the lifetime of the patient and would reduce or eliminate the need for chronic infusions of replacement proteins or clotting factor products.
Emerging from our expertise in molecular biology and AAV manufacturing and delivery, our gene therapy approach for hemophilia A comprises an AAV-based cDNA construct with an optimized B-domain deleted human Factor 8 gene (hF8) driven by our proprietary, synthetic liver-specific promoter.
Data from preclinical studies in mouse models and non-human primates (NHPs) have demonstrated production of supraphysiologic levels of human Factor VIII clotting protein (hFVIII) after a single dose of our AAV-hF8 cDNA construct. Overall mean hFVIII expression levels of 331% of normal were observed in mouse models of hemophilia A resulting in a significant reduction in bleeding time, in treated animals, from 38.3 minutes to 2.5 minutes (in line with wild-type mice). Additional dose-ranging studies in NHPs resulted in the dose-dependent expression of circulating hFVIII at therapeutic levels that ranged from 5% to 230% of normal with doses from 6x1011 to 6x1012 vector genomes per kilogram (vg/kg). These data demonstrate the most potent dose response observed in NHPs thus far. We believe that the potency of our optimized AAV-cDNA hF8 construct, which may enable clinically relevant levels of hFVIII to be expressed using lower vector doses, has the potential to provide a better therapeutic risk/benefit profile for patients than other competing products under development.