Determining the Minimally Effective Dose of a Clinical Candidate AAV Vector in a Mouse Model of Hemophilia A.

Abstract

Hemophilia A, a bleeding disorder, affects 1:5,000 males and is caused by deficient human blood coagulation factor VIII (hFVIII). Studies in mice and macaques identified AAVhu37.E03.TTR.hFVIIIco-SQ.PA75 as a clinical candidate gene therapy vector to treat hemophilia A. Here, we sought to determine the minimally effective dose of this vector in a hemophilia A mouse model. Mice received one of four vector doses (3x1011 - 1x1013 GC/kg) via intravenous tail vein injection; one cohort received vehicle as a control. Animals were monitored daily after vector/vehicle administration. Blood samples were collected to evaluate hFVIII activity levels and anti-hFVIII antibodies. Animals were sacrificed and necropsied on days 28 and 56; tissues were harvested for histopathological examination and blood was collected for a serum chemistry panel. We found no significant differences in liver transaminases in mice administered any vector dose compared to mice administered vehicle (except for one group administered 3x1011 GC/kg). Total bilirubin levels were significantly elevated compared to the vehicle group following two vector doses at day 56 (1x1012 and 1x1013 GC/kg). We observed no vector-related gross or histological findings. Most microscopic findings were in the vehicle group and considered secondary to blood loss, an expected phenotype of this mouse model. Since we observed no dose-limiting safety markers, we determined the maximally tolerated dose was greater than or equal to the highest dose tested (1x1013 GC/kg). Since we detected hFVIII activity in all cohorts administered vector, we conclude the minimally effective dose is 3x1011 GC/kg-the lowest dose evaluated in this study.