Identification of Safe and Effective Intravenous Dose of AAVrh.10hFXN to Treat the Cardiac Manifestations of Friedreich's Ataxia.

Overview

abstract

Friedreich's ataxia (FA) is a life-threatening autosomal recessive disorder characterized by neurologic and cardiac dysfunction. Arrythmias and heart failure are the main cause of premature death. From prior studies in murine models of FA, AAV serotype rh.10 expressing human frataxin (AAVrh.10hFXN) effectively treated the cardiac manifestations of the disease. However, the therapeutic dose window is limited by high level of human frataxin (FXN) gene expression associated with toxicity. As a therapeutic goal, since FA heterozygotes have no clinical manifestations of FA, we estimated the level of FXN necessary to convert the heart of a homozygote to that of a heterozygote. In non-cardiac cells, FA heterozygotes have 30-80% of normal FXN levels (17.7-47.2 ng/mg, average 32.5 ng/mg), and FA homozygotes 2-30% normal levels (1.2-17.7 ng/mg, average 9.4 ng/mg). Therefore, an AAV vector would need to augment endogenous in an FA homozygote by >8.3 ng/mg. To determine the required dose of AAVrh.10hFXN, we administered 1.8x1011, 5.7x1011 or 1.8x1012 gc/kg of AAVrh.10hFXN intravenously to MCK mice, a cardiac and skeletal FXN knockout model. The minimally effective dose was 5.7x1011 gc/kg, resulting in cardiac human FXN levels of 6.1±4.2 ng/mg and a mild (p<0.01 compared to PBS controls) improvement in mortality. A dose of 1.8x1012 gc/kg resulted in cardiac human FXN levels of 33.7±6.4 ng/mg, a significant improvement in ejection fraction and fractional shortening (p<0.05, both comparisons), and a 21.5% improvement in mortality (p<0.001). To determine if the significantly effective dose of 1.8x1012 gc/kg could achieve human FA heterozygote levels in a large animal, this dose was administered intravenously to nonhuman primates. After 12 weeks, the vector expressed FXN in the heart was 17.8±4.9 ng/mg, comparable to the target human levels. These data identify both minimally and significantly effective therapeutic doses that are clinically relevant for the treatment of the cardiac manifestations of FA.