Glucosylceramide synthase inhibition with venglustat in classic fabry disease patients leads to progressive reduction of endothelial cell globotriaosylceramide inclusion volume

Background: Fabry disease (FD) is a rare disorder caused by mutations in the gene for the lysosomal enzyme alpha-galactosidase A (αGal-A). Progressive accumulation of globotriaosylceramide (GL-3) in vascular endothelial and other cell types leads over decades to renal, cardiovascular, and other severe clinical manifestations. In a phase 2 study, glucosylceramide synthase inhibition with venglustat led to reduction in microscopic (LM) scores of lysosomal GL-3 inclusions in skin capillary endothelial cells (EC) after 3 years, although not after 6 months. We applied quantitative unbiased stereological methods to better characterize the effect of venglustat on skin EC GL-3 inclusions. Methods: Skin biopsies were obtained from classic male Fabry disease patients (N = 11) at baseline and during daily treatment with venglustat (NCT02228460, NCT02489344). Images from at least 50 randomly selected superficial skin capillaries per biopsy were obtained using transmission electron microscopy (EM) at 7,500 X magnification. The fraction of the volume (Vv) of EC cytoplasm occupied by GL-3 inclusions [Vv(Inc/Endo)] was estimated using point counting by a masked reader. Twosided paired t tests were used to evaluate changes from baseline to post-treatment values at each time point. Results: Venglustat therapy led to a significant reduction from baseline in Vv(Inc/ Endo) of 0.062 (21.1%; p=0.001) after 6 months and 0.119 (38.7%; p=0.001) after 3 years of treatment. Conclusions: Treatment with venglustat led to reduction in skin capillary GL-3 inclusion fractional volume which was detectable after 6 months using precise quantitative EM methods, but not by LM scoring. This was followed by further reduction over the next 2 1/2 years. We posit that, in the absence of αGal-A activity, inhibition of GL-3 production with venglustat allowed other enzymatic or non-enzymatic mechanisms to progressively reduce EC lysosomal GL-3 content. Long-term venglustat therapy may therefore prevent or reverse progressive tissue injury in Fabry disease.