Alpha-Galactosidase A deficiency (Fabry Disease) is an inherited X-linked disorder resulting in deficient activity of the lysosomal hydrolase, alpha-galactosidase A. In hemizygous males the phenotypic expression of this disorder is manifest by the accumulation of glycosphingolipids containing alpha-galactosyl linkages. These glycosphingolipids include galabiosylceramide and globotriosylceramide, glycolipids which serve as the receptor for shigatoxin. With the exception of galabiosylceramide, the accumulated glycolipids contain glucosylceramide as their base cerebroside. Globotriosylceramide is the primary lipid which accumulates in the tissues of affected patients. The major clinical manifestations of Fabry disease include renal failure, cerebral vascular disease, myocardial infarction, intense pain due to peripheral nervous system involvement, and skin lesions termed angiokeratomas. We propose as the primary hypothesis for these studies that the selective pharmacological inhibition of glucosylceramide synthase will prevent the complications of Fabry disease. The following specific aims are proposed to test this hypothesis. 1. To generate a combinatorial library of D-threo-PDMP congeners by diversifying the aromatic function and tertiary amine of the parent compound. To screen this library for inhibition of glucosylceramide synthase. 2. To test novel glucosylceramide synthase inhibitors for their therapeutic index by comparing the inhibitory activity against glucosylceramide synthase, galactosylceramide synthase, and ceramide transacylase. 3. To evaluate the role of candidate glucosylceramide synthase inhibitors in reversing the phenotype of primary and immortalized cells from alpha-galactosidase knockout mice and from immortalized cells from patients with Fabry disease. 4. To test candidate inhibitors in alpha-galactosidase deficient knockout mice for prevention of the Fabry phenotype.