Lysosomal storage diseases arise from a genetic loss-of-function defect in 
enzymes mediating key catabolic steps resulting in accumulation of substrate 
within the lysosome. Treatment of several of these disorders has been achieved 
by enzyme replacement therapy (ERT), in which a recombinant version of the 
defective enzyme is expressed in vitro and administered by infusion. However, in 
many cases the biodistribution of the administered protein does not match that 
of the accumulated substrate due to the glycosylation-mediated clearance of the 
enzymes from circulation, resulting in poor or absent substrate clearance from 
some tissues. To overcome this limitation, we have evaluated several 
peptide-based targeting motifs to redirect recombinant human alpha-galactosidase 
(rhalphaGal) to specific receptors. A reversible thiol-based PEGylation 
chemistry was developed to achieve multivalent peptide display with lysosomal 
release. In vitro, cell uptake was peptide dependent and independent of the 
normal mannose-6-phosphate receptor mediated pathway. Surprisingly, despite 
increased plasma half-life and decreased liver uptake, none of the peptide 
conjugates showed significantly altered biodistribution in alphaGal-knockout 
mice. This suggests that these peptide-based targeting motifs are unlikely to 
provide substantial therapeutic benefit likely due to the complexity of factors 
affecting PK and biodistribution.