LAUSR

The pharmacological chaperone migalastat is indicated for the treatment of Fabry disease in patients with an amenable GLA variant. Amenability is determined by an in vitro, good laboratory practice (GLP)-validated assay using HEK293 cells (GLP-HEK assay) performed at a single, highly experienced, GLP-certified laboratory using rigorous standards and extensive analytical validation to limit inter-assay variability [1]. The recent report by Oommen et al. entitled “Inter-assay variability influences migalastat amenability assessments among Fabry disease variants” showed [2], despite technical differences between a non-GLP-validated assay and the GLP-HEK assay, 53 out of the 59 GLA variants tested in the non-GLP assay matched the GLP-HEK amenability classification [1]. Considering the non-GLP assay was done without identical procedures and validated quality standards as in the GLP-HEK assay, differences in results are expected. We noted at least two deviations from the GLP-HEK assay that likely account for the discrepancies reported for 6 variants (Table 1). First, the GLP-HEK assay uses qPCR to directly measure the amount of transfected plasmid DNA for transfection efficiency control [1]. The method employed by Oommen et al., an indirect measurement of co-transfected, secreted embryonic alkaline phosphatase (SEAP), may be inaccurate because overexpression of mutant α-galactosidase A (α-Gal A) can affect trafficking and secretion of SEAP [2]. Second, Oommen et al. used the relative activity (% of wild type) instead of absolute activity (nmol/mg/ h) to calculate the fold-increase in α-Gal A activity in response to migalastat, causing values for 4 variants to narrowly miss the amenability criteria (Table 1). In conclusion, the concern over assay variability seems unfounded, since amenability to migalastat is determined in a single GLP-certified laboratory. We believe physicians can have a high level of confidence in the approved GLP-HEK assay, which identifies GLA variants with the potential to respond to migalastat. Of course, individual response will need to be assessed clinically.