Vasculopathy in patients with Fabry disease: current controversies and research directions

Fabry disease and early stroke

Fabry disease, an X-linked lysosomal storage disorder, results from deficient activity of the enzyme α-galactosidase A. Affected males with the classic phoenotype have acroparaesthesias, hypohidrosis, and corneal opacities in childhood and develop renal failure, cardiac hypertrophy or strokes in the third to fifth decade of life. Some female heterozygotes are asymptomatic, some as severely affected as males. The natural history of Fabry patients includes transitory cerebral ischaemia and strokes, even in very young persons of both genders. The mechanism is partly due to vascular endothelial accumulation of GL-3. White matter lesions on MRI occur. Both males and females can be safely treated with enzyme replacement; and thus screening for Fabry disease of young stroke populations should be considered. There are, however, no hard data of treatment effect on mortality and morbidity. The analyses of results from ongoing studirs will add to the decision on whether or not to screen young stroke patients for Fabry disease. Finally, stroke prophylactic therapy should be used liberally in patients of both genders with verified Fabry disease. This includes primary prevention such as lifestyle counseling, targeting blood pressure, managing atrial fibrillation, diabetes mellitus, hyperlipidaemia, and ASA.

Biomarkers in the diagnosis of lysosomal storage disorders: proteins, lipids, and inhibodies

A biomarker is an analyte indicating the presence of a biological process linked to the clinical manifestations and outcome of a particular disease. In the case of lysosomal storage disorders (LSDs), primary and secondary accumulating metabolites or proteins specifically secreted by storage cells are good candidates for biomarkers. Clinical applications of biomarkers are found in improved diagnosis, monitoring disease progression, and assessing therapeutic correction. These are illustrated by reviewing the discovery and use of biomarkers for Gaucher disease and Fabry disease. In addition, recently developed chemical tools allowing specific visualization of enzymatically active lysosomal glucocerebrosidase are described. Such probes, coined inhibodies, offer entirely new possibilities for more sophisticated molecular diagnosis, enzyme replacement therapy monitoring, and fundamental research.

Limitations of drug registries to evaluate orphan medicinal products for the treatment of lysosomal storage disorders

Orphan drugs are often approved under exceptional circumstances, requiring submission of additional data on safety and effectiveness through registries. These registries are mainly focused on one drug only and data is frequently incomplete. Some registries also address phenotypic heterogeneity and natural history data and publications on these aspects have contributed to the knowledge and awareness of these rare diseases. However, for the assessment of long-term outcomes and for cost-effectiveness, the incompleteness and variable quality of the data raises concerns on the usefulness of these registries. The existing registries for orphan drug treatments for lysosomal storage disorders (LSD's) illustrate these limitations. LSD's are inherited disorders of lysosomal metabolism with a wide variety in clinical symptoms, ranging from severe life-threatening neurological disease to mild or even asymptomatic cases. Their prevalence is extremely low and thus data is scarce and scattered all over Europe. In the past few years, several enzyme replacement therapies and an oral substrate inhibitor have been developed which provide lifelong treatment of LSD's. For Fabry disease, two enzymes were authorized at the same time resulting in two different drug registries being required by the European Medicines Agency (EMA) to monitor effectiveness and safety. This has lead to patient data being divided between two separate registries which may have contributed to delays in the assessment of important outcomes. Three treatments (including a recently approved new enzyme) have now been authorized for Gaucher Disease and two other potential therapies are in the pipeline. Dividing up the data on Gaucher disease patients in to five separate registries benefits nobody. We argue that disease specific (rather than drug specific) registries, supervised by independent clinicians are urgently needed for the best long-term evaluation of treatments of these rare diseases.

Enzyme replacement therapy for Fabry disease: some answers but more questions

Fabry disease (FD) is a multisystem, X-linked disorder of glycosphingolipid metabolism caused by enzyme deficiency of α-galactosidase A. Affected patients have symptoms including acroparesthesias, angiokeratomas, and hypohidrosis. More serious manifestations include debilitating pain and gastrointestinal symptoms, proteinuria and gradual deterioration of renal function leading to end-stage renal disease, hypertrophic cardiomyopathy, and stroke. Heterozygous females may have symptoms as severe as males with the classic phenotype. Before 2001, treatment of patients with FD was supportive. The successful development of enzyme replacement therapy (ERT) has been a great advancement in the treatment of patients with FD and can stabilize renal function and cardiac size, as well as improve pain and quality of life of patients with FD. In this review, we have provided a critical appraisal of the literature on the effects of ERT for FD. This analysis shows that data available on the treatment of FD are often derived from studies which are not controlled, rely on surrogate markers, and are of insufficient power to detect differences on hard clinical endpoints. Further studies of higher quality are needed to answer the questions that remain concerning the efficacy of ERT for FD.

Pharmacological small molecules for the treatment of lysosomal storage disorders

IMPORTANCE OF THE FIELD

Inherited lysosomal storage diseases often cause severe disability and have a devastating effect on quality of life. Enzyme replacement therapy (ERT) forms a cornerstone in the treatment of lysosomal enzyme deficiencies. Although for some lysosomal disorders ERT is lifesaving, important intrinsic restrictions of the approach are limited access of infused enzyme to less accessible body compartments such as the CNS, the burden of frequent intravenous administration, the emergence of antibodies and the high associated costs. Pharmacological small molecules may overcome these limitations.

AREAS COVERED IN THIS REVIEW

Several novel therapeutic approaches using small molecules are emerging: substrate reduction therapy, pharmacological chaperone therapy, premature nonsense mutation suppressors and proteostasis regulators.

WHAT THE READER WILL GAIN

Based on an extensive literature search up until June 2010, we here review the various therapeutic approaches with small compounds, including those currently in clinical use and those that have entered clinical trials. Compounds that are still in the preclinical phase are also briefly discussed.

TAKE HOME MESSAGE

pharmacological small molecules are a new class of agents that show great promise for the treatment of lysosomal storage disorders.