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[Orphan drugs and metabolic disorders].

INTRODUCTION
Over the past twenty years the legal and philosophical concept of orphan diseases has developed to include the diseases with an incidence in the general population of less than 1/5,000. Treatment of these conditions, which is very specific, requires drugs which will be used by a very small number of patients and are therefore not profitable from the financial point of view. This gives rise to the concept of orphan drugs which lack sponsorship, are expensive to investigate and develop, are little used and therefore there is little incentive to market them. All metabolic disorders due to genetic defects may be considered to be orphan diseases , since their incidence in the population is less than 1/5,000 and there may be only a negligible incidence of 1/37,000,000.


DEVELOPMENT
In this study we discuss the treatment of three orphan metabolic diseases, which severely affect the central nervous system by different mechanisms, by three orphan drugs which solve the problems of only a few patients. We describe the treatment of: (1) the deficiency of the synthesis of tetrahydrobiopterin, which causes neurotransmitter deficiency, with tetrahydrobiopterin, (2) N acetylglutamate sythetase deficiency, which causes severe hyperammonaemia and cerebral oedema, with N carbamyl glutamate (3) cystathionine synthetase deficiency which causes hyperhomocyteinaemia and a high risk of thromboembolic accidents, with Betaine.