The Food and Drug Administration (FDA) has approved a reduced dosing frequency for Orfadin (nitisinone; Sobi) in the treatment of hereditary tyrosinemia type 1 (HT-1). Orfadin can now be dosed once daily, rather than twice daily, in patients 5 years of age and older who have undetectable serum and urine succinylacetone concentrations after a minimum of 4 weeks on a stable dosage of nitisinone.

The approval of the once-daily dosing option was based on a clinical study that compared a 4-week once-daily regimen to a 4-week twice daily regimen; both regimens showed comparable blood levels of nitisinone in the 16 HT-1 patients treated. 

Orfadin is a competitive 4-hydroxyphenyl-pyruvate dioxygenase inhibitor that works by blocking the normal catabolism of tyrosine, thus preventing the accumulation of catabolic intermediates maleylacetoacetate and fumarylacetoacetate. In patients with HT-1, these intermediates are converted to toxic metabolites that are responsible for hepatic and renal toxicity.

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Orfadin is available in both capsule (2mg, 5mg, 10mg, 20mg) and oral suspension (4mg/mL) formulations.

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