Nitisinone-13C6 isinone-13C6 is intended for use as an internal standard for the quantification of nitisinone by GC- or LC-MS. Nitisinone (T1684) is an inhibitor of 4-hydroxyphenylpyruvate dioxygenase (HPPD),which converts 4-hydroxyphenylpyruvate (HPPA)to homogentisate in the tyrosine catabolic pathway.1Nitisinone increases urinary levels of HPPA and 4-hydroxyphenyllactate (HPLA)in rats when administered at a dose of 10 mg/kg. Nitisinone (T1684) (3 mg/kg)prevents the neonatal lethality of fumarylacetoacetate hydrolase (FAH)deficiency in mice when administered to pregnant dams.2It exhibits hepatoprotective effects inFAH-/-mice,such as prevention of increases in plasma levels of aspartate serine aminotransferase (AST)and conjugated bilirubin,when administration is continued following birth at a dose of 1 mg/kg. Nitisinone (T1684) (100 μg)decreases urinary excretion of homogentisate and increases urinary excretion of HPPA,HPLA,and 4-hydroxyphenylacetate in a mouse model of alkaptonuria induced by ethylnitrosourea.3Formulations containing nitisinone have been used in the treatment of hereditary tyrosinemia type 1 (HT-1).