Nitisinone, 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) is a triketone with herbicidal activity. Orfadin® capsules contain nitisinone used in the treatment of hereditary tyrosinemia type 1 (HT-1). Nitisinone is a competitive inhibitor of 4-hydroxyphenyl-pyruvate dioxygenase, an enzyme upstream of fumarylacetoacetase in the tyrosine catabolic pathway. By inhibiting the normal catabolism of tyrosine in patients with HT-1, nitisinone prevents the accumulation of the catabolic intermediates maleylacetoacetate and fumarylacetoacetate. In patients with HT-1, these catabolic intermediates are converted to the toxic metabolites succinylacetone and succinylacetoacetate, which are responsible for the observed liver and kidney toxicity. Succinylacetone can also inhibit the porphyrin synthesis pathway leading to the accumulation of 5-aminolevulinate, a neurotoxin responsible for the porphyric crises characteristic of HT-1. Zeneca Agrochemicals and Zeneca Pharmaceuticals made NTBC available for clinical use and, with the approval of the Swedish Medical Products Agency, a seriously ill child with an acute form of tyrosinaemia type 1 was successfully treated in February 1991. Nitisinone is investigated as a potential treatment for other disorders of tyrosine metabolism including alkaptonuria.