Aims NTBC (2‐(2‐nitro‐4‐fluoromethylbenzoyl)‐1,3‐cyclohexanedione) and mesotrione (2‐(4‐methylsulphonyl‐2‐nitrobenzoyl)‐1,3‐cyclohexanedione) are inhibitors of 4‐hydroxyphenyl pyruvate dioxygenase (HPPD). NTBC has been successfully used as a treatment for hereditary tyrosinaemia type 1 (HT‐1), while mesotrione has been developed as an herbicide. The pharmacokinetics of the two compounds were investigated in healthy male volunteers following single oral administration. The aim of the NTBC study was to assess the bioequivalence of two different formulations and to determine the extent of the induced tyrosinaemia. The mesotrione study was performed to determine the magnitude and duration of the effect on tyrosine catabolism. Additionally, the urinary excretion of unchanged mesotrione was measured to assess the importance of this route of clearance and to help develop a strategy for monitoring occupational exposure.

Methods A total of 28 volunteers participated in two separate studies with the compounds. In the first study, the relative bioavailability of NTBC from liquid and capsule formulations was compared and the effect on plasma tyrosine concentrations measured. In the second study the pharmacokinetics of mesotrione were determined at three doses. Plasma tyrosine concentrations were monitored and the urinary excretion of mesotrione and tyrosine metabolites was measured.

Results Both compounds were well tolerated at the dose levels studied. Peak plasma concentrations of NTBC were rapidly attained following a single oral dose of 1 mg kg⁻¹ body weight of either formulation and the half‐life in plasma was approximately 54 h. There were no statistical differences in mean (± s.d.) AUC(0,∞) (capsule 602 ± 154 vs solution 602 ± 146 µg ml⁻¹ h) or t_½ (capsule 55 ± 13 vs solution 54 ± 8 h) and these parameters supported the bioequivalence of the two formulations. Mesotrione was also rapidly absorbed, with a significant proportion of the dose eliminated unchanged in urine. The plasma half‐life was approximately 1 h and was independent of dose and AUC(0,∞) and C_max increased linearly with dose. Following administration of 1 mg NTBC kg⁻¹ in either formulation, the concentrations of tyrosine in plasma increased to approximately 1100 nmol ml⁻¹. Concentrations were still approximately 8 times those of background at 14 days after dosing, but had returned to background levels within 2 months of the second dose. Administration of mesotrione resulted in an increase in tyrosine concentrations which reached a maximum of approximately 300 nmol ml⁻¹ following a dose of 4 mg kg⁻¹ body weight. Concentrations returned to those of background within 2 days of dosing. Urinary excretion of tyrosine metabolites was increased during the 24 h immediately following a dose of 4 mg mesotrione kg⁻¹, but returned to background levels during the following 24 h period.

Conclusions NTBC and mesotrione are both inhibitors of HPPD, although the magnitude and duration of their effect on tyrosine concentrations are very different. When normalized for dose, the extent of the induced tyrosinaemia after administration of NTBC and over the duration of these studies, was approximately 400 fold greater than that following administration of mesotrione. The persistent and significant effect on HPPD following administration of NTBC make it suitable for the treatment of patients with hereditary tyrosinaemia type 1 (HT‐1), whilst the minimal and transient effects of mesotrione minimize the likelihood of a clinical effect in the event of systemic exposure …