Nitisinone

Nitisinone is a competitive inhibitor of 4-hydroxyphenylpyruvate dioxygenase, the second step in the tyrosine metabolism. By inhibiting the normal catabolism of tyrosine in patients with HT-1 and AKU, nitisinone prevents the accumulation of harmful metabolites downstream of 4-hydroxyphenylpyruvate dioxygenase.

The biochemical defect in HT-1 is a deficiency of fumarylacetoacetate hydrolase, which is the final enzyme of the tyrosine catabolic pathway. Nitisinone prevents the accumulation of the toxic intermediates maleylacetoacetate and fumarylacetoacetate. These intermediates are otherwise converted to the toxic metabolites succinylacetone and succinylacetoacetate. Succinylacetone inhibits the porphyrin synthesis pathway leading to the accumulation of 5-aminolevulinate.

The biochemical defect in AKU is a deficiency of homogentisate 1,2 dioxygenase, the third enzyme of the tyrosine catabolic pathway. Nitisinone prevents the accumulation of the harmful metabolite homogentisic acid (HGA), which otherwise leads to ochronosis of joints and cartilage and thereby the development of the clinical features of the disease.

In patients with HT-1, nitisinone treatment leads to normalised porphyrin metabolism with normal erythrocyte porphobilinogen synthase activity and urine 5-aminolevulinate, decreased urinary excretion of succinylacetone, increased plasma tyrosine concentration and increased urinary excretion of phenolic acids. Available data from a clinical study indicates that in more than 90% of the patients urine succinylacetone was normalised during the first week of treatment. Succinylacetone should not be detectable in urine or plasma when the nitisinone dose is properly adjusted.

In patients with AKU, nitisinone treatment reduces the accumulation of HGA. Available data from a clinical study shows a 99.7% reduction of urinary HGA, and a 98.8% reduction of serum HGA, following nitisinone treatment compared to untreated control patients after 12 months of treatment.

Formal absorption, distribution, metabolism and elimination studies have not been performed with nitisinone. In 23 healthy volunteers, after administration of a single dose of nitisinone tablets (10 mg) the terminal half-life (median) of nitisinone in plasma was 59 hours (ranging from 41 to 74 hours).

A population pharmacokinetic analysis has been conducted on a group of 207 HT-1 patients. The clearance and half-life were determined to be 0.0956 l/kg body weight/day and 52.1 hours respectively.

in vitro studies using human liver microsomes and cDNA-expressed P450 enzymes have shown limited CYP3A4-mediated metabolism.

Based on data from a clinical interaction study with 80 mg nitisinone at steady-state, nitisinone caused a 2.3-fold increase in AUC_∞ of the CYP2C9 substrate tolbutamide, which is indicative of a moderate inhibition of CYP2C9. Nitisinone caused an approximate 30% decrease in chlorzoxazone AUC_∞, indicative of a weak induction of CYP2E1. Nitisinone does not inhibit CYP2D6 since metoprolol AUC_∞ was not affected by the administration of nitisinone. Furosemide AUC_∞ was increased 1.7-fold, indicating a weak inhibition of OAT1/OAT3.

Based on in vitro studies, nitisinone is not expected to inhibit CYP1A2, 2C19 or 3A4-mediated metabolism or to induce CYP1A2, 2B6 or 3A4/5. Nitisinone is not expected to inhibit P-gp, BCRP or OCT2-mediated transport. Nitisinone plasma concentration reached in clinical setting is not expected to inhibit OATP1B1, OATP1B3 mediated transport.

Nitisinone has shown embryo-foetal toxicity in the mouse and rabbit at clinically relevant dose levels. In the rabbit, nitisinone induced a dose-related increase in malformations (umbilical hernia and gastroschisis) from a dose level 2.5-fold higher than the maximum recommended human dose (2 mg/kg/day).

A pre- and postnatal development study in the mouse showed statistically significantly reduced pup survival and pup growth during the weaning period at dose levels 125- and 25-fold higher, respectively, than the maximum recommended human dose, with a trend toward a negative effect on pup survival starting from the dose of 5 mg/kg/day. In rats, exposure via milk resulted in reduced mean pup weight and corneal lesions.

No mutagenic but a weak clastogenic activity was observed in in vitro studies. There was no evidence of in vivo genotoxicity (mouse micronucleus assay and mouse liver unscheduled DNA synthesis assay). Nitisinone did not show carcinogenic potential in a 26-week carcinogenicity study in transgenic mice (TgrasH2).