Nitisinone (NTBC) In Different Age Groups Of Patients With Alkaptonuria

Overview

Nitisinone is a potent inhibitor of the enzyme that catalyzes the formation of homogentisic acid, and should be an even more logical treatment for alkaptonuria than for tyrosinemia, for which it has been approved by the FDA.The objective of this research is to explore reported age related differences in toxicity of nitisinone and its pharmacokinetic underpinnings and to develop an optimal therapeutic requirement for a targeted population of presymptomatic patients. The additional effect of mixtures of amino acids excluding tyrosine will be explored to take advantage of protein synthesis to avoid elevations of tyrosine that would otherwise limit the optimal dosage of nitisinone. The study is designed to treat patients and find the optimal dosage of nitisinone to obtain maximal reduction in levels of homogentisic acid and maintain safe levels of tyrosine. The long term objective in the target population of pre-symptomatic patients is the prevention of the characteristic effects on joint cartilage and tendons.

Study procedures are designed to: 1. Develop a method for nitisinone measurement via tandem mass spectrometry (MS/MS). 2. Determine whether differences between adult and children could be erased by employing a dosage regimen based on m2 of body surface area as opposed to per kg of body weight, as we have found in a recent study of dichloroacetate 3. Determine optimal dosage for reduction of urinary levels of homogentisic acid and minimal elevation of plasma levels of tyrosine in the target population of pre-symptomatic patients. 4. Determine optimal doses of Tyrex to prevent hypertyrosinemia and allow maximal reduction in homogentisic acid. Baseline studies will include ophthalmologic examinations, echocardiogram, X-rays of all joints, MRI of selected joints when financially plausible; history and physical examinations with emphasis on joints and tendons. Ideally MRI will be repeated at 12 month intervals thereafter. Ophthalmologic exam, x-rays of the kidneys and prostate, and echocardiogram will be repeated approximately every 12 months, depending on the age of the subject and the nitisinone dose. Patients will be seen every 3 months for the first year, then at 6 month intervals to month 36. The timing of the visits may be altered in response to modifications of nitisinone and/or Tyrex doses. Complete or near complete elimination of homogentisic acid excretion will be sought. Optimal NTBC dosage would be judged to yield plasma concentration of tyrosine less than 1000 mmol/L with the concomitant use of tyrosine free amino acid supplement. Dosage will be escalated or reduced dependent on evidence of accumulation of nitisinone and urinary levels of HGA. The maximum initial dose of nitisinone for adult and adolescent patients will be 0.2/mg/kg/day, younger patients may require a larger dose; the standard dose in hypertyrosinemia is 1mg/kg/day, which will be tentatively used as the maximum dose for all populations. Patients will be asked to collect first morning or 12 hour urine collections for homogentisic acid and p-hydroxyphenyllactic acids and to monitor levels of tyrosine. Accumulation will be tested for by repeated studies after 3 months of treatment. Trough levels for nitisinone will be collected prior to and 5-7 days after dose increases and/or at months 6, 12, 18, 24 and every 12 months. The SF-36 Health Status Survey, a two page quality of life questionnaire which asks about ADLs and emotional/social impacts of disease, will be completed by the patient or patient's parent at baseline and every six months. At any point during the study if the plasma tyrosine level is greater than 900umol/L the amount of Tyrex or dietary protein intake may be modified, or the nitisinone dose may be decreased. Protein Saver blood spot cards, which can be done in the patient's home and mailed to our lab, will be given to the patients with instructions on blood collection to check the tyrosine level after 5-7 days of the drug/dietary modifications. These steps-the dietary modification with diet records if needed, addition or adjustment of Tyrex, adjustment of the nitisinone dose, and repeat tyrosine analysis-will be repeated as necessary.