Characterization of changes in the tyrosine pathway by 24-h profiling during nitisinone treatment in alkaptonuria

Abstract

Background: Although changes in the tyrosine pathway during nitisinone therapy are known, a complete characterization of the induced tyrosinaemia is lacking to improve disease management.

Patients and methods: Our research aims were addressed by 24-h blood sampling. 40 patients with alkaptonuria (AKU), treated with 0, 1, 2, 4 and 8 mg nitisinone daily (n = 8), were studied over four weeks. Serum homogentisic acid (sHGA), tyrosine (sTYR), phenylalanine (sPHE), hydroxyphenylpyruvate (sHPPA), hydroxyphenyllactate (sHPLA) and nitisinone (sNIT) were measured at baseline and after four weeks.

Results: sNIT showed a clear dose-proportional response. sTYR increased markedly but with less clear-cut dose responses after nitisinone. Fasting and average 24-h (C_av) sTYR responses were similar. Individual patient sTYR 24-h profiles showed significant fluctuations during nitisinone therapy. At week 4, sTYR, sHPPA and sHPPL all showed dose-related increases compared to V0, with the greatest difference between 1 and 8 mg nitisinone seen for HPLA, while there was no change from V0 in sPHE. sHGA decreased to values around the lower limit of quantitation.

Discussion: There was sustained tyrosinaemia after four weeks of nitisinone therapy with significant fluctuations over the day in individual patients. Diet and degree of conversion of HPPA to HPLA may determine extent of nitisinone-induced tyrosinaemia.

Conclusion: A fasting blood sample is recommended to monitor sTYR during nitisinone therapy Adaptations in HPPA metabolites as well as the inhibition of tyrosine aminotransferase could be contributing factors generating tyrosinaemia during nitisinone therapy.

Keywords: 24-h sampling; Alkaptonuria; Homogentisic acid; Hydroxyphenyllactate; Hydroxyphenylpyruvate; Nitisinone; Phenylalanine; Tyrosine.

Fig. 1

Suitability Of Nitisinone In Alkaptonuria 1 (SONIA 1) study design. The study consisted of two main periods: treatment and follow-up. After screening, patients were randomised at baseline (V0) (1:1:1:1:1) to no treatment (control), and oral daily doses of nitisinone of 1 mg, 2 mg, 4 mg and 8 mg. The treatment period consisted of 4 weeks, during which study drug was administered. At 6 weeks a follow-up telephone call concluded the study. Visits are shown as V0, V1, V2 and V3 for screening baseline, two weeks, four weeks and telephone close-out (Abbreviations: S + R = screening, baseline and randomisation visit; F = final treatment visit; T = telephone follow-up visit).

Fig. 2

sNIT 24-h profiles shown at week 4. The legend indicates the various dose groups of nitisinone in SONIA 1 (n = 8 in each dose group). The time points are 0, 0.5, 1, 1.5, 2.5, 3.5, 4.5, 6.5, 8.5, 10.5, 12.5, 15.5, 18.5, and 24.5 h after breaking overnight fast. Breakfast, lunch and dinner were given at 0, 4.5 and 10.5 h in the time course approximately. Error bars are not shown for the purposes of clarity and can be found instead in Tables S2 and S3. Dose 0 mg removed from sNIT figure as values below lower limit of quantification (LLOQ).

Fig. 3

sTYR and sPHE 24-h profiles shown at week 4. The legend indicates the various dose groups of nitisinone in SONIA 1 (n = 8 in each dose group). The time points are 0, 0.5, 1, 1.5, 2.5, 3.5, 4.5, 6.5, 8.5, 10.5, 12.5, 15.5, 18.5, and 24.5 h after breaking overnight fast. Breakfast, lunch and dinner were given at 0, 4.5 and 10.5 h in the time course approximately. Error bars are not shown for the purposes of clarity and can be found instead in Tables S4 and S5.

Fig. 4

Individual patient sTYR 24-h profiles showing the variability between minimum and maximum values at the different doses of nitisinone (0, 1, 2, 4, 8 mg). Each dose had same number of patients (n = 8). The time points are 0, 0.5, 1, 1.5, 2.5, 3.5, 4.5, 6.5, 8.5, 10.5, 12.5, 15.5, 18.5, and 24.5 h after breaking overnight fast. Breakfast, lunch and dinner were given at 0, 4.5 and 10.5 h in the time course approximately. Different scales were used to better indicate the fluctuations in sTYR for the doses shown. Dotted black lines of the C_av for each dose show how this can misinform the fluctuations seen over the day.

Fig. 5

sHPPA and HPLA 24-h profiles shown at week 4. The legend indicates the various dose groups of nitisinone in SONIA 1 (n = 8 in each dose group). The time points are 0, 0.5, 1, 1.5, 2.5, 3.5, 4.5, 6.5, 8.5, 10.5, 12.5, 15.5, 18.5, and 24.5 h after breaking overnight fast. Breakfast, lunch and dinner were given at 0, 4.5 and 10.5 h in the time course approximately. Error bars are not shown for the purposes of clarity and can be found instead in Tables S6 and S7. Dose 0 mg removed as values below lower limit of quantification (LLOQ).