Evaluation of a casein glycomacropeptide-based protein substitute, in the dietary management of NTBC-induced tyrosinaemia in patients with alkaptonuria: A prospective open-label study.

BACKGROUND: 2-(2-Nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) treatment of alkaptonuria (AKU) leads to increased blood tyrosine levels, causing skin issues and potentially sight-threatening corneal keratopathy. Adherence to dietary management of NTBC-induced tyrosinemia, a low-protein diet with or without protein substitutes, can be difficult for patients. This 28-day interventional study evaluated a low tyrosine casein glycomacropeptide (cGMP) protein substitute (TYR sphere)®, a 20 g protein equivalent, cGMP-based protein substitute, in terms of adherence, palatability, usability, comparison to amino acid (AA)-based protein substitutes, gastrointestinal tolerance and metabolic control in adults with NTBC-induced tyrosinaemia. METHODS: Four adults (mean 61.1 years, range 53.3-69.3 years) with AKU and NTBC-induced tyrosinaemia were recruited from the United Kingdom National Alkaptonuria Centre (NAC). The cGMP protein substitute was prescribed based on individual nutritional requirements, replacing ≥1 AA-based protein substitute. Participants recorded product-related data in study diaries, using five-point Likert scales and daily and weekly logs. To determine metabolic control, prestudy blood tyrosine levels were compared to weekly blood spot tests during the study. RESULTS: Median cGMP protein substitute adherence was 98%. Most participants rated palatability and usability positively, and preferred cGMP protein substitute to AA-based products. There were no notable gastrointestinal changes, and metabolic control was maintained. CONCLUSIONS: cGMP protein substitute is a palatable and well-tolerated option in the dietary management of AKU patients with NTBC-induced tyrosinaemia.

Temporal adaptations in the phenylalanine/tyrosine pathway and related factors during nitisinone-induced tyrosinaemia in alkaptonuria.

BACKGROUND: Adaptations within the phenylalanine (PHE)/tyrosine (TYR) pathway during nitisinone (NIT) are not fully understood. OBJECTIVE: To characterise the temporal changes in metabolic features in NIT-treated patients with alkaptonuria. PATIENTS AND METHODS: Serum (s) and 24-urine (u) homogentisic acid (sHGA, uHGA24), TYR (sTYR, uTYR24), PHE (sPHE, uPHE24), hydroxyphenylpyruvate (sHPPA, uHPPA24), hydroxyphenyllactate (sHPLA, uHPLA24) and sNIT were measured at baseline (V1) and until month 48 (V6) in 69 NIT-treated patients, recommended to reduce protein intake. The 24-h urine urea (uUREA24), creatinine (uCREAT24) and body weight were also measured. Amounts of tyrosine metabolites in total body water (TBW) were derived by multiplying the serum concentrations by 60% body weight, and sum of TBW and urine metabolites resulted in combined values (c). RESULTS: uUREA24 and uCREAT24 decreased between V1 and V6 during NIT, whereas body weight and sNIT increased. Linear regression coefficient between uUREA24 and uCREAT24 was extremely strong (R = 0.84). sPHE, TBWPHE and cPHE24 increased gradually from V1 to V6. A decrease in cTYR24/cPHE24, sTYR/sPHE and TBWTYR/TBWPHE was seen from V2 to V6. Serum, 24-urine and combined TYR, HPPA and HPLA either remained stable or decreased from V2 to V6. DISCUSSION: The gradual increase in PHE suggests adaptation to increasing TYR during NIT therapy. The decrease in protein intake resulted in decreased muscle mass and increased weight gain. CONCLUSION: Progressive adaptation by decreasing PHE conversion to TYR occurs over time during NIT therapy. A low protein diet results in loss of muscle mass but also weight gain suggesting an increase in fat mass.

Nitisinone causes acquired tyrosinosis in alkaptonuria.

For over two decades, nitisinone (NTBC) has been successfully used to manipulate the tyrosine degradation pathway and save the lives of many children with hereditary tyrosinaemia type 1. More recently, NTBC has been used to halt homogentisic acid accumulation in alkaptonuria (AKU) with evidence suggesting its efficacy as a disease modifying agent. NTBC-induced hypertyrosinaemia has been associated with cognitive impairment and potentially sight-threatening keratopathy. In the context of a non-lethal condition (ie, AKU), these serious risks call for an evaluation of the wider impact of NTBC on the tyrosine pathway. We hypothesised that NTBC increases the tyrosine pool size and concentrations in tissues. In AKU mice tyrosine concentrations of tissue homogenates were measured before and after treatment with NTBC. In humans, pulse injection with l-[13 C9 ]tyrosine and l-[d8 ]phenylalanine was used along with compartmental modelling to estimate the size of tyrosine pools before and after treatment with NTBC. We found that NTBC increased tyrosine concentrations in murine tissues by five to nine folds. It also significantly increased the tyrosine pool size in humans (P < .001), suggesting that NTBC increases tyrosine not just in serum but also in tissues (ie, acquired tyrosinosis). This study provides, for the first time, the experimental proof for the magnitude of NTBC-related acquired tyrosinosis which should be overcome to ensure the safe use of NTBC in AKU.

From Weed Killer to Wonder Drug.

The discovery that a natural product leptospermone had herbicidal activity formed the starting point for chemical synthesis to find more activity and selectivity. A series of molecules called triketones were found to possess good activity and 2-(2-nitro-4-trifluoromethylbenzoyl)-cyclohexane-1,3-dione (NTBC) was selected for toxicology testing. NTBC fed at low doses to rats and dogs caused keratopathy, which on cessation of the diet recovered. Mice, rabbits and monkeys fed NTBC did not show this response. Research discovered that NTBC caused tyrosinaemia which was due to inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase in both mammals and plants thereby finding a novel target for killing plants. NTBC was also used sucessfully as a drug to treat a rare inborn error of metabolism, tyrosinaemia type I, in collaboration with Professor's Sven Lindstedt and Elisabeth Holme. Understanding the mechanism of toxicity of NTBC led to novel herbicide discovery and saved the lives of children with acute tyrosinaemia type I.

Diagnosing Hepatorenal Tyrosinaemia in Europe: Newborn Mass Screening Versus Selective Screening.

Hepatorenal tyrosinaemia (HT1) is a serious condition that used to be fatal before the advent of nitisinone (NTBC, Orfadine®) as a therapeutic option. We have recently shown that selective screening is inadequate as initial symptoms are often uncharacteristic which leads to a considerable delay in diagnosis and treatment. This has a negative impact on morbidity and mortality as well as long-term outcome. For example, the odds ratio to develop hepatocellular carcinoma is 12.7 when treatment is initiated after the first birthday compared to start of treatment in the neonatal period. Timely diagnosis is only possible when neonatal mass screening is operational. HT1 meets all the criteria for neonatal mass screening at a clinical and analytical level. The natural course of the disease is well known, clinically there is a latent phase in most patients when presymptomatic treatment can be initiated. There are no mild phenotypes which do not require treatment. Using succinylacetone as the screening parameter a highly specific and sensitive test is available with acceptable financial burden. Neonatal mass screening for HT1 is acceptable to the target population as it can be performed simultaneously with the already existing screening tests in dried blood, there are no false negative and false positive cases and the financial burden to the health system is moderate. An efficient treatment is available with nitisinone and protein-reduced diet supplemented with special amino acid mixtures. Despite compelling evidence in favour of a neonatal mass screening for HT1 only 57% of European centres taking part in our recent cross-sectional study have included HT1 in their newborn screening programme.

Childhood hepatocellular carcinoma: a clinicopathological study of 12 cases with special reference to EpCAM.

AIMS: To elucidate the characteristics of hepatocellular carcinoma (HCC) in children. METHODS AND RESULTS: A retrospective search of our database identified 12 children with HCC (aged 10 months to 11 years; male/female ratio of 5:7). Their pathological features were compared with those of adult HCCs (n = 20), fibrolamellar HCCs (n = 14), and hepatoblastomas (n = 15). All childhood HCCs developed on a background of cirrhosis resulting from tyrosinaemia type 1 (n = 4), bile salt export transporter deficiency (n = 4), biliary atresia (n = 3), and long-standing total parenteral nutrition (n = 1). HCCs in cases of tyrosinaemia type 1 always had clear cell changes, solid architecture, and only mild nuclear atypia, whereas the morphological features of HCCs in the other conditions were basically similar to those of adult HCCs. On immunostaining, all cases of childhood HCC were positive for epithelial cell adhesion molecule (EpCAM); expression was diffuse (>50% of cancer cells) in 11 cases, and particularly strong in six children, all aged <3 years. In contrast, EpCAM was only focally expressed in three cases of adult HCC (15%). EpCAM was also expressed in most fibrolamellar HCCs and hepatoblastomas, but these two neoplasms differed from childhood HCCs in the expression of CK7, ß-catenin, and p53. CONCLUSIONS: The diffuse expression of EpCAM characterizes childhood HCC, and may indicate immaturity of neoplastic cells.

Anthropometric, Body Composition, and Nutritional Indicators with and without Nutritional Intervention during Nitisinone Therapy in Alkaptonuria.

INTRODUCTION: Protein nutrition disorder in alkaptonuria (AKU), resulting in increased homogentisic acid (HGA) before nitisinone therapy and increased tyrosine (TYR) during nitisinone therapy, may benefit from dietetic intervention. The aim of this study was to characterise the diet and their effects prospectively in those who received formal dietetic intervention in the nitisinone-receiving National Alkaptonuria Centre (NAC) patients with those who did not in no-nitisinone Suitability of Nitisinone in Alkaptonuria 2 (SN2 N-) and nitisinone-treated SN2 (SN2 N+) randomised study groups. PATIENTS AND METHODS: A total of 63, 69, and 69 AKU patients from the NAC, SN2 N-, and SN2 N+ were studied for anthropometric (weight, BMI), body composition (including muscle mass, %body fat, hand grip strength), chemical characteristics (serum TYR, serum phenylalanine, urine urea or uUREA, and urine creatinine or uCREAT), and corneal keratopathy. Nitisinone 2 mg and 10 mg were employed in the NAC and SN2 N+ groups, respectively. Dieticians managed protein intake in the NAC, while the SN2 N- and SN2 N+ groups only received advice on self-directed protein restriction during four years of study duration. RESULTS: uUREA decreased in the NAC, SN2 N-, and SN2 N+ groups, showing that protein restriction was achieved in these groups. Body weight and BMI increased in the NAC and SN2 N+ groups. uCREAT decreased significantly in SN2 N- and SN2 N+ compared with the NAC over four years of study. Corneal keratopathy was less frequent in the NAC than in the SN2 N+ group. Active dietetic intervention in NAC stabilised lean body mass (muscle mass, hand grip strength) despite a decrease in uUREA and uCREAT, as well as sTYR. CONCLUSION: Ongoing dietetic intervention prevented loss of lean body mass despite protein restriction and moderated serum tyrosine increase, leading to less prevalent corneal keratopathy. Protein restriction risks fat mass gain.

Emotional and behavioural functioning in children with tyrosinaemia type 1.

INTRODUCTION: Tyrosinaemia type I is a rare hereditary metabolic disease caused by deficiency of the enzyme involved in the breakdown of tyrosine. Since the use of nitisinone in addition to diet in 1992, survival rates have increased significantly, but more and more socio-emotional problems have become apparent. The aim of the study was the assessment the relationship between variations in serum tyrosine and phenylalanine levels and measurements of socio-emotional functioning and determination of patients' IQs. THE AIM OF THE STUDY: was the assessment the relationship between variations in serum tyrosine and phenylalanine levels and measurements of socio-emotional functioning and determination of patients' IQs. MATERIAL AND METHODS: Twelve children were studied, from a single centre, born between 1994 and 2012, treated with nitisinone and a low-phenylalanine and -tyrosine diet. The psychological evaluation was conducted using the parent form of the Child Behaviour Checklist (CBCL)/4-18. Additionally, the patients' IQs were measured using the Stanford-Binet 5 (SB5) Intelligence Scale. Statistical analyses were performed using PAWS software suite version 26. We found that phenylalanine variability over time correlated with measures of emotional and behavioural functioning. This relationship holds true for externalising behaviour, associated with the experience of maladjustment and aggression. Total score intellectual and cognitive function was within the norm for all patients. CONCLUSIONS: To maintain better quality of life for patients and their families in terms of emotional and behavioural functioning, it may be important to avoid spikes (significant fluctuations) in phenylalanine levels. Regular, detailed psychological evaluations are recommended to detect potential problems and implement interventions aimed at achieving the best possible individual development and realise the intellectual and behavioural potential, thereby improving the patient's and her family's quality of life.

Hereditary tyrosinaemia type 1 in the absence of succinylacetone: 4-oxo 6-hydroxyhepanoate (4OHHA), a putative diagnostic biomarker.

Hereditary tyrosinemia type 1 (HT1) is a rare metabolic disease resulting in acute liver failure in early infancy, hypophosphataemic rickets, neurological crises, liver cirrhosis and risk of hepatocellular carcinoma later on in life. It is caused by the deficiency of the enzyme fumarylacetoacetate hydrolase which is involved in the terminal step of the catabolic pathway of tyrosine. Diagnosis is made through clinical suspicion supported by biochemical abnormalities that result from accumulation of upstream metabolites. Detection of succinylacetone (SA) in dried blood spot or urine remains pathognomonic, however it is not always detectable. Here we describe three cases of HT1 presenting with atypical biochemistry, where SA was not always detectable, highlighting the importance of an additional disease biomarker, 4-oxo-6-hydroxyheptanoate.

Determination of the Protein and Amino Acid Content of Fruit, Vegetables and Starchy Roots for Use in Inherited Metabolic Disorders.

Amino acid (AA)-related inherited metabolic disorders (IMDs) and urea cycle disorders (UCDs) require strict dietary management including foods low in protein such as fruits, vegetables and starchy roots. Despite this recommendation, there are limited data on the AA content of many of these foods. The aim of this study is to describe an analysis of the protein and AA content of a range of fruits, vegetables and starchy roots, specifically focusing on amino acids (AAs) relevant to AA-related IMDs such as phenylalanine (Phe), methionine (Met), leucine (Leu), lysine (Lys) and tyrosine (Tyr). AA analysis was performed using high-performance liquid chromatography (HPLC) on 165 food samples. Protein analysis was also carried out using the Dumas method. Foods were classified as either 'Fruits', 'Dried fruits', 'Cruciferous vegetables', 'Legumes', 'Other vegetables' or 'Starchy roots'. 'Dried fruits' and 'Legumes' had the highest median values of protein, while 'Fruits' and 'Cruciferous vegetables' contained the lowest median results. 'Legumes' contained the highest and 'Fruits' had the lowest median values for all five AAs. Variations were seen in AA content for individual foods. The results presented in this study provide useful data on the protein and AA content of fruits, vegetables and starchy roots which can be used in clinical practice. This further expansion of the current literature will help to improve diet quality and metabolic control among individuals with AA-related IMDs and UCDs.

The Discovery of the Mode of Action of Nitisinone.

This review briefly discusses the discovery of the mode of action of the triketone herbicide, 2-(2-nitro-4-trifluormethylbenzoyl)-1,3-cyclohexanedione and its use as a drug Nitisinone for the treatment of inborn errors of tyrosine metabolism. Nitisinone is a potent reversible tight-binding inhibitor of the enzyme 4-hydroxyphenylpyruvate dioxygenase, involved in the catabolism of the amino acid tyrosine. Nitisinone is used to treat the rare disease hereditary tyrosinaemia type 1 where the last enzyme in the breakdown of tyrosine, fumarylacetoacetase is deficient. Nitisinone is also used to treat patients with alkaptonuria where the enzyme homogentisic acid oxidase is deficient. Articles in this issue discuss metabolites of tyrosine catabolism in healthy patients and those with alkaptonuria.

Comparing the Phenylalanine/Tyrosine Pathway and Related Factors between Keratopathy and No-Keratopathy Groups as Well as between Genders in Alkaptonuria during Nitisinone Treatment.

Nitisinone (NIT) causes tyrosinaemia and corneal keratopathy (KP), especially in men. However, the adaptation within the phenylalanine (PHE)/tyrosine (TYR) catabolic pathway during KP is not understood. The objective of this study is to assess potential differences in the PHE/TYR pathway during KP and the influence of gender in NIT-induced tyrosinaemia in alkaptonuria (AKU). Samples of serum and 24 h urine collected from patients treated with NIT during a 4-year randomized study in NIT vs. no-treatment controls (SONIA 2; Suitability Of Nitisinone In Alkaptonuria 2; EudraCT no. 2013-001633-41) at months 3 (V2), 12 (V3), 24 (V4), 36 (V5) and 48 (V6) were included in these analyses. Homogentisic acid (HGA), TYR, PHE, hydroxyphenylpyruvate (HPPA), hydroxyphenyllactate (HPLA) and sNIT were analysed at all time-points in serum and urine in the NIT-group. All statistical analyses were post hoc. Keratopathy occurred in 10 out of 69 AKU patients, eight of them male. Thirty-five sampling points (serum and 24 h urine) were analysed in patients experiencing KP and 272 in those with no-KP (NKP) during NIT therapy. The KP group had a lower HPLA/TYR ratio and a higher TYR/PHE ratio compared with the NKP group (p < 0.05 for both). There were 24, 45, 100 and 207 sampling points (serum and 24 h urine) in the NIT group which were pre-NIT female, pre-NIT male, NIT female and NIT male, respectively. The PHE/TYR ratio and the HPLA/TYR ratio were lower in males (p < 0.001 and p < 0.01, respectively). In the KP group and in the male group during NIT therapy, adaptive responses to minimise TYR formation were impaired compared to NKP group and females, respectively.

UK Dietary Practices for Tyrosinaemias: Time for Change.

In the UK, different dietary systems are used to calculate protein or tyrosine/phenylalanine intake in the dietary management of hereditary tyrosinaemia, HTI, II and III (HT), with no systematic evidence comparing the merits and inadequacies of each. This study aimed to examine the current UK dietary practices in all HTs and, using Delphi methodology, to reach consensus agreement about the best dietary management system. Over 12 months, five meetings were held with UK paediatric and adult dietitians working in inherited metabolic disorders (IMDs) managing HTs. Eleven statements on the dietary system for calculating protein or tyrosine/phenylalanine intake were discussed. Dietitians from 12 of 14 IMD centres caring for HT patients participated, and 7/11 statements were agreed with one Delphi round. Nine centres (three abstentions) supported a 1 g protein exchange system for all foods except fruit and vegetables. The same definitions used in the UK for phenylketonuria (PKU) were adopted to define when to calculate foods as part of a protein exchange system or permit them without measurement. Fruit and vegetables contain a lower amount of tyrosine/phenylalanine per 1 g of protein than animal and cereal foods. The correlation of tyrosine vs. phenylalanine (mg/100 g) for vegetables and fruits was high (r = 0.9). In Delphi round 2, agreement was reached to use the tyrosine/phenylalanine analyses of fruits/vegetables, for their allocation within the HT diet. This allowed larger portion sizes of measured fruits and vegetables and increased the variety of fruit and vegetables that could be eaten without measurement. In HTs, a combined dietary management system will be used: 1 g protein exchanges for cereal and milk protein sources and tyrosine/phenylalanine exchanges for fruit and vegetables. Intensive, systematic communication with IMD dietitians and reappraisal of the evidence has redefined and harmonised HT dietary practice across the UK.

Physical Growth of Patients with Hereditary Tyrosinaemia Type I: A Single-Centre Retrospective Study.

In a retrospective review, we aimed to assess long-term growth in 17 patients (n = 11 males) with hereditary tyrosinaemia type I (HTI). Median age at assessment was 15.6 years (5.7-26.6 years) and median age at diagnosis was 1 month (range: 0-16 months), with 35% (n = 6/17) symptomatic on presentation. From the age of 8 years, there was a noticeable change in median height, weight, and body-mass-index [BMI]-z-scores. Median height-for-age z-scores were consistently ≤ -1 (IQR -1.6, -0.5) during the first 8 years of life but increased with age. Weight-for-age z-scores ranged between -1 to 0 (IQR -1.2, 0.1) in the first 8 years; then increased to > 0.5 (IQR -0.3, 1.3) by age 16 years, and BMI-for-age z-scores ranged from 0 to 1 (IQR -0.7, 1.3) up to 8 years, and >1 (IQR -0.2, 1.9) until 16 years. The percentage of overweight and obesity was lowest in children aged < 5 years, and consistently > 40% in patients aged between 7 to 16 years. The prescribed total protein intake was associated with improved height growth (p < 0.01). Impaired growth in early life improved with age achieving normal population standards. Further studies are needed to investigate factors that influence growth outcome in HTI patients.

Natural Protein Tolerance and Metabolic Control in Patients with Hereditary Tyrosinaemia Type 1.

In a longitudinal retrospective study, we aimed to assess natural protein (NP) tolerance and metabolic control in a cohort of 20 Hereditary Tyrosinaemia type I (HTI) patients. Their median age was 12 years ([3.2-17.7 years], n = 11 female, n = 8 Caucasian, n = 8 Asian origin, n = 2 Arabic and n = 2 Indian). All were on nitisinone (NTBC) with a median dose of 0.7 g/kg/day (range 0.4-1.5 g/kg/day) and were prescribed a tyrosine (Tyr)/phenylalanine (Phe)-restricted diet supplemented with Tyr/Phe-free L-amino acids. Data were collected on clinical signs at presentation, medical history, annual dietary prescriptions, and blood Phe and Tyr levels from diagnosis until transition to the adult service (aged 16-18 years) or liver transplantation (if it preceded transition). The median age of diagnosis was 2 months (range: 0 to 24 months), with n = 1 diagnosed by newborn screening, n = 3 following phenylketonuria (PKU) screening and n = 7 by sibling screening. Five patients were transplanted (median age 6.3 years), and one died due to liver cancer. The median follow-up was 10 years (3-16 years), and daily prescribed NP intake increased from a median of 5 to 24 g/day. Lifetime median blood Tyr (370 µmol/L, range 280-420 µmol/L) and Phe (50 µmol/L, 45-70 µmol/L) were maintained within the target recommended ranges. This cohort of HTI patients were able to increase the daily NP intake with age while maintaining good metabolic control. Extra NP may improve lifelong adherence to the diet.

Hepatorenal Tyrosinaemia: Impact of a Simplified Diet on Metabolic Control and Clinical Outcome.

Background: Tyrosinaemia type 1 is a rare inherited metabolic disease caused by an enzyme defect in the tyrosine degradation pathway. It is treated using nitisinone and a low-protein diet. In a workshop in 2013, a group of nutritional specialists from Germany, Switzerland and Austria agreed to advocate a simplified low-protein diet and to allow more natural protein intake in patients with tyrosinaemia type 1. This retrospective study evaluates the recommendations made at different treatment centers and their impact on clinical symptoms and metabolic control. Methods: For this multicenter study, questionnaires were sent to nine participating treatment centers to collect data on the general therapeutic approach and data of 47 individual patients treated by those centers. Results: Dietary simplification allocating food to 3 categories led to increased tyrosine and phenylalanine blood concentrations without weighing food. Phenylalanine levels were significantly higher in comparison to a strict dietary regimen whereas tyrosine levels in plasma did not change. Non-inferiority was shown for the simplification and liberalization of the diet. Compliance with dietary recommendations was higher using the simplified diet in comparison to the stricter approach. Age correlates negatively with compliance. Conclusions: Simplification of the diet with increased natural protein intake based on three categories of food may be implemented in the diet of patients with tyrosinaemia type 1 without significantly altering metabolic control. Patient compliance is strongly influencing tyrosine blood concentrations. A subsequent prospective study with a larger sample size is necessary to get a better insight into the effect of dietary recommendations on metabolic control.

Predicting tyrosinaemia: a mathematical model of 4-hydroxyphenylpyruvate dioxygenase inhibition by nitisinone in rats.

Nitisinone or 2-(2-nitro-4-trifluoromethylbenzoyl)cyclohexane-1,3-dione is a reversible inhibitor of 4-hydroxyphenylpyruvate dioxygenase (HPPD), an enzyme important in tyrosine catabolism. Today, nitisinone is successfully used to treat Hereditary Tyrosinaemia type 1, although its original expected role was as a herbicide. In laboratory animals, treatment with nitisinone leads to the elevation of plasma tyrosine (tyrosinaemia). In rats and Beagle dogs, repeat low-dose exposure to nitisinone leads to corneal opacities whilst similar studies in the mouse and Rhesus monkey showed no comparable toxicities or other treatment related findings. The differences in toxicological sensitivities have been related to the upper limit of the concentration of tyrosine that accumulates in plasma, which is driven by the amount/activity of tyrosine aminotransferase. A physiologically based, pharmacodynamics ordinary differential equation model of HPPD inhibition to bolus exposure of nitisinone in vivo is presented. Going beyond traditional approaches, asymptotic analysis is used to separate the different timescales of events involved in HPPD inhibition and tyrosinaemia. This analysis elucidates, in terms of the model parameters, a critical inhibitor concentration (at which tyrosine concentration starts to rise) and highlights the contribution of in vitro measured parameters to events in an in vivo system. Furthermore, using parameter-fitting methods, a systematically derived reduced model is shown to fit well to rat data, making explicit how the parameters are informed by such data. This model in combination with in vitro descriptors has potential as a surrogate for animal experimentation to predict tyrosinaemia, and further development can extend its application to other related medical scenarios.