J Inherit Metab Dis DOI 10.1007/s10545-015-9844-6
REVIEW
Old treatments for new insights and strategies: proposed management in adults and children with alkaptonuria Jean-Baptiste Arnoux 1 & Kim-Hanh Le Quan Sang 1 & Anais Brassier 1 & Coraline Grisel 1 & Aude Servais 2 & Julien Wippf 3 & Sandrine Dubois 1 & Nicolas Sireau 4 & Chantal Job-Deslandre 3 & Lakshminarayan Ranganath 5 & Pascale de Lonlay 1,6
Received: 28 November 2014 / Revised: 6 March 2015 / Accepted: 18 March 2015 # SSIEM 2015
Abstract Alkaptonuria (AKU) is caused by deficiency of the enzyme homogentisate 1,2 dioxygenase. It results in an accumulation of homogentisate which oxidizes spontaneously to benzoquinone acetate, a highly oxidant compound, which polymerises to a melanin-like structure, in a process called ochronosis. Asymptomatic during childhood, this accumulation will lead from the second decade of life to a progressive and severe spondylo-arthopathy, associated with multisystem involvement: osteoporosis/fractures, stones (renal, prostatic, gall bladder, salivary glands), ruptures of tendons/muscle/ligaments, renal failure and aortic valve disease. The pathophysiological mechanisms of AKU remain poorly understood, but recent advances lead us to reconsider the treatment strategy in AKU patients. Besides the supporting therapies (pain killers, Communicated by: Bruce A Barshop * Jean-Baptiste Arnoux [email protected] 1
Reference Centre for Inherited Metabolic Diseases Necker-Enfants Malades Hospital, Assistance Publique - Hôpitaux de Paris, 149 rue de Sèvres, Paris 75015, France
2
Nephrology and Metabolism Department Necker-Enfants Malades Hospital, Assistance Publique - Hôpitaux de Paris, 149 rue de Sèvres, Paris 75015, France
3
Rheumatology B department Cochin Hospital, Assistance Publique Hôpitaux de Paris, 27 rue du Faubourg Saint Jacques, Paris 75014, France
4
AKU society AdviceSpace, 66 Devonshire Road, Cambridge CB1 2BL, UK
5
Department of Clinical Biochemistry and Metabolic Medicine, Royal Liverpool University Hospital, 4th Floor Duncan Building, Daulby St, Liverpool L7 8XP, UK
6
Paris Descartes University, 12 rue de l’école de medicine, Paris 75006, France
anti-inflammatory drugs, physiotherapy, joints replacements and others), specific therapies have been considered (anti-oxidant, low protein diet, nitisinone), but clinical studies have failed to prove efficiency on the rheumatological lesions of the disease. Here we propose a treatment strategy for children and adults with AKU, based on a review of the latest findings on AKU and lessons from other aminoacipathies, especially tyrosinemias.
Introduction The natural history of alkaptonuria (AKU) is complex and involves virtually all connective tissues. This genetic disease is characterized by ochronosis (dark blue pigmentation) due to high circulating homogentisic acid (HGA). Although the genetic defect is present at birth, leading to increased plasma and urinary HGA, the onset of clinical symptoms (except black urine) is delayed until the second or third decade of life, whereas pre-symptomatic tissues lesions are supposed to develop from childhood. Then, AKU is seen to affect all connective tissues, particularly bones and joints (mainly spine and large joints). Other system involvements may include stones (renal, prostatic, gall bladder and salivary), heart valve stenosis, ruptures of muscles/tendons, eyes and ears pigmentation and renal insufficiency. Over time, the evolution appears to be slowly progressive, as shown by the AKU severity score index, which follows a steady progression from early adulthood, leading to severe motor disabilities (Cox and Ranganath 2011). Recent advances in the pathophysiology of AKU redefine the treatment strategy over the life span. AKU is an autosomal recessive genetic disease explained by the deficiency in the enzyme homogentisate 1,2 dioxygenase (HGD). The upstream substrate of HGD, HGA, accumulates in plasma and urine, and auto-oxidizes in tissues to benzoquinone acetic acid
J Inherit Metab Dis
(BQA), which polymerizes to a melanin-like pigment, called ochronotic pigment (Zannoni et al 1969). HGA and BQA are notorious oxidative factors, but also play a role in the multisystem amyloidosis observed in AKU patients, by promoting local inflammation, serum amyloid A (SAA) production and amyloid fibrils formation (Millucci et al 2012). Only two drugs have been proposed as a specific treatment AKU: vitamin C and nitisinone. Nitisinone seems the most promising, because it has revolutionized the outcome of tyrosinemia type I patients (a disease involving the same metabolic tyrosine degradation pathway as AKU), but also experiences in HGD deficient rats have shown prevention of ochronotic osteoarthropathy thanks to an early-stage treatment with nitisinone (Preston et al 2014). We propose here to review the specific treatments for AKU and see, far from being competitors, how they might be sequentially and/or synergistically used over the life span. We won’t focus in this review on future therapies (enzyme replacement therapy, gene therapy), neither on hepatocytes nor liver transplantation, which seem inappropriate in a disease where life expectancy is generally preserved.
Treatments Low tyrosine and phenylalanine diet The enzyme HGD is metabolically located in the phenylalanine and tyrosine degradation pathway, directly downstream to 4 hydroxyphenylpyruvate dioxygenase (HGA producing enzyme). Thus, HGA derives from protein degradation. A low protein diet is theoretically an option to decrease the HGA production, since this therapeutic strategy is successfully used for some other inherited metabolic diseases (e.g. phenylketonuria, maple syrup urine diseases, glutaric aciduria type 1) with an amazingly positive effect on neurological outcome and/or survival (Hilliges et al 1993; Viau et al 2012; Waisbren et al 2007). Three short-term studies (1 month) translated this experience to AKU and reported the effect of dietary interventions on urine HGA. Two of these studies were single-case studies. First, it was shown that a high protein diet in children induced an increased excretion of HGA in urine (de Haas et al 1998). Second, a strictly low protein diet in one adult (0.3 g/Kg/d, thus a priori a strictly vegetarian diet without dairy products) led to a moderate decrease (−25 %) of plasma HGA and its urine excretion. Finally, only a very low protein diet (3.5 g/d) in a 5 months old child allowed an 89 % decrease in HGA excretion (Wolff et al 1989). Unfortunately, the long-term effect of such a diet on the symptoms of AKU was never studied. We can hypothesize that lower HGA in urine and plasma might delay the onset of ochronosis and reduce its severity. However, we will expose later the difficulty of keeping a diet throughout adulthood.
Anti-oxidants The HGD deficiency results in a high concentration of HGA in blood and plasma, that polymerizes in tissues after autooxidation to benzoquinone-acetic acid (BQA) (Martin and Batkoff 1987). Thus, ochronotic pigments are found in the extra cellular matrix but also within cells (fibroblast, chondrocytes, osteosarcoma cell lines). HGA and BQA promote a significant oxidative stress, especially through lipid peroxidation and serum protein oxidation. Moreover, they induce a dysfunction of the chondrocytes. AKU chondrocytes release more nitric oxide and pro-inflammatory cytokines (mainly IL-6, IL-8) and display evidence of oxidative stress: decreased activity of the enzyme glutathione peroxidase, depletion of thiol groups, lipid peroxidation and protein carbonylation, leading to an increased apoptosis rate (Braconi et al 2010, 2011 & 2012). AKU chondrocytes release less collagen to the extra-cellular matrix, and this inhibition of the collagen synthesis in vitro is correlated with the medium HGA concentration (Tinti et al 2010). Thus BQA and oxidative stress appears to be the key mechanism leading to the severe osteoarthritis of AKU patients. Consequently, it can be hypothesized that an appropriate antioxidant therapy, especially if preventive, may have a positive effect in delaying the disease or slowing its progression. For example, the pathological examinations of 14-month old mouse models of AKU (high urine HGA and null HGD activity) displayed no ochronosis in knee and hip cartilages, intervertebral disks and tendons (Kamoun et al 1992). This non-appearance of ochronosis in HGD deficient mice may be explained by an increased renal clearance of HGA and/or by the endogenous production of ascorbic acid in mice (33.6– 275.0 mg/Kg/d) (Levine 1986). In a human chondrocytic cell AKU model, ascorbic acid, N-acetylcysteine, taurine, phytic acid and lipoic acid significantly inhibited both the HGA-induced amyloid production and the pro-inflammatory cytokines release. Indeed, the birefringent microscopic examination of Congo red stained cells observed a 50 to 70 % decrease of amyloid deposition in cultured AKU chondrocytes when the culture medium was treated with one of these anti-oxidant drugs. The most favourable effect (86 to 97 % decreased) was obtained with a dose dependant and synergistic combination of ascorbic acid and N-acetylcysteine (0.1 to 10 μmol/L each) (Braconi et al 2010; Spreafico et al 2013). With this combination of treatment at 10 μmol/L in the culture medium, the serum amyloid A (SAA production) and pro-inflammatory cytokines of cultured AKU chondrocytes were restored to control values. By comparison, the plasma concentrations of ascorbate in adults and infants treated with high dose (10 g/d in adults; 200 mg/ Kg/d for infants) were 1.5 to 2 mmol/L (Wolff et al 1989), even though the HGA concentration in the culture medium was ten times higher (Spreafico et al 2013) than in the
J Inherit Metab Dis
patients’ plasma (Wolff et al 1989). Further, in vivo study in a rat model of AKU found a 2.5 to 3.5 fold decrease of the fixation of 14C-HGA on connective tissue (xyphoid cartilage and tail tendon) when the rats were treated with an ascorbate intake equivalent to 8 g/day/70Kg of weight (Lustberg et al 1970). Concerning N-acetylcysteine, when given orally twice daily at the dose of 1200 mg/d to healthy adult control subjects, its plasma concentration rose to a peak after 1.9 h at an average of 15 μmol/L, then decreased to a trough level of 3.7 μmol/L, thus in the range used by Spreafico as exposed upper (Nolin et al 2010). In human cases of AKU, ascorbic acid was considered as a possible treatment since the 1940s (Sealock et al 1940). All the studies showed that ascorbic acid treatment led to a (near) total suppression of urinary BQA excretion in all treated patients (Mayatepek et al 1998; Wolff et al 1990), while urinary HGA excretion remained stable (Mayatepek et al 1998; Wolff et al 1998; Phornphutkul et al 2002). HGA levels even increased in infants, which might be the consequence of an activation of the HGA producing enzyme 4-hydroxyphenylpyravate dioxygenase by its cofactor ascorbic acid. In these studies, the doses were various: 500 mg bid (Forstling et al 1988), between 0.25 and 4 g/d (Phornphutkul et al 2002), 10 mg/Kg/d in adults and 200 mg/Kg/d in infants (Wolff et al 1990), 1 g/d then 10 g/d (Mayatepek et al 1998). In this latter, the urinary BQA reductions were 99.6 and 99.9 % compared to untreated patients’ results. Thus, the use of high dose of ascorbate does not seem necessary. Finally, because of their small cohort size and their short duration—both inadequate for a slowly progressive disease—none of these studies ascertained whether ascorbate could have a positive clinical effect. Nitisinone 2–(2-nitro-4-trifluoromethylbenzoyl)-cyclohexane-1,3-dione (also called NTBC or nitisinone) is a triketone herbicide derived from the leptospermone produced by the bottlebrush plant. Its inhibition of the HGA producing enzyme 4hydroxyphenylpyruvate dioxygenase (4HPPD) is fast, strong (50 % inhibitory concentration: 40 nM) and reversible (Ellis et al 1995). In human metabolism, this enzyme is located in the tyrosine and phenylalanine degradation pathway, whose end products are fumarate and acetoacetate. Besides AKU, some other diseases involve this pathway: tyrosinemia type 1 (HT1, liver and kidney presentation; fumarylacetoacetase deficiency), type 2 (ocular and skin symptoms; tyrosine aminotransferase deficiency) and type 3 (neurological presentation; 4-hyroxyphenylpyruvate dioxygenase, 4HPPD deficiency). Nitisinone has a market authorization for the treatment of HT1, in association with a low phenylalanine and tyrosine diet. The classical presentation of HT1 exposes children to a lethal liver insufficiency during the first months of life. The inhibition of 4HPPD enzyme by nitisinone, located upstream
to fumarylacetoacetase, Bshifts^ the enzymatic block and subsequently transforms the lethal HT1 disease to a nonlethal tyrosinemia type 3. The tissue distribution of nitisinone is good in liver and kidneys, low in lungs and eyes, and not studied in cartilages and bones (Lock et al 2000). Because of its dramatic effect on patients with HT1, nitisinone was also proposed as a treatment for AKU. However, in trials involving adult AKU patients, nitisinone was proven to be efficient only on the biological parameters: with a daily dose of 2 mg/d given once or twice daily, the urine and plasma HGA concentrations dropped by 95 %, and this result could be sustained during 3 years (Phornphutkul et al 2002; Suwannarat et al 2005; Introne et al 2011). No longterm benefit of nitisinone was found on the rheumatologic parameters and, at best, it suggested a limited progression of the associated aortic valve disease (Introne et al 2011). The only side effect of nitisinone is the expected rise of plasma tyrosine. In AKU patients, a dose of 2 mg/d led to an increase of plasma tyrosine to an average of 700 to 800 μmol/ L (extremes 332–1528 μmol/L; normal value
REVIEW
Old treatments for new insights and strategies: proposed management in adults and children with alkaptonuria Jean-Baptiste Arnoux 1 & Kim-Hanh Le Quan Sang 1 & Anais Brassier 1 & Coraline Grisel 1 & Aude Servais 2 & Julien Wippf 3 & Sandrine Dubois 1 & Nicolas Sireau 4 & Chantal Job-Deslandre 3 & Lakshminarayan Ranganath 5 & Pascale de Lonlay 1,6
Received: 28 November 2014 / Revised: 6 March 2015 / Accepted: 18 March 2015 # SSIEM 2015
Abstract Alkaptonuria (AKU) is caused by deficiency of the enzyme homogentisate 1,2 dioxygenase. It results in an accumulation of homogentisate which oxidizes spontaneously to benzoquinone acetate, a highly oxidant compound, which polymerises to a melanin-like structure, in a process called ochronosis. Asymptomatic during childhood, this accumulation will lead from the second decade of life to a progressive and severe spondylo-arthopathy, associated with multisystem involvement: osteoporosis/fractures, stones (renal, prostatic, gall bladder, salivary glands), ruptures of tendons/muscle/ligaments, renal failure and aortic valve disease. The pathophysiological mechanisms of AKU remain poorly understood, but recent advances lead us to reconsider the treatment strategy in AKU patients. Besides the supporting therapies (pain killers, Communicated by: Bruce A Barshop * Jean-Baptiste Arnoux [email protected] 1
Reference Centre for Inherited Metabolic Diseases Necker-Enfants Malades Hospital, Assistance Publique - Hôpitaux de Paris, 149 rue de Sèvres, Paris 75015, France
2
Nephrology and Metabolism Department Necker-Enfants Malades Hospital, Assistance Publique - Hôpitaux de Paris, 149 rue de Sèvres, Paris 75015, France
3
Rheumatology B department Cochin Hospital, Assistance Publique Hôpitaux de Paris, 27 rue du Faubourg Saint Jacques, Paris 75014, France
4
AKU society AdviceSpace, 66 Devonshire Road, Cambridge CB1 2BL, UK
5
Department of Clinical Biochemistry and Metabolic Medicine, Royal Liverpool University Hospital, 4th Floor Duncan Building, Daulby St, Liverpool L7 8XP, UK
6
Paris Descartes University, 12 rue de l’école de medicine, Paris 75006, France
anti-inflammatory drugs, physiotherapy, joints replacements and others), specific therapies have been considered (anti-oxidant, low protein diet, nitisinone), but clinical studies have failed to prove efficiency on the rheumatological lesions of the disease. Here we propose a treatment strategy for children and adults with AKU, based on a review of the latest findings on AKU and lessons from other aminoacipathies, especially tyrosinemias.
Introduction The natural history of alkaptonuria (AKU) is complex and involves virtually all connective tissues. This genetic disease is characterized by ochronosis (dark blue pigmentation) due to high circulating homogentisic acid (HGA). Although the genetic defect is present at birth, leading to increased plasma and urinary HGA, the onset of clinical symptoms (except black urine) is delayed until the second or third decade of life, whereas pre-symptomatic tissues lesions are supposed to develop from childhood. Then, AKU is seen to affect all connective tissues, particularly bones and joints (mainly spine and large joints). Other system involvements may include stones (renal, prostatic, gall bladder and salivary), heart valve stenosis, ruptures of muscles/tendons, eyes and ears pigmentation and renal insufficiency. Over time, the evolution appears to be slowly progressive, as shown by the AKU severity score index, which follows a steady progression from early adulthood, leading to severe motor disabilities (Cox and Ranganath 2011). Recent advances in the pathophysiology of AKU redefine the treatment strategy over the life span. AKU is an autosomal recessive genetic disease explained by the deficiency in the enzyme homogentisate 1,2 dioxygenase (HGD). The upstream substrate of HGD, HGA, accumulates in plasma and urine, and auto-oxidizes in tissues to benzoquinone acetic acid
J Inherit Metab Dis
(BQA), which polymerizes to a melanin-like pigment, called ochronotic pigment (Zannoni et al 1969). HGA and BQA are notorious oxidative factors, but also play a role in the multisystem amyloidosis observed in AKU patients, by promoting local inflammation, serum amyloid A (SAA) production and amyloid fibrils formation (Millucci et al 2012). Only two drugs have been proposed as a specific treatment AKU: vitamin C and nitisinone. Nitisinone seems the most promising, because it has revolutionized the outcome of tyrosinemia type I patients (a disease involving the same metabolic tyrosine degradation pathway as AKU), but also experiences in HGD deficient rats have shown prevention of ochronotic osteoarthropathy thanks to an early-stage treatment with nitisinone (Preston et al 2014). We propose here to review the specific treatments for AKU and see, far from being competitors, how they might be sequentially and/or synergistically used over the life span. We won’t focus in this review on future therapies (enzyme replacement therapy, gene therapy), neither on hepatocytes nor liver transplantation, which seem inappropriate in a disease where life expectancy is generally preserved.
Treatments Low tyrosine and phenylalanine diet The enzyme HGD is metabolically located in the phenylalanine and tyrosine degradation pathway, directly downstream to 4 hydroxyphenylpyruvate dioxygenase (HGA producing enzyme). Thus, HGA derives from protein degradation. A low protein diet is theoretically an option to decrease the HGA production, since this therapeutic strategy is successfully used for some other inherited metabolic diseases (e.g. phenylketonuria, maple syrup urine diseases, glutaric aciduria type 1) with an amazingly positive effect on neurological outcome and/or survival (Hilliges et al 1993; Viau et al 2012; Waisbren et al 2007). Three short-term studies (1 month) translated this experience to AKU and reported the effect of dietary interventions on urine HGA. Two of these studies were single-case studies. First, it was shown that a high protein diet in children induced an increased excretion of HGA in urine (de Haas et al 1998). Second, a strictly low protein diet in one adult (0.3 g/Kg/d, thus a priori a strictly vegetarian diet without dairy products) led to a moderate decrease (−25 %) of plasma HGA and its urine excretion. Finally, only a very low protein diet (3.5 g/d) in a 5 months old child allowed an 89 % decrease in HGA excretion (Wolff et al 1989). Unfortunately, the long-term effect of such a diet on the symptoms of AKU was never studied. We can hypothesize that lower HGA in urine and plasma might delay the onset of ochronosis and reduce its severity. However, we will expose later the difficulty of keeping a diet throughout adulthood.
Anti-oxidants The HGD deficiency results in a high concentration of HGA in blood and plasma, that polymerizes in tissues after autooxidation to benzoquinone-acetic acid (BQA) (Martin and Batkoff 1987). Thus, ochronotic pigments are found in the extra cellular matrix but also within cells (fibroblast, chondrocytes, osteosarcoma cell lines). HGA and BQA promote a significant oxidative stress, especially through lipid peroxidation and serum protein oxidation. Moreover, they induce a dysfunction of the chondrocytes. AKU chondrocytes release more nitric oxide and pro-inflammatory cytokines (mainly IL-6, IL-8) and display evidence of oxidative stress: decreased activity of the enzyme glutathione peroxidase, depletion of thiol groups, lipid peroxidation and protein carbonylation, leading to an increased apoptosis rate (Braconi et al 2010, 2011 & 2012). AKU chondrocytes release less collagen to the extra-cellular matrix, and this inhibition of the collagen synthesis in vitro is correlated with the medium HGA concentration (Tinti et al 2010). Thus BQA and oxidative stress appears to be the key mechanism leading to the severe osteoarthritis of AKU patients. Consequently, it can be hypothesized that an appropriate antioxidant therapy, especially if preventive, may have a positive effect in delaying the disease or slowing its progression. For example, the pathological examinations of 14-month old mouse models of AKU (high urine HGA and null HGD activity) displayed no ochronosis in knee and hip cartilages, intervertebral disks and tendons (Kamoun et al 1992). This non-appearance of ochronosis in HGD deficient mice may be explained by an increased renal clearance of HGA and/or by the endogenous production of ascorbic acid in mice (33.6– 275.0 mg/Kg/d) (Levine 1986). In a human chondrocytic cell AKU model, ascorbic acid, N-acetylcysteine, taurine, phytic acid and lipoic acid significantly inhibited both the HGA-induced amyloid production and the pro-inflammatory cytokines release. Indeed, the birefringent microscopic examination of Congo red stained cells observed a 50 to 70 % decrease of amyloid deposition in cultured AKU chondrocytes when the culture medium was treated with one of these anti-oxidant drugs. The most favourable effect (86 to 97 % decreased) was obtained with a dose dependant and synergistic combination of ascorbic acid and N-acetylcysteine (0.1 to 10 μmol/L each) (Braconi et al 2010; Spreafico et al 2013). With this combination of treatment at 10 μmol/L in the culture medium, the serum amyloid A (SAA production) and pro-inflammatory cytokines of cultured AKU chondrocytes were restored to control values. By comparison, the plasma concentrations of ascorbate in adults and infants treated with high dose (10 g/d in adults; 200 mg/ Kg/d for infants) were 1.5 to 2 mmol/L (Wolff et al 1989), even though the HGA concentration in the culture medium was ten times higher (Spreafico et al 2013) than in the
J Inherit Metab Dis
patients’ plasma (Wolff et al 1989). Further, in vivo study in a rat model of AKU found a 2.5 to 3.5 fold decrease of the fixation of 14C-HGA on connective tissue (xyphoid cartilage and tail tendon) when the rats were treated with an ascorbate intake equivalent to 8 g/day/70Kg of weight (Lustberg et al 1970). Concerning N-acetylcysteine, when given orally twice daily at the dose of 1200 mg/d to healthy adult control subjects, its plasma concentration rose to a peak after 1.9 h at an average of 15 μmol/L, then decreased to a trough level of 3.7 μmol/L, thus in the range used by Spreafico as exposed upper (Nolin et al 2010). In human cases of AKU, ascorbic acid was considered as a possible treatment since the 1940s (Sealock et al 1940). All the studies showed that ascorbic acid treatment led to a (near) total suppression of urinary BQA excretion in all treated patients (Mayatepek et al 1998; Wolff et al 1990), while urinary HGA excretion remained stable (Mayatepek et al 1998; Wolff et al 1998; Phornphutkul et al 2002). HGA levels even increased in infants, which might be the consequence of an activation of the HGA producing enzyme 4-hydroxyphenylpyravate dioxygenase by its cofactor ascorbic acid. In these studies, the doses were various: 500 mg bid (Forstling et al 1988), between 0.25 and 4 g/d (Phornphutkul et al 2002), 10 mg/Kg/d in adults and 200 mg/Kg/d in infants (Wolff et al 1990), 1 g/d then 10 g/d (Mayatepek et al 1998). In this latter, the urinary BQA reductions were 99.6 and 99.9 % compared to untreated patients’ results. Thus, the use of high dose of ascorbate does not seem necessary. Finally, because of their small cohort size and their short duration—both inadequate for a slowly progressive disease—none of these studies ascertained whether ascorbate could have a positive clinical effect. Nitisinone 2–(2-nitro-4-trifluoromethylbenzoyl)-cyclohexane-1,3-dione (also called NTBC or nitisinone) is a triketone herbicide derived from the leptospermone produced by the bottlebrush plant. Its inhibition of the HGA producing enzyme 4hydroxyphenylpyruvate dioxygenase (4HPPD) is fast, strong (50 % inhibitory concentration: 40 nM) and reversible (Ellis et al 1995). In human metabolism, this enzyme is located in the tyrosine and phenylalanine degradation pathway, whose end products are fumarate and acetoacetate. Besides AKU, some other diseases involve this pathway: tyrosinemia type 1 (HT1, liver and kidney presentation; fumarylacetoacetase deficiency), type 2 (ocular and skin symptoms; tyrosine aminotransferase deficiency) and type 3 (neurological presentation; 4-hyroxyphenylpyruvate dioxygenase, 4HPPD deficiency). Nitisinone has a market authorization for the treatment of HT1, in association with a low phenylalanine and tyrosine diet. The classical presentation of HT1 exposes children to a lethal liver insufficiency during the first months of life. The inhibition of 4HPPD enzyme by nitisinone, located upstream
to fumarylacetoacetase, Bshifts^ the enzymatic block and subsequently transforms the lethal HT1 disease to a nonlethal tyrosinemia type 3. The tissue distribution of nitisinone is good in liver and kidneys, low in lungs and eyes, and not studied in cartilages and bones (Lock et al 2000). Because of its dramatic effect on patients with HT1, nitisinone was also proposed as a treatment for AKU. However, in trials involving adult AKU patients, nitisinone was proven to be efficient only on the biological parameters: with a daily dose of 2 mg/d given once or twice daily, the urine and plasma HGA concentrations dropped by 95 %, and this result could be sustained during 3 years (Phornphutkul et al 2002; Suwannarat et al 2005; Introne et al 2011). No longterm benefit of nitisinone was found on the rheumatologic parameters and, at best, it suggested a limited progression of the associated aortic valve disease (Introne et al 2011). The only side effect of nitisinone is the expected rise of plasma tyrosine. In AKU patients, a dose of 2 mg/d led to an increase of plasma tyrosine to an average of 700 to 800 μmol/ L (extremes 332–1528 μmol/L; normal value
