Monitoring tyrosinaemia type I: Blood spot test for nitisinone (NTBC)

Clinical and genetic characteristics of two patients with tyrosinemia type 1 in Slovenia – A novel fumarylacetoacetate hydrolase (FAH) intronic disease-causing variant

Tyrosinemia type 1 (HT1) is an inborn error of tyrosine catabolism that leads to severe liver, kidney, and neurological dysfunction. Newborn screening (NBS) can enable a timely diagnosis and early initiation of treatment.

We presented the follow up of the only two Slovenian patients diagnosed with HT1. Metabolic control was monitored by measuring tyrosine, phenylalanine and succinylacetone from dried blood spots (DBSs). Retrograde screening of HT1 was performed from DBSs taken at birth using tandem mass spectrometry.

First patient was diagnosed at the age of 6 months in the asymptomatic phase due to an abnormal liver echogenicity, the other presented at 2.5 months with an acute liver failure and needed a liver transplantation. The first was a compound heterozygote for a novel FAH intronic variant c.607-21A>G and c.192G>T whereas the second was homozygous for c.192G>T. At the non-transplanted patient, 66% of tyrosine and 79% of phenylalanine measurements were in strict reference ranges of 200–400 μmol/L and >30 μmol/L, respectively, which resulted in a favorable cognitive outcome at 3.6 years. On retrograde screening, both patients had elevated SA levels; on the other hand, tyrosine was elevated only at one.

We showed that non-coding regions should be analyzed when clinical and biochemical markers are characteristic of HT1. DBSs represent a convenient sample type for frequent amino acid monitoring. Retrograde diagnosis of HT1 was possible after more than three years of birth with SA as a primary marker, complemented by tyrosine.

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Non-coding region variants of FAH gene can result in a symptomatic HT1.

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Retrograde screening for HT1 is technically possible even three years after birth.

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DBS are convenient for monitoring HT1 patients and are family-friendly.

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Regular monitoring in HT1 patients can result in a favorable cognitive outcome.

NTBC Treatment Monitoring in Chilean Patients with Tyrosinemia Type 1 and Its Association with Biochemical Parameters and Liver Biomarkers

Treatment and follow-up in Hereditary Tyrosinemia type 1 (HT-1) patients require comprehensive clinical and dietary management, which involves drug therapy with NTBC and the laboratory monitoring of parameters, including NTBC levels, succinylacetone (SA), amino acids, and various biomarkers of liver and kidney function. Good adherence to treatment and optimal adjustment of the NTBC dose, according to clinical manifestations and laboratory parameters, can prevent severe liver complications such as hepatocarcinogenesis (HCC). We analyzed several laboratory parameters for 15 HT-1 patients over one year of follow-up in a cohort that included long-term NTBC-treated patients (more than 20 years), as well as short-term patients (one year). Based on this analysis, we described the overall adherence by our cohort of 70% adherence to drug and dietary treatment. A positive correlation was found between blood and plasma NTBC concentration with a conversion factor of 2.57. Nonetheless, there was no correlation of the NTBC level with SA levels, αFP, liver biomarkers, and amino acids in paired samples analysis. By separating according to the range of the NTBC concentration, we therefore determined the mean concentration of each biochemical marker, for NTBC ranges above 15–25 μmol/L. SA in urine and αFP showed mean levels within controlled parameters in our group of patients. Future studies analyzing a longer follow-up period, as well as SA determination in the blood, are encouraged to confirm the present findings.

β-Cyclodextrin Derivative Grafted on Silica Gel Represents a New Polymeric Sorbent for Extracting Nitisinone from Model Physiological Fluids

Nitisinone (NTBC) is used in the treatment of disorders affecting the tyrosine pathway, including hereditary tyrosinemia type I, alkaptonuria, and neuroblastoma. An inappropriate dosage of this therapeutic drug causes side effects; therefore, it is necessary to develop a rapid and sensitive method to monitor the content of NTBC in patients' blood. This study aimed to develop anew polymeric sorbent containing β-cyclodextrin (β-CD) derivatives grafted on silica gel to effectively extract NTBC from model physiological fluids. The inclusion complex formed between β-CD and NTBC was examined by proton nuclear magnetic resonance spectroscopy. The novel sorbents with derivatives of β-CD were prepared on modified silica gel using styrene as a comonomer, ethylene glycol dimethacrylate as a crosslinking agent, and 2,2'-azo-bis-isobutyronitrile as a polymerization initiator. The obtained products were characterized via Fourier transform infrared spectroscopy and then used as sorbents as part of a solid phase extraction technique. High NTBC recovery (70%indicated that the developed polymeric sorbent may be suitable for extracting this compound from patients' blood samples.

Clinical utilization of dried blood spot nitisinone (NTBC) and succinylacetone (SA) concentrations in hereditary tyrosinaemia type 1 - A UK centre experience

BACKGROUND

Dried blood spot monitoring of nitisinone and succinylacetone in hereditary tyrosinaemia type 1 patients is not widely available in the United Kingdom. Currently, biochemical monitoring utilizes urinary succinylacetone, blood spot tyrosine and phenylalanine monitoring, which can lack in convenience and accuracy, respectively.

METHODS

We report the development of a dried blood spot assay for nitisinone and succinylacetone and analysed retrospective clinical and biochemical data for hereditary tyrosinaemia type 1 patients from a single UK centre.

RESULTS

A total of 13 hereditary tyrosinaemia type 1 patients were evaluated. Eleven presented with liver dysfunction (two with associated renal tubulopathy) and two were detected by early sibling screening. All patients (age 0.03-22 months) were commenced on a tyrosine-/phenylalanine-restricted diet and nitisinone at diagnosis. Ten patients were on twice daily dosing and three were on single daily dosing at the start of monitoring. One patient from each dosing group swapped between dosing regimens at 20 years of age and 8 months of age, respectively. A total of 684 dried blood spot samples were analysed; 80% of nitisinone concentrations were between 9.2 and 27 µmol/L when succinylacetone was <0.3 µmol/L. Patients on twice daily dosing regimens had significantly higher nitisinone concentration compared with those on once daily dosing (P < 0.0001). The median dose required in the twice daily doing group was significantly lower when compared with once daily dosing.

CONCLUSIONS

Dried blood spot monitoring for nitisinone and succinylacetone concentrations in hereditary tyrosinaemia type 1 patients is a rapid and convenient method which allows physicians to individualize treatment plans and observe adherence to treatment.

Use of Dried Blood Spot Specimens to Monitor Patients with Inherited Metabolic Disorders

Monitoring of patients with inherited metabolic disorders (IMDs) using dried blood spot (DBS) specimens has been routinely used since the inception of newborn screening (NBS) for phenylketonuria in the 1960s. The introduction of flow injection analysis tandem mass spectrometry (FIA-MS/MS) in the 1990s facilitated the expansion of NBS for IMDs. This has led to increased identification of patients who require biochemical monitoring. Monitoring of IMD patients using DBS specimens is widely favoured due to the convenience of collecting blood from a finger prick onto filter paper devices in the patient's home, which can then be mailed directly to the laboratory. Ideally, analytical methodologies with a short analysis time and high sample throughput are required to enable results to be communicated to patients in a timely manner, allowing prompt therapy adjustment. The development of ultra-performance liquid chromatography (UPLC-MS/MS), means that metabolic laboratories now have the capability to routinely analyse DBS specimens with superior specificity and sensitivity. This advancement in analytical technology has led to the development of numerous assays to detect analytes at low concentrations (pmol/L) in DBS specimens that can be used to monitor IMD patients. In this review, we discuss the pre-analytical, analytical and post-analytical variables that may affect the final test result obtained using DBS specimens used for monitoring of patients with an IMD.

Inter-laboratory analytical improvement of succinylacetone and nitisinone quantification from dried blood spot samples

BACKGROUND

Nitisinone is used to treat hereditary tyrosinemia type 1 (HT-1) by preventing accumulation of toxic metabolites, including succinylacetone (SA). Accurate quantification of SA during newborn screening is essential, as is quantification of both SA and nitisinone for disease monitoring and optimization of treatment. Analysis of dried blood spots (DBS) rather than plasma samples is a convenient method, but interlaboratory differences and comparability of DBS to serum/plasma may be issues to consider.

METHODS

Eight laboratories with experience in newborn screening and/or monitoring of patients with HT-1 across Europe participated in this study to assess variability and improve SA and nitisinone concentration measurements from DBS by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Quantification of nitisinone from both DBS and plasma was performed to assess sample comparability. In addition, efforts to harmonize laboratory procedures of SA and nitisinone quantifications during 5 rounds of analysis are described.

RESULTS

Nitisinone levels measured from DBS and plasma strongly correlated (R 2 = 0.93). Due to partitioning of nitisinone to the plasma, levels were higher in plasma by a factor of 2.34. In the initial assessment of laboratory performance, all had linear calibrations of SA and nitisinone although there was large inter-laboratory variability in actual concentration measurements. Subsequent analytical rounds demonstrated markedly improved spread and precision over previous rounds, an outcome confirmed in a final re-test round.

CONCLUSION

The study provides guidance for the determination of nitisinone and SA from DBS and the interpretation of results in the clinic. Inter-laboratory analytical harmonization was demonstrated through calibration improvements.

LC-MS/MS study of the degradation processes of nitisinone and its by-products

Nitisinone (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione, NTBC) was the first synthetically produced triketone herbicide. However, its unsatisfactory herbicidal properties, negative impact on the natural environment and the high cost of synthesis have hindered its commercialization as a plant protection agent. Nevertheless, NTBC has become the medical treatment of choice for a rare hereditary metabolic disease -hepatorenal tyrosinemia. Literature review shows that most research on nitisinone focuses on its medical applications, while there are neither in-depth studies of its stability nor its degradation pathways. Therefore, the aim of our study was to employ liquid chromatography coupled with mass spectrometry (LC-MS/MS) to determine the stability of NTBC in different experimental conditions (pH of solution, temperature, time of incubation, ultraviolet radiation), identify its degradation products and determine the stability of the latter. Electrospray ionization (ESI) in the negative ion mode was used as an ionization method and the analytes were detected by multiple reaction monitoring. We show that nitisinone stability increases with increasing pH of the solution. At pH similar to that of gastric juice in the human stomach, two major products of NTBC degradation are formed: 2-amino-4-(trifluoromethyl)benzoic acid (ATFA) and 2-nitro-4-(trifluoromethyl)benzoic acid (NTFA), which show considerable stability under studied conditions. The results of these studies shed new light on the properties of NTBC, therefore contributing to better understanding of possible risks and benefits of its medical application.

Long-Term Outcomes and Practical Considerations in the Pharmacological Management of Tyrosinemia Type 1

Tyrosinemia type 1 (TT1) is a rare metabolic disease caused by a defect in tyrosine catabolism. TT1 is clinically characterized by acute liver failure, development of hepatocellular carcinoma, renal and neurological problems, and consequently an extremely poor outcome. This review showed that the introduction of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) in 1992 has revolutionized the outcome of TT1 patients, especially when started pre-clinically. If started early, NTBC can prevent liver failure, renal problems, and neurological attacks and decrease the risk for hepatocellular carcinoma. NTBC has been shown to be safe and well tolerated, although the long-term effectiveness of treatment with NTBC needs to be awaited. The high tyrosine concentrations caused by treatment with NTBC could result in ophthalmological and skin problems and requires life-long dietary restriction of tyrosine and its precursor phenylalanine, which could be strenuous to adhere to. In addition, neurocognitive problems have been reported since the introduction of NTBC, with hypothesized but as yet unproven pathophysiological mechanisms. Further research should be done to investigate the possible relationship between important clinical outcomes and blood concentrations of biochemical parameters such as phenylalanine, tyrosine, succinylacetone, and NTBC, and to develop clear guidelines for treatment and follow-up with reliable measurements. This all in order to ultimately improve the combined NTBC and dietary treatment and limit possible complications such as hepatocellular carcinoma development, neurocognitive problems, and impaired quality of life.

A simple RP-HPLC method for determination of nitisinone in pharmaceuticals

Nitisinone is a reversible inhibitor of 4-hydroxyphenylpyruvate dioxygenase and an active substance in the orphan drug used for the treatment of hereditary tyrosinemia type I, a rare genetic disease caused by mutations in the gene for the enzyme fumarylacetoacetase. The aim of our study was to develop a simple and accurate RP–HPLC method with UV detection for routine determination of nitisinone in commercially available pharmaceutical forms. The chromatographic separation was achieved on a reversed-phase column Purospher STAR® RP–8 end–capped (150 x 4.6 mm i.d., particle size 5 μm), with a mobile phase consisted of a mixture of acetonitrile and water acidified with o-phosphoric acid with pH adjusted to 3.0, 65:35 (V/V), filtered through 0.45μm nylon filter. The flow rate was kept at 1 mL/min. A diode array detector measured the UV absorbance at 272 nm. The injection volume was 10 μL. The method was validated by a determination of system suitability, specificity, linearity, range, accuracy, precision, detection limit and quantitation limit. Then, the method was applied for determination of nitisinon in commercially available capsules. The proposed RP-HPLC method allows a simple, accurate, precise and rapid determination of nitisinone in pharmaceuticals. The advantages of the method include simple sample treatment, good precision (RSD less than 2%) and high recovery (greater than 99%). The method could be recommended for routine analysis in quality control laboratories, in stability studies as well as for the evaluation of potentially counterfeit capsules containing nitisinone. Key words: Nitisinone; High performance liquid chromatography; Method validation; Hereditary tyrosinemia type I

Clinical utility of nitisinone for the treatment of hereditary tyrosinemia type-1 (HT-1)

Medical therapy for hereditary hepatorenal tyrosinemia (hereditary tyrosinemia type 1, HT-1) with nitisinone was discovered incidentally, and is a by-product of agrochemistry. It blocks the catabolic pathway of tyrosine, thereby leading to a reduction in the accumulation of toxic metabolites in HT-1. It has to be combined with a low-protein diet supplemented with amino acid mixtures devoid of tyrosine and phenylalanine. This treatment option has completely changed the clinical course of patients suffering from HT-1 who used to die in the first few months to years of life from liver failure, renal dysfunction, and/or hepatocellular carcinoma (HCC). It is essential to start nitisinone therapy early in life to avoid sequelae; beginning treatment in the newborn period is ideal. As initial clinical symptoms of HT-1 are often atypical and because there is a clinically latent phase during the first few months of life in many patients, newborn screening is required to secure early diagnosis. Succinylacetone in blood is a reliable screening parameter whereas tyrosine is neither specific nor sensitive. Especially HCC, but also liver and kidney dysfunction, rickets, and neurological crises can be prevented in most patients if nitisinone therapy is started in the newborn period. It is essential to adhere to a low-protein diet to avoid tyrosine toxicity. Reversible eye symptoms may occur as a side-effect of nitisinone, but other side effects are rare. Neurocognitive development is impaired in some patients, and the reason for this is unclear. Metabolic monitoring includes measurement of tyrosine, succinylacetone, and nitisinone concentrations in blood.

A Rapid Screening Test on Dried Blood for the Neonatal Diagnosis of Tyrosinemia Type I

Background:

Tyrosinemia is an inherited metabolic disorder characterized by elevated levels of tyrosine and its metabolites in plasma. Without treatment, the disease will progress to hepatic and renal failure, so that without liver transplantation will cause death in less than 10 years of age. So, early diagnosis and treatment can be life saving and crucial. It means that with early treatment starting in the neonatal period, the patient can have normal life with very few restrictions in diets containing tyrosine and phenylalanine.

Objectives:

In this study we wanted to evaluate an easy to perform, rapid and sensitive qualitative test with low cost, as a part of neonatal screening tests to help early diagnosis and treatment of hereditary tyrosinemia.

Patients and Methods:

In this cross sectional study, during the study period (2013 - 2014), 100 patients were selected. Fifty three (53) of these patients had proven tyrosinemia and the other 47 cases biliary atresia, paucity of intrahepatic bile ducts, cytomegalovirus (CMV) hepatitis, galactosemia and storage diseases.

Results:

There were 2 false negative and 14 false positive cases of hereditary tyrosinemia (HT-1) in the test. Six cases of biliary atresia, 7 cases of paucity of intrahepatic bile ducts and one patient with cytomegalovirus (CMV) hepatitis were falsely positive with the test. Sensitivity of the test was 96.23%, specificity 71.43%, positive predictive value (PPV) 78.46%, and negative predictive value (NPV) 94.59%.

Conclusions:

This rapid qualitative test on dried blood sample is an easy, cheap, and feasible method for the screening of hereditary tyrosinemia in neonatal period.

A Stability Indicating HPLC Method for the Determination of Nitisinone in Capsules

In this study a simple and efficient stability-indicating HPLC method with short run time was developed for the determination of nitisinone. The stress degradation of nitisinone was studied in different acidic, basic, oxidative, thermal and photolytic conditions. The chromatographic separation was achieved on a Nova-Pak C18 column using a mixture of 50 mM NaH₂PO₄ (pH 2.5) and acetonitrile (45:55, v/v) as mobile phase. UV detection was performed at 280 nm. Good linearity was observed over the concentration range of 0.5-50 μg/ml with r²>0.999. The within-day and between-day precision values were less than 2%. The proposed method could be used for the determination of nitisinone in the presence of its degradation products and also dosage form excipients for the quality control purposes.

Metabolic Effects of Increasing Doses of Nitisinone in the Treatment of Alkaptonuria

Alkaptonuria is an autosomal recessive disease involving a deficiency of the enzyme homogentisate dioxygenase, which is involved in the tyrosine degradation pathway. The enzymatic deficiency results in high concentrations of homogentisic acid (HGA), which results in orthopedic and cardiac complications, among other symptoms. Nitisinone (NTBC) has been shown to effectively treat alkaptonuria by blocking the conversion of 4-hydroxyphenylpyruvate to HGA, but there have been concerns that using doses higher than about 2 mg/day could cause excessively high levels of tyrosine, resulting in crystal deposition and corneal pathology. We have enrolled seven patients in a study to determine whether higher doses of NTBC were effective at further reducing HGA levels while maintaining tyrosine at acceptable levels. Patients were given varying doses of NTBC (ranging from 2 to 8 mg/day) over the course of between 0.5 and 3.5 years. Urine HGA, plasma tyrosine levels, and plasma NTBC were then measured longitudinally at various doses. We found that tyrosine concentrations plateaued and did not reach significantly higher levels as NTBC doses were increased above 2 mg/day, while a significant drop in HGA continued from 2 to 4 mg/day, with no significant changes at higher doses. We also demonstrated using untargeted metabolomics that elevations in tyrosine from treatment resulted in proportional elevations in alternative tyrosine metabolic products, that of N-acetyltyrosine and γ-glutamyltyrosine.

Cross-sectional study of 168 patients with hepatorenal tyrosinaemia and implications for clinical practice

BACKGROUND

Hepatorenal tyrosinaemia (Tyr 1) is a rare inborn error of tyrosine metabolism. Without treatment, patients are at high risk of developing acute liver failure, renal dysfunction and in the long run hepatocellular carcinoma. The aim of our study was to collect cross-sectional data.

METHODS

Via questionnaires we collected retrospective data of 168 patients with Tyr 1 from 21 centres (Europe, Turkey and Israel) about diagnosis, treatment, monitoring and outcome. In a subsequent consensus workshop, we discussed data and clinical implications.

RESULTS

Early treatment by NTBC accompanied by diet is essential to prevent serious complications such as liver failure, hepatocellular carcinoma and renal disease. As patients may remain initially asymptomatic or develop uncharacteristic clinical symptoms in the first months of life newborn mass screening using succinylacetone (SA) as a screening parameter in dried blood is mandatory for early diagnosis. NTBC-treatment has to be combined with natural protein restriction supplemented with essential amino acids. NTBC dosage should be reduced to the minimal dose allowing metabolic control, once daily dosing may be an option in older children and adults in order to increase compliance. Metabolic control is judged by SA (below detection limit) in dried blood or urine, plasma tyrosine (<400 μM) and NTBC-levels in the therapeutic range (20-40 μM). Side effects of NTBC are mild and often transient. Indications for liver transplantation are hepatocellular carcinoma or failure to respond to NTBC. Follow-up procedures should include liver and kidney function tests, tumor markers and imaging, ophthalmological examination, blood count, psychomotor and intelligence testing as well as therapeutic monitoring (SA, tyrosine, NTBC in blood).

CONCLUSION

Based on the data from 21 centres treating 168 patients we were able to characterize current practice and clinical experience in Tyr 1. This information could form the basis for clinical practice recommendations, however further prospective data are required to underpin some of the recommendations.

Single dose NTBC-treatment of hereditary tyrosinemia type I

NTBC (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3cyclohexanedione) is the mainstay of treatment in tyrosinemia type 1 (HT 1). The current recommendation is to divide the total daily dose of NTBC into two doses. We monitored the plasma NTBC concentrations in a series of seven patients who were changed from multiple divided doses to a single daily dose of NTBC. Two additional patients were started on a single daily dose of NTBC after the diagnosis of HT 1 was established. In three patients, NTBC kinetics were performed over 6 and 24 hours, respectively. The use of multiple divided doses or a single daily dose did not significantly affect plasma NTBC concentrations or the mean daily dose needed to attain therapeutic plasma NTBC concentrations. Moreover, kinetic studies demonstrated that plasma NTBC concentrations were completely stable over a period of 24 hours with a single dose regimen, as expected given the known NTBC plasma half life of 54 hours. Although these preliminary results need to be confirmed in more patients, our findings show that administration of NTBC in a single daily dose may be as effective as a multiple-dose regimen in reaching therapeutic plasma NTBC concentrations and suppressing succinylacetone formation in patients with HT 1. In fact, single dose treatment may increase patients' compliance with the drug treatment and improve metabolic control.

LC-MS/MS method for simultaneous determination on a dried blood spot of multiple analytes relevant for treatment monitoring in patients with tyrosinemia type I

Tyrosinemia type 1 is caused by deficiency of fumarylacetoacetate hydrolase. The enzymatic defect impairs the conversion of fumarylacetoacetate to fumarate, causing accumulation of succinylacetone which induces severe liver and kidney dysfunction along with mutagenic changes and hepatocellular carcinoma. Treatment is based on nitisinone (NTBC), an enzymatic inhibitor which suppresses succinylacetone production. NTBC, which has dramatically changed the disease course improving liver and kidney functions and reducing risk of liver cancer, causes a side effect of the increase of tyrosine levels. Treatment is therefore based on the combination of NTBC with a protein-restricted diet to prevent the potential toxicity of excessive tyrosine accumulation. Long-term therapy requires a careful monitoring in blood of NTBC levels along with other disease biomarkers, which include succinylacetone, and a selected panel of circulating aminoacids. We have developed a straightforward and fast MS/MS method for the simultaneous determination of NTBC, succinylacetone, tyrosine, phenylalanine, and methionine on a dried blood spot requiring a 2 min run. A single assay suitable for quantitative evaluation of all biochemical markers is of great advance over conventional methods, especially in pediatric patients, since it reduces laboratory costs and blood sampling, is less invasive and particularly suitable for pediatric patients, and allows easier storage and shipping.

Rapid steroid hormone quantification for congenital adrenal hyperplasia (CAH) in dried blood spots using UPLC liquid chromatography-tandem mass spectrometry

Newborn screening for congenital adrenal hyperplasia (CAH) is usually done by quantifying 17α-hydroxyprogesterone using immunoassay. However, this test produces high rates of false positive results caused by cross reacting steroids. Therefore we have developed a selective and specific method with a short run time (1.25 min) for quantification of 17α-hydroxyprogesterone, 21-deoxycortisol, 11-deoxycortisol, 11-deoxycorticosterone and cortisol from dried blood spots. The extraction procedure is very simple and steroid separation is ensured on a BEH C18 column and an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Analysis was done in positive ionization mode (ESI+) and recorded in multiple reaction monitoring mode (MRM). The method gave linear results for all steroids over a range of 5-200 (cortisol: 12.5-500)nmol/L with coefficients of regression >0.992. Absolute recovery was >64.1%. Across the analytical range the inter-assay coefficient of variation (CV) was <3%. Newborn blood samples of patients with confirmed 21-CAH and 11-CAH could clearly be distinguished from samples of unaffected newborns falsely positive on immunoassay. The method is not influenced by cross reactions as found on immunoassay. Analysis of dried blood spots shows that this method is sensitive and fast enough to allow rapid analysis and can therefore improve the newborn screening program.

Comparison of plasma and dry blood spots as samples for the determination of nitisinone (NTBC) by high-performance liquid chromatography-tandem mass spectrometry. Study of the stability of the samples at different temperatures

Tyrosinemia is an inborn error of metabolism characterized by the accumulation of tyrosine as well as toxic by-products. NTBC or nitisinone is a drug currently used for the treatment of tyrosinemia that avoids the formation of these toxic substances. This paper presents the determination of NTBC in plasma and dry blood spots by high-performance liquid chromatography (HPLC) coupled to tandem mass spectrometry. The concentration of NTBC in matched plasma-dry blood spots was compared and the study of degradation of NTBC in plasma and dry spots at different temperatures is presented.

METHOD

For sample preparation, plasma proteins were precipitated with acetonitrile and 3-mm discs were extracted with methanol. ESI(+) was used as inozation method and the analytes were detected by multiple reaction monitoring using the transitions 330>218 for NTBC and 340>228 for mesotrione, used as internal standard.

RESULTS

There is good correlation between concentrations obtained in dry blood spots and plasma (r(2)=0.83), although values are 2.4 times higher in plasma samples. NTBC in plasma is stable at least for 45 days frozen at -30°C and refrigerated at 4°C. However, it shows slow decomposition at room temperature, approximately 30% after 45 days. The method shows good precision, accuracy and linearity and the detection limit is 50 nmol/L and paper samples are appropriate for the monitorization of NTBC.