J Inherit Metab Dis (2010) 33 (Suppl 3):S321-S328
`DOl 10.1007/s10545-010-9172-9
`
`RESEARCH REPORT
`
`Simultaneous LC-MS/MS determination of phenylbutyrate,
`phenylacetate benzoate and their corresponding metabolites
`phenylacetylglutamine and hippurate in blood
`and urine
`
`Maurice D. Laryea • Diran Herebian •
`Thomas Meissner • Ertan Mayatepek
`
`Received: 13 January 2010 /Revised: 15 June 2010 I Accepted: 7 July 2010 /Published online: 7 August 2010
`© SSIEM and Springer 2010
`
`Abstract Inborn errors of urea metabolism result in hyper(cid:173)
`ammonemia. Treatment of urea cycle disorders can effectively
`lower plasma ammonium levels and results in survival in the
`majority of patients. Available medications for treating urea
`cycle disorders include sodium benzoate (BA), sodium
`phenylacetate (PAA), and sodium phenylbutyrate (PBA) and
`are given to provide alternate routes for disposition of waste
`nitrogen excretion. In this study, we develop and validate a
`liquid chromatography tandem mass spectrometry (LC-MS/
`MS) method for simultaneous determination ofbenzoic acid,
`phenylacetic acid, phenylbutyric acid, phenylacetylglutamine,
`and hippuric acid in plasma and urine from children with
`inborn errors of urea synthesis. Plasma extracts and diluted
`mine samples were injected on a reverse-phase column and
`identified and quantified by selected reaction monitoring
`(SRM) in negative ion mode. Deuterated analogues served as
`internal standards. Analysis time was 7 min. Assay precision,
`accuracy, and linearity and sample stability were determined
`using enriched samples. Quantification limits of the method
`were 100 ng/ml (0.3-0.8 11-mol/L) for all analytes, and
`recoveries were >90%. Inter- and intraday relative standard
`deviations were <10%. Our newly developed LC-MS/MS
`represents a robust, sensitive, and rapid method that allows
`simultaneous determination of the five compounds in plasma
`and urine.
`
`Communicated by: Marinus Duran
`
`Competing interest: None declared.
`
`M. D. Laryea (~) · D. Herebian · T. Meissner· E. Mayatepek
`Department of General Pediatrics, University Children's Hospital,
`Heinrich-Heine-University,
`MoorenstraJ3e 5,
`40225 Dusseldorf, Germany
`e-mail: Laryea@med.uni-duesseldorf.de
`
`Abbreviations
`LC-MS/MS Liquid chromatography tandem mass
`spectrometry
`Benzoate
`Phenylacetate
`Phenylbutyrate
`Hippurate
`Phenylacetylglutamine
`Internal standard
`
`BA
`PAA
`PBA
`HA
`PAG
`IS
`
`Introduction
`
`Ammonia is a toxic compound produced in the body from
`catabolism of amino acids and protein. Ammonia is
`converted to urea in the liver cells by urea-cycle enzymes
`and eliminated in the urine as nitrogenous waste (Shih
`2007). Inborn errors of urea metabolism result in hyper(cid:173)
`ammonemia, and prompt recognition and treatment of urea
`cycle disorders can effectively lower plasma ammonium
`levels and results in survival in the majority of patients
`(Enns et al. 2007). Treatment of urea cycle disorders
`consists of a protein-restricted diet and, in some cases,
`essential amino acid supplementation (Wilcken 2004).
`Nowadays, urea cycle defects are part of extended tandem
`mass spectrometry (MS/MS)-based newborn screening
`programs, as reviewed extensively by Garg and Dasouki
`(2006).
`Available medications for treating urea cycle disorders
`include sodium benzoate (BA), sodium phenylacetate
`(PAA), and sodium phenylbutyrate (PBA) and are given
`to provide alternate routes for disposition of waste nitrogen
`
`~Springer
`
`Par Pharmaceutical, Inc. Ex. 1026
`Par v. Horizon, IPR of Patent No. 9,561,197
`Page 1 of 8
`
`

`

`S322
`
`J Inherit Metab Dis (2010) 33 (Suppl3):S321-S328
`
`excretion. The administration of BA results in conjugation
`of BA with glycine to form benzoyl glycine, also known as
`hippuric acid (HA), which is rapidly cleared by the kidney.
`Similarly, the administration of PAA results in conjugation
`with glutamine and excretion of phenylacetylglutamine
`(PAG) (Webster et al. 1976), a normal constituent of human
`urine (Stein et al. 1954). The molecular structures of the
`target analytes are depicted in Fig. 1. The phenylacetyl
`moiety of PAG probably is a product of phenylalanine
`metabolism and occurs in the human circulation in low
`concentrations. Both PAA and BA have been administered
`to children with hyperammonemia due to inborn errors of
`urea synthesis (Batshaw 1983 ; Brusilow et al. 1984;
`Brusilow 1991). BA was recently shown to be effective
`when infused prenatally (Das et al. 2009). However, PAA
`has an unpleasant odor that limits acceptability by patients;
`recently, PBA, which is rapidly converted to PAA by
`mitochondrial j3-oxidation, has been used as a pro-drug
`(Piscitelli et al. 1995).
`Knowledge regarding concentrations of BA, PAA, and
`PBA and their metabolites HA and PAG in urine and blood
`is a prerequisite for detailed studies on their metabolism
`and for pharmacokinetic and evaluation studies. Further(cid:173)
`more, individual dosage and therapy optimization are
`highly important in children with inborn errors of urea
`synthesis. However, drug monitoring is only of value if the
`results are indeed rapidly available. Procedures described for
`analysis of these compounds are time consuming and require
`large sample volumes because separate chromatographic
`methods are needed to measure all five compounds. To date,
`several studies have addressed the effect and pharmacokinet(cid:173)
`ics of some ofBA, PAA, and PBA in biological systems using
`high-performance liquid chromatography and gas chromatog-
`
`raphy (HPLC-GC) coupled with mass spectrometry (MS)
`(Hommes 1999; Kubota et al. 1988; Thibault et al. 1994b;
`Yu et al. 2001 ; Zimmerman et al. 1990). However, none of
`these methods describes the measurement of all the five
`compounds in a single analytical run, which is very
`important when the compounds are coadministered in the
`manner currently used to treat the various hyperammonemias
`(Enns et al. 2007).
`This article describes the development and validation of an
`LC-MS/MS method to simultaneously determine PAA, PBA,
`and BA and their metabolites PAG and HA in body fluids,
`which requires minimal sample volume. The method was
`successfully applied to detect levels ofBA and PBA and their
`metabolites after oral administration to children with hyper(cid:173)
`ammonemia due to inborn errors of urea synthesis.
`
`Materials and methods
`
`Reagents
`
`Benzoylaminoacetic acid (hippuric acid), phenylacetic acid,
`phenylbutyric acid, and benzoic acids were obtained from
`Sigma-Aldrich (Deisenhofen, Germany); PAG was purchased
`from Bachem (Bubendorf, Switzerland); and formic acid was
`from Merck (Darmstadt Germany). Deuterium-labelled inter(cid:173)
`nal standards were purchased from CDN Isotopes (Pointe(cid:173)
`Claire Quebec, Canada). LC-MS-grade methanol and water
`were from Fischer Scientific (Fair Lawn, NJ, USA).
`
`Specimen
`
`Pooled blank blood samples used to develop and validate
`the procedure were obtained from the Heinrich-Heine
`University Blood Bank. Blank urine samples were collected
`from healthy laboratory members.
`This study was approved by the ethical committee of the
`~OH
`medical faculty of the Heinrich-Heine University of
`0
`Dusseldorf (Germany), filed under study number 3415.
`
`v
`
`HOOO
`I""'
`0
`
`BA
`
`HA
`
`PAA
`
`Patients
`
`~OH
`6
`
`v
`
`Twelve patients with hyperammonemia due to various
`inborn errors of urea synthesis were studied. Six patients
`had ornithine transcarbamoylase deficiency, three had
`argininosuccinate synthetase deficiency, and three had
`arginase deficiency.
`
`PBA
`
`PAG
`
`Preparation of standards
`
`Fig. 1 Molecular structures of the analytes benzoate (BA), hippurate
`(HA), phenylacetate (PAA), phenylbutyrate (PBA), and phenylacetyl(cid:173)
`glutamine (PAG)
`
`Stock solutions ofPAG, HA, PBA, PAA, and BA (3.7, 5.5,
`6.1, 7.3, and 8.3 mmol/L), respectively, were prepared in
`70% ethanol and stored at -20°C. Combined working
`
`~Springer
`
`Par Pharmaceutical, Inc. Ex. 1026
`Par v. Horizon, IPR of Patent No. 9,561,197
`Page 2 of 8
`
`

`

`J Inherit Metab Dis (20 10) 33 (Suppl 3):S32 1-S328
`
`S323
`
`solution was prepared by diluting the stock solution to give
`concentrations of37, 55, 66, 73, and 83 ~-tmol/L PAG, HA,
`PBA, PAA, and BA, respectively. This mixed standard
`solution was further serially diluted with methanol to obtain
`methanol calibration standards in the range ofO.l-37 ~-tmol/L
`for PAG, 0.1-55 ~-tmol/L for HA, 0.1-62 ~-tmol/L for PBA,
`0.2-73 ~-tmol/L for PAA, and 0.2-82 ~-tmol/L for BA. Similar
`calibration standards were also prepared by serially diluting
`the mixed standard solution using plasma and urine.
`Three quality control (QC) samples with low (1.2 ~-tmol/L
`PAG, 1.7 ~-tmol/L HA, 1.9 ~-tmol/L PBA, 2.3 ~-tmol/L PAA,
`and2.6~-tmol/LBA), medium(9.4~-tmol/LPAG, 13.9~-tmol/L
`HA, 15.2 ~-tmol/L PBA, 18.4 ~-tmol/L PAA, and 20.5 ~-tmol/L
`BA), and high (37.7.4 ~-tmol!L PAG, 55.7 ~-tmol!L, HA,
`60.8 ~-tmol/L PBA, 73.6 ~-tmol/L PAA, and 81.9 ~-tmol!L BA)
`were prepared by spiking the mixed standard solution to
`plasma and urine pools. Calibration standards and quality
`control samples were prepared fresh on each day of validation.
`A solution of 10,000 ng/ml each of the deuterated
`labelled analytes in methanol was used as internal standard
`(IS) and consequently for precipitation of protein from
`serum or plasma samples.
`
`Sample preparation
`
`Plasma
`
`One hundred microliters of each sample or standard
`solution containing the analytes of interest were placed
`into a 1.5-ml conical plastic centrifuge tube, and 100 ~-tl of
`IS solution in methanol and a further 100 ~-tl methanol were
`added to the tube to precipitate the proteins in the sample.
`The tubes were capped, vortexed for at least 30 s, and
`centrifuged at 1,000xg for 5 min. Supernatant was
`transferred into autosampler vials.
`
`Urine
`
`As the urine concentrations of HA and PAG were expected
`to be high in urine samples, sample preparation could be
`simplified by a dilution step. Urine of patients was first
`diluted in a ratio of 1:100 with water, and urine of drug-free
`patients was diluted in a ratio of 1:10. Urine was worked up
`using the same procedure as described above.
`
`Liquid chromatography tandem mass spectrometry
`
`The LC-MS/MS system used consisted of a Waters
`Alliance 2795 separation module (Waters, Milford, UK)
`coupled to a Quattro Micro mass spectrometry system
`(Micro Mass, Manchester, UK). Guard and analytical
`columns were Phenomenex Gemini NX (4.0x3 mm,
`5 ~-tm) and Gemini NX (2.1 x 15 mm, 3 ~-tm) , respectively.
`
`Sample elution was isocratic over 7 min using a mobile
`phase containing 0.01% formic acid/methanol (35:65, v/v).
`Ten microliters of the supernatant containing the analytes
`were injected into the HPLC.
`Electrospray ionization was performed in the negative
`ionization mode (ESI-). The following conditions were
`found to be optimal for the analysis: capillary voltage 3 kV;
`source block temperature l20°C; desolvation gas (nitrogen)
`heated to 350°C and delivered at a flow rate of 625 L/h.
`The appropriate selected reaction monitoring (SRM) con(cid:173)
`ditions for the individual analytes and their respective
`deuterated analogues were determined by direct infusion
`into the MS/MS. The cone voltage (CV) was adjusted to
`maximize the intensity of the deprotonated molecular
`species [M-HL and collision-induced dissociation of each
`deprotonated molecule was performed. Collision gas
`(argon) pressure was maintained at 2.7 10- 3 mbar and
`the collision energy ( e V) adjusted to optimize the signal
`for the most abundant product ions, which were subse(cid:173)
`quently used for MS. The instrument response for the
`analytes was optimized by infusion experiments of the
`pure compounds dissolved in 70% ethanol at a flow rate of
`10 ~-tllmin.
`All aspects of system operation and data acquisition
`were controlled using Mass Lynx NT 4.0 software with
`automated data processing using the quantify option of
`Mass Lynx software. Statistical analysis was carried out
`using Microsoft Excel.
`
`Method validation
`
`Linearity was assessed by analyzing calibrators ranging in
`concentrations from 0-37.8 ~-tmol/L for PAG, 0-55.8 ~-tmol/
`L for HA, 0-73.4 ~-tmol/L for PAA, 0-61 ~-tmol/L for PBA,
`and 0-82.5 ~-tmol/L for BA using 10-~-tl injection volumes.
`Identities of BA, HA, PAG, PAA, and PBA peaks were
`verified by analyzing the compound-specific mass spectra
`after addition of calibrators. The lower limit of detection
`(LLOD)-as defined by a signal-to-noise ratio (SNR) of 3
`of the lower limit of quantitation (LLOQ), as defined by an
`SNR of 10-were determined in both plasma and urine.
`Total and within-run imprecision measured on plasma and
`urine at three concentrations was assessed by analyzing the
`samples six times within 1 day, using >20 different assays
`within a 4-week period. The amounts added to the speci(cid:173)
`mens were chosen to cover the ranges of calibration curves
`and to include all specimens of high value encountered in
`patients being treated with sodium BA, PBA, and PAA.
`Accuracy and recovery of the method was calculated from
`the same samples used for total and within-run imprecision
`measurements. Five assays at each concentration were
`performed. Analyte stability determinations were performed
`as freeze-and-thaw cycle and autosampler stability over
`
`~Springer
`
`Par Pharmaceutical, Inc. Ex. 1026
`Par v. Horizon, IPR of Patent No. 9,561,197
`Page 3 of 8
`
`

`

`S324
`
`Table 1 Selected reaction mon(cid:173)
`itoring (SRM) transitions and
`conditions for all compounds
`and their deuterated analogues
`
`Balded transitions used as
`qualifiers.
`
`J Inherit Metab Dis (2010) 33 (Suppl3):S321-S328
`
`Precursor ion (m/z)
`
`Product ion (m/z)
`
`Collision energy (eV)
`
`Benzoic acid
`d5-Benzoic acid
`Phenylacetic acid
`d5-Phenylacetic acid
`Phenylbutyric acid
`dn-Phenylbutyric acid
`Hippuric acid
`d5-Hippuric acid
`Pheny lacety !glutamine
`d5-Phenylyacety !glutamine
`
`121
`126
`135
`140
`163
`174
`178
`183
`263
`268
`
`77
`82
`91
`96
`91
`98
`77/134
`82/139
`127/145
`127/145
`
`10
`10
`15
`15
`15
`10
`10
`15
`25/15
`25/15
`
`3 days at room temperature (RT). For storage stability tests,
`plasma and urine samples were stored up to 7 days at 4 °C
`or RT and at -20°C for 3 months.
`
`Results
`
`Chromatography and tandem mass spectrometry
`
`SRM transitions and fragmentation conditions selected for the
`analytes are shown in Table 1, and corresponding represen(cid:173)
`tative LC-MS/MS chromatograms of the five analytes are
`shown in Fig. 2.
`
`Linearity, LLOD, and LLOQ
`
`Standard calibration curves were generated by plotting the
`response, which is defined as the ratio between the area of
`the target analyte to its IS multiplied by the concentration of
`IS against the concentrations of PAG, HA, PAA, BA, and
`PBA in methanol, plasma, and urine. All calibration curves
`were reproducible, and correlation coefficients (r2
`) of the
`curves were >0.99 for all five analytes. All standard curves
`were linear up to the maximum concentrations measured
`(Fig. 3). The slopes of the calibration curves between pure
`solvent and pooled urine solutions were nearly the same,
`probably due to the high dilution of the urine samples.
`
`Fig. 2 Selected reaction moni(cid:173)
`toring (SRM) chromatograms of
`benzoate (BA), phenylacetate
`(PAA), and phenylbutyrate
`(PBA) and their metabolites
`phenylacetylglutamine (PAG)
`and hippuric acid (HA) in pure
`solvent
`
`~Springer
`
`2.99
`
`2.99
`
`177.9 > 133.9
`
`P G
`
`100
`
`:
`
`''~~]~~~P~A~G~~~~~~~TIITTITI~~~~~~~~~~~~~~~~~n~~~~~:~6~3.2> 145
`3::1 ::;::::::;:==A;::::::;::::::;:::::;:::::;::::::::;::;:::;::::::;:::=;:::f\~____:;=:::;:::::;:::::::;==:;::::::;:::::;::::;::::::;:==;:::::::;:2=63 _ 2 > 1 2 7
`li
`":~
`W:--f--n-TTTI-;-rr~H~~rrn--rrn-rrr;~TTTTTTT'~'"l ~rrrr-rrr;-,-,-f,~~2 ~-9rr4,--;ni'l"Fr.,.,..,"'l'ffFFFF'FT'l"T~--rrr;-;-n-TTTI~TTTrn-r~1~7-n7~- 9 > 7 6 .6
`
`4.76
`
`1 0~ ],~~--rrrP~A~~~~]TTT~~~rrn-~~~,.~:'--rn-~-n"TTTTj'TTTT'f"TTT"rrrrrrn-~~~~~1"3-n4~-~7 > 90.8
`
`3.22
`
`3.31
`10~ ~
`BA
`(\
`0 '6, ~~-~~~~-~~----....-i-~~,--,--;o;.~~-------~~-Time
`
`0.50
`
`1.00
`
`1.50
`
`2.00
`
`2.50
`
`3.00
`
`3.50
`
`4.00
`
`4.50
`
`5.00
`
`5.50
`
`120.7 > 76.6
`
`Par Pharmaceutical, Inc. Ex. 1026
`Par v. Horizon, IPR of Patent No. 9,561,197
`Page 4 of 8
`
`

`

`J Inherit Metab Dis (2010) 33 (Suppl 3):S321-S328
`
`S325
`
`12000
`
`10000
`
`BA
`
`8000
`
`Cl)
`1/)
`s:::
`0 c.. 6000
`Cl) a:
`
`1/)
`
`4000
`
`2000
`
`0
`
`0
`
`10000
`
`8000
`
`6000
`
`4000
`
`2000
`
`PBA
`
`•
`
`Pure solvent
`y = 0.8825x - 10.022
`R2 = 0.9976
`
`Plasma
`y = 0.7553x- 33.256
`R2 = 0.9987
`
`PAA
`
`60000
`
`50000
`
`40000
`
`30000
`
`20000
`
`10000
`
`Cl)
`1/)
`s:::
`0 c..
`Cl) a:
`
`1/)
`
`Pure solvent
`y = 5.4963x + 682.72
`R2 = 0.9942
`
`Plasma
`y = 5.6652x- 188.23
`R2 = 0.9935
`
`Cl)
`
`1/) s:::
`0 c..
`Cl) a:
`
`1/)
`
`Pure solvent
`y = 1.1138x + 40.527
`R2 = 0.9996
`
`Plasma
`y = 0.9574x + 304.74
`R2 = 0.9941
`
`10000
`5000
`[ng/ml]
`
`15000
`
`0
`
`0
`
`10000
`5000
`[ng/ml]
`
`15000
`
`0
`
`0
`
`10000
`5000
`[ng/ml]
`
`15000
`
`17500
`
`15000
`
`HA
`
`Cl)
`1/)
`s:::
`0 c..
`Cl) a:
`
`1/)
`
`12500
`
`10000
`
`7500
`
`5000
`
`2500
`
`Pure solvent
`y = 1.5847x + 77.226
`R2 = 0.9988
`Plasma
`y = 1.427x + 548.29
`R2 = 0.9802
`
`0
`
`0
`
`5000
`
`10000
`
`15000
`
`[ng/ml)
`
`6000
`
`5000
`
`PAG
`
`Cl) 4000
`1/)
`s:::
`0
`m- 3000
`a:
`
`2000
`
`Pure solvent
`y = 0.5188x + 7.2338
`R2 = 0.9985
`
`1000
`
`Plasma
`y = 0.4928x + 64.828
`R2 = 0.9956
`0 +-------,-------,-------,
`0
`5000
`10000
`15000
`
`[ng/ml]
`
`Fig. 3 Calibration curves of benzoate (BA), phenylacetate (PAA), and phenyl butyrate (PBA) and their metabolites phenylacetylglutamine (PAG)
`and hippuric acid (HA) in plasma ( .._) and pure solvent ( •)
`
`Regression equations of plasma probes and standard
`solutions (pure solvent) are depicted in Fig. 3. LLOD
`values for the target analytes were 28-34 ng/ml (0.1-
`0.3 ~J.mol/L), whereas LLOQ values were 100 ng/ml (0.3-
`0.8 ~J.mol/L).
`
`up to 3 days. Furthermore, samples were found to be stable
`after three freeze/thaw cycles. No analyte degraded under
`the assay conditions for at least 72 h in the autosampler.
`
`Clinical application of the method
`
`Precision, recovery, and stability
`
`Precision and recovery data for each analyte are summa(cid:173)
`rized in Table 2. Mean recovery of all analytes was >90%
`(range 95-103%). The intra- and interassay precisions were
`highly satisfactory, with all relative size distribution (RSD)
`values being <10%. Ranges were 3.2-7.3% and 3.1-7.0%
`for within- and between-day, respectively.
`All analytes in the samples were found to be stable in
`plasma and urine when stored at -2ooc for 3 months and
`were also stable when incubated at ooc as well as at RT for
`
`A preliminary study for determining PBA, PAA, PAG, HA,
`and BA in plasma and urine samples demonstrated the
`clinical applicability of the assay for therapeutic monitor(cid:173)
`ing. In total, 12 patients with hyperammonemia due to
`various inborn errors of urea synthesis were studied.
`Ornithine transcarbamoylase deficiency (six patients), argi(cid:173)
`ninosuccinate synthetase deficiency (three patients), and
`arginase deficiency (three patients) were treated with BA
`alone or in combination with PBA in doses of 1.5-6 g/d.
`Blood and urine samples were taken approximately 2-4
`h after intake. In patients receiving either PBA alone (nine
`
`~Springer
`
`Par Pharmaceutical, Inc. Ex. 1026
`Par v. Horizon, IPR of Patent No. 9,561,197
`Page 5 of 8
`
`

`

`S326
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`J Inherit Metab Dis (2010) 33 (Suppl3):S321-S328
`
`Table 2 Accmacy, precision, and recovery data for plasma and urine samples
`
`Calibration range
`(J.tmol/L)
`
`Amount added
`(J.tmol!L)
`
`Recovery run (n=20)
`(%)
`
`Within-run (n=5)
`(% RSD)
`
`Between-run
`(% RSD)
`
`BA (plasma)
`
`0.08- 82.5
`
`PAA (plasma)
`
`0.07- 73.4
`
`PBA (plasma)
`
`0.06-61.0
`
`HA (urine)
`
`0.06-55.8
`
`PAG (mine)
`
`0.04- 37.8
`
`2.6
`20.5
`81.9
`2.3
`18.4
`73.6
`1.9
`15.2
`60.8
`1.7
`13.9
`55.7
`1.2
`9.4
`37.7
`
`98
`99
`98
`96
`97
`101
`105
`99
`104
`95
`99
`99
`105
`98
`98
`
`4.0
`3.4
`3.7
`3.4
`4.7
`3.9
`4.2
`5.1
`6.3
`6.2
`7.3
`3.2
`4.1
`3.2
`3.2
`
`3.1
`3.9
`4.2
`4.7
`5.3
`5.0
`4.4
`7.0
`4.3
`5.8
`6.3
`2.9
`3.8
`2.8
`3.4
`
`Pooled mine/plasma calibrators were used to analyze target analytes
`BA benzoic acid, PAA phenylacetic acid, PBA phenyl butyric acid, HA hippmic acid, PAG phenylacetylglutamine, RSD relative standard deviations
`
`patients) or in combination with BA (three patients), plasma
`levels of PAA, PBA, and PAG were within the ranges of
`100-250, 10-300, and 200-650 1-1-mol/L, respectively. For
`the three patients who received BA in addition to PBA,
`plasma BA was in the range of 9-59.2 1-1-mol/L. Moreover,
`plasma HA was high in these patients (range 50-160 1-1-mol/
`L). In urine of BA-treated patients, concentrations of HA
`were highly increased, range 6,000-30,000 mmol/mol
`creatinine. These high concentrations of HP demonstrate
`its clinical purpose for disposition of waste nitrogen. Urine
`BA levels of patients receiving BA was 14- 107 mmol/mol
`creatinine. In patients receiving either PAG alone or in
`combination with BA, urinary levels of PAA, PBA, and
`PAG were in the ranges of 20-200, 12-411, and 800-
`20,000 mmollmol creatinine, respectively. These findings,
`on a limited number of unselected patients, obviously do
`not represent reference values, but clearly demonstrate the
`applicability of the method.
`
`Discussion
`
`Knowledge of concentrations ofBA, PAA, and PBA and their
`metabolites HA and PAG in urine and blood is a prerequisite
`for detailed studies on their metabolism as well as for
`pharmacokinetic and evaluation studies. In children with
`inborn errors of urea synthesis, individual dosage and therapy
`optimization are highly important as, for instance, higher
`doses of BA are known to be toxic (Van Hove et al. 2005).
`
`~Springer
`
`Coadministration of BA and PBA necessitates the
`simultaneous determination of these compounds and their
`major metabolites in a single run (Thibault et al. 1994b; Yu
`et al. 2001 ). Pharmacokinetics studies on individual as well
`as coadministered sodium PAA and sodium BA have been
`described in the literature (Camacho et al. 2007; Carducci et
`al. 2001 ; Kubota and Ishizaki 1991 ; MacArthur et al. 2004;
`Piscitelli et al. 1995; Thibault et al. 1994a). These studies
`were conducted mostly by intravenous administration of the
`compounds, and the results cannot necessarily be extended
`to oral administration, which is common practice nowa(cid:173)
`days. Further well-designed studies are thus called for to
`optimize levels for maximum elimination of waste nitrogen
`while keeping plasma and urine concentrations below
`levels associated with adverse effects. Requirement for
`such studies will be the determination of the analytes and
`their metabolites in both plasma as well as urine samples.
`Our goal was to develop an easy-to-operate and
`validated method for quantifying BA, PAA, and PBA and
`their metabolites HA and PAG in plasma and urine. The
`newly developed method described here offers the advan(cid:173)
`tage of a 7 -min run time for simultaneous quantitation of all
`five target analytes. The strength of the method is the
`simple sample extraction procedure and the increased
`reliability of quantification using deuterated IS. For the
`chromatographic procedure, we tested several combinations
`of columns and different mobile phases with different pH
`ranges. Careful selection of pH and water/methanol amount
`in the mobile phase was found to be highly effective for
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`Par Pharmaceutical, Inc. Ex. 1026
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`J Inherit Metab Dis (2010) 33 (Suppl 3):S321-S328
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`satisfactory separation of the analytes. A Phenomenex
`Gemini NX column allowed for good chromatographic
`peak shape and acceptable retention times of the analytes.
`The applied isocratic mobile phase (methanol 0.01% formic
`acid, 65:35 v/v) allowed sufficient separation and good
`symmetry of the compounds. Compounds PAG/HA and
`PAA/BA showed similar retention times on the chromato(cid:173)
`graphic peak scale. This, however, did not affect their
`quantification due to different precursor and product ions.
`With the parent and daughter ions of each analyte being
`monitored in SRM mode, the method yielded a mean
`recovery >90% for all analytes. Parent and daughter ion
`selection was achieved by acquiring full-scan mass spectra
`of each analyte using ESI-negative ion mode, as all
`analyzed compounds were found to exhibit much better
`ionization efficiency in that mode.
`Special attention is necessary to evaluate matrix effects
`when developing a quantitative LC-MS/MS method.
`Coeluting, undetected compounds originating from the
`plasma or urine matrix can cause signal suppression or
`enhancement. Hence, matrix effect can affect the accuracy
`and reproducibility of the developed quantitative method.
`The effect of the matrix is also strongly dependent on the
`degree of sample cleanup (selective extraction) and degree
`of chromatographic separation and retention of the analytes
`on the HPLC column. Optimizing these two effects may
`improve significantly the efficiency and reproducibility of
`the electrospray ionization process occurring in the ion
`source of the mass spectrometer. On the other hand,
`extensive sample preparation methods are not desirable in
`clinical applications due to the labor-intensive and time(cid:173)
`consuming process. One method, which is still employed
`during LC-MS/MS experiments to control matrix effects, is
`the use of labelled IS. These stable isotopically labelled
`analogues are believed to be the most appropriate IS in the
`quantitative LC-MS/MS due to their nearly identical
`physicochemical properties to the unlabeled target analytes.
`They should not only help in correcting sample preparation
`during the extraction procedure, but also help for compen(cid:173)
`sating the variable MS response in ESI. In many cases,
`stable isotopically labelled analogues as IS are not always
`available. Fortunately, we obtained for this method all five
`target analytes in the form of the deuterated analogues. By
`using an appropriate concentration of the deuterated IS, the
`precision and accuracy of the method were adequate to
`support our clinical applications.
`This newly developed method was successful in a
`preliminary study on determining PBA, PAA, PAG, HA,
`and BA in patient plasma and urine samples, clearly
`demonstrating the clinical applicability of our assay.
`Furthermore, therapeutic monitoring of the analytes may
`well be relevant in cancer research because PAA and PBA
`have been found to suppress tumour growth and promote
`
`cellular differentiation in multiple tumour models (Camacho
`et al. 2007; Carducci et al. 2001 ; Piscitelli et al. 1995; Samid
`et al. 1992; Thibault et al. 1994a).
`In conclusion, a robust, fast, and sensitive LC-MS/MS
`method has been developed to simultaneously determine
`BA, PBA, PAA and their metabolites PAG and HA in
`human plasma and urine. The method offers rapid, accurate,
`and clinically useful means of monitoring the therapeutic
`course. Furthermore, this method can be applied for
`pharmacokinetic and toxicity studies.
`
`Acknowledgment The authors thank Dr. F. ter Veld for reviewing
`this manuscript
`
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