`
`ONCOLOGY
`
`Disposition of Phenylbutyrate andits
`Metabolites, Phenylacetate and
`Phenylacetylglutamine
`StephenC.Piscitelli, PharmD, Alain Thibault, MD, William D. Figg, PharmD,
`Anne Tompkins, RN, Donna Headlee, RN, Ronald Lieberman, MD,
`Dvorit Samid, PhD, and Charles E. Myers, MD
`
`
`
`Phenylacetate, an inducer of tumor cytostasis and differentiation, shows promise as a
`relatively nontoxic antincoplastic agent. Phenylacetate, however, has an unpleasant odor
`that might limit patient acceptability. Phenylbutyrate, an odorless compoundthat also
`has activity in tumor models, is known to undergo rapid conversion to phenylacetate by
`beta-oxidation in vivo. This phase I study examined the pharmacokinetics of phenylbu-
`tyrale and characterized the disposition of the two metabolites, phenylacetate and phe-
`nylacetylglutamine. Fourteen patients with cancer (aged 51.8 + 13.8 years) received a 30-
`minute infusion of phenylbutyrate at 3 dose levels (600, 1200, and 2000 mg/m’). Serial
`blood samples and 24-hour urine collections were obtained. Samples were assayed by
`high-performance liquid chromatography. A model to simultaneously describe the phar-
`macokineticsof all three compounds was developed using ADAPT II. Data were modeled
`as molar equivalents. The modelfit the data well as shown by mean (+SD) coefficients of
`determination (r?) for phenylbutyrate, phenylacetate, and phenylacetylglutamine, which
`were 0,96 + 0.07, 0.88 + 0.10, and 0.92 + 0.06, respectively. The intrapatient coefficient of
`variation percentage (CV%) around the parameter estimates were small (range 7.2-
`33,5%). Phenylbutyrate achieved peak concentrations in the range of in vitro tumor ac-
`tivity (500-2000 pmol/L) and exhibited saturable elimination (Kj, = 34.1 + 18.1 we/mL
`and Vinax = 18.1 + 18 mg/h/kg). Metabolism was rapid; the times to maximum concentro-
`tion for phenylacetate and phenylacetylglutamine were 1 and 2 hours, respectively. The
`conversion of phenylbutyrate to phenylacetate was extensive (80 + 12.6%), but serum
`concentrations of phenylacetate were low owing to rapid, subsequent conversion to phe-
`nylacetylglutamine. The ratio of phenylbutyrate AUC to phenylacetate AUC was 2.66.
`Thus, phenylbutyrate may not be a prodrug for phenylacetate und should be pursued us
`an independent antitumor agent.
`
`Te amino acid phenylalanine is degraded by a
`combination of hydroxylation and deamination,
`leading to a range of metabolic products including
`
`From the Clinical Pharmacokinetics Research Laboratory, Pharmacy
`Department, Clinical Center, National Institutes of Health, Bethesda,
`Maryland (Dr. Piscitelli); the Clinical Pharmacology Branch, National
`Cancer Institute, National Institutes of Health, Bethesda, Maryland
`(Mss. Tompkins and Headlee, and Drs. Thibault, Figg, Samid, and My-
`ers); and the Center for Drug Evaluation and Research, Food and Drug
`Administration, Rockville, Maryland (Dr. Lieberman). Address for corre-
`spondence: William D, Figg, PharmD, Clinical Pharmacokinetics, Sec-
`tion, Clinical Pharmacology Branch, National CancerInstitute, Building
`10, Room 5401, Bethesda, MD 20892.
`
`368 © JClin Pharmacol! 1995;35:368-373
`
`phenylacetate, a compound used to treat children
`with hyperammonemic urea cycle disorders.’ Man
`and higher primates conjugate phenylacetate with
`glutamine to form phenylacetylglutamine, whereas
`in rodents this compound is conjugated with gly-
`cine.’ Thefact that phenylacetate is conjugated with
`and depletes circujating glutamineis of special inter-
`est, because tumorcells are highly dependentonthis
`amino acid, rendering glutamine a target for thera-
`peutic intervention. In addition to potential gluta-
`mine starvation, phenylacetate can arrest
`tumor
`growth by modulating the expression of genescriti-
`cal to growth control and differentiation.**
`Recently, phenylacetate has been shownto possess
`cytostatic and differentiating properties against a va-
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`PHENYLBUTYRATE AND ITS METABOLITES, PHENYLACETATE AND PHENYLACETYLGLUTAMINE
`
`
`riety of hematologic and solid tumors in laboratory
`sion of phenylbutyrate, and serial blood samples
`models,*® Whengivento healthy subjects, phenyla-
`werecollected before, immediately post-dose, and at
`cetate undergoes hepatic conjugation with gluta-
`0.16, 0.3, 0.5, 0.75, 1, 1.5, 2.5, 3.5, and 5 hours after
`mine by phenylacetyl coenzymeA: glutamineacyl-
`the infusion. Blood samples (5 mL) were collected in
`transferase, which yields phenylacetylglutamine,
`5-mL glass tubes (Vacutainer®; Becton Dickinson,
`the major urinary metabolite.” Although previously
`Rutherford, NJ) either via an intravenous catheter
`shownto follow first-order pharmacokinetics,’ the
`(separate from the drug administration catheter) or
`drug exhibits nonlinear, saturable pharmacokinetics
`venipuncture. Blood was centrifuged, and the serum
`at doses currently being evaluated in patients with
`wastransferred to 5-mL polypropylene tubesandfro-
`cancer.® Phenylacetate, however, has an unpleasant
`zen at —85°C until the time of analysis. A 24-hour
`odor that might limit its acceptance and develop-
`urine collection for cumulative phenylacetylglutam-
`ment as an oral drug.
`ine excretion was done in a subsetofpatients.
`The reversed phase high-performanceliquid chro-
`In contrast, phenylbutyrate is an odorless com-
`matography method for measuring serum concentra-
`pound andhasalso been safely given to children for
`hyperammonemic urea cycle disorders.*’® Recent
`tions of phenylacetate, phenylbutyrate, and phe-
`nylacetylglutamine has been previously described.’*
`laboratory studies have documentedthat phenylbu-
`tyrate, like phenylacetate, can (1) induce selective
`Briefly, 100 wL of 10% perchloric acid was used to
`precipitate the proteins of a 200-yL aliquot of serum,
`cytostasis and maturation of cultured tumorcells de-
`which was then centrifuged. The supernatant was
`rived from various erythropoietic and solid neo-
`neutralized with 25 uL ofa 20% solution ofpotassium
`plasms(including adenocarcinomasofthe prostate,
`breast, ovary, colon, and lung,as well as central ner-
`bicarbonate. After centrifugation, 20 uL of superna-
`tant was injected onto a C-18 column heatedat 60°C.
`vous system tumors and malignant melanoma); (2)
`Urine samples were processedsimilarly, after a 1:20
`modulate the expression of genes implicated in tu-
`dilution with water. Elution was done with an in-
`morgrowth, metastasis, and immunogenicity; and(3)
`creasing gradient of acetonitrile in water from 5 to
`enhancetheefficacy of other agentsof clinical inter-
`30% over 45 minutes. Its progress was followed by
`est including retinoids, interferon alfa, suramin, 5-
`monitoring ultraviolet absorbance at 208 nm. Char-
`aza-2’-deoxycytidine, and hydroxyurea (Samid et
`acteristic elution times for phenylacetylglutamine,
`al.®; Liu et al., Hudginset al., Figg et al., submitted:
`phenylacetate, and phenylbutyrate were 10.1, 17.4,
`Sand et al., unpublished data). Phenylbutyrate is
`and 27.8 minutes, respectively. The assay yielded a
`converted in vivo to phenylacetate by mitochondrial
`lower limit of detection of 2 ug/ml and waslinear
`beta-oxidation.’' Therefore, phenylbutyrate is cur-
`rently being investigated as a new antineoplastic
`for concentrations as high as 2,000 ng/mL. Between
`20 and 1,000 ng/mL, the interassay CV% was less
`agent, and asaprodrugfor phenylacetate in thetreat-
`than 10%.
`mentof cancer.
`A model to simultaneously describe the pharma-
`To better understand the disposition of these com-
`cokinetics of all three compoundswas developed us-
`poundsafter intravenous administration of pheny]-
`ing ADAPTII."* Several models were constructed to
`butyrate, a pharmacokinetic model
`that simulta-
`compare one and two compartments for each drug,
`neously characterizes the disposition of phenylbu-
`as well as the possibility of nonlinear pharmacoki-
`tyrate, phenylacetate, and phenylacetylglutamine
`netics. Model selection was determined by Akaike’s
`was developed from plasma and urine data collected
`Information Criterion (AIC),’° and by visual inspec-
`during a phaseI clinicaltrial.
`tion of the difference between measured: and com-
`puter-fitted concentrations (residuals). Data were
`modeled as molar equivalents. The pharmacokinetic
`parameters wereestimated using weighted nonlin-
`ear least squares by an adaptive processthat usedse-
`quential updating of priors for parameter values.
`Weighting was by the inverse of the observation vari-
`ancefor all compounds.
`:
`Drug input was by intermittent intravenous infu-
`sion. To make the modelidentifiable, the volume of
`distribution of phenylacetate was fixed at'0.3 L/kg
`based on previousphase | data in which phenylacet-
`ate wasgiven intravenously.° Complete conversion
`of phenylacetate to phenylacetylglutamine and
`elimination of all phenylacetylglutamine in the
`
`Adults with advancedsolid tumors refractory to con-
`ventional therapy, a performancestatus greater than
`60% on Karnofsky’s scale,’* normal hepatic transam-
`inases andbilirubin, a serum creatinineless than 1.5
`mg/dL, and normal leukocyte and platelet counts
`wereeligible for this study. The clinical protocol was
`approved by the National CancerInstitute’s Institu-
`tional Review Board, and al! patients gave written in-
`formed consent before participating in the study.
`Patients were enrolled into the study in cohorts of
`at least 3 per dosage level (600, 1200, and 2000 mg/
`m2). Each patient received a single 30-minute infu-
`
`METHODS AND DEVELOPMENT OF MODEL
`
`ONCOLOGY
`
`369
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`PISCITELLI ET AL
`
`
`
`
`
`Figure 1. Modelto describe the disposition of phenylbutyrate (PB),
`phenylacetate (PA), and phenylacetylglutamine (PAG)illustrating
`the pharmacokinetic parameters.
`Abbreviations: Vpg, volume of distribution for PB; Vp,, volume
`of distribution for PA; Veac. volume of distribution for PAG; MM,
`Michaelis-Menten elimination; K,,, Michaelis-Menten constant;
`Vinex Maximumclimination rate; CL;, formational clearance of PA
`to PAG: CL,, clearance of PAG into the urine; CL,,, clearance of PB
`out of the central compartment.
`
`ination rate constant for the metabolites owing to
`sparsedata describing the terminal slope.
`
`RESULTS
`
`Patients
`
`Fourteen male patients were included in the study.
`Three patients received 600 mg/m’ of phenylbutyr-
`ate, 8 received 1200 mg/m’, and 3 received 2000 mg/
`m*. Patient demographics are shownin TableI.
`
`Pharmacokinetics
`
`The modelfit the data well as shown by mean (+SD)
`coefficients of determination(r*) for phenylbutyrate,
`phenylacetate, and phenylacetylglutamine, which
`were 0.96 + 0.07, 0.88 + 0.10, and 0.92 + 0.06, respec-
`tively. Pharmacokinetic parameters are shownin
`Table Il. The intrapatient CV% around the parame-
`ter estimates were small, ranging from 7.2 to 33.5%
`of the fitted values. The mean interpatient CV%for
`parametervalues ranged from 11.85 to 34.6%.
`Serum concentration-time plots for a representa-
`tive patient in each dosage group are shownin Figure
`2. Similarfits were seen for the other patients. Peak
`concentrations of phenylbutyrate after 600 mg/m*
`ranged from 31 to 57 ug/mL. After 1200 mg/m* and
`2000 mg/m”, peak concentrations in serum ranged
`from 57 to 115 ng/mL and 114 to 184 wg/mL,respec-
`tively. Concentrations at 5 hours after dosing were 2
`
`
`
`TABLE |!
`
`Individual and Mean Patient Demographics
`Patient
`Age
`No.
`yn
`
`Height
`(cm)
`
`Weight
`(kg)
`
`urine was also assumed based on ourprevious phase
`I experience.*® Thus, the fraction of phenylbutyrate
`converted ta phenylacetate was determined using
`the following equation:
`
`urinary phenylacetylglutamine (umol)
`dose of pheny!butyrate (umol)
`The pharmacokinetic parameters for phenylacet-
`ylglutamine are dependent onthis fraction, whichis
`analogousto oral drugs where clearance and volume
`are dependent on the value of bioavailability [i.c.,
`CL/For V../F).
`The model eventually used was a one-compart-
`ment nonlinear model for phenylbutyrate with con-
`version to a one-compartment linear mode! for phe-
`nylacetate, and further conversion of phenylacetate
`to phenylacetylglutamine (one-compartment). Phe-
`nylbutyrate was parameterized by a central volume
`(Vpg), a minor elimination pathway (CL,,), and a non-
`linear function consisting of intrinsic clearance
`(CL,,,) and the Michaelis-Mentenconstant(K,,). The
`Vmax is equal to CLin:K,- The CL, and CL, describe
`the clearances of phenylacetate to phenylacetylglu-
`tamine and phenylacetylglutamine into the urine,
`respectively. The Vpac describes the volumeof dis-
`tribution (V.) for phenylacetylglutamine. The Vpa
`represents the volumeofdistribution of phenylacet-
`ate. The model displaying the pharmacokinetic pa-
`rameters is shownin Figure 1.
`The area under the serum concentration versus
`time curve (AUC) wascalculated by the trapezoidal
`rule according to Gibaldi and Perrier.’° The AUC was
`determined from time zero until the last time point
`{5 hours), because concentrations of each compound
`77.6
`173.0
`51.8
`Mean
`were usually below detectable limits at this point
`16.7
`10.2
`13.8
`sD
`aSSE
`and becauseofthe difficulty in determining an elim-
`
`Dose/m*
`
`Dose
`(mg)
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`14
`
`75
`60
`55
`66
`55
`61
`39
`29
`48
`35
`42
`46
`71
`43
`
`174
`188
`177
`164
`180
`180
`167
`179
`170
`169
`184
`152
`180
`158
`
`63.9
`87.0
`79.9
`70.6
`101.4
`101.4
`80.2
`70.6
`71.4
`72.5
`103.0
`54.0
`82.0
`48.7
`
`600
`600
`600
`1200
`1200
`1200
`1200
`1200
`1200
`1200
`1200
`2000
`2000
`2000
`
`1080
`1278
`1188
`2112
`2640
`2352
`2292
`2340
`2148
`2196
`2700
`3000
`4020
`2940
`
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`PHENYLBUTYRATE AND ITS METABOLITES, PHENYLACETATE AND PHENYLACETYLGLUTAMINE
`
`
`
`Pharmacokinetic Parameters Derived
`From the Model
`
`Mean
`$D
`
`
`0.08
`0.21
`Vpp (L/kg)
`0.04
`0.10
`CL,,, (L/hr/kg)
`18.1
`34.1
`Kn (ug/mL)
`0.30
`0.50
`Clint (L/hr/kg)
`18.0
`18.1
`Vinax (me/hr/kg)
`Fixed
`0.30
`Vea (L/ke)
`0.13
`0.37
`CLt1 (L/hr/kg)
`0.11
`0.19
`Vpac (L/kg)
`0.11
`0.17
`CLt2 (L/hr/kg)
`139.6
`265.4
`AUC PB 600 mg/m?
`167.9
`557.8
`AUC PB 1200 mg/m?
`689.2
`1214.5
`AUC PB 2000 mg/m?
`16.6
`120.0
`AUC PA 600 mg/m?
`90.6
`220.2
`AUC PA 1200 mg/m?
`160.0
`608.3
`AUC PA 2000 mg/m?
`119.2
`401.3
`AUC PAG 600 mg/m?
`269.7
`438.0
`AUC PAG 1200 mg/m?
`
`
`AUC PAG 2000 mg/m? 389.6 1055.4
`Vex = volume of distribution for PB; Vp, = volume ofdistribution for PA; Vas =
`volume of distribution for PAG; K,, = Michaelis-Menten constant; CL,, = intrinsic
`clearance; Vinae = Maximum elimination rate; CLt1 = formational clearance of PA
`to PAG; CLt2 = clearance of PAG into the urine; CLm = clearance of PB out of
`central compartment; AUC = area under the curve from time 0 to 5 hours post-
`dose; PB = phenylbutyrate; PA = phenylacetate; PAG = phenylacetylglutamine.
`AUC values are represented as :mol-hr/L.
`
`ug/mL or lowerin all patients. Phenylbutyrate ex-
`hibited saturable elimination pharmacokinetics as
`evidenced by concaveJog-linear plots on visual in-
`spection, an AUC,_; disproportionate to dose (Figure
`3), and improvedfits at high doses using a nonlinear
`function as assessed by AIC. Final estimates for Mi-
`chaelis-Menten parameters were a K,, of 34.1 + 18.1
`ug/mL and a Vinay Of 18.1 + 18.0 mg/h/kg.
`Six patients had complete 24-hour urine collec-
`tions for determination of phenylbutyrate conver-
`sion to phenylacetate, 1 at 600 mg/m’, and 5 at 1200
`mg/m’. The percentage of conversion was high with
`a mean (SD) of 80.0 + 12.6%. The conversion ranged
`from 68 to 100% of phenylbutyrate accountedforin
`the urine by phenylacetylglutamine.
`Phenylacetate was detectable in plasma immedi-
`ately after phenylbutyrate infusion with mean (+SD)
`peak concentrations of 20.7 + 13.6 wg/mL. The time
`to maximum concentration most commonly oc-
`curred 30 to 60 minutesafter the infusion. The serum
`concentrations of phenylacetate that were seen in
`this study were muchlowerthanthoseafter intrave-
`nous administration of phenylacetate.* After pheny!-
`butyrate administration, phenylacetate followed
`
`ONCOLOGY
`
`first-order elimination. The Michaelis-Menten con-
`stant of phenylacetate from our previoustrial® was
`105.1 + 44.5 wg/mL. The highest concentration of
`phenylacetate achieved in this study was 57 ng/mL
`with 11 of the 14 patients exhibiting peak concentra-
`tions less than 30 ug/mL. Because the peak phenyla-
`cetate concentrations were less than or equalto one-
`half the K,,, the nonlinear function of phenylacetate
`collapsesto a first-order rate constant.'® As a compar-
`ison of the total exposure between the 2 compounds,
`the mean (+SD)ratio of phenylbutyrate AUC to phe-
`nylacetate AUC was 2.66 + 1.57.
`Phenylacetylglutamine
`serum concentrations
`were also observed immediately after phenylbutyr-
`ate dosing. However, peak concentrations appeared
`1 to 3.5 hours after the infusion, which was later
`than those of phenylacetate. Phenylacetylglutamine
`achieved maximum serum concentrations of 59.5 +
`34.2 ug/mL, which ranged from 27 to 129% of those
`of phenylbutyrate (mean + SD, 61.2 + 29.9%}. Com-
`paratively, phenylacetate achieved peak concentra-
`tions that were only 38.8 + 19.2% of those of pheny-
`lacetylglutamine.
`
`DISCUSSION
`
`Pharmacokinetic models of anticancer agents can be
`used for a variety of purposes.In addition to describ-
`ing a drug’s disposition, these models can be used to
`(1) determine sampling schemes based on a small
`number of blacd samples (using optimal sampling
`theory’); (2) predict plasma concentrations of new
`regimens; or(3) optimize dosing for maximalefficacy
`and minimaltoxicity in patients receiving multiple
`courses of therapy.
`The simultaneous modeling approach usedinthis
`analysis accurately characterized the conversion
`anddisposition of phenylbutyrate andits two metab-
`olites, phenylacetate and phenylacetylglutamine.
`Thereis increasing interest in phenylacetate as a rel-
`atively nontoxic antitumor agent.*** The unpleas-
`ant odor of phenylacetate, however, maylimit its ac-
`ceptance bypatients. Phenylbutyrate is the odorless
`precursor of phenylacetate, with demonstrable anti-
`tumoractivity in laboratory models. Phenylbutyrate
`was converted to phenylacetate with subsequent
`conversion to phenylacetylglutamine. These conver-
`sions were rapid with detectable amounts of both
`metabolites occurring less than 10 minutesafterini-
`tiation of the phenylbutyrate infusion. Phenylbutyr-
`ate was characterized by nonlinear elimination
`pharmacokinetics with a K,, of 34.1 ug/mL and a
`Vinax Of 18.1 mg/h/kg.
`Ourgroup has previously reported the results of a
`phase J study of intravenous phenylacetate that
`showednonlinear pharmacokinetics for phenylacet-
`
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`curve. Line ofbestfit is shown; y = 105.95. 10°09") (R = .78).
`
`2000
`
`3000
`
`4000
`
`5000
`
`time (h)
`
`Figure 3. Plot of phenylbutyrate dose (mg) and area under the
`
`372 @ JClin Pharmacol 1995;35:368-373
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`concentration(uM)
`
`time (h)
`
`—=3= €5=od
`
`l=
`
`e2oc°°
`
`o
`
`
`
`concentration(uM)
`
`PISCITELLI ET AL
`
`Figure 2. Actual(squares) and computer-fitted (line) concentration-
`time profiles of phenylbutyrate, phenylacetate, and phenylacety!-
`glutaminein (A) patient 1 receiving 600 mg/m? of phenylbutyrate;
`(R) patient 5 receiving 1200 mg/m? of pheny!butyraie; (C) patient
`12 receiving 2000 mg/m?of phenylbutyrate.
`$<
`
`ate characterized by saturable metabolism to phe-
`nylacetylglutamine.* In this study, where concentra-
`tions of phenylacetate were smaller than the re-
`ported K,,, the Michaelis-Menten function reduces
`to a first-order rate constant. Thus, no saturability of
`phenylacetate was observed. The low concentrations
`of phenylacetate seen in this study may also bere-
`lated to the small doses of phenylbutyrate used here
`compared with the initial phase I trial, which used
`a 150-mg/kg (approximately 6000 mg/m’) bolusof
`phenylacetate."
`Preclinical antitumor activity has been observed
`for phenylbutyrate at concentrations of 500 to 2000
`uwmol/L (94-376 ug/mL). This concentration range
`was shownhereto be clinically achievable after a
`30-minute infusion. It will be important to further
`evaluate the pharmacokinetics of phenylbutyrate
`using alternative dosing strategies (e.g., continuous
`infusion) or higher doses to determine whetherthese
`concentrations can be maintained for longer periods
`of time. In addition, continuous infusion may yield
`higher phenylacetate concentrations, especially if
`saturation of phenylacetate is achieved.
`Phenylbutyrate is known to undergo rapid conver-
`sion to phenylacetate in vivo by beta-oxidation.
`
`-£*=== o=<
`
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`PHENYLBUTYRATE AND ITS METABOLITES, PHENYLACETATE AND PHENYLACETYLGLUTAMINE
`
`REFERENCES
`
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`
`«
`
`However,this conversion has not been extensively
`studied to date. Therefore, one purposeofthis study
`was to examine the percentage of phenylbutyrate
`that is metabolized to phenylacetate. Comparison of
`phenylacetate and phenylbutyrate AUC would be
`inappropriate because metabolites often have con-
`siderable differences in clearance and half-life, as
`well as different distributional properties. More im-
`portantly, phenylacetateis only an intermediate me-
`tabolite and undergoes further metabolism to phe-
`nylacetylglutamine; rapid conjugation with gluta-
`mine would clearly result in a low AUCfor phenyla-
`cetate. We therefore determined the conversion of
`phenylbutyrate based on the amountof phenylacet-
`ylglutamine recovered in the urine. This methodis
`valid based on the more than 99% conversion of phe-
`nyl-acetate to phenylacetylglutaminethat our group
`has previously shown.* Although onepatient did ex-
`hibit a 100% recovery, several patients showed in-
`complete conversion. The latter could be owing to
`assay variance of 10% for each compound and any
`error incurred in collection of 24-hour urine sam-
`ples. Another possible factor may have been a urine
`collection of only 24 hours for determination of uri-
`nary phenylacetylglutamine. An additional period of
`8. Thibault A, Cooper MR, Figg WD, Venzon DJ, Sartor AO, Tam-
`pkins AC, Weinberger MS, Headlee DJ, McCall] NA, Samid D, My-
`collection mayhaveyielded a greater recovery. Also,
`ers CE: A phase | and pharmacokinetic study of intravenous phe-
`small traces of phenylbutyrate and phenylacetate,
`nylacetale in patients with cancer. Cancer Res 1994;54:1690-1694.
`belowthe limit of quantification of the assay, were
`9. Brusilow SW, Horowich AL: Urea cycle enzymes, in Scriber C,
`observed in the urine. The combination of all these
`Beaudet A, Sly W, Valle DR (eds.); Metabolic Basics of Inherited
`factors is likely to play a role in explaining the in-
`Diseases. New York: McGraw Hill, 1989;629-664.
`complete conversion.
`10. Brusilow SW: Phenylacetylglutamine may replace urea as a
`vehicle for waste nitrogen excretion. Pediatr Nes 1991; 29:147-150.
`In summary, phenylbutyrate exhibits saturable,
`nonlinear pharmacokinetics after intravenous ad-
`11, Knoop F: Der Abbau aromatischer fettsaure Tierkorper. Bietr
`ChemPhysiol Pathol 1905; 6:150-162.
`ministration and achieves peak concentrations in the
`12. Karnofsky DA, Abelman WH, Craver LF, Burchenal JH: The
`range of in vitro antitumor activity. Concentrations
`use of the nitrogen mustards in the palliative treatment of carci-
`of the active, intermediate metabolite (phenylacet-
`noma. Cancer 1948; 1:634-656.,
`ate) were low inthis study and did not achievelevels
`13. Thibault A, Figg WD, McCall N, Myers CE, Cooper MR: A si-
`at which saturation occurs. The conversion of phe-
`multaneous assay of the differentiating agents phenylacetate and
`nylbutyrate to phenylacetate was high (80%), but the
`phenylbutyrate, and one of their metabolites, phenylacetylglu-
`rapid, subsequent conversion to phenylacctylglu-
`tamine, by reversed-phase, high performance liquid chromatogra-
`phy. J Lig Chromatogr 1994; 17:2895-2900,
`tamine resulted in serum levels of phenylacetate
`14. D'Argenio DZ, Schumitzky A: ADAPT User's Guide, Biomedi-
`that were much lowerthan those seen when the drug
`cul Simulations Resource. Los Angeles: University of Southern
`is given intravenously. We conclude that phenylbu-
`California, 1990.
`tyrate should not be consideredaclinically useful
`15. Yamaoka K, Nakagawa T, Uno T: Application of Akaike's In-
`prodrugof phenylacetate andthat both phenylbutyr-
`formation Criterion in the evaluation of linear pharmacokinetic
`ate and phenylacetate should be pursued as indepen-
`equations. J Pharmacokinet Biopharm 1978;6:165-175.
`dent antineoplastic agents.
`16. Gibaldi M, Perrier D: Pharmacokinetics, 2nd ed. New York:
`Marcel Dekker, Inc., 1982.
`17. D’Argenio DZ: Optimal sampling times for pharmacokinetic
`experiments. J Pharmacokinet Biopharm 1981; 9:739-756.
`
`The authors thank Natalie McCall, Bernadette Allman,and Jen-
`nifer Stevensfor laboratory assistance.
`
`ONCOLOGY
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`373
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`LUPIN EX. 1010
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`LUPIN EX. 1010
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