`
`ONCOLOGY
`
`Disposition of Phenylbutyrate and its
`Metabolites, Phenylacetate and
`Phenylacetylglutamine
`
`Stephen C. Piscitelli, PhermD, Alain Thibault, MD, William D. Figg. PharmD.
`Anne Tompkins, RN. Donna Headlcc. RN, Ronald Lieberman. MD.
`Dvorit Samid, PhD. and Charles E. Myers, MD
`
`
`
`Phenylocetate. an inducer of tumor cylastasis and differentiation, shows promise as a
`relatively nontoxic antincaplastic agent. Phenylacetate. however. has an unpleasant odor
`that might limit patient acceptability. Phenylbutyrate. on odorless compound that also
`has activity in tumor models. is known to undergo rapid canvarsion to phenylacetate by
`beta-oxidation in viva. This phase l study examined the pharmacokinetics of phenylbu-
`tyrale and characterized the disposition of the two metnholites. phenylnr:elale anrl phe—
`nylacetylglutamine. Fourteen patients with cancer [aged 51.8 4: 13.8 years} received n 30
`minute infusion of phenylbutyrate at 3 dose levels (000, i200. 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-
`macokinetics ofall three compounds was developed using ADAPT Il. Dula were modeled
`as molar equivalents. The model fit the data well as shown by mean [180] coefficients of
`determination in“) for phenylhutyrate. phenylacetate, and phenylacetylglutamine. which
`were 0.96 :r 0.07. 0.83 i 0.10, and 0.92 i- 006. respectively. The intrapatient coefficient of
`variation percentage [CV%} around the parameter estimates were small [range 7.2—
`33.5%l. Phenylbutyrate achieved peak concentrations in the range ofin vitro tumor oc-
`tivily [500-2000 Jpanel/Ll and exhibited saturablc elimination (K... = 34.1 i.- 18.1 pg/mL
`and Vnm = 18.1 I 18 mgfll/ltgl. Metabolism was rapid: the times to maximum concentra—
`tion for phenylacetalc and phenytacetylglotamine Were I and 2 hours. respectively. The
`conversion of phenylbutyrate to phenylacetote was extensive {30 :t 12.0%], but serum
`concentrations ofphenylocetote were low owing to rapid. subsequent canversion to phe-
`nylacetylglutamine. The ratio of phenylbutyrate AUG to phenylacetate AUC was 2.66.
`Thus, phenylhutyrote may not be a prodrug for phenyloceiate and should be pursued as
`an independent antitumor agent.
`
`The amino acid phenylalanine is degraded by a
`combination of hydroxyiatiun and deainination.
`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. Fiscitelli); the Clinical Pharmacology Branch, National
`Cancer Institute. National Institutes of Health. Bethesda, Maryland
`{Mss Tompkins and Headlee. and Drs. Thibault. Fig. 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. Fig. PharmD. Clinical Pharmacokinetics, Sec-
`tion. Clinical Pharmacology Branch. National Cancer Institute. Building
`10. Room 5A0], Bethesda. MD 20392.
`
`368 0 J Clin 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.z The fact that phenylacetate is conjugated with
`and depletes circulating glutamine is ofspecia] inter-
`est, because tumor cells are highly dependent on this
`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 genes criti-
`cal to growth control and differentiation.“
`Recently. phenylacetate has been shovvn topnssess
`cytostatic and differentiating properties against a va-
`
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`PHENYIBUTYHATE AND IT'S METABOUTES, PHENYLACETATE AND PHENYLACETYLGLU'I‘AMINE
`
`riety of hematologic and solid tumors in laboratory
`models.3'fi When given to healthy subjects, phenyla-
`cetatc undergoes hepatic conjugation with gluta-
`mine by phenylacetyl coenzyme A: glutamine acyl-
`transferase, which yields phenylacetylglutamine,
`the major urinary metabolite.2 Although previously
`shown to follow first-order pharmacoltinetics,7 the
`drug exhibits nonlinear, saturable pharmacokinetics
`at doses currently being evaluated in patients with
`cancer.“ Phenylacetate. however. has an unpleasant
`odor that might limit its acceptance and develop-
`ment as an oral drug.
`In contrast. phenylbutyrate is an odorless com-
`pound and has also been safely given to children for
`hyperainmonemic urea cycle disordersg'm Recent
`laboratory studies have documented that phenylbu-
`tyrate. like phenylacetate. can [1) induce selective
`cytostasis and maturation of cultu red tumor cells de—
`rived from' various erythropoietic and solid neo-
`plasms {including adenocarcinumas of the prostate,
`breast. ovary. colon. and lung, as well as central ner-
`vous system tumors and malignant melanoma]: [2)
`modulate the expression of genes implicated in tu-
`morgrowth, metastasis, and immunogenicity; and [3]
`enhance the efficacy of other agents of clinical inter-
`est including retinoids. interferon alfa, suramin. 5-
`aza-2'-deoxycytidine, and hydroxyurea {Sarnid et
`al.5; Liu et al., lludgins et al.. Fig et al., submitted:
`Sand et al., unpublished data]. Phenylbutyrate is
`converted in vivo to phenylacetate by mitochondrial
`beta—oxidation.‘1 Therefore. phenylbutyrate is cur-
`rently being investigated as a new antineoplastic
`agent, and as a prodrug for phenylacetate in the treat-
`ment of cancer.
`To better understand the disposition ofthese com-
`pounds after intravenous administration of phenyl-
`butyrate. a pharmacokinetic model
`that simulta-
`neously characterizes the disposition of phenylbu-
`tyrate, phenylacetate, and phenylacetylglutamine
`was developed from plasma and urine data collected
`during a phase I clinical trial.
`
`METHODS AND DEVELOPMENT 01" MODEL
`
`Adults with adva nced solid tumors refractory to con-
`ventional therapy. a performance status greater than
`60% on Karnofsky‘s scale.12 normal hepatic transam-
`inases and bilirubin. a serum creatinine less than 1.5
`mg/dL. and normal leukocyte and platelet counts
`were eligible for this study. The clinical protocol was
`approved by the National Cancer Institute’s Institu-
`tional Review Board, and all 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/
`n12]. Each patient received a single 30-minute infu-
`
`ONCOLOGY
`
`sion of phenylbutyrate, and serial blood samples
`were collected before, immediately post—dose. and at
`0.15. 0.3. 0.5. 0.75. 1, 1.5, 2.5, 3.5, and 5 hours after
`the infusion. Blood samples (5 mL) were collected in
`5-mL glass tubes {Vacutainer®; Becton Dickinson.
`Rutherford. NJ] either via an intraVenous catheter
`[separate from the drug administration catheter} or
`venipuncture. Blood was centrifuged. and the serum
`was transferred to 5-mL polypropylene tubes and fro-
`zen at —85°C until the time of analysis. A 24-hour
`urine collection for cumulative phenylacetylglutam-
`inc excretion was done in a subset ofpatients.
`The reversed phase high—performance liquid chro—
`matography method for measuring serum concentra-
`tions of phenylacetate, phenylbutyrale. and phe-
`nylacetylglutamine has been previously described."3
`Briefly. 100 at. of 10% perchloric acid was used to
`precipitate the proteins ofa ZOO-pl. aliquot of serum,
`which was then centrifuged. The supernatant was
`neutralized with 25 pL ofa 20% solution ofpolassium
`bicarbonate. After centrifugation. 20 pL of superna-
`tant was injected onto a (3-13 column heated at 60°C.
`Urine samples were processed similarly. after a 1:20
`dilution with water. Elution was done with an in-
`
`creasing gradienl of acetonitrile in waler from 5 to
`30% over 45 minutes. Its progress was followed by
`monitoring ultraviolet absorbance at 203 nm. Char-
`acteristic elution times for phenylacctylglutamine.
`phenylacetate. and phenylbutyrate were 10.1, 17.4,
`and 27.8 minutes. respectively. The assay yielded a
`lower limit of detection of 2 pig/It'll. and was linear
`for concentrations as high as 2,000 ,ug/mL. Between
`20 and 1.000 pg/mL. the interassay CV% was less
`than 10%.
`A model to simultaneously describe the pharma-
`cokinetics ofall three compounds was developed us-
`ing ADAPT It.” Several models were constructed to
`compare one and two compartments for each drug.
`as well as the possibility of nonlinear pharmacoki-
`netics. Model selection was determined by Akaike’s
`Information Criterion {MC}.15 and by visual inspec~
`tion of the difference between measured and corn-~
`puter-fitted concentrations {residuals}. Data were
`modeled as molar equivalents. The pharmacokinetic
`parameters were estimated using weighted nonlin-
`ear least squeres by an adaptive process that used se-
`quential updating of priors for parameter values.
`Weighting was by the inverse of the observation vari-
`an ce for all compounds.
`-
`Drug input was by intermittent intravenous infu-
`sion. To make the model identifiable. the volume of
`distribution of phenylacetate was fixed at 0.3 L/kg
`based on previous phase I data in which phenylacet-
`ate was given intravenously.B Complete conversion
`of phenylacetate to phenylacetyiglutamine and
`elimination of all phenylacetylglutamine in the
`
`369
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`PtSCl'l‘ELLI ET AL
`
`
`
`
`
`Figure 1. Model to describe the disposition of phenylhutymte [PB],
`phenylocctote tPAl. and phcnvlocctylglutomint: lPACl illustrating
`the pharmacokinetic parameters.
`Abbreviations: V”, volume of distribution for PR: V”. volume
`of distribution for PA; VH5. volume of distribution Jfor FAG; MM.
`Michaelis-Menten elimination: Km. Michaelis-Menten constant:
`Um," maximum elimination rate: CL, . liormational clearance ofPA
`to PAC; CL}. clearance of PAC into the urine: Cl...” clearance of PB
`out of the central compartment.
`
`urine was also assumed based on our previous phase
`I experience.8 Thus, the fraction of phenylbutyratc
`converted to phenylacatate was determined using
`the following equation:
`
`urinary phenylacetylglutamine [enrol]
`
`dose of phenylbutyrate [,umol}
`
`The pharmacokinetic parameters for phenylacet-
`ylglntamine are dependent on this fraction, which is
`analogous to oral drugs where clearance and volume
`are dependent on the value of bioavailability [i.c.,
`CL/F or Vfi/F].
`The model eventually used was a one-compart-
`ment nonlinear model for phenylbutyrate with con-
`version to a one-compartment linear model for phe-
`nylacetate, and further conversion of phenylacetatc
`t0 phenylacetylglutamine {one-compartment}. Phe-
`nylbutyratc was parameterized by a central volume
`[V135], a minor elimination pathway [Chm]. and a non-
`linear function consisting of intrinsic clearance
`[CLm] and the Michaelis—Menten constant {Km}. The
`Vmax is equal to Chm-Km. The CLl and [IL2 describe
`the clearances of phenylacetate to phenylacetylglu-
`tamine and phenylacetylglutamine into the urine.
`respectively. The Vmg describes the volume of dis
`tribution [V2] for phenylecetylglutamine. The V”
`represents the volume of distribution of phenylacet-
`ate. The model displaying the pharmacokinetic pa-
`rameters is shown in Figure 1.
`The area under the serum concentration versus
`
`time curve [AUC] was calculated by the trapezoidal
`rule according to Gibaldi and Perrier.“i The .MJC was
`determined from time zero until the last time point
`{5 hours]. because concentrations of each compound
`ware usually below detectable limits at this point
`and because of the difficulty in determining an elim-
`
`370 a JClinPharmacol1995;35:368—373
`
`ination rate constant for the metabolites owing to
`sparse data describing the terminal slope.
`
`RESULTS
`
`Pa lien is
`
`Fourteen male patients were included in the study.
`Three patients received 600 ting/tn2 of phenyl'ouiyr-
`ate. 8 received 1200 rag/m2, and 3 received 2000 mg/
`mg. Patient demographics are shown in Table I.
`
`Pharmacokinetics
`
`The model fit the data well as shown by mean [15D]
`coefficients of determination {r2} for phenytbutyrate,
`phenylacetate. and phenylacetylglutamine, which
`were 0.96 1' 0.07, 0.88 :t 0.10, and 0.92 i 0.05, respec-
`tively. Pharmacokinetic parameters are shown in
`Table I]. The intrapatient CV21“: around the parame-
`ter estimates were small, ranging from 7.2 to 33.5%
`of the fitted values. The mean interpatient CV% for
`parameter values ranged from 11.85 to 34.6%.
`Serum concentration-lime plots for a representa-
`tive patient in each dosage group are shown in Figure
`2. Similar fits were seen for the other patients. Peak
`concentrations of phenylhutyrate after 600 mg/m2
`ranged from 31 to 57 pg/mL. After 1200 rag/m2 and
`2000 mg/mz, peak concentrations in serum ranged
`from 57 to 115 ,ug/mL and 114 to 184 pg/mL. respec-
`tively. Concentrations at 5 hours after dosing were 2
`
`
`
`TABLE I
`
`Individual and Mean Patient Demographics
`Patient
`Age
`Height
`Weight
`Dose
`
`No.
`(yr)
`Dose/m2
`(I118)
`(amt
`(kg)
`
`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
`2143
`2196
`2700
`3000
`4020
`2940
`
`77.6
`173.0
`51.8
`Mean
`16.7
`10.2
`13.8
`so
`W
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`PHENYLBUTYHATE AND ITS METABOLITES, PHENYLACETATE AND PHENYLACETYLGLU'I'AMINE
`
`
`
`TABLE il
`
`Pharmacoklnetic Parameters Derived
`From the Model
`
`
`
` Mean SD
`
`.
`
`0.08
`0.21
`VPB [L/kg)
`0.04
`0.1 0
`01.... (L/hr/kg)
`181
`34.1
`K... (lug/mo
`0.30
`0.50
`CL...“ (L/hr/kg)
`18.0
`18.1
`vm, (mg/hr/kg)
`Fixed
`0.30
`v... (L/kg)
`0.13
`0.37
`CLt1(L/hrjkg)
`0.11
`0.19
`vm (L/kg)
`0.11
`0.17
`cm (L/hr/kg)
`139.6
`265.4
`A00 PB 600 mg/m’
`167.9
`557.8
`AUC PB 1200 ring/m2
`689.2
`1214.5
`400 PB 2000 mg/m"
`16.6
`120.0
`AUC PA 600 mg/m2
`90.6
`220.2
`sec PA 1200 Eng/m2.
`160.0
`608.3
`AUC PA 2000 mg/m?
`119.2
`401.3
`AUC PAG 600 mg/m2
`269.7
`438.0
`AUC FAQ 1200 mg/m’
`
`
`1055.4AUG FAG 2000 mg/rn2 389.6
`Visa — volume of distribution for PE; V9. — volume of distn'butien for PA; V»; =-
`vulume oidisttibution for P146; K... = Michaelis-Menteri constant; CLM — intrinsic
`clearancezvm. = maximum elimination rate: cut = lormaiienalclearance of PA
`to FAG: CLtZ = clearance of FAG into the urine: CLm = clearance 0i PB out at
`central compartment: AUC = area under the curve trorn time 0 to 5 hours post-
`dose: PB = phenylbutyrate: PA = phenyiacetate; PM} = phenyiacetylglutamine.
`AUC values are represented as areal -hr/L_
`
`Jug/mL or lower in all patients. Phenylbutyrate ex-
`hibited saturable elimination pharmacokinotics as
`evidenced by concave log-linear plots on visual io—
`spection, an AUCH, disproportionate to dose {Figure
`3), and improved fits at high doses using a nonlinear
`function as assessed by AIC. Final estimates for Mi-
`chaelis-Menten parameters were 61 Km of 34.1 i 18.1
`lug/ml. and a Vmax of18.1 i 18.0 mg/h/kg.
`Six patients had complete 24-hour urine collec—
`tions for determination of phenytbutyrate conver-
`sion to phenylacetate, 1 at 600 rug/m2, and 5 at 1200
`mg/rnz. The percentage of conversion was high with
`a mean (:SD] of80.0 i 12.6%. The conversion ranged
`from 68 to 100% of phenylbutyrate accounted for in
`the urine by phenylacetylglutamine.
`Phenylacelate was detectable in plasma immedi-
`ately after phenylbutyrate infusion with mean [:SD]
`peak concentrations of 20.7 t 13.6 pg/mL. The time
`to maximum concentration most commonly oc—
`curred 30 to 60 minutes after the infusion. The serum
`concentrations of phenylacetate that were seen in
`this study were much lower than those after intrave-
`nous administration ofphenyiacetate.a After phenyl-
`butyrate administration. phenylacetate followed
`
`ONCO L0 6‘!
`
`first-order elimination. The Michaelis-Menten con-
`stant of phenylacetate from our previous trial'1 was
`105.1 i 44.5 pg/mL. The highest concentration of
`phenyiacetaie achieved in this study was 57 pg/mL
`with 11 of the 14 patients exhibiting peak concentra-
`tions less than 30 rig/roll. Because the peak phenyla-
`cetate concentrations were less than or equal to one-
`half the Km, the nonlinear function of phenylacetate
`collapses to a first-order rate constant.” As a compar-
`ison ofthe total exposure between the 2 compounds,
`the mean [:80] ratio of phenylbutyrate AUC to phe-
`nylacetate AUC was 2.66 i 1.57.
`Phenylacetylglutamine
`serum concentrations
`were also observed immediately after phenylbutyr-
`ate dosing. However, peak concentrations appeared
`1 to 3.5 hours after the infuaion. which was later
`than those of phenylacetate. Phenylacetylglutamine
`achieved maximum serum concentrations of 59.5 t
`
`34.2 ng/mL. which ranged from 27 to 129% of those
`of plionylbulyrate [mean 1 SD. 61.2 i 29.0%]. Comv
`paratively. phenylacetate achieved peak concentra-
`tions that were only 38.8 i 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 dispoaition. these models can be used to
`[I] determine sampling schemes based on a small
`number of blood samples [using optimal sampling
`theory“]; [2] predict plasma concentrations of new
`regimens: or [3] optimize dosing for maximal efficacy
`and minimal toxicity in patients receiving multiple
`courses oftherapy.
`The simultaneous modeling approach used in this
`analysis accurately characterized the conversion
`and disposition ofphenylbutyrate and its two metab-
`olites, phenylacetate and phenylacetylglutamine.
`There is increasing interest in phenylacetate as a rel-
`atively nontoxic antitumor ag,erit.3“"a The unpleas-
`ant odor of phenylacetate, however, may limit its ac-
`ceptance by patients. Phenylbutyrate is the odorless
`precursor of phenylacetate. with demonstrable anti-
`tumor activity 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 minutes after ini-
`tiation of the phenylbutyrate infusion. Phenylbutyr—
`ate was characterized by nonlinear elimination
`pharmacokinctics with a Km of 34.1 pg/rnL and a
`V,"ax of18.11ng/h/kg.
`Our group has previously reported the results of a
`phase I study of intravenous phenylacetate that
`showad nonlinear pharmacokineties for phenylacet-
`
`371
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`

`PISCITELLI ET AL
`
`Figure 2. Actual {squares} and computer-fitted {line} concentration-
`time profiles of phenylbutyrute, phenylucetuto, and phenylncetyi-
`glutcmine in {A} patient 1 receiving 600 l'ng,."mz of phenylbulyrute;
`{B} patient 5 receiving 1200 [Hg/m" of plienylbutymic; [Ci patient
`12 receiving 2090 mg/m2 of phenylbutyrote.
`4—-—-—-——-——————
`
`ate characterized by saturable metabolism to phe-
`nylacflylglulamine.” In this study. where concentra-
`tions of phenylacetate were smaller than the re-
`ported Km. 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 be re-
`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/m2] bolus of
`phenylacetate."l
`Preclinical antitumor activity has been observed
`for phenylbutyrate at concentrations of 500 to 2000
`,umol/L [94-376 ,ug/mlJ. This concentration range
`was shown here to be clinically achievable after a
`30-minute infusion. It will be important to further
`evaluate the pharmacokinetics of phonylbutyrate
`using alternative dosing strategies [e.g., continuous
`infusion] or higher doses to determine whether these
`concentrations can be maintained for longer periods
`of time. In addition, continuous infusion may yield
`higher phenylacetate concentrations. especially if
`saturation of phenylacetale is achieved.
`Phenylbutyrate is known to undergo rapid conver—
`sion to phenylacetate in vivo by beta—oxidation.
`
`Dose [mg]
`
`1cm
`
`HSlE3
`
`0aa
`
`t
`
`200 D
`
`3000
`
`4 D D El
`
`
`
`
`
`concentration(nu)
`
`.—
`E:Ia.
`I:o
`.7.ash..
`n0I:
`ou
`
`EI
`
`
`
`concentrallen(UM)
`
`372 I J Clin Pharmac011995:35:358—373
`
`FigUm 3- Plot of phefll’lbutymte dose (mg) unti area under the
`curve. Line of best fit is shown: 3! = I05.95 - Ifllfl'm’s’”, [H = .78].
`
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`PHENYLBUTYRATE AND ”'8 ME’I'ABULITES, PHENYLACE'I‘ATE AND PHENYLACETYLGLUTAMINE
`
`However, this conversion has not been extensively
`studied to date. Therefore, one purpose of this 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—
`porlantly. phenylacetate is only an intermediate me-
`tabolite and undergoes further metabolism to phe-
`nylacetylglutalnine; rapid conjugation with gluta-
`mine would clearly result in a low AUC for phenyla-
`cetale. We therefore determined the conversion of
`
`phenylbutyrate based on the amount of phenylacet-
`ylglutamioe recovered in the urine. This method is
`valid based on the more than 00% conversion of phe-
`nyt-acetate to phenylacetylglutamine that our group
`has previously shown.’3 Although one patient 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
`collection may have yielded a greater recovery. Also,
`small traces of phenylbutyrate and phenylacetate,
`below the limit of quantification of the assay. were
`observed in the urine. The combination of all these
`
`factors is likely to play a role in explaining the in-
`complete conversion.
`In summary, phenylhutyrate exhibits saturablc.
`nonlinear pharmacokinetics after intravenous ad-
`ministration and achieves peak concentrations in the
`range of in vitro antitumor activity. Concentrations
`of the active, intermediate metabolite (phenylacet-
`ate) were low in this study and did not achieve levels
`at which saturation occurs. The conversion of phe-
`nylbutyrate to phenylacetate was high [80%], but the
`rapid. subsequent conversion to phenylacetylglu—
`tamine resulted in serum levels of phenylacetate
`that were much lower than those seen when the drug
`is given intravenously. We conclude that phenylbu-
`tyrate should not be considered a clinically useful
`prodrug ofphenylacetate and that both phenylbutyr-
`ate and phenylacetate should be pursued as indepen—
`dent antineoplastic agents.
`
`The authors thank Natalie McCall. Bernadette Altman. and len—
`nifer Stevens for laboratory assistance.
`
`REFERENCES
`
`‘l. Brusilow SW, Denney M. Weber L], Batshaw M. Burton B. Lev-
`itsky l.. Roth K. McKeethren C. Ward I: Treatment of episodic hy-
`perarnrnooemia in children wilh inborn errors of urea synthesis.
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`LUPIN EX. 1010
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