2932
`
`Phase I Study of Phenylacetate
`Administered Twice Daily to Patients
`with Cancer
`
`Alain Tliibaalt, M.D.,* Dvorit Samid, Plz.D.,* Michael R. Cooper, MD,“
`
`William D. Figg, Pharm.D.,* Anne C. Tompkins, RN," Nicholas Patronas, M.D.,T
`
`Donna]. Headlee, R.N.,* David R. KOl’llt’l‘, Pliariii.D.,1 David I. Verizon, Ph.D.,§
`
`and Charles E. Myers, M.D.*
`
`Background. The growth-inhibiting and dilferenli—
`ating effects of sodium phenylacetate against hematopoi—
`etic and solid tumor cell lines has aroused clinical inter—
`
`est in its use as an anticancer drug. In an earlier Phase I
`trial of phenylacetate aimed at maintaining serum drug
`concentrations in the range that proved active in vitro
`(>250 pig/ml) for 2 consecutive weeks, infusion rates ap-
`preached the maximum velocity of drug elimination and
`commonly resulted in drug accumulation and reversible
`dose-limiting neurologic toxicity. In this study, the au—
`thors described the nonlinear pharmacokinetics, metab—
`olism, toxicity, and clinical activity of phenylacetate.
`Methods. The treatment regimen of this Phase I study
`was designed to expose patients intermittently to drug
`concentrations exceeding 250 pig/ml and to allow time for
`drug elimination to occur between doses to minimize ac-
`cumulation. Sodium phenylacetate was administered as
`a 1-hour infusion twice daily (8 a.m., 5 pm.) at two dose
`levels of 125 and 150 mg/kg for a 2—week period. Therapy
`was repeated at 4—week intervals for patients who did not
`experience dose-limiting toxicity or disease progression.
`Results. Eighteen patients (4 of whom previously
`were treated with phenylacetate by continuous intrave-
`nous infusion] received 27 cycles of therapy. Detailed
`
`From the *Clinical Pharmacology Branch and the §Biostatistics
`and Data Management Section, National Cancer Institute, National
`Institutes of Health, and the TDiagnostic Radiology Department and
`iPharmacy Department, Warren G. Magnuson Clinical Center, Na?
`tional Institutes of Health, Bethesda, Maryland.
`Supported by the intramural program of the National Cancer
`Institute and by a grant from Elan Pharmaceutical Company.
`The authors thank the medical staff fellows and nursing staff of
`the NC] for their skillful care of patients treated on this study, as well
`as Natalie McCall, BS, and Kara Ammerman for theirinvaluable lat»
`oratory assistance.
`Address for reprints: Alain Thibault, M.D., National Institutes
`of Health, National Cancer Institute, Clinical Pharmacology Branch,
`Building 10, Room 12 N 214, Bethesda, MD 2089271576.
`Received September 12, 1994; revision received Ianuary 13,
`1995; accepted February 27, 1995.
`
`pharmacokinetic studies for eight patients indicated that
`phenylacetate induced its own clearance by a factor of
`27% in a 2—week period. Dose-limiting toxicity, consisting
`of reversible central nervous system depression, was ob-
`served for three patients at the second dose level. One pa-
`tient with refractory malignant glioma had a partial re-
`sponse, and one with hormone-independent prostate can—
`(:er achieved a 50% decline in prostate specific antigen
`level, which was maintained for 1 month.
`Conclusions. Phenylacetate administered at a dose of
`125 mg/kg twice daily for 2 consecutive weeks is well tol-
`erated. High grade gliomas and advanced prostate cancer
`are reasonable targets for Phase II clinical trials. Cancer
`1995;75:2932—8.
`
`Key words: phenylacetate, pharmacokinetics. differen-
`tiation. glioma, prostate cancer.
`
`Phenylacetate is a minor product of phenylalanine me—
`tabolism normally found at micromolar concentrations
`in the plasma and cerebrospinal
`fluid of humans.1
`Higher concentrations (250—700 pg/ml) sustained for a
`minimum of 7 days induce cytostasis and differentia—
`tion in a variety of hematologic and solid tumor models,
`including cultures of prostate and brain tumor cells.“’
`We recently reported the first Phase I trial of phenyle
`acetate,7 in which we administered the drug by contin—
`uous intravenous infusion in an attempt to maintain
`drug concentrations in the range associated with pre—
`clinical activity. Under these conditions, phenylacetate
`displayed saturable elimination and evidence for induc—
`tion of its own metabolism (pharmacokinetic parame—
`ters, mean '1 standard deviation (SD): Km (Michaelis—
`Menten constant) : 105 i 45 og/ml, V max (maximum
`metabolic rate) : 24 i 5.2 mg/kg/hour, and VD (vol-
`ume of distribution) = 19 i 3.3 I. Clinical improvement
`was noted in several patients with malignant gliomas
`
`‘1of7
`
`Horizon Exhibit 2017
`Horizon Exhibit 2017
`Lupin v. Horizon
`Lupin v. Horizon
`IPR2018-00459
`|PR2018-00459
`
`1 of 7
`
`

`

`Phase I Study of Phenylacetate/Tlnlmult et al.
`
`2933
`
`and metastatic hormonedndependent prostate cancer
`who achieved serum phenylacetate concentrations of
`150—200 rig/ml. However, infusion rates close to the V
`max of
`the metabolizing enzyme were required to
`achieve concentrations of 250 ng/ml or more. This of—
`ten resulted in rapid drug accumulation, which was as—
`sociated with neurologic toxicity once phenylacetate
`concentrations exceeded 800 jig/ml.
`Postulating that intermittent drug administration
`might obviate this problem at clinically tolerable doses,
`we used the one—compartment nonlinear model and
`population parameters derived from our previous expe-
`rience to simulate the course of several intermittent dos—
`
`ing regimens. Our primary objective was to design one
`that would expose most patients to transient phenyl-
`acetate concentrations in excess of 250 ng/ml and
`maintain trough concentrations not exceeding 100 ng/
`ml, which had been well tolerated by patients for as
`long as 2 weeks. Our secondary objective was to char-
`acterize the toxicity and identify the maximum tolerated
`dose of this dosing schedule, wherein the drug was
`given as a 1-hour infusion twice daily over 14 consecu-
`tive days.
`
`Methods
`
`Patient Population
`
`Adults with advanced solid tumors refractory to con—
`ventional therapy, a performance status greater than
`60% on Karnofsky’s scale, normal hepatic transamie
`nases and bilirubin, a serum creatinine level below 1.5
`
`mg/dl, and normal leukocyte and platelet counts were
`eligible for this study. The clinical protocol was re-
`viewed and approved by the National Cancer Institute
`Institutional Review Board, and all patients gave writ
`ten informed consent before participating in the study.
`Eighteen patients, 15 men and 3 women, with a median
`age of 55 years (range, 32~76 years), were enrolled be—
`tween ]uly and October 1993. No selection process took
`place to ensure a balanced malezfemale ratio. Disease
`distribution included metastatic, hormone—indepen—
`dent prostate cancer (9 patients), primary central ner—
`vous system (CNS) tumors (7 patients), renal cell cancer
`(1 patient), and sarcoma (1 patient). Patients with pros—
`tate cancer who had not undergone orchiectomy main—
`tained medical castration with leuprolide acetate. They
`were required to have discontinued flutamide for at
`least 1 month before enrollment and to thereafter have
`
`three sequentially increasing prostate specific antigen
`(PSA) measurements. Patients who had previously re—
`ceived suramin as an experimental treatment for pros-
`tate cancer continued to take hydrocortisone (20 mg ev—
`ery morning, 10 mg every evening) as replacement for
`
`20f7
`
`insufficiency. Patients with
`suramin~induced adrenal
`primary CNS tumors who were taking corticosteroids at
`the time of enrollment were maintained at the same or
`
`lower dose of the corticosteroid while participating in
`the study. Four patients, two with gliomas and two with
`prostate cancer, had received prior treatment with phe—
`nylacetate given by continuous intravenous infusion.
`The latter patients were entered on study because they
`appeared to have had clinically benefitted from the ad—
`ministration of phenylacetate with no evidence of cu—
`mulative toxicity. No other form of antitumor therapy
`was allowed during the study period.
`
`Drug Preparation and Administration
`
`An injectable formulation of sodium phenylacetate was
`manufactured by the Pharmaceutical Development
`Section of the National Institutes of Health Clinical
`
`Center Pharmacy Department from bulk phenylacetate
`powder supplied by Elan Pharmaceutical Research Cor—
`poration (Cainesville, GA). The finished drug product
`contained sodium phenylacetate 500 mg/ml in sterile
`water for injection, USP, with sodium hydroxide and/
`or hydrochloric acid added to adjust the pH to approxi—
`mately 8.5. For administration to patients, this solution
`was further diluted in water for injection, USP, to a total
`volume of 250 ml and administered over 1 h0ur with a
`
`portable pump (CADDePLUS; Pharmacia Deltec, Inc.,
`St. Paul, MN).
`
`Phenylacetate was delivered at two dose levels: 125
`and 150 mg/kg/dose, twice daily (8 am. and 5 pm.)
`for 14 consecutive days. (As had been customary for
`the treatment of urea-cycle disorders in children with
`phenylacetate,
`the dosing schedule was calculated
`based on body weight, rather than on surface area).8
`Cycles of therapy were repeated every 4 weeks. lndi—
`vidual patients could escalate from one dose level to the
`next with sequential cycles, provided they had experi—
`enced no more than Grade 1 drug—related toxicity and
`their disease was stable or improved. The maximum tol—
`erated dose was defined as the dose at which two or
`
`more patients developed dose—limiting toxicity, defined
`as Grade 3 toxicity (or Grade 2 if involving the CNS)
`according to the National Cancer lnstitute's Common
`Toxicity Criteriaf’
`
`Sampling Schedule
`
`Serum drug concentrations were measured twice a day
`in all patients immediately before (trough level) and 15
`minutes after the administration of the 5 pm. infusion
`(peak level). To assess the possibility that phenylacetate
`induces its own clearance, eight patients were randomly
`chosen to undergo more intensive drug level monitor—
`
`2 of 7
`
`

`

`2934
`
`CANCER June 15, 1995, Volume 75,N0.12
`
`ing on days 1, 2, or 3 and days 12, 13, or 14 of the 2
`weeks of therapy. In these patients, blood was also ob
`tained at 0, 65, 90, 105, 120, 150, 180, 210, 240, 300,
`
`and 360 minutes from the beginning of the 8 am. infu-
`sion. This allowed for a comparison to be made between
`area under the serum concentration versus time curves
`
`(area under the curve [AUC]) generated from identical
`doses of phenylacetate at the beginning and at the end
`of therapy, with any difference reflecting a change in
`drug clearance over this period.
`
`Analytic Method
`
`Concentrations of phenylacetate and phenylacetyl—
`glutamine were measured in serum with the use of
`high—performance liquid chromatography.7
`
`Phamacokinetic Methods
`
`Initial estimates of Km and V max were obtained from
`
`our prior experience with phenylacetate. We used a
`one-compartment nonlinear model and a Bayesian
`modification of the Nelder—Mead iterative algorithm
`(Abbottbase Pharmacokinetic System software pro—
`gram, Abbott Laboratories, Abbott Park, IL; version 1.0)
`to calculate the parameters for the patients in this study.
`In the eight patients that underwent extensive pharma—
`cokinetic sampling, the AUCs were determined using
`the trapezoidal rule and compared using the rule of su-
`perposition.
`
`Determination ofResponses to Treatment
`
`The response status of malignancies other than prostate
`cancer and primary CNS tumors was determined
`monthly, before each cycle of therapy, using conven—
`tional anatomic criteria.10 For patients with prostate
`cancer, criteria from the National Prostate Cancer Proj~
`ect and published criteria based on declines in PSA con*
`centrations were used.”12 A technetium bone scan was
`
`obtained every 3 months if initially positive or in the
`presence of new bone symptoms.
`The assessment of patients with gliomas is compli~
`cated by the variability in tumor—associated edema and
`its response to steroid therapy as well as technical fac~
`tors that preclude using the intensity of gadolinium en
`hancement on magnetic resonance imaging to deter~
`mine tumor response. For these reasons, special atten»
`tion was paid to changes in performance status and
`steroid requirements, which were assessed at each visit
`Complete response was defined as complete disappear—
`ance of lesions on magnetic resonance imaging (assess—
`ment done in two different planes) and weaning from
`steroids. Partial response was defined by conventional
`
`3of7
`
`590*
`
`500C
`
`..
`
`,
`
`
`“0i [ll lll ill l ill
`
`g 3201i“
`
`50
`
`13
`SnmelDZYSlM1O n 12
`2
`1
`O
`Figure I Modeled pharmacokinetic course of a 70—kg man given
`phenylacetate at 125 mg/kg/dose twice daily The simulation
`predicts peak levels between 150 and 450 ug/ml, with trough
`concentrations below 50 ng/ml and no drug accumulation (95%
`confidence intel‘Vals).
`
`14
`
`anatomic criteria, absence of deterioration in perfor—
`mance status, and stable or decreased corticosteroid re—
`
`quirements. Minor response was defined similarly, us-
`ing 25% as the minimal limit of size reduction. Progres—
`sive disease was defined either by anatomic criteria,
`deterioration in performance status by at least 20 points
`on Karnofsky's scale, or the need for increasing steroid
`doses to maintain function. Disease stabilization was
`
`defined as the absence of a significant (>250/0) increase
`or decrease in tumor size while the patient maintained
`or improved his/her performance status at his/her pre~
`treatment leVe], To be scored as significant, disease sta—
`bilization in these patients had to be maintained for at
`least 3 months.
`
`Statistical Methods
`
`To determine whether phenylacetate induces its own
`clearance, the AUCs after a single dose of the drug at
`the beginning and end of phenylacetate therapy in eight
`patients were compared using the Wilcoxon signed rank
`test for paired data.
`
`Results
`
`Pharmacokinetic Simulation and Clinical Findings
`
`Several intermittent dosing schedules were modeled us—
`ing the pharmacokinetic parameters derived from our
`previous trial of phenylacetate. Figure 1 illustrates the
`course of a hypothetical 70-kg man given phenylacetate
`at 125 mg/kg/dose twice daily (8 am. and 5 p.m.). The
`simulation predicts, with a 95% confidence interval,
`
`3 of 7
`
`

`

`Phase I Study of Phenylacetate/Thibault et a].
`
`peak concentrations of 150—450 ug/ml, trough conceir
`. trations below 50 pg/ml, and no drug accumulation
`over time.
`
`Of the 18 patients entered on study, twelve re
`ceived 14 cycles of therapy at the 125 mg/kg dose level.
`Four of them were allowed to escalate to the 150 mg/
`kg dose level for a second cycle. Of the latter, two rer
`ceived a third cycle at the higher dose level. Six addir
`tional patients were entered at the 150 mg/kg dose
`level, of whom one went on to a second cycle. Analysis
`of drug concentrations shows peak serum concentra
`tions (mean i SD) of 490 i 78 pg/ml (n : 14 cycles)
`and 623 i 110 pg/ml (n = 13 cycles) at the two dose
`levels, respectively. Corresponding trough concentra—
`tions were 15 i 18 ug/ml and 62 :r 48 pg/ml, respec~
`tively. The time spent at serum concentrations above
`250 pg/ml corresponded to 32 i 10% (mean + SD) of
`the total treatment time at the first dose level and 48 i
`
`12% (mean + SD) at the second dose level. Drug accu—
`mulation associated with neurologic toxicity occurred in
`one patient treated at the second dose level. The last
`(and highest) phenylacetate concentrations measured
`in this patient before interrupting therapy were 1155
`pg/ml (estimated fitted concentration at the end of in—
`fusion: 1501 ug/ml) and 549 pg/ml (trough). The pharr
`macokinetic parameters of each patient were deter—
`mined by Bayesian fitting a one compartment nonlinear
`model
`to each patient's serum concentration versus
`time curve, using as initial parameters estimates of the
`mean values reported previously.5 Because the 4 pa—
`tients previously treated with phenylacetate by contine
`uous intravenous infusion did not differ from the other
`
`14 patients, the individuals' parameters were then av—
`eraged and expressed as mean parameter values with
`associated SD: Km = 106 i 22 ag/ml,
`
`V max = 29 i 6.3 mg/kg/hour, and
`
`VD = 21 i 4.81.
`
`Metabolism of Phenylacetate
`
`The molar excretion of phenylacetylglutamine was de~
`termined from 24—hour urine collections. It accounted
`
`for 76% i 15% (mean _+_ SD, n = 24) of the dose of
`phenylacetate given over the same period of time. The
`recovery of free, nonmetabolized drug was 3% i 1% of
`the administered dose.
`
`Autoinduction 0f Phenylacetate Clearance
`
`We tested the hypothesis that phenylacetate induces its
`own clearance by comparing AUCs after the 8 am. in—
`fusion of phenylacetate at the beginning (days 1—3) and
`at the end of therapy (days 12—14) in eight patients
`
`40f7
`
`2935
`
`Table 1. 'l‘oxicilies (125 rug/kg BID): Grade, Type and
`Frequency
`
`Grade 1
`Grade 2
`Grade 3
`
`Type
`(11 ; '19)
`(n : 0)
`(n : 1)
`
`Neurologic
`Somnolence
`
`Fatigue
`Headache
`
`lightheadedness
`Dysgeusia
`Cardiovascular
`l’edal edema
`Gastrointestinal
`Nausea
`
`13
`6
`
`3
`l
`
`2
`1
`3
`3
`2
`l
`
`0
`~
`
`—
`—
`
`~
`~
`0
`—
`0
`—
`
`l
`
`—
`—
`
`~
`~
`0
`~
`0
`~
`
`‘
`—
`1
`Vomiting
`fl
`~
`1
`l’)ermatologic
`
`Rash — 1 ~
`
`
`Bll): twice a day.
`
`(seven at the 125 mg/kg and one at the 150 mg/kg dose
`level, respectively). All exhibited a decrease in AUC, the
`mean value of which was 27 i 10% (P value = 0.008).
`
`Toxicities
`
`The recorded toxicities associated with the administra—
`
`tion of phenylacetate on a twice—daily schedule are
`listed in Table 1 and Table 2. Seventeen patients (94%)
`experienced at least Grade 1 toxicity (mean peak serum
`concentration i SD equal to 553 i 114 ug/ml). Four
`patients (9%) had Grade 2 and three (7%), Grade 3 tox—
`icity. Rapidly reversible neurologic toxicity was the
`most common side effect (71% of all episodes). Except
`for one occurrence, the cases of dose—limiting toxicity
`were seen at the 150 mg/kg dose level. They were all
`neurologic in nature: four patients experienced pro—
`found somnolence (one of whom was taking high doses
`of opiates while being treated at the 125 mg/kg dose
`level), and one patient suffered from confusion, one
`from hypoacusis, and one from an- exacerbation of a
`preexisting, suramin—induced, peripheral sensory neu-
`ropathy. The latter three patients had achieved high
`peak drug concentrations (mean .+_ SD: 682 i 290 ag/
`ml,- range: 499 to 1016 pg/ml) and one experienced
`drug accumulation. The patient who suffered an ex—
`acerbation of neuropathy experienced gradual deterio—
`ration of his condition over the first 10 days of pheny-
`lacetate administration (peak and trough levels, mean i
`SD: 574 i 52 and 95 i 59 ug/ml) before therapy was
`discontinued. This complication partially improved
`over the ensuing 3 months.
`Three patients (7%) with a history of angina pecto—
`ris, supraventricular tachycardia, or palpitations associ-
`
`4 of 7
`
`

`

`2936
`
`CANCER lune 15, 1995, Volume 75. No. 12
`
`Table 2. Toxicities (150 nig/kg BID): Grade.
`Type and Frequency
`Grade 1
`Grade 2
`(n , 4)
`(n > 20)
`
`TYPE
`
`Grade 3
`tn * 2)
`
`Neurologic
`Somnole nce
`
`Fatigue
`Headache
`
`Lightheadedness
`Hypoacusis
`Disorien ta tion
`
`Exacerbation of neuropathy
`Impaired memory
`Cardiovascular
`Pedal edema
`
`l
`
`WN-Lht-AJLJ
`
`«7
`_
`
`re
`1
`6
`
`[\J
`
`i
`
`N
`l
`
`l
`~
`0
`
`i
`
`l
`w
`
`r
`«~
`0
`
`Angina
`Arrhythmias
`Gastrointestinal
`Nausea
`BID: twice a day.
`
`tll
`
`#Hygfiw
`
`ated with mitral valve prolapse reported reversible ex»
`acerbation of their usual symptoms during the infusion
`of phenylacetate‘ This was probably related to signifi-
`cant fluid shifts induced by the high sodium content of
`the drug formulation. No such symptoms were noted in
`patients free of cardiovascular impairment. ln addition,
`six (14%) patients developed pedal edema that was eas~
`ily controlled with short courses of diuretic therapy.
`
`Antitumor Activity
`
`One patient with recurrent anaplastic astrocytoma had
`a partial response accompanied by subjective improve—
`ment in short—term memory (Fig. 2). She was also able
`to reduce her daily corticosteroid dose by 50%, from 8
`to 4 mg of dexamethasone. One patient with hormone—
`independent prostate cancer experienced a greater than
`50% decline in PSA sustained for a month and has
`
`maintained a performance status of 100% on Karnof«
`sky's scale for more than 5 months.
`
`Discussion
`
`This trial was designed to overcome the problem of
`rapid drug accumulation associated with the unin«
`terrupted delivery of high doses of phenylacetate by
`continuous intravenous infusion. The goal was to
`achieve transient peak drug concentrations in the range
`found to be active preclinically (>250 ug/ml) and to al~
`low enough time for drug elimination between each
`dose of phenylacetate. We used the pharmacokinetic in~
`formation derived from our first trial of phenylacetate
`
`50f7
`
`to stimulate several intermittent dos1ng regimens, which
`successfully eliminated the need for multiple escalation
`steps and allowed the clinical questions to be answered
`rapidly liurthei‘
`research is
`required to establish
`a
`
`a
`
`g!
`
`na
`
`u
`
`n
`
`V
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`“I“!
`
`
`
`
`
`
`
`E
`
`-tu:
`tau-.2645? Level= _'1184
`er: BBS/86‘
`ma: 814447813
`
`‘-
`lb 1 5
`
`.. .
`
`Figure 2. (Top) Pretreatment gadoliniumrenhanced brain magnetic
`resonance imaging in a patient with recurrent glioblastoma
`multiforme and shortsterm memory loss (Bottom) Posttreatment
`gadolinium—enhanced magnetic resonance imaging after three cycles
`of phenylacetate (150 mg/kg twice daily), illustrating partial
`response. There was concurrent improvement in memory.
`
`5 of 7
`
`

`

`Phase I Study of Phenylacetate/Thibault et a].
`
`whether this approach will be applicable to the design
`. of Phase I studies with other agents.
`Administering 125 mg/kg/dose of phenylacetate
`twice daily (9 hours apart) as l—hour infusions was pre—
`dicted to achieve serum drug concentrations between
`150 and 450 rig/ml without drug accumulation in more
`than 95% of patients. This dose was therefore chosen
`as the starting point for the trial. The results confirm
`that most patients treated with 125 and 150 mg/kg/
`dose of phenylacetate achieve peak serum drug concene
`trations in the range of 500 rig/ml. No patient treated at
`the first dose level experienced undesirable drug accue
`mulation, which occurred in l of 10 patients treated at
`the higher level. Patients’ exPosure to preclinically ac—
`tive concentrations of phenylacetate was equal to ap
`proximately 40% of their total treatment time. Of note,
`the maximum tolerated dose of phenylacetate in this
`trial (125 mg/kg/dose, twice daily) is quantitatively
`equal to the one identified in the continuous infusion
`study (250 mg/kg/day). The dose-limiting toxicity is
`also similar in its nature and course. The major differ-
`ence resides in the drug concentrations achieved: the
`patients on the continuous infusion regimen main—
`tained an average phenylacetate concentration of 104 i
`40 pg/ml, well below the values associated with in vitro
`activity. Assuming the values pertaining to the in vitro
`values apply in vivo, an interrupted dosing regimen of
`phenylacetate appears to be superior to continuous inv
`travenous administration.
`
`Indirect evidence for the induction of phenylacetate
`clearance by the hepatic enzyme phenylacetyl Coen-
`zyme Azglutamine acyltransferasem’I4 is available from
`our first trial of phenylacetate.7 With the current fixed—
`dosing regimen, we have shown a 27% mean decline in
`the AUC associated with identical doses of drug given
`at the beginning and end of the 2—week treatment in
`eight patients. This observation may justify the need for
`dose modification over time if longer treatment periods
`are considered.
`
`Although a larger number of patients will be neces~
`sary to establish the therapeutic value of phenylacetate,
`the experience gained suggests that it may have activity
`against hormone-independent prostate cancer and re—
`current high grade gliomas.7 Assessing the antitumor
`activity of a differentiating agent, however, can be
`problematic in patients with prostate cancer, for whom
`PSA has been proposed as the best monitoring tool
`available.15 PSA production is organ-specific and di—
`rectly correlated with the degree of tumor differentia-
`tion.16 Phenotypic reversion induced by phenylacetate
`could therefore be associated with rising concentrations
`of the marker. This would paradoxically invalidate the
`use of PSA as an index of tumor burden. Because the
`
`vast majority of patients with advanced prostate cancer
`
`60f7
`
`2937
`
`lack measurable soft
`
`tissue disease,11 drug activity
`should be described both in terms of anatomic and per—
`formance status criteria until more experience with PSA
`is acquired.
`Reduction in tumor size in the context of differen—
`
`tiation therapy could be secondary to enhanced tumor
`immunogenicity leading to subsequent cell death. In
`laboratory models, phenotypic reversion induced by
`phenylacetate in human prostate3 and brain tumor cell
`lines]7 is accompanied by reduced production of
`transforming growth factor beta~2, an immunosuppres-
`sive cytokine,18 and increased expression of major his-
`tocompatibility complex Class I antigens,3 known to
`evoke proliferatii'e and cytotoxic T—cell responses in
`vivo. An alternative mechanism is tied to the depletion
`of glutamine induced by the metabolism of phenylace—
`tate. Glutamine donates the nitrogen groups required
`for DNA, RNA, and protein synthesis. It is also a major
`energy source for various tumor cell types.” Although
`we could not demonstrate sustained declines in plasma
`glutamine concentrations after repeated administration
`of phenylacetate, the urinary excretion of glutamine (as
`phenylacetylglutamine) from a 70—kg patient receiving
`125 mg/kg/dose of phenylacetate twice daily would
`nevertheless exceed 90 mol per day. Whether this re—
`sults in glutamine depletion at the tumor site is not
`known. Finally, phenylacetate inhibits the mevalonate
`pathway of cholesterol synthesis and protein prenyla-
`tion by interfering with the use of acetylcoenzyme
`AM”: Because malignant astroglia and prostate ade—
`nocarcinoma cells
`rely on this pathway for
`their
`growth?23 25 a partial decline in the use of acetylcoen—
`zyme A may result in faulty intracellular signaling26 and
`cell death, thus contributing to the observed clinical
`effects.
`
`A more severe decline in the use of acetylcoenzyme
`A may, conversely, inhibit choline acetyltransferase ac—
`tivity in neurons.27 The resulting deficiency in acetyl—
`choline, a ubiquitous CNS transmitter, could account
`for the neurologic side effects observed clinically. In this
`regard, this trial has enabled us to characterize the tox—
`icity of phenylacetate with respect‘to peak drug levels.
`The 125 mg/kg dose level was associated with a mean
`peak serum concentration of 490 pg/ml and Grade 1
`neurocortical toxicity (somnolence). The temporal rela—
`tionship between drug infusion and the onset of som—
`nolence was noted in all patients treated at this dose
`level. Gradual recovery between each infusion was the
`rule. The second dose level (150 mg/kg twice daily) was
`associated with Grade 1 neurotoxicity in five patients
`(mean peak serum concentration, 623 pg/ml) and more
`severe toxicity in three patients whose mean peak drug
`concentration was 682 rig/ml. Except for the deteriora—
`tion seen in a patient with preexisting suramin-induced
`
`6 of 7
`
`

`

`2938
`
`CANCER lune 15, 1995, Volume 75,N0.12
`
`sensory neuropathy, neurotoxicity from phenylacetate
`has been acute and reversible.
`‘
`
`We conclude that phenylacetate given at a dose of
`125 mg/kg twice daily for two consecutive weeks is
`well tolerated and that high grade gliomas and ad
`vanced prostate cancer are reasonable targets for Phase
`II clinical trials of this drug.
`
`References
`
`Sandler M, Ruthven CR], Goodwin BL, Less A, Stern GM. Pher-
`nylacetic acid in human body fluids: high correlation between
`plasma and cerebrospinal fluid concentrations values. J Ncm-nl
`Neurosurg Psych 1982;45:366~8.
`Samid D, Ram Z, Hudgins R, Shack S, Liu L, Walbridge S. Selece
`tive activity of phenylacetate against malignant gliomas: resems
`blance to fetal brain damage in phenylketonuria. Cancer Res
`1994;54:89175.
`Samid D, Shack S, Myers CE. Selective growth arrest and phe-
`notypic reversion of prostate cancer cells in vitro by nontoxic
`pharmacological concentrations of phenylacetate. [C1111 Inzws/
`1993;91:2288495.
`Samid D, Shack S, Sherman LT. Phenylacetate: a novel nontoxic
`inducer of tumor cell differentiation Cancer Res 1992;132:1988‘
`92.
`
`Samid D, Yeh A, Prasanna P. lnduction of erythroid different}
`ation and fetal hemoglobin production in human leukemic cells
`treated with phenylacetate. Blood 1992;80:1576481.
`Liu L, Shack S, Steler-Stevenson WG, liudgins WR, Samid D.
`Differentiation of cultured human melanoma cells induced by
`the aromatic fatty acid phenylacetate and phenylbutyrate. ] In—
`vest Dermalal 1994;103:335—40
`Thibault A, Cooper MR, Figg WD, Venzon D], Sartor AO,
`Tompkin AC, et al. A phase 1 and pharmacokinetic study of ins
`travenous phenylacetate in patients with cancer. Cancer Res
`1994;52:1690—4.
`Batshaw ML, Monahan PS. Treatment of urea cycle disorders.
`Enzyme 1987;38:242—50.
`Wittes RE. Manual of oncologic therapeutics. 3rd ed. Philadel-
`phia: IB. Lippincott, 1989.
`Miller AB, Hoogstraten B, Staquet M, Winkler A. Reporting re»
`sults of cancer treatment. Cancer 1981; 47:20744.
`Scher HI, Yagoda A. Clinical trials in prostatic cancer: methods
`ology and controversies. In: Bruce AW, Trachtenberg J, editors.
`Adenocarcinoma of the prostate. New York: SpringenVerlag.
`1987:197-220.
`
`Slack NH, Murphy GP. Criteria for evaluating patient responses
`to treatment modalities for prostate cancer. Ural czm N0rH/.Am
`1984;11:3371741.
`
`10,
`
`11.
`
`12.
`
`14
`
`UI
`
`16.
`
`18.
`
`19.
`
`21.
`
`Is.) IX)
`
`23.
`
`24.
`
`26.
`
`27.
`
`James MO, Smith RL, Williams FRS, Reidcnberg M The conju—
`gation of phenylacctic acid in man, sulvhuman primates and
`stilllL’ noneprnnate spt‘ciL‘s. Prof Row)! 501‘ London, Series B,
`W72: WEE; 735.
`
`Moldai‘c K, Meister A. Synthesis of phenylacctylglutamine by
`human tissue. / Biol Chum 1957;229:463/70.
`Andriole (ll . Serum prostate speClfiC antigen: expanding its role
`as a measure of treatment response in patients with prostate can<
`cer. ) C1171 Oncni 19911159677.
`l’artin AW, Carter 1 1B, Chan DW. Prostate specific antigen in the
`staging of localized prostate cancer: influence of tumor differv
`entialion,
`tumor volume and benign hyperplasia.
`I Urol
`U990; 1437-17752.
`Liu l_, BarrNcr M, Weber J, Danielpour D, Qian SW, Shearer
`CM, cl al. Enhancement of tumor immunogenicity by phenyl—
`acctate and derivatives: changes in surface antigens and tumor—
`deriied iininunosuppressive factors [abstract]. Proc Am Assoc
`Ciltlt‘l't RPS 1994;35:481.
`Maxwell M, Galanopoulos T, Neville—Golden ], Antoniades HN.
`Effect of the expression of transforming growth factor—beta 2 in
`primary human glioblastomas on immunosuppression and loss
`of immune surveillance.)Nr’m‘nsurg1992;76:799—804.
`Weber (3. Biochemical strategy of cancer cells and the design of
`chcmotherapy. Camrr Res 1983;43:3466-92.
`Castillo M, Zafra MF, Garcia—Peregrin E. Inhibition of brain and
`liver 3shydroxyfi~methylglutaryl—CoA reductase and mevalo~
`nate 5*pyr0ph0sphate dccarboxylase in experimental hyper—
`phenylalaninemia. Ncm‘oclicm Res 1988;13:55175.
`Castillo M,
`lglesias J, Zafra MF, Garcia»Peregrin E. Inhibition
`of chick brain cholesterogenic enzymes by phenyl and phenolic
`derivatives of phenylalanine. NeuroC/iem Int 1991;18:171—4.
`Castillo M, lVlartinez~Cayuela M, Zafra MF, Garcia—Peregrin E.
`Effect of phenylalanine derivatives on the main regulatory en—
`zymes of hepatic cholesterogenesis. Mol Ccll Biorhem 1991;105:
`21~5.
`
`Kandutsh AA, Saucier 5E. Regulation of sterol synthesis in de—
`veloping brains of normal and jimpy mice. Arch Biochem Biophy
`1969;135:2(lls8.
`Crossi F l’aoletti P, Paoletti R. An analysis of brain cholesterol
`and fatty acid biosynthesis. Arch Int Plii/siol Biochem 1958,66:
`564s72.
`
`Fumagalli R, Grossi E, Paoletti P, Paoletti R. Studies on lipids in
`brain tumors. J, Occurrence and significance of sterol precursors
`of cholesterol in human brain tumors. I Neurochem 1964,11:
`561775.
`
`Marshall C]. I’mtein prenylation: a mediator of protein~pr0tein
`interactions. Science 1993;259:1865~6.
`Potempska A, Loo YH, Wisniewski HM. On the possible mech—
`anism of phenylacetate neurotoxicity: inhibition of choline ace»
`tyltransferase by phenylacetyl—COA,
`I chzrochcm 1984;42:
`14997501.
`'
`
`7of7
`
`7 of 7
`
`