`
`Phase I Study of Phenylacetate
`Administered Twice Daily to Patients
`with Cancer
`
`Alain Thibault, M.D.,* Duortt Samid, Ph.D.,* Michael R. Cooper, M.D.*
`William D. Figg, Pharm.D.,* Anne C. Tompkins, R.N..* Nicholas Patronas, M.D.,t
`Donna J. Headlee, R.N.,* David R. Kohler, Pharm.D. David J. Venzon, Ph.D_§
`and Charles E. Myers, M.D.*
`
`Background. The growth-inhibiting and differenti-
`ating effects of sodium phenylacetate against hematopoi-
`etic and solid tumorcell lines has aroused clinical inter-
`est in its use as an anticancer drug. In an earlier Phase J
`trial of phenylacetate aimed at maintaining serum drug
`concentrations in the range that proved active in vitro
`(> 250 ug/ml) for 2 consecutive weeks, infusion rates ap-
`proached 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 palients intermittently to drug
`concentrations exceeding 250 ug/ml andto allow timefor
`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 p.m.) al two dose
`levels of 125 and 150 mg/kg for a 2-week period. Therapy
`wasrepeated at 4-weekintervals for patients whodid 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 {Diagnostic Radiology Department and
`Pharmacy Department, Warren G. Magnuson Clinical Center, Na-
`tional Institutes of Health, Bethesda, Maryland.
`Supported by the intramural program of the National Cancer
`Institute and bya grant from Elan Pharmaceutical Company.
`The authors thank the medicalstaff fellows and nursing staff of
`the NC] fortheir skillful care of patients treated on this study, as well
`as Natalie McCall, B.S., and Kara Ammermanfortheir invaluable lab-
`oratory assistance.
`Address for reprints: Alain Thibault, M.D., National Institutes
`of Health, National CancerInstitute, Clinical Pharmacology Branch,
`Building 10, Room 12 N 214, Bethesda, MD 20892-1576.
`Received September 12, 1994; revision received January 13,
`1995; accepted February 27, 1995.
`
`pharmacokinetic studies for eight patients indicated that
`phenylacetate induced its own clearance by a factor of
`27%ina 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-
`cer 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 advancedprostate cancer
`are reasonable targets for Phase I] clinical trials. Cancer
`1995: 75:2932~-8.
`
`Key words: phenylacetate, pharmacokinetics, differen-
`tiation. glioma, prostate cancer.
`
`Phenvlacetate is a minor product of phenylalanine me-
`tabolism normally found at micromolar concentrations
`in the plasma and cerebrospinal
`fluid of humans.'
`Higher concentrations (250-700 ug/ml) sustained fora
`minimum of 7 days induce cytostasis and differentia-
`tion in a variety of hematologic and solid tumor models,
`including cultures of prostate and brain tumorcells.*°
`Werecently reported the first Phase | trial of phenyl-
`acetate,’ 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 evidencefor induc-
`tion of its own metabolism (pharmacokinetic parame-
`ters, mean + standard deviation (SD): Km (Michaelis-
`Menten constant) = 105 + 45 ne/ml, V max (maximum
`metabolic rate) = 24 + 5.2 mg/kg/hour, and Vp (vol-
`ume of distribution) = 19 + 3.3 |. 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
`IPR2018-00459
`
`1 of 7
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`PhaseI Studyof Phenylacetate/Thibault et al.
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`2933
`
`insufficiency. Patients with
`suramin-induced adrenal
`primary CNS tumors who were taking corticosteroidsat
`the time of enrollment were maintained at the same or
`lower dose of the corticosteroid while participating in
`the study. Fourpatients, tvo 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 hadclinically benefitted from the ad-
`ministration of phenyjacetate with no evidence of cu-
`mulative toxicity. No other form of antitumor therapy
`was allowed during the studyperiod.
`
`Drug Preparation and Administration
`
`and metastatic hormone-independent prostate cancer
`who achieved scrum phenylacetate concentrations of
`150-200 ug/ml. However, infusion rates close to the V
`max of
`the metabolizing enzyme were required to
`achieve concentrations of 250 ug/ml or more. This of-
`ten resulted in rapid drug accumulation, which was as-
`sociated with neurologic toxicity once phenylacetate
`concentrations exceeded 800 ug/ml.
`Postulating that intermittent drug administration
`might obviate this problemat 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 ug/ml and
`maintain trough concentrations not exceeding 100 ug/
`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 maximumtolerated
`dose of this dosing schedule, wherein the drug was
`given as a 1-hour infusion twice daily over 14 consecu-
`tive days.
`
`Aninjectable formulation of sodium phenylacetate was
`manufactured by the Pharmaceutical Development
`Section of the National Institutes of Health Clinical
`Center Pharmacy Department from bulk phenylacetate
`powdersupplied by Elan Pharmaceutical Research Cor-
`poration (Gainesville, GA). The finished drug product
`contained sodium phenylacetate 500 mg/mlin sterile
`waterfor 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
`Methods
`wasfurther diluted in water for injection, USP,toatotal
`volume of 250 ml and administered over 1 hour with a
`Patient Population
`portable pump (CADD-PLUS; Pharmacia Deltec, Inc.,
`St. Paul, MN).
`Phenylacetate was delivered at two doselevels: 125
`and 150 mg/kg/dose, twice daily (8 a.m. and 5 p.m.)
`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).®
`Cycles of therapy were repeated every 4 weeks. Indi-
`vidual patients could escalate from one doselevelto 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 maximumtol-
`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 Institute’s Common
`Toxicity Criteria.’
`
`Adults with advanced solid tumors refractory to con-
`ventional therapy, a performance status greater than
`60% on Karnofsky’s scale, normal hepatic transami-
`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, andall patients gave writ-
`ten informed consent before participating in the study.
`Eighteenpatients, 15 men and 3 women,with a median
`age of 55 years (range, 32-76 years), were enrolled be-
`tween July and October 1993. No selection process took
`place to ensure a balanced male:female 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 previouslyre-
`ceived suramin as an experimental treatment for pros-
`tate cancer continuedto take hydrocortisone (20 mg ev-
`ery morning, 10 mg every evening) as replacement for
`
`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 p.m. infusion
`(peak Jevel). To assess the possibility that phenylacetate
`induces its own clearance, eight patients were randomly
`chosen to undergo moreintensive drug level monitor-
`
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`2934
`
`CANCER June 15, 1995, Volume 75, No. 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 a.m. infu-
`sion. This allowed for a comparison to be made between
`area underthe 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
`
`50
`
`
`
`590 -
`
`[
`500 |-
`
`§ a0}
`2
`8
`3207p
`2 230
`ao
`x sao
`
`-T
`
`T
`
`ty
`
`T
`
`Ny
`
`Hy
`
`t
`i
`
`|
`
`Concentrations of phenylacetate and phenylacety!-
`glutamine were measured in serum with the use of
`high-performance liquid chromatography.’
`
`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-Meaditerative algorithm
`(Abbottbase Pharmacokinetic System software pro-
`gram, Abbott Laboratories, Abbott Park, IL; version 1.0)
`to calculate the parametersfor 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 usingtherule of su-
`perposition.
`
`Determination of Responses to Treatment
`
`The responsestatus of malignancies other than prostate
`cancer and primary CNS tumors was determined
`monthly, before each cycle of therapy, using conven-
`tional anatomic criteria.'° For patients with prostate
`cancer, criteria from the National Prostate Cancer Proj-
`ect and publishedcriteria based on declines in PSA con-
`centrations were used.!’* A technetium bone scan was
`obtained every 3 monthsif initially positive or in the
`presence of new bone symptoms.
`The assessmentof 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 assessedat 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
`
`3 of 7
`
`§ 0 12°13 14
`
`64 7 3°46 6°7 8
`Time [Days]
`Figure 1. Modeled pharmacokinetic course of a 70-kg man given
`phenylacetate at 125 mg/kg/dose twice daily. The simulation
`predicts peak Jevels between 150 and 450 ug/ml, with trough
`concentrations below50 ug/ml and no drug accumulation (95%
`confidence intervals).
`
`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 performancestatus byat 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 (>25%) increase
`or decrease in tumorsize while the patient maintained
`or improved his/her performancestatus at his/her pre-
`treatment level. To be scored as significant, disease sta-
`bilization in these patients had to be maintainedfor at
`least 3 months.
`
`Statistical Methods
`
`To determine whether phenylacetate induces its own
`clearance, the AUCsafter 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 tria] of phenylacetate. Figure 1 illustrates the
`course of a hypothetical 70-kg man given phenylacetate
`at 125 mg/kg/dose twice daily (8 a.m. and 5 p.m.). The
`simulation predicts, with a 95% confidence interval,
`
`3 of 7
`
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`
`Phase I Study of Phenylacetate/ Thibault etal.
`
`2935
`
`Table 1, Toxicities (125 mg/kg BED): Grade, Type and
`Frequency
`
`peak concentrations of 150-450 ug/ml, trough concen-
`. 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 re-
`ceived a third cycle at the higher dose level. Six addi-
`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 + SD) of 490 + 78 wg/ml (n = 14 cycles)
`and 623 + 110 ug/ml (n = 13 cycles) at the two dose
`levels, respectively. Corresponding trough concentra-
`tions were 15 + 18 ng/ml and 62 + 48 we/ml, respec-
`tively. The time spent at serum concentrations above
`250 ug/ml corresponded to 32 + 10% (mean + SD) of
`the total treatment time at the first dose level and 48 +
`12% (mean + SD)at the second doselevel. 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
`ug/ml (estimated fitted concentration at the end of in-
`fusion: 1501 ug/ml) and 549 pg/ml (trough). The phar-
`macokinetic parameters of each patient were deter-
`mined by Bayesianfitting a one compartmentnonlinear
`model
`to each patient’s serum concentration versus
`time curve, using as initial parameters estimates of the
`mean values reported previously.” Because the 4 pa-
`tients previously treated with phenylacetate by contin-
`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 + 22 pg/ml,
`
`V max = 29 + 6.3 mg/kg/hour, and
`
`Vp = 2144.81.
`
`Metabolism of Phenylacetate
`
`The molar excretion of phenylacetylglutamine was de-
`termined from 24-hour urine collections. It accounted
`for 76% + 15% (mean + SD, n = 24) of the dose of
`phenylacetate given over the sameperiod of time. The
`recovery of free, nonmetabolized drug was 3% + 1%of
`the administered dose.
`
`Autoinduction of Phenylacetate Clearance
`
`Wetested the hypothesis that phenylacetate inducesits
`own clearance by comparing AUCsafter the 8 a.m. in-
`fusion of phenylacetate at the beginning (days 1-3) and
`at the end of therapy (days 12-14) in eight patients
`
`4of7
`
`Grade 2
`(n = 0)
`
`Grade 3
`(n = 1)
`
`2|
`
`2|
`
`Type
`Neurologic
`Somnolence
`
`Fatigue
`Headache
`
`Lightheadedness
`Dyspeusia
`Cardiovascular
`Pedal edema
`Gastromitestinal
`Nausea
`
`Grade 1
`(n = 19)
`1
`
`SPPerNYwWWOHNKFWAWwW
`
`Vomiting
`Dermatologic
`Rash
`BID): twice a day.
`
`(seven at the 125 mg/kg and oneat the 150 mg/kg dose
`level, respectively). All exhibited a decrease in AUC, the
`mean value of which was 27 + 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 + SD equal to 553 + 114 ug/ml). Four
`patients (9%) had Grade 2 and three (7%), Grade 3 tox-
`icity. Rapidly reversible neurologic toxicity was the
`most commonside 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 wereall
`neurologic in nature: four patients experienced pro-
`found somnolence (one of whom wastaking 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 anexacerbation of a
`preexisting, suramin-induced, peripheral sensory neu-
`ropathy. The latter three patients had achieved high
`peak drug concentrations (mean + SD: 682 + 290 pg/
`ml; range: 499 to 1016 g/ml) and one experienced
`drug accumulation. The patient who suffered an ex-
`acerbation of neuropathy experienced gradual deterio-
`ration of his condition overthefirst 10 days of pheny-
`lacetate administration (peak and trough levels, mean +
`SD: 574 + 52 and 95 + 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
`
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`
`2936
`
`CANCER June 15, 1995, Volume 75, No. 12
`
`Table 2. Toxicities (150 mg/kg BID): Grade,
`Type and Frequency
`Grade 1
`Grade 2
`{n = 4)
`(n = 20)
`
`Type
`
`Grade 3
`(n = 2)
`
`to simulate several intermittent dosing regimens, which
`successfully eliminated the need for multiple escalation
`steps and allowed the clinical questions to be answered
`rapidly. Turther
`research is
`required to establish
`
`=
`
`=
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`
`Neurologic
`Somnolence
`
`Fatigue
`Headache
`
`Lightheadedness
`Hypoacusis
`Disorientahion
`
`Exacerbation of neuropathy
`Impaired memory
`Cardiovascular
`Pedal edema
`
`we
`
`—_
`
`3 1
`
`2
`
`1 1
`
`Angina
`Arrhythmias
`Gastrointestinal
`Nausea
`BID: twice a day.
`
`ated with mitral valve prolapse reported reversible ex-
`acerbation of their usual symptoms during the infusion
`of phenylacetate. This was probably related to signifi-
`cantfluid shifts induced by the high sodium content of
`the drug formulation. No such symptoms were noted in
`patients free of cardiovascular impairment. In addition,
`six (14%) patients developed pedal edemathat was eas-
`ily controlled with short courses of diuretic therapy.
`
`Antitumor Activity
`
`Onepatient with recurrent anaplastic astrocytoma had
`a partial response accompanied bysubjective 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
`
`5 of 7
`
`
`
`
`
`a
`
`Ferre ec ACY
`Btu:
`Soy
`LaGOAa)
`
`=
`pee
`
`ane
`
`Figure 2. (Top) Pretreatment gadolinium-enhanced brain magnetic
`resonance imaging in a patient with recurrent glioblastoma
`multiforme and short-term memoryloss. (Bottom) Posttreatment
`gadolinium-enhanced magnetic resonance imaging after three cycles
`of phenylacetate (150 mg/kg twicedaily), illustrating partial
`response. There was concurrent improvement in memory.
`
`5 of 7
`
`
`
`Phase I Study of Phenylacetate/Thibault et al.
`
`whetherthis approach will be applicable to the design
`_of Phase | studies with other agents.
`Administering 125 mg/kg/dose of phenylacetate
`twice daily (9 hours apart) as 1-hour infusions waspre-
`dicted to achieve serum drug concentrations between
`150 and 450 ug/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 concen-
`trations in the range of 500 ug/ml. Nopatienttreated at
`the first dose level experienced undesirable drug accu-
`mulation, which occurred in 1 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 +
`40 ug/ml, well below the values associated with invitro
`activity. Assuming the values pertaining to the in vitro
`values apply in vivo, an interrupted dosing regimen of
`phenylacetate appears to be superior to continuousin-
`travenous administration.
`Indirect evidence for the induction of phenylacetate
`clearance by the hepatic enzyme phenylacetyl Coen-
`zyme A:glutamine acyltransferase’*"* is available from
`ourfirst trial of phenylacetate.” With the currentfixed-
`dosing regimen, we have shown a 27% meandecline in
`the AUCassociated with identical doses of drug given
`at the beginning and end of the 2-week treatment in
`eight patients. This observation mayjustify 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 haveactivity
`against hormone-independent prostate cancer andre-
`current high grade gliomas.’ 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.” PSA production is organ-specific and di-
`rectly correlated with the degree of tumor differentia-
`tion.’* 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
`
`6 of 7
`
`2937
`
`tissue disease,’ drug activity
`lack measurable soft
`should be described both in terms of anatomic and per-
`formancestatus criteria until more experience with PSA
`15 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 prostate® and brain tumorcell
`lines’
`is accompanied by reduced production of
`transforming growthfactor beta-2, an immunosuppres-
`sive cytokine," and increased expression of major his-
`tocompatibility complex Class I antigens,’ known to
`evoke proliferative and cytotoxic T-cell responses in
`vivo. Analternative mechanism is tied to the depletion
`of glutamine induced by the metabolism of phenylace-
`tate. Glutamine donates the nitrogen groups required
`for DNA, RNA,andprotein synthesis. It is also a major
`energysource for various tumorcell 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
`A?°"*? Because malignant astroglia and prostate ade-
`nocarcinoma cells
`rely on this pathway for
`their
`growth,’*? *° a partial decline in the use of acetylcoen-
`zyme A may result in faulty intracellular signaling”® 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.’’ The resulting deficiency in acetyl-
`choline, a ubiquitous CNS transmitter, could account
`for the neurologic side effects observedclinically. 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 ug/ml and Grade 1
`neurocortical toxicity (somnolence). The temporal rela-
`tionship between drug infusion and the onset of som-
`nolence was noted inall patients treated at this dose
`level. Gradual recovery between each infusion was the
`tule. The second dose level (150 mg/kg twice daily) was
`associated with Grade 1 neurotoxicity in five patients
`(mean peak serum concentration, 623 ug/ml) and more
`severe toxicity in three patients whose mean peak drug
`concentration was 682 uve/ml. Except for the deteriora-
`tion seen in a patient with preexisting suramin-induced
`
`6 of 7
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`
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`2938
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`,
`
`ao
`
`16,
`
`21.
`
`nN ho
`
`23.
`
`24.
`
`26.
`
`27.
`
`sensory neuropathy, neurotoxicity from phenylacetate
`has been acute andreversible.
`,
`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
`Hclinicaltrials of this drug.
`
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