Molecular Genetics and Metabolism 110 (2013) 446–453
`
`Contents lists available at ScienceDirect
`
`Molecular Genetics and Metabolism
`
`j o ur n a l h o m e p a g e : w ww . e l s e v i e r . c om / l o c a t e / y mg m e
`
`Elevated phenylacetic acid levels do not correlate with adverse events in patients with
`urea cycle disorders or hepatic encephalopathy and can be predicted based on the
`plasma PAA to PAGN ratio☆
`M. Mokhtarani a,⁎, G.A. Diaz b, W. Rhead c, S.A. Berry d, U. Lichter-Konecki e, A. Feigenbaum f, A. Schulze f,
`N. Longo g, J. Bartley h, W. Berquist i, R. Gallagher j, W. Smith k, S.E. McCandless l, C. Harding m, D.C. Rockey n,
`J.M. Vierling o, P. Mantry p, M. Ghabril q, R.S. Brown Jr. r, K. Dickinson a, T. Moors a, C. Norris a, D. Coakley a,
`D.A. Milikien s, S.C. Nagamani t, C. LeMons u, B. Lee t, B.F. Scharschmidt a
`a Hyperion Therapeutics, 601 Gateway Blvd., Suite 200, South San Francisco, CA 94080, USA
`b Icahn School of Medicine at Mount Sinai, Department of Genetics and Genomic Sciences, Department of Pediatrics, 1428 Madison Ave., New York, NY 10029, USA
`c The Medical College of Wisconsin, 9000 W. Wisconsin Ave., Milwaukee, WI 53226, USA
`d University of Minnesota, Minneapolis, 420 Delaware St., SE MMC 75, Minneapolis, MN 55455, USA
`e Children's National Medical Center, 111 Michigan Ave., NW #1950, Washington, DC 20010, USA
`f The Hospital for Sick Children and University of Toronto, Division of Clinical and Metabolic Genetics, 555 University Avenue, Toronto, ON M5G1X8, Canada
`g The University of Utah, Division of Medical Genetics, 2C412 SOM, 50 North Mario Capecchi Drive, Salt Lake City, UT 94132, USA
`h Long Beach Memorial Hospital, 2865 Atlantic Avenue, #104, Long Beach, CA 90806, USA
`i Stanford University, 750 Welch Road, #116, Palo Alto, CA 94305, USA
`j Children's Hospital Colorado, 13123 East 16th Avenue, B153, Aurora, CO 80045, USA
`k Maine Medical Center, 1577 Congress Street, 2nd Floor, Portland, ME 04102, USA
`l Center for Human Genetics, Case Western Reserve University and University Hospitals, Case Medical Center, 11100 Euclid Avenue, Cleveland, OH 44106, USA
`m Oregon Health Sciences University, 3181 SW Sam Jackson Park Road, A036/B198, Mailcode L103, Portland, OR 97239, USA
`n Medical University of South Carolina, Department of Medicine, 96 Jonathan Lucas St. 803CSB, Charleston, SC 29425, USA
`o Baylor College of Medicine, Departments of Medicine and Surgery, 6620 Main St #1475, Houston, TX 77030, USA
`p Liver Institute at Methodist Dallas Medical Center, Dallas 1411 N. Beckley Ave., Pavilion III, STE 268, Dallas, TX 75203, USA
`q Indiana University, Gastroenterology and Hepatology, Regenstrief Health Center, Room 4100, 1050 Wishard Blvd., Indianapolis, IN 46202, USA
`r Columbia University Medical Center, Department of Surgery, Room PH 14, 105, 622 West 168th St., New York, NY 10032, USA
`s Accudata Solutions, Inc., 886 Oak St., Lafayette, CA 94549, USA
`t Baylor College of Medicine, One Baylor Plaza, Room R814, Houston, TX, USA
`u The National Urea Cycle Disorders Foundation, Pasadena, CA, 75 South Grand Avenue, Pasadena, CA 91105, USA
`
`a r t i c l e
`
`i n f o
`
`a b s t r a c t
`
`Article history:
`Received 26 September 2013
`Accepted 29 September 2013
`Available online 8 October 2013
`
`Keywords:
`BUPHENYL
`Glycerol phenylbutyrate
`HPN-100
`Neurological adverse events
`RAVICTI
`Sodium phenylbutyrate
`
`Background: Phenylacetic acid (PAA) is the active moiety in sodium phenylbutyrate (NaPBA) and glycerol
`phenylbutyrate (GPB, HPN-100). Both are approved for treatment of urea cycle disorders (UCDs) — rare genetic
`disorders characterized by hyperammonemia. PAA is conjugated with glutamine in the liver to form
`phenylacetyleglutamine (PAGN), which is excreted in urine. PAA plasma levels ≥ 500 μg/dL have been reported
`to be associated with reversible neurological adverse events (AEs) in cancer patients receiving PAA intravenous-
`ly. Therefore, we have investigated the relationship between PAA levels and neurological AEs in patients treated
`with these PAA pro-drugs as well as approaches to identifying patients most likely to experience high PAA levels.
`Methods: The relationship between nervous system AEs, PAA levels and the ratio of plasma PAA to PAGN were
`examined in 4683 blood samples taken serially from: [1] healthy adults [2], UCD patients of ≥2 months of age,
`and [3] patients with cirrhosis and hepatic encephalopathy (HE). The plasma ratio of PAA to PAGN was analyzed
`with respect to its utility in identifying patients at risk of high PAA values.
`Results: Only 0.2% (11) of 4683 samples exceeded 500μg/ml. There was no relationship between neurological AEs
`and PAA levels in UCD or HE patients, but transient AEs including headache and nausea that correlated with PAA
`levels were observed in healthy adults. Irrespective of population, a curvilinear relationship was observed be-
`tween PAA levels and the plasma PAA:PAGN ratio, and a ratio N 2.5 (both in μg/mL) in a random blood draw iden-
`tified patients at risk for PAA levels N 500 μg/ml.
`
`Abbreviations: GEE, generalized estimating equations; GPB, glycerol phenylbutyrate (generic name for glyceryl tri (4-phenylbutyrate), also referred to as HPN-100 or RAVICTI®); HE,
`hepatic encephalopathy; NaPBA, sodium phenylbutyrate (BUPHENYL®); PAA, phenylacetic acid; PAA:PAGN ratio, ratio of the concentrations in μg/mL of PAA to PAGN in plasma; PAGN,
`phenylacetylglutamine; PBA, phenylbutyric acid; SE, safety extension; SO, switchover; UCD, urea cycle disorder.
`☆ ClinicalTrials.gov identifiers: ClinicalTrials.gov NCT00551200, NCT00947544, NCT00992459, NCT00947297, NCT00999167, and NCT 01347073.
`⁎ Corresponding author at: VP, Clinical Development & Medical Affairs, Hyperion Therapeutics, Inc., 601 Gateway Blvd., Ste. 200, South San Francisco, CA 94080, USA. Fax: +1 650 745 3581.
`E-mail address: Masoud.mokhtarani@hyperiontx.com (M. Mokhtarani).
`
`1096-7192/$ – see front matter © 2013 Elsevier Inc. All rights reserved.
`http://dx.doi.org/10.1016/j.ymgme.2013.09.017
`
`[6.1.2.9] [Mokhtarani et al - PAA and PAA-PAGN ratio on line publication.pdf] [Page 1 of 8]
`
`Horizon Exhibit 2018
`Lupin v. Horizon
`IPR2018-00459
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`M. Mokhtarani et al. / Molecular Genetics and Metabolism 110 (2013) 446–453
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`447
`
`Conclusions: The presence of a relationship between PAA levels and reversible AEs in healthy adults but not in
`UCD or HE patients may reflect intrinsic differences among the populations and/or metabolic adaptation with
`continued dosing. The plasma PAA:PAGN ratio is a functional measure of the rate of PAA metabolism and repre-
`sents a useful dosing biomarker.
`
`© 2013 Elsevier Inc. All rights reserved.
`
`1. Introduction
`
`Glycerol phenylbutyrate, a sodium- and sugar-free phenylbutyrate
`derivative, and sodium phenylbutyrate are approved as ammonia
`lowering agents in patients with urea cycle disorders (UCDs). Both
`are pro-drugs of phenylacetic acid (PAA), which is formed by
`beta-oxidation from phenylbutyric acid (PBA) delivered either
`as glycerol phenylbutyrate following its intestinal hydrolysis by
`pancreatic lipases [1] or as sodium phenylbutyrate following disso-
`ciation in the stomach. PAA is conjugated with glutamine by
`glutamine-N-phenylacetyltransferase, largely in the liver and to a
`lesser extent in the kidney [2], to form phenylacetylglutamine
`(PAGN), which is excreted in urine, thereby providing an alternate
`pathway to urea for waste nitrogen excretion. In controlled studies
`population pharmacokinetic analyses of sodium phenylbutyrate
`and glycerol phenylbutyrate, it has been shown that the gastrointes-
`tinal absorption of PBA is approximately 75% slower when delivered
`as glycerol phenylbutyrate vs. sodium phenylbutyrate and that plas-
`ma PAA and PAGN levels show less variability during glycerol
`phenylbutyrate dosing [3–7].
`There are over 30 reports of the administration of sodium
`phenylacetate or sodium phenylbutyrate to healthy volunteers, patients
`with UCDs or other metabolic disorders and patients with cancer, many
`of which reported some adverse events (AEs) attributed to PAA (Sup-
`plemental Table 1) [8–36]. These reversible AEs in cancer patients
`were reported in studies involving continuous or intermittent intrave-
`nous administration designed to maintain high levels of PAA, suggesting
`that duration of exposure as well as peak PAA levels are important
`[3,35].
`The AEs reportedly associated with high levels of PAA have
`most commonly included nausea, headache, emesis, fatigue, weak-
`ness, lethargy, somnolence, dizziness, slurred speech, memory loss,
`confusion, and disorientation [35,36]. Except for the symptoms of
`Kussmaul respiration, metabolic acidosis, cerebral edema, and
`coma associated with a fatal overdose of sodium phenylacetate/
`sodium benzoate (AMMONUL®) [13], the symptoms were rapidly
`reversible with reduced dosing or interruption of dosing. Based on a de-
`tailed analysis of the timing of the AEs in relation to blood PAA concen-
`trations, Simell calculated the safe upper PAA concentration limit to be
`3.5 mmol/L, equivalent to 476 μg/mL [22], and Thibault reported that
`AEs were associated with PAA levels ranging from 499–1285 μg/mL
`[35,36].
`Sodium phenylbutyrate (BUPHENYL®) has been used for over
`three decades in the treatment of UCDs. Despite the fact that the
`AEs reportedly associated with elevated plasma PAA levels can
`mimic those associated with hyperammonemia, little is known
`regarding the relationship between PAA levels and AEs in UCD pa-
`tients. The clinical trials of glycerol phenylbutyrate (RAVICTI®,
`HPN-100), which included over 100 UCD patients, 80 of whom
`underwent a comparative study of sodium phenylbutyrate and
`glycerol phenylbutyrate [3–6] (the largest prospectively studied
`group of patients with this rare disorder), 193 patients with ad-
`vanced cirrhosis complicated by hepatic encephalopathy (HE)
`[37], and more than 90 healthy adult subjects have afforded a
`unique dataset and opportunity to systematically examine the
`relationship between PAA levels and AEs and to explore bio-
`markers indicative of patients most likely to experience elevated
`PAA levels.
`
`2. Methods
`
`2.1. Clinical studies (Table 1)
`
`Data from a thorough QTc study in healthy adults, five clinical stud-
`ies in UCD patients and an open label safety and dose escalation study as
`well as a randomized, double-blinded controlled phase 2 study of
`patients with decompensated cirrhosis complicated by HE formed the
`basis for these analyses.
`
`2.2. UCD patients
`
`Eighty UCD patients completed 4 short-term (10 to 28 days) cross-
`over studies of sodium phenylbutyrate vs. glycerol phenylbutyrate
`(Table 1). The short-term UCD study population included 26 pediatric
`patients of ages ≥ 2 mos through 17 years who received a mean
`(range) dose of 8 (1–19) g/day of glycerol phenylbutyrate or an equiva-
`lent dose of sodium phenylbutyrate and 54 adults patients ages 18 years
`or older who received a mean (range) dose of 13 (2–34) g/day of glycerol
`phenylbutyrate or an equivalent dose of sodium phenylbutyrate [3–6]. In
`addition, data from 100 UCD patients enrolled in 12-month glycerol
`phenylbutyrate treatment protocols including 49 children and 51 adults
`were analyzed in relation to PAA levels over time and the occurrence of
`the symptoms reported in cancer patients by Thibault [4–6,35,36] during
`12 months treatment.
`
`2.3. Patients with cirrhosis and HE
`
`Data from a 4-week safety and dose escalation study and a multicen-
`ter, randomized placebo-controlled study of 178 patients with cirrhosis
`and hepatic encephalopathy who received 13.2 g/day of glycerol
`phenylbutyrate (N = 90) or placebo (N = 88) for 16 weeks were
`analyzed [37,38] (Table 1). The patients were monitored for safety and
`frequent PK samples were taken over the course of the study.
`
`2.4. Healthy adults
`
`A total of 98 healthy adults (mean age of 28; 53 male 45 female)
`participated in a blinded, randomized, cross over study to assess effects
`of glycerol phenylbutyrate and its metabolites on QTc and other ECG pa-
`rameters (Table 1). In this protocol 12 subjects received 29.7g/day, 4 sub-
`jects 39.6 g/day of glycerol phenylbutyrate and 68 subjects received
`placebo, moxifloxacin as the positive control and glycerol phenylbutyrate
`at doses of 13.2g/day and 19.8g/day administered three times daily for 3
`days.
`
`2.5. Adverse event mapping
`
`All treatment emergent adverse events (AEs) coded as to body
`system as nervous system disorders using the Medical Dictionary for
`Regulatory Activities (MedDRA) in the subjects enrolled in these studies
`were included in the analyses. For UCD patients, the specific toxicities
`reported by Thibault [35,36], including nausea, headache, emesis,
`fatigue, weakness, lethargy, somnolence, dizziness, slurred speech,
`memory loss, confusion, and disorientation, exacerbation of neu-
`ropathy, pedal edema, hearing loss, abnormal taste, arrhythmia,
`rash, Kussmaul respiration, metabolic acidosis, increased anion gap,
`tachypnea, abdominal discomfort, cerebral edema, and obtundation or
`
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`M. Mokhtarani et al. / Molecular Genetics and Metabolism 110 (2013) 446–453
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`Table 1
`Clinical studies and subject disposition.
`
`Populations
`
`Study ID, design and objectives
`
`Healthy adult volunteers
`
`HPN-100-010
`Thorough QT/QTc study
`Arm 1: safety run-in
`Arm 2: double-blind, randomized, crossover
`
`UCD patients
`
`UP 1204-003
`Phase 2, open-label, fixed-sequence, switch-over study
`
`HPN-100-006
`Phase 3, randomized, double-blind, crossover,
`active-controlled, multiple-dose study
`HPN-100-005SO
`Phase 2, open-label, fixed-sequence, switch-over,
`multiple-dose study with 12-month safety extension
`HPN-100-012SO
`Phase 3b, open-label, fixed-sequence, switch-over study
`
`HPN-100-005SE
`Phase 2, open-label 12-month
`safety-extension study
`HPN-100-012SE
`Phase 2, open-label 12-month safety-extension study
`
`HPN-100-007
`Phase 3, open-label, 12-month safety-extension study
`
`HE Patients
`
`HPN-100-008 (Part A)
`open label, safety and dose-escalation study
`HPN-100-008a
`randomized, double-blind, placebo-controlled phase 2 study
`
`Ages/no. treated
`Adults ≥ 18 N:98
`Arm 1: 9 mL TID: 12
`12 mL TID: 4
`Arm 2: 4 mL TID: 68
`6 mL TID: 68
`Adults ≥ 18
`N:10
`
`Adults ≥ 18
`N: 44
`
`Pediatric, patients ages 6–17
`N:11
`
`Pediatric, patients ages 2 months
`to b6 yrs
`N:15
`Pediatric ages
`6–17 years
`N: 17
`Pediatric ages
`29 days to b6 years
`N: 23
`Adult and pediatric
`ages ≥ 6
`N: 60 (51 adults, 9 pediatric patients)
`Adults ≥ 18
`N: 15
`Adults ≥ 18
`N: 178
`
`Study drug/Duration
`
`GPB
`3 days
`
`Open label, fixed sequence, NaPBA to
`GPB switchover
`7 days on each drug
`GPB and NaPBA
`14 days randomized, double blind,
`double dummy cross over
`Open label, fixed sequence, NaPBA to
`GPB switchover
`7 days on each drug
`Open label, fixed sequence, NaPBA to
`GPB switchover
`≤7 days on each drug
`GPB
`12 months
`
`GPB
`12 months
`
`GPB
`12 months
`
`GPB
`4 weeks
`GPB
`16 weeks
`
`GPB — glycerol phenylbutyrate; HE — hepatic encephalopathy; NaPBA — sodium phenylbutyrate; SE — safety extension; SO — switchover; UCD — urea cycle disorders.
`a Used for analysis of PAA levels in relation to AEs only.
`
`coma, were mapped to the MedDRA preferred terms in the clinical trial
`databases.
`
`2.7. Analysis of PAA in relation to plasma PAA:PAGN ratio
`
`2.6. Analysis of AEs in relation to PAA levels
`
`Analyses were based on (a) 2126 samples from 98 healthy adults,
`(b) 1281 blood PAA and PAGN values derived from 80 UCD patients
`during the short term-switchover studies who received both sodium
`phenylbutyrate and glycerol phenylbutyrate, and (c) 428 samples
`from 90 patients with cirrhosis and HE who received glycerol
`phenylbutyrate. Because plasma PAA levels were not always available
`at the time the patient was experiencing an AE, the following rules
`were applied to associate an AE to a known PAA level. For healthy sub-
`jects, maximum PAA values recorded after the first dose but within 24h
`of the last dose and the incidence of neurological AEs (yes/no) were
`summarized by dosing period; for periods where subjects received pla-
`cebo or moxifloxacin, the PAA levels were set to 0. For UCD patients,
`maximum PAA values (Cmax) recorded during each dosing period
`and the incidence of neurological AEs were summarized by treatment
`(glycerol phenylbutyrate or sodium phenylbutyrate). For HE patients,
`each AE was attributed to the PAA result that was closest in time to
`the AE.
`The contribution of a 20μg/mL increase in PAA levels to the probabil-
`ity of a neurological AE regardless of relationship to the study drug
`was examined using generalized estimating equations [39]. For healthy
`subjects, the data were summarized for each dose group. Since the UCD
`patients received a range of doses, the data were summarized for pa-
`tients receiving a dose greater or less than the median dose (equivalent
`to 11.7 g/day). For the HE patients, neurological AEs were examined
`both in relation to the blinded treatment group assignment; i.e. glycerol
`phenylbutyrate or placebo, as well as in relation to PAA levels among
`the patients treated with glycerol phenylbutyrate.
`
`GEE were used to model the predictive value of the plasma PAA:
`PAGN ratio in identifying patients at risk of a high plasma PAA
`level as defined to have a PAA level equal or greater than 400 μg/mL or
`500 μg/mL during 24 h of dosing. The plasma PAA:PAGN ratios were
`grouped into binary categorical ranges of less than 2.5 or greater than
`2.5. The repeated measure categorical outcome was modeled using
`GEE with a logit link function, ratio category as the independent
`variable, and the individual subject ID as the repeated measures factor.
`Confidence intervals for the predicted probabilities were computed
`by bootstrap estimation of 1000 re-samplings of the original data, as
`detailed in Davison and Hinkley [40].
`
`3. Results
`
`3.1. UCD patients (Table 2, Fig. 1)
`
`Common AEs reported by at least 10% of the patients during short-
`term treatment with either drug included diarrhea, flatulence, and
`headache. Neurological AEs reported by more than 1 UCD patient in-
`cluded headache, dizziness and dysgeusia. The mean (SD) PAA Cmax
`was similar in patients who reported at least one neurological AE, as
`compared with those who did not (50.8 (34.5) μg/mL vs. 51.5 (49.23)
`μg/mL respectively). There was no statistically significant relationship
`in the UCD patients between the presence or absence of neurological
`AEs and PAA levels during either glycerol phenylbutyrate or sodium
`phenylbutyrate treatment. The odds ratio of a neurological AE occurring
`for each 20 μg/mL increase in PAA levels for the two drugs combined,
`controlling for dose level, was 0.929, very close to 1 indicating that in-
`creasing levels of PAA were not associated with an increase in neurolog-
`ical AEs in these studies. There was no difference in the frequency of the
`PAA-associated AEs reported in the cancer patients by Thibault [35,36]
`
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`in adult vs. pediatric UCD patients in the short-term controlled studies,
`despite the generally higher PAA levels in the pediatric patients
`(Supplemental Table 2).
`A total of 100 UCD patients enrolled in 12-month studies of glycerol
`phenylbutyrate received a mean (SD) total dose of 11.01 (5.970) g/day
`(range: 0.8–34.3 g). Overall common AEs reported in at least 10% of
`UCD patients during long-term treatment included vomiting, upper re-
`spiratory tract infection, nausea, nasopharyngitis, diarrhea, headache,
`hyperammonemia, decreased appetite, cough, fatigue, dizziness, and
`oropharyngeal pain. There was no increase either in plasma PAA levels
`(Supplemental Figure 1) or the rate of AEs over time. Just as in the
`short-term studies there was no difference between pediatric and
`adult patients in the frequency of the PAA-associated AEs reported in
`cancer patients by Thibault (Supplemental Table 2).
`
`3.2. Patients with cirrhosis and HE (Table 2, Fig. 1)
`
`Of the 88 patients randomized to placebo, 48.9% reported a neuro-
`logical AE as compared to 40.9% of the 90 patients randomized to glyc-
`erol phenylbutyrate. Of the 428 PAA data points from the patients
`randomized to glycerol phenylbutyrate, 46 were in the patients who re-
`ported a neurological AE and 382 in the patients who did not. The mean
`(SD) PAA value closest to occurrence of an AE was 61.4 (75.3) μg/mL
`while the mean PAA value not temporally associated with an AE was
`36.4 (55.6) μg/mL (p = 0.77) (Fig. 2). Similar to the UCD patients,
`there was no increase in the odds of experiencing a neurological AE
`with each 20 μg/mL increment in PAA levels in the cirrhosis patients
`(odds ratio 1.086; p = 0.172) indicating that at the dose of 13.2 g/day
`the odds of experiencing a neurological AE did not increase with an
`increase in PAA level.
`
`3.3. Healthy subjects
`
`Common AEs in ≥10% of healthy volunteers included headache,
`nausea, and dizziness. Neurological AEs increased in frequency with in-
`creasing dose, ranging from 26.5% for 13.2g/day to 91.7% for 29.7g/day.
`Among those who reported a neurological AE, PAA values were higher
`for the 19.8 g/day, 29.7 g/day, and 39.6 g/day dosing periods than for
`the 13.2 g/day dosing period (Fig. 2, Table 2). PAA levels increased as
`the dose of glycerol phenylbutyrate increased. In the case of the 13.2
`g/day dose group, the difference was statistically significant (73.3 vs.
`41.6, p b 0.001) (Table 2). Logistic regression analysis indicated that
`each increment in PAA of 20 μg/mL was associated with increasing
`
`odds of experiencing a neurological AE (odds ratio = 1.75; p = 0.006).
`Individual AEs reported by healthy adults were generally transient
`and typically began within 36 h of dosing and generally resolved with
`continued dosing, as depicted in Supplemental Figure 2.
`
`3.4. Plasma PAA:PAGN ratio as a predictor of elevated PAA levels
`
`PAA levels showed considerable variation over a 24-hr period in all
`the patients regardless of the dose, drug and population (Fig. 3). Unlike
`PAA, the ratio of PAA:PAGN was comparatively constant over 24h (data
`not shown). A curvilinear relationship was observed between PAA and
`PAA:PAGN in all the populations, with a sharp upward inflection begin-
`ning with PAA concentrations approaching 200 μg/ml and a PAA:PAGN
`of approximately 2.5 or greater (Fig. 4). Only 11 of a total of 4683 sam-
`ples exceeded the 500 μg/ml threshold level reported by Thibault to be
`associated with the occurrence of neurological AEs in cancer patients.
`The estimated probabilities of correctly detecting a ratio ≥ 2.0 based
`on a single plasma sample taken at any time between the fasting morn-
`ing sample (0 h time point) and early evening (12 h time point)
`remained relatively constant (77% to 84%), indicating that the timing
`of blood draw did not have an impact on the ratio of PAA:PAGN in
`plasma regardless of the PAA concentration. Patients with a ratio ≥ 2.5
`had significantly higher PAA levels than those with a ratio ≤ 2.5
`(p b 0.0001) and PAA:PAGN ratios ≥ 2.5 had an approximately 20
`times higher probability of being associated with PAA levels N 400
`μg/ml (0.8% vs. 19.1%) or 500 μg/ml (0.3% vs. 8.4%) (Table 3).
`
`4. Discussion
`
`No relationship was observed among UCD patients between PAA
`levels and either neurological AEs, or the specific AEs reported by
`Thibault, during treatment with either glycerol phenylbutyrate or sodi-
`um phenylbutyrate. This is supported by (a) the absence of a relation-
`ship during short term treatment in UCD patients, in which the odds
`ratio for the likelihood of a neurological AE for every 20 μg/mL increase
`in PAA levels was 0.929, (b) the absence of a difference in the frequency
`of AEs similar to those reported in cancer patients by Thibault between
`pediatric and adult UCD patients during short or long-term treatment,
`despite generally higher PAA levels in pediatric patients, and (c) the ab-
`sence of any change in either PAA levels or the pattern of AEs during 12
`months of dosing. Similarly, no statistical relationship was noted be-
`tween PAA levels and neurological AEs among HE patients treated
`with 13.2 g/day of glycerol phenylbutyrate for 16 weeks, as there was
`
`Table 2
`Analysis of the relationship between PAA and neurological adverse events.
`
`Healthy subjects
`
`Dose
`
`13.2 g/d
`
`19.8 g/d
`
`29.7 g/d
`
`39.6 g/d
`
`AE Reported
`
`n
`
`Yes
`15
`
`Plasma PAA (μg/mL)
`Mean
`32.9
`SD
`13.1
`Median
`35.5
`p-valuea
`0.076
`1.75b
`Odds ratio
`p-valued
`0.006
`
`No
`51
`
`26.9
`12.2
`24.6
`
`Yes
`31
`
`86.3
`53.9
`73.3
`b0.001
`
`No
`38
`
`48.1
`30.2
`41.6
`
`Yes
`8
`
`211.1
`118.4
`194.8
`0.222
`
`No
`1
`
`65.0
`–
`65.0
`
`Yes
`3
`
`282.4
`179.5
`328.0
`0.500
`
`No
`1
`
`64.8
`–
`64.8
`
`UCD patients
`
`b=11.7 g/d
`
`N11.7 g/d
`
`HE patients
`
`13.2 g/day
`
`No
`75
`
`55.3
`91.4
`24.3
`
`Yes
`11
`
`58.7
`38.2
`70.1
`0.935
`
`No
`68
`
`64.6
`54.1
`48.1
`
`Yes
`4
`
`30.9
`7.4
`30.6
`0.627
`0.929c
`0.529
`
`No
`52
`
`36.4
`55.6
`20.9
`
`Yes
`36
`
`61.4
`75.3
`29.9
`0.77
`1.086c
`0.172
`
`AE — adverse event; HE — hepatic encephalopathy; PAA — phenylacetic acid; UCD — urea cycle disorders.
`UCD patient data are derived from studies UP-1204-003, HPN-100-005SO, HPN-100-006, and HPN-100-012SO. HE patient data are derived from study HPN-100-012 Part B only.
`a p-value comparing subjects reporting a neurological AE within each dose group using an exact non-parametric Mann–Whitney test.
`b Odds ratio of experiencing a neurological AE associated with a 20 μg/mL increase in PAA, Cmax controlling for dose group, including placebo and moxifloxacin, for which PAA was
`assumed to be zero.
`c Odds ratio (p-value) of incidence of neurological AE associated with a 20-μg/mL increase in PAA Cmax.
`d p-value for the odds ratio.
`
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`Experienced neurological AE
`
`No
`
`Yes
`
`Sodium Phenylbutyrate
`
`Experienced neurological AE
`
`No
`
`Yes
`
`Glycerol Phenylbutyrate
`
`Experienced neurological AE
`
`No
`
`Yes
`
`Glycerol Phenylbutyrate
`
`Fig. 1. Lack of relationship between PAA levels and neurological AEs in UCD and HE pa-
`tients. The top and middle panels depict box-and-whisker plots for the mean maximal
`(Cmax) concentration of PAA during dosing of UCD patients with sodium phenylbutyrate
`and glycerol phenylbutyrate, respectively. There was no statistical difference in maximal
`PAA levels between UCD patients who did or did not report neurological AEs. The bottom
`panel depicts mean PAA concentrations (mean [SD]= 61.4 [75.3] vs. 36.4 [55.6]; p = 0.77)
`among patients with cirrhosis and hepatic encephalopathy randomized to treatment with
`glycerol phenylbutyrate who reported neurological adverse events. The range of PAA con-
`centrations as reflected by the box (25th to 75th percentile) are similar for patients who
`did or did not report an AE. The dots depict individual values (See Table 2 for statistical
`summary).
`
`Fig. 2. PAA levels in healthy adults reporting a nervous system adverse event (AE) grouped
`by dose. The maximum PAA value (Cmax) is displayed in relation to dose of glycerol
`phenylbutyrate for patients who did or did not report a neurological adverse event (AE),
`regardless of relationship to study drug or timing relative to blood draw for PAA. The
`box and whisker plots depict mean (horizontal line), 25–75 percentiles (box) and 10
`and 90% confidence intervals. Note that a wide range of PAA levels was observed at each
`dose and among patients with or without AEs. PAA levels were significantly higher
`among patients with AEs as compared to those without at the 6 mL TID dose, but not at
`the 4 mL TID dose. All but 1 subject in the 9 and 12 mL dose groups reported a neurological
`AE (See Table 2 for statistical summary).
`
`no difference in neurological AEs between the glycerol phenylbutyrate
`and placebo treatment arms, nor was there a relationship between
`PAA levels and the occurrence of neurological AEs.
`Among the healthy adult volunteers, a relationship was observed
`between PAA levels and the occurrence of any neurological AE
`(e.g. headache, dizziness, vomiting and nausea). These AEs were gener-
`ally mild, started early in the dosing period, and disappeared with
`continued dosing.
`The theoretical risk of PAA toxicity is expected to be similar for sodi-
`um phenylbutyrate or glycerol phenylbutyrate, as both drugs convert to
`PAA upon absorption. The AEs reported by healthy volunteers in these
`studies receiving glycerol phenylbutyrate are generally consistent
`with prior reports involving administration of sodium phenylbutyrate.
`The mechanism for these AEs is unknown, although interference with
`brain biochemical function has been suggested [41].
`These differences between populations may be attributable either
`to metabolic differences between UCD and HE patients, who exhibit
`pathological nitrogen retention and high glutamine levels, as compared
`with healthy adults, and/or metabolic adaptation that may occur with
`continued exposure to PAA in chronically treated patients. Consistent
`with adaptation are the findings that AEs tended to disappear with con-
`tinued dosing in healthy adults and that the UCD patients enrolled in
`these studies had been treated with sodium phenylbutyrate for an
`average of more than 9 years.
`While most human tissues are capable of beta-oxidation and, hence,
`conversion of phenylbutyrate to PAA [42], enzymatic conversion of PAA
`to PAGN occurs primarily in the liver [2]. This may explain why conver-
`sion of PAA to PAGN appears to be a rate-limiting step in the metabolism
`of PAA prodrugs and why PAA metabolism may be compromised when
`liver function is poor, when the availability of the precursor glutamine
`may be limited as in healthy subjects, and/or when the capacity of the
`enzymatic conversion may be limited as in very young children [7].
`Regardless of the reason, decreases in the rate of PAGN formation are
`associated with an increased ratio of PAA to PAGN in plasma. It is inter-
`esting in this regard that the upward inflection in PAA values assessed as
`a function of the PAA:PAGN ratio occurs at a concentration similar to the
`estimated Km of this reaction based on population PK modeling, which
`is approximately 190 μM as previously described by Monteleone et al.
`[7].
`
`In clinical practice, interpretation of an individual PAA value is com-
`promised by the fact that concentrations vary considerably over the
`
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`Fig. 3. Plasma PAA intra-subject variability. Healthy subjects and patients with UCD or HE underwent serial blood sampling over 12 to 24 h. The figure depicts the coefficient of variation
`(CV%) as an indicator of intra-subject variability. Regardless of the dose or population, there is high degree of variability among all subjects.
`
`course of the day due to the relatively short half-lives of PBA and PAA.
`For example among the clinical trials comprising the present analyses,
`the plasma PAA fluctuation index varied from 843%–3931%; and fasting
`and maximal PAA levels in HE patients ranged from 0 to 1.3 μg/mL and
`248 to 532 μg/mL, respectively. As compared with measurement of
`PAA alone, measurement of the PAA:PAGN ratio appears to be a useful
`proxy for the efficiency with which an individual patient converts PAA
`to PAGN, and a predictor of patients at risk of having an elevated PAA
`level. The PAA:PAGN ratio also has an important clinical advantage in
`that it remains comparatively constant over the day and, therefore, is
`more readily interpretable in a random blood draw.
`These analyses have several limitations. First, although pharmacoki-
`netic and safety data were derived from controlled prospective studies,
`the analyses of the frequency of the specific AEs reported by Thibault
`et al. [35,36] were done as post hoc analysis. Second, PAA levels were
`not always available at precisely the time of occurrence of neurological
`AEs, though a conservative approach was taken in these analyses by
`
`Fig. 4. Plasma PAA vs. plasma PAA:PAGN ratio. PAA levels in μg/mL (Y axis) are plotted in
`relation to the ratio of PAA to PAGN concentration (both expressed as μg/mL) in plasma (X
`axis) in that same sample. This plot includes N3500 samples from all populations, includ-
`ing healthy adults (Healthy), patients with cirrhosis and hepatic encephalopathy (Hepat-
`ic), and patients with urea cycle disorders (UCD). All populations exhibit a similar
`relationship, with the upward inflection point occurring at ratios exceeding approximately
`2.5 and PAA concentrations in the range of 100–200 μg/mL.
`
`utilizing the highest recorded PAA for that dosing period. Finally, these
`conclusions pertaining to the absence of a