The
Pharmacokinetics
of Injectable
Allopurinol
in Newborns
With
the
Hypoplastic
Left
Heart
Syndrome
Susan Phillips McGaurn, PharmD*; Lisa E. Davis, PharmD; Motria M. Krawczeniuk, PharmD;
John D. Murphy, MD; Marshall L. Jacobs, MD#; William I. Norwood, MD#; and Robert R. Clancy, MD*
ABSTRACT. Objective. The purpose of this investiga-lion was to determine the pharmacokinetic disposition of intravenous allopurinol and its metabolite oxypurinol in neonates with the hypoplastic left heart syndrome (HLHS) and to evaluate the subsequent degree of
xan-thine oxidase inhibition using serum uric acid as a
marker.
Methods. Pharmacokinetic data were evaluated in 12
stable preoperative neonates with HLHS after a single intravenous allopurinol administration of 5 mg/kg or 10
mg/kg. Pharmacokinetic parameters were determined for
elimination half-life, clearance, volume of distribution, and mean residence time. Xanthine oxidase inhibition, measured by serum uric acid reduction, was also mea-sured.
Results. Pharmacokinetic parameters revealed no
sta-tistically significant differences between a 5-mg/kg and 10-mg/kg dose of intravenous allopurinol on elimination half-life, clearance, volume of distribution, and mean residence time. Mean serum uric acid levels were signif-icantly reduced from baseline by 39.99 and 42.94%, re-spectively, in the 5- and 10-mg/kg treatment groups.
Discussion. The enzyme xanthine oxidase plays a key biochemical role in the generation of toxic
oxygen-de-rived free radicals during ischemia-reperfusion condi-tions. Allopurinol and its active metabolite oxypurinol
inhibit xanthine oxidase, and significantly reduce the conversion of hypoxanthine to xanthine and xanthine to uric acid. Cell injury may be caused by toxic oxygen free radicals produced by ischemia-reperfusion injury such as could occur during the repair of HLHS under hypother-mic total circulatory arrest. We hypothesize that allopuri-nol may provide protection from cellular injury in this clinical context. Pediatrics 199494:820-823; allopurinol,
ischemia-reperfusion, hypoplastic left heart syndrome,
hypothermic total circulatory arrest, neuroprotection,
pharmacokinetics.
ABBREVIATIONS. CHD, congenital heart disease; Cmax,
maxi-mum serum concentration; time to maximum serum
concen-tration; AUC, area under the serum concentration versus time curve; AUMC, area under the first moment curve.
PURPOSE
The purpose of this investigation was to determine the pharmacokinetic disposition of intravenous
allo-purinol and its metabolite oxypurinol and to
evalu-ate the degree of xanthine oxidase inhibition using serum uric acid as a marker.
Under normal conditions nucleic acids are sequen-tially converted to adenosine, inosine, hypoxanthine, xanthine, and uric acid. Xanthine oxidase in the
pres-ence of oxygen is the enzyme responsible for the
conversion of hypoxanthine to xanthine and
xan-thine to uric acid. However, during periods of
hypoxia-ischemia, the energy-rich nucleotide adeno-sine triphosphate is rapidly catabolized to
hypoxan-thine. Upon a replenished supply of oxygen or
reper-fusion, hypoxanthine and xanthine oxidase form
toxic oxygen free radicals. Although other sources of
toxic-derived free radicals exist, these oxygen free
radicals have been identified as an important medi-ator in the biochemical cascade operative in the pro-duction of cellular injury and tissue necrosis due to ischemia-reperfusion injury.’3 Allopurinol and its metabolite oxypurmnol inhibit xanthine oxidase and this ability may provide a therapeutic intervention to prevent or ameliorate some features of ischemia-reperfusion injury caused by oxygen free radicals.4’#{176}
This study was conducted to aid in the design of
an optimum dosing strategy for a large scale, long-term therapeutic trial to assess allopurinol’s abifity to protect against ischemia-reperfusion injury in neo-nates with congenital heart disease (CHD). The
mod-em conduct of surgical repair of complex CHD is
usually performed under hypothermic total circula-tory arrest. At the completion of the operation,
cir-culation is restored. This clinical practice parallels
the ischemia-reperfusion model of injury with the
exception that circulatory arrest occurs at a body temperature <20#{176}Cand may not result in tissue
injury in the time frame of ischemia.
METHODS
From the *vision of Neurology, The Children’s Hospital of Philadelphia, and the Departments of Neurology and Pediatrics of the University of Pennsylvania School of Medicine; the IDepartment of Pharmacy Practice,
The Philadelphia College of Pharmacy and Science; the §Division of
Car-diology, The Children’s Hospital of Philadelphia, and the Department of Pediatrics of the University of Pennsylvania School of Medicine; and the #Division of Cardiothoracic Surgery, The Children’s Hospital of
Philadel-phia, and the Department of Surgery of the University of Pennsylvania School of Medicine.
Received for publication Sep 29, 1993; accepted Mar 31, 1994.
PEDIATRICS (ISSN 0031 4005). Copyright © 1994 by the American
Acad-emy of Pediatrics.
Study Population
The study population was composed of twelve neonates ad-mitted to the Pediatric Intensive Care Unit or the Infant Intensive
Care Unit at the Children’s Hospital of Philadelphia with an established diagnosis of hypoplastic left heart syndrome requiring
surgery under hypothermic total circulatory arrest between
December 1991 and May 1992. The protocol was approved by the Committee for Protection of Human Subjects Institutional Review
Board. Written informed consent was obtained from a parent of each neonate.
All neonates were studied approximately 36 hours before their
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Serum concentration versus time data were analyzed sepa-rately for each neonate to determine pharmacokinetic parameters of allopurinol using noncompartmental methods.’2 The observed peaks of serum allopurinol, oxypurinol, uric acid, and hypoxan-thine concentrations (C) and corresponding time to reach peak
concentrations (T) were determined by visual inspection of the
0 Allopurinol . Oxypurinol
V Uric Acid
V Hypoxanthine
10
0.1
TABLE 1. Allopurinol Dose and Patient Demographics
Patient Demographics Allopurinol Dose
5 mg/kg 10 mg/kg
(n=6) (n=6) Male/female
Mean postnatal age in days Range
Mean weight in kg
Range
Mean BUN* in mg/dL
Range
Mean creatinine in mg/dL
Range 3/3 2/4 6.2 3-9 3-21 3.0 2.9 2.1-4.1 2.1-4.0 18.7 14.0 8-31 8-25 1.0 0.82 0.6-1.7 0.6-1.4
* BUN, blood urea nitrogen.
0 4 8 12 16
Time (hrs)
*U#{149}jAcid. mg/dl
20 24
Fig 1. TypiCal serum concentration versus time proffle, 5 mg/kg.
ARTICLES 821
scheduled cardiac repair surgery. Neonates were between 3 and
21 days old with weights ranging from 2100 g to 4100 g. All had
normal renal and hepatic function. Six patients received a single, intravenous infusion of 5 mg/kg of allopurinol. The other six
patients received a single allopurinol dose of 10 mg/kg. Patient demographics are provided in Table 1. The groups were similar
(P > .05) with respect to mean postnatal age, body weight, blood
urea nitrogen, and serum creatinine. All of the neonates, with the exception of one in the 10 mg/kg group, were 9 days old.
Drug Administration and Blood Sampling
Sodium allopurinol was the study drug provided (lot #OZ2790
by the Burroughs Wellcome Company, Research Triangle Park,
NC). Each dose was diluted in 5% dextrose to a final concentration of 5 mg/mL and infused over 20 minutes.
Twelve 0.75 mL blood samples were collected via an indwelling
venous or arterial line at the following times: before drug infusion (at time 0), and at times 10 minutes, 20 minutes, I h, 2 h, 3 h, 5 h,
7 h, 9 h, 12 h, 18 h, and 24 h after the start of infusion.
After collection, blood samples were immediately placed on ice
then centrifuged. The serum was separated and stored at -70#{176}C until analysis.
Sample Analysis
Plasma concentrations of allopurinol, oxypurinol, uric acid, and hypoxanthine were measured using a modification of a high
per-formance liquid-chromatography assay procedure by Wung et
al.” All analytic studies were performed at the Philadelphia Col-lege of Pharmacy and Science. Standards for high performance
liquid-chromatography analysis were also provided by The
Bur-roughs Weilcome Company. Samples were thawed and then
deproteinated via ultrafiltration by adding 150 mL of serum to an
Amicon MPS-1 micropartition system (Amicon, Danvers, MA)
which was centrifuged at 2900 rpm for 40 minutes at 25#{176}C.A
50-microliter aliquot of the ultrafiltrate was injected onto a
Beck-man System Gold, Model IIOB (Beckman Instruments, Inc. San Ramon, CA) solvent delivery module using a Model 166
ultravi-olet detector with the absorbance set at 254 nm. Sensitivity of
allopurinol, oxypurinol, and uric acid was 100 ng/mL and 125
ng/mL for hypoxanthine. This assay utilized a5micron,
reversed-phase Ultrasphere (Beckman) C18 250 x 4.6 mm column with a
Direct-Connect (Ailtech, Deerfield, IL) Spherisorb ODS-2 5 micron guard column. The mobile phase consisted of a 99% 0.05 M
potassium phosphate buffer in 1% acetonitrile adjusted to pH 4.6
at a flow rate of I mL/minute. The interday coefficients of
varia-lion of allopurinol, oxypurinol, uric acid, and hypoxanthine were
2.4%, 3.2%, 3.9%, and 1.6%, respectively. Recoveries of each com-pound were >95%. Elution of uric acid, hypoxanthine, oxypurinol,
and allopurinol occurred at approximately 4.6, 6.9, 10.8, and 13
minutes, respectively.
Data Analysis
data. The apparent first-order terminal elimination rate constant
(lc; h’) for allopurinol was calculated by linear regression
analy-sis of the log serum concentration versus time plot for each set of data. The total area under the serum concentration (AUC) versus
time curve from time 0 to infinity (AUC; mg x h/L) and the
area under the first moment curve (AUMC mg x h2/L) were
calculated using the linear trapezoidal rule method. The AUC was extrapolated to infinite time by adding the quotient of the last extrapolated serum concentration and the elimination rate con-stant. Additional pharmacokinetic parameters were calculated us-ing the following formulas: total dearance, (CL; L/h/kg) =
Dose/AUC; mean residence time, (MRT; .h) = AUMC/AUC
+ infusion tiine/2; and the calculated steady state volume of
distribution, (V L/kg) = (Dose x AUMC)/AUC - (Dose x infusion time)/(2 x AUC). Differences between pharmacokinetic
parameters of each group were compared using the
Mann-Whitney LI test. Linear associations between allopurinol and
oxypurinol pharmacokinetic parameters and degree of uric acid reduction as well as increase in serum hypoxanthine
concen-trations were examined using calculated Pearson correlation
coefficients. The a priori significance level for all comparisons
was
P
< .05. Data was analyzed using the SPSS/PC+ computer software package.RESULTS
Figures 1 and 2 demonstrate typical plots that
depict changes in serum allopurinol, oxypurinol, uric
acid, and hypoxanthine concentrations for neonates
receiving 5-mg/kg and 10-mg/kg, respectively. The
mean serum allopurinol concentration versus time
profile is depicted in Figure 3. Mean
pharmacoki-netic parameters following single intravenous doses
of allopurinol at 5 mg/kg and 10 mg/kg are
sum-marized in Table 2. There are no statistically
signifi-cant differences in pharmacokinetic parameters
between the 5-mg/kg and 10-mg/kg groups.
Allo-purinol is rapidly eliminated from the body as sug-gested by a mean allopurinoi plasma half-life (t#{189})of approximately 2.5 hours.
The mean serum oxypurinol concentration versus
time proffle is shown in Figure 4. Serum oxypurinol
C, and T are summarized in Table 3.
Oxypuri-nol C appears to be dose-related.
The effect of the 5-mg/kg and 10-mg/kg doses of
allopurinol on serum uric acid concentrations is
shown in Table 4. The time to uric acid nadir is
0 (0 ..-) Q) 0 0 0
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10
0 Allopurinol
. Oxypurinol
V Uric Acid
V Hypoxanthine E C 0 a C C 1) C 0 0 01
. 5 mg/kg
=. 10 0 10 mg/kg
as C 0 a I. C a) 0 C 0 C) 0.1 0 25
Fig 4. Mean serum oxypurinol concentration versus time.
0 4 8 12 16 20 24
‘Uric Acid, mg,’dl
Time (hrs)
5 10 15 20
Time (hours)
. 5 mg/kg
0 10 mg/kg
0 5 10
Time (hours)
. 5 mg/kg
0 10 mg/kg
Time (hours) Fig 2. Typical serum concentration versus time profile, 10 mg/kg.
E a) C 0 0 C a 0 C 0 C) 100 10 0.1
Fig 3. Mean serum allopurinol concentration versus time.
15
TABLE 2. Allo purinol Pharmaco kinetic Data
Parameter 5 mg/kg 10 mg/kg
P
Value Mean ± SD (n = 6) (n = 6)K: (h-’) 0.317 ± 0.225 0.360 ± 0.159 NS
C01(L/h/kg) 0.608 ± 0.567 0.310 ± 0.148 NS
VJL/kg) 1.646 ± 1.095 0.815 ±0.267 NS
MRT (h) 3.77 ±2.74 2.77 ± 0.55 NS
* Abbreviations: K, first-order terminal elimination rate constant; CLoiai, total clearance; VI, steady state volume of distribution;
MRT, mean residence time; NS, not significant.
similar in both groups and occurs at approximately
16 hours. The reduction in mean serum uric acid
level was greater in those who received a 10-mg/kg dose of allopurinol, but the difference was not sta-tistically significant. The mean initial uric acid level was 3.69 mg/dL ± 2.06 mg/dl (SD) and significantly
fell to a mean nadir value of 2.125 mg/dL ± 1.29
mg/dL (SD) after allopurinol administration
TABLE 3. Serum Oxypurinol Results
Parameter 5 mg/kg 10 mg/kg
P
ValueMean ± SD (n = 6) (n = 6)
Cm): (mg/mL) 6.08 ± 5.82 11.92 ± 2.07 NS
T (h) 7.28 ±5.97 11.63 ± 5.02 NS
* Abbreviations: C maximum serum concentration; T time
to maximum serum concentration; NS, not significant.
TABLE 4. Serum Un c Acid (UA) Results
Parameter
Mean ± SD
5 mg/kg
(n = 6)
10 mg/kg
(n =6)
P
ValueBaseline UA (mg/dL) UA nadir (mg/dL) Time to nadir (h)
Max UA reduction (%)
3.65 ±2.60 2.15 ±1.62 15.83 ± 9.35
39.99 ±14.39
3.73 ±1.02 2.10 ± 0.68 15.63 ±5.91 43.94 ± 11.88
NS NS NS NS 0 0) E C 0 0 C a 0 C 0 I.) 6 4, 2 0
-5 0 5 10 15 20 25
Fig 5. Mean serum uric acid concentration versus time.
(P = .0001). A mean uric acid concentration versus
time plot is shown in Figure 5. DISCUSSION
The medical approach to the treatment of the
ARTICLES 823
circulation to the whole body and prevent
hypoxic-ischemic injury until palliative or corrective surgery restores a more normal circulatory physiology.
Surgery in the neonate with some forms of CHD,
particularly the hypoplastic left heart syndrome, re-quires a period of hypothermic total circulatory ar-rest. The sharp reduction in metabolic demands
in-duced by the hypothermia preserves much of the
cellular integrity during total circulatory arrest. At the completion of surgical repair circulation is re-stored. It is at this time of reperfusion that production of free radicals can occur when oxygen is converted by newly activated xanthine oxidase in the presence of hypoxanthine to toxic species of free radicals.
We have hypothesized that cell injury by such a mechanism may be reduced by preoperative admin-istration of allopurinol and its metabolite oxypurinol via xanthine oxidase inhibition, the enzyme cata-lyzing the conversion of hypoxanthine to xanthine and xanthine to uric acid. Uric acid reduction reflects xanthine oxidase inhibition.
This pharmacokinetic pilot study reveals allopuri-nol’s ability to inhibit xanthine oxidase, as measured by a prompt and significant reduction in serum uric acid by about 40% from baseline. Our data also show that there are no statistically significant differences in the
values of measured pharmacokinetic parameters or
percent uric acid reduction between a 5-mg/kg dose
and a 10-mg/kg dose of intravenous allopurinoL
Therapeutic strategies such as preoperative
ad-ministration of allopurinol are intended to augment
the neuroprotective benefits of deep hypothermia and inhibit or prevent cell injury at the time of reper-fusion after ischemia. This may reduce the threat of brain or multiorgan damage and preserve the long-term neurologic integrity of survivors.
We are now conducting a long-term, blinded,
ran-domized controlled treatment trial to determine
whether preoperative administration of allopurmnol
reduces the risk of death or brain, myocardial, or
other organ damage in neonates with CHD requiring
hypothermic total circulatory arrest.
ACKNOWLEDGMENT
This work was supported by NIH Contract NO1-NS-1-2315
(Ors Clancy, McGaurn, and Murphy).
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A PLEA FOR SIMPLICITY AND LARGE SAMPLE SIZE
“Most [clinical] trials would be of much greater scientific value if they collected
ten times less data, both at entry and during follow-up, and were therefore much
larger.
Peto R, Coffins R, Gray R. Large-scale randomized evidence: large, simple trials and overviews of trials. In: Warren KS, Mosteller F, eds. Doing More Good Than Harm: The Evaluation of Health Care Interventions.”
Ann NY Acad Sci. 1993;703:314-340.
Submitted by Student
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1994;94;820
Pediatrics
Murphy, Marshall L. Jacobs and William I. Norwood
Susan Phillips McGaurn, Robert R. Clancy, Lisa E. Davis, Motria M. Krawczeniuk, John D.
Left Heart Syndrome
The Pharmacokinetics of Injectable Allopurinol in Newborns With the Hypoplastic
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1994;94;820
Pediatrics
Murphy, Marshall L. Jacobs and William I. Norwood
Susan Phillips McGaurn, Robert R. Clancy, Lisa E. Davis, Motria M. Krawczeniuk, John D.
Left Heart Syndrome
The Pharmacokinetics of Injectable Allopurinol in Newborns With the Hypoplastic
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