Pharmacokinetics of 6-thioguanine nucleotide and 6

ARTIGO ORIGINAL
Pharmacokinetics of 6-thioguanine nucleotide
and 6-methyl-mercaptopurine in a case of inadvertent
combination therapy of azathioprine with allopurinol
MAURÍLIO PACHECO-NETO1, ATECLA N. L. ALVES1, ALEXANDRE S. FORTINI1, NAIRO M. SUMITA1, MARIA E. MENDES1, LARISSA H. L. TORRES2,
JOHN A. DULEY3, WILLIAM C. NAHAS4, PEDRO R. CHOCAIR4
1
Hospital das Clínicas, Medical School, Universidade de São Paulo (HC-FMUSP), Central Laboratory Division, Laboratory of Medical Investigation 03, Department of Clinical Pathology,
FMUSP, SP, Brazil
2
Department of Toxicology, Pharmaceutical Sciences School, Universidade de São Paulo (USP), SP, Brazil
3
School of Pharmacy, University of Queensland, and Mater Medical Research Institute, Brisbane, Australia
4
Renal Transplantation Unit, HC-FMUSP, SP, Brazil
SUMMARY
Background: Allopurinol was invented originally to improve response to thiopurine drugs, such as azathioprine (AZA) and mercaptopurine, but if they are given in combination then the thiopurine dose
must be drastically reduced to about one third of a normal dose. Failure to reduce the thiopurine dose
can cause severe toxicity, and has resulted in allopurinol usually being contraindicated for patients taking
thiopurines. Case report: We present a case of a 44 year old female patient who received a renal transplant in 2001, with mycophenylate/tacrolimus/prednisolone immunosuppression. In 2004 the patient
experienced gout symptoms and was prescribed 100 mg allopurinol per day. In 2008, her mycophenylate
was replaced with 150 mg AZA. Within four weeks the patient was hospitalized suffering from severe
myelotoxicity, with high blood levels of the AZA metabolite thioguanine nucleotide (6-TGN). AZA was
stopped, with recovery of hematological parameters and elimination of AZA metabolites requiring a
further two weeks. Discussion: This case demonstrates the risk of rapid-onset myelotoxicity due to AZA/
allopurinol co-therapy without correct dose adjustment of these drugs. The availability of routine analysis
of AZA metabolites was useful for rapidly diagnosing the cause of the toxicity and monitoring recovery.
Interestingly, the half-life of AZA metabolites after cessation of therapy (5.5 days for 6-TGN, 4 days for
6-MMP) was comparable to values in the absence of allopurinol: this excluded the elevation of 6-TGN
being caused by an increased half-life.
Keywords: Azathioprine; allopurinol; co-therapy; renal transplantation; 6-TGN half-life; myelotoxicity.
©2012 Elsevier Editora Ltda. All rights reserved.
RESUMO
Farmacocinética dos nucleotídeos de 6-tioguanina e
6-metil-mercaptopurina em um caso de terapia combinada e
inadvertida de azatioprina com alopurinol
Correspondence to:
Pedro R. Chocair
Hospital Alemão Oswaldo Cruz
Diretoria Clínica
Rua João Julião, 331 – Paraíso
CEP: 01323-903
São Paulo, SP, Brazil
Phone/Fax: +55 (11) 3549-0000
3287-8177
[email protected]
Introdução: O alopurinol foi desenvolvido para melhorar a resposta às tiopurinas, por exemplo, azatioprina (AZA) e mercaptopurina. Se as tiopurinas forem administradas em associação com alopurinol, a
dose de tiopurina deve ser reduzida para aproximadamente um terço da usual. A falta de redução da dose
de tiopurina pode produzir toxicidade grave, e esse efeito tem causado a contraindicação de alopurinol
para pacientes que têm tiopurinas prescritas. Relato de caso: Apresentamos o caso de uma paciente do
sexo feminino, 44 anos, que recebeu transplante renal em 2001, com imunossupressão constituída de
micofenolato/ tacrolimus/ prednisona. Em 2004, a paciente apresentou sintomas de gota, e foi prescrito
alopurinol, 100 mg diários. Em 2008, o micofenolato foi substituído por 150 mg AZA, devido a efeitos
colaterais gástricos. Em quatro semanas, a paciente foi internada sofrendo de mielotoxicidade grave, com
altos níveis sanguíneos do metabólito ativo da AZA, nucleotídeo de 6-tioguanina (6-TGN). A terapia
com AZA foi interrompida; a recuperação dos parâmetros hematológicos e a eliminação dos metabolitos
da AZA exigiram um período de duas semanas. Discussão: Este estudo de caso demonstrou o risco da
mielotoxicidade rápida causada por coterapia AZA-alopurinol quando o ajuste da dose correta desses
medicamentos for esquecido. A disponibilidade da análise de rotina dos metabólitos da AZA foi útil para
o diagnóstico rápido da causa da toxicidade e para o monitoramento da recuperação. Curiosamente, a
meia-vida dos metabólitos da AZA após a interrupção do tratamento (5,5 dias para a 6-TGN, 4 dias para
a 6-MMP) foi comparável com os valores na ausência de alopurinol: isto exclui a elevação da 6-TGN
sendo causada por aumento na meia-vida.
Unitermos: Azatioprina; alopurinol; coterapia; transplante renal; meia-vida 6-TGN; mielotoxicidade.
Conflict of interest: None.
14
©2012 Elsevier Editora Ltda. Todos os direitos reservados.
PHARMACOKINETICS
OF
6-THIOGUANINE NUCLEOTIDE AND 6-METHYL-MERCAPTOPURINE IN A CASE OF INADVERTENT COMBINATION THERAPY OF AZATHIOPRINE WITH ALLOPURINOL
INTRODUCTION
CASE
Azathioprine (AZA) was developed to prevent rejection following renal transplantation, although in recent years its usage has been replaced or complemented by cyclosporine-A
(including Neoral) and mycophenylate. Following absorption, AZA is converted to 6-mercaptopurine (6-MP), which
is then catabolized either by thiopurine methyltransferase
(TPMT) to form methyl-6-MP (6-MMP) or by xanthine
oxidase (XO), the latter being blocked by allopurinol. Hypoxanthine phosphoribosyl transferase (HPRT) initiates
the activation of thiopurines to 6-thioguanine nucleotides
(6-TGN) (Figure 1). Co-prescription of AZA with allopurinol reduces the catabolism of 6-MP by xanthine oxidase and
also appears to reduce methylation of the 6-MP by TPMT.
For a normal dose regimen, the result is high 6-TGN levels,
which usually cause serious myelosuppression.
A review of five cases of AZA/allopurinol myelotoxicity reported that clinical signs arose 4-6 weeks following commencement of co-therapy, and recovery required
4-8 weeks following interruption of either drug1. Such reports led to the contraindication of these two drugs unless
there was dose adjustment: reduction of the usual AZA
dose to one third is typically recommended2. We describe
here grossly raised levels of both 6-TGN and 6-MMP metabolites discovered in a kidney transplant recipient who
presented with severe myelotoxicity due to inadvertent coprescription of AZA and allopurinol.
A 44-year-old Brazilian Caucasian woman received a kidney transplant in 2001, with a maintenance immunosuppressive regimen initially consisting of mycophenolate,
tacrolimus and prednisone. In 2004, due to symptomatic
hyperuricemia, 100 mg daily allopurinol was started.
In 2008, AZA (150 mg or 2 mg/kg daily) was prescribed in place of mycophenolate, due to gastrointestinal
side effects, but allopurinol was not interrupted. After 26
days the patient was hospitalized with severe myelotoxicity and a raised creatinine of approximately 2.5 mg/dL.
During this period hemoglobin levels decreased
from 11.0 to 5.9 g/dL and leukocyte count from 9,200 to
900/mm3.
Both AZA and allopurinol were discontinued and
measurement of AZA metabolites performed one day
following drug withdrawal showed levels were 1,165 and
1,471 nmol/8 x 108 RBC of 6-TGN and 6-MMP, respectively. Granulocyte colony-stimulating factor (Granulokine®) and two units of packed red blood cells were
administered. Serial measurements of AZA metabolites
were performed until day 14 following medication discontinuation, when concentrations had decreased to 201
and < 60 nmol/8 x 108 RBC of 6-TGN and 6-MMP, respectively. Gradual recovery of hematological parameters
was observed during 15 days (Figure 2).
H3C
S
H
N
N
N
N
H
O
PRESENTATION
Hepatotoxicity
6-MMP
+
N O
N
N
H3C
TPMT
S
S
H
N
N
N
N
N
H
N
AZA
H
N
N
GSTs
6-MP
xo
Allopurinol
6-TU
HPRT
6-MeTIMP
TPMT
6-TIMP
IMPDH
6-TGMP
6-TXMP
GMPS
6-TGDP
6-TGTP
Myelotoxicity
6-TGN
HPRT
SH
6-TG
H
N
N
H2N
N
N
Figure 1 – Azathioprine (AZA) is converted into 6-MP by glutathione S-transferase (GST). 6-MP may follow 3 enzyme
pathways: methylation by TPMT to form 6-MMP and 6-MeTIMP; oxidation by XO to 6-thiouric acid (6-TU), which is blocked
by allopurinol; or activation by HPRT to form 6-thioinosine monophosphate (6-TIMP). IMPDH converts 6-thio-IMP (6-TIMP)
to 6-thioxanthosine monophosphate (6-TXMP) then GMPS leads to the 6-thioguanine nucleotides (6-TGN): 6-thioguanine
mono-, di- and tri-phosphate (6-TGMP, 6-TGDP, 6-TGTP). 6-Thioguanine is converted more directly by HPRT to 6-TGN.
Rev Assoc Med Bras 2012; 58(Suppl 1):S14-17
15
MAURÍLIO PACHECO-NETO ET AL.
14
AZA and
12
12
11 allopurinol
discontinuation
10
G-CSF
9.2
9.5
9
8
6
6
6
5.9
Hemoglobin
Leokocytes (×1,000)
6
5.9
4
2
2.2
1.8
1.2
0.9
0
-1
3
0
6
2nd
12
15
Days
4th
5th
6th
7th
14th
6-TGN
1165 1253
(pmol/8 × 108 RBC)
744
680
545
649
201
6-MMP
1471
(pmol/8 × 108 RBC)
941
1157
156
230
< 60
Day
1st
9
527
Figure 2 – Patient parameters during 14 days following medication withdrawal (day 0), showing hemoglobin (g/dL) and
leukocytes (thousands/mm3) changes, and 6-TGN and 6-MMP levels (nmol/8x108 RBC). G-CSF, granulocyte colonystimulating factor (Granulokine®); CH, packed red cell transfusion.
DISCUSSION
There is a strong synergistic effect by co-administration of
allopurinol and thiopurine drugs, so the risk is high for
severe and rapid-onset myelotoxicity due to AZA and allopurinol co-therapy without correct AZA dose adjustment.
Our case demonstrated the potential toxicity of inadvertent full-dose AZA given with allopurinol and stressed the
need for careful monitoring.
Allopurinol was originally invented not for gout, but
to improve thiopurine response in leukemic patients3. This
co-therapy was first pioneered as a successful strategy at
our institution4, for renal transplant patients. More recently, AZA and allopurinol co-therapy has been developed
with good effect as treatment for inflammatory bowel
disease in patients who have thiopurine resistance or nonresponsiveness5,6.
The availability of routine therapeutic drug monitoring of AZA metabolites facilitated clarification of the myelotoxicity pathogenesis, which was presumably caused by
acute formation of high 6-TGN levels. Normalization of
hematological parameters required only two weeks, compared to earlier reports of 4-8 weeks, presumably aided
by granulocyte colony-stimulating factor. Interestingly,
the decay of 6-TGN could theoretically be inhibited by
16
Rev Assoc Med Bras 2012; 58(Suppl 1):S14-17
co-therapy with allopurinol, in particular by its metabolite
oxypurinol – which has a half-life of about one day7 – but
in our patient’s case it showed no apparent effect on 6-TGN
elimination. During recovery, the decay of patient’s red cell
6-TGN provided a half-life estimate of approximately 5.5
days, which was close to a typical 6-TGN half-life estimate
of five days8,9. Red cell concentrations of 6-MMP, which
followed a more irregular decline, corresponded to a halflife of approximately four days: this clearance was slightly
faster than that of 6-TGN, possibly because elimination of
methylated metabolites may be aided via biliary excretion.
ACKNOWLEDGMENTS
This paper was presented in part to the III International
Thiopurine Symposium, held at the Instituto de Educação
e Ciências, Hospital Alemão Oswaldo Cruz, São Paulo,
September 30th – October 2nd, 2010.
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