Alves, Atecla N. L.,

Chocair, Pedro R.

Duley, John A.

Fortini, Alexandre S.

Mendes, Maria E.

Nahas, William C.

Pacheco-Neto, Maurílio

Sumita, Nairo M.

Torres, Larissa H. L.

University of Queensland eSpace

ARTIGO ORIGINAL
SUMMARY
Background: Allopurinol was invented originally to improve response to thiopurine drugs, such as aza-
thioprine (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 trans-
plant 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
Introdução: O alopurinol foi desenvolvido para melhorar a resposta às tiopurinas, por exemplo, azatio-
prina (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.
©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
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
1Hospital 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
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
diretoriaclinica@haoc.com.br
Conflict of interest: None.
14
PHARMACOKINETICS OF 6-THIOGUANINE NUCLEOTIDE AND 6-METHYL-MERCAPTOPURINE IN A CASE OF INADVERTENT COMBINATION THERAPY OF AZATHIOPRINE WITH ALLOPURINOL
15Rev Assoc Med Bras 2012; 58(Suppl 1):S14-17
INTRODUCTION
Azathioprine (AZA) was developed to prevent rejection fol-
lowing renal transplantation, although in recent years its us-
age has been replaced or complemented by cyclosporine-A 
(including Neoral) and mycophenylate. Following absorp-
tion, 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. Hy-
poxanthine phosphoribosyl transferase (HPRT) initiates 
the activation of thiopurines to 6-thioguanine nucleotides 
(6-TGN) (Figure 1). Co-prescription of AZA with allopuri-
nol 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 myelotox-
icity reported that clinical signs arose 4-6 weeks follow-
ing commencement of co-therapy, and recovery required 
4-8 weeks following interruption of either drug1. Such re-
ports 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 me-
tabolites discovered in a kidney transplant recipient who 
presented with severe myelotoxicity due to inadvertent co-
prescription of AZA and allopurinol.
CASE PRESENTATION
A 44-year-old Brazilian Caucasian woman received a kid-
ney transplant in 2001, with a maintenance immunosup-
pressive 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 pre-
scribed in place of mycophenolate, due to gastrointestinal 
side effects, but allopurinol was not interrupted. After 26 
days the patient was hospitalized with severe myelotoxic-
ity 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, respec-
tively. Granulocyte colony-stimulating factor (Granu-
lokine®) and two units of packed red blood cells were 
administered. Serial measurements of AZA metabolites 
were performed until day 14 following medication dis-
continuation, when concentrations had decreased to 201 
and < 60 nmol/8 x 108 RBC of 6-TGN and 6-MMP, re-
spectively. Gradual recovery of hematological parameters 
was observed during 15 days (Figure 2).
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.
H3C
H3C
H
H
Hepatotoxicity
Myelotoxicity
S
N N
NN
H
H
S
TPMT
TPMT
HPRT
6-TG
6-TGTP
6-TGDP
6-TGMP
6-MeTIMP
6-TIMP6-MP
6-TU
6-TXMP
GMPSIMPDHHPRT
N N
NN
N
O
O
S
N
N
N
N
+
H
N
N
GSTs
xo
Allopurinol
6-MMP
6-TGNAZA
H
H2N
SH
N N
NN
MAURÍLIO PACHECO-NETO ET AL.
16 Rev Assoc Med Bras 2012; 58(Suppl 1):S14-17
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 allo-
purinol co-therapy without correct AZA dose adjustment. 
Our case demonstrated the potential toxicity of inadver-
tent 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 recent-
ly, AZA and allopurinol co-therapy has been developed 
with good effect as treatment for inflammatory bowel 
disease in patients who have thiopurine resistance or non-
responsiveness5,6.
The availability of routine therapeutic drug monitor-
ing of AZA metabolites facilitated clarification of the my-
elotoxicity pathogenesis, which was presumably caused by 
acute formation of high 6-TGN levels. Normalization of 
hematological parameters required only two weeks, com-
pared 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 
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 half-
life 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.
REFERENCES
1. Venkat Raman G, Sharman VL, Lee HA. Azathioprine and allopurinol: 
a potentially dangerous combination. J Intern Med. 1990;228:69-71.
2. EGoRT E. European best practice guidelines for renal transplanta-
tion, Section IV: Long-term management of the transplant recipi-
ent, IV.9.2 Haematological complications: Leukopenia. Nephrol Dial 
Transplant. 2002;17(Suppl. 4): 49.
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 colony-
stimulating factor (Granulokine®); CH, packed red cell transfusion.
14
12
10
8
6
4
2
0
0
11
9.2
6 6 6
9 9.5
12
5.9
G-CSF
AZA and
allopurinol
discontinuation
5.9
2.2
0.9
1.8
1.2
3 6 9 12 15 Days
Hemoglobin
Leokocytes (×1,000)
-1
Day 1st 2nd 4th 5th 6th 7th 14th
6-TGN 
(pmol/8 × 108 RBC)
1165 1253 744 680 545 649 201
6-MMP 
(pmol/8 × 108 RBC)
1471 527 941 1157 156 230 < 60
PHARMACOKINETICS OF 6-THIOGUANINE NUCLEOTIDE AND 6-METHYL-MERCAPTOPURINE IN A CASE OF INADVERTENT COMBINATION THERAPY OF AZATHIOPRINE WITH ALLOPURINOL
17Rev Assoc Med Bras 2012; 58(Suppl 1):S14-17
3. Elion GB. The purine path to chemotherapy. Science. 1989;244:41-47.
4. Chocair PR, Duley JA, Simmonds HA, Cameron JS, Ianhez LE, 
Arap S, et al. Low dose allopurinol, plus azathioprine/cyclospo-
rin/prednisolone, a novel immunosuppressive regimen. Lancet. 
1993;342:83-84.
5. Sparrow MP, Hande SA, Friedman S, Lim WC, Reddy SI, Cao D, 
et al. Allopurinol safely and effectively optimizes tioguanine me-
tabolites in inflammatory bowel disease patients not responding 
to azathioprine and mercaptopurine. Aliment Pharmacol Ther. 
2005;22:441-446.
6. Ansari AR, Patel N, Sanderson J, O’Donohue J, Duley JA, Florin 
THJ. Low dose azathioprine or 6-mercaptopurine in combination 
with allopurinol can bypass many adverse drug reactions in pa-
tients with inflammatory bowel disease. Aliment Pharmacol Ther. 
2010;31:640-647.
7. Day RO, Graham GG, Hicks M, McLachlan AJ, Stocker SL, 
Williams KM. Clinical pharmacokinetics and pharmacodynamics of 
allopurinol and oxypurinol. Clin Pharmacokinet. 2007;46:623-644.
8. Derijks LJ, Gilissen LP, Engels LG, Bos LP, Bus PJ, Lohman JJ, et al. 
Pharmacokinetics of 6-thioguanine in patients with inflammatory 
bowel disease. Ther Drug Monit. 2006;28:45-50.
9. Derijks LJ, Gilissen LP, Engels LG, Bos LP, Bus PJ, Lohman JJ, et 
al. Pharmacokinetics of 6-mercaptopurine in patients with inflam-
matory bowel disease: implications for therapy, Ther Drug Monit. 
2004;26:311-318.


Pharmacokinetics of 6-thioguanine nucleotide and 6-methyl-mercaptopurine in a case of inadvertent combination therapy of azathioprine with allopurinol

http://espace.library.uq.edu.au/view/UQ:272953/UQ272953_OA.pdf

Pharmacokinetics of 6-thioguanine nucleotide and 6-methyl-mercaptopurine in a case of inadvertent combination therapy of azathioprine with allopurinol

Abstract

Similar works

Full text

Available Versions

University of Queensland eSpace