Physiologically based pharmacokinetic modelling of methotrexate and 6-mercaptopurine in adults and children. Part 2: 6-mercaptopurine and its interaction with methotrexate

  • Kayode Ogungbenro
  • Leon Aarons
  • The CRESim & Epi-CRESim Project Groups
Original Paper

Abstract

6-mercaptopurine (6-MP) is a purine antimetabolite and prodrug that undergoes extensive intracellular metabolism to produce thionucleotides, active metabolites which have cytotoxic and immunosuppressive properties. Combination therapies involving 6-MP and methotrexate have shown remarkable results in the cure of childhood acute lymphoblastic leukaemia (ALL) in the last 30 years. 6-MP undergoes very extensive intestinal and hepatic metabolism following oral dosing due to the activity of xanthine oxidase leading to very low and highly variable bioavailability and methotrexate has been demonstrated as an inhibitor of xanthine oxidase. Despite the success recorded in the use of 6-MP in ALL, there is still lack of effect and life threatening toxicity in some patients due to variability in the pharmacokinetics of 6-MP. Also, dose adjustment during treatment is still based on toxicity. The aim of the current work was to develop a mechanistic model that can be used to simulate trial outcomes and help to improve dose individualisation and dosage regimen optimisation. A physiological based pharmacokinetic model was proposed for 6-MP, this model has compartments for stomach, gut lumen, enterocyte, gut tissue, spleen, liver vascular, liver tissue, kidney vascular, kidney tissue, skin, bone marrow, thymus, muscle, rest of body and red blood cells. The model was based on the assumption of the same elimination pathways in adults and children. Parameters of the model include physiological parameters and drug-specific parameter which were obtained from the literature or estimated using plasma and red blood cell concentration data. Age-dependent changes in parameters were implemented for scaling and variability was also introduced on the parameters for prediction. Inhibition of 6-MP first-pass effect by methotrexate was implemented to predict observed clinical interaction between the two drugs. The model was developed successfully and plasma and red blood cell concentrations were adequately predicted both in terms of mean prediction and variability. The predicted interaction between 6-MP and methotrexate was slightly lower than the reported clinical interaction between the two drugs. The model can be used to predict plasma and tissue concentration in adults and children following oral and intravenous dosing and may ultimately help to improve treatment outcome in childhood ALL patients.

Keywords

6-mercaptopurine Pharmacokinetics PBPK Leukaemia Arthritis Modelling 

Abbreviation

6-MP

6-Mercaptopurine

MTX

Methotrexate

ALL

Acute lymphoblastic leukaemia

PK

Pharmacokinetics

PBPK

Physiologically based pharmacokinetic

RBC

Red blood cell

XO

Xanthine oxidase

HPRT

Hypoxanthine phosphoribosyltransferase

TPMT

Thiopurine methyltransferase

6-TGN

6-Thioguanine nucleotides

6-mMPN

6-Methylmercaptopurine nucleotide

BSA

Body surface area

BW

Body weight

HT

Height

AUC

Area under the concentration

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  • Kayode Ogungbenro
    • 1
  • Leon Aarons
    • 1
  • The CRESim & Epi-CRESim Project Groups
  1. 1.Centre for Applied Pharmacokinetic Research, Manchester Pharmacy SchoolThe University of ManchesterManchesterUK

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