Research articleSynthesis and positron emission tomography studies of carbon-11-labeled imatinib (Gleevec)
Introduction
Positron emission tomography (PET), coupled with a radiolabeled drug, is a powerful tool for determining drug distribution and pharmacokinetics [1]. This information would be of major importance in determining whether a chemotherapeutic drug targets a tumor and in determining other organs where the drug or its metabolites accumulate [2]. In addition, a study design in which a labeled chemotherapeutic drug is paired with a functional radiotracer such as 18FDG or 18FLT offers the potential to plan and monitor therapy and to make changes depending on individual response.
Imatinib mesylate (Gleevec; Fig. 1) ([4-[(4-methyl-1-piperazinyl)methyl]-N-[4-methyl-3-[[4-(3-pyridyl)-2-pyrimidinyl]amino]phenyl]benzamide methanesulfonate; STI571) is a member of a new class of signal transduction inhibitors used to treat chronic myeloid leukemia (CML) and gastrointestinal stromal tumor (GIST). It was the first drug to be rationally designed based on a molecular abnormality in CML and to be approved by the Food and Drug Administration in 2001 after remarkable success in the treatment of chronic-phase CML patients [3], [4]. In fact, PET studies with 18FDG have documented a dramatic reduction in 18FDG uptake after the oral administration of 300–400 mg/day imatinib mesylate in GIST patients [5], [6].
CML is a myeloproliferative disorder [7] triggered by genetic translocation between chromosome 9 and chromosome 22, producing aberrant Philadelphia chromosome [8], [9]. This chromosome expresses the abnormal protein enzyme bcr-abl tyrosine kinase. Bcr-abl tyrosine kinase is constitutively altered so that it is no longer dependent on normal signal transduction induced by the interaction between cytokine (interleukin-3) and its receptor. Therefore, hematopoietic stem cells comprising bcr-abl tyrosine kinase exert enhanced cell function to produce white blood cells and blasts, as well as cell proliferation. For these reasons, bcr-abl tyrosine kinase serves as a good molecular target for CML treatment.
Imatinib, the active ingredient of Gleevec, was originally designed as a competitive inhibitor of bcr-abl tyrosine kinase in leukemic cells and of c-abl tyrosine kinase in normal cells (IC50=0.025 μM for both cells). Since c-abl tyrosine kinase does not play an important role in the cell survival of normal cells, imatinib spares normal cells while killing leukemic cells [10], [11], [12]. However, in spite of its efficacy, treatment resistance emerges due to the mutation of bcr-abl tyrosine kinase, which interferes with drug binding [13], [14].
Imatinib also blocks other tyrosine kinases such as c-kit and platelet-derived growth factor (PDGF) [10], [15] [IC50=0.41 μM (c-kit), IC50=0.38 μM (PDGF)]. c-kit contributes to the unique pathology of GIST [16], which is initiated in interstitial cells of Cajal, which play an essential role in intestinal motility [17]. c-kit undergoes a gain-of-function mutation in GIST [18], [19], [20]. Imatinib reduces tumor size in a significant fraction of GIST patients [21]. Unfortunately, similar to CML, resistance against imatinib also occurs in GIST by a secondary mutation. From this perspective, we reasoned that [N-11C-methyl]imatinib may be useful in determining whether drug pharmacokinetics changes when drug resistance develops.
In addition to chemotherapeutic applications for CML and GIST, preclinical studies show that imatinib has some potency against the deposition and accumulation of amyloid β-peptide, a characteristic peptide plaque found in the brain of Alzheimer's disease patients although blood–brain barrier penetration is a limitation [22], [23]. Other conditions where imatinib shows promise are hepatocellular carcinoma [24], [25], liver fibrosis [26], [27] and pulmonary fibrosis [28], [29], which are characterized by the expression of abl kinase, c-kit kinase or PDGF kinase.
Here we synthesized [N-11C-methyl]imatinib according to a recent patent procedure in which a nor precursor was prepared via the demethylation of imatinib [30]. We also prepared norimatinib from the corresponding amine and acid. We measured the distribution of [N-11C-methyl]imatinib and/or its labeled metabolites in the brain and in peripheral organs in baboons at tracer doses and after treatment with a single dose of imatinib. This information is of relevance in determining the organs targeted by imatinib and its labeled metabolites.
Section snippets
General
All chemicals used in synthesis were purchased from Sigma Aldrich Chemical Co. (Milwaukee, WI) and were used without any further purification. 1H nuclear magnetic resonance (NMR) spectra were obtained in CDCl3 solution (unless specified) using Bruker Avance 400 MHz NMR spectrometer (400 MHz for 1H and 100 MHz for 13C) (Bruker Instruments, Inc., Billerica, MA) and were reported in parts per million downfield from tetramethylsilane as internal standard. Melting points were measured by
Chemistry and carbon-11 labeling
Demethylation of imatinib with 3-chloroperbenzoic acid (m-CPBA) and iron sulfate by nonclassical Polonovski reaction according to the patent procedure gave a modest yield (34.5%) [30], [34]. Norimatinib (2) and imatinib (1) were also synthesized from commercially available starting materials by adapting a series of literature methods [33] (Fig. 2). Norimatinib was labeled successfully at 80°C for 10 min without any base by modifying the labeling scheme in a published patent [30] (Fig. 2). [N-11
Conclusions
[N-11C-methyl]imatinib has potential for assessing the regional distribution and kinetics of imatinib in the human body to determine whether the drug targets tumors and to identify other organs to which the drug or its labeled metabolites distribute. Paired with tracers such as 18FDG and 18FLT, [N-11C-methyl]imatinib may be a useful radiotracer in planning chemotherapy, in monitoring response to treatment and in assessing the role of drug pharmacokinetics in drug resistance. Although the use of
Acknowledgments
This work was carried out at the Brookhaven National Laboratory under contract DE-AC02-98CH10886 with the US Department of Energy and supported by its Office of Biological and Environmental Research and by the National Institute on Drug Abuse (K05DA020001). The authors thank Michael Schueller and David Schlyer for the operation of cyclotron, Donald Warner for PET operations, Payton King and Pauline Carter for performance of baboon studies. The authors are also grateful to Michael Viola for
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