Although most gastrointestinal stromal tumor (GIST) patients show a good response to imatinib, some patients may develop resistance after a long period of treatment with this inhibitor of the c-Kit tyrosine kinase, which has led to the development of novel, more effective inhibitors. AMN107 (Novartis Pharma AG), a novel and highly potent inhibitor of the tyrosine kinase activity of the Bcr-Abl oncoprotein, is undergoing Phase II clinical trials in chronic myelogenous leukemia patients. Although highly selective for Bcr-Abl, AMN107 also targets the KIT and PDGFR tyrosine kinases with affinities very similar to those of imatinib, and is currently also being evaluated in GIST. Several studies have reported on the role of P-glycoprotein (PGP) as a transporter of imatinib in transformed cell lines, indicating that MDR1 expression confers resistance. High levels of PGP in tumor cells may efficiently reduce intracellular drug levels. Preliminary data in MDR1 over-expressing CCRF cells indicate favorable cellular uptake of AMN107 when compared to imatinib. This investigation assessed the cellular uptake of imatinib in comparison with AMN107 in two human GIST patient-derived cell lines (GIST882 and GIST GDG1) which express constitutively activated KIT. Whereas GIST882 is sensitive to imatinib, GIST GDG1 is an imatinib-resistant cell line derived from a patient that progressed during imatinib treatment. Determination of the proliferative activity in the GIST882 cell line demonstrates that AMN107 potently inhibits KIT-dependent cell proliferation (IC50: 40 ± 3 nM) with potency similar to that of imatinib. Neither compounds showed activity in the GIST GDG1 cell line. A validated high-performance liquid chromatographic (HPLC) method has been used for the determination of imatinib and AMN107 concentrations in cultured tumor cells. The human GIST cells were treated with 3 different concentrations (0.1, 1, 10 μM) of either AMN107 or imatinib for 2 hours. HPLC analysis in GIST882 cells revealed an increased uptake by 3-fold for AMN107 when compared with imatinib. Moreover, in the GIST GDG1 cell line the increased uptake for AMN107 was even 10-fold when compared with imatinib. This finding suggests that AMN107 might be less susceptible to multi-drug resistance pump driven imatinib-resistance. In conclusion, our data indicate favorable cellular uptake of AMN107 when compared to imatinib in GIST cell lines. This strongly increased exposition of GIST cells to a highly active agent could be important in the treatment of GIST patients in whom resistance had been developed against imatinib based on imatinib cellular transport mechanisms for which AMN107 is not a substrate. The exact mechanism of this difference in cellular uptake is still under investigation.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]