Sensitivity and resistance of tumor cells to the antiangiogenic tyrosine kinase inhibitor sunitinib
K.J. Gotink (1), H.J. Broxterman (1), R.R. de Haas (1), H. Dekker (1), R.J. Honeywell (1), M. Labots (1), Y.G. Assaraf (1), R. Pili (2), G.J. Peters (1), H.M.W. Verheul (1)
1 Dept. of Medical Oncology, VU University Medical Center, Amsterdam, The Netherlands, 2 Roswell Park Cancer Institute, Buffalo, New York, USA
Sunitinib, a multi-targeted antiangiogenic TKI, has demonstrated clinical efficacy in advanced renal cell cancer (RCC) and gastrointestinal stromal tumors. However, treatment with antiangiogenic TKIs is hampered by resistance, but the underlying mechanisms of resistance are poorly understood. In this study we aimed to unravel mechanisms of sunitinib resistance.
Two cancer cell lines, 786-O and HT-29, which are sensitive to sunitinib treatment, were continuously exposed to sunitinib for > 1 year to induce resistance. The resistant tumor cells continued to stably grow on exposures to clinically achieved intratumoral concentrations, whereas their parental cells died at these concentrations. Staining parental and resistant tumor cells with a lysosome-specific fluorescent dye revealed accumulation of sunitinib in lysosomes, which was increased in resistant tumor cells compared to their parental cells. Increased lysosomal accumulation capacity was confirmed by flow cytometry and Western blot analysis (LAMP-1 and LAMP-2 expression). Intracellular concentrations were increased in the resistant tumor cells compared to their parental cells, 4.6±1.1 mM and 2.3±0.4 mM, respectively, and were up to 1000-fold higher than the micromolar concentrations used for in vitro exposure. Growth of resistant cells in drug-free medium resulted in restoration of drug sensitivity and normalization of lysosomal drug accumulation capacity within 12 weeks.
In conclusion, we showed that tumor cells acquire a transient drug-resistant phenotype under continuous exposure to sunitinib. Resistant tumor cells demonstrated increased lysosomal accumulation capacity and increased sunitinib uptake. Our results indicate that lysosomal sequestration of sunitinib in tumor cells may contribute to sunitinib resistance.