Drug Accumulation and Efflux Do not Contribute to Acquired Gefitinib Resistance

Abstrcat

Gefitinib is one of tyrosine kinase inhibitors and often has dramatic effect on non-small-cell lung cancer (NSCLC) expressed mutant EGFR. However, most of the patients who respond to gefitinib eventually experience tumor recurrence. To clarify the mechanism of this acquired resistance, we examined cellular accumulation and efflux of gefitinib using gefitinib sensitive and resistant NSCLC cells. We used four NSCLC cell lines; PC-9: hypersensitive to gefitinib, expressed a 15 bp deletion mutant EGFR, PC-9/ZD2001 and PC-9/ZD1kl: acquired-resistant to gefitinib, PC-9/ZD2001R: a revertant reacquired sensitivity to gefitinib. To measure the cellular accumulation, cells were exposed to 1 µM of [14C]gefitinib for 3 to 30 min at 37 °C For the measuring of drug efflux, cells were exposed gefitinib for 30 min, and then cells were washed and further incubated in drug free medium. After the incubation, cells were lysed and the radioactivities were counted. There was no significant difference of gefitinib accumulation in those cell lines (range 642–731 µmol/g protein at 30 min). The efficiencies of gefitinib-efflux were almost the same in those cell lines (about 80% of gefitinb was discharged at 15 min). According these findings, we demonstrated that acquired resistance to gefitinib did not depend on cellular accumulation or efflux of this drug.