Induction of resistance to the Abelson inhibitor ST1571 in human leukemic cells through gene amplification

The 2-phenylaminopyrimidine derivative STI571 has been shown to selectively inhibit the tyrosine kinase domain of the oncogenic bcr/abl fusion protein . The activity of this inhibitor has been demonstrated so far both in vitro with bcr/abl expressing cells derived from leukemic patients, and in vivo on nude mice inoculated with bcr/abl positive cells, Yet, no information is available on whether leukemic cells can develop resistance to bcr/abl inhi bition, The human bcr/abl expressing cell line LAMA84 was cultured with inc reasing concentrations of STI571, After approximately 6 months of culture, a new cell line was obtained and named LAMA84R, This newly selected cell li ne showed an IC50 for the STI571 (1.0 mu M) 10-fold higher than the IC50 (0 .1 mu M) of the parental sensitive cell line. Treatment with STI571 was sho wn to increase both the early and late apoptotic fraction in LAMA84 but not in LAMA84R, The induction of apoptosis in LAMA84 was associated with the a ctivation of caspase 3-like activity, which did not develop in the resistan t LAMA84R cell line. LAMA84R cells showed increased levels of bcr/abl prote in and mRNA when compared to LAMA84 cells. FISH analysis with BCR- and ABL- specific probes in LAMA84R cells revealed the presence of a marker chromoso me containing approximately 13 to 14 copies of the BCR/ABL gene. Thus, over expression of the Bcr/Abl protein mediated through gene amplification is as sociated with and probably determines resistance of human leukemic cells to STI571 in vitro, (C) 2000 by The American Society of Hematology.