The effect of STI571 withdrawal.

(A) Viability of Baf/BCR-ABL–r1 and LAMA84-r cells upon withdrawal of STI571 from the culture. On day 0, exponentially growing cells were washed off the compound and replated in RF-10 media (R−/−), whereas control cells were maintained in media supplemented with 1 μM STI571 (R+/+). Subculturing with the respective media was done whenever necessary for each cell line. On day 7 (indicated by the black arrow), one half of the RF-10 culture was reexposed to 1 μM STI571 (R−/+) and the 3 cultures were observed for 1 additional week. Results are shown as the mean and range percentage of trypan blue–negative cells out of 200 cell counts from duplicate culture aliquots, in 1 of 3 representative experiments. (B) Caspase-3 activity in LAMA84-s and LAMA84-r cells after 3 days in culture with (+) and without (−) 1 μM STI571. Results represent the mean ± SD fluorescent units of quadruplicate cultures.