The invention provides methods for overriding cell cycle arrest in a tumor cell, which comprise inducing DNA damage in the cell, and contacting the cell with an amount of bosutinib or a bosutinib isomer effective to inhibit one or more kinase constituents of a DNA damage checkpoint pathway. The invention also provides novel bosutinib isomers, as well as compositions of the novel isomers and the bosutinib isomer 3,5-dichloro-4-methoxyaniline.
