The kinase inhibitor STI571 (Gleevec; Novartis) can induce complete remission in patients with chronic-phase chronic myelogenous leukaemia (CML). Patients whose disease has advanced to blast crisis, however, frequently become resistant to the drug, due to mutations in the BCR–ABL kinase domain. As reported in Cell, Azam et al. have developed an in vitro screen to survey mutagenized forms of BCR–ABL, and have obtained a more comprehensive picture of mutations that confer drug resistance.

Kinases typically exist in equilibrium between 'open' (active) states, or a 'closed' (autoinhibited) state. Co-crystallization studies of STI571 and the ABL kinase domain have shown that the drug achieves its specificity by trapping the kinase in the closed conformation. The majority of patients that become resistant to STI571 therefore harbour mutations within the BCR–ABL kinase domain.

Azam et al. reasoned that mutations in other domains of the protein, in addition to the kinase domain, might mediate resistance. To look for these, they randomly mutagenized the BCR–ABL gene through propagation of the gene in a bacterial strain that is deficient in DNA repair. The screen led to the identification of 59 protein variants that were resistant to STI571 treatment, of which only 13 had been previously identified in patients with drug-resistant CML.

So how do these mutations confer drug resistance? Twenty-six resistance-associated mutations were found to lie outside the kinase domain. Structural modelling studies indicated that many of these mutations destabilize the closed conformation of the ABL kinase, shifting the protein equilibrium toward the open, active kinase conformation, which precludes drug binding. Some of the mutations were also associated with increased kinase activity and accelerated disease progression.

The in vitro screening strategy reported by Azam et al. could potentially be used for the many other kinase targets for which compounds are presently in development, to predict the mutations that are likely to be problematic in the clinic. Identification of the most refractory drug-resistant variants could be valuable in the development of next-generation drugs.