In chronic myelogenous leukaemia (CML), a reciprocal translocation between chromosomes 9 and 22 results in a gene encoding BCR–ABL, a fused deregulated kinase that stimulates cellular proliferation and mediates resistance to apoptosis. Although inhibitors directed against the ATP-binding site of the kinase, such as imatinib (Glivec/Gleevec; Novartis), lead to clinical remission in the early stage of the disease, most patients with advanced disease develop drug resistance associated with mutations in the ABL kinase domain. Reporting in Nature, Gray and colleagues identify and characterize a selective allosteric inhibitor of BCR–ABL, GNF-5, that cooperates with ATP competitive inhibitors to inhibit both wild-type and mutant forms of the kinase.

In 2006, the authors reported GNF-2 as a BCR–ABL inhibitor, and provided initial evidence that it binds to the autoregulatory myristate binding cleft of ABL, which is spatially distant from the ATP-binding site of ABL kinase. Using nuclear magnetic resonance spectroscopy and X-ray crystallography, they have now confirmed these findings, identifying the precise binding site of GNF-2 in the myristate cleft.

By selecting for BCR–ABL alleles that are resistant to GNF-2 in vitro, they also identified residues both within and outside the myristate cleft that are required for drug efficacy. It seems that mutations in the myristate cleft prevent GNF-2 binding, whereas non-myristate-site mutants probably prevent the enzyme from adopting its inhibited conformation.

The authors then found that combining GNF-2 with imatinib suppressed the emergence of resistance mutations in vitro. Using GNF-5, an N-hydroxyethyl carboxamide analogue of GNF-2 with similar inhibitory activity but with more favourable pharmacokinetic properties, they further investigated this cooperative effect. When GNF-5 was administered in combination with imatinib or the related compound nilotinib (Tasigna; Novartis), an additive inhibitory effect on BCR–ABL was observed in vitro and in a CML mouse model. Even in mice transplanted with bone-marrow cells bearing the BCR–ABL imatinib-resistant T315I mutation, administration of GNF-5 and nilotinib resulted in significant antitumour activity and improved survival compared with treatment with either agent alone.

Using hydrogen-exchange mass spectrometry, the authors were able to examine the mechanism underlying this cooperative effect. Their results suggest that GNF-5 binding at the myristate-binding site influences the conformation of the ATP-binding site and support further exploration of treatments that combine ATP and non-ATP competitive inhibitors in the treatment of diseases that are characterized by deregulated kinases.