rgoing , it is very common for those that are diagnosed and treated during advanced stages of the disease. Currently, it is estimated that about BMS-806 BMS 378806 thirty percent of patients undergoing imatinib therapy will switch to an alternative treatment within five years due to side effects and the development of drug resistance. For patients undergoing treatment with imatinib, relapse occurs through re activation of the BCR ABL pathway in the presence of the drug. The most frequent route for the development of resistance to imatinib is through mutations in the kinase domain of ABL. To date, over 50 different point mutations in the ABL kinase domain have been detected in imatinib resistant CML patients.
Despite the large AZ 960 number of mutations that have been identified, imatinib resistance frequently occurs through several common mechanisms. While resistance mutations have been identified throughout the catalytic and regulatory domains of ABL, a large percentage localize to a region called the phosphate binding loop or glycine rich loop. The P loop is a flexible, glycine rich loop that makes contact with the and phosphates of ATP . X ray crystal structures of the imatinib ABL complex have demonstrated that the P loop adopts a unique kinked conformation, which shields the pyridine and pyrimidine rings of the drug from solvent . The ordered nature of the P loop when ABL is bound to imatinib has been confirmed in solution by NMR spectroscopy. The two most commonly observed sites of mutation in the P loop are Tyr253 and Glu255, which account for over 30% of all clinically observed imatinib resistance mutations.
Commonly, Tyr253 is mutated to a His or Phe residue and Glu255 to a Lys or Val. In vitro activity assays with purified ABL kinase have demonstrated that the Tyr253His and Tyr253Phe mutations result in a 18 and 15 fold loss in drug sensitivity, respectively. Analysis of the imatinib ABL complex has shown that there are likely two reasons that these mutations result in the observed loss in potency of imatinib. First, conversion of Tyr253 to a phenylalanine or histidine residue most likely leads to a less favorable face to edge aromatic interaction between this side chain and Krishnamurty and Maly Page 3 ACS Chem Biol. Author manuscript, available in PMC 2011 January 15. NIH PA Author Manuscript NIH PA Author Manuscript NIH PA Author Manuscript the pyrimidine ring of the drug.
In addition, these mutations remove the ability of this sidechain to hydrogen bond with Asn322 in the C lobe which most likely results in disruption of the distorted conformation of the P loop. Glu255 mutations result in a similar loss in potency, with the Glu255Val and Glu255Lys mutants of ABL showing 13 and 18 fold less sensitivity to imatinib, respectively. Unlike Tyr253, the side chain of Glu255 does not make direct contact with the drug. Rather, the carboxylate from this residue forms a hydrogen bonding network with Lys247 and Tyr257 that stabilizes the anti parallel strand of the P loop. Mutating Glu to a Lys or Val residue disrupts these interactions and most likely destabilizes the conformation of the P loop. It has been hypothesized that mutations in the P loop contribute to imatinib resistance by destabilizing the inactive DFG out conformation of ABL. While this may be true in a cellular context, several recent studies show that this is