Thomas O’Hare

Imatinib, a Bcr-Abl tyrosine kinase inhibitor, is a highly effective therapy for patients with chronic myelogenous leukemia (CML). Despite durable responses in most chronic phase patients, relapses have been observed and are much more prevalent in patients with advanced disease. The most common mechanism of acquired imatinib resistance has been traced to Bcr-Abl kinase domain mutations with decreased imatinib sensitivity. Thus, alternate Bcr-Abl kinase inhibitors that have activity against imatinib-resistant mutants would be useful for patients who relapse on imatinib therapy. Two such Bcr-Abl inhibitors are currently being evaluated in clinical trials: the improved potency, selective Abl inhibitor AMN107 and the highly potent dual Src/Abl inhibitor BMS-354825. In the current article, we compared imatinib, AMN107, and BMS-354825 in cellular and biochemical assays against a panel of 16 kinase domain mutants representing >90% of clinical isolates. We report that AMN107 and BMS-354825 are 20-fold and 325-fold more potent than imatinib against cells expressing wild-type Bcr-Abl and that similar improvements are maintained for all imatinib-resistant mutants tested, with the exception of T315I. Thus, both inhibitors hold promise for treating imatinib-refractory CML.

BACKGROUND: Resistance to tyrosine kinase inhibitors in patients with chronic myeloid leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph-positive ALL) is frequently caused by mutations in the BCR-ABL kinase domain. Ponatinib (AP24534) is a potent oral tyrosine kinase inhibitor that blocks native and mutated BCR-ABL, including the gatekeeper mutant T315I, which is uniformly resistant to tyrosine kinase inhibitors. METHODS: In this phase 1 dose-escalation study, we enrolled 81 patients with resistant hematologic cancers, including 60 with CML and 5 with Ph-positive ALL. Ponatinib was administered once daily at doses ranging from 2 to 60 mg. Median follow-up was 56 weeks (range, 2 to 140). RESULTS: Dose-limiting toxic effects included elevated lipase or amylase levels and pancreatitis. Common adverse events were rash, myelosuppression, and constitutional symptoms. Among Ph-positive patients, 91% had received two or more approved tyrosine kinase inhibitors, and 51% had received all three approved tyrosine kinase inhibitors. Of 43 patients with chronic-phase CML, 98% had a complete hematologic response, 72% had a major cytogenetic response, and 44% had a major molecular response. Of 12 patients who had chronic-phase CML with the T315I mutation, 100% had a complete hematologic response and 92% had a major cytogenetic response. Of 13 patients with chronic-phase CML without detectable mutations, 100% had a complete hematologic response and 62% had a major cytogenetic response. Responses among patients with chronic-phase CML were durable. Of 22 patients with accelerated-phase or blast-phase CML or Ph-positive ALL, 36% had a major hematologic response and 32% had a major cytogenetic response. CONCLUSIONS: Ponatinib was highly active in heavily pretreated patients with Ph-positive leukemias with resistance to tyrosine kinase inhibitors, including patients with the BCR-ABL T315I mutation, other mutations, or no mutations. (Funded by Ariad Pharmaceuticals and others; ClinicalTrials.gov number, NCT00660920.).

Mutations in the kinase domain (KD) of BCR-ABL are the most prevalent mechanism of acquired imatinib resistance in patients with chronic myeloid leukemia (CML). Here we examine predisposing factors underlying acquisition of KD mutations, evidence for acquisition of mutations before and during therapy, and whether the detection of a KD mutation universally implies resistance. We also provide a perspective on how the second-line Abl inhibitors dasatinib and nilotinib are faring in the treatment of imatinib-resistant CML, especially in relation to specific KD mutations. We discuss the growing importance of the multi-inhibitor-resistant 315T>I mutant and the therapeutic potential that a 315T>I inhibitor would have. Last, we assess the potential of Abl kinase inhibitor combinations to induce stable responses even in advanced CML and interpret the emerging data in the context of CML pathogenesis.

BMS-354825 (dasatinib) and AMN107 (nilotinib) are potent alternate Abl inhibitors with activity against many imatinib mesylate-resistant BCR-ABL kinase domain (KD) mutants, except T315I. We used N-ethyl-N-nitrosourea (ENU)-exposed Ba/F3-p210(BCR-ABL) cells to compare incidence and types of KD mutants emerging in the presence of imatinib mesylate, dasatinib, and nilotinib, alone and in dual combinations. Although ENU is expected to induce mutations in multiple proteins, resistant clones were almost exclusively BCR-ABL KD mutant at relevant concentrations of nilotinib and dasatinib, consistent with a central role of KD mutations for resistance to these drugs. Twenty different mutations were identified with imatinib mesylate, 10 with nilotinib (including only 1 novel mutation, E292V) and 9 with dasatinib. At intermediate drug levels the spectrum narrowed to F317V and T315I for dasatinib and Y253H, E255V, and T315I for nilotinib. Thus, cross-resistance is limited to T315I, which is also the only mutant isolated at drug concentrations equivalent to maximal achievable plasma trough levels. With drug combinations maximal suppression of resistant clone outgrowth was achieved at lower concentrations compared with single agents, suggesting that such combinations may be equipotent to higher-dose single agents. However, sequencing uniformly revealed T315I, consistent with the need for a T315I inhibitor, to completely block resistance.