DCC-2036: A Novel Switch Pocket Inhibitor of ABL Tyrosine Kinase with Therapeutic Efficacy Against BCR-ABL T315I In Vitro and in a CML Mouse Model.

Therapy with ATP-competitive ABL kinase inhibitors, including imatinib (Gleevec), dasatinib (Sprycel), and nilotinib (Tasigna), has revolutionized the treatment of chronic myeloid leukemia (CML), but a substantial proportion of patients develop resistance to these agents. A major mechanism of acquired resistance is mutations in the ABL kinase domain that render BCR-ABL insensitive to the drug; in particular, mutation of the ABL kinase “gatekeeper” Thr315 residue to Ile (T315I) confers pan-resistance to all ATP-competitive ABL inhibitors. To address this need, we developed and characterized a novel chemical class of compounds that bind to five structural pockets involved in the endogenous “switch” mechanism used by the ABL kinase to conformationally control its activity state. Using a structure-based drug design approach, diversity in these “switch pockets” between kinases can be exploited to develop inhibitors with high potency and specificity. The resulting ABL inhibitors have several attractive features, as they:

1. act through a non-ATP-competitive mechanism;

2. avoid steric clash with Ile315;

3. inhibit purified ABL that is either unphosphorylated (switch-off) or phosphorylated (switch-on) at Tyr393 with IC₅₀ of 0.8–4.0 nM;

4. co-crystallize with phospho-Y393-ABL WT and T315I; and

5. have very prolonged residency time on the kinase (off-rate ∼400 min vs. 3 min for imatinib).

A lead compound, DCC-2036, is highly selective for ABL, FLT3, and SRC family kinases, and has favorable pharmacokinetic and toxicity profiles in animals. In vitro, DCC-2036 inhibited proliferation and induced apoptosis of Ba/F3 cells expressing BCR-ABL WT or several common imatinib-resistant mutants (Y253F, T315I, M351T) with IC₅₀ from 5 to 25 nM, without appreciable inhibition of parental Ba/F3 cells in IL-3 (IC₅₀ >1000 nM). In Balb/c mice injected with Ba/F3-BCR-ABL T315I cells, pharmacodynamic studies indicated >8 hour inhibition of phospho-Stat5 and phospho-ABL in the leukemic cells following a single oral 100 mg/kg dose of DCC-2036, whereas daily dosing of DCC-2036 significantly prolonged survival. In the mouse retroviral bone marrow transduction/transplantation model of CML, DCC-2036 at 100 mg/kg/d also reduced peripheral blood leukocyte counts and significantly prolonged survival vs. vehicle-treated control mice. These results demonstrate that ABL switch pocket inhibitors are a promising new pharmacologic weapon for the treatment of de novo and drug-resistant CML, including BCR-ABL T315I.