Cryptic Intracellular Retention of ABL Tyrosine Kinase Inhibitors within CML Cells Mediates Apoptosis Commitment Following Acute Drug Exposure,

Abstract 3504

The imatinib paradigm established continuous BCR-ABL inhibition as a design principle for ABL tyrosine kinase inhibitors (TKIs). However, once-daily dasatinib (serum half-life: 3–5 h) is clinically effective despite only transient BCR-ABL inhibition, opening an opportunity for in-depth study of the mechanistic requirements for ABL TKI-induced CML cell death. Apoptosis commitment after potent, transient target inhibition is observed with ABL TKIs in vitro (Blood, 114, 2009, 3459–63), although variations in required TKI concentrations relative to their activity against BCR-ABL kinase suggest involvement of previously unrecognized factors. The “oncogenic shock” concept holds that temporary disruption of BCR-ABL-mediated prosurvival and proapoptotic signaling sets up a kinetic imbalance in favor of apoptosis. We have undertaken a comprehensive mechanistic exploration of this issue, wherein CML cells were transiently exposed to the ABL TKIs imatinib (50 and 500 nM), nilotinib (50 and 500 nM), dasatinib (10 and 100 nM), and ponatinib (AP24534; 10 and 100 nM) and then investigated with respect to pathways critical to drug efficacy and intracellular residence time. Cellular studies utilized multi-parameter intracellular FACS and immunoblot analysis, liquid chromatography-mass spectrometry, and high-throughput real-time qPCR expression analysis. Corresponding biochemical studies to determine ABL kinase/inhibitor dissociation parameters were also performed. All four ABL TKIs tested were capable of triggering apoptosis following transient exposure, although nilotinib and imatinib (which feature much narrower kinase target profiles than dasatinib and ponatinib) did so only at high concentrations. In contrast to potent, transient inhibition of BCR-ABL being necessary and sufficient for commitment of CML cells to apoptosis, we found that apoptosis could be reversed under conditions involving extensive additional TKI washout protocols. Consistent with the best indicator of apoptosis induction in our experiments being incomplete restoration of BCR-ABL signaling activity to pre-treatment levels, in all cases for which apoptosis commitment was irreversible, we identified a small, functionally important pool of intracellular TKI after washout of drug from the media. This property correlated with results of ABL kinase/inhibitor dissociation studies. For example, we found that ponatinib is a tight-binding inhibitor with a remarkably slow off-rate (t_1/2 >95 h). Transient exposure to ponatinib followed by thorough washout committed CML cells to apoptosis despite very low intracellular concentrations that did not completely inhibit BCR-ABL. Based on these findings, we explored the possibility that effective TKIs are inhibiting additional targets involved in committing cells to an apoptotic fate, and used high-throughput qPCR assays to identify a preliminary profile of 30 apoptosis-related genes differentially expressed in TKI-treatment conditions that do and do not irrevocably commit CML cells to apoptosis. Taken together, our findings reveal that even slightly attenuated restoration of BCR-ABL signaling correlates with apoptosis commitment and that cryptic cellular retention of ABL TKIs is important in mediating this effect, potentially via sustained low-level inhibition of auxiliary targets. By extension, monitoring intracellular drug levels by LC-MS may be informative, especially for short serum half-life kinase inhibitors such as dasatinib. These studies further establish and refine the guiding principles of commitment of CML cells to apoptosis and improve our ability to design kinase inhibitors for CML and other malignancies.