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The imatinib paradigm in chronic myelogenous leukemia (CML) established continuous BCR-ABL inhibition as a design principle for ABL tyrosine kinase inhibitors (TKI). However, clinical responses seen in patients treated with the ABL TKI dasatinib despite its much shorter plasma half-life and the apparent rapid restoration of BCR-ABL signaling activity following once-daily dosing suggested acute, potent inhibition of kinase activity may be sufficient to irrevocably commit CML cells to apoptosis. To determine the specific requirements for ABL TKI-induced CML cell death for a panel of clinically important ABL TKIs (imatinib, nilotinib, dasatinib, ponatinib, and DCC-2036), we interrogated response of CML cell lines and primary CML cells following acute drug exposure using intracellular fluorescence-activated cell sorting and immunoblot analyses of BCR-ABL signaling, apoptosis measurements, liquid chromatography/tandem mass spectrometry of intracellular drug levels, and biochemical TKI dissociation studies. Importantly, significant intracellular TKI stores were detected following drug washout, levels of which tracked with onset of apoptosis and incomplete return of BCR-ABL signaling, particularly pSTAT5, to baseline. Among TKIs tested, ponatinib showed the most robust capacity for apoptotic commitment showing sustained suppression of BCR-ABL signaling even at low intracellular levels following extensive washout, consistent with high-affinity binding and slow dissociation from ABL kinase. Together, our findings suggest commitment of CML cells to apoptosis requires protracted incomplete restoration of BCR-ABL signaling mediated by intracellular retention of TKIs above a quantifiable threshold. These studies refine our understanding of apoptotic commitment in CML cells and highlight parameters important to design of therapeutic kinase inhibitors for CML and other malignancies. Cancer Res; 73(11); 3356–70. ©2013 AACR.

Analytical cytometry has significant potential beyond cellular analysis. The inherent capability of flow cytometers to efficiently discriminate between uniformly sized particles based on their intrinsic properties provides the foundation for multiplex bead assays. The technology can be exploited in designing immunoassays, Western blot-like antibody assays, and nucleic acid hybridization assays. This chapter focuses on immunoassay applications. The multiplex bead assays have recently evolved as a new and increasingly popular area for flow cytometry, becoming a good alternative to enzyme-linked immunosorbent assay for efficient evaluation of panels of analytes. This chapter provides detailed information about two bead platforms, the BD Cytometric Bead Array kits and the BD Cytometric Bead Array Flex Set Assays.