The major mechanism of imatinib (IM) resistance of CML is the reactivation of ABL kinase either through BCR‐ABL gene amplification or mutation. We investigated the cytotoxicity of a pan‐ABL tyrosine kinase inhibitor, ponatinib, and a pan‐histone deacetylase inhibitor, panobinostat, against IM‐resistant CML cells in vitro. Two different IM‐resistant cell lines, K562/IM‐R1 and Ba/F3/T315I were evaluated in comparison with their respective, parental cell lines, K562 and Ba/F3. K562/IM‐R1 overexpressed BCR‐ABL due to gene amplification. Ba/F3/T315I was transfected with a BCR‐ABL gene encoding T315I‐mutated BCR‐ABL. Ponatinib inhibited the growth of both K562/IM‐R1 and Ba/F3/T315I as potently as it inhibited their parental cells with an IC 50 of 2–30 nM. Panobinostat also similarly inhibited the growth of all of the cell lines with an IC 50 of 40–51 nM. This was accompanied by reduced histone deacetylase activity, induced histone H3 acetylation, and an increased protein level of heat shock protein 70, which suggested disruption of heat shock protein 90 chaperone function for BCR‐ABL and its degradation. Importantly, the combination of ponatinib with panobinostat showed synergistic growth inhibition and induced a higher level of apoptosis than the sum of the apoptosis induced by each agent alone in all of the cell lines. Ponatinib inhibited phosphorylation not only of BCR‐ABL but also of downstream signal transducer and activator of transcription 5, protein kinase B, and ERK1/2 in both K562/IM‐R1 and Ba/F3/T315I, and the addition of panobinostat to ponatinib further inhibited these phosphorylations. In conclusion, panobinostat enhanced the cytotoxicity of ponatinib towards IM‐resistant CML cells including those with T315I‐mutated BCR‐ABL.

BackgroundNine-beta-D-arabinofuranosylguanine (ara-G), an active metabolite of nelarabine, enters leukemic cells through human Equilibrative Nucleoside Transporter 1, and is then phosphorylated to an intracellular active metabolite ara-G triphosphate (ara-GTP) by both cytosolic deoxycytidine kinase and mitochondrial deoxyguanosine kinase. Ara-GTP is subsequently incorporated into DNA, thereby inhibiting DNA synthesis.MethodsIn the present study, we developed a novel ara-G-resistant variant (CEM/ara-G) of human T-lymphoblastic leukemia cell line CCRF-CEM, and elucidated its mechanism of ara-G resistance. The cytotoxicity was measured by using the growth inhibition assay and the induction of apoptosis. Intracellular triphosphate concentrations were quantitated by using HPLC. DNA synthesis was evaluated by the incorporation of tritiated thymidine into DNA. Protein expression levels were determined by using Western blotting.ResultsCEM/ara-G cells were 70-fold more ara-G-resistant than were CEM cells. CEM/ara-G cells were also refractory to ara-G-mediated apoptosis. The transcript level of human Equilibrative Nucleoside Transporter 1 was lowered, and the protein levels of deoxycytidine kinase and deoxyguanosine kinase were decreased in CEM/ara-G cells. The subsequent production of intracellular ara-GTP (21.3 pmol/107 cells) was one-fourth that of CEM cells (83.9 pmol/107 cells) after incubation for 6 h with 10 μM ara-G. Upon ara-G treatment, ara-G incorporation into nuclear and mitochondrial DNA resulted in the inhibition of DNA synthesis of both fractions in CEM cells. However, DNA synthesis was not inhibited in CEM/ara-G cells due to reduced ara-G incorporation into DNA. Mitochondrial DNA-depleted CEM cells, which were generated by treating CEM cells with ethidium bromide, were as sensitive to ara-G as CEM cells. Anti-apoptotic Bcl-xL was increased and pro-apoptotic Bax and Bad were reduced in CEM/ara-G cells.ConclusionsAn ara-G-resistant CEM variant was successfully established. The mechanisms of resistance included reduced drug incorporation into nuclear DNA and antiapoptosis.

Barasertib, an aurora B inhibitor, terminates cell division, introduces polyploidy, and consequently causes apoptosis. In the present study, we evaluated the effect of the combination of barasertib and cytarabine (ara-C), a key agent for leukemia chemotherapy, on leukemic cells in vitro. Human leukemia HL-60 cells and HL-60 ⁄ ara-C20 cells, a 20-fold ara-C-resistant variant, were used. The 50% growth inhibitory concentrations of an active metabolite of barasertib, barasertib-hydroxyquinazolinepyrazol-aniline (Barasertib-HQPA), and ara-C were 51 nM and 300 nM for HL-60 cells and 70 nM and 5300 nM for HL-60 ⁄ ara-C20 cells, respectively. Barasertib-HQPA induced polyploidy with a subsequent induction of sub-G1 phase apoptosis, indicating the M-phase specific cytotoxicity. Cells treated with the S-phase specific ara-C accumulated in S phase and subsequently died through apoptosis. When HL-60 cells were treated with barasertib-HQPA and ara-C in combination, a greater-than-additive apoptosis was induced. This enhancement was obtained when the cells were treated with barasertib-HQPA prior to ara-C (37.9% sub-G1) or with both concurrently (31.2% sub-G1), but not with ara-C prior to barasertib-HQPA (17.8% sub-G1). The combination effects were similarly obtained in HL-60 ⁄ ara-C20 cells with 19.7% sub-G1 for barasertib-HQPA?ara-C, 18.4% sub-G1 for both concurrently, and 13.8% sub-G1 for ara-C?barasertib-HQPA, and another leukemic U937 cells with 25.4% sub-G1 for barasertib-HQPA?ara-C, 28.2% sub-G1 for both concurrently, and 16.0% sub-G1 for ara-C?barasertib-HQPA. Barasertib-HQPA inhibited aurora B autophosphorylation and histone H3 phosphorylation in all the cell lines. Barasertib-HQPA did not inhibit DNA synthesis, allowing ara-C incorporation into DNA for its cytotoxicity. Thus, barasertib-HQPA and ara-C provided a greater-than-additive cytotoxicity in leukemic cells in vitro. (Cancer Sci 2013; 104: 926-933)

Abstract. The cytotoxicity of the monofunctional alkylator, temozolomide (TMZ), is known to be mediated by mismatch repair (MMR) triggered by O 6 -alkylguanine. By contrast, the cytotoxicity of bifunctional alkylators, including carmustine (BCNU) and melphalan (MEL), depends on interstrand crosslinks formed through O 6 -alkylguanine, which is repaired by nucleotide excision repair and recombination. O 6 -alkylguanine is removed by O 6 -methylguanine-DNA methyltransferase (MGMT). The aim of the present study was to evaluate the cytotoxicity of TMZ, BCNU and MEL in two different leukemic cell lines (HL-60 and MOLT-4) in the context of DNA repair. The transcript levels of MGMT, ERCC1, hMLH1 and hMSH2 were determined using reverse transcription-quantitative polymerase chain reaction. In addition, the proliferation was measured using the trypan blue exclusion assay. Drug sensitivity was found to vary between the two cell lines. Treatment of the cells with TMZ, BCNU or MEL in combination with O 6 -benzylguanine, an MGMT inhibitor, was demonstrated to sensitize the two cell lines to these agents. However, the extent of sensitization was not found to be correlated with the expression levels of MGMT transcripts. Furthermore, the drug sensitivity was also not associated with the transcript levels of ERCC1, hMLH1 and hMSH2. Thus, leukemic cells were sensitized to alkylating agents by the inhibition of MGMT.

Current first-line treatment options for chronic myeloid leukemia (CML) include imatinib (IM) and the second-generation agents nilotinib and dasatinib. Despite the effectiveness of these tyrosine kinase inhibitors, a small percentage of chronic-phase CML patients are primarily refractory or acquire secondary resistance to these agents. Moreover, the prognosis of patients in blast crisis is still poor despite the use of the current treatment modalities because of drug resistance. The major mechanism of drug resistance is the re-activation of ABL kinase either through mutations in the BCR-ABL gene or by BCR-ABLgene amplification. A novel pan-histone deacetylase inhibitor, panobinostat (formerly LBH589), induces the acetylation of heat shock protein 90, thereby inhibiting its chaperone function in association with its client proteins BCR-ABL, leading to the degradation of BCR-ABL. A new pan–ABL tyrosine kinase inhibitor, ponatinib, is a promising therapeutic option in patients with all kinds of BCR-ABLmutation including T315I. It was thus hypothesized that the combination of panobinostat and ponatinib exerts synergistic cytotoxicity against IM-resistant cells through a mechanism of action different from that of each agent. To test this hypothesis, K562/IM-R1 and Ba/F3/T315I cell lines were evaluated for the cytotoxicity of panobinostat and ponatinib in vitro. K562/IM-R1 cells, established in our previous study, showed BCR-ABL overexpression due to BCR-ABL gene amplification. The Ba/F3/T315I cell line showed BCR-ABL with a T315I mutation. The XTT proliferation assay revealed that K562/IM-R1 cells were 12-fold more IM-resistant (50%-inhibitory concentration (IC50), 7.6 µM) than K562 cells (IC50, 0.6 µM). Ba/F3/T315I cells were refractory to IM treatment (IC50, 34.1 µM) compared with Ba/F3 cells (IC50, 3.4 µM). Panobinostat overcame IM-resistance and inhibited similarly the growth of K562, K562/IM-R1, Ba/F3, and Ba/F3/T315I cells, with IC50 values of around 50 nM (40.0 - 51.0 nM). Ponatinib inhibited the growth of both K562/IM-R1 cells (IC50, 4 nM) and Ba/F3/T315I cells (25 nM) as potently as parental K562 cells (IC50, 2 nM) and Ba/F3 cells (IC50, 5 nM). Importantly, the combination of panobinostat and ponatinib exhibited enhanced growth inhibition effects on all cell lines. The IC50 values for this combination were 0.7 nM for K562 cells, 1.3 nM for K562/IM-R1 cells, 3.7 nM for Ba/F3 cells, and 10 nM for Ba/F3/T315I cells. The combination index clearly showed synergism, with values of 0.5 for K562 cells, 0.28 for K562/IM-R1 cells, 0.9 for Ba/F3 cells, and 0.4 for Ba/F3/T315I cells. When the cells were treated with 10 nM panobinostat or 10 nM ponatinib for 48 h alone or in combination, the combination of the 2 agents led to greater-than-additive apoptotic cell death than each agent alone in all cell lines, evaluated by annexin V-positivity. Western blotting was used to evaluate the protein expression levels of BCR-ABL and phospho-BCR-ABL in cells after treatment with panobinostat or ponatinib or both in combination. BCR-ABL expression was greater in K562/IM-R1 than K562 cells. Phospho-BCR-ABL expression was not inhibited by IM in K562/IM-R1 or Ba/F3/T315I cells. However, ponatinib inhibited the autophosphorylation of BCR-ABL in these cell lines. Treatment with panobinostat reduced the BCR-ABL and phospho-BCR-ABL expression levels in K562/IM-R1 and Ba/F3/T315I cells. The combination of the 2 agents augmented inhibition of the autophosphorylation of BCR-ABL in these IM-resistant cell lines. The activity of histone deacetylase, determined using the HDAC assay Kit (Active Motif, Carlsbad, CA, USA), was inhibited by panobinostat in all cell lines regardless of IM sensitivity. In comparison, IM did not alter cellular histone deacetylase activity. Upon treatment of K562 cells with panobinostat, the protein expression levels of acetylated histone H3 and H4 were increased, suggesting the consequence of the inhibition of histone deacetylase. In conclusion, we firstly reported that panobinostat and ponatinib demonstrated synergistic cytotoxicity against IM-resistant cell lines not only due to BCR-ABL gene amplification but also BCR-ABL T315I mutation. The synergism is attributable to the greater inhibition of ABL kinase activity through a mechanism of action different from that of each agent. Disclosures: No relevant conflicts of interest to declare.