Evaluation of methemoglobin as an intravascular contrast agent: T1 relaxation time effect in a rabbit model

OBJECTIVE

Alternative contrast agents for MRI are needed for individuals who may respond adversely to gadolinium, and need an intravascular agent for specific indications. One potential contrast agent is intracellular methemoglobin, a paramagnetic molecule that is normally present in small amounts in red blood cells. An animal model was used to determine whether methemoglobin modulation with intravenous sodium nitrite transiently changes the T1 relaxation of blood.

METHODS

Four adult New Zealand white rabbits were treated with 30 mg intravenous sodium nitrite. 3D TOF and 3D MPRAGE images were acquired before (baseline) and after methemoglobin modulation. T1 of blood was measured with 2D ss EPl acquisitions with inversion recovery preparation performed at two-minute intervals up to 30 min. T1 maps were calculated by fitting the signal recovery curve within major blood vessels.

RESULTS

Baseline T1 was 1758 ± 53 ms in carotid arteries and 1716 ± 41 ms in jugular veins. Sodium nitrite significantly changed intravascular T1 relaxation. The mean minimum value of T1 was 1126 ± 28 ms in carotid arteries 8 to 10 min after the injection of sodium nitrite. The mean minimum value of T1 was 1171 ± 52 ms in jugular veins 10 to 14 min after the injection of sodium nitrite. Arterial and venous T1 recovered to baseline after a period of 30 min.

CONCLUSION

Methemoglobin modulation produces intravascular contrast on T1-weighted MRI in vivo. Additional studies are needed to safely optimize methemoglobin modulation and sequence parameters for maximal tissue contrast.

Non-invasive central nervous system assessment of a porcine model of neuropathic pain demonstrates increased latency of somatosensory-evoked potentials

BACKGROUND

The study of chronic pain and its treatments requires a robust animal model with objective and quantifiable metrics. Porcine neuropathic pain models have been assessed with peripheral pain recordings and behavioral responses, but thus far central nervous system electrophysiology has not been investigated. This work aimed to record non-invasive, somatosensory-evoked potentials (SEPs) via electroencephalography in order to quantitatively assess chronic neuropathic pain induced in a porcine model.

NEW METHOD

Peripheral neuritis trauma (PNT) was induced unilaterally in the common peroneal nerve of domestic farm pigs, with the contralateral leg serving as the control for each animal. SEPs were generated by stimulation of the peripheral nerves distal to the PNT and were recorded non-invasively using transcranial electroencephalography (EEG). The P30 wave of the SEP was analyzed for latency changes.

RESULTS

P30 SEPs were successfully recorded with non-invasive EEG. PNT resulted in significantly longer P30 SEP latencies (p < 0.01 [n = 8]) with a median latency increase of 14.3 [IQR 5.0 - 17.5] ms. Histological results confirmed perineural inflammatory response and nerve damage around the PNT nerves.

COMPARISON WITH EXISTING METHOD(S)

Control P30 SEPs were similar in latency and amplitude to those previously recorded invasively in healthy pigs. Non-invasive recordings have numerous advantages over invasive measures.

CONCLUSIONS

P30 SEP latency can serve as a quantifiable neurological measure that reflects central nervous system processing in a porcine model of chronic pain. Advancing the development of a porcine chronic pain model will facilitate the translation of experimental therapies into human clinical trials.

Real-Time Monitoring and Modulation of Blood Pressure in a Rabbit Model of Ischemic Stroke

Control of blood pressure, in terms of both absolute values and its variability, affects outcomes in ischemic stroke patients. However, it remains challenging to identify the mechanisms that lead to poor outcomes or evaluate measures by which these effects can be mitigated because of the prohibitive limitations inherent to human data. In such cases, animal models can be utilized to conduct rigorous and reproducible evaluations of diseases. Here we report refinement of a previously described model of ischemic stroke in rabbits that is augmented with continuous blood pressure recording to assess the impacts of modulation on blood pressure. Under general anesthesia, femoral arteries are exposed through surgical cutdowns to place arterial sheaths bilaterally. Under fluoroscopic visualization and roadmap guidance, a microcatheter is advanced into an artery of the posterior circulation of the brain. An angiogram is performed by injecting the contralateral vertebral artery to confirm occlusion of the target artery. With the occlusive catheter remaining in position for a fixed duration, blood pressure is continuously recorded to allow for tight titration of blood pressure manipulations, whether through mechanical or pharmacological means. At the completion of the occlusion interval, the microcatheter is removed, and the animal is maintained under general anesthesia for a prescribed length of reperfusion. For acute studies, the animal is then euthanized and decapitated. The brain is harvested and processed to measure the infarct volume under light microscopy and further assessed with various histopathological stains or spatial transcriptomic analysis. This protocol provides a reproducible model that can be utilized for more thorough preclinical studies on the effects of blood pressure parameters during ischemic stroke. It also facilitates effective preclinical evaluation of novel neuroprotective interventions that might improve care for ischemic stroke patients.

Spatially resolved transcriptomics for evaluation of intracranial vessels in a rabbit model: Proof of concept

BACKGROUND

Better understanding of vessel biology and vascular pathophysiology is needed to improve understanding of cerebrovascular disorders. Tissue from diseased vessels can offer the best data. Rabbit models can be effective for studying intracranial vessels, filling gaps resulting from difficulties acquiring human tissue. Spatially-resolved transcriptomics (SRT) in particular hold promise for studying such models as they build on RNA sequencing methods, augmenting such data with histopathology.

METHODS

Rabbit brains with intact arteries were flash frozen, cryosectioned, and stained with H&E to confirm adequate inclusion of intracranial vessels before proceeding with tissue optimization and gene expression analysis using the Visium SRT platform. SRT results were analyzed with k-means clustering analysis, and differential gene expression was examined, comparing arteries to veins.

RESULTS

Cryosections were successfully mounted on Visium proprietary slides. Quality control thresholds were met. Optimum permeabilization was determined to be 24 min for the tissue optimization step. In analysis of SRT data, k-means clustering distinguished vascular tissue from parenchyma. When comparing gene expression traits, the most differentially expressed genes were those found in smooth muscle cells. These genes were more commonly expressed in arteries compared to veins.

CONCLUSIONS

Intracranial vessels from model rabbits can be processed and analyzed with the Visium SRT platform. Face validity is found in the ability of SRT data to distinguish vessels from parenchymal tissue and differential expression analysis accurately distinguishing arteries from veins. SRT should be considered for future animal model investigations into cerebrovascular diseases.

MS4A3 promotes differentiation in chronic myeloid leukemia by enhancing common β-chain cytokine receptor endocytosis

The chronic phase of chronic myeloid leukemia (CP-CML) is characterized by the excessive production of maturating myeloid cells. As CML stem/progenitor cells (LSPCs) are poised to cycle and differentiate, LSPCs must balance conservation and differentiation to avoid exhaustion, similar to normal hematopoiesis under stress. Since BCR-ABL1 tyrosine kinase inhibitors (TKIs) eliminate differentiating cells but spare BCR-ABL1-independent LSPCs, understanding the mechanisms that regulate LSPC differentiation may inform strategies to eliminate LSPCs. Upon performing a meta-analysis of published CML transcriptomes, we discovered that low expression of the MS4A3 transmembrane protein is a universal characteristic of LSPC quiescence, BCR-ABL1 independence, and transformation to blast phase (BP). Several mechanisms are involved in suppressing MS4A3, including aberrant methylation and a MECOM-C/EBPε axis. Contrary to previous reports, we find that MS4A3 does not function as a G1/S phase inhibitor but promotes endocytosis of common β-chain (βc) cytokine receptors upon GM-CSF/IL-3 stimulation, enhancing downstream signaling and cellular differentiation. This suggests that LSPCs downregulate MS4A3 to evade βc cytokine-induced differentiation and maintain a more primitive, TKI-insensitive state. Accordingly, knockdown (KD) or deletion of MS4A3/Ms4a3 promotes TKI resistance and survival of CML cells ex vivo and enhances leukemogenesis in vivo, while targeted delivery of exogenous MS4A3 protein promotes differentiation. These data support a model in which MS4A3 governs response to differentiating myeloid cytokines, providing a unifying mechanism for the differentiation block characteristic of CML quiescence and BP-CML. Promoting MS4A3 reexpression or delivery of ectopic MS4A3 may help eliminate LSPCs in vivo.

Abstract LB109: A critical role for SIRT5 in acute myeloid leukemia metabolism

Standard of care for AML includes chemotherapy and stem cell transplant, with 5-year survival rates &lt;30%. We sought to identify genes critical to AML cells, irrespective of mutational status, and performed an shRNA screen targeting 1,287 genes on 12 AML patient samples. This screen identified Sirtuin 5 (SIRT5) as a top candidate. SIRT5 is the only known enzyme with desuccinylase, demalonylase, and/or deglutarylase activity and we are the first to report the dependence of AML cells on SIRT5. Next, we stably transduced a panel of AML cell lines with doxycycline (dox)-inducible shSIRT5 (dox-shSIRT5). SIRT5 knockdown (KD) strongly inhibited cell growth, colony formation and increased apoptosis in 15/22 lines (SIRT5-dependent), while 7/22 lines were SIRT5-independent. SIRT5 dependence did not correlate with AML-related mutations nor basal SIRT5 expression. SIRT5 KD in primary AML samples (N=25) revealed a therapeutic window (~50% reduction), with no effect in CB samples (N=5). We examined the requirement of SIRT5 in vivo using three mouse models of leukemia. In a xenograft model with AML cell lines, SIRT5 KD indefinitely prolonged survival of mice injected with SIRT5-dependent cells with no sign of leukemia. Bone marrow transplant with transduced (MLL-AF9 or BCR-ABL1) SIRT5 null cells showed reduced leukemia cell burden and splenomegaly, and significantly prolonged survival. FLT3-ITD-driven disease was also blunted by the absence of SIRT5 in a genetic knockout mouse model. Mechanically, SIRT5 KD profoundly reduced oxidative phosphorylation (OXPHOS) and glycolysis. Additionally, SIRT5 KD increased mitochondrial superoxide selectively in annexin V-negative, SIRT5-dependent cells. Concomitant, ectopic expression of SOD2 abrogated the increase in superoxide, rescued cells from apoptosis, and rescued the colony formation deficit. Untargeted metabolomics revealed RNA charging and alanine and serine metabolism as top metabolic pathways regulated by SIRT5, with glutaminase (GLS) and α-ketoglutarate identified as potential upstream regulators. Metabolic tracing experiments with [13C5,15N2]-glutamine confirmed disrupted glutamine metabolism in SIRT5-dependent cells. Together, these results indicate that SIRT5 is required to regulate glutamine flux to sustain redox homeostasis and/or anabolism. NRD167, a novel SIRT5 inhibitor, was used to target SIRT5 in AML. NRD167 reduced cell proliferation, induced apoptosis, and reduced OXPHOS in SIRT5-dependent but not SIRT5-independent cells. NRD167 inhibited colony formation from AML patient samples, but not in CB samples. An AML patient-derived xenograft model trended toward prolonged survival following ex vivo treatment with NRD167. Our data suggest that the majority of AML samples are dependent on SIRT5 and that inhibition preferentially targets AML cells, implicating SIRT5 as a therapy target in AML.

Citation Format: Dongqing Yan, Anca Franzini, Anthony D. Pomicter, Brayden J. Halverson, Orlando Antelope, Clinton C. Mason, Jonathan M. Ahmann, Anna V. Senina, Courtney L. L. Jones, Matthew S. Zabriskie, Hein Than, Michael J. Xiao, Alexandria van Scoyk, Ami B. Patel, William L. L. Heaton, Shawn C. Owen, Joshua L. Andersen, Christina M. Egbert, Julie A. Reisz, Angelo D'Alessandro, James E. Cox, Kevin C. Gantz, Hannah M. Redwine, Siddharth M. Iyer, Jamshid S. Khorashad, Nima Rajabi, Christian A. Olsen, Thomas O'Hare, Michael W. Deininger. A critical role for SIRT5 in acute myeloid leukemia metabolism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB109.

Rabbit models of intracranial atherosclerotic disease for pathological validation of vessel wall MRI

INTRODUCTION

Vessel wall magnetic resonance imaging can improve the evaluation of intracranial atherosclerotic disease. However, pathological validation is needed to improve vessel wall magnetic resonance imaging techniques. Human pathology samples are not practical for such analysis, so an animal model is therefore needed.

MATERIALS AND METHODS

Watanabe heritable hyperlipidemic rabbits and apolipoprotein E knockout rabbits were evaluated against New Zealand white wild-type rabbits. Evaluation of intracranial arteries was performed with vessel wall magnetic resonance imaging and pathological analysis, rating the presence and severity of disease in each segment. Two-tailed t-tests were performed to compare disease occurrence and severity prevalence among rabbit subtypes. Sensitivity and specificity were calculated to assess the diagnostic accuracy of vessel wall magnetic resonance imaging.

RESULTS

Seventeen rabbits (five Watanabe heritable hyperlipidemic, four apolipoprotein E knockout and eight New Zealand white) were analysed for a total of 51 artery segments. Eleven segments (five Watanabe heritable hyperlipidemic and six apolipoprotein E knockout) demonstrated intracranial atherosclerotic disease on pathology. Disease model animals had lesions more frequently than New Zealand white animals (P<0.001). The sensitivity and specificity of vessel wall magnetic resonance imaging for the detection of intracranial atherosclerotic disease were 68.8% and 95.2%, respectively. When excluding mild cases to assess vessel wall magnetic resonance imaging accuracy for detecting moderate to severe intracranial atherosclerotic disease lesions, sensitivity improved to 100% with unchanged specificity.

CONCLUSION

Intracranial atherosclerotic disease can be reliably produced and detected using 3T vessel wall magnetic resonance imaging-compatible Watanabe heritable hyperlipidemic and ApoE rabbit models. Further analysis is needed to characterize better the development and progression of the disease to correlate tissue-validated animal findings with those in human vessel wall magnetic resonance imaging studies.

Apolipoprotein E knockout rabbit model of intracranial atherosclerotic disease

Intracranial atherosclerotic disease (ICAD) is the most common cause of ischemic stroke. Poor understanding of the disease due to limited human data leads to imprecise treatment. Apolipoprotein E knockout (ApoE-KO) rabbits were compared to an existing model, the Watanabe heritable hyperlipidemic (WHHL) rabbit, and wild-type New Zealand white (NZW) rabbit controls. Intracranial artery samples were assessed on histopathology for the presence of ICAD. Logistic and ordinal regression analyses were performed to assess for disease presence and severity, respectively. Eighteen rabbits and 54 artery segments were analyzed. Univariate logistic analysis confirmed the presence of ICAD in model rabbits (P < .001), while no difference was found between WHHL and ApoE-KO rabbits (P = .178). In multivariate analysis, only classification as a model vs wild-type animal (P < .001) was associated with the presence of ICAD. Univariate ordinal regression analysis demonstrated an association between ICAD severity and model animals (P = .001), with no difference was noted between WHHL and ApoE-KO rabbits (P = .528). In multivariate ordinal regression analysis, only classification as a model retained significance (P < .001). ICAD can be reliably produced in ApoE-KO rabbits, developing the disease comparably to the older WHHL model. Further analysis is warranted to optimize accelerated development of ICAD in ApoE-KO rabbits to more efficiently study this disease.

SIRT5 IS A DRUGGABLE METABOLIC VULNERABILITY IN ACUTE MYELOID LEUKEMIA

We discovered that the survival and growth of many primary acute myeloid leukemia (AML) samples and cell lines, but not normal CD34+ cells, are dependent on SIRT5, a lysine deacylase implicated in regulating multiple metabolic pathways. Dependence on SIRT5 is genotype-agnostic and extends to RAS- and p53-mutated AML. Results were comparable between SIRT5 knockdown and SIRT5 inhibition using NRD167, a potent and selective SIRT5 inhibitor. Apoptosis induced by SIRT5 disruption is preceded by reductions in oxidative phosphorylation and glutamine utilization, and an increase in mitochondrial superoxide that is attenuated by ectopic superoxide dismutase 2. These data indicate that SIRT5 controls and coordinates several key metabolic pathways in AML and implicate SIRT5 as a vulnerability in AML.

Combining the Allosteric Inhibitor Asciminib with Ponatinib Suppresses Emergence of and Restores Efficacy against Highly Resistant BCR-ABL1 Mutants

BCR-ABL1 point mutation-mediated resistance to tyrosine kinase inhibitor (TKI) therapy in Philadelphia chromosome-positive (Ph+) leukemia is effectively managed with several approved drugs, including ponatinib for BCR-ABL1T315I-mutant disease. However, therapy options are limited for patients with leukemic clones bearing multiple BCR-ABL1 mutations. Asciminib, an allosteric inhibitor targeting the myristoyl-binding pocket of BCR-ABL1, is active against most single mutants but ineffective against all tested compound mutants. We demonstrate that combining asciminib with ATP site TKIs enhances target inhibition and suppression of resistant outgrowth in Ph+ clinical isolates and cell lines. Inclusion of asciminib restores ponatinib's effectiveness against currently untreatable compound mutants at clinically achievable concentrations. Our findings support combining asciminib with ponatinib as a treatment strategy for this molecularly defined group of patients.

A Novel Crizotinib-Resistant Solvent-Front Mutation Responsive to Cabozantinib Therapy in a Patient with ROS1-Rearranged Lung Cancer

PURPOSE

Rearranged ROS1 is a crizotinib-sensitive oncogenic driver in lung cancer. The development of acquired resistance, however, poses a serious clinical challenge. Consequently, experimental and clinical validation of resistance mechanisms and potential second-line therapies is essential.

EXPERIMENTAL DESIGN

We report the discovery of a novel, solvent-front ROS1(D2033N) mutation in a patient with CD74-ROS1-rearranged lung adenocarcinoma and acquired resistance to crizotinib. Crizotinib resistance of CD74-ROS1(D2033N) was functionally evaluated using cell-based assays and structural modeling.

RESULTS

In biochemical and cell-based assays, the CD74-ROS1(D2033N) mutant demonstrated significantly decreased sensitivity to crizotinib. Molecular dynamics simulation revealed compromised crizotinib binding due to drastic changes in the electrostatic interaction between the D2033 residue and crizotinib and reorientation of neighboring residues. In contrast, cabozantinib binding was unaffected by the D2033N substitution, and inhibitory potency against the mutant was retained. Notably, cabozantinib treatment resulted in a rapid clinical and near-complete radiographic response in this patient.

CONCLUSIONS

These results provide the first example of successful therapeutic intervention with targeted therapy to overcome crizotinib resistance in a ROS1-rearranged cancer. Clin Cancer Res; 22(10); 2351-8. ©2015 AACR.

Age-related mutations and chronic myelomonocytic leukemia

Chronic myelomonocytic leukemia (CMML) is a hematologic malignancy nearly confined to the elderly. Previous studies to determine incidence and prognostic significance of somatic mutations in CMML have relied on candidate gene sequencing, although an unbiased mutational search has not been conducted. As many of the genes commonly mutated in CMML were recently associated with age-related clonal hematopoiesis (ARCH) and aged hematopoiesis is characterized by a myelomonocytic differentiation bias, we hypothesized that CMML and aged hematopoiesis may be closely related. We initially established the somatic mutation landscape of CMML by whole exome sequencing followed by gene-targeted validation. Genes mutated in ⩾10% of patients were SRSF2, TET2, ASXL1, RUNX1, SETBP1, KRAS, EZH2, CBL and NRAS, as well as the novel CMML genes FAT4, ARIH1, DNAH2 and CSMD1. Most CMML patients (71%) had mutations in ⩾2 ARCH genes and 52% had ⩾7 mutations overall. Higher mutation burden was associated with shorter survival. Age-adjusted population incidence and reported ARCH mutation rates are consistent with a model in which clinical CMML ensues when a sufficient number of stochastically acquired age-related mutations has accumulated, suggesting that CMML represents the leukemic conversion of the myelomonocytic-lineage-biased aged hematopoietic system.

shRNA library screening identifies nucleocytoplasmic transport as a mediator of BCR-ABL1 kinase-independent resistance

The mechanisms underlying tyrosine kinase inhibitor (TKI) resistance in chronic myeloid leukemia (CML) patients lacking explanatory BCR-ABL1 kinase domain mutations are incompletely understood. To identify mechanisms of TKI resistance that are independent of BCR-ABL1 kinase activity, we introduced a lentiviral short hairpin RNA (shRNA) library targeting ∼5000 cell signaling genes into K562(R), a CML cell line with BCR-ABL1 kinase-independent TKI resistance expressing exclusively native BCR-ABL1. A customized algorithm identified genes whose shRNA-mediated knockdown markedly impaired growth of K562(R) cells compared with TKI-sensitive controls. Among the top candidates were 2 components of the nucleocytoplasmic transport complex, RAN and XPO1 (CRM1). shRNA-mediated RAN inhibition or treatment of cells with the XPO1 inhibitor, KPT-330 (Selinexor), increased the imatinib sensitivity of CML cell lines with kinase-independent TKI resistance. Inhibition of either RAN or XPO1 impaired colony formation of CD34(+) cells from newly diagnosed and TKI-resistant CML patients in the presence of imatinib, without effects on CD34(+) cells from normal cord blood or from a patient harboring the BCR-ABL1(T315I) mutant. These data implicate RAN in BCR-ABL1 kinase-independent imatinib resistance and show that shRNA library screens are useful to identify alternative pathways critical to drug resistance in CML.

A coiled-coil mimetic intercepts BCR-ABL1 dimerization in native and kinase-mutant chronic myeloid leukemia

Targeted therapy of chronic myeloid leukemia is currently based on small-molecule inhibitors that directly bind the tyrosine kinase domain of BCR-ABL1. This strategy has generally been successful, but is subject to drug resistance due to point mutations in the kinase domain. Kinase activity requires transactivation of BCR-ABL1 following an oligomerization event, which is mediated by the coiled-coil (CC) domain at the N-terminus of the protein. Here, we describe a rationally engineered mutant version of the CC domain, called CC^mut3, which interferes with BCR-ABL1 oligomerization and promotes apoptosis in BCR-ABL1-expressing cells, regardless of kinase domain mutation status. CC^mut3 exhibits strong pro-apoptotic and anti-proliferative activity in cell lines expressing native BCR-ABL1, single kinase domain mutant BCR-ABL1 (E255V and T315I) or compound mutant BCR-ABL1 (E255V/T315I). Moreover, CC^mut3 inhibits colony formation by primary CML CD34⁺ cells ex vivo, including a sample expressing the T315I mutant. These data suggest that targeting BCR-ABL1 with CC mutants may provide a novel alternative strategy for treating patients with resistance to current targeted therapies.

BCR-ABL1 compound mutations combining key kinase domain positions confer clinical resistance to ponatinib in Ph chromosome-positive leukemia

Ponatinib is the only currently approved tyrosine kinase inhibitor (TKI) that suppresses all BCR-ABL1 single mutants in Philadelphia chromosome-positive (Ph(+)) leukemia, including the recalcitrant BCR-ABL1(T315I) mutant. However, emergence of compound mutations in a BCR-ABL1 allele may confer ponatinib resistance. We found that clinically reported BCR-ABL1 compound mutants center on 12 key positions and confer varying resistance to imatinib, nilotinib, dasatinib, ponatinib, rebastinib, and bosutinib. T315I-inclusive compound mutants confer high-level resistance to TKIs, including ponatinib. In vitro resistance profiling was predictive of treatment outcomes in Ph(+) leukemia patients. Structural explanations for compound mutation-based resistance were obtained through molecular dynamics simulations. Our findings demonstrate that BCR-ABL1 compound mutants confer different levels of TKI resistance, necessitating rational treatment selection to optimize clinical outcome.

Combined STAT3 and BCR-ABL1 inhibition induces synthetic lethality in therapy-resistant chronic myeloid leukemia

Mutations in the BCR-ABL1 kinase domain are an established mechanism of tyrosine kinase inhibitor (TKI) resistance in Philadelphia chromosome-positive leukemia, but fail to explain many cases of clinical TKI failure. In contrast, it is largely unknown why some patients fail TKI therapy despite continued suppression of BCR-ABL1 kinase activity, a situation termed BCRABL1 kinase-independent TKI resistance. Here, we identified activation of signal transducer and activator of transcription 3 (STAT3) by extrinsic or intrinsic mechanisms as an essential feature of BCR-ABL1 kinase-independent TKI resistance. By combining synthetic chemistry, in vitro reporter assays, and molecular dynamics-guided rational inhibitor design and high-throughput screening, we discovered BP-5-087, a potent and selective STAT3 SH2 domain inhibitor that reduces STAT3 phosphorylation and nuclear transactivation. Computational simulations, fluorescence polarization assays, and hydrogen-deuterium exchange assays establish direct engagement of STAT3 by BP-5-087 and provide a high-resolution view of the STAT3 SH2 domain/BP-5-087 interface. In primary cells from CML patients with BCR-ABL1 kinase-independent TKI resistance, BP-5-087 (1.0 μM) restored TKI sensitivity to therapy-resistant CML progenitor cells, including leukemic stem cells (LSCs). Our findings implicate STAT3 as a critical signaling node in BCR-ABL1 kinase-independent TKI resistance, and suggest that BP-5-087 has clinical utility for treating malignancies characterized by STAT3 activation.

Threshold levels of ABL tyrosine kinase inhibitors retained in chronic myeloid leukemia cells determine their commitment to apoptosis

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.

Pushing the limits of targeted therapy in chronic myeloid leukaemia

Tyrosine kinase inhibitor (TKI) therapy targeting the BCR-ABL1 kinase is effective against chronic myeloid leukaemia (CML), but is not curative for most patients. Minimal residual disease (MRD) is thought to reside in TKI-insensitive leukaemia stem cells (LSCs) that are not fully addicted to BCR-ABL1. Recent conceptual advances in both CML biology and therapeutic intervention have increased the potential for the elimination of CML cells, including LSCs, through simultaneous inhibition of BCR-ABL1 and other newly identified, crucial targets.

Nilotinib and MEK inhibitors induce synthetic lethality through paradoxical activation of RAF in drug-resistant chronic myeloid leukemia

We show that imatinib, nilotinib, and dasatinib possess weak off-target activity against RAF and, therefore, drive paradoxical activation of BRAF and CRAF in a RAS-dependent manner. Critically, because RAS is activated by BCR-ABL, in drug-resistant chronic myeloid leukemia (CML) cells, RAS activity persists in the presence of these drugs, driving paradoxical activation of BRAF, CRAF, MEK, and ERK, and leading to an unexpected dependency on the pathway. Consequently, nilotinib synergizes with MEK inhibitors to kill drug-resistant CML cells and block tumor growth in mice. Thus, we show that imatinib, nilotinib, and dasatinib drive paradoxical RAF/MEK/ERK pathway activation and have uncovered a synthetic lethal interaction that can be used to kill drug-resistant CML cells in vitro and in vivo.