Combined Targeting of the Glutathione and Thioredoxin Antioxidant Systems in Pancreatic Cancer.

Pancreatic ductal adenocarcinoma is characterized by increased generation of reactive oxygen species that can cause lethal oxidative stress. Here, we evaluated the combined inhibition of the glutathione and thioredoxin antioxidant systems in preclinical models of pancreatic ductal adenocarcinoma, using buthionine sulfoximine (BSO) that targets glutathione synthesis, and auranofin that targets thioredoxin recycling. BSO potentiated the cytotoxicity of auranofin and induced lethal oxidative stress in primary pancreatic cancer cells. As assessed by the cellular thermal shift assay, auranofin engaged with thioredoxin reductase 1 in primary cells at concentrations known to induce cell death. Moreover, we used imaging mass cytometry to map the biodistribution of atomic gold in patient-derived xenografts treated with auranofin, and the drug was readily detectable throughout the epithelial and stromal compartments after treatment with a clinically relevant dose. In conclusion, combinatorial treatment with BSO and auranofin could serve as a potential therapeutic strategy in pancreatic ductal adenocarcinoma.

An Iodinated DAPI-Based Reagent for Mass Cytometry.

Multiparametric single-cell analysis has seen dramatic improvements with the introduction of mass cytometry (MC) and imaging mass cytometry (IMC™ ). These technologies expanded the number of biomarkers that can be identified simultaneously by using heavy-isotope-tagged antibody reagents. Small-molecule probes bearing heavy isotopes are emerging as additional useful functional reporters of cellular features. Realizing this, we explored the iodination of DAPI to produce a heavy-atom-substituted derivative of the commonly used fluorescent DNA stain. Although exhibiting a drastically reduced fluorescence emission profile, I-DAPI retains strong binding affinity for DNA. I-DAPI was used to detect cellular DNA in MC and IMC™ assays with comparable efficiency to known Ir-containing DNA intercalators. This work suggests repurposing well-known colorimetric stains through simple reactions could be an effective strategy to develop new, functional MC and IMC™ reagents.

Mobilization of Leukemic Cells Using Plerixafor as Part of a Myeloablative Preparative Regimen for Patients with Acute Myelogenous Leukemia Undergoing Allografting: Assessment of Safety and Tolerability.

Allogeneic hematopoietic cell transplantation (HCT) is potentially curative for acute myelogenous leukemia (AML); however, a major cause of treatment failure is disease relapse. The purpose of this single-center Phase I study was to determine the safety and tolerability of administration of the CXCR4 inhibitor plerixafor (Mozobil; Sanofi Genzyme) along with myeloablative conditioning in patients with AML undergoing allogeneic HCT. The rationale was that plerixafor may mobilize leukemic stem cells, making them more susceptible to the conditioning chemotherapy (registered at ClinicalTrials.gov; identifier NCT01141543). Three patients were enrolled into each of 4 sequential cohorts (12 patients total). Patients in the first cohort received 1 dose of plerixafor (240 µg/kg s.c.) before the first dose of fludarabine and busulfan, and subsequent cohorts received injections before 2, 3, and 4 days of conditioning chemotherapy. The median age at HCT was 49 years (range, 38 to 58 years). All patients engrafted following HCT, with an absolute neutrophil count ≥.5 × 109/L observed at a median of 14 days (range, 11 to 18 days). Adverse events possibly related to plerixafor were transient and not severe. Main adverse events following the injection were nausea and dizziness in 4 patients (33%) and fatigue in 4 patients (33%). Among the 12 patients, 2 patients (17%) relapsed post-HCT and 6 (50%) were alive at the last follow-up. The median follow-up of survivors was 67 months (range, 53 to 82 months). In conclusion, plerixafor administration is safe and well tolerated when included in a myeloablative conditioning regimen for allogeneic HCT for AML. Further study in a larger cohort is warranted for the investigation of the impact of plerixafor on post-allogeneic HCT outcomes.

A phase I trial of the aurora kinase inhibitor, ENMD-2076, in patients with relapsed or refractory acute myeloid leukemia or chronic myelomonocytic leukemia.

ENMD-2076 is a novel, orally-active molecule that inhibits Aurora A kinase, as well as c-Kit, FLT3 and VEGFR2. A phase I study was conducted to determine the maximum tolerated dose (MTD), recommended phase 2 dose (RP2D) and toxicities of ENMD-2076 in patients with acute myeloid leukemia (AML) and chronic myelomonocytic leukemia (CMML). Patients received escalating doses of ENMD-2076 administered orally daily [225 mg (n = 7), 375 mg (n = 6), 325 mg (n = 9), or 275 mg (n = 5)]. Twenty-seven patients were treated (26 AML; 1 CMML-2). The most common non-hematological toxicities of any grade, regardless of association with drug, were fatigue, diarrhea, dysphonia, dyspnea, hypertension, constipation, and abdominal pain. Dose-limiting toxicities (DLTs) consisted of grade 3 fatigue, grade 3 typhilitis, grade 3 syncope and grade 3 QTc prolongation). Of the 16 evaluable patients, one patient achieved a complete remission with incomplete count recovery (CRi), three experienced a morphologic leukemia-free state (MLFS) with a major hematologic improvement in platelets (HI-P), and 5 other patients had a reduction in marrow blast percentage (i.e. 11-65 %). The RP2D in this patient population is 225 mg orally once daily.

Phase shifts to light are altered by antagonists to neuropeptide receptors.

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) is a heterogeneous structure. Two key populations of cells that receive retinal input and are believed to participate in circadian responses to light are cells that contain vasoactive intestinal polypeptide (VIP) and gastrin-releasing peptide (GRP). VIP acts primarily through the VPAC2 receptor, while GRP works primarily through the BB2 receptor. Both VIP and GRP phase shift the circadian clock in a manner similar to light when applied to the SCN, both in vivo and in vitro, indicating that they are sufficient to elicit photic-like phase shifts. However, it is not known if they are necessary signals for light to elicit phase shifts. Here we test the hypothesis that GRP and VIP are necessary signaling components for the photic phase shifting of the hamster circadian clock by examining two antagonists for each of these neuropeptides. The BB2 antagonist PD176252 had no effect on light-induced delays on its own, while the BB2 antagonist RC-3095 had the unexpected effect of significantly potentiating both phase delays and advances. Neither of the VIP antagonists ([d-p-Cl-Phe6, Leu17]-VIP, or PG99-465) altered phase shifting responses to light on their own. When the BB2 antagonist PD176252 and the VPAC2 antagonist PG99-465 were delivered together to the SCN, phase delays were significantly attenuated. These results indicate that photic phase shifting requires participation of either VIP or GRP; phase shifts to light are only impaired when signalling in both pathways are inhibited. Additionally, the unexpected potentiation of light-induced phase shifts by RC-3095 should be investigated further for potential chronobiotic applications.

A phase I trial of ANG1/2-Tie2 inhibitor trebaninib (AMG386) and temsirolimus in advanced solid tumors (PJC008/NCI♯9041).

BACKGROUND: There is crosstalk between the ANG-Tie2 and the PI3K/Akt/mTOR pathways. Combined ANG1/2 and mTOR blockade may have additive anti-cancer activity. The combination of trebananib, an inhibitor of ANG1/2-Tie2 interaction, with temsirolimus was evaluated in patients with advanced solid tumors to determine tolerability, maximum tolerated dose (MTD), and preliminary antitumor activity. METHODS: Patients were enrolled using 3 + 3 design, and were given intravenous trebananib and temsirolimus on Day 1, 8, 15 and 22 of a 28-day cycle. Dose limiting toxicities (DLTs) were evaluated during cycle 1. Peripheral blood was collected for evaluation of Tie2-expressing monocytes (TEMs) and thymidine phosphorylase (TP). Sparse pharmacokinetic (PK) sampling for trebananib drug levels was performed on Day 1 and 8 of cycle 2. RESULTS: Twenty-one patients were enrolled, 6 at dose level (DL) 1, 7 at DL -1, and 8 at DL -2. No effect of temsirolimus on trebananib PK was observed. The most common treatment-related adverse events (AEs) were: fatigue (81 %), edema (62 %), anorexia (57 %), nausea (52 %), rash (43 %) and mucositis (43 %). The most common grade ≥ 3 AEs included lymphopenia (28 %) and fatigue (28 %). The MTD was exceeded at DL-2. Of 18 response evaluable patients, 1 partial response was observed (ER+/HER2-/PIK3CA mutant breast cancer) and 4 patients had prolonged SD ≥ 24 weeks. No correlation with clinical benefit was observed with change in number TEMs or TP expression in TEMs with treatment. CONCLUSIONS: The MTD was exceeded at trebananib 10 mg/kg weekly and temsirolimus 20 mg weekly, with frequent overlapping toxicities including fatigue, edema, and anorexia.

The study of epigenetic mechanisms based on the analysis of histone modification patterns by flow cytometry.

Epigenetic regulation of genes involved in cell growth, survival, or differentiation through histone modifications is an important determinant of cancer development and outcome. The basic science of epigenetics uses analytical tools that, although powerful, are not well suited to the analysis of heterogeneous cell populations found in human cancers, or for monitoring the effects of drugs designed to modulate epigenetic mechanisms in patients. To address this, we selected three clinically relevant histone marks (H3K27me3, H3K9ac, and H3K9me2), modulated their expression levels by in vitro treatments to generate high and low expressing control cells, and tested the relative sensitivity of candidate antibodies to detect the differences in expression levels by flow cytoametry using a range of sample preparation techniques. We identified monoclonal antibodies to all three histone marks that were suitable for flow cytoametry. Staining intensities were reduced with increasing formaldehyde concentration, and were not affected by ionic strength or by alcohol treatment. A protocol suitable for clinical samples was then developed, to allow combined labeling of histone marks and surface antigens while preserving light scatter signals. This was applied to normal donor blood, and to samples obtained from 25 patients with leukemia (predominantly acute myeloid leukemia). Significant cellular heterogeneity in H3K9ac and H3K27me3 staining was seen in normal peripheral blood, but the patterns were very similar between individual donors. In contrast, H3K27me3 in particular showed considerable inter-patient heterogeneity in the leukemia cell populations. Although further refinements are likely needed to fully optimize sample staining protocols, "flow epigenetics" appears to be technically feasible, and to have potential both in basic research, and in clinical application.

Glycogen synthase kinase-3 inhibition sensitizes pancreatic cancer cells to TRAIL-induced apoptosis.

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) induces apoptosis in a variety of cancer cell lines with little or no effect on normal cells. However, its effect is limited as some cancers including pancreatic cancer show de novo resistance to TRAIL induced apoptosis. In this study we report that GSK-3 inhibition using the pharmacologic agent AR-18, enhanced TRAIL sensitivity in a range of pancreatic and prostate cancer cell lines. This sensitization was found to be caspase-dependent, and both pharmacological and genetic knock-down of GSK-3 isoforms resulted in apoptotic features as shown by cleavage of PARP and caspase-3. Elevated levels of reactive oxygen intermediates and disturbance of mitochondrial membrane potential point to a mitochondrial amplification loop for TRAIL-induced apoptosis after GSK-3 inhibition. Consistent with this, overexpression of anti-apoptotic mitochondrial targets such as Bcl-XL, Mcl-1, and Bcl-2 rescued PANC-1 and PPC-1 cells from TRAIL sensitization. However, overexpression of the caspase-8 inhibitor CrmA also inhibited the sensitizing effects of GSK-3 inhibitor, suggesting an additional role for GSK-3 that inhibits death receptor signaling. Acute treatment of mice bearing PANC-1 xenografts with a combination of AR-18 and TRAIL also resulted in a significant increase in apoptosis, as measured by caspase-3 cleavage. Sensitization to TRAIL occurred despite an increase in ß-catenin due to GSK-3 inhibition, suggesting that the approach might be effective even in cancers with dysregulated ß-catenin. These results suggest that GSK-3 inhibitors might be effectively combined with TRAIL for the treatment of pancreatic cancer.

Cytometry of intracellular signaling: from laboratory bench to clinical application.

The analysis of signaling pathways based on combinations of phospho-specific antibodies is now a well-recognized flow cytometry technique. Despite its wide-ranging potential in the fields of biology, industry, and medicine, it has been relatively slow to gain widespread use, and is often considered to be technically challenging. In this chapter, we detail protocols developed in our laboratory for monitoring signaling pathways in blood samples based on combinations of phospho-specific antibodies. Emphasis is placed on clinical application. The assays have a modular design, with a core protocol for whole blood fixation and lysis, a suite of agents that can acutely activate or inhibit the different signaling pathways, and a wide range of phospho-specific antibodies as the readout.

Tenofovir selectively regulates production of inflammatory cytokines and shifts the IL-12/IL-10 balance in human primary cells.

OBJECTIVES: In this study, we aimed to investigate the possible immune modulatory effects of HIV nucleoside reverse transcriptase inhibitors during secondary infections and inflammation, focusing on inflammatory cytokine responses and the interleukin (IL)-12/IL-10 balance. METHODS: We investigated the in vitro effect of tenofovir and zidovudine (AZT) on production of proinflammatory cytokines in monocytes and human peripheral blood mononuclear cells (PBMCs). Stimulation panels included Toll-Like receptor (TLR) ligands; the inflammation mediator tumor necrosis factor-α; and the pathogens cytomegalovirus, Neisseria meningitides, Escherichia coli, and Streptococcus pneumoniae. Cytokine levels were measured using enzyme-linked immunosorbent assay and luminex technology. RNA levels were assessed using real-time polymerase chain reaction. Activity of mitogen-activated protein kinase and NF-κB signaling was evaluated using flow cytometry and multispectral imaging cytometry, respectively. RESULTS: Tenofovir decreased and AZT increased both IL-8 and CCL3 production from monocytes after stimulation with TLR ligands, tumor necrosis factor-α, or live pathogens. Similarly, tenofovir decreased CCL3 levels in human PBMCs. Furthermore, tenofovir strongly decreased induction of IL-10 but increased levels of IL-12. AZT did not affect IL-12 or IL-10 levels. The observed drug-induced changes in cytokine production were independent from transcriptional regulation through the mitogen-activated protein kinase and nuclear factor kappa B pathways. CONCLUSIONS: Our data suggest divergent effects of tenofovir and AZT on proinflammatory responses in monocytes (CCL3 and IL-8) and PBMCs (CCL3). Moreover, tenofovir shifts the IL-10/IL-12 balance after cell stimulation with TLR ligands or infection with live bacteria, thus suggesting that the choice of nucleoside reverse transcriptase inhibitor affects overall inflammation and early immune responses against secondary pathogens.

Potential use of cetrimonium bromide as an apoptosis-promoting anticancer agent for head and neck cancer.

A potential therapeutic agent for human head and neck cancer (HNC), cetrimonium bromide (CTAB), was identified through a cell-based phenotype-driven high-throughput screen (HTS) of 2000 biologically active or clinically used compounds, followed by in vitro and in vivo characterization of its antitumor efficacy. The preliminary and secondary screens were performed on FaDu (hypopharyngeal squamous cancer) and GM05757 (primary normal fibroblasts), respectively. Potential hit compounds were further evaluated for their anticancer specificity and efficacy in combination with standard therapeutics on a panel of normal and cancer cell lines. Mechanism of action, in vivo antitumor efficacy, and potential lead compound optimizations were also investigated. In vitro, CTAB interacted additively with gamma radiation and cisplatin, two standard HNC therapeutic agents. CTAB exhibited anticancer cytotoxicity against several HNC cell lines, with minimal effects on normal fibroblasts; a selectivity that exploits cancer-specific metabolic aberrations. The central mode of cytotoxicity was mitochondria-mediated apoptosis via inhibition of H(+)-ATP synthase activity and mitochondrial membrane potential depolarization, which in turn was associated with reduced intracellular ATP levels, caspase activation, elevated sub-G(1) cell population, and chromatin condensation. In vivo, CTAB ablated tumor-forming capacity of FaDu cells and delayed growth of established tumors. Thus, using an HTS approach, CTAB was identified as a potential apoptogenic quaternary ammonium compound possessing in vitro and in vivo efficacy against HNC models.

Whole blood processing for measurement of signaling proteins by flow cytometry.

Signal transduction pathways link external stimuli with cellular responses, which normally regulate cell proliferation, death, and differentiation. The study of signal transduction was revolutionized through the development of phospho-specific antibodies that recognize proteins only when they are phosphorylated at specific sites. As discussed by Nolan and co-workers (UNIT ), one of the unique features of flow cytometry is its ability to perform correlated measurements of multiple phosphorylation states at the single cell level. This provides insight into the complexity of signaling networks that is not obtained by standard biochemical techniques. Furthermore, in combination with other phenotypic markers, flow cytometry can measure alterations in signaling pathways in subpopulations of cells. This clearly has wide potential for studying disorders of the hematopoietic and immune systems.

Cyproheptadine displays preclinical activity in myeloma and leukemia.

D-cyclins are regulators of cell division that act in a complex with cyclin-dependent kinases to commit cells to a program of DNA replication. D-cyclins are overexpressed in many tumors, including multiple myeloma and leukemia, and contribute to disease progression and chemoresistance. To better understand the role and impact of D-cyclins in hematologic malignancies, we conducted a high throughput screen for inhibitors of the cyclin D2 promoter and identified the drug cyproheptadine. In myeloma and leukemia cells, cyproheptadine decreased expression of cyclins D1, D2, and D3 and arrested these cells in the G(0)/G(1) phase. After D-cyclin suppression, cyproheptadine induced apoptosis in myeloma and leukemia cell lines and primary patient samples preferentially over normal hematopoietic cells. In mouse models of myeloma and leukemia, cyproheptadine inhibited tumor growth without significant toxicity. Cyproheptadine-induced apoptosis was preceded by activation of the mitochondrial pathway of caspase activation and was independent of the drug's known activity as an H1 histamine and serotonin receptor antagonist. Thus, cyproheptadine represents a lead for a novel therapeutic agent for the treatment of malignancy. Because the drug is well tolerated and already approved in multiple countries for clinical use as an antihistamine and appetite stimulant, it could be moved directly into clinical trials for cancer.

Pharmacodynamic monitoring of molecular-targeted agents in the peripheral blood of leukemia patients using flow cytometry.

The introduction of specific, molecular-targeted drugs is radically changing cancer treatment. Pharmacodynamics, which measures drug effects on the host, is key during early-phase clinical trials of novel agents to determine the relations between drug dose and target inhibition as well as measure the downstream effects of target inhibition on the cancer. In this article, we describe the application of flow cytometry to the pharmacodynamic monitoring of molecular-targeted agents in leukemia patients. The methods are based on current clinical flow-cytometry applications, with the addition of phosphospecific antibodies to measure the activation states of intracellular signaling elements and the introduction of techniques that maintain drug-target equilibrium during sample preparation. Using this approach, we successfully showed dose-dependent inhibition of c-Kit during a phase I clinical trial treating acute leukemia patients with the novel agent sorafenib. Further refinements identify considerable interpatient variation in signaling activity within leukemic blast populations, suggesting that an individualized approach to treatment based on flow cytometric monitoring might be advantageous. Improvements in sample turnaround offer the potential to introduce real-time pharmacodynamic monitoring during early-phase clinical trials.

Benzethonium chloride: a novel anticancer agent identified by using a cell-based small-molecule screen.

PURPOSE: This study aims to identify a novel therapeutic agent for head and neck cancer and to evaluate its antitumor efficacy. EXPERIMENTAL DESIGN: A cell-based and phenotype-driven high-throughput screening of approximately 2,400 biologically active or clinically used compounds was done using a tetrazolium-based assay on FaDu (hypopharyngeal squamous cancer) and NIH 3T3 (untransformed mouse embryonic fibroblast) cells, with secondary screening done on C666-1 (nasopharyngeal cancer) and GM05757 (primary normal human fibroblast) lines. The "hit" compound was assayed for efficacy in combination with standard therapeutics on a panel of human cancer cell lines. Furthermore, its mode of action (using transmission electron microscopy and flow cytometry) and its in vivo efficacy (using xenograft models) were evaluated. RESULTS: Benzethonium chloride was identified as a novel cancer-specific compound. For benzethonium (48-hour incubation), the dose required to reduce cell viability by 50% was 3.8 micromol/L in FaDu, 42.2 micromol/L in NIH 3T3, 5.3 micromol/L in C666-1, and 17.0 micromol/L in GM05757. In vitro, this compound did not interfere with the effects of cisplatin, 5-fluorouracil, or gamma-irradiation. Benzethonium chloride induced apoptosis and activated caspases after 12 hours. Loss of mitochondrial membrane potential (DeltaPsiM) preceded cytosolic Ca2+ increase and cell death. In vivo, benzethonium chloride ablated the tumor-forming ability of FaDu cells, delayed the growth of xenograft tumors, and combined additively with local tumor radiation therapy. Evaluation of benzethonium chloride on the National Cancer Institute/NIH Developmental Therapeutics Program 60 human cancer cell lines revealed broad-range antitumor activity. CONCLUSIONS: This high-throughput screening identified a novel antimicrobial compound with significant broad-spectrum anticancer activity.

Constitutive phosphorylation of the S6 ribosomal protein via mTOR and ERK signaling in the peripheral blasts of acute leukemia patients.

OBJECTIVE: The phosphorylation state of the S6 ribosomal protein was measured in the peripheral blasts of 19 newly diagnosed patients with acute leukemia. METHODS: We employed a flow cytometry protocol that enabled correlated measurement of pS6, phosphorylation of extracellular signal-regulated kinase (pERK), and cluster differentiation surface markers. Baseline levels of pS6 in leukemic blasts were compared with those found when the samples were activated using stem cell factor, or exposed to rapamycin, LY294002, or the mitogen-activated protein kinase inhibitor U0126. RESULTS: Results showed a considerable degree of intra- and intertumoral heterogeneity in the constitutive levels of pS6. Rapamycin and LY294002 suppressed pS6 in 10 of 11 cases that showed increased basal levels, consistent with phosphatidylinositol 3 (PI3)-kinase/Akt/mTOR signaling being the predominant upstream signaling pathway. However, in 6 of 11 cases pS6 was also suppressed by U0126, indicating that the ERK pathway can significantly input to pS6. CONCLUSIONS: The constitutive activation of pS6 in acute leukemia patients likely reflects alterations in growth factor signaling that can be mediated by the ERK as well as the mTOR pathway, and could potentially have prognostic significance. As well as identifying aberrant signal transduction in leukemia patients, the flow cytometry methodology has potential for the pharmacodynamic monitoring of novel agents that inhibit ERK or PI3-kinase/Akt/mTOR signaling.

Disruption of the endoplasmic reticulum and increases in cytoplasmic calcium are early events in cell death induced by the natural triterpenoid Asiatic acid.

Triterpenoids are a novel class of compounds being investigated as potential therapeutic agents for the treatment of prostate cancer and other malignancies. Asiatic acid (AA) is a member of the ursane family of triterpenoids and has anticancer activity, but its mechanism of action is not completely understood. To investigate its mechanism of action, PPC-1 prostate cancer cells were treated with AA at increasing concentrations and times. AA induced rapid caspase-dependent and independent cell death that peaked within 8 h of treatment. AA-induced death was associated with early activation of caspases 2, 3, and 8, but not caspase 9. Within 2.5 h of treatment, release of calcium from intracellular stores and dilatation of the endoplasmic reticulum was observed. Thus, disruption of the endoplasmic reticulum and alterations in calcium homeostasis are early events in AA-induced death.

Pharmacodynamic monitoring of BAY 43-9006 (Sorafenib) in phase I clinical trials involving solid tumor and AML/MDS patients, using flow cytometry to monitor activation of the ERK pathway in peripheral blood cells.

BACKGROUND: We previously reported a flow cytometry technique to monitor pharmacodynamic effects of the raf kinase inhibitor BAY 43-9006 based on the ability of phorbol ester (PMA) to phosphorylate extracellular-regulated kinase (ERK) in peripheral blood (Chow et al., Cytometry 2001;46:72-78). In this article, we describe its application to phase I trials of BAY 43-9006 in solid tumor and AML/MDS patients. METHODS: The previously described whole blood lysis method was used to monitor BAY 43-9006 effects on peripheral T-cells of solid tumor patients. A modified whole blood fixation protocol was developed for the AML/MDS trial, using the c-kit ligand stem cell factor (SCF) to activate ERK as an alternative to PMA, and incorporating immunophenotypic markers to identify leukemic blasts. RESULTS: At all dose levels of BAY 43-9006 used to treat solid tumor patients, ERK could be activated by PMA in peripheral T-cells and we were not able to show inhibition of raf kinase. A similar effect was seen in the lymphocytes of AML/MDS patients during treatment with BAY 43-9006. However, we found strong inhibition when ERK was activated via c-kit using SCF. Furthermore, normal donor CD34+ve stem cells were much more sensitive to BAY 43-9006 when ERK was activated by SCF, compared to PMA. CONCLUSIONS: These findings support the further development of flow cytometry applications to monitor signal transduction inhibitors during early phase clinical trials.