SU14813: a novel multiple receptor tyrosine kinase inhibitor with potent antiangiogenic and antitumor activity.

Receptor tyrosine kinases (RTK), such as vascular endothelial growth factor receptor (VEGFR), platelet-derived growth factor receptor (PDGFR), stem cell factor receptor (KIT), and fms-like tyrosine kinase 3 (FLT3), are expressed in malignant tissues and act in concert, playing diverse and major roles in angiogenesis, tumor growth, and metastasis. With the exception of a few malignancies, seemingly driven by a single genetic mutation in a signaling protein, most tumors are the product of multiple mutations in multiple aberrant signaling pathways. Consequently, simultaneous targeted inhibition of multiple signaling pathways could be more effective than inhibiting a single pathway in cancer therapies. Such a multitargeted strategy has recently been validated in a number of preclinical and clinical studies using RTK inhibitors with broad target selectivity. SU14813, a small molecule identified from the same chemical library used to isolate sunitinib, has broad-spectrum RTK inhibitory activity through binding to and inhibition of VEGFR, PDGFR, KIT, and FLT3. In cellular assays, SU14813 inhibited ligand-dependent and ligand-independent proliferation, migration, and survival of endothelial cells and/or tumor cells expressing these targets. SU14813 inhibited VEGFR-2, PDGFR-beta, and FLT3 phosphorylation in xenograft tumors in a dose- and time-dependent fashion. The plasma concentration required for in vivo target inhibition was estimated to be 100 to 200 ng/mL. Used as monotherapy, SU14813 exhibited broad and potent antitumor activity resulting in regression, growth arrest, or substantially reduced growth of various established xenografts derived from human or rat tumor cell lines. Treatment in combination with docetaxel significantly enhanced both the inhibition of primary tumor growth and the survival of the tumor-bearing mice compared with administration of either agent alone. In summary, SU14813 inhibited target RTK activity in vivo in association with reduction in angiogenesis, target RTK-mediated proliferation, and survival of tumor cells, leading to broad and potent antitumor efficacy. These data support the ongoing phase I clinical evaluation of SU14813 in advanced malignancies.

A phase 1 study of SU11248 in the treatment of patients with refractory or resistant acute myeloid leukemia (AML) or not amenable to conventional therapy for the disease.

Fifteen patients with refractory AML were treated in a phase 1 study with SU11248, an oral kinase inhibitor of fms-like tyrosine kinase 3 (Flt3), Kit, vascular endothelial growth factor (VEGF), and platelet-derived growth factor (PDGF) receptors. Separate cohorts of patients received SU11248 for 4-week cycles followed by either a 2- or a 1-week rest period. At the starting dose level of 50 mg (n = 13), no dose-limiting toxicities were observed. The most frequent grade 2 toxicities were edema, fatigue, and oral ulcerations. Two fatal bleedings possibly related to the disease, one from a concomitant lung cancer and one cerebral bleeding, were observed. At the 75 mg dose level (n = 2), one case each of grade 4 fatigue, hypertension, and cardiac failure was observed, and this dose level was abandoned. All patients with FLT3 mutations (n = 4) had morphologic or partial responses compared with 2 of 10 evaluable patients with wild-type FLT3. Responses, although longer in patients with mutated FLT3, were of short duration. Reductions of cellularity and numbers of Ki-67(+), phospho-Kit(+), phospho-kinase domain-containing receptor-positive (phospho-KDR(+)), phospho-signal transducer and activator of transcription 5-positive (phospho-STAT5(+)), and phospho-Akt(+) cells were detected in bone marrow histology analysis. In summary, monotherapy with SU11248 induced partial remissions of short duration in acute myeloid leukemia (AML) patients. Further evaluation of this compound, for example in combination with chemotherapy, is warranted.

Synergistic effect of SU11248 with cytarabine or daunorubicin on FLT3 ITD-positive leukemic cells.

Fetal liver tyrosine kinase 3 internal tandem duplication (FLT3 ITD) mutations are the most common molecular abnormality associated with adult acute myeloid leukemia (AML). To exploit this molecular target, a number of potent and specific FLT3 kinase inhibitors have been developed and are currently being tested in early phase clinical trials of patients with refractory AML. To explore the efficacy of combining a FLT3 inhibitor with standard AML chemotherapy drugs, we tested the effect of combining the FLT3 inhibitor SU11248 with cytarabine or daunorubicin on the proliferation and survival of cell lines expressing either mutant (FLT3 ITD or FLT3 D835V) or wild-type (WT) FLT3. SU11248 had additive-to-synergistic inhibitory effects on FLT3-dependent leukemic cell proliferation when combined with cytarabine or daunorubicin. The synergistic interaction of SU11248 and the traditional antileukemic agents was more pronounced for induction of apoptosis. SU11248 inhibited the proliferation of primary AML myeloblasts expressing mutant FLT3 ITD but not WT FLT3 protein. Combining SU11248 and cytarabine synergistically inhibited the proliferation of primary AML myeloblasts expressing FLT3 ITD but not WT FLT3 protein. These data suggest that the addition of potent FLT3 inhibitors such as SU11248 to AML chemotherapy regimens could result in improved treatment results.

Effects of SU5416, a small molecule tyrosine kinase receptor inhibitor, on FLT3 expression and phosphorylation in patients with refractory acute myeloid leukemia.

Acute myeloid leukemia (AML) is associated with dysregulated hematopoietic cell proliferation and increased bone marrow angiogenesis, each regulated by signaling through receptor tyrosine kinases (RTKs). SU5416 is a small molecule inhibitor of VEGF receptors, c-kit and FLT3 and therefore provides a novel opportunity to target both angiogenesis and proliferation in AML. SU5416 was assessed in a phase II hematological malignancy trial in the US, where partial responses were observed in two of 33 patients. Since AML provides a unique platform to evaluate mechanism of action of small molecule inhibitors, investigation of the effect of SU5416 on FLT3 expression and phosphorylation in blood and bone marrow was an additional focus of this trial. Phosphorylated FLT3 was detected by immunoprecipitation/Western analysis in peripheral blood samples from 17 of 22 patients, and seven exhibited strong inhibition of phosphorylation immediately following a 1h SU5416 infusion, demonstrating that SU5416 can modulate RTK phosphorylation in humans. Although no clear correlation with clinical response was observed, analysis of patient plasma drug levels suggested that a threshold SU5416 concentration of 15 microM was associated with FLT3 inhibition. This observation was supported by data from an ex vivo model where AML cells were spiked into human blood, established to mimic the clinical setting and enable more rigorous analysis of effect of SU5416. In addition, FLT3 protein levels were downregulated in patient bone marrow samples, analyzed by an RIA assay. To identify putative predictors of response, patient plasma was analyzed for levels of secreted ligands of SU5416 targets; SCF and FLT3 ligand. Baseline levels of SCF in patients with stable or progressive disease were significantly higher than those in normal donors, whereas FLT3 ligand levels in patients who exhibited progressive disease were significantly lower than those in normal donors. The translational and clinical analyses described in this report provide some insights into the mechanism and duration of action of SU5416.

An innovative phase I clinical study demonstrates inhibition of FLT3 phosphorylation by SU11248 in acute myeloid leukemia patients.

PURPOSE: Obtaining direct and rapid proof of molecular activity in early clinical trials is critical for optimal clinical development of novel targeted therapies. SU11248 is an oral multitargeted kinase inhibitor with selectivity for fms-related tyrosine kinase 3/Flk2 (FLT3), platelet-derived growth factor receptor alpha/beta, vascular endothelial growth factor receptor 1/2, and KIT receptor tyrosine kinases. FLT3 is a promising candidate for targeted therapy in acute myeloid leukemia (AML), because activating mutations occur in up to 30% of patients. We conducted an innovative single-dose clinical study with a primary objective to demonstrate inhibition of FLT3 phosphorylation by SU11248 in AML. EXPERIMENTAL DESIGN: Twenty-nine AML patients each received a single dose of SU11248, escalated from 50 to 350 mg, in increments of 50 mg and cohorts of three to six patients. FLT3 phosphorylation and plasma pharmacokinetics were evaluated at seven time points over 48 h after SU11248 administration, and FLT3 genotype was determined. Study drug-related adverse events occurred in 31% of patients, mainly grade 1 or 2 diarrhea and nausea, at higher dose levels. RESULTS: Inhibition of FLT3 phosphorylation was apparent in 50% of FLT3-wild-type (WT) patients and in 100% of FLT3-mutant patients. FLT3 internal tandem duplication (ITD) mutants showed increased sensitivity relative to FLT3-WT, consistent with preclinical predictions. The primary end point, strong inhibition of FLT3 phosphorylation in >50% patients, was reached in 200 mg and higher dose cohorts. Downstream signaling pathways were also inhibited; signal transducer and activator of transcription 5 (STAT5) was reduced primarily in internal tandem duplication patients and at late time points in FLT3-WT patients, whereas extracellular signal-regulated kinase (ERK) activity was reduced in the majority of patients, independent of FLT3 inhibition. CONCLUSIONS: This novel translational study bridges preclinical models to the patient setting and provides the first evidence of anti-FLT3 activity in patients. Proof of target inhibition accomplishes a crucial milestone in the development of novel oncology therapeutics.

Expression profiling of blood samples from an SU5416 Phase III metastatic colorectal cancer clinical trial: a novel strategy for biomarker identification.

BACKGROUND: Microarray-based gene expression profiling is a powerful approach for the identification of molecular biomarkers of disease, particularly in human cancers. Utility of this approach to measure responses to therapy is less well established, in part due to challenges in obtaining serial biopsies. Identification of suitable surrogate tissues will help minimize limitations imposed by those challenges. This study describes an approach used to identify gene expression changes that might serve as surrogate biomarkers of drug activity. METHODS: Expression profiling using microarrays was applied to peripheral blood mononuclear cell (PBMC) samples obtained from patients with advanced colorectal cancer participating in a Phase III clinical trial. The PBMC samples were harvested pre-treatment and at the end of the first 6-week cycle from patients receiving standard of care chemotherapy or standard of care plus SU5416, a vascular endothelial growth factor (VEGF) receptor tyrosine kinase (RTK) inhibitor. Results from matched pairs of PBMC samples from 23 patients were queried for expression changes that consistently correlated with SU5416 administration. RESULTS: Thirteen transcripts met this selection criterion; six were further tested by quantitative RT-PCR analysis of 62 additional samples from this trial and a second SU5416 Phase III trial of similar design. This method confirmed four of these transcripts (CD24, lactoferrin, lipocalin 2, and MMP-9) as potential biomarkers of drug treatment. Discriminant analysis showed that expression profiles of these 4 transcripts could be used to classify patients by treatment arm in a predictive fashion. CONCLUSIONS: These results establish a foundation for the further exploration of peripheral blood cells as a surrogate system for biomarker analyses in clinical oncology studies.

SU5416, a small molecule tyrosine kinase receptor inhibitor, has biologic activity in patients with refractory acute myeloid leukemia or myelodysplastic syndromes.

Increased bone marrow angiogenesis and vascular endothelial growth factor (VEGF) levels are adverse prognostic features in patients with acute myeloid leukemia (AML) or myelodysplastic syndromes (MDSs). VEGF is a soluble circulating angiogenic molecule that stimulates signaling via receptor tyrosine kinases (RTKs), including VEGF receptor 2 (VEGFR-2). AML blasts may express VEGFR-2, c-kit, and FLT3. SU5416 is a small molecule RTK inhibitor (RTKI) of VEGFR-2, c-kit, and both wild-type and mutant FLT3. A multicenter phase 2 study of SU5416 was conducted in patients with refractory AML or MDS. For a median of 9 weeks (range, 1-55 weeks), 55 patients (33 AML: 10 [30%] primary refractory, 23 [70%] relapsed; 22 MDS: 15 [68%] relapsed) received 145 mg/m2 SU5416 twice weekly intravenously. Grade 3 or 4 drug-related toxicities included headaches (14%), infusion-related reactions (11%), dyspnea (14%), fatigue (7%), thrombotic episodes (7%), bone pain (5%), and gastrointestinal disturbance (4%). There were 11 patients (20%) who did not complete 4 weeks of therapy (10 progressive disease, 1 adverse event); 3 patients (5%) who achieved partial responses; and 1 (2%) who achieved hematologic improvement. Single agent SU5416 had biologic and modest clinical activity in refractory AML/MDS. Overall median survival was 12 weeks in AML patients (range, 4-41 weeks) and not reached in MDS patients. Most observed toxicities were attributable to drug formulation (polyoxyl 35 castor oil or hyperosmolarity of the SU5416 preparation). Studies of other RTKI and/or other antiangiogenic approaches, with correlative studies to examine biologic effects, may be warranted in patients with AML/MDS.

SU11248 is a novel FLT3 tyrosine kinase inhibitor with potent activity in vitro and in vivo.

FLT3 (fms-related tyrosine kinase/Flk2/Stk-2) is a receptor tyrosine kinase (RTK) primarily expressed on hematopoietic cells. In blasts from acute myelogenous leukemia (AML) patients, 2 classes of FLT3 activating mutations have been identified: internal tandem duplication (ITD) mutations in the juxtamembrane domain (25%-30% of patients) and point mutations in the kinase domain activation loop (7%-8% of patients). FLT3-ITD mutations are the most common molecular defect identified in AML and have been shown to be an independent prognostic factor for decreased survival. FLT3-ITD is therefore an attractive molecular target for therapy. SU11248 is a recently described selective inhibitor with selectivity for split kinase domain RTKs, including platelet-derived growth factor receptors, vascular endothelial growth factor receptors, and KIT. We show that SU11248 also has potent activity against wild-type FLT3 (FLT3-WT), FLT3-ITD, and FLT3 activation loop (FLT3-Asp835) mutants in phosphorylation assays. SU11248 inhibits FLT3-driven phosphorylation and induces apoptosis in vitro. In addition, SU11248 inhibits FLT3-induced VEGF production. The in vivo efficacy of SU11248 was investigated in 2 FLT3-ITD models: a subcutaneous tumor xenograft model and a bone marrow engraftment model. We show that SU11248 (20 mg/kg/d) dramatically regresses FLT3-ITD tumors in the subcutaneous tumor xenograft model and prolongs survival in the bone marrow engraftment model. Pharmacokinetic and pharmacodynamic analysis in subcutaneous tumors showed that a single administration of an efficacious drug dose potently inhibits FLT3-ITD phosphorylation for up to 16 hours following a single dose. These results suggest that further exploration of SU11248 activity in AML patients is warranted.

A model of APL with FLT3 mutation is responsive to retinoic acid and a receptor tyrosine kinase inhibitor, SU11657.

The PML-RAR alpha fusion protein is central to the pathogenesis of acute promyelocytic leukemia (APL). Expression of this protein in transgenic mice initiates myeloid leukemias with features of human APL, but only after a long latency (8.5 months in MRP8 PML-RARA mice). Thus, additional changes contribute to leukemic transformation. Activating mutations of the FLT3 receptor tyrosine kinase are common in human acute myeloid leukemias and are frequent in human APL. To assess how activating mutations of FLT3 contribute to APL pathogenesis and impact therapy, we used retroviral transduction to introduce an activated allele of FLT3 into control and MRP8 PML-RARA transgenic bone marrow. Activated FLT3 cooperated with PML-RAR alpha to induce leukemias in 62 to 299 days (median latency, 105 days). In contrast to the leukemias that arose spontaneously in MRP8 PML-RARA mice, the activated FLT3/PML-RAR alpha leukemias were characterized by leukocytosis, similar to human APL with FLT3 mutations. Cytogenetic analysis revealed clonal karyotypic abnormalities, which may contribute to pathogenesis or progression. SU11657, a selective, oral, multitargeted tyrosine kinase inhibitor that targets FLT3, cooperated with all-trans retinoic acid to rapidly cause regression of leukemia. Our results suggest that the acquisition of FLT3 mutations by cells with a pre-existing t(15;17) is a frequent pathway to the development of APL. Our findings also indicate that APL patients with FLT3 mutations may benefit from combination therapy with all-trans retinoic acid plus an FLT3 inhibitor.

SU11248 inhibits tumor growth and CSF-1R-dependent osteolysis in an experimental breast cancer bone metastasis model.

The aim of the study was to investigate inhibitory effects of the receptor tyrosine kinase (RTK) inhibitor SU11248 against CSF-1R and osteoclast (OC) formation. We developed an in vivo model of breast cancer metastasis to evaluate efficacy of SU11248 against tumor growth and tumor-induced osteolysis in bone. The in vitro effects of SU11248 on CSF-1R phosphorylation, OC formation and function were evaluated. Effects on 435/HAL-Luc tumor growth in bone were monitored by in vivo bioluminescence imaging (BLI), and inhibition of osteolysis was evaluated by measurement of serum pyridinoline (PYD) concentration and histology. Phosphorylation of the receptor for M-CSF (CSF-1R) expressed by NIH3T3 cells was inhibited by SU11248 with an IC50 of 50-100 nM, consistent with CSF-1R belonging to the class III split kinase domain RTK family. The early M-CSF-dependent phase of in vitro murine OC development and function were inhibited by SU11248 at 10-100 nM. In vivo inhibition of osteolysis was confirmed by significant lowering of serum PYD levels following SU11248 treatment of tumor-bearing mice (P = 0.047). Using BLI, SU11248 treatment at 40 mg/kg/day for 21 days showed 64% inhibition of tumor growth in bone (P = 0.006), and at 80 mg/kg/day showed 89% inhibition (P = 0.001). Collectively, these data suggest that SU11248 may be an effective and tolerated therapy to inhibit growth of breast cancer bone metastases, with the additional advantage of inhibiting tumor-associated osteolysis.

SU5416 and SU5614 inhibit kinase activity of wild-type and mutant FLT3 receptor tyrosine kinase.

Internal tandem duplication (ITD) in the juxtamembrane portion of Fms-like tyrosine kinase 3 (FLT3), a type III receptor tyrosine kinase (RTK), is the most common molecular defect associated with acute myeloid leukemia (AML). The high prevalence of this activating mutation makes it a potential target for molecularly based therapy. Indolinone tyrosine kinase inhibitors have known activity against KIT, another member of the type III RTK family. Given the conserved homology between members of this family, we postulated that the activity of some KIT inhibitors would extend to FLT3. We used various leukemic cell lines (BaF3, MV 4-11, RS 4;11) to test the activity of indolinone compounds against the FLT3 kinase activity of both wild-type (WT) and ITD isoforms. Both SU5416 and SU5614 were capable of inhibiting autophosphorylation of ITD and WT FLT3 (SU5416 concentration that inhibits 50% [IC(50)], 100 nM; and SU5614 IC(50) 10 nM). FLT3-dependent activation of the downstream signaling proteins mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription 5 (STAT5) was also inhibited by treatment in the same concentration ranges. FLT3 inhibition by SU5416 and SU5614 resulted in reduced proliferation (IC(50), 250 nM and 100 nM, respectively) and induction of apoptosis of FLT3 ITD-positive leukemic cell lines. Treatment of these cells with an alternative growth factor (granulocyte-macrophage colony-stimulating factor [GM-CSF]) restored MAPK signaling and cellular proliferation, demonstrating specificity of the observed inhibitory effects. We conclude that SU5416 and SU5614 are potent inhibitors of FLT3. Our finding that inhibition of FLT3 induces apoptosis of leukemic cells supports the feasibility of targeting FLT3 as a novel treatment strategy for AML.

A receptor for the heterodimeric cytokine IL-23 is composed of IL-12Rbeta1 and a novel cytokine receptor subunit, IL-23R.

IL-23 is a heterodimeric cytokine composed of the IL-12p40 "soluble receptor" subunit and a novel cytokine-like subunit related to IL-12p35, termed p19. Human and mouse IL-23 exhibit some activities similar to IL-12, but differ in their capacities to stimulate particular populations of memory T cells. Like IL-12, IL-23 binds to the IL-12R subunit IL-12Rbeta1. However, it does not use IL-12Rbeta2. In this study, we identify a novel member of the hemopoietin receptor family as a subunit of the receptor for IL-23, "IL-23R." IL-23R pairs with IL-12Rbeta1 to confer IL-23 responsiveness on cells expressing both subunits. Human IL-23, but not IL-12, exhibits detectable affinity for human IL-23R. Anti-IL-12Rbeta1 and anti-IL-23R Abs block IL-23 responses of an NK cell line and Ba/F3 cells expressing the two receptor chains. IL-23 activates the same Jak-stat signaling molecules as IL-12: Jak2, Tyk2, and stat1, -3, -4, and -5, but stat4 activation is substantially weaker and different DNA-binding stat complexes form in response to IL-23 compared with IL-12. IL-23R associates constitutively with Jak2 and in a ligand-dependent manner with stat3. The ability of cells to respond to IL-23 or IL-12 correlates with expression of IL-23R or IL-12Rbeta2, respectively. The human IL-23R gene is on human chromosome 1 within 150 kb of IL-12Rbeta2.