Targeting of the AXL receptor tyrosine kinase by small molecule inhibitor leads to AXL cell surface accumulation by impairing the ubiquitin-dependent receptor degradation.

BACKGROUND: Overexpression of AXL receptor tyrosine kinase (AXL) in various human cancers correlates with reduced patients overall survival and resistance to first line therapies. Therefore, several AXL tyrosine kinase inhibitors (TKIs) are currently under clinical evaluation. RESULTS: AXL TKI BMS777607 treatment increased AXL protein levels after 24 h as observed by Western blot and flow cytometry analysis. Mechanistically, this inhibition-induced AXL cell surface accumulation was neither associated with epigenetic modifications, nor altered transcriptional and translational regulation. Further, we saw no impact on glycosylation and receptor shedding by α-secretases. However, we observed that BMS777607 increased the glycosylated 140 kDa AXL protein abundance, which was impaired in the kinase dead mutant AXL (K567R). We demonstrated that AXL kinase activity and subsequent kinase phosphorylation is necessary for GAS6-dependent receptor internalization and degradation. Blocking of kinase function by BMS777607 resulted in ubiquitination prohibition, impaired internalization and subsequent cell surface accumulation. Subsequently, AXL cell surface accumulation was accompanied by increased proliferation of 3D-Speroids induced by low µM levels of BMS777607 treatment. CONCLUSION: Our data suggest a re-evaluation of anti-AXL clinical protocols due to possible feedback loops and resistance formation to targeted AXL therapy. An alternative strategy to circumvent feedback loops for AXL targeting therapies may exist in linkage of AXL TKIs to a degradation machinery recruiting unit, as already demonstrated with PROTACs for EGFR, HER2, and c-Met. This might result in a sustained inhibition and depletion of the AXL from tumor cell surface and enhance the efficacy of targeted anti-AXL therapies in the clinic.

Discovery of N-[4-(Quinolin-4-yloxy)phenyl]benzenesulfonamides as Novel AXL Kinase Inhibitors.

The overexpression of AXL kinase has been described in many types of cancer. Due to its role in proliferation, survival, migration, and resistance, AXL represents a promising target in the treatment of the disease. In this study we present a novel compound family that successfully targets the AXL kinase. Through optimization and detailed SAR studies we developed low nanomolar inhibitors, and after further biological characterization we identified a potent AXL kinase inhibitor with favorable pharmacokinetic profile. The antitumor activity was determined in xenograft models, and the lead compounds reduced the tumor size by 40% with no observed toxicity as well as lung metastasis formation by 66% when compared to vehicle control.

Phospho-AXL is widely expressed in glioblastoma and associated with significant shorter overall survival.

Receptor tyrosine kinase AXL (RTK-AXL) is regarded as a suitable target in glioblastoma (GBM) therapy. Since AXL kinase inhibitors are about to get approval for clinical use, patients with a potential benefit from therapy targeting AXL need to be identified. We therefore assessed the expression pattern of Phospho-AXL (P-AXL), the biologically active form of AXL, in 90 patients with newly diagnosed GBM, which was found to be detectable in 67 patients (corresponding to 74%). We identified three main P-AXL expression patterns: i) exclusively in the tumor vasculature (13%), ii) in areas of hypercellularity (35%), or iii) both, in the tumor vasculature and in hypercellular areas of the tumor tissue (52%). Pattern iii) is associated with significant decrease in overall survival (Hazard ratio 2.349, 95% confidence interval 1.069 to 5.162, *p=0.03). Our data suggest that P-AXL may serve as a therapeutic target in the majority of GBM patients.

GAS6-expressing and self-sustaining cancer cells in 3D spheroids activate the PDK-RSK-mTOR pathway for survival and drug resistance.

AXL receptor tyrosine kinase (RTK) inhibition presents a promising therapeutic strategy for aggressive tumor subtypes, as AXL signaling is upregulated in many cancers resistant to first-line treatments. Furthermore, the AXL ligand growth arrest-specific gene 6 (GAS6) has recently been linked to cancer drug resistance. Here, we established that challenging conditions, such as serum deprivation, divide AXL-overexpressing tumor cell lines into non-self-sustaining and self-sustaining subtypes in 3D spheroid culture. Self-sustaining cells are characterized by excessive GAS6 secretion and TAM-PDK-RSK-mTOR pathway activation. In 3D spheroid culture, the activation of the TAM-PDK-RSK-mTOR pathway proves crucial following treatment with AXL/MET inhibitor BMS777607, when the self-sustaining tumor cells react with TAM-RSK hyperactivation and enhanced SRC-AKT-mTOR signaling. Thus, bidirectional activated mTOR leads to enhanced proliferation and counteracts the drug effect. mTOR activation is accompanied by an enhanced AXL expression and hyperphosphorylation following 24 h of treatment with BMS777607. Therefore, we elucidate a double role of AXL that can be assigned to RSK-mTOR as well as SRC-AKT-mTOR pathway activation, specifically through AXL Y779 phosphorylation. This phosphosite fuels the resistance mechanism in 3D spheroids, alongside further SRC-dependent EGFR Y1173 and/or MET Y1349 phosphorylation which is defined by the cell-specific addiction. In conclusion, self-sustenance in cancer cells is based on a signaling synergy, individually balanced between GAS6 TAM-dependent PDK-RSK-mTOR survival pathway and the AXLY779/EGFR/MET-driven SRC-mTOR pathway.

Inhibiting receptor tyrosine kinase AXL with small molecule inhibitor BMS-777607 reduces glioblastoma growth, migration, and invasion in vitro and in vivo.

PURPOSE: Receptor tyrosine kinase AXL (RTK-AXL) is regarded as suitable target in glioma therapy. Here we evaluate the anti-tumoral effect of small molecule inhibitor BMS-777607 targeting RTK-AXL in a preclinical glioma model and provide evidence that RTK-AXL is expressed and phosphorylated in primary and recurrent glioblastoma multiforme (GBM). EXPERIMENTAL DESIGN: We studied the impact of BMS-777607 targeting RTK-AXL in GBM models in vitro and in vivo utilizing glioma cells SF126 and U118MG. Impact on proliferation, apoptosis and angiogenesis was investigated by immunohistochemistry (IHC) and functional assays in vitro and in vivo. Tumor growth was assessed with MRI. Human GBM tissue was analyzed in terms of RTK-AXL phosphorylation by immunoprecipitation and immunohistochemistry. RESULTS: BMS-777607 displayed various anti-cancer effects dependent on increased apoptosis, decreased proliferation and migration in vitro and ex vivo in SF126 and U118 GBM cells. In vivo we observed a 56% tumor volume reduction in SF126 xenografts and remission in U118MG xenografts of more than 91%. The tube formation assay confirmed the anti-angiogenic effect of BMS-777607, which became also apparent in tumor xenografts. IHC of human GBM tissue localized phosphorylated RTK-AXL in hypercellular tumor regions, the migratory front of tumor cells in pseudo-palisades, and in vascular proliferates within the tumor. We further proved RTK-AXL phosphorylation in primary and recurrent disease state. CONCLUSION: Collectively, these data strongly suggest that targeting RTK-AXL with BMS-777607 could represent a novel and potent regimen for the treatment of primary and recurrent GBM.

Combined inhibition of AXL, Lyn and p130Cas kinases block migration of triple negative breast cancer cells.

Blocking the migration of metastatic cancer cells is a major goal in the therapy of cancer. The receptor tyrosine kinase AXL is one of the main triggers for cancer cell migration in neoplasia of breast, colon, skin, thyroid and prostate. In our study we analyzed the effect of AXL inhibition on cell motility and viability in triple negative breast cancer cell lines overexpressing AXL. Thereby we reveal that the compound BMS777607, exhibiting the lowest IC50 values for inhibition of AXL kinase activity in the studied cell lines, attenuates cell motility to a lower extent than the kinase inhibitors MPCD84111 and SKI606. By analyzing the target kinases of MPCD84111 and SKI606 with kinase profiling assays we identified Lyn, a Src family kinase, as a target of both compounds. Knockdown of Lyn and the migration-related CRK-associated substrate (p130Cas), had a significant inhibitory effect on cell migration. Taken together, our findings highlight the importance of combinatorial or multikinase inhibition of non-receptor tyrosine kinases and AXL receptor tyrosine kinase in the therapy of triple negative breast cancer.

Discovery and Biological Evaluation of Novel Dual EGFR/c-Met Inhibitors.

Activating mutations in the epidermal growth factor receptor (EGFR) have been identified in a subset of non-small cell lung cancer (NSCLC), which is one of the leading cancer types worldwide. Application of EGFR tyrosine kinase inhibitors leads to acquired resistance by secondary EGFR mutations or by amplification of the hepatocyte growth factor receptor (c-Met) gene. Although several EGFR and c-Met inhibitors have been reported, potent dual EGFR/c-Met inhibitors, which can overcome this latter resistance mechanism, have hitherto not been published and have not reached clinical trials. In the present study we have identified dual EGFR/c-Met inhibitors and designed novel N-[4-(quinolin-4-yloxy)-phenyl]-biarylsulfonamide derivatives, which inhibit the c-Met receptor and both the wild-type and the activating mutant EGFR kinases in nanomolar range. We have demonstrated by Western blot analysis that compound 10 inhibits EGFR and c-Met phosphorylation at cellular level and effectively inhibits viability of the NSCLC cell lines.

Activation of HER3 interferes with antitumor effects of Axl receptor tyrosine kinase inhibitors: suggestion of combination therapy.

The Axl receptor tyrosine kinase (RTK) has been established as a strong candidate for targeted therapy of cancer. However, the benefits of targeted therapies are limited due to acquired resistance and activation of alternative RTKs. Therefore, we asked if cancer cells are able to overcome targeted Axl therapies. Here, we demonstrate that inhibition of Axl by short interfering RNA or the tyrosine kinase inhibitor (TKI) BMS777607 induces the expression of human epidermal growth factor receptor 3 (HER3) and the neuregulin 1(NRG1)-dependent phosphorylation of HER3 in MDA-MB231 and Ovcar8 cells. Moreover, analysis of 20 Axl-expressing cancer cell lines of different tissue origin indicates a low basal phosphorylation of RAC-α serine/threonine-protein kinase (AKT) as a general requirement for HER3 activation on Axl inhibition. Consequently, phosphorylation of AKT arises as an independent biomarker for Axl treatment. Additionally, we introduce phosphorylation of HER3 as an independent pharmacodynamic biomarker for monitoring of anti-Axl therapy response. Inhibition of cell viability by BMS777607 could be rescued by NRG1-dependent activation of HER3, suggesting an escape mechanism by tumor microenvironment. The Axl-TKI MPCD84111 simultaneously blocked Axl and HER2/3 signaling and thereby prohibited HER3 feedback activation. Furthermore, dual inhibition of Axl and HER2/3 using BMS777607 and lapatinib led to a significant inhibition of cell viability in Axl-expressing MDA-MB231 and Ovcar8 cells. Therefore, we conclude that, in patient cohorts with expression of Axl and low basal activity of AKT, a combined inhibition of Axl and HER2/3 kinase would be beneficial to overcome acquired resistance to Axl-targeted therapies.

The E3 ligase Cbl-b and TAM receptors regulate cancer metastasis via natural killer cells.

Tumour metastasis is the primary cause of mortality in cancer patients and remains the key challenge for cancer therapy. New therapeutic approaches to block inhibitory pathways of the immune system have renewed hopes for the utility of such therapies. Here we show that genetic deletion of the E3 ubiquitin ligase Cbl-b (casitas B-lineage lymphoma-b) or targeted inactivation of its E3 ligase activity licenses natural killer (NK) cells to spontaneously reject metastatic tumours. The TAM tyrosine kinase receptors Tyro3, Axl and Mer (also known as Mertk) were identified as ubiquitylation substrates for Cbl-b. Treatment of wild-type NK cells with a newly developed small molecule TAM kinase inhibitor conferred therapeutic potential, efficiently enhancing anti-metastatic NK cell activity in vivo. Oral or intraperitoneal administration using this TAM inhibitor markedly reduced murine mammary cancer and melanoma metastases dependent on NK cells. We further report that the anticoagulant warfarin exerts anti-metastatic activity in mice via Cbl-b/TAM receptors in NK cells, providing a molecular explanation for a 50-year-old puzzle in cancer biology. This novel TAM/Cbl-b inhibitory pathway shows that it might be possible to develop a 'pill' that awakens the innate immune system to kill cancer metastases.

Integrin-linked kinase is required for epidermal and hair follicle morphogenesis.

Integrin-linked kinase (ILK) links integrins to the actin cytoskeleton and is believed to phosphorylate several target proteins. We report that a keratinocyte-restricted deletion of the ILK gene leads to epidermal defects and hair loss. ILK-deficient epidermal keratinocytes exhibited a pronounced integrin-mediated adhesion defect leading to epidermal detachment and blister formation, disruption of the epidermal-dermal basement membrane, and the translocation of proliferating, integrin-expressing keratinocytes to suprabasal epidermal cell layers. The mutant hair follicles were capable of producing hair shaft and inner root sheath cells and contained stem cells and generated proliferating progenitor cells, which were impaired in their downward migration and hence accumulated in the outer root sheath and failed to replenish the hair matrix. In vitro studies with primary ILK-deficient keratinocytes attributed the migration defect to a reduced migration velocity and an impaired stabilization of the leading-edge lamellipodia, which compromised directional and persistent migration. We conclude that ILK plays important roles for epidermis and hair follicle morphogenesis by modulating integrin-mediated adhesion, actin reorganization, and plasma membrane dynamics in keratinocytes.

ROCK signaling mediates the adoption of different modes of migration and invasion in human mammary epithelial tumor cells.

For the invasive migration of tumor cells, at least two mechanisms are currently discussed: (1) the mesenchymal mode depending on extracellular proteolysis and (2) the proteolysis-independent amoeboid mode depending on the activity of the Rho kinase ROCK. The ability of tumor cells to switch between different modes of motility has been shown to limit the efficiency of agents aimed to reduce invasion. Here we show by combining 2D and 3D migration assays that human mammary tumor cells exhibited a strongly reduced migration velocity as compared to their normal counterparts indicating that high invasiveness is not necessarily correlated with high migratory capacity in 2D assays. This reduced migration was apparently due to significant differences in actin organization, decreased persistence of lamellipodia by 50% and increased cell substrate adhesion. These differences resulted from a 2.5-fold higher activity of ROCK and were mediated by its downstream effectors myosin light chain kinase and cofilin. Thus, inhibition of ROCK activity caused a marked increase in 2D migration efficiency by 40%, without, however, affecting 3D invasion. A massive reduction of invasion by 60% was achieved by the simultaneous inhibition of the ROCK-dependent amoeboid and the extracellular proteolysis-dependent mesenchymal mode. These results may point to a new efficient strategy for blocking tumor cell invasion in vivo.

Determination of features indicating progression in atypical squamous cells with undetermined significance: human papillomavirus typing and DNA ploidy analysis from liquid-based cytologic samples.

BACKGROUND: The Bethesda System of cervical cytologic findings introduced the term ASCUS (atypical squamous cells of undetermined significance) to cover the broad zone separating normal cytomorphology from definitive squamous intraepithelial lesions (SILs). The management of patients with ASCUS is particularly problematic as approximately 10% of ASCUS patients develop SIL and 1 per 1000 develop cervical carcinoma. METHODS: Our aim was to demonstrate the combined use of polymerase chain reaction for human papillomavirus (HPV) typing and laser scanning cytometry for DNA content measurements in the subcategorization of ASCUS cases according to the risk for progression toward cancer. Liquid- based monolayer preparation (ThinPrep, Cytyc, Boston, MA) of the cytologic material was used for cytomorphologic analysis. DNA content measurements using laser scanning cytometry and direct sequencing of HPV using the consensus primers GP5+/GP6+ and MY09/MY11 were performed from the same material. RESULTS: Twelve of the 44 cases (27.2%) with ASCUS carried a high-risk HPV genome whereas only 3 of the 195 normal control cases (1.5%) showed positivity for a high-risk HPV genome. Six of 12 (50%) of the high-risk HPV-positive ASCUS cases presented isolated cells with a DNA content above 5c, whereas cells with a DNA content above 9c were found in 3 of 12 cases (25%) and were exclusively found in combination with high-risk HPV infection. In these three cases, the histologic follow-up resulted in cervical intraepithelial neoplasia (CIN) I (one case) and CIN III (two cases). None of the other ASCUS or normal cases displayed DNA aneuploidy above 9c. They returned to normal cytology (within normal limits/benign cellular changes) in the follow-up. CONCLUSIONS: Human papillomavirus typing and DNA content measurements may delineate a distinct group of ASCUS. Our preliminary data suggest that ASCUS cases with high-risk HPV positivity and with rare cells with abnormally high DNA content represent similar biologic features as high-grade SIL and are at elevated risk to develop cancer.

Human papillomavirus typing and DNA ploidy determination of squamous intraepithelial lesions in liquid-based cytologic samples.

BACKGROUND: Infection by high-risk human papillomavirus (HPV) plays a role in the evolution of cervical carcinoma. Cellular atypia and consecutive DNA content alterations in cytologic samples are consequences of a preexisting viral infection. METHODS: We analyzed the frequency and association of HPV types and the presence of rare cells with abnormally high DNA content. We also evaluated whether these findings support the cytologic diagnosis in 112 routine cases with low and high-grade squamous intraepithelial lesions (LSIL/HSIL) when performed from liquid-based cytologic samples (ThinPrep). For DNA content measurements, laser scanning cytometry was applied and at least 10,000 cells were analyzed. HPV typing was performed by a direct sequencing approach using the consensus primers GP5+/GP6+ and MY09/MY11. RESULTS: Of 112 SIL cases, 110 (98.2%) were HPV positive and 95 (84.8%) had a high-risk type HPV infection. Almost one-half of the cases (46 of 95, 48.4%) with a high-risk HPV infection presented aneuploid squamous cells with greater than 9c DNA content, whereas none of the low-risk HPV-positive or HPV-negative SIL cases showed any aneuploid cells in this range. Although 91.8% of the HSIL cases displayed greater than 9c aneuploid cells, only 7.9% of the LSIL cases were positive for such cells with abnormally high DNA content. CONCLUSIONS: HPV typing and DNA measurements help in the objectivation of cytologic atypia and both can be performed efficiently from the same liquid-based cytologic samples.

Determination of ploidy and steroid receptor status in breast cancer by laser scanning cytometry.

BACKGROUND: Measurements on DNA content and steroid receptor status in breast cancer are of great clinical interest. Objective determination of estrogen and progesterone receptor expression should help to define the lowest levels of positivity still responding to adjuvant antihormonal therapy. For this purpose, a simple protocol for laser scanning cytometry is presented. METHODS: Analysis of 54 routine breast cancer samples was performed by laser scanning cytometry (LSC). To obtain single cell preparations from fresh tumor tissue, slides were prepared using the Cervisoft cytological device. Exact determination of tumor cell DNA content was done by referring to the CD45-positive tissue leukocyte fraction as the internal diploid reference cell population. Steroid receptor-expressing cells were detected by indirect immunolabeling. RESULTS: Indirect immunofluorescence allowed the best quantification of both the estrogen and progesterone receptor-expressing cell fractions by LSC. The number of receptor-expressing cells could be given in percentage. For comparison, the 10% cutoff value was used to determine receptor positivity. CONCLUSION: LSC enabled a simple, reliable, and inexpensive determination of DNA index and steroid receptor expression in breast cancer specimens by objective criteria.