Targeting Src signaling in metastatic bone disease

Enhanced Anticancer Activity of Nanoformulation of Dasatinib against Triple-Negative Breast Cancer

Triple negative breast cancer (TNBC) is the most aggressive breast cancer accounting for around 15% of identified breast cancer cases. TNBC lacks human epidermal growth factor receptor 2 (HER2) amplification, is hormone independent estrogen (ER) and progesterone receptors (PR) negative, and is not reactive to current targeted therapies. Existing treatment relies on chemotherapeutic treatment, but in spite of an initial response to chemotherapy, the inception of resistance and relapse is unfortunately common. Dasatinib is an approved second-generation inhibitor of multiple tyrosine kinases, and literature data strongly support its use in the management of TNBC. However, dasatinib binds to plasma proteins and undergoes extensive metabolism through oxidation and conjugation. To protect dasatinib from fast pharmacokinetic degradation and to prolong its activity, it was encapsulated on poly(styrene-co-maleic acid) (SMA) micelles. The obtained SMA-dasatinib nanoparticles (NPs) were evaluated for their physicochemical properties, in vitro antiproliferative activity in different TNBC cell lines, and in vivo anticancer activity in a syngeneic model of breast cancer. Obtained results showed that SMA-dasatinib is more potent against 4T1 TNBC tumor growth in vivo compared to free drug. This enhanced effect was ascribed to the encapsulation of the drug protecting it from a rapid metabolism. Our finding highlights the often-overlooked value of nanoformulations in protecting its cargo from degradation. Overall, results may provide an alternative therapeutic strategy for TNBC management.

Structure and Characterization of a Covalent Inhibitor of Src Kinase

Unregulated Src activity promotes malignant processes in cancer, but no Src-directed targeted therapies are used clinically, possibly because early Src inhibitors produce off-target effects leading to toxicity. Improved selective Src inhibitors may enable Src-directed therapies. Previously, we reported an irreversible Src inhibitor, DGY-06-116, based on the hybridization of dasatinib and a promiscuous covalent kinase probe SM1-71. Here, we report biochemical and biophysical characterization of this compound. An x-ray co-crystal structure of DGY-06-116: Src shows a covalent interaction with the kinase p-loop and occupancy of the back hydrophobic kinase pocket, explaining its high potency, and selectivity. However, a reversible analog also shows similar potency. Kinetic analysis shows a slow inactivation rate compared to other clinically approved covalent kinase inhibitors, consistent with a need for p-loop movement prior to covalent bond formation. Overall, these results suggest that a strong reversible interaction is required to allow sufficient time for the covalent reaction to occur. Further optimization of the covalent linker may improve the kinetics of covalent bond formation.

Exploring Poly(Ethylene Glycol)-Poly(Trimethylene Carbonate) Nanoparticles as Carriers of Hydrophobic Drugs to Modulate Osteoblastic Activity

Current treatment options for bone-related disorders rely on a systemic administration of therapeutic agents that possess low solubility and intracellular bioavailability, as well as a high pharmacokinetic variability, which in turn lead to major off-target side effects. Hence, there is an unmet need of developing drug delivery systems that can improve the clinical efficacy of such therapeutic agents. Nanoparticle delivery systems might serve as promising carriers of hydrophobic molecules. Here, we propose 2 nanoparticle-based delivery systems based on monomethoxy poly(ethylene glycol)-poly(trimethyl carbonate) (mPEG-PTMC) and poly(lactide-co-glycolide) for the intracellular controlled release of a small hydrophobic drug (dexamethasone) to osteoblast cells in vitro. mPEG-PTMC self-assembles into stable nanoparticles in the absence of surfactant and shows a greater entrapment capacity of dexamethasone, while assuring bioactivity in MC3T3-E1 and bone marrow stromal cells cultured under apoptotic and osteogenic conditions, respectively. The mPEG-PTMC nanoparticles represent a potential vector for the intracellular delivery of hydrophobic drugs in the framework of bone-related diseases.

Randomized phase-II evaluation of letrozole plus dasatinib in hormone receptor positive metastatic breast cancer patients

The non-receptor tyrosine kinase Src activation plays a role in the malignant progression of breast cancer, including development of endocrine therapy resistance and survival of bone metastases. This study investigated whether adding Src kinase inhibitor dasatinib to aromatase inhibitor (AI) therapy improved outcomes in estrogen receptor (ER)-positive, HER2-negative metastatic breast cancer (MBC). Postmenopausal patients with ER-positive, HER2-negative MBC (0-1 prior chemotherapies and no prior AI for MBC) were eligible for this non-comparative, parallel group, phase-II study. Patients were randomized to letrozole (2.5 mg/day PO) alone or with dasatinib (100 mg/day PO). Patients with disease progression on letrozole alone could crossover to dasatinib plus continued letrozole. The primary endpoint was clinical-benefit-rate (CBR; complete response + partial response + stable disease ≥6 months). A total of 120 patients were randomized. The CBR of 71% (95% CI 58-83%) was observed with letrozole + dasatinib versus the projected CBR of the combination of 56%. The CBR of 66% (95% CI 52-77%) with letrozole alone also exceeded the projected CBR of 39% with letrozole alone. The CBR was 23% in the crossover arm of letrozole plus dasatinib in patients progressing on letrozole alone. Median progression-free survival with the combination was 20.1 months and 9.9 months with letrozole alone. Letrozole plus dasatinib was well tolerated, although 26% of patients required dasatinib dose reductions. In this non-comparative phase-II trial, the CBR of 71% and the median PFS of 20.1 months with letrozole + dasatinib are encouraging and suggest that dasatinib may inhibit the emergence of acquired resistance to AI therapy.

Complete response in advanced breast cancer patient treated with a combination of capecitabine, oral vinorelbine and dasatinib

Background

Currently, there are no data available on the best choice of treatment in heavily pretreated patients with advanced breast cancer. However, the combination of oral vinorelbine and capecitabine has been demonstrated to be effective and safe in patients with advanced breast cancer pretreated with anthracycline. Furthermore, some studies assessed the activity of dasatinib, an oral tyrosine kinase inhibitor that inhibits five oncogenic tyrosine kinase families, alone or in combination with different chemotherapy in patients affected with advanced breast cancer.

Case presentation

A patient with metastatic breast cancer, hormone receptor positive and human epidermal grow factor receptor 2 negative, pretreated with epirubicine, taxanes and nab-paclitaxel, was submitted to third line chemotherapy with vinorelbine 60 mg/m² on day 1, 8 plus capecitabine 1000 mg/m² twice daily from day 1 to day 14 every 21 days. The patient was taking also dasatinib 100 mg once daily for chronic myeloid leukemia. The treatment was well tolerated and, after 15 months, computed tomography scan showed a complete response of liver metastases and bone stable disease. After another 28 months, a 18-fluorodeoxyglucose positron emission tomography scan showed a metabolic response of bone metastases without other site of disease.

Conclusions

This is the first case in literature about activity of dasatinib in combination with a chemotherapy schedule of oral vinorelbine and capecitabine in advanced breast cancer. This treatment showed both good tolerability and great activity with a long progression free survival of 54 months.

The Src family kinase inhibitor dasatinib delays pain-related behaviour and conserves bone in a rat model of cancer-induced bone pain

Pain is a severe and debilitating complication of metastatic bone cancer. Current analgesics do not provide sufficient pain relief for all patients, creating a great need for new treatment options. The Src kinase, a non-receptor protein tyrosine kinase, is implicated in processes involved in cancer-induced bone pain, including cancer growth, osteoclastic bone degradation and nociceptive signalling. Here we investigate the role of dasatinib, an oral Src kinase family and Bcr-Abl tyrosine kinase inhibitor, in an animal model of cancer-induced bone pain. Daily administration of dasatinib (15 mg/kg, p.o.) from day 7 after inoculation of MRMT-1 mammary carcinoma cells significantly attenuated movement-evoked and non-evoked pain behaviour in cancer-bearing rats. Radiographic - and microcomputed tomographic analyses showed significantly higher relative bone density and considerably preserved bone micro-architecture in the dasatinib treated groups, suggesting a bone-preserving effect. This was supported by a significant reduction of serum TRACP 5b levels in cancer-bearing rats treated with 15 mg/kg dasatinib. Furthermore, immunoblotting of lumbar spinal segments showed an increased activation of Src but not the NMDA receptor subunit 2B. These findings support a role of dasatinib as a disease modifying drug in pain pathologies characterized by increased osteoclast activity, such as bone metastases.

Src signaling pathways in prostate cancer

Knowledge of the molecular events that contribute to prostate cancer progression has created opportunities to develop novel therapy strategies. It is now well established that c-Src, a non-receptor tyrosine kinase, regulates a complex signaling network that drives the development of castrate-resistance and bone metastases, events that signal the lethal phenotype of advanced disease. Preclinical studies have established a role for c-Src and Src Family Kinases (SFKs) in proliferation, angiogenesis, invasion and bone metabolism, thus implicating Src signaling in both epithelial and stromal mechanisms of disease progression. A number of small molecule inhibitors of SFK now exist, many of which have demonstrated efficacy in preclinical models and several that have been tested in patients with metastatic castrate-resistant prostate cancer. These agents have demonstrated provocative clinic activity, particularly in modulating the bone microenvironment in a therapeutically favorable manner. Here, we review the discovery and basic biology of c-Src and further discuss the role of SFK inhibitors in the treatment of advanced prostate cancer.

Loss of Androgen-Regulated MicroRNA 1 Activates SRC and Promotes Prostate Cancer Bone Metastasis

Bone metastasis is the hallmark of progressive and castration-resistant prostate cancers. MicroRNA 1 (miR-1) levels are decreased in clinical samples of primary prostate cancer and further reduced in metastases. SRC has been implicated as a critical factor in bone metastasis, and here we show that SRC is a direct target of miR-1. In prostate cancer patient samples, miR-1 levels are inversely correlated with SRC expression and a SRC-dependent gene signature. Ectopic miR-1 expression inhibited extracellular signal-regulated kinase (ERK) signaling and bone metastasis in a xenograft model. In contrast, SRC overexpression was sufficient to reconstitute bone metastasis and ERK signaling in cells expressing high levels of miR-1. Androgen receptor (AR) activity, defined by an AR output signature, is low in a portion of castration-resistant prostate cancer. We show that AR binds to the miR-1-2 regulatory region and regulates miR-1 transcription. Patients with low miR-1 levels displayed correlated low canonical AR gene signatures. Our data support the existence of an AR-miR-1-SRC regulatory network. We propose that loss of miR-1 is one mechanistic link between low canonical AR output and SRC-promoted metastatic phenotypes.

BMP-7 enhances cell migration and αvβ3 integrin expression via a c-Src-dependent pathway in human chondrosarcoma cells

Bone morphogenic protein (BMP)-7 is a member of the transforming growth factor (TGF)-beta superfamily, which is originally identified based on its ability to induce cartilage and bone formation. In recent years, BMP-7 is also defined as a potent promoter of cell motility, invasion, and metastasis. However, there is little knowledge of the role of BMP-7 and its cellular function in chondrosarcoma cells. In the present study, we investigated the biological impact of BMP-7 on cell motility using transwell assay. In addition, the intracellular signaling pathways were also investigated by pharmacological and genetic approaches. Our results demonstrated that treatment with exogenous BMP-7 markedly increased cell migration by activating c-Src/PI3K/Akt/IKK/NF-κB signaling pathway, resulting in the transactivation of αvβ3 integrin expression. Indeed, abrogation of signaling activation, by chemical inhibition or expression of a kinase dead form of the protein attenuated BMP-7-induced expression of integrin αvβ3 and cell migration. These findings may provide a useful tool for diagnostic/prognostic purposes and even therapeutically in late-stage chondrosarcoma as an anti-metastatic agent.

Osteolytic bone metastasis is hampered by impinging on the interplay among autophagy, anoikis and ossification

Here we show that the fate of osteolytic bone metastasis depends on the balance among autophagy, anoikis resistance and ossification, and that the hepatocyte growth factor (HGF) signaling pathway seems to have an important role in orchestrating bone colonization. These findings are consistent with the pathophysiology of bone metastasis that is influenced by the cross-talk of supportive and neoplastic cells through molecular signaling networks. We adopted the strategy to target metastasis and stroma with the use of adenovirally expressed NK4 (AdNK4) and Dasatinib to block HGF/Met axis and Src activity. In human bone metastatic 1833 cells, HGF conferred anoikis resistance via Akt and Src activities and HIF-1α induction, leading to Bim isoforms degradation. When Src and Met activities were inhibited with Dasatinib, the Bim isoforms accumulated conferring anoikis sensitivity. The proviability effect of HGF, under low-nutrient stress condition, was related to a faster autophagy deactivation with respect to HGF plus Dasatinib. In the 1833 xenograft model, AdNK4 switched metastasis vasculature to blood lacunae, increasing HIF-1α in metastasis. The combination of AdNK4 plus Dasatinib gave the most relevant results for mice survival, and the following molecular and cellular changes were found to be responsible. In bone metastasis, we observed a hypoxic condition – marked by HIF-1α – and an autophagy failure – marked by p62 without Beclin-1. Then, osteolytic bone metastases were largely prevented, because of autophagy failure in metastasis and ossification in bone marrow, with osteocalcin deposition. The abnormal repair process was triggered by the dysfunctional autophagy/anoikis interplay. In conclusion, the concomitant blockade of HGF/Met axis and Src activity seemed to induce HIF-1α in metastasis, whereas the bone marrow hypoxic response was reduced. As a consequence, anoikis resistance might be hampered favoring, instead, autophagy failure and neoformation of woven bone trabeculae. Mice survival was, therefore, prolonged by overcoming an escape strategy adopted by metastatic cells by disruption of tumor–stroma coevolution, showing the importance of autophagy inhibition for the therapy of bone metastasis.

Ethanol Extracts of Fresh Davallia formosana (WL1101) Inhibit Osteoclast Differentiation by Suppressing RANKL-Induced Nuclear Factor- κ B Activation

The rhizome of Davallia formosana is commonly used to treat bone disease including bone fracture, arthritis, and osteoporosis in Chinese herbal medicine. Here, we report the effects of WL1101, the ethanol extracts of fresh rhizomes of Davallia formosana on ovariectomy-induced osteoporosis. In addition, excess activated bone-resorbing osteoclasts play crucial roles in inflammation-induced bone loss diseases, including rheumatoid arthritis and osteoporosis. In this study, we examined the effects of WL1101 on receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis. Treatment with WL1101 significantly inhibited RANKL-stimulated osteoclastogenesis. Two isolated active compounds, ((−)-epicatechin) or WL14 (4-hydroxy-3-aminobenzoic acid) could also inhibit RANKL-induced osteoclastogenesis. WL1101 suppressed the RANKL-induced nuclear factor-κB (NF-κB) activation and nuclear translocation, which is the key process during osteoclastogenesis, by inhibiting the activation of IκB kinase (IKK) and IκBα. In animal model, oral administration of WL1101 (50 or 200 mg/kg/day) effectively decreased the excess bone resorption and significantly antagonized the trabecular bone loss in ovariectomized rats. Our results demonstrate that the ethanol extracts of fresh rhizomes of Davallia formosana inhibit osteoclast differentiation via the inhibition of NF-κB activation and effectively ameliorate ovariectomy-induced osteoporosis. WL1101 may thus have therapeutic potential for the treatment of diseases associated with excessive osteoclastic activity.

Dextromethorphan inhibits osteoclast differentiation by suppressing RANKL-induced nuclear factor-κB activation

Dextromethorphan (DXM), a commonly used antitussive, is a dextrorotatory morphinan. Here, we report that DXM inhibits the receptor activator of nuclear factor kappa B ligand (RANKL)-induced osteoclastogenesis and bone resorption by abrogating the activation of NF-κB signalling in vitro. Oral administration of DXM ameliorates ovariectomy (OVX)-induced osteoporosis in vivo.

INTRODUCTION

DXM was reported to possess anti-inflammatory properties through inhibition of the release of pro-inflammatory factors. However, the potential role and action mechanism of DXM on osteoclasts and osteoblasts remain unclear. In this study, in vitro and in vivo studies were performed to investigate the potential effects of DXM on osteoclastogenesis and OVX-induced bone loss.

METHODS

Osteoclastogenesis was examined by the TRAP staining, pit resorption, TNF-α release, and CCR2 and CALCR gene expression. Osteoblast differentiation was analyzed by calcium deposition. Osteogenic and adipogenic genes were measured by real-time PCR. Signaling pathways were explored using Western blot. ICR mice were used in an OVX-induced osteoporosis model. Tibiae were measured by µCT and serum markers were examined with ELISA kits.

RESULTS

DXM inhibited RANKL-induced osteoclastogenesis. DXM mainly inhibited osteoclastogenesis via abrogation of IKK-IκBα-NF-κB pathways. However, a higher dosage of DXM antagonized the differentiation of osteoblasts via the inhibition of osteogenic signals and increase of adipogenic signals. Oral administration of DXM (20 mg/kg/day) partially reduced trabecular bone loss in ovariectomized mice.

CONCLUSION

DXM inhibits osteoclast differentiation and activity by affecting NF-κB signaling. Therefore, DXM at suitable doses may have new therapeutic applications for the treatment of diseases associated with excessive osteoclastic activity.

Bone targeted therapies in metastatic castration-resistant prostate cancer

Prostate cancer is the most common male cancer. About 90% of metastatic patients will develop bone metastases. Bone disease is responsible of pain, deterioration of quality of life and serious bone complications. Proliferation of prostate cancer cells in the bone marrow induces osteoclast activation and osteolysis. Targeting the bone microenvironment reduces morbidity. Relevant preclinical and clinical studies of bone-targeted therapies in castration-resistant prostate cancer were identified in PubMed and clinical trial databases. Different drugs are available or in development that target bone resorption (bisphosphonates, RANK ligand inhibitors), bone formation (endothelin 1 inhibitors), cancer cell migration (SRC-family kinase inhibitors, vascular endothelial growth factor-MET inhibitors), and survival (radiopharmaceuticals). In phase III trials, zoledronic acid, denosumab, and radium-223 were shown to significantly delay skeletal-related events. Radium-223 was also shown to improve overall survival. Biomarkers of bone resorption (urinary N-telopeptide) and bone making (alkaline phosphatase) have an independent prognostic impact. Targeting the bone microenvironment is an important component of castration-resistant prostate cancer management to reduce bone complications and improve overall survival. Biomarkers of bone turnover have an independent prognostic impact.

miR-30 as a tumor suppressor connects EGF/Src signal to ERG and EMT

Src tyrosine kinase (Src) is implicated in the development of bone metastasis and castration resistance of prostate cancer. Src inhibitors are currently being tested in clinical trials for such diseases. Understanding the molecular and cellular actions of Src inhibitors holds the key to future improvement of this line of therapy. Here we describe the microRNA expression profiles modulated by two Src inhibitors and demonstrate that the miR-30 family members are the most prominently induced species. Consistent with its tumor suppressor role, miR-30 is downmodulated by oncogenic signals such as epidermal growth factor (EGF) and hepatocyte growth factor, and is generally underexpressed in prostate cancer specimens. A number of epithelial-to-mesenchymal transition (EMT)-associated genes are predicted targets of miR-30. Among these genes the Ets-related gene (ERG) is the most frequently overexpressed oncogene in prostate cancer activated by genomic fusion events between promoter upstream sequences of the TMPRSS2 and coding sequences of ERG. We showed by ERG 3' untranslated region reporter and mutagenesis assays that ERG is a direct target of miR-30. Overexpression of miR-30 in prostate cancer cells suppresses EMT phenotypes and inhibits cell migration and invasion. It also inhibits the in vitro and in vivo growth of VCaP cells, which depends on TMPRSS2-ERG for proliferation. TMPRSS2-ERG is generally regulated by androgen at the transcriptional level. Our finding reveals a new post-transcriptional mechanism of TMPRSS2-ERG regulation by Src and growth signals via miR-30 providing a rationale for targeting ERG-positive castration-resistant tumors with Src inhibitors.

A phase 2 study of KX2-391, an oral inhibitor of Src kinase and tubulin polymerization, in men with bone-metastatic castration-resistant prostate cancer

PURPOSE

KX2-391 is an oral non-ATP-competitive inhibitor of Src kinase and tubulin polymerization. In phase 1 trials, prostate-specific antigen (PSA) declines were seen in patients with advanced prostate cancer. We conducted a single-arm phase 2 study evaluating KX2-391 in men with chemotherapy-naïve bone-metastatic castration-resistant prostate cancer (CRPC).

METHODS

We treated 31 patients with oral KX2-391 (40 mg twice-daily) until disease progression or unacceptable toxicity. The primary endpoint was 24-week progression-free survival (PFS); a 50 % success rate was pre-defined as clinically significant. Secondary endpoints included PSA progression-free survival (PPFS) and PSA response rates. Exploratory outcomes included pharmacokinetic studies, circulating tumor cell (CTC) enumeration, and analysis of markers of bone resorption [urinary N-telopeptide (uNTx); C-telopeptide (CTx)] and formation [bone alkaline phosphatase (BAP); osteocalcin].

RESULTS

The trial closed early after accrual of 31 patients, due to a pre-specified futility rule. PFS at 24 weeks was 8 %, and median PFS was 18.6 weeks. The PSA response rate (≥ 30 % decline) was 10 %, and median PPFS was 5.0 weeks. Additionally, 18 % of men with unfavorable (≥ 5) CTCs at baseline converted to favorable (<5) CTCs with treatment. The proportion of men with declines in bone turnover markers was 32 % for uNTx, 21 % for CTx, 10 % for BAP, and 25 % for osteocalcin. In pharmacokinetic studies, median C max was 61 (range 16-129) ng/mL, and median AUC was 156 (35-348) ng h/mL. Common toxicities included hepatic derangements, myelosuppression, fatigue, nausea, and constipation.

CONCLUSION

KX2-391 dosed at 40 mg twice-daily lacks antitumor activity in men with CRPC, but has modest effects on bone turnover markers. Because a C max of ≥142 ng/mL is required for tubulin polymerization inhibition (defined from preclinical studies), higher once-daily dosing will be used in future trials.

Recent developments in treatments targeting castration-resistant prostate cancer bone metastases

BACKGROUND

Prostate cancer is the most common male cancer and one of the top causes of male cancer-related death. Most patients with prostate cancer respond to initial androgen deprivation therapy before progressing to castration-resistant prostate cancer (CRPC) and eventually developing bone metastases. Growth of prostate cancer metastases in the bone microenvironment produces numerous factors that disrupt the dynamic equilibrium of osteogenesis and osteolysis existing in healthy bone, leading to progressive morbidity, poor quality of life, and increased treatment costs.

MATERIALS AND METHODS

Relevant studies of CRPC and targeted therapies were identified from literature and clinical trial databases, websites, and conference abstracts.

RESULTS

Available data on agents potentially targeting bone metastatic CRPC or the bone microenvironment in patients with CRPC are discussed, including inhibitors of tumor growth/survival and bone turnover (SRC family kinase inhibitors, endothelin-1 inhibitors, MET inhibitors, and thalidomide and its derivatives), inhibitors of bone turnover (bisphosphonates and receptor activator of nuclear factor-kB ligand inhibitors), antiangiogenic agents (vascular endothelial growth factor receptor and platelet-derived growth factor blockers), prostate cancer vaccines, and bone-directed radiopharmaceuticals.

CONCLUSIONS

With increasing data availability demonstrating tumor-bone microenvironment interactions and routine incorporation of bone-related end points into CRPC trials, bone microenvironment-targeted agents are likely to become an increasingly important component of CRPC treatment.

Effects of Src kinase inhibition by saracatinib (AZD0530) on bone turnover in advanced malignancy in a Phase I study

Saracatinib (AZD0530) is an orally active once-daily Src kinase inhibitor which modulates key signaling pathways in cancer cells. In a Phase I study in patients with advanced solid malignancies resistant to standard treatment we assessed the effect of saracatinib on bone turnover. Fifty-one patients were randomized into three parallel groups to receive saracatinib 50, 125 or 175 mg/day. After a single dose followed by a 7-day washout, patients received once-daily doses for 21 days. Bone turnover markers were measured in serum and urine samples collected before dosing on days 1, 2, 3, 17 and 28. Samples were available at baseline and more than one other time point for 44 patients. Bone resorption markers were significantly decreased by saracatinib. Serum cross-linked C-terminal telopeptide of type I collagen (sCTX) changed in the 50, 125 and 175 mg/day groups by -36% (95% CI -58, -4), -64% (95% CI -75, -48) and -75% (95% CI -83, -61), respectively, at day 28. Urinary cross-linked N-terminal telopeptide of type I collagen/creatinine ratio (uNTX/Cr) changed in the 50, 125 and 175 mg/day groups by; -13% (95% CI -33, 13), -48% (95% CI -59, -34) and -50% (95% CI -62, -35), respectively, at day 28. The significant decreases in bone resorption markers indicate that suppression of Src kinase inhibits osteoclast activity in patients with advanced cancer. This result suggests that saracatinib may have therapeutic benefit in metastatic bone disease.

Src inhibitors in the treatment of metastatic bone disease: rationale and clinical data

Src is a nonreceptor tyrosine kinase essential for the activation of osteoclasts, the cells that degrade bone. Src also regulates normal cell functions, cancer cell growth and metastasis to organs, including bone where tumor cells induce bone destruction by osteoclasts. Src inhibitors prevent bone destruction and tumor cell growth in animal models of metastatic bone disease, and some are being investigated in clinical trials, particularly in patients with prostate cancer, which has high bone metastatic potential. Here, we review how Src regulates osteoclast formation, activation and survival and the results of preclinical and clinical trials of Src inhibitors, which show some promise in inhibiting the effects of tumor cells on the skeleton.

C-Src-mediated RANKL-induced breast cancer cell migration by activation of the ERK and Akt pathway

The receptor activator for nuclear factor κB ligand/receptor activator for nuclear factor κB (RANKL/RANK) pathway is critical for RANK-expressing cancer cells to home to bones, and c-Src is critical for cancer progression. The objective of this study was to explore the effect of c-Src in the RANKL/RANK pathway and migration activity in human breast cancer cells. Breast cancer cell lines MCF-7, MDA-MB-231 and BT-474 were obtained and cultured. Flow cytometry was used to examine RANK expression. The results showed that RANK was expressed in breast cancer cell lines MCF-7, MDA-MB-231 and BT-474, and soluble RANKL (sRANKL)-triggered migration of breast cancer cells by activating ERK1/2, Akt and c-Src. The sRANKL-induced migration was blocked with RANKL inhibitor osteoprotegerin (OPG), MEK inhibitor PD98059, PI3K inhibitor LY294002 and Src inhibitor PP2. Inhibition of c-Src function with PP2 blocked the activation of Akt and ERK1/2, resulting in the inhibition of RANKL-induced migration. In conclusion, RANKL was found to increase the migration of breast cancer cells by activating the c-Src-Akt and c-Src-ERK signaling pathways.

Emerging therapeutic approaches in the management of metastatic castration-resistant prostate cancer

Although treatment options for men with castration-resistant prostate cancer (CRPC) have improved with the recent and anticipated approvals of novel immunotherapeutic, hormonal, chemotherapeutic and bone-targeted agents, clinical benefit with these systemic therapies is transient and survival times remain unacceptably short. Thus, we devote the second section of this two-part review to discussing emerging therapeutic paradigms and research strategies that are entering phase II and III clinical testing for men with metastatic CRPC. We will discuss a range of emerging hormonal, immunomodulatory, antiangiogenic, epigenetic and cell survival pathway inhibitors in current clinical trials, with an emphasis on how these therapies may complement our existing treatment options.

Src inhibitors are promising therapy molecules for human cervical carcinomas

Metastatic cervical cancer is a major cause of cancer mortality in women worldwide. In parallel, it is well established that high-risk human papillomaviruses (HPVs) are important factors in the progression of this cancer. Meanwhile, the overall 5 year-survival rate for patients diagnosed with cervical cancer is approximately 50% and has not significantly improved over the past two decades. Therefore, new strategies for the treatment of this cancer, especially the metastatic form, are becoming a major focus of investigation. Alternatively, Src family kinase activities are elevated in several human carcinomas, including cervical, and are often associated with aggressive cancer. Moreover, Src activities are deregulated by E6/E7 onco-proteins of high-risk HPV which are expressed in the majority of human cervical cancers. This raises the question whether Src inhibitors play a significant role in the treatment of human cervical carcinomas. In this paper, we propose the hypothesis that the Src family can be an important target for the treatment of this cancer. Although, we believe that significant studies, using different cells and animal models as well as clinical trails, are necessary to verify this hypothesis.

Comparative role of acetylation along c-SRC/ETS1 signaling pathway in bone metastatic and invasive mammary cell phenotypes

Metastatic cells switch between different modes of migration through supramolecular plasticity mechanism(s) still largely unknown. The aim of the present paper was to clarify some molecular aspects of the epigenetic control of migration of 1833-bone metastatic cells compared to MDA-MB231-parental mammary carcinoma cells. Active c-Src overexpression enhanced 1833-cell spontaneous migration and CXCR4-mediated chemoinvasion toward CXCL12 ligand. Only in metastatic cells, in fact, c-Src seemed to stabilize nuclear CXCR4-protein receptor possibly due to tyrosine phosphorylation, by impairing protein-degradative smear and causing instead an electrophoretic-mobility shift; the cytosolic steady-state level of CXCR4 was enhanced, and the protein appeared also phosphorylated. These findings suggested the triggering of unique signaling pathways in metastasis for homing of breast-cancer cells to congenial environment of specific organs. Microenvironmental stimuli activating c-Src might influence Ets1 binding to CXCR4 promoter and consequent transactivation, as well as CXCR4 post-translational regulatory mechanisms such as phosphorylation. Enhancement of Ets1 activity and CXCR4 induction by c-Src overexpression were prevented by histone deacetylase (HDAC) blockade. In contrast, HDAC inhibition with trichostatin A increased cytosolic phosphorylated CXCR4 expression in MDA-MB231 cells, but Ets1 involvement was practically unneeded. c-Src might be suggested as a bio-marker predicting metastasis sensitivity patterns to HDAC inhibitors. Rationally designed and individualized therapy may become possible as more is learned about the target molecules of HDAC's inhibitory agents and their roles, as undertaken for CXCR4 that is likely to be crucial for homing, angiogenesis and survival in a c-Src-dependent manner in bone-metastatic mammary cells.

Future directions in castrate-resistant prostate cancer therapy

Although several new therapies have recently become available for the treatment of castrate-resistant prostate cancer (CRPC), the disease remains universally incurable and demands novel therapeutic approaches. To this end, great strides have been made in our understanding of the biologic and molecular mechanisms driving prostate cancer growth and progression in the past few years, resulting in widespread clinical investigation of numerous new targeted therapies. This review will highlight some of the key therapeutic agents that (in the opinion of the authors) may have the largest effect on the future management of CRPC, with a focus on both molecular targets and clinical trial design. These agents include angiogenesis inhibitors, mTOR pathway inhibitors, apoptosis-inducing drugs, IGF pathway inhibitors, Src family inhibitors, Hedgehog pathway antagonists, epigenetic therapies, PARP inhibitors, and prodrug approaches. The future of CRPC therapy appears brighter than ever before.

Castration-resistant prostate cancer: targeted therapies and individualized treatment

Various molecular mechanisms have been implicated in the progression from hormone-sensitive to castration-resistant prostate cancer (CRPC). Novel targeted agents to treat CRPC have been developed that inhibit either androgen receptor (AR)-mediated signaling (AR antagonists and inhibitors of androgen synthesis) or non-AR-mediated signaling (inhibitors of Src, mammalian target of rapamycin, chaperone proteins, insulin-like growth factor-1 receptor, vascular endothelial growth factor, and endothelin-A receptor) pathways. However, variable efficacy has been observed in clinical trials, most likely because of the biologic heterogeneity of CRPC. To account for potential differences in disease biology, a more individualized approach to treatment, based on genomic and/or proteomic analyses of individual tumors, is being investigated. By identifying tumors with a characteristic molecular subtype and assigning treatment accordingly, it is hoped that a higher proportion of patients will benefit from targeted therapy. Additionally, lessons learned through the application of these technologies to prostate cancer may subsequently influence therapeutic development in other solid tumors.

Breast cancer at bone metastatic sites: recent discoveries and treatment targets

Breast carcinoma is the most common cancer of women. Bones are often involved with breast carcinoma metastases with the resulting morbidity and reduced quality of life. Breast cancer cells arriving at bone tissues mount supportive microenvironment by recruiting and modulating the activity of several host tissue cell types including the specialized bone cells osteoblasts and osteoclasts. Pathologically activated osteoclasts produce osteolytic lesions associated with bone pain, pathological fractures, cord compression and other complications of metastatic breast carcinoma at bone. Over the last decade there has been enormous growth of knowledge in the field of osteoclasts biology both in the physiological state and in the tumor microenvironment. This knowledge allowed the development and implementation of several targeted therapeutics that expanded the armamentarium of the oncologists dealing with the metastases-associated osteolytic disease. While the interactions of cancer cells with resident bone cells at the established metastatic gross lesions are well-studied, the preclinical events that underlie the progression of disseminated tumor cells into micrometastases and then into clinically-overt macrometastases are just starting to be uncovered. In this review, we discuss the established information and the most recent discoveries in the pathogenesis of osteolytic metastases of breast cancer, as well as the corresponding investigational drugs that have been introduced into clinical development.

Dasatinib: a potent SRC inhibitor in clinical development for the treatment of solid tumors

SRC is a tyrosine kinase that plays a role in oncogenic, invasive and bone-metastatic processes. It has therefore been prioritized as a candidate therapeutic target in patients with solid tumors. Several SRC inhibitors are now in development, of which dasatinib has been most explored. Preclinical studies in a wide variety of solid tumor cell lines, including prostate, breast and glioma, have shown that that dasatinib acts as a cytostatic agent, inhibiting the processes of cell proliferation, invasion and metastasis. Dasatinib also inhibits the activity of osteoclasts, which have a major role in the development of metastatic bone lesions. Dasatinib has additive or synergistic activity in combination with a number of other agents, including cytotoxic agents and targeted therapies, providing a rationale for combination treatment in a clinical setting. Emerging clinical data with dasatinib support experimental observations, with preliminary phase 1 and 2 data demonstrating activity, both as a single agent and as combination therapy, in a range of solid tumors. Future clinical trials will further assess the clinical value of SRC inhibition with dasatinib.

Src kinase inhibitors: an emerging therapeutic treatment option for prostate cancer

IMPORTANCE OF THE FIELD

Once prostate cancer becomes castration-resistant, bone metastases are a significant problem and treatment options are limited. As a result, there is a need for more effective therapies that have antitumor and anti-bone metastatic effects. Because Src and Src-family kinases (SFKs) are involved in multiple signaling pathways central to prostate cancer development, progression, and metastasis, in addition to normal and pathologic osteoclast activities, Src inhibition represents a valid therapeutic strategy for investigation.

AREAS COVERED IN THIS REVIEW

Here, current treatment options for advanced prostate cancer, the preclinical rationale behind using Src inhibitors, emerging data from clinical trials of Src inhibitors in prostate cancer, and future therapeutic directions are described. Data published in peer-reviewed journals within the last 20 years or presented at recent European or American Society of Clinical Oncology conferences have been reviewed.

WHAT THE READER WILL GAIN

Readers will gain an insight into the development of therapeutic Src inhibitors, including dasatinib and saracatinib; an understanding of their effects on prostate cancer cells and the bone microenvironment; and emerging clinical data.

TAKE HOME MESSAGE

Src is implicated in prostate cancer progression and metastasis, therefore treatment with Src inhibitors warrants further investigation.

Shedding of RANKL by tumor-associated MT1-MMP activates Src-dependent prostate cancer cell migration

Membrane type 1 matrix metalloproteinase (MT1-MMP) plays an essential role in protease-mediated extracellular matrix (ECM) degradation, but it also functions as a sheddase releasing non-ECM substrates such as receptor activator of NF-kappaB ligand (RANKL), an osteoclastogenic factor typically confined to the surface of osteoblasts. We previously found high expression of MT1-MMP in skeletal metastasis of prostate cancer patients, in a pattern similar to RANKL expression. We also showed that overexpression of MT1-MMP in prostate cancer cells increases tumor growth and osteolysis in an intratibial mouse model of bone metastasis, and that soluble factor(s) shed by tumor-derived MT1-MMP enhance osteoclast differentiation in a RANKL-dependent manner. Recent evidence indicates that the cognate receptor for RANKL, RANK, is expressed in prostate cancer cells, suggesting the presence of an autocrine pathway. In this study, we show that MT1-MMP-expressing LNCaP prostate cancer cells display enhanced migration. Moreover, conditioned medium from LNCaP cells expressing both RANKL and MT1-MMP stimulates the migration of MT1-MMP-deficient C42b prostate cancer cells. This enhanced chemotaxis can be abrogated by osteoprotegerin (soluble decoy receptor of RANKL), MIK-G2 (a selective inhibitor for MT1-MMP), and PP2 (a Src inhibitor). These findings indicate that tumor-derived MT1-MMP enhances tumor cell migration through initiation of an autocrine loop requiring ectodomain shedding of membrane-bound RANKL in prostate cancer cells, and that Src is a key downstream mediator of RANKL-induced migration of prostate cancer cells.

SKI-606 (Bosutinib) blocks prostate cancer invasion, growth, and metastasis in vitro and in vivo through regulation of genes involved in cancer growth and skeletal metastasis

In the current study, we have examined the efficacy of a Src/Abl kinase inhibitor SKI-606 (Bosutinib) for its effect on prostate cancer growth and skeletal metastasis. Treatment of highly invasive human prostate cancer cells PC-3 and DU-145 with different doses of SKI-606 decreased Src activation, cell proliferation, migration, and invasion as determined by Matrigel Boyden chamber invasion assay. For in vivo studies, PC-3 cells were inoculated through s.c. or i.t. route into male BALB/c nu/nu or Fox Chase severe combined immunodeficient mice, respectively. Experimental animals treated with SKI-606 developed tumors of a significantly smaller volume and a significant decrease (50%) in experimental skeletal lesion area. A marked increase (32%) in bone volume to tumor volume ratio was also seen by micro-computed tomography analysis of tibias from control and experimental groups of animals. Western blot analysis showed the ability of SKI-606 to significantly decrease the phosphorylation of signaling molecules (AKT, mitogen-activated protein kinase, focal adhesion kinase) and the expression of tumor progression-associated genes uPAR, MMP-2, MMP-9, N-cadherin, fibronectin, BMP-2 (bone morphogenetic protein 2), BMP-6 (bone morphogenetic protein 6), IL-8 (interleukin 8), and TGF-beta (transforming growth factor beta) in prostate cancer cells. SKI-606 is currently in clinical trials for breast cancer and chronic myelogenous leukemia. Results from these studies provide convincing evidence for evaluating its efficacy in prostate cancer patients.

Dasatinib in solid tumors

IMPORTANCE OF THE FIELD

Dasatinib is an oral, potent adenosine triphosphate-competitive inhibitor of multiple tyrosine kinases including BCR-ABL, c-KIT, platelet-derived growth factor receptor, and Src family kinases (SFKs). It has gained much attention for its use in chronic myeloid leukemia and for the treatment of adults with Philadelphia chromosome-positive acute lymphoblastic leukemia. However, dasatinib is also being explored in solid tumors in ongoing Phase I and II clinical trials.

AREAS COVERED IN THIS REVIEW

The clinical efficacy of dasatinib in a wide variety of solid tumors and important Phase I/II studies utilizing dasatinib and the optimal dosage used in solid tumors. A literature search was conducted using PubMed/MEDLINE, www.clinicaltrials.gov , and the American Society of Clinical Oncology websites to find relevant Phase I/II clinical trials during 1987-2009.

WHAT THE READER WILL GAIN

The understanding that the biology and mechanism of Src activation in tumors are not well understood and finding the optimal use of SFK inhibitors in the clinical setting requires further investigation.

TAKE HOME MESSAGE

In reviewing the clinical safety data of dasatinib in its current use as a Src inhibitor in a wide variety of solid malignancies, dasatinib appears to be safe and is a promising agent for the treatment of metastatic solid tumors refractory to standard therapies.

Dasatinib inhibits site-specific tyrosine phosphorylation of androgen receptor by Ack1 and Src kinases

Activation of androgen receptor (AR) may play a role in the development of castration resistant prostate cancer. Two intracellular tyrosine kinases, Ack1 (activated cdc42-associated kinase) and Src, phosphorylate and enhance AR activity and promote prostate xenograft tumor growth in castrated animals. However, the upstream signals that activate these kinases and lead to AR activation are incompletely characterized. In this study, we investigated AR phosphorylation in response to non-androgen ligand stimulation using phospho-specific antibodies. Treatment of LNCaP and LAPC-4 cells with epidermal growth factor (EGF), heregulin, Gas6 (ligand binding to Mer receptor tyrosine kinase and activating Ack1 downstream), interleukin (IL)-6 or bombesin stimulated cell proliferation in the absence of androgen. Treatment of LNCaP and LAPC-4 cells with EGF, heregulin, or Gas6 induced AR phosphorylation at Tyr-267; IL-6 or bombesin treatment did not. AR phosphorylation at Tyr-534 was induced by treatment with EGF, IL-6 or bombesin, but not by heregulin or Gas6. siRNA-mediated knockdown of Ack1 or Src showed that Ack1 mediates heregulin- and Gas6-induced AR Tyr-267 phosphorylation whereas Src mediates Tyr-534 phosphorylation induced by EGF, IL-6, and bombesin. Dasatinib, a Src inhibitor, blocked EGF-induced Tyr-534 phosphorylation. In addition, we show dasatinib also inhibited Ack1 kinase. Dasatinib inhibited heregulin-induced Ack1 kinase activity and AR Tyr-267 phosphorylation. Dasatinib inhibited heregulin-induced AR-dependent reporter activity. Dasatinib also inhibited heregulin-induced expression of endogenous AR target genes. Dasatinib inhibited Ack1-dependent colony formation and prostate xenograft tumor growth in castrated mice. Interestingly, Ack1 or Src knockdown or dasatinib did not inhibit EGF-induced AR Tyr-267 phosphorylation or EGF-stimulated AR activity, suggesting the existence of an additional tyrosine kinase that phosphorylates AR at Tyr-267. These data suggest that specific tyrosine kinases phosphorylate AR at distinct sites and that dasatinib may exert anti-tumor activity in prostate cancer through inhibition of Ack1.

Src family kinase/abl inhibitor dasatinib suppresses proliferation and enhances differentiation of osteoblasts

Dasatinib, a dual Src family kinase and Abl inhibitor, is being tested clinically for the treatment of prostate cancer bone metastasis. Bidirectional interactions between osteoblasts and prostate cancer cells are critical in the progression of prostate cancer in bone, but the effect of dasatinib on osteoblasts is unknown. We found that dasatinib inhibited proliferation of primary mouse osteoblasts isolated from mouse calvaria and the immortalized MC3T3-E1 cell line. In calvarial osteoblasts from Col-luc transgenic mice carrying osteoblast-specific Col1alpha1 promoter reporter, luciferase activity was inhibited. Dasatinib also inhibited fibroblast growth factor-2-induced osteoblast proliferation, but strongly promoted osteoblast differentiation, as reflected by stimulation of alkaline phosphatase activity, osteocalcin secretion and osteoblast mineralization. To determine how dasatinib blocks proliferative signaling in osteoblasts, we analyzed the expression of a panel of tyrosine kinases, including Src, Lyn, Fyn, Yes and Abl, in osteoblasts. In the Src family kinases, only Src was activated at a high level. Abl was expressed at a low level in osteoblasts. Phosphorylation of Src-Y419 or Abl-Y245 was inhibited by dasatinib treatment. Knockdown of either Src or Abl by lenti-shRNA in osteoblasts enhances osteoblast differentiation, suggesting that dasatinib enhances osteoblast differentiation through inhibition of both Src and Abl.

Phase I/II study of the Src inhibitor dasatinib in combination with erlotinib in advanced non-small-cell lung cancer

PURPOSE

Src family kinase (SFK) proteins are frequently activated in cancer and can coordinate tumor cell growth, survival, invasion, and angiogenesis. Given the importance of SFK signaling in cancer, known cooperation between SFK and epidermal growth factor receptor (EGFR) signaling, and efficacy of EGFR inhibitors, we performed a phase I trial combining dasatinib, an SFK and multikinase inhibitor, with erlotinib, an EGFR inhibitor, in patients with advanced non-small-cell lung cancer.

PATIENTS AND METHODS

Patients received erlotinib for 1 week before addition of dasatinib; pharmacokinetics were performed after weeks 1 and 2. Four cohorts were examined, including twice-daily and daily dasatinib dosing. Responses were assessed after 8 weeks. Plasma levels of angiogenic markers (vascular endothelial growth factor [VEGF], interleukin-8, and basic fibroblast growth factor [bFGF]) were determined before and during treatment.

RESULTS

Thirty-four patients were enrolled. The average duration of treatment was 73 days. The main adverse events include GI (diarrhea, anorexia, and nausea), skin rash, cytopenias, pleural effusions, and fatigue. No effect of escalating doses of dasatinib was observed on erlotinib pharmacokinetics. Two partial responses and one bone response were observed, and the disease control rate was 63%. Reductions in plasma VEGF and bFGF were observed, and reductions in VEGF correlated with disease control.

CONCLUSION

The combination of erlotinib and dasatinib is tolerable, with adverse effects consistent with the two agents. Disease control and inhibition of plasma angiogenesis markers were observed. Personalized strategies for deployment of SFK should receive further attention.

Emerging therapeutic targets in breast cancer bone metastasis

In the past decade, our understanding of the molecular mechanisms that underlie breast cancer pathology and progression has dramatically improved. Using this knowledge, we have identified additional targets and developed novel therapeutic interventions in breast cancer. Together, these translational research efforts are helping to usher us into an age of personalized cancer therapy. Metastasis to bone is a common and devastating consequence of breast cancer. Bisphosphonates, which represent the current gold standard in bone metastasis therapies, are being improved with newer and more efficacious generations of these compounds being developed. Breast cancer growth in the bone requires activation of various signaling pathways in both cancer cells and stromal cells, including those that are stimulated by TGF-β and RANKL, and mediated through the Src tyrosine kinase. Bone cells and cancer cells alike express promising targets for therapeutic intervention, including Cathepsin K, CXCR4 and GPNMB. In this article we discuss the molecular mechanisms behind these pro-metastatic molecules and review the most recent findings in the clinical development of their associated targeted therapies.

Increased levels of active c-Src distinguish invasive from in situ lobular lesions

Introduction

Mounting molecular evidence suggests that invasive lobular carcinoma (ILC) is developing from in situ lesions, atypical lobular hyperplasia (ALH), and lobular carcinoma in situ (LCIS). However, little is known about the mechanisms promoting the progression of lobular breast cancer (LBC) to invasive disease. Here, we investigated whether c-Src kinase, an established inducer of invasive states, contributes to the progression from ALH/LCIS to ILC.

Methods

Immunochemistry for c-Src and other cancer-related molecules was performed on archived tissue specimens from 57 LBC patients. Relative c-Src activity was estimated by comparing fluorescence intensity of ILC with that of adjacent ALH/LCIS and nonneoplastic epithelia after staining with an antibody against active c-Src. Expression of active c-Src was correlated with markers of invasion and malignancy and with relapse among LBC patients.

Results

Levels of activated c-Src were increased in ILC relative to ALH/LCIS (1.63-fold ± 0.24 SD) and nonneoplastic epithelia (1.47 ± 0.18 SD). Increased c-Src levels correlated with the activation of c-Src downstream targets (Fak, Stat-3) and the expression of mesenchymal markers. ILC cells with activated c-Src co-expressed metastatic markers (Opn, Cxcr4) and included cells positive for the cancer stem cell marker Aldh1. A tendency for high c-Src levels (P = 0.072) was observed among the seven LBC patients with relapsed disease.

Conclusions

Our data indicate elevated c-Src activity in ILC relative to noninvasive neoplastic tissue. The associated molecular changes suggest that c-Src promotes LBC invasiveness by inducing an epithelial-mesenchymal transition. Therefore, c-Src antagonists might counteract the acquisition of invasiveness during LBC progression. Inhibition of c-Src may also affect ILC cells thought to have a high metastatic potential and to be capable of initiating/maintaining tumor growth. Together with the possible association between high c-Src levels and disease recurrence, our findings encourage the evaluation of c-Src antagonists for the treatment of LBC.

Protons make tumor cells move like clockwork

Cancer accounts for 13% of the yearly total mortality worldwide. Most cancer deaths are the sequel of metastatic diseases rather than of primary tumor growth. Thus, the major challenge in tumor therapy is the tumor cells' ability to metastasize. The extent to which a tumor metastasizes correlates with the tumor cells' migratory activity. Tumor cell migration requires a coordinated formation and release of cell adhesion contacts, a controlled cytoskeletal dynamics, the digestion and reorganization of the extracellular matrix, and local ion and water transport across the plasma membrane. All of these operations depend on intracellular pH (pH(i)) and extracellular pH (pH(e)). Numerous H(+), HCO (3) (-) , and monocarboxylate transporters as well as different carbonic anhydrase isozymes have considerable impact on pH(i) and pH(e) which spotlights them as possible, potential targets for anticancer therapeutics. Especially in solid tumors whose vascularization is often not sufficient, tumor cells cope with hypoxia and the resulting glycolysis by overexpressing the Na(+)/H(+) exchanger NHE1, monocarboxylate transporters MCT1 and/or MCT4, and the carbonic anhydrase CA IX. NHE1, MCT, and CA IX activity lead to an acidification of the extracellular space in order to maintain the cytosolic pH homeostasis stable. The present article gives a review on how this characteristic, acidic tumor micro- and nanoenvironment controls tumor cell migration.