Imatinib inhibits the in vitro development of the monocyte/macrophage lineage from normal human bone marrow progenitors

Nilotinib exhibits less toxicity than imatinib and influences the immune state by modulating iNOS, p-p38 and p-JNK in LPS/IFN gamma-activated macrophages

In this study, we aimed to analyze the effects of first and second-generation Bcr-Abl tyrosine kinase inhibitors, imatinib and nilotinib on LPS/IFN gamma activated RAW 264.7 macrophages. Our data revealed that imatinib was less effective on nitrite levels and more toxic on macrophages compared to nilotinib. Therefore, we further analysed the effect of nilotinib on various inflammatory markers including iNOS, COX-2, NFkB, IL-6, p-ERK, p-p38 and p-JNK in LPS/IFN gamma activated RAW264.7 macrophages. Spectrophotometric viability test and Griess assay,western blot, RT-PCR and luciferase reporter assays were used to analyze the biological activity of nilotinib. Our findings revealed that nilotinib decreases nitrite levels, iNOS mRNA, iNOS and p-p38 protein expressions significantly whereas induces IL-6 mRNA and p-JNK protein expressions at particular doses. We did not find significant effect of nilotinib on COX-2, p-ERK and nuclear p65 proteins and NFkB transcriptional activity. In addition, the binding mode of nilotinib to iNOS protein was predicted by molecular docking. According to the docking analyses, nilotinib exhibited hydrophobic interactions between MET349, ALA191, VAL346, PHE363, TYR367, MET368, CYS194, TRP366 residues at the binding pocket and the molecule as well as van der Waals interactions at specific residues. In conclusion, our results reveal that, in addition to its anticancer activity, nilotinib can exhibit immune modulatory effects on macrophages through its effects on iNOS, IL-6, p-p38 and p-JNK.

Kinase inhibitors developed for treatment of hematologic malignancies: implications for immune modulation in COVID-19

Tyrosine kinase inhibitors (TKIs) are used to target dysregulated signaling pathways in virtually all hematologic malignancies. Many of the targeted signaling pathways are also essential in nonmalignant immune cells. The current coronavirus severe acute respiratory syndrome coronavirus 2 pandemic catalyzed clinical exploration of TKIs in the treatment of the various stages of COVID-19, which are characterized by distinct immune-related complications. Most of the reported effects of TKIs on immune regulation have been explored in vitro, with different class-specific drugs having nonoverlapping target affinities. Moreover, many of the reported in vivo effects are based on artificial animal models or on observations made in symptomatic patients with a hematologic malignancy who often already suffer from disturbed immune regulation. Based on in vitro and clinical observations, we attempt to decipher the impact of the main TKIs approved or in late-stage development for the treatment of hematological malignancies, including inhibitors of Bruton’s tyrosine kinase, spleen tyrosine kinase, BCR-Abl, phosphatidylinositol 3-kinase/ mammalian target of rapamycin, JAK/STAT, and FMS-like tyrosine kinase 3, to provide a rationale for how such inhibitors could modify clinical courses of diseases, such as COVID-19.

Evaluation of Different Concentrations of Imatinib on the Viability of Leishmania major: An In Vitro Study

BACKGROUND

Leishmaniasis is an infectious disease caused by an intracellular parasite of Leishmania and is transmitted through the female sandflies bite and may lead to severe skin lesions. Although drugs such as antimony compounds are available, their side effects such as toxicity, low efficacy, and emergence of resistance have raised the importance of effective replacement. Imatinib, as an inhibitor of tyrosine kinase (TK) of Leishmania, stops abnormal function of TK such as Bcr-Abl through assembling into transmembrane pores in a sterol-dependent manner. Hence, the evaluation of killing effects of different concentrations of imatinib against Leishmania major amastigotes and promastigotes in vitro were the objectives of the present study.

MATERIALS AND METHODS

The killing effects of different concentrations of imatinib (25, 50, and 100 μg) and 25 μg amphotericin B (as positive control) were evaluated against RPMI 1640-cultured promastigotes and the amastigote/macrophage model by MTS cell proliferation assay kit (ab197010) and Giemsa staining method during 24, 48, and 72 h.

RESULTS

The results showed anti-Leishmania effect of imatinib in concentration and time-dependent manner. The lowest number of live promastigotes and amastigotes were obtained due to treat with 100 μg/ml imatinib at 72 h. Furthermore, 100 μg concentration of imatinib had the same effect as 25 μg amphotericin B on both L. major promastigotes and amastigotes (P < 0.001).

CONCLUSION

The anti-Leishmania effect of imatinib was confirmed by MTS and direct microscopy. Further study is recommended for evaluating possible therapeutic effects of imatinib on leishmaniasis in vivo.

Osteonecrosis of the jaw in the era of targeted therapy and immunotherapy in oncology

Osteonecrosis of the jaw (ONJ) is a well-known pathological condition in oncology derived from the use of bisphosphonates (BPs) and denosumab. Many molecular and immunological targets have been introduced for daily use in cancer treatment in recent years; consequently, new cases of ONJ have been reported in association with these drugs, especially if administered with BPs and denosumab. When the drugs are administered alone, ONJ is rarely seen. The objective of our study was to analyze the recent literature relative to the association of ONJ with these new drugs highlighting the pathogenic, clinical and therapeutic aspects. The close collaboration between maxillofacial surgeon, oncologist, dentist, and dental hygienist remains the most important aspect for the prevention, prompt recognition, and treatment of this pathology.

BCR-ABL Tyrosine Kinase Inhibitors: Which Mechanism(s) May Explain the Risk of Thrombosis?

Imatinib, the first-in-class BCR-ABL tyrosine kinase inhibitor (TKI), had been a revolution for the treatment of chronic myeloid leukemia (CML) and had greatly enhanced patient survival. Second- (dasatinib, nilotinib, and bosutinib) and third-generation (ponatinib) TKIs have been developed to be effective against BCR-ABL mutations making imatinib less effective. However, these treatments have been associated with arterial occlusive events. This review gathers clinical data and experiments about the pathophysiology of these arterial occlusive events with BCR-ABL TKIs. Imatinib is associated with very low rates of thrombosis, suggesting a potentially protecting cardiovascular effect of this treatment in patients with BCR-ABL CML. This protective effect might be mediated by decreased platelet secretion and activation, decreased leukocyte recruitment, and anti-inflammatory or antifibrotic effects. Clinical data have guided mechanistic studies toward alteration of platelet functions and atherosclerosis development, which might be secondary to metabolism impairment. Dasatinib, nilotinib, and ponatinib affect endothelial cells and might induce atherogenesis through increased vascular permeability. Nilotinib also impairs platelet functions and induces hyperglycemia and dyslipidemia that might contribute to atherosclerosis development. Description of the pathophysiology of arterial thrombotic events is necessary to implement risk minimization strategies.

Off‑target effect of imatinib and nilotinib on human vitamin D3 metabolism

Prolonged treatment with tyrosine kinase inhibitors (TKI) including imatinib (IMA) or nilotinib (NIL), induces severe disturbances of bone metabolism in patients with chronic myeloid leukaemia. As vitamin D3 (VD3) is involved in the complex cycle of bone remodelling, the present study investigated in vitro, the influence of IMA and NIL on VD3 metabolism i) in HaCaT cells and ii) in cultured outer root sheath keratinocytes (ORS‑KC) from hair follicles of IMA treated children. Cells were incubated in the presence of IMA or NIL. Concomitantly, specific inhibitors were applied to analyze the inhibition of the VD3 processing cytochrome P450 isoenzyme family by TKIs. In vitro, IMA and NIL significantly impaired the production of calcitriol in HaCaT and cultured ORS‑KC cells from hair follicles of IMA treated children. For NIL, this inhibitory effect demonstrated a 4‑fold increase. In HaCaT and ORS‑KC, application of specific CYP450 inhibitors revealed that CYP27B1 was impaired by IMA and NIL leading to an intracellular accumulation of calcidiol. However, during TKI treatment, KC of IMA treated children revealed no differences in calcidiol and calcitriol levels. In conclusion, IMA and NIL interfere with the vitamin D3 cascade due to their metabolism by CYP27B1.

Multiparameter Analysis of Off-Target Effects of Dasatinib on Bone Homeostasis in Patients With Newly Diagnosed Chronic Myelogenous Leukemia

BACKGROUND

We assessed patients with chronic myelogenous leukemia (CML) for serum calcium (Ca), phosphate (PO4), bone alkaline phosphatase, N-telopeptide (NTx), osteoprotegerin (OPG) levels, and trabecular bone area (TBA) in bone marrow (BM) specimens before and after treatment with dasatinib. We identified a significant increase in percentage of TBA in postdasatinib BM (P = .022). This suggests that dasatinib therapy can increase TBA without significant changes in bone and mineral metabolism. Interferences with bone homeostasis and mineral metabolism have been described in patients receiving imatinib for CML or gastrointestinal stromal tumors. Dasatinib is a potent second-generation tyrosine kinase inhibitor designed to inhibit ABL and SRC kinases while also interfering with the c-Kit, platelet-derived growth factor receptor, and STAT5 pathways.

PATIENTS AND METHODS

We used a multiparameter approach to examine the off-target effects of dasatinib in 30 patients with CML treated between 2009 and 2012. We recorded serum Ca and PO4 levels, analyzed markers of bone formation (bone alkaline phosphatase/bone-specific alkaline phosphatase [BAP]) and bone resorption (NTx), measured OPG levels, and digitally analyzed changes in TBA in paired BM biopsy specimens before and after treatment. We correlated all findings with each other and with the results of conventional cytogenetic and molecular analyses.

RESULTS

We identified a significant increase in the percentage of TBA in postdasatinib BM biopsy specimens (P = .022) and noted a decrease in serum OPG levels in 75% of patients. Ca, PO4, BAP, and NTx levels remained steady, without significant changes. There was no correlation between biomarker levels, percentage of TBA, and/or cytogenetic or molecular response.

CONCLUSION

These findings suggest that dasatinib therapy (within the therapeutic range) can increase trabecular bone, without causing significant changes in bone and mineral metabolism. Nonetheless, monitoring of bone health and skeletal integrity should be included into the long-term management of patients treated with dasatinib to further enhance our understanding of its safety profile and its potential role as a treatment modality for other bone diseases.

Imatinib and Nilotinib Off-Target Effects on Human NK Cells, Monocytes, and M2 Macrophages

Tyrosine kinase inhibitors (TKIs) are used in the clinical management of hematological neoplasms. Moreover, in solid tumors such as stage 4 neuroblastomas (NB), imatinib showed benefits that might depend on both on-target and immunological off-target effects. We investigated the effects of imatinib and nilotinib on human NK cells, monocytes, and macrophages. High numbers of monocytes died upon exposure to TKI concentrations similar to those achieved in patients. Conversely, NK cells were highly resistant to the TKI cytotoxic effect, were properly activated by immunostimulatory cytokines, and degranulated in the presence of NB cells. In NB, neither drug reduced the expression of ligands for activating NK receptors or upregulated that of HLA class I, B7-H3, PD-L1, and PD-L2, molecules that might limit NK cell function. Interestingly, TKIs modulated the chemokine receptor repertoire of immune cells. Acting at the transcriptional level, they increased the surface expression of CXCR4, an effect observed also in NK cells and monocytes of patients receiving imatinib for chronic myeloid leukemia. Moreover, TKIs reduced the expression of CXCR3 (in NK cells) and CCR1 (in monocytes). Monocytes also decreased the expression of M-CSFR, and low numbers of cells underwent differentiation toward macrophages. M0 and M2 macrophages were highly resistant to TKIs and maintained their phenotypic and functional characteristics. Importantly, also in the presence of TKIs, the M2 immunosuppressive polarization was reverted by TLR engagement, and M1-oriented macrophages fully activated autologous NK cells. Our results contribute to better interpreting the off-target efficacy of TKIs in tumors and to envisaging strategies aimed at facilitating antitumor immune responses.

Impact of long-term exposure to the tyrosine kinase inhibitor imatinib on the skeleton of growing rats

The tyrosine kinase (TK) inhibitor imatinib provides a highly effective therapy for chronic myeloid leukemia (CML) via inhibition of the oncogenic TK BCR-ABL1. However, off-target TKs like platelet-derived growth factor receptors (PDGF-R) and colony-stimulating factor-1 receptor (c-fms), involved in bone remodeling, are also inhibited. Thus, pediatric patients with CML on imatinib exhibit altered bone metabolism, leading to linear growth failure. As TKI treatment might be necessary for a lifetime, long-term effects exerted on bone in children are of major concern. Therefore, we studied the skeletal long-term effects of continuous and intermittent imatinib exposure in a juvenile rat model. Four-weeks-old male Wistar rats were chronically exposed to imatinib via drinking water over a period of 10 weeks. Animals were exposed to a standard and high imatinib dosage continuously and to the high imatinib dose intermittently. Bone mass and strength were assessed using pQCT, micro-computed tomography (μCT), and biomechanical testing at the prepubertal, pubertal, and postpubertal age. Bone length and vertebral height as well as biochemical markers of bone turnover were analyzed. Femoral and tibial bone length were dose-dependently reduced by up to 24% (p<0.0001), femoral and tibial trabecular bone mass density (BMD) were reduced by up to 25% (p<0.01), and femoral breaking strength was lowered by up to 20% (p<0.05). Intermittent exposure mitigated these skeletal effects. Long-term exposure resulted in reduced vertebral height by 15% and lower trabecular BMD by 5%. Skeletal changes were associated with suppressed serum osteocalcin (p<0.01) and non-significantly elevated serum CTX-I and PINP levels. In conclusion, imatinib mainly impaired longitudinal growth of long bones rather than the vertebrae of growing rats. Interestingly, intermittent imatinib exposure has less skeletal side effects, which may be beneficial in pediatric patients taking imatinib.

Dendritic cell c-kit signaling and adaptive immunity: implications for the upper airways

PURPOSE OF REVIEW

Binding of the receptor tyrosine kinase, c-kit, to its ligand, stem cell factor (SCF), mediates numerous biological functions. Important roles for c-kit in hematopoiesis, melanogenesis, erythropoiesis, spermatogenesis, and carcinogenesis are well documented. Similarly, activation of mast cells and eosinophils by c-kit ligation has long been known to result in degranulation with concomitant release of pro-inflammatory mediators including cytokines. This review will highlight a recently discovered function of c-kit in regulating the adaptive immune responses with relevance to allergic diseases.

RECENT FINDINGS

Recent studies in a number of laboratories including our own highlight the previously unappreciated functions for c-kit in immunological processes. Increased expression of c-kit and its ligand, SCF, on dendritic cells by Th2/Th17-inducing stimuli leads to c-kit activation and immune skewing toward these subsets and away from Th1 responses. Treatment of dendritic cells with inhibitors of c-kit activation such as imatinib mesylate (Gleevec) induces breach of T-cell tolerance, skewing of responses toward Th1, and activation of natural killer cells.

SUMMARY

Taken together, these observations suggest that the c-kit/SCF axis may be a useful target for redirecting deleterious immune responses in various disease settings, including allergic diseases that are often associated with Th2 and Th17 responses.

Digital image analysis as a tool to assess the effects of imatinib on trabecular bone in patients with chronic myelogenous leukemia

Skeletal integrity is sustained by osteoblast-osteoclast interactions, controlled by several signaling pathways that include tyrosine kinases. Imatinib is a tyrosine kinase inhibitor with an extended therapeutic range based on its ability to differentially bind to receptor and nonreceptor tyrosine kinases. In this study, we used digital image analysis to assess changes in trabecular bone surface area within bone marrow biopsy specimens of 34 patients with chronic phase chronic myelogenous leukemia treated with single-agent imatinib. These patients were 25 men and 9 women with a median age of 59 years. We selected representative areas of paired bone marrow biopsy specimens obtained at baseline and within the subsequent 48 months. Computer-assisted analysis was performed to calculate trabecular bone area (TBA) within the sample by using the equation TBA% = sum of trabecular area/total biopsy specimen area. Percentage changes were defined as ΔTBA% and were arbitrarily subdivided into marked (>50%), moderate (10%-50%), and mild (<10%). During the study interval, TBA% increased in 24 patients (71%) and decreased in 10 patients (29%). Overall, there was a significant increase in TBA% (P = .02). No correlation was found between changes in trabecular bone area and either clinical or cytogenetic response (P = .25). The results show that imatinib therapy can alter trabecular bone in bone marrow biopsy specimens of chronic myelogenous leukemia patients, most often resulting in an increase in TBA%.

The skeletal effects of the tyrosine kinase inhibitor nilotinib

Nilotinib is a tyrosine kinase inhibitor (TKI) developed to manage imatinib-resistance in patients with chronic myeloid leukemia (CML). It inhibits similar molecular targets to imatinib, but is a significantly more potent inhibitor of Bcr-Abl. Nilotinib exhibits off-target effects in other tissues, and of relevance to bone metabolism, hypophosphataemia has been reported in up to 30% of patients receiving nilotinib. We have assessed the effects of nilotinib on bone cells in vitro and on bone metabolism in patients receiving nilotinib for treatment of CML. We firstly investigated the effects of nilotinib on proliferating and differentiating osteoblastic cells, and on osteoclastogenesis in murine bone marrow cultures and RAW264.7 cells. Nilotinib potently inhibited osteoblast proliferation (0.01-1uM), through inhibition of the platelet-derived growth factor (PDGFR). There was a biphasic effect on osteoblast differentiation such that it was reduced by lower concentrations of nilotinib (0.1-0.5uM), with no effect at higher concentrations (1uM). Nilotinib also potently inhibited osteoclastogenesis, predominantly by stromal-cell dependent mechanisms. Thus, nilotinib decreased osteoclast development in murine bone marrow cultures, but did not affect osteoclastogenesis in RAW264.7 cells. Nilotinib treatment of osteoblastic cells increased expression and secretion of OPG and decreased expression of RANKL. In 10 patients receiving nilotinib, levels of bone turnover markers were in the low-normal range, despite secondary hyperparathyroidism, findings that are similar to those in patients treated with imatinib. Bone density tended to be higher than age and gender-matched normal values. These data suggest that nilotinib may have important effects on bone metabolism. Prospective studies should be conducted to determine the long-term effects of nilotinib on bone density and calcium metabolism.

Nilotinib attenuates renal injury and prolongs survival in chronic kidney disease

The tyrosine kinase inhibitor imatinib is beneficial in experimental renal diseases, but the effect of the new tyrosine kinase inhibitor nilotinib on the progression of renal failure is unknown. We administered either nilotinib or vehicle to Sprague-Dawley rats beginning 2 weeks after 5/6 nephrectomy (Nx) or laparotomy and continuing for 8 weeks. Serum creatinine levels were significantly lower in the nilotinib group after 6 and 8 weeks of treatment. Furthermore, nilotinib-treated rats had less proteinuria, attenuated glomerulosclerosis and tubulointerstitial damage, and reduced macrophage infiltration into the tubulointerstitium. Treatment with nilotinib also significantly decreased renal cortical expression of profibrogenic genes, such as IL-1β and monocyte chemotactic protein-1, which correlated closely with the tubulointerstitial damage score and ED1-positive macrophages score. In addition, nilotinib treatment significantly prolonged survival. Taken together, these results suggest that nilotinib may limit the progression of chronic kidney disease.

[Novel immunopathological approaches to pulmonary arterial hypertension]

Inflammation is important for the initiation and the maintenance of vascular remodeling in the most commun animal models of pulmonary hypertension (PH), and its therapeutical targeting blocks PH development in these models. In human, pulmonary vascular lesions of PH are also the source of an intense chemokine production, linked to inflammatory cell recruitment. However, arteritis is uncommon in PH patients. Of note, current PH treatments have immunomodulatory properties. In addition, some studies have shown a correlation between levels of circulating inflammatory mediators and patients' survival. The study of autoimmunity in the pathophysiology of pulmonary arterial hypertension is becoming an area of intense investigation. New immunopathological approaches to PH should allow the development of innovative treatments for this very severe condition.

Anti-inflammatory M2 type macrophages characterize metastasized and tyrosine kinase inhibitor-treated gastrointestinal stromal tumors

We have made a detailed inventory of the immune infiltrate of gastrointestinal stromal tumors (GISTs), which originate from mesenchymal cells in the intestinal tract. These sarcomas are heavily infiltrated with macrophages and T cells, while immune cells of other lineages were much less abundant. Dissecting the functional subtypes of T cells with multicolor fluorescent microscopy revealed substantial populations of cytotoxic T cells, helper T cells and FoxP3(+) regulatory T cells. The balance of cytotoxic T cells and FoxP3(+) T cells was toward immune suppression. Analysis of the macrophage population also showed a dominance of anti-inflammatory cells, as the M2 type scavenger receptor CD163 was abundantly present. Other subsets of macrophages (CD14(+)CD163(-)) were occasionally detected. M2 type CD163(+) macrophages were associated with the number of infiltrating FoxP3(+) regulatory T cells and twice as many macrophages were found in metastatic GIST compared to primary lesions. Most metastatic GISTs had been treated with the tyrosine kinase inhibitors imatinib and sunitinib, but the high macrophage infiltrate was not related to this treatment. However, imatinib and sunitinib did induce secretion of anti-inflammatory IL-10 in macrophage cultures, indicating that treatment with these inhibitors might contribute to an immune suppressive microenvironment in GIST. Overall, our data reveal a picture of GIST as an active site of tumor-immune interaction in which suppressive mechanisms overrule potential antitumor responses. Tyrosine kinase inhibitors might promote this negative balance.

Signaling of c-kit in dendritic cells influences adaptive immunity

The binding of the receptor tyrosine kinase, c-kit, to its ligand, stem cell factor (SCF), mediates numerous biological functions. Important roles for c-kit in hematopoiesis, melanogenesis, erythropoiesis, spermatogenesis, and carcinogenesis are well documented. Similarly, activation of granulocytes, mast cells, and of eosinophils in particular, by c-kit ligation has long been known to result in degranulation with concomitant release of pro-inflammatory mediators, including cytokines. However, recent work from a number of laboratories, including our own, highlights previously unappreciated functions for c-kit in immunologic processes. These novel findings strongly suggest that signaling through the c-kit-SCF axis could have a significant impact on the pathogenesis of diseases associated with an immunologic component. In our own studies, c-kit upregulation on dendritic cells via T helper (Th)2- and Th17-inducing stimuli led to c-kit activation and immune skewing toward these T helper subsets and away from Th1 responses. Others have shown that dendritic cell treatment with inhibitors of c-kit activation, such as imatinib mesylate (Gleevec), favored breaking of T-cell tolerance, skewing of responses toward production of Th1 cytokines, and activation of natural killer cells. These data all indicate that deeper understanding of, and ability to control, the c-kit-SCF axis could lead to improved treatment modalities aimed at redirecting unwanted and/or deleterious immune responses in a wide variety of conditions.

EGFR tyrosine kinase inhibitor (PD153035) improves glucose tolerance and insulin action in high-fat diet-fed mice

OBJECTIVE

In obesity, an increased macrophage infiltration in adipose tissue occurs, contributing to low-grade inflammation and insulin resistance. Epidermal growth factor receptor (EGFR) mediates both chemotaxis and proliferation in monocytes and macrophages. However, the role of EGFR inhibitors in this subclinical inflammation has not yet been investigated. We investigated, herein, in vivo efficacy and associated molecular mechanisms by which PD153035, an EGFR tyrosine kinase inhibitor, improved diabetes control and insulin action.

RESEARCH DESIGN AND METHODS

The effect of PD153035 was investigated on insulin sensitivity, insulin signaling, and c-Jun NH(2)-terminal kinase (JNK) and nuclear factor (NF)-kappaB activity in tissues of high-fat diet (HFD)-fed mice and also on infiltration and the activation state of adipose tissue macrophages (ATMs) in these mice.

RESULTS

PD153035 treatment for 1 day decreased the protein expression of inducible nitric oxide synthase, tumor necrosis factor (TNF)-alpha, and interleukin (IL)-6 in the stroma vascular fraction, suggesting that this drug reduces the M1 proinflammatory state in ATMs, as an initial effect, in turn reducing the circulating levels of TNF-alpha and IL-6, and initiating an improvement in insulin signaling and sensitivity. After 14 days of drug administration, there was a marked improvement in glucose tolerance; a reduction in insulin resistance; a reduction in macrophage infiltration in adipose tissue and in TNF-alpha, IL-6, and free fatty acids; accompanied by an improvement in insulin signaling in liver, muscle, and adipose tissue; and also a decrease in insulin receptor substrate-1 Ser(307) phosphorylation in JNK and inhibitor of NF-kappaB kinase (IKKbeta) activation in these tissues.

CONCLUSIONS

Treatment with PD153035 improves glucose tolerance, insulin sensitivity, and signaling and reduces subclinical inflammation in HFD-fed mice.

Dysregulation of bone remodeling by imatinib mesylate

Imatinib mesylate is a rationally designed tyrosine kinase inhibitor that has revolutionized the treatment of chronic myeloid leukemia and gastrointestinal stromal tumors. Although the efficacy and tolerability of imatinib are a vast improvement over conventional chemotherapies, the drug exhibits off-target effects. An unanticipated side effect of imatinib therapy is hypophosphatemia and hypocalcemia, which in part has been attributed to drug-mediated changes to renal and gastrointestinal handling of phosphate and calcium. However, emerging data suggest that imatinib also targets cells of the skeleton, stimulating the retention and sequestration of calcium and phosphate to bone, leading to decreased circulating levels of these minerals. The aim of this review is to highlight our current understanding of the mechanisms surrounding the effects of imatinib on the skeleton. In particular, it examines recent studies suggesting that imatinib has direct effects on bone-resorbing osteoclasts and bone-forming osteoblasts through inhibition of c-fms, c-kit, carbonic anhydrase II, and the platelet-derived growth factor receptor. The potential application of imatinib in the treatment of cancer-induced osteolysis will also be discussed.

Differential effects of imatinib mesylate against uveal melanoma in vitro and in vivo

Uveal melanoma is refractory to chemotherapy. The receptor tyrosine kinase inhibitor, imatinib mesylate, has demonstrated antiproliferative effects against uveal melanoma cells in vitro. The effects of imatinib mesylate, alone and combined with the alklyating agent, temozolomide, were examined in vivo as well as in vitro. Proliferation and angiogenic factor production of human uveal melanoma cell lines in response to imatinib mesylate and temozolomide were examined in vitro. Tumor growth, angiogenic factor production, tumor interstitial fluid pressure, and stroma constituents in response to imatinib mesylate and temozolomide were examined in vivo in mice bearing human uveal melanoma xenografts. Imatinib mesylate in vitro antagonized the antiproliferative effects of temozolomide and increased the production of angiogenic factors. In contrast, pretreatment with imatinib mesylate in vivo could improve the antitumor activity of temozolomide. Imatinib mesylate in vivo decreased the production of angiogenic factors in the tumor stroma and tumor interstitial fluid pressure. These effects were transient. Increases in angiogenic factors, interstitial fluid pressure, and tumor infiltrating macrophages were observed with continued imatinib mesylate treatment in vivo. The antitumor effects of imatinib mesylate can vary in vivo when compared with in vitro. Imatinib mesylate can both positively and negatively modify host-tumor interactions in uveal melanoma.

Circulating biomarkers for vascular endothelial growth factor inhibitors in renal cell carcinoma

In recent years, there has been significant progress in the clinical development and application of antiangiogenic therapies in renal cell carcinomas, particularly inhibitors of the vascular endothelial growth factor (VEGF) pathway. Despite this progress, no validated methods are currently available for identifying which patients are most likely to respond to treatment or experience toxic effects, selecting the optimal dose, or determining whether the intended molecular target has been effectively inhibited. However, recent studies have suggested that some of the biomarkers currently under investigation in clear cell renal cell carcinoma for VEGF pathway inhibitors are promising. These biomarkers include circulating proangiogenic factors and receptors; markers of hypoxia and endothelial damage; and cellular populations in peripheral blood, such as circulating endothelial cells. Further preclinical and translational validation studies are still needed to determine their practical utility in the clinical setting.

Immunomodulatory effects of imatinib and second-generation tyrosine kinase inhibitors on T cells and dendritic cells: an update

The discovery of new drugs has occasionally led to a better understanding of biologic processes and unforeseen therapeutic applications. One such example is the new group of tyrosine kinase inhibitors, exemplified by the Bcr-Abl inhibitor imatinib (Glivec). In the last 10 years, these so-called 'small molecules' have started to enter the clinic with the promise of cancer treatments targeted at the underlying molecular changes that are responsible for specific malignant phenotypes. The aim of these small molecules has been to avoid the side-effects of systemic chemotherapies and the high morbidity/mortality risks associated with hematopoietic stem cell transplantation. Concurrently, however, increasing evidence has emerged to indicate that these drugs exert profound immunomodulatory effects on T cells and antigen-presenting cells, such as dendritic cells, which play major roles in immune tumor surveillance and the outcome of hematopoietic stem cell transplantation. Targeted tyrosine kinase inhibitor therapy may thus control cancer cell growth both directly and indirectly by changing the immunologic microenvironment. Furthermore, such molecules might help to unravel the complexities of the human immune system and could find therapeutic application in conditions as diverse as autoimmune diseases and certain infectious processes. In this brief review, we discuss recent developments in this fast evolving field.

Preventive and therapeutic effects of imatinib in Wistar-Kyoto rats with anti-glomerular basement membrane glomerulonephritis

Imatinib is a selective tyrosine kinase inhibitor that can block activity of the platelet-derived growth factor receptor (PDGFR) and that has immunomodulatory effects on various cell types. Here we measured the protective effects of imatinib in Wistar-Kyoto rats with nephrotoxic serum nephritis, a kidney disease model where CD8+ T cells and macrophages play pathogenetic roles. Groups of animals were given imatinib from one day before up to 13 days following induction of nephritis and from day 7 to 20 following disease induction. Compared to control rats, at each time point imatinib treatment caused significantly less proteinuria, lowered serum blood urea nitrogen and creatinine, and decreased the number of glomeruli with necrosis, crescents, and fibrin deposits. Imatinib-treated rats had a significant reduction in glomerular macrophage accumulation and reduced renal cortical PDGFR-beta and M-CSF receptor mRNA expression. Using colocalization we found that glomerular macrophages had reduced IL-1beta and MCP-1 protein expression. Late imatinib treatment significantly reduced proteinuria, serum blood urea nitrogen, and creatinine, and reversed renal histopathological changes. We show that imatinib has renoprotective and therapeutic properties and provide pre-clinical work that will need to be confirmed in patients with crescentic glomerulonephritis.

Blood-based biomarkers of SU11248 activity and clinical outcome in patients with metastatic imatinib-resistant gastrointestinal stromal tumor

PURPOSE

There is an unmet need for noninvasive markers to measure the biological effects of targeted agents, particularly those inhibiting the vascular endothelial growth factor (VEGF) receptor (VEGFR) pathway, and identify patients most likely to benefit from treatment. In this study, we investigated potential blood-based biomarkers for SU11248 (sunitinib malate), a multitargeted tyrosine kinase inhibitor, in patients with metastatic imatinib-refractory gastrointestinal stromal tumors.

EXPERIMENTAL DESIGN

Patients (n=73) enrolled in a phase I/II trial received SU11248 daily for 14 or 28 days followed by 14 days without treatment per cycle. Clinical benefit was defined as progression-free survival of >6 months. We assessed plasma markers, including VEGF and soluble VEGFR-2 (sVEGFR-2), and two cellular populations bearing VEGF receptors: monocytes and, in a subset of patients, mature circulating endothelial cells (CEC).

RESULTS

Compared to patients with progressive disease, patients with clinical benefit had significantly greater increases in CECs (0.52 versus -0.01 CEC/microL/d, P=0.03) and smaller decreases in monocyte levels (47% versus 60%, P=0.007) during cycle 1. VEGF increased by 2.2-fold and sVEGFR-2 decreased 25% during the first 2 weeks of treatment. Neither plasma marker correlated with clinical outcome although a modest inverse correlation was observed between sVEGFR-2 changes and plasma drug levels. Monocytes, VEGF, and sVEGFR-2 all rebounded towards baseline off treatment.

CONCLUSIONS

Monocytes, VEGF, and sVEGFR-2 were consistently modulated by treatment, suggesting that they may serve as pharmacodynamic markers for SU11248. Changes in CECs and monocytes, but not the plasma markers, differed between the patients with clinical benefit and those with progressive disease. These end points merit further investigation in future trials to determine their utility as markers of SU11248 activity and clinical benefit in gastrointestinal stromal tumors and other tumor types.

The 2.7 Å Crystal Structure of the Autoinhibited Human c-Fms Kinase Domain

c-Fms, a member of the Platelet-derived Growth Factor (PDGF) receptor family of receptor tyrosine kinases (RTKs), is the receptor for macrophage colony stimulating factor (CSF-1) that regulates proliferation, differentiation and survival of cells of the mononuclear phagocyte lineage. Abnormal expression of c-fms proto-oncogene is associated with a significant number of human pathologies, including a variety of cancers and rheumatoid arthritis. Accordingly, c-Fms represents an attractive therapeutic target. To further understand the regulation of c-Fms, we determined the 2.7 A resolution crystal structure of the cytosolic domain of c-Fms that comprised the kinase domain and the juxtamembrane domain. The structure reveals the crucial inhibitory role of the juxtamembrane domain (JM) that binds to a hydrophobic site immediately adjacent to the ATP binding pocket. This interaction prevents the activation loop from adopting an active conformation thereby locking the c-Fms kinase into an autoinhibited state. As observed for other members of the PDGF receptor family, namely c-Kit and Flt3, three JM-derived tyrosine residues primarily drive the mechanism for autoinhibition in c-Fms, therefore defining a common autoinhibitory mechanism within this family. Moreover the structure provides an understanding of c-Fms inhibition by Gleevec as well as providing a platform for the development of more selective inhibitors that target the inactive conformation of c-Fms kinase.

Imatinib Mesylate Inhibits Antigen-Specific Memory CD8 T Cell Responses In Vivo

Imatinib mesylate (IM) is effective at inducing complete cytogenetic remission in patients with chronic myelogenous leukemia. Because its influence on CD8 T cell responsiveness in vivo is unknown, we investigated the effects of IM by analyzing the response of OT-1 CD8 T cells to Listeria monocytogenes (LM) that express the cognate epitope OVA(257-264) (LM-OVA). In vitro, IM had no effect on Ag-specific expansion, cell division, cell cycle progression, or IFN-gamma expression in naive or memory OT-1 T cells. However, IM induced apoptosis of naive and memory OT-1 T cells at doses of >5 microM. At 15 microM IM, OT-1 T cells did not survive in in vitro cultures. The primary response of OT-1 T cells in vivo to LM-OVA infection was unaltered. In contrast, continuous IM treatment resulted in a diminished memory OT-1 response. The expression of IL-7Ralpha, a receptor required for memory cell survival, was lower (on OT-1 cells) in animals receiving IM. These results indicate that IM treatment affects the ability of the CD8 memory pool to respond to Ag and has the potential to increase susceptibility to infection.

Effects of imatinib on normal hematopoiesis and immune activation

The selective tyrosine kinase inhibitor imatinib (Glivec; Novartis International, Basel, Switzerland, http://www.glivec.com/content/home.jsp) is increasingly used for the treatment of Philadelphia chromosome-positive leukemias and other malignancies. In principle, the drug is well tolerated and clinical side effects are mostly moderate. However, it was shown that imatinib can affect the function of normal, nonmalignant cells, resulting in myelosuppression in treated patients. Recently, it has been demonstrated that imatinib might affect mobilization, proliferation, and differentiation of hematopoietic progenitor cells while leaving hematopoietic stem cells unaffected. Furthermore, in several in vitro studies and animal models, it was demonstrated that imatinib can affect the function and differentiation of antigen-presenting cells and inhibit the effector functions of T lymphocytes. Moreover, the induction of specific cytotoxic T cells seems to be impaired in chronic myeloid leukemia (CML) patients treated with imatinib compared with patients receiving interferon-alpha. This is of importance because some of the therapeutic effects in the treatment of patients with CML are mediated by the induction of leukemia-specific T-cell responses. Further studies investigating the effects of imatinib on normal hematopoiesis are of interest as they might lead to a better understanding of the clinically observed side effects and also might help identify new therapeutic applications of the drug, possibly in Bcr-Abl-negative myeloproliferative disorders and potentially as an immunomodulatory agent.

Inhibition of phosphorylation of the colony-stimulating factor-1 receptor (c-Fms) tyrosine kinase in transfected cells by ABT-869 and other tyrosine kinase inhibitors

The properties of several multitargeted receptor tyrosine kinase inhibitors have been studied for their inhibition of colony-stimulating factor-1 receptor (CSF-1R) signaling. A structurally novel, multitargeted tyrosine kinase inhibitor (ABT-869), imatinib (STI571), and four compounds currently in clinical development (AG013736, BAY 43-9006, CHIR258, and SU11248) were tested for inhibition of CSF-1R signaling in both the enzymatic and cellular assays. ABT-869 showed potent CSF-1R inhibition in both the enzyme and cell-based assays (IC50s < 20 nmol/L). In contrast to a previous report, we have found that imatinib has activity against human CSF-1R in both assays at submicromolar concentrations. In enzyme assays, we have found that the inhibition of CSF-1R by both ABT-869 and imatinib are competitive with ATP, with Ki values of 3 and 120 nmol/L, respectively. SU11248 is a potent inhibitor of CSF-1R in the enzyme assay (IC50 = 7 nmol/L) and inhibits receptor phosphorylation in the cellular assay (IC50 = 61 nmol/L). AG013736 was also a potent inhibitor of CSF-1R in both assays (enzyme, IC50 = 16 nmol/L; cellular, IC50 = 21 nmol/L), whereas BAY 43-9006 is less potent in the enzyme assay (IC50 = 107 nmol/L) than in the cellular system (IC50 = 20 nmol/L). In contrast, we found that CHIR258 had less activity in the cellular assay (IC50 = 535 nmol/L) relative to its enzymatic potency (IC50 = 26 nmol/L). These results show the use of a cell-based assay to confirm the inhibitory activity of lead compounds and drug candidates, such as ABT-869, against the CSF-1R protein in situ.

The Role of Platelet-Derived Growth Factor Receptor in Eotaxin Signaling of Eosinophils

BACKGROUND

Receptor tyrosine kinases (RTKs) such as epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR) are capable of eliciting kinase activity after ligand binding. In several cells, RTKs are activated via the G-protein-coupled receptor independent of the ligand-RTK interaction. We have previously found that EGFR is transactivated via CC chemokine receptor 3 in bronchial epithelial cells and that this pathway is important for mitogen-activated protein (MAP) kinase activation and cytokine production. It has recently been suggested that hypereosinophilic syndrome results from the fusion tyrosine kinase FIP1L1-PDGFRA. Although it is possible that the PDGFR signal is involved in eosinophil function, the details are still unclear.

METHODS

Blood eosinophils were purified using Percoll and anti-CD16 antibody-coated magnetic beads. Expression of PDGFR mRNA was examined by RT-PCR. After stimulating eosinophils with eotaxin, the phosphorylation of MAP kinases was examined by Western blotting with the antiphosphospecific MAP kinase antibody. The eotaxin-induced eosinophil chemotaxis was studied using Boyden chambers.

RESULTS

Eosinophils expressed PDGFRbeta mRNA in 4 out of 8 donors, while PDGFRalpha mRNA was expressed in only 1 donor. Protein expression of PDGFR was also detectable in eosinophils from some donors. AG1295, a specific inhibitor of PDGFR, showed dose-dependent inhibition of eotaxin-induced MAP kinase phosphorylation in the eosinophils expressing PDGFRbeta mRNA. The chemotaxis of these eosinophils was significantly inhibited by AG1295 (n = 3).

CONCLUSIONS

Our results suggest that PDGFR modifies the CCR3-MAP kinase signaling pathway and chemotactic response in some donors. The pharmacological targeting of PDGFR may be a new strategy to treat eosinophilic disorders.

In vivo effects of imatinib mesylate on human haematopoietic progenitor cells

BACKGROUND

Imatinib mesylate has considerable antineoplastic activity in patients with chronic myeloid leukaemia (CML) and some solid tumours. Although originally regarded as nontoxic for normal haematopoiesis, mild to moderate myelosuppression is a commonly observed side-effect of this treatment. Recently, this molecule has been shown to suppress normal haematopoietic progenitor cells in vitro. This is the first study that has investigated the effect of imatinib on haematopoietic progenitor cells in vivo.

MATERIALS AND METHODS

We investigated the number of circulating haematopoietic progenitor cells in 79 patients with CML and five patients with solid tumours who were treated with imatinib for at least 3 months. Bone marrow progenitor cells were assessed in a subgroup of 18 patients with CML after 12 months of imatinib treatment. Results were compared with haematopoietic progenitor cell numbers of normal controls.

RESULTS

Circulating progenitors of all classes were significantly decreased in CML up to 24 months of imatinib therapy compared with healthy controls (median progenitor cells in CML after 12 months: CFU-GM 62, range 0-2543; BFU-E 216, range 0-3259; CFU-GEMM 0, range 0-139; versus controls: CFU-GM 208, range 50-936; BFU-E 690, range 120-1862; CFU-GEMM 20, range 4-77; P < 0.001). Similar reductions in the number of progenitor cells derived from bone marrow were found in a subgroup of 18 patients with CML. In patients with solid tumours the number of circulating progenitor cells was significantly lower under treatment with imatinib when compared with the controls. Withdrawal of imatinib in a patient with a malignant brain tumour resulted in a prompt normalization of circulating progenitors.

CONCLUSIONS

This study suggests that imatinib exerts myelosuppressive effects through inhibition of haematopoietic progenitor cells.

Effect of Tyrosine Kinase Inhibition Using Imatinib on Normal Lymphohematopoietic Cells

Imatinib is a selective tyrosine kinase inhibitor used for the treatment of Philadelphia chromosome-positive leukemias and other malignancies. An important clinical observation is that imatinib can affect the function of normal nonmalignant cells resulting in myelosuppression in treated patients. This observation is supported by the recent findings suggesting that imatinib might affect mobilization, proliferation, and differentiation of hematopoietic progenitor cells while leaving hematopoietic stem cells unaffected. Furthermore, the induction of a specific T cell response seems to be impaired in chronic myeloid leukemia (CML) patients treated with imatinib in contrast to patients receiving interferon-alpha. Recent studies demonstrate that in vitro exposure of mobilized human CD34(+) progenitors to imatinib inhibits their differentiation into dendritic cells. This is of importance as some of the therapeutic effects in the treatment of patients with CML are mediated by the induction of leukemia-specific T cell responses. Studies investigating the effects of imatinib on normal hematopoiesis are of interest because they might help us better understand the side effects observed clinically and might lead to the identification of novel therapeutic applications of the drug (e.g., in Bcr-Abl(-) myeloproliferative disorders and potentially as an immunomodulatory agent).

Imatinib mesylate inhibits osteoclastogenesis and joint destruction in rats with collagen-induced arthritis (CIA)

Macrophage colony-stimulating factor (M-CSF) is a key factor for osteoclastogenesis at the bone-pannus interface in patients with rheumatoid arthritis as well as a receptor activator of NF-kappaB ligand (RANKL). Imatinib mesylate inhibits the phosphorylation of c-fms, a receptor for M-CSF. The present study investigates the effect of imatinib mesylate on joint destruction in rats with collagen-induced arthritis (CIA) and on osteoclastogenesis in vitro. Imatinib mesylate (50 or 150 mg/kg), dexamethasone, or vehicle was administered daily to CIA rats for 4 weeks from the onset of arthritis. Hind-paw swelling and body weight were measured weekly. At weeks 2 and 4, the metatarsophalangeal (MTP) joints and the ankle and subtalar joints were radiographically and histologically assessed. The effect of imatinib mesylate on osteoclast formation from rat bone marrow cells with M-CSF and soluble RANKL (sRANKL) in vitro was also examined. Radiographic assessment showed that 150 mg/kg imatinib mesylate suppressed the destruction of the MTP and the ankle and subtalar joints at week 2, and MTP joint destruction at week 4 in CIA rats, although hind-paw swelling was not suppressed. The number of TRAP-positive cells at the bone-pannus interface was significantly reduced in the group administered with 150 mg/kg imatinib mesylate compared with that given vehicle at week 4. Imatinib mesylate dose-dependently inhibited the proliferation of M-CSF-dependent osteoclast precursor cells in vitro as well as osteoclast formation induced by M-CSF and sRANKL. These findings suggest that imatinib mesylate could prevent joint destruction in patients with rheumatoid arthritis.

Autocrine signaling of platelet-derived growth factor regulates disabled-2 expression during megakaryocytic differentiation of K562 cells

Platelet-derived growth factor (PDGF) is involved in megakaryocytopoiesis and is secreted into the culture medium during megakaryocytic differentiation of human leukemic cells. We investigate whether PDGF plays a role in the regulation of the adapter protein Disabled-2 (DAB2) that expresses abundantly in platelets and megakaryocytes. Western blot analysis revealed that conditioned medium from 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated, megakaryocytic differentiating K562 cells upregulated DAB2 expression. DAB2 induction and megakaryocytic differentiation was abrogated when cells were co-treated with the PDGF receptor inhibitor STI571 or when the conditioned medium was derived from TPA-plus STI571-treated cells. Although the level of PDGF mRNA was not altered by STI571, an approximate 44% decrease in PDGF in the conditioned medium was observed. Consistent with these findings, interfering PDGF signaling by PDGF neutralization antibody or dominant negative PDGF receptors attenuated DAB2 expression. Accordingly, transfection of an expression plasmid encoding secreted PDGF upregulated DAB2. This study shows for the first time that PDGF autocrine signaling regulates DAB2 expression during megakaryocytic differentiation.

FMS receptor for M-CSF (CSF-1) is sensitive to the kinase inhibitor imatinib and mutation of Asp-802 to Val confers resistance

The kinase inhibitor imatinib is used in the treatment of chronic myeloid leukaemia, where it targets the intracellular Bcr-Abl tyrosine kinase, and gastrointestinal stromal tumours, where it targets either the KIT or PDGF tyrosine kinase receptors. Here, we report that imatinib is also an effective inhibitor of the closely related FMS receptor for macrophage colony stimulating factor and that mutation of Asp 802 of FMS to Val confers imatinib resistance. Imatinib readily reverted the transformed phenotype of haemopoietic and fibroblast cell lines that express the oncogene v-fms and also inhibited the growth of the Bacl.2F5 macrophage cell line. The cellular IC50 value of imatinib for FMS was similar to those for Bcr-Abl and KIT. Consequently, imatinib may also prove effective for the treatment of diseases whose progression is dependent upon macrophage-colony stimulating factor, this includes certain aspects of cancer and inflammation.

Fibroblast phenotype plasticity: relevance for understanding heterogeneity in "fibroblastic" tumors

Cellular transformations, reflecting phenotypic plasticity, characterize embryonic life, would-repair, physiological adaptation, and neoplasia. Fibroblastic tumors show a range of cellular differentiation, which can be rationalized in terms of phenotypic plasticity of the "normal" fibroblast. In this paper, the various kinds of fibroblast transformation are discussed, and some insights provided into the molecular mechanisms involved. Comparable molecular events may take place in neoplastic fibroblasts to produce the heterogeneous tumors nevertheless identified as fibroblastic. The following transformations are discussed: histiocytic, and fibrohistiocytic tumors; adipocytic, and lipogenic tumors; myofibroblastic, and myofibroblastic tumors. A definition of the fibroblast is required. This consists of spindle-cell morphology, vimentin-staining, and abundant rough endoplasmic reticulum. Transformation to histiocytic, lipogenic and myofibroblastic phenotypes requires the development of lysosomes, lipid droplets and lamina, and peripheral myofilaments and fibronexuses respectively. These occur in non-malignant transforming (transdifferentiating) fibroblasts, and also in tumors identified as fibrohistiocytic, lipogenic and myofibroblastic. The molecular basis of the myofibroblast transformation is probably the best studied. It is driven primarily by transforming growth factor beta. Investigations into the mechanisms of differentiation in normal fibrobiasts could prove fertile ground for defining comparable differentiation in tumors. In this respect, there are very few publications on the presence of growth factors in tumors or tumor-like lesions. There is, however, increasing investigation into gene expression and gene products in tumors, which bear on the differentiation process. Ultimately, our understanding of the molecular events controlling differentiation in cancer will lead to control, cure and prevention.

Imatinib mesylate (STI-571) enhances antigen-presenting cell function and overcomes tumor-induced CD4+ T-cell tolerance

Tumor antigen–specific T-cell tolerance imposes a significant barrier to the development of effective therapeutic cancer vaccines. Bone marrow–derived antigen-presenting cells (APCs) are critical in the induction of this unresponsive state. Here we show that in vitro treatment of APCs with the tyrosine kinase inhibitor, imatinib mesylate (STI-571), enhances the activation of naive antigen-specific T cells and restores the responsiveness of tolerant T cells from tumor-bearing hosts. Furthermore, in vivo treatment with STI-571 not only prevented the induction of tolerance in tumor-specific CD4+ T cells, preserving their responsiveness to a subsequent immunization, but also resulted in enhanced vaccine efficacy. These findings demonstrate that tolerance to tumor antigens is not an insurmountable obstacle and points to modulation of APC function as a promising strategy in the immunotherapy of cancer.

Macrophage colony-stimulating factor receptor c-fms is a novel target of imatinib

Imatinib is a tyrosine kinase inhibitor that suppresses the growth of bcr-abl-expressing chronic myeloid leukemia (CML) progenitor cells by blockade of the adenosine triphosphate (ATP)-binding site of the kinase domain of bcr-abl. Imatinib also inhibits the c-abl, platelet-derived growth factor (PDGF) receptor, abl-related gene (ARG) and stem-cell factor (SCF) receptor tyrosine kinases, and has been used clinically to inhibit the growth of malignant cells in patients with CML and gastrointestinal stromal tumors (GISTs). Although initially considered to have minimal effects of normal hematopoiesis, recent studies show that imatinib also inhibits the growth of some nonmalignant hematopoietic cells, including monocyte/macrophages. This inhibition could not be attributed to the known activity profile of imatinib. Here, we demonstrate for the first time that imatinib targets the macrophage colony-stimulating factor (M-CSF) receptor c-fms. Phosphorylation of c-fms was inhibited by therapeutic concentrations of imatinib, and this was not due to down-regulation in c-fms expression. Imatinib was also found to inhibit M-CSF-induced proliferation of a cytokine-dependent cell line, further supporting the hypothesis that imatinib affects the growth and development of monocyte and/or macrophages through inhibition of c-fms signaling. Importantly, these results identify an additional biologic target to those already defined for imatinib. Imatinib should now be assessed for activity in diseases where c-fms activation is implicated, including breast and ovarian cancer and inflammatory conditions.

Imatinib inhibits the functional capacity of cultured human monocytes

Imatinib is a tyrosine kinase inhibitor that has been reported to specifically inhibit the growth of bcr-abl expressing chronic myeloid leukaemia progenitors. This drug functions by blocking the ATP-binding site of the kinase domain of bcr-abl, and has also been found to inhibit the c-abl, platelet-derived growth factor receptor, ARG and stem cell factor receptor tyrosine kinases. Reports have recently emerged demonstrating that imatinib also inhibits the growth of non-malignant haemopoietic cells. Here, we demonstrate that concentrations of imatinib within the therapeutic dose range inhibit the function of cultured monocytes (CM) from normal donors. A decrease in the response of CM to LPS was observed morphologically and functionally, with CM grown in the presence of imatinib showing decreased pseudopodia formation and inhibition of IL-6 and TNF-alpha production following LPS stimulation. Imatinib also reduced the ability of M-CSF and GM-CSF stimulated CM to phagocytose zymosan particles, with uptake of non-opsonized zymosan by M-CSF stimulated CM (M-CM) being most affected. M-CM that had been cultured in the presence of imatinib were also impaired in their ability to stimulate responder cells in a mixed lymphocyte reaction. These results demonstrate that human monocytes cultured in the presence of imatinib are functionally impaired, and suggest that imatinib displays inhibitory activity against other kinase(s) that play a role in monocyte/macrophage development.

RNA interference: a potent tool for gene-specific therapeutics

RNA interference (RNAi) is a process through which double-stranded RNA induces the activation of cellular pathways, leading to potent and selective silencing of genes with homology to the double strand. Much excitement surrounding small interfering RNA (siRNA)-mediated therapeutics arises from the fact that this approach overcomes many of the shortcomings previously experienced with approaches such as antibodies, antisense oligonucleotides and pharmacological inhibitors. Induction of RNAi through administration of siRNA has been successfully used in treatment of hepatitis, viral infections, and cancer. In this review we will present a brief history of RNAi, methods of inducing RNAi, application of RNAi in the therapeutic setting, and the possibilities of using this highly promising approach in the context of transplantation.