Inhibition of tumor angiogenesis as a strategy to circumvent acquired resistance to anti-cancer therapeutic agents

Antibodies for angiogenesis inhibition, vascular targeting and endothelial cell transcytosis

The endothelium is increasingly recognized as a target for biomedical intervention, not only for its accessibility to molecular agents coming from the blood-stream, but also for the active role played by endothelial cell proliferation to support diseases such as cancer, blinding ocular disorders and chronic inflammatory conditions. The notion that solid tumors cannot grow beyond a size of few millimeters without inducing the proliferation of new blood vessels has stimulated the search for mediators of angiogenesis and for inhibitors of this process, culminating in the approval of a humanized monoclonal antibody to VEGF-A for oncology applications. In parallel, researchers have begun to consider imaging and therapeutic strategies based on the selective delivery of bioactive agents to the new blood vessels, mediated by monoclonal antibody derivatives. Recently, the field of vascular targeting research has been extended to the investigation of molecular agents that may mediate endothelial cell transcytosis, in the hope to overcome this body barrier for drug delivery. This article reviews some of the most significant advances in these areas, and outlines future challenges and opportunities.

SSeCKS metastasis-suppressing activity in MatLyLu prostate cancer cells correlates with vascular endothelial growth factor inhibition

SSeCKS, a Src-suppressed protein kinase C substrate with metastasis suppressor activity, is the rodent orthologue of human gravin/AKAP12, a scaffolding protein for protein kinase A and protein kinase C. We show here that the tetracycline-regulated reexpression of SSeCKS in MatLyLu (MLL) prostate cancer cells suppressed formation of macroscopic lung metastases in both spontaneous and experimental models of in vivo metastasis while having minimal inhibitory effects on the growth of primary-site s.c. tumors. SSeCKS decreased angiogenesis in vitro and in vivo by suppressing vascular endothelial growth factor (VEGF) expression in MLL tumor cells as well as in stromal cells. The forced reexpression of VEGF(165) and VEGF(121) isoforms was sufficient to reverse aspects of SSeCKS metastasis-suppressor activity in both the experimental and spontaneous models. SSeCKS reexpression in MLL cells resulted in the down-regulation of proangiogenic genes, such as osteopontin, tenascin C, KGF, angiopoietin, HIF-1alpha, and PDGFRbeta, and the up-regulation of antiangiogenic genes, such as vasostatin and collagen 18a1, a precursor of endostatin. These results suggest that SSeCKS suppresses formation of metastatic lesions by inhibiting VEGF expression and by inducing soluble antiangiogenic factors.

Antiangiogenesis response of endothelial cells to the antitumour drug 10-methoxy-9-nitrocamptothecin

10-Methoxy-9-nitrocamptothecin (MONCPT), a topoisomerase I inhibitor, exhibited high anticancer activity in solid tumour xenograft animal models in our previous study. We hypothesized that this phenomenon was associated with antiangiogenesis response. In the present study, we found that MONCPT exhibited high antiproliferation action in human EA.hy926 endothelial cells and the IC(50) value was 0.13+/-0.04microM (MTT assay). With AO/EB stain, MONCPT (50-5000nM)-mediated apoptosis was observed in EA.hy926 cells, and the similar results were shown in flow cytometry assay, the percentage of apoptotic cells induced by MONCPT (50-5000nM) was 9.2-58.5%. In Chick embryo chorioallantoic membrane (CAM) assay, MONCPT (1-5microg) resulted in a dose-dependent angiogenic inhibition. In addition, MONCPT significantly inhibited chemotactic-migration invasion on gelatin and tube formation on Matrigel of HUVECs. These results suggest that MONCPT has potential property for inhibiting angiogenesis which is involved in its antitumour activity.

Targeting platelet-derived growth factor receptor on endothelial cells of multidrug-resistant prostate cancer

BACKGROUND

Inhibiting phosphorylation of platelet-derived growth factor receptor (PDGFR) by treatment with the PDGFR kinase inhibitor imatinib and the chemotherapeutic agent paclitaxel reduces the incidence and size of human prostate cancer bone lesions in nude mice. Because tumor cells and tumor-associated endothelial cells express activated PDGFR, the primary target for imatinib has been unclear.

METHODS

We selected multidrug-resistant human PC-3MM2 prostate cancer cells (termed PC-3MM2-MDR cells) by culturing them in increasing concentrations of paclitaxel. PC-3MM2-MDR cells were implanted into one tibia of 80 nude mice. Two weeks later, the mice were randomly assigned to receive distilled water (control group), paclitaxel, imatinib, or imatinib plus paclitaxel for 10 weeks (20 mice per group). Tumor incidence and weight, bone structure preservation and osteolysis, and the incidence of lymph node metastasis were determined. The phosphorylation status of PDGFR on tumor cells and tumor-associated endothelial cells and levels of apoptosis were examined with immunohistochemical analyses. Microvessel density was assessed as the number of cells expressing CD31/platelet endothelial cell adhesion molecule 1 (PECAM-1). All statistical tests were two-sided.

RESULTS

PC-3MM2-MDR cells were resistant to paclitaxel and imatinib in vitro. Treatment of implanted mice with imatinib plus paclitaxel led to statistically significant decreases in bone tumor incidence (control = 19 mice with tumors of 19 mice total; imatinib plus paclitaxel = four of 18 mice; P < .001), median tumor weight (control = 1.3 g, interquartile range [IQR] = 1.0-1.9; imatinib plus paclitaxel = 0.1 g, IQR = 0-0.3; P < .001), bone lysis, and the incidence of lymph node metastasis (control = 19 of 19 mice total; imatinib plus paclitaxel = three of 18 mice; P < .001). Treatment with imatinib alone had similar effects, and imatinib treatment also inhibited phosphorylation of PDGFR on tumor cells and tumor-associated endothelial cells and increased the level of apoptosis of endothelial cells, but not tumor cells. Treatment with imatinib and more so with imatinib and paclitaxel decreased mean vessel density (three CD31/PECAM-1-positive cells, 95% confidence interval [CI] = 0 to 9; and control group = 38 CD31/PECAM-1-positive cells, 95% CI = 17 to 59) (P < .001), which was followed by apoptosis of tumor cells.

CONCLUSION

Tumor-associated endothelial cells, rather than tumor cells themselves, appear to be the target for imatinib in prostate cancer bone metastasis.

Combination of MIG (CXCL9) chemokine gene therapy with low-dose cisplatin improves therapeutic efficacy against murine carcinoma

MIG (monokine induced by interferon-gamma) is a CXC chemokine ligand (CXCL9) that can potently inhibit angiogenesis, and displays thymus-dependent antitumor effects. The effectiveness of a treatment combining gene therapy with plasmid-borne MIG (pORF-MIG) and low-dose cisplatin chemotherapy was determined using colon carcinoma (CT26) and Lewis lung carcinoma (LL/2c) murine models. The program was carried out via intramuscular delivery of pORF-MIG at 100 mug/mouse twice a week for 4 weeks, and/or intraperitoneal delivery of cisplatin at 0.6 mg/kg/mouse every 3 days for 48 days. Tumor volume and survival time were evaluated after treatment. CD31 immunohistochemical staining in tumor tissues and alginate capsule models in vivo was used to evaluate angiogenesis. Induction of apoptosis and cytotoxic T-lymphocyte (CTL) activity were also assessed. The combination of pORF-MIG and low-dose cisplatin produced significant antitumor activity, with complete tumor regression in 4/10 of CT26 colon carcinomas and 3/10 of LL/2c lung carcinomas, low vascularity, in alginate capsules, apparently degraded tumor microvessel density, and increased induction of apoptotic and CTL activities compared with either treatment alone. This study suggests that the combination of pORF-MIG plus cisplatin augments the inhibition of angiogenesis and the induction of apoptosis or CTL activity, all of which enhance antitumor activity. These findings may prove useful in further explorations of the application of combinatorial approaches to the treatment of solid tumors.

Selective occlusion of tumor blood vessels by targeted delivery of an antibody-photosensitizer conjugate

The irregular vasculature and high interstitial pressure of solid tumors hinder the delivery of cytotoxic agents to cancer cells. As a consequence, the doses of chemotherapy necessary to achieve complete tumor eradication are associated with unacceptably high toxicities. The selective thrombosis of tumor blood vessels has been postulated as an alternative avenue for combating cancer, depriving tumors of nutrients and oxygen and causing an avalanche of tumor cell deaths. The human antibody L19, specific to the EDB domain of fibronectin, a marker of angiogenesis, is capable of selective in vivo localization around tumor blood vessels and is thus a suitable agent for delivering toxic payloads to the tumor neovasculature. Here we show that a chemical conjugate of the L19 antibody with the photosensitizer bis(triethanolamine)Sn(IV) chlorin e(6), after intravenous injection and irradiation with red light, caused an arrest of tumor growth in mice with subcutaneous tumors. By contrast, a photosensitizer conjugate obtained with an antibody of identical pharmacokinetic properties but irrelevant specificity did not exhibit a significant therapeutic effect. These results confirm that vascular targeting strategies, aimed at the selective occlusion/disruption of tumor blood vessels, have a significant anticancer therapeutic potential and encourage the use of antibody-photosensitizer conjugates for the therapy of superficial tumors and possibly other angiogenesis-related pathologies.

Phase I and pharmacokinetic study of oral thalidomide in patients with advanced hepatocellular carcinoma

PURPOSE

To evaluate the dose-limiting toxicities (DLT), maximum tolerated dose (MTD), and pharmacokinetics of thalidomide in patients with advanced hepatocellular carcinoma (HCC).

METHODS

Patients with advanced HCC who were not feasible for definitive local therapy were eligible. Patients were enrolled in a cohort of three to receive thalidomide twice daily for 1 week to determine the MTD. Intra-patient dose escalation was permitted. Pharmacokinetic studies were performed at the first dose level and repeated at the second dose level of each patient.

RESULTS

Fifteen patients were accrued at four dose levels with the starting dose range 100-400 mg/day. Two patients at 400 mg/day experienced DLT (grade 3 angioedema and dyspnea, respectively). The MTD of twice-daily schedule was determined as 300 mg/day. The mean steady-state maximal blood concentration and mean steady-state area under the curve had a trend toward positive correlation, but non-linear proportionate, to the daily dose of thalidomide. Pharmacokinetic parameters are comparable for patients of Child-Pugh's A and B. Apparent mild, transient drug-induced transaminitis was early onset, self-limited, which occurred in 30.7% of patients. Serum hepatitis B or C viral titers was largely not affected.

CONCLUSION

The absorption and elimination of thalidomide are not significantly different in HCC patients with compensated or decompensated hepatic dysfunction.

YM-359445, an Orally Bioavailable Vascular Endothelial Growth Factor Receptor-2 Tyrosine Kinase Inhibitor, Has Highly Potent Antitumor Activity against Established Tumors

PURPOSE

The vascular endothelial growth factor receptor-2 (VEGFR2) tyrosine kinase has been implicated in the pathologic angiogenesis associated with tumor growth. YM-359445 was a (3Z)-3-quinolin-2(1H)-ylidene-1,3-dihydro-2H-indol-2-one derivative found while screening based on the inhibition of VEGFR2 tyrosine kinase. The aim of this study was to analyze the efficacy of this compound both in vitro and in vivo.

EXPERIMENTAL DESIGN

We tested the effects of YM-359445 on VEGFR2 tyrosine kinase activity, cell proliferation, and angiogenesis. The antitumor activity of YM-359445 was also tested in nude mice bearing various established tumors and compared with other VEGFR2 tyrosine kinase inhibitors (ZD6474, CP-547632, CGP79787, SU11248, and AZD2171), a cytotoxic agent (paclitaxel), and an epidermal growth factor receptor tyrosine kinase inhibitor (gefitinib).

RESULTS

The IC50 of YM-359445 for VEGFR2 tyrosine kinase was 0.0085 micromol/L. In human vascular endothelial cells, the compound inhibited VEGF-dependent proliferation, VEGFR2 autophosphorylation, and sprout formation at concentrations of 0.001 to 0.003 micromol/L. These concentrations had no direct cytotoxic effect on cancer cells. In mice bearing various established tumors, including paclitaxel-resistant tumors, once daily oral administration of YM-359445 at doses of 0.5 to 4 mg/kg not only inhibited tumor growth but also reduced its vasculature. YM-359445 had greater antitumor activity than other VEGFR2 tyrosine kinase inhibitors. Moreover, in human lung cancer A549 xenografts, YM-359445 markedly regressed the tumors (73%) at a dose of 4 mg/kg, whereas gefitinib caused no regression even at 100 mg/kg.

CONCLUSION

Our results show that YM-359445 is more potent than orally bioavailable VEGFR2 tyrosine kinase inhibitors, which leads to great expectations for clinical applicability.

2-(Quinazolin-4-ylamino)-[1,4]benzoquinones as covalent-binding, irreversible inhibitors of the kinase domain of vascular endothelial growth factor receptor-2

A series of 2-(quinazolin-4-ylamino)-[1,4] benzoquinone derivatives that function as potent covalent-binding, irreversible inhibitors of the kinase domain of vascular endothelial growth factor receptor-2 (VEGFR-2) has been prepared by ceric ammonium nitrate oxidation of substituted (2,5-dimethoxyphenyl)(6,7-disubstituted-quinazolin-4-yl)amines and by displacement of the chlorine atom of substituted 2-chloro-5-(6,7-disubstituted-quinazolin-4-ylamino)-[1,4]benzoquinones with various amines, anilines, phenols, and alcohols. Enzyme studies were conducted in the absence and presence of glutathione and plasma. Several of the compounds inhibit VEGF-stimulated autophosphorylation in intact cells. Kinetic experiments were performed to study the reactivity of selected inhibitors toward glutathione. Reactivities correlated with LUMO energies calculated as averages of those of individual conformers weighted by the Boltzmann distribution. These results and molecular modeling were used to rationalize the biological observations. The compounds behave as non-ATP-competitive inhibitors. Unequivocal evidence, from mass spectral studies, indicates that these inhibitors form a covalent interaction with Cys-1045. One member of this series displays antitumor activity in an in vivo model.

Low-Dose Metronomic Daily Cyclophosphamide and Weekly Tirapazamine: A Well-Tolerated Combination Regimen with Enhanced Efficacy That Exploits Tumor Hypoxia

The recent clinical successes of antiangiogenic drug-based therapies have also served to highlight the problem of acquired resistance because, similar to other types of cancer therapy, tumors that initially respond eventually stop doing so. Consequently, strategies designed to delay resistance or treat resistant subpopulations when they arise have assumed considerable importance. This requires a better understanding of the various possible mechanisms for resistance. In this regard, reduced oxygenation is thought to be a key mediator of the antitumor effects of antiangiogenic therapies; accordingly, increased hypoxia tolerance of the tumor cells presents a potential mechanism of resistance. However, hypoxia can also be exploited therapeutically through the use of hypoxic cell cytotoxins, such as tirapazamine. With this in mind, we measured the oxygenation of PC-3 human prostate cancer xenografts subjected to chronic low-dose metronomic (LDM) antiangiogenic chemotherapy using cyclophosphamide given through the drinking water. We found that LDM cyclophosphamide impairs the oxygenation of PC-3 xenografts even during relapse, coinciding with reduced microvessel density. Combination of LDM cyclophosphamide with tirapazamine results in significantly improved tumor control in the PC-3, HT-29 colon adenocarcinoma, and MDA-MB-231 breast cancer human xenograft models without having a negative effect on the favorable toxicity profile of LDM cyclophosphamide. These results provide further evidence that reduced vascular dependence/increased hypoxia tolerance may be a basis for eventual resistance of tumors exposed to long-term LDM chemotherapy.

Ganoderma lucidum polysaccharides peptide inhibits the growth of vascular endothelial cell and the induction of VEGF in human lung cancer cell

Ganoderma lucidum Polysaccharide Peptide (Gl-PP) has shown some effects as anti-tumors in mice and potential anti-angiogenesis. In this study, we elucidated the possible mechanism of Gl-PP action on anti-angiogenesis of tumor. Our research indicated that the proliferation of HUVECs was inhibited by Gl-PP in a dose-dependent fashion, but not because of cytotoxicity. Flow cytometric studies revealed that Gl-PP treatment of HUVECs could induce cell apoptosis directly. Moreover, addition of Gl-PP also led to a reduction of Bcl-2 anti-apoptotic protein expression and an increase of Bax pro-apoptotic protein expression of HUVECs. Therefore, inducing cell apoptosis by Gl-PP might be the mechanism of inhibiting HUVEC proliferation. Human lung carcinoma cells PG when exposed to high dose of Gl-PP in hypoxia for 18 h resulted in a decrease in the secreted VEGF. Taken together, these findings support the hypothesis that the key attribute of the anti-angiogenic potential of Gl-PP is that it may directly inhibit vascular endothelial cell proliferation or indirectly decrease growth factor expression of tumor cells.

Antiangiogenic chimeric anti-endoglin (CD105) antibody: pharmacokinetics and immunogenicity in nonhuman primates and effects of doxorubicin

We generated a human/mouse chimeric antibody c-SN6j of human IgG1 isotype from a murine anti-human endoglin (EDG) monoclonal antibody (mAb) SN6j that suppressed angiogenesis, tumor growth and metastasis in mice. We determined pharmacokinetics (PKs) and immunogenicity of c-SN6j in monkeys after multiple i.v. injections. A dose-escalation study was performed by administration of c-SN6j into six monkeys at the dose of 1 mg, 3 mg and 10 mg per kg body weight. In addition, both c-SN6j (3 mg/kg) and doxorubicin (0.275 mg/kg) were injected into two monkeys. c-SN6j and doxorubicin were injected twice a week for 3 weeks. We developed a unique and sensitive ELISA by sequentially targeting the common and idiotypic epitopes of c-SN6j-Fv to quantify plasma c-SN6j. Application of the ELISA showed that increasing the c-SN6j dose resulted in a proportional increase in the circulating c-SN6j after the first injection. In addition, the estimated area under the curve (AUC) for the first injection of c-SN6j is proportional to dose. We carried out detailed analyses of PKs of c-SN6j during and after the repeated injections. Our model of PKs fitted the empirical data well. Addition of doxorubicin modulated the PK parameters. We developed two ELISAs to separately determine the immune responses to the murine part and the human part of c-SN6j in monkeys. Interestingly, the murine part induced a weaker immune response than the human part. Doxorubicin potentiated the immune responses. Increasing the dose of c-SN6j increased plasma levels of c-SN6j but did not increase the immune responses to c-SN6j.

Concepts and clinical trials of dose-dense chemotherapy for breast cancer

This article will review the strategy of dose-dense administration of chemotherapy for breast cancer. Increased dose density is achieved by reducing the interval between each dose of chemotherapy. The cumulative drug dose remains constant, but the same amount of drug is administered over a shorter period. Mathematical models of tumor growth have provided the basis for the clinical application of dose-dense chemotherapy. The Norton-Simon model suggests that increasing the dose density of chemotherapy will increase efficacy by minimizing the opportunity for regrowth of tumor cells between cycles of chemotherapy. Intergroup trial 9741, coordinated by the Cancer and Leukemia Group B (CALGB), tested the 2 hypotheses that dose-dense and sequential administration of chemotherapy regimens incorporating doxorubicin, cyclophosphamide, and paclitaxel would improve disease-free survival and overall survival. A statistically significant 4-year disease-free survival advantage was detected for the 2 dose-dense regimens compared with the regimens administered every 3 weeks. The mathematical concepts and previous clinical trials of dose density that contributed to the design of CALGB 9741 will be reviewed. The strengths and limitations of CALGB 9741 will then be discussed before the presentation of future directions of research and recommendations for clinical practice today.

Polysaccharides from the root of Angelica sinensis protect bone marrow and gastrointestinal tissues against the cytotoxicity of cyclophosphamide in mice

Cyclophosphamide (CY) is a cytostatic agent that produces systemic toxicity especially on cells with high proliferative capacity, while polysaccharides from Angelica sinensis (AP) have been shown to increase the turnover of gastrointestinal mucosal and hemopoietic stem cells. It is not known whether AP has an effect on CY-induced cytotoxicity on bone marrow and gastrointestinal tract. In this study, we assessed the protective actions of AP on CY-induced leukopenia and proliferative arrest in the gastroduodenal mucosa in mice. Subcutaneous injection of CY (200 mg/kg) provoked dramatic decrease in white blood cell (WBC) count and number of blood vessels and proliferating cells in both the gastric and duodenal mucosae. Subcutaneous injection of AP significantly promoted the recovery from leukopenia and increased number of blood vessels and proliferating cells in both the gastric and duodenal tissues. Western blotting revealed that CY significantly down-regulated the protein expression of vascular endothelial growth factor (VEGF), c-Myc and ornithine decarboxylase (ODC) in gastric mucosae but had no effect on epidermal growth factor (EGF) expression. AP also reversed the dampening effect of CY on VEGF expression in the gastric mucosa. These data suggest that AP is a cytoprotective agent which can protect against the cytotoxicity of CY on hematopoietic and gastrointestinal tissues when the polysaccharide is co-administered with CY in cancer patients during treatment regimen.

Endothelin B receptor agonist, IRL 1620, enhances the anti-tumor efficacy of paclitaxel in breast tumor rats

Pharmacological agents that increase tumor blood flow could be utilized to promote the delivery of anti-cancer drugs. We have demonstrated that administration of endothelin-1 (ET-1) to breast tumor bearing rats transiently increased tumor blood flow by stimulating endothelin B (ET(B)) receptors. The present study evaluated the effect of ET(B) receptor agonist, IRL 1620, on breast tumor perfusion, concentration of [3H]paclitaxel in tumor and tissues, and efficacy of paclitaxel in N-methyl nitrosourea induced breast tumor bearing rats. Administration of IRL 1620 (3 and 9 nmol/kg) significantly increased (203 and 140%, respectively) breast tumor perfusion. BQ 788, an ET(B) receptor antagonist, pretreatment completely abolished IRL 1620 induced increase in tumor perfusion. Tumor [3H]paclitaxel concentration was increased by 308% when [3H]paclitaxel was administered 15 min after IRL 1620 (3 nmol/kg) compared to vehicle treated rats. However, IRL 1620 did not increase [3H]paclitaxel concentrations in other organs. Efficacy study showed that paclitaxel (5 mg/kg) administration on every third day for a total of five doses produced 60.0, 4.5 and 0% reduction in tumor volume, tumor progression and complete tumor remission, respectively, compared to saline treated rats. However, paclitaxel (5 mg/kg) when administered 15 min after IRL 1620 (3 nmol/kg) produced 268.9, 210.3 and 20% reduction in tumor volume, tumor progression and complete remission of tumors, respectively, compared to saline treated rats. In conclusion, IRL 1620 significantly enhanced delivery and effectiveness of paclitaxel in an animal model of breast cancer.

Vascular Endothelial Growth Factor Trap Combined with Paclitaxel Strikingly Inhibits Tumor and Ascites, Prolonging Survival in a Human Ovarian Cancer Model

Ovarian cancer is characterized by i.p. carcinomatosis and massive ascites. Vascular endothelial growth factor (VEGF) plays a pivotal role in tumor angiogenesis and vascular leakage leading to ascites. We assessed the efficacy of a soluble decoy receptor (VEGF Trap) combined with paclitaxel, in a mouse model of human ovarian cancer. Tumor burden after VEGF Trap plus paclitaxel was reduced by approximately 98% versus controls. No measurable ascites developed in the treated group. Morphologic studies showed that most residual tumor had degenerative changes. Diaphragmatic and hepatic tumors were not found in the VEGF Trap plus paclitaxel group in contrast to controls, indicating lack of metastasis. In vivo FITC-lectin tumor vessel imaging showed sparse, short, straight vessels in treated mice as compared to controls, in which vessels were numerous, irregular, tortuous, and leaky. In a survival study, all controls underwent euthanasia between 29 and 58 days after tumor cell inoculation (cachexia, extensive ascites, and tumor masses). In the VEGF Trap plus paclitaxel group, mice were ambulating and eating normally with no signs of disease for at least 81 days after tumor cell inoculation, and survival occurred for 129.9 +/- 38.88 days with no further treatment. We conclude that combination therapy with VEGF Trap plus paclitaxel may provide a novel, long-lasting therapeutic strategy for treatment of patients with ovarian cancer associated with ascites.

Therapeutic implications of intrinsic or induced angiogenic growth factor redundancy in tumors revealed

There is a large family of known pro-angiogenic growth factors, many of which can be expressed by a single tumor, especially in advanced stages of disease. Such redundancy, which can be amplified by hypoxia, has long been suspected as a potential cause of acquired resistance when tumors are treated with highly specific targeted antiangiogenic drugs. Definitive preclinical evidence for antiangiogenic drug evasion by alternate pathways of angiogenesis in tumor cells, likely induced by antiangiogenic drug-mediated increases in tumor hypoxia, is reported in this issue of Cancer Cell (casanovas et al.,2005); it has major implications for the development of strategies to prolong the effectiveness of antiangiogenic drugs as monotherapies, and for their use as chemosensitizing agents in combination treatment strategies.

Anti-angiogenic treatment of breast cancer using metronomic low-dose chemotherapy

We have been studying the molecular and cellular basis of chronic low-dose, frequently administered, metronomic chemotherapy regimens for the treatment of cancer in a variety of preclinical models, including human breast cancer xenografts. The advantages of metronomic-maintenance-type chemotherapy regimens include significantly reduced host toxicity, potentially reduced costs, increased convenience for patients when oral chemotherapy drugs are used, and the possibility of adopting chronic combination therapies involving conventional chemotherapy drugs and cytostatic molecularly targeted therapies. However, a disadvantage is the empiricism associated with determining the optimal biologic dose (OBD). Recently, we have developed a surrogate biomarker approach involving measurement of circulating endothelial progenitor cells (CEPs) in peripheral blood to help determine the OBD of anti-angiogenic drugs or treatments, including metronomic chemotherapy. Using this approach we determined the OBD for different metronomic chemotherapy regimens and then tested the effect of such drugs for the treatment of established, advanced (high volume) and widespread human breast cancer metastases in immunodeficient mice. This treatment strategy, which was maintained for over 6 months, with no breaks, resulted in marked prolongation of survival and was devoid of overt toxicity. These results suggest the possibility of using metronomic chemotherapy regimens as an adjuvant therapy for early-stage disease, including breast cancer, as was demonstrated recently using long-term daily low-dose UFT for the treatment of early-stage resected non-small cell lung cancer or UFT in combination for early stage breast cancer combined with tamoxifen.

Combination of low-dose gemcitabine and recombinant quail vascular endothelial growth factor receptor-2 as a vaccine induces synergistic antitumor activities

Vascular endothelial growth factor receptor-2 (VEGFR-2) has been shown to play a major role in inducing the full spectrum of VEGF biological response which is essential for tumor angiogenesis. We have demonstrated that immunotherapy of tumors with a vaccine based on quail homologous VEGFR-2 (qVEGFR) was effective in providing both protective and therapeutic antitumor immunity in several tumor models in mice. The purpose of this study was to determine whether the combination therapy of low-dose gemcitabine with qVEGFR as a vaccine could inhibit tumor growth to a greater extent. To test this concept, H22 hepatoma and Lewis lung carcinoma models were established in BALB/c mice and C57BL/6 mice, respectively. Mice were treated with either qVEGFR as a protein vaccine, gemcitabine, or both agents together. qVEGFR or low-dose chemotherapy treatment individually resulted in tumor inhibition to a certain extent.Remarkably, the combination therapy resulted in synergistic antitumor activity. Histological examination revealed that there was endothelial deposition of immunoglobulins within tumor tissues from mice treated with vaccine or combination therapy, especially intratumor angiogenesis was suppressed more significantly for the combination group. Also, ELISPOT analysis showed that mice treated with either qVEGFR alone or in combination with low-dose chemotherapy produced similar amount of anti-VEGFR antibody-producing B cells, which suggested that low-dose gemcitabine did not suppress the host's immune response, but potentiated the antitumor activity of the qVEGFR vaccine. Furthermore, TUNEL staining demonstrated a significant increase in the number of TUNEL-positive cells in the combination group compared with those of other groups. The observations may provide a new bio-chemotherapeutic approach for cancer.

alpha-fetoprotein response predicts survival benefits of thalidomide in advanced hepatocellular carcinoma

BACKGROUND

Radiographic measurements do not always reflect the biological response of hepatocellular carcinoma to drug therapy.

AIMS

To evaluate the clinical implications of tumour marker (alpha-fetoprotein) response in advanced hepatocellular carcinoma patients with thalidomide treatment.

PATIENTS AND METHODS

Forty-two advanced hepatocellular carcinoma patients with baseline alpha-fetoprotein levels above 200 ng/mL and thalidomide therapy were included. Serum alpha-fetoprotein levels were measured every 4 weeks. alpha-fetoprotein response was defined as a 50% or greater reduction of alpha-fetoprotein levels for 4 or more weeks during treatment. Radiographic response was assessed by World Health Organization criteria; survivals were estimated by Kaplan-Meier method and prognostic factors were assessed by Cox's proportional hazard model.

RESULTS

With intention-to-treat analysis, radiographic response and alpha-fetoprotein response were obtained in 7% (three of 42, 95% confidence interval: 0-15) and 24% (10 of 42, 95% CI: 10-38) of patients, respectively. All radiographic response was observed in alpha-fetoprotein responders. Multivariate analyses showed alpha-fetoprotein response was independent prognostic factor for both progression-free survival (relative risk = 0.394, 95% CI: 0.189-0.820, P = 0.013) and overall survival (relative risk = 0.241, 95% CI: 0.096-0.606, P =0.003), whereas radiographic response was not.

CONCLUSION

alpha-fetoprotein response can more accurately reflect the biological response of advanced hepatocellular carcinoma to thalidomide therapy than radiographic response.

Specific Occlusion of Murine and Human Tumor Vasculature by VCAM-1–Targeted Recombinant Fusion Proteins

BACKGROUND

The tumor vasculature is increasingly recognized as a target for cancer therapy. We developed and evaluated recombinant fusion proteins targeting the coagulation-inducing protein soluble tissue factor (sTF) to the luminal tumor endothelial antigen vascular cell adhesion molecule 1 (VCAM-1, CD106).

METHODS

We generated fusion proteins consisting of sTF fused to antibody fragments directed against mouse or human VCAM-1 and characterized them in vitro by flow cytometry, surface plasmon resonance, and two-stage coagulation assays. Their therapeutic effects were tested in three human xenograft tumor models: L540rec Hodgkin lymphoma, Colo677 small-cell lung carcinoma, and Colo677/HDMEC small-cell lung carcinoma with human vasculature. Toxicity was analyzed by histologic examination of organs and determination of laboratory blood parameters.

RESULTS

The fusion proteins bound VCAM-1 with nanomolar affinities and had the same coagulation activity as an sTF standard. Xenograft tumor-bearing mice treated with fusion protein (FP) alone or in combination with lipopolysaccharide (FP/L) or doxorubicin (FP/D) exhibited tumor-selective necrosis (L540rec tumors: 74% tumor necrosis [95% confidence interval {CI} = 55% to 93%] with FP/L versus 13% tumor necrosis [95% CI = 4% to 22%] with vehicle; Colo677 tumors: 26% [95% CI = 16% to 36%] with FP versus 8% [95% CI = 2% to 14%] with vehicle); tumor growth delay (Colo677/HDMEC: mean tumor weights after 3 days = 42 mg in FP-treated mice versus 71 mg in vehicle-treated mice, difference = 29 mg, 95% CI = 8 to 100, Mann-Whitney P = .008); and some tumor regressions (one of seven FP-treated Colo677 tumor-bearing mice and two of seven FP/D-treated mice). The fusion protein was well tolerated.

CONCLUSIONS

Recombinant tissue factor-based fusion proteins directed against an intraluminal tumor endothelial cell marker induce tumor-selective intravascular coagulation, tumor tissue necrosis, and tumor growth delay.

Combination of antiangiogenic therapy with other anticancer therapies: results, challenges, and open questions

Angiogenesis is necessary for tumor growth. Drug discovery efforts have identified several potential therapeutic targets on endothelial cells and selective inhibitors capable of slowing tumor growth or producing tumor regression by blocking angiogenesis in in vivo tumor models. Certain antiangiogenic therapeutics increase the activity of cytotoxic anticancer treatments in preclinical models. More than 75 antiangiogenic compounds have entered clinical trials. Most of the early clinical testing was conducted in patients with advanced disease resistant to standard therapies. Several phase III trials have been undertaken to compare the efficacy of standard chemotherapy versus the same in combination with an experimental angiogenesis inhibitor. Preliminary results of the clinical studies suggest that single-agent antiangiogenic therapy is poorly active in advanced tumors. Although some of the results of combination trials are controversial, recent positive outcomes with an antivascular endothelial growth factor antibody combined with chemotherapy as front-line therapy of metastatic colorectal cancer have renewed enthusiasm for this therapeutic strategy. This article presents an overview of experimental and clinical studies of combined therapy with antiangiogenic agents and highlights the challenges related to the appropriate strategies for selection of the patients, study design, and choice of proper end points for preclinical and clinical studies using these agents.

Mechanisms of drug resistance in cancer chemotherapy

The management of cancer involves procedures, which include surgery, radiotherapy and chemotherapy. Development of chemoresistance is a persistent problem during the treatment of local and disseminated disease. A plethora of cytotoxic drugs that selectively, but not exclusively, target actively proliferating cells include such diverse groups as DNA alkylating agents, antimetabolites, intercalating agents and mitotic inhibitors. Resistance constitutes a lack of response to drug-induced tumour growth inhibition; it may be inherent in a subpopulation of heterogeneous cancer cells or be acquired as a cellular response to drug exposure. Resistance varies. Although regulatory approval may require efficacy in as few as 20% of trial cohorts, a drug may subsequently be used in unselected patients displaying resistance to the treatment. Principal mechanisms may include altered membrane transport involving the P-glycoprotein product of the multidrug resistance (MDR) gene as well as other associated proteins, altered target enzyme (e.g. mutated topoisomerase II), decreased drug activation, increased drug degradation due to altered expression of drug-metabolising enzymes, drug inactivation due to conjugation with increased glutathione, subcellular redistribution, drug interaction, enhanced DNA repair and failure to apoptose as a result of mutated cell cycle proteins such as p53. Attempts to overcome resistance mainly involve the use of combination drug therapy using different classes of drugs with minimally overlapping toxicities to allow maximal dosages and with narrowest cycle intervals, necessary for bone marrow recovery. Adjuvant therapy with P-glycoprotein inhibitors and, in specific instances, the use of growth factor and protein kinase C inhibitors are newer experimental approaches that may also prove effective in abrogating or delaying onset of resistance. Gene knockout using antisense molecules may be another effective way of blocking drug resistance genes. Conversely, drug resistance may also be used to good purpose by transplanting retrovirally transformed CD34 cells expressing the MDR gene to protect the bone marrow during high-dose chemotherapy.

YM-231146, a Novel Orally Bioavailable Inhibitor of Vascular Endothelial Growth Factor Receptor-2, Is Effective against Paclitaxel Resistant Tumors

Chemotherapy using anticancer drugs induces serious the problem of multidrug resistance (MDR) in the cancer cells. In contrast, endothelial cells so rarely acquire MDR that antiangiogenesis therapy has recently been considered as an effective means for cancer chemotherapy. We screened compounds in the chemical library to find a novel and orally active antitumor agent with vascular endothelial growth factor receptor-2 tyrosine kinase (VEGF-R2 TK) inhibition. The result was YM-231146 (IC50=0.080 microM). YM-231146 inhibited VEGF-stimulated proliferation, VEGF-R2 autophosphorylation, and vessel sprout formation of human vascular endothelial cells at concentrations between 0.15-0.30 microM. However, YM-231146 did not inhibit cancer cell proliferation at these concentrations (IC50>5 microM). In the in vivo studies, once-daily oral dosing of YM-231146 to human cancer xenografts elicited antitumor activity at doses of 3-100 mg/kg. Moreover, YM-231146 completely inhibited tumor growth of paclitaxel-resistant cancer cells without decreasing body weight at a dose of 100 mg/kg. These results suggest that YM-231146 is a novel orally bioavailable inhibitor of VEGF-R2 that would be useful for the treatment of multidrug resistant tumors.

Metronomic low-dose chemotherapy as antiangiogenic therapeutic strategy for cancer. Metronomische niedrig-dosierte Chemotherapie als antiangiogene Therapiestrategie fur Tumorerkrankungen

New blood vessel formation is essential for the growth and metastasis of many cancers. As a result, antitumor activities of various angiogenesis inhibitors have been intensely explored in various tumors. Recent preclinical studies suggest that certain conventional cytotoxic agents can function as antiangiogenic drugs when administered at comparatively low doses on a continuous or very frequent schedule. Such antiangiogenic 'metronomic' scheduling of chemotherapy without extended rest periods has been shown to exert significant therapeutic antitumor efficacy with very limited toxicity in different tumor models. Combining metronomic low-dose chemotherapy regimens with specific angiogenesis inhibitors further increases efficacy. Based on the promising preclinical studies, it is anticipated that metronomic chemotherapy in combination with angiogenesis inhibitors will prove effective in clinical trials in terms of survival prolongation. While considerable progress may derive from larger randomized clinical studies, only joint efforts between basic and clinical research will ultimately advance the new paradigm of long-term metronomic antiangiogenic chemotherapy, which carries the prospect of turning cancer into a more controllable chronic disease at minimal toxicity.

A Selective Retinoid X Receptor Agonist Bexarotene (Targretin) Prevents and Overcomes Acquired Paclitaxel (Taxol) Resistance in Human Non–Small Cell Lung Cancer

PURPOSE

Paclitaxel is an important anticancer agent for the treatment of non-small cell lung cancer (NSCLC). However, its use in cancer therapy is limited by development of acquired drug resistance. The goal of this study was to determine the effect of bexarotene on development of acquired paclitaxel resistance in NSCLC.

EXPERIMENTAL DESIGN

Human NSCLC Calu3 cells were repeatedly treated in culture with intermittent paclitaxel alone or in combination with continuous bexarotene for 3 months. Thereafter, cells were isolated and characterized for their drug sensitivity in vitro and in vivo.

RESULTS

Repeat exposure to paclitaxel alone resulted in development of paclitaxel resistance with cross-resistance to multidrug resistance P-glycoprotein substrates, whereas the bexarotene/paclitaxel combination prevented the development of drug resistance and the cells remained chemosensitive. Furthermore, paclitaxel resistance could be overcome when the resistant cells were treated with the combination regimen. Fluctuation analysis showed that treatment with bexarotene decreased the rate of spontaneous development of paclitaxel resistance. In vivo, the bexarotene/paclitaxel combination regimen produced a statistically significant decrease in tumor growth in a Calu3 NSCLC xenograft model compared with the single agents (two-tailed, P < 0.05). In addition, paclitaxel-resistant Calu3 tumors treated with the bexarotene/paclitaxel combination showed greater delay in tumor growth compared with those treated with paclitaxel alone.

CONCLUSIONS

Our results suggest that bexarotene may offer a novel approach to prevent and overcome paclitaxel resistance in patients with NSCLC.

Measuring tumour vascular response to antivascular and antiangiogenic drugs

The tumour vasculature is an attractive target for therapy because of its accessibility to blood-borne anticancer agents and the reliance of most tumour cells on an intact vascular supply for their survival. For convenience, therapeutic targeting of the tumour vasculature can be divided into antiangiogenic approaches, which target the process of new blood vessel development and antivascular approaches, which target the established tumour vasculature. Many agents are now in clinical trial for the treatment of cancer by these methods. The main aim of this article is to describe the vascular effects of some of these agents and identify suitable end-points for measuring efficacy in early clinical trials. For drugs which are active below their maximum tolerated dose (MTD), measurement of vascular end-points is required to determine the most effective dosing/scheduling protocols. In addition, many of the current and developing antiangiogenic agents have additional mechanisms of action unrelated to angiogenesis per se, requiring measurement of vascular end-points to understand their mechanisms of action. Measurement of tumour microvascular density (MVD) from tumour biopsies is a common method for assessing the efficacy of antiangiogenic drugs. The limitations of this method and alternative end-points, which take into account vascular function, are discussed. Pre-clinical data regarding tumour response to the antivascular agent combretastatin A-4 3-0-phosphate (CA-4-P) are discussed in the context of guiding clinical trial planning. Finally, the accessibility of vascular end-points for clinical imaging is addressed.

Anti-vascular tumor therapy: recent advances, pitfalls and clinical perspectives

Anti-vascular tumor therapy represents a promising new strategy for cancer treatment. Anti-vascular treatment may be divided in anti-angiogenic and vascular targeting therapy. Whereas anti-angiogenic drugs aim on the inhibition of new vessel formation, vascular targeting compounds are designed to selectively destruct preexisting tumor blood vessels leading to secondary tumor cell death. Both anti-angiogenic drugs and vascular targeting agents have proven effective anti-tumoral activity in numerous preclinical studies over the last decade. In vivo, a combination with anti-vascular tumor therapy enhances the effects of other treatment modalities as chemo- and radiotherapy. Phase I clinical studies revealed a number of well-tolerated candidates. As monotherapy, however, anti-angiogenic treatment lacked efficacy in randomized clinical studies so far. In contrast, combination of anti-angiogenic therapy with chemotherapy was highly effective in an encouraging, large randomized phase III trial on metastatic colorectal cancer. This review will outline recent advances in the preclinical and clinical development of anti-vascular therapy with focus on vascular targeting. Conceptual differences between anti-angiogenic and vascular targeting therapies will be discussed with emphasis on specific problems and pitfalls in the conversion into the clinic.

Novel therapeutic targets in squamous cell carcinoma of the head and neck

The treatment of squamous cell carcinoma of the head and neck (SCCHN) has recently witnessed the introduction of molecularly targeted agents based on disease biology, target discovery, and validation. One class of agents, the epidermal growth factor receptor (EGFR) inhibitors, is currently in phase III trials. There are multiple processes, however, that appear to be suitable for targeted therapy beyond EGFR. These include signal transduction, cell cycle control, prostaglandin synthesis, protein degradation, hypoxia, and angiogenesis. These systems and specific protein targets will be reviewed in detail with emphasis on promising preclinical and early clinical evidence of activity.

Therapeutic targeting of the tumor vasculature

A functional tumor vasculature is essential for tumor growth and metastasis and makes an attractive target for therapy. Both antiangiogenic and antivascular approaches are being developed for this purpose. In this article, the current antiangiogenic and antivascular approaches to cancer therapy, potential for their combination with radiotherapy, methods for identifying new targets on the tumor vasculature, and methods for evaluating new vascular-targeted strategies in in vivo model systems are reviewed.

Molecular targeting of angiogenesis

The majority of pharmacological approaches for the treatment of solid tumors suffer from poor selectivity, thus limiting dose escalation (i.e., the doses of drug which are required to kill tumor cells cause unacceptable toxicities to normal tissues). The situation is made more dramatic by the fact that the majority of anticancer drugs accumulate preferentially in normal tissues rather than in neoplastic sites, due to the irregular vasculature and to the high interstitial pressure of solid tumors. One avenue towards the development of more efficacious and better tolerated anti-cancer drugs relies on the targeted delivery of therapeutic agents to the tumor environment, thus sparing normal tissues. Molecular markers which are selectively expressed in the stroma and in neo-vascular sites of aggressive solid tumors appear to be particularly suited for ligand-based tumor targeting strategies. Tumor blood vessels are accessible to agents coming from the bloodstream, and their occlusion may result in an avalanche of tumor cell death. Furthermore, endothelial cells and stromal cells are genetically more stable than tumor cells and can produce abundant markers, which are ideally suited for tumor targeting strategies. This review focuses on recent advances in the development of ligands for the selective targeting of tumor blood vessels and new blood vessels in other angiogenesis-related diseases.

Tumor endothelial markers: new targets for cancer therapy

PURPOSE OF REVIEW

Targeting the endothelial cells that line tumor infiltrating blood vessels is a new anticancer strategy that has gained widespread support from biologists and clinicians. Here we highlight different approaches currently being used to target tumor endothelium and discuss new avenues for intervention that have been opened through the recent identification of tumor endothelial markers (TEMs).

RECENT FINDINGS

The ability of Avastin to prolong survival in a Phase III clinical trial of human colorectal cancer has established the validity of the anti-angiogenic approach. However, realization of the full potential of a vascular targeting strategy may require the exploitation of molecules which are highly restricted in expression to tumor endothelium. Here we explore the potential of TEMs as new targets for cancer therapy. Current knowledge of these markers and their relation to other family members in the context of tumor angiogenesis is discussed. In particular, we highlight those molecules which, by virtue of their structure, cell-surface location and expression pattern, appear to hold promise as targets for future drug development. The identification of TEM8 as the anthrax toxin receptor and the successful targeting of this receptor in preclinical tumor models make this molecule a particularly attractive candidate for future vascular targeting studies.

SUMMARY

Technological advances in cellular fractionation and genomics enabled the identification of several markers preferentially expressed on human tumor endothelium. Studies of these TEMs are expected to aid in our understanding of angiogenesis and could lead to the development of new imaging and diagnostic agents for cancer.

Bypass of tumor drug resistance by antivascular therapy

Multidrug resistance (MDR) presents a major obstacle for the successful chemotherapy of cancer. Its emergence during chemotherapy is attributed to a selective process, which gives a growth advantage to MDR cells within the genetically unstable neoplastic cell population. The pleiotropic nature of clinical MDR poses a great difficulty for the development of treatment strategies that aim at blocking MDR at the tumor cell level. Targeting treatment to the nonmalignant vascular network-the lifeline of the tumor-is a promising alternative for the treatment of drug-resistant tumors. The present study demonstrates that MDR in cancer can be successfully circumvented by photodynamic therapy (PDT) using an antivascular treatment protocol. We show that, although P-glycoprotein-expressing human HT29/MDR colon carcinoma cells in culture are resistant to PDT with Pd-bacteriopheophorbide (TOOKAD), the same treatment induces tumor necrosis with equal efficacy (88% vs 82%) in HT29/MDR-derived xenografts and their wild type counterparts, respectively. These results are ascribed to the rapid antivascular effects of the treatment, supporting the hypothesis that MDR tumors can be successfully eradicated by indirect approaches that bypass their inherent drug resistance. We suggest that with progress in ongoing clinical trials, TOOKAD-PDT may offer a novel option for local treatment of MDR tumors.

Angiogenesis in acute and chronic lymphocytic leukemia

The bone marrow microenvironment plays a crucial role in leukemogenesis. Recent studies suggest that its vascularity changes significantly during this process and that angiogenic factors are of major importance in leukemia. This review summarizes the literature concerning the relationship between angiogenesis and the progression of acute and chronic lymphocytic leukemia. It is becoming increasingly evident that agents which interfere with angiogenesis also block tumor progression and anti-angiogenic management has become a prominent aspect of pre-clinical and clinical assessment. Recent applications of anti-angiogenic agents which interfere with or block leukemia progression are reviewed.

Oncogenes and tumor angiogenesis

Among novel promising approaches that have recently entered the scene of anti-cancer therapy angiogenesis inhibition and targeting cancer-causing genes (e.g. oncogenes) are of particular interest as potentially highly synergistic. One reason for this is that transforming genetic lesions driving cancer progression (e.g. mutations of ras and/or p53) are thought to be causative for the onset of tumor angiogenesis and thereby responsible for build up of vascular supply which is essential for cancer cell survival, malignant growth, invasion and metastasis. However, many of the same genetic alterations that emerge during disease progression and repeated rounds of mutagenic and/or apoptosis causing therapy could alter cellular hypoxia-, growth factor- and apoptotic pathways in such a manner, as to also render cancer cells (partially) refractory to the detrimental consequences of poor blood vessel accessibility (density), ischemia, hypoxia and growth factor deprivation. As recent experimental evidence suggests, such cancer cells could therefore display a reduced vascular demand and remain viable even in poorly perfused regions of the tumor as well as possess an overall growth/survival advantage. The latter circumstance may lead to (predict) diminished efficacy of anti-angiogenic agents in certain malignancies. Therefore, we propose that analysis of oncogenic pathways and gene expression profiling of cancer cells may lead to important clues as to potential efficacy of anti-angiogenic therapies, the direct target of which is the host vasculature, but which are ultimately aimed at (indirect) destruction/control of the cancer cells population. We also suggest that oncogene (tumor suppressor)-directed therapies may help reverse diminished vascular demand of highly transformed cancer cells and thereby facilitate (sensitize tumors to) therapies directed against vascular supply of cancers and their metastases.

Acquired resistance to EGFR inhibitors: mechanisms and prevention strategies

Potent and specific, or relatively specific, inhibitors of epidermal growth factor receptor (EGFR) signaling, including monoclonal antibodies and small molecular weight compounds, have been successfully developed. Both types of agent have been found to have significant antitumor activity, especially when used in combination with radio- hormone- and chemotherapy in preclinical studies. Because of the potentiation of the conventional drug activity in these combination settings, inhibitors of EGFR signaling have often been referred to as sensitizers for chemotherapy or radiation, as well as drug resistance reversal agents. Phase II clinical trials in head-and-neck as well as lung cancer suggested this concept of chemosensitization might translate into the clinic, but this remains to be definitively proven in randomized, double-blind Phase III trials. Given the extensive preclinical literature on EGFR blocking drugs and the advanced clinical development of such agents, it is surprising that the possibility of development of acquired resistance to the EGFR inhibitors themselves, a common clinical problem with virtually all other currently used anticancer drugs, remains a largely unexplored subject of investigation. Here we summarize some of the possible mechanisms that can result in acquired resistance to EGFR-targeting drugs. Alternative combination therapies to circumvent and delay this problem are suggested.

Metronomic therapy for breast cancer

Conventional cytotoxic chemotherapeutic drugs treat cancer either by direct killing or by inhibition of growth of cycling tumor cells. In addition, evidence suggests that cytotoxic agents may inhibit tumor growth through an antiangiogenic mechanism. "Metronomic" or frequent continuous administration of the same chemotherapeutic agents at lower doses may optimize their antiangiogenic properties. The effectiveness of metronomic chemotherapy regimens can be improved significantly by concurrent administration of antiangiogenic, endothelial-specific drugs. Preclinical studies have shown that integrating chemotherapy with antiangiogenic drugs can improve efficacy and circumvent the toxicity and drug resistance associated with standard or high-dose chemotherapy. Preliminary clinical studies have shown similar results. Further confirmation of this concept is required with randomized, controlled clinical trials.

Anti-Tumor Efficacy of Human Angiostatin Using Liver-Mediated Adeno-Associated Virus Gene Therapy

Angiostatin is a potent endogenous inhibitor of angiogenesis and tumor growth in vivo. The therapeutic potential of adeno-associated viral (AAV) gene delivery of angiostatin in modulating tumor growth in vivo was evaluated. Sustained levels of angiostatin were detected in the sera of mice for up to 6 months after they received a single injection of AAV-angiostatin. AAV-mediated stable expression of angiostatin inhibited tumor burden in the highly aggressive B16F10 melanoma and Lewis lung carcinoma (LLC) models of experimental metastasis. Moreover, AAV-angiostatin prolonged survival in B16F10 and LLC tumor-bearing mice compared to control groups. Anti-tumor efficacy was consistently observed when angiostatin serum levels of 15-50 ng/ml were detected following gene transfer, but the effect was minimal when the levels were lower or higher than this range. The combination of AAV-angiostatin gene therapy with chemotherapy was also shown to extend marginally the survival of mice bearing preestablished human tumors; however, the effect was evident only within a narrow dose of circulating angiostatin. These studies demonstrate the feasibility of using AAV anti-angiogenic gene therapy as a cancer treatment modality and suggest that the optimal anti-tumor efficacy of angiostatin following gene transfer may be limited to a narrow dose range.

Regulation of tumor angiogenesis by integrin-linked kinase (ILK)

We show that integrin-linked kinase (ILK) stimulates the expression of VEGF by stimulating HIF-1alpha protein expression in a PKB/Akt- and mTOR/FRAP-dependent manner. In human prostate cancer cells, knockdown of ILK expression with siRNA, or inhibition of ILK activity, results in significant inhibition of HIF-1alpha and VEGF expression. In endothelial cells, VEGF stimulates ILK activity, and inhibition of ILK expression or activity results in the inhibition of VEGF-mediated endothelial cell migration, capillary formation in vitro, and angiogenesis in vivo. Inhibition of ILK activity also inhibits prostate tumor angiogenesis and suppresses tumor growth. These data demonstrate an important and essential role of ILK in two key aspects of tumor angiogenesis: VEGF expression by tumor cells and VEGF-stimulated blood vessel formation.

Negative regulatory role of PI3-kinase in TNF-induced tumor necrosis

Tissue factor is the prime initiator of blood coagulation. Expression of tissue factor in tumor endothelial cells leads to thrombus formation, occlusion of vessels and development of hemorrhagic infarctions in the tumor tissue, often followed by regression of the tumor. Tumor cells produce endogenous vascular endothelial growth factor (VEGF), which sensitizes endothelial cells for systemically administered tumor necrosis factor alpha (TNF alpha) and synergistically enhances the TNF-induced expression of tissue factor. We have analyzed the pathways involved in the induction of tissue factor in human umbilical cord vein endothelial cells (HUVECs) after combined stimulation with TNF and VEGF. By using specific low molecular weight inhibitors, we demonstrated that protein kinase C (PKC), p44/42 and p38 mitogen-activated protein (MAP) kinases, and stress-activated protein kinase (JNK) are essentially involved in the induction of tissue factor. In contrast, the application of wortmannin, an inhibitor of phosphatidylinositol 3 (PI3)-kinase, led to strongly enhanced expression of tissue factor in TNF- and VEGF-treated cells, implicating a negative regulatory role for PI3-kinase. In vivo, the application of wortmannin promoted the formation of TNF-induced hemorrhages and intratumoral necroses in murine meth A tumors. The co-injection of wortmannin lowered the effective dose of applied TNF. Therefore, it is conceivable that the treatment of TNF-sensitive tumors with a combination of TNF and wortmannin will ensure the selective damage of the tumor endothelium and minimize the risk of systemic toxicity of TNF. TNF-treatment in combination with specific inhibition of PI3-kinase is a novel concept in anti-cancer therapy.

Thrombospondin 1, a mediator of the antiangiogenic effects of low-dose metronomic chemotherapy

Chemotherapeutic drugs chronically administered to tumor-bearing mice, using a frequent schedule at doses substantially lower than the maximum tolerated dose (MTD) (i.e., metronomic dosing), can cause sustained and potent antiangiogenic effects by targeting the endothelial cells of newly growing tumor blood vessels. These effects appear to occur in the absence of an increase in the severity of side effects caused by destruction of other cell types normally sensitive to MTD chemotherapy, suggesting a marked and selective sensitivity of activated endothelial cells, the basis of which is unknown. Here we report that protracted exposure of endothelial cells in vitro to low concentrations of several different anticancer agents, including microtubule inhibitors and an alkylating agent, caused marked induction of gene and protein expression of TSP-1, a potent and endothelial-specific inhibitor of angiogenesis. Increases in circulating TSP-1 were also detected in the plasma of human tumor-bearing severe combined immunodeficient mice treated with metronomic low-dose cyclophosphamide. Most importantly, the antiangiogenic and antitumor effects of low-dose continuous cyclophosphamide were lost in TSP-1-null C57BL/6 mice, whereas, in contrast, these effects were retained by using a MTD schedule of the same drug. Taken together, the results implicate TSP-1 as a secondary mediator of the antiangiogenic effects of at least some low-dose metronomic chemotherapy regimens.

New non-angiogenesis dependent pathways for tumour growth

The current wisdom is that tumours are endowed with an angiogenic capability and that their growth, invasion and metastasis are angiogenesis-dependent. This article summarises the literature concerning recent histomorphological studies that indicate that some tumours may be vascularised without significant angiogenesis, probably by using existing vessels, a process later described as vascular co-option, or even by forming vascular channels on their own through a non-endothelial cell process called "vascular mimicry". Moreover, the possibility that bone marrow-derived stem cells may also be a source of endothelial precursor cells recruited for tumour-induced neovascularisation, is reviewed. In fact, it has been assumed that the additional endothelial cells required to construct new tumour vessels come from the division and proliferation of local endothelial cells and that endothelial cells incorporated into sites of neovascularisation, including tumour-induced new blood vessels, may be derived from these precursor cells. Finally, lymphoangiogenesis as a mechanism of de novo formation of lymphatics, favouring the metastatic dissemination of tumour cells, is summarised. Potential therapeutic applications are also discussed.

In vitro and in vivo evaluations of THAM derived telomers bearing RGD and Ara-C for tumour neovasculature targeting

As an approach to the development of specific drug delivery systems, a new class of low macromolecular carriers called 'telomers' endowed with an antitumour agent, such as arabinofuranosylcytosine (Ara-C), RGDSK peptidic sequences, as tumour targeting moieties, and tyrosine groups labelled with 125I atoms allowing the in vivo scintigraphic follow up, were synthesized. Their tumour targeting ability was assessed in vivo in mice bearing a murine B16 melanoma. The biological results showed that the presence of RGDSK sequences onto the macromolecules leads to the selective targeting and the accumulation of telomers within the vascularized zone of the tumour. Moreover, such compounds exhibited in vitro a better IC(50) (0.015 muM) than pure Ara-C and in vivo an oncostatic index higher than 160%.

Molecular targets in the inhibition of angiogenesis

Angiogenesis, the process of blood vessel formation, is crucial for malignant tumour growth and metastases; therefore, it has become an attractive target for anticancer therapy. Theoretically applicable to most solid tumours, this therapy may be advantageous over existing cytotoxic therapy, since it is directed at genetically stable endothelium growing within tumours rather than at malignant cells, which acquire resistance to treatment. Many promising angiogenesis inhibitors have been developed, although their activity has yet to be demonstrated in human clinical trials. To improve therapeutic benefit, this may require further insight into tumour angiogenesis, development of appropriate surrogate markers of activity, treatment of early stage neoplastic disease and probably a combination of different classes of antiangiogenesis agents to overcome redundant mechanisms of angiogenesis control.

COX-2 inhibitors and cancer therapeutics: potential roles for inhibitors of COX-2 in combination with cytotoxic therapy: reports from a symposium held in conjunction with the Radiation Therapy Oncology Group June 2001 Meeting

Tumor growth and angiogenesis are interdependent. Cyclooxygenase (COX) catalyzes the synthesis of prostaglandins (PGs) from arachidonic acid. Nonsteroidal antiinflammatory drugs (NSAIDs) inhibit COX-mediated synthesis of PGs. Although COX-1 is constitutively expressed in a wide range of tissues, COX-2 is cytokine inducible. Enhanced COX-2 expression has been attributed a key role in the development of inflammation and related processes observed in pathologically altered disease states. Two specific COX-2 inhibitors, namely, rofecoxib (Vioxx) and celecoxib (Celebrex), both oral agents and FDA approved, have been shown preclinically and clinically to have efficacy comparable to that of NSAIDs for relief of pain and inflammation in osteoarthritis, with decreased risk of gastrointestinal damage. Significant preclinical evidence strongly supports the potential role for these inhibitors for the treatment of cancer. On June 29, 2001, the Radiation Therapy Oncology Group (www.rtog.org), a National Cancer Institute-sponsored cooperative group, held a 1-day symposium focusing on the potential role of inhibitors of COX-2 in the treatment of cancer. This issue of the American Journal of Clinical Oncology contains the summary of those presentations.

Inhibition of angiogenesis by non-toxic doses of temozolomide

It is well established that certain chemotherapeutic agents have potent antiangiogenic properties which may be part of their antitumor activity. Temozolomide (TMZ) is a lipophilic methylating agent used in the therapy of malignant melanoma and other tumors. We sought to determine whether TMZ is capable of inhibiting angiogenesis or influencing endothelial function. We used the in vivo chorioallantoic membrane (CAM) assay, and HUVEC-based in vitro Matrigel, adhesion and proliferation assays to determine the antiangiogenic effects of different doses of TMZ. In the CAM assay, angiogenesis was significantly inhibited by 5 microM TMZ, a concentration also found to be effective in interfering with in vitro angiogenesis as measured by the Matrigel assay. For the inhibition of basic fibroblast growth factor (bFGF)-, vascular endothelial growth factor (VEGF)- or beta-phorbol 12-myristate-13-acetate (PMA)-induced endothelial cell proliferation or endothelial cell adhesion to fibronectin, TMZ concentrations of at least 25 microM were necessary, indicating that bFGF-, VEGF- or protein kinase C-mediated pathways may not primarily be involved in the observed antiangiogenic effect. Thus, we could demonstrate that TMZ inhibits angiogenesis at low, non-toxic doses that correspond to the plasma concentrations achieved by an oral application of 20 mg/m2 every 8 h. This 'metronomic' scheduling has already been used in phase I studies and has produced antitumor effects. Therefore, the antitumor activity of TMZ may, at least in part, be due to its antiangiogenic properties. The precise mechanism of its antiangiogenic action remains to be elucidated.

Bone marrow angiogenesis and progression in multiple myeloma: clinical significance and therapeutic approach

It is now well established that solid tumors depend on angiogenesis. Promoters and inhibitors of angiogenesis are in balance and antiangiogenic strategies aim at repressing the angiogenic process, thus retarding solid tumor progression. Recent data suggest the importance of angiogenesis in hematologic malignancies and several studies reveal an increased angiogenesis in active multiple myeloma. Angiogenesis seems to be a prominent feature of MM progression, and seems to be correlated with the prognosis and the resistance of MM to chemotherapy. Numerous cell populations and cytokines are involved in angiogenesis in multiple myeloma and antiangiogenic therapy with thalidomide is effective in patients with refractory or relapsed disease. The combination of thalidomide and of other immunomodulatory agents with other therapeutic regimens could lead to more effective management of patients with multiple myeloma.

Resistance to cytotoxic and anti-angiogenic anticancer agents: similarities and differences

Intrinsic resistance to anticancer drugs, or resistance developed during chemotherapy, remains a major obstacle to successful treatment. This is the case both for resistance to cytotoxic agents, directed at malignant cells, and for resistance to anti-angiogenic agents, directed at non-malignant endothelial cells. In this review, we will discuss mechanisms of resistance which have a bearing on both these conceptually different classes of drugs. The complexity of drug resistance, involving drug transporters, such as P-glycoprotein, as well as resistance related to the tissue structure of solid tumors and its consequences for drug delivery is discussed. Possible mechanisms of resistance to endothelial cell-targeted drugs, including inhibitors of the VEGF receptor and EGF receptor family, are reviewed. The resistance of cancer cells as well as endothelial cells related to anti-apoptotic signaling events initiated by cell integrin-matrix interactions is discussed. Current strategies to overcome resistance mechanisms are summarized; they include high-dose chemotherapy, tumor targeting of cytotoxics to improve tumor uptake, low-dose protracted (metronomic) chemotherapy and combinations of classical agents with anti-angiogenic agents. This review discusses primarily literature published in 2001 and 2002.

Targeting angiogenic processes by combination low-dose paclitaxel and radiation therapy

Tumor growth and angiogenesis are interdependent. Paclitaxel and radiation therapy are commonly used in the clinic, in a number of disease sites, requiring high dosages of both drug and radiation for cure. Paclitaxel (Taxol) is a diterpenoid with antitumor activity against a variety of human neoplasms and can amplify the cytotoxic effect of ionizing radiation in vitro, presumably by inducing arrest at metaphase, known to be a very radiosensitive phase of the cell cycle. Little is known about how angiogenesis is affected by paclitaxel when the combination of paclitaxel and radiation are used. We have evaluated the combination of paclitaxel and radiation at various concentrations, on cytokine-induced angiogenesis in vitro with the goal of determining whether reduction of radiation and paclitaxel doses is possible without sacrificing efficacy. We have found that paclitaxel inhibited endothelial cell proliferation, migration, and tube formation (differentiation) at one-tenth the concentration needed to achieve a similar effect on tumor cell lines. In combination with radiation, inhibition of endothelial cell function was additive and increased twofold. The combination of low-dose paclitaxel and radiation suggests a complementary strategy with potential clinical ramifications to target angiogenesis-dependent malignancies.