| Sign In to gain access to subscriptions and/or personal tools. |
Angiogenesis Inhibitors and the Need for Anti-angiogenic Therapeutics
1 Department of Orthopaedics, University of Melbourne, St. Vincent’s Health, P.O. Box 2900, Fitzroy, 3065, Melbourne, Australia; and Correspondence: * corresponding author, crispin.dass{at}svhm.org.au
Angiogenesis is the formation of new blood vessels from pre-existing vessels to form capillary networks, which, among other diseases, such as diabetic retinopathy and macular degeneration, is particularly important for tumor growth and metastasis. Thus, depriving a tumor of its vascular supply by means of anti-angiogenic agents has been of great interest since its proposal in the 1970s. This review looks at the common angiogenic inhibitors (angiostatin, endostatin, maspin, pigment epithelium-derived factor, bevacizumab and other monoclonal antibodies, and zoledronic acid) and their current status in clinical trials.
Key Words: angiogenesis • therapy • blood vessel • cancer • vasculature
Angiogenesis is the formation of a new capillary network from pre-existing vessels and is required for normal and tumor vasculature. It can be found in normal physiological processes, such as growth and development in embryos and adults, and wound healing, as well as in the menstrual cycle (Folkman, 1992). Angiogenesis is particularly important in tumor growth and metastasis. A solid tumor usually begins small and is localized, due to the lack of a vascular supply. It then progresses to make an ’angiogenic switch’, which allows it to promote the formation of new capillary tubes from host vessels (Folkman, 1992). This is a critical process in which the dynamic balance (Fig  .) between pro-and anti-angiogenic factors is shifted to the former by conditions created by the tumor and its environment, including hypoxia, inflammation, and mutation in oncogenes or tumor suppressor genes, such as p53. Commonly known pro-angiogenic factors are vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), placental growth factor (PlGF), and matrix metalloproteinases (MMPs). Endogenous anti-angiogenic factors include thrombospondin, angiostatin, tumstatin, and endostatin. A companion article in this issue of the JDR describes VEGF and its importance in oral biology (Dai and Rabie, 2007).
The mechanism of angiogenesis mainly involves ’sprouting’ of pre-existing host blood vessels and ’intussusception’. Sprouting angiogenesis involves proliferation and migration of endothelial cells of pre-existing vessels, but not bone-marrow-derived endothelial progenitors. As the tumor grows into an avascular area, there is an increasing need for nutrients and oxygen supply from the blood, leading to induced transcription of the gene encoding VEGF, an important angiogenic factor. Interaction between VEGF and its receptor VEGFR results in proliferation and migration of bone-marrow-derived circulating endothelial precursors, which subsequently differentiate into vascular endothelial cells (VECs; Calfa et al., 2006). At the same time, tumor-related monocytes and macrophages, derived from hemopoietic stem cells, secrete angiogenic growth factors as well as proteases to remodel the neovasculature. Later, tumor cells grow into the newly formed vessel and functionally divide it into two branches, in a process called ’intussusception’, which eventually results in a new capillary network (Carmeliet and Jain, 2000). As the tumor grows larger, increasing need for oxygen causes hypoxia in the tissue, leading to increased production of angiogenic factors, which, in turn, stimulate angiogenesis further (Cao, 2004). The structure of tumor vasculature differs greatly from that of its normal counterpart, which determines its physiology and hence the delivery of therapeutic agents through it. Tumor vessels are usually tortuous and dilated, with uneven lumen diameter (Carmeliet and Jain, 2000). They are excessively branched, particularly by trifurcation, a feature that is rarely found in normal vessels (Dass and Su, 2000). Cells of the vessel walls are often a combination of VECs and tumor cells, up to 15% in some parts. Tumor vascular endothelia are activated cells and are highly proliferative. They express an abnormally large number of markers that sensitize them to pro-angiogenic molecules, such as VEGF (Carmeliet and Jain, 2000; Dass and Su, 2000). As a result of abnormally constructed vessels, turbulent flow and arteriovenous shunts usually occur. In addition, frequent blood stasis and changes in flow direction cause increased hypoxia, which is a potent stimulus for angiogenesis (Adair et al., 1990), with increased vascular permeability (Olesen, 1986). Furthermore, discontinuous or absent basement membrane and lack of lymphatic drainage often result in interstitial hypertension within tumors (Carmeliet and Jain, 2000). Besides these phenomena, the lack of perivascular cells such as pericytes and smooth-muscle cells makes tumor vessels less responsive to vasoactive stimuli (Dass and Su, 2000). Ultimately, angiogenesis is an essential process for the survival and metastasis of a tumor. As a result, there has been a growing number of studies on anti-angiogenic therapy, which aims to slow, stop, or potentially reverse neovascularization to manage cancer. There are four main pathways to target angiogenesis: (1) via VEGF/VEGFR or its signal transduction pathway; (2) targeting the integrin receptors or extracellular matrix; (3) using endogenous angiogenesis inhibitors; and (4) via other, lesser known, growth-factor-mediated pathways, such as cytokines. VECs are particular targets because of the many favorable characteristics over tumor cells: They are genetically stable and less heterogenous than their tumor counterparts, and hence are less likely to develop resistance and are suitable for prolonged treatment (Dass and Su, 2000; reviewed by Fayette et al., 2005).
Since tumor VECs proliferate from 50- to 200-fold faster than normal cells, they express specific markers that can be exploited by means of therapeutic agents. One vascular endothelial cell has been found to support up to 100 tumor cells; hence, the destruction of even one single blood vessel can eradicate a considerable number of tumor cells. Besides, angiogenesis is generally limited in adults; hence, anti-angiogenic therapy would be less likely to cause adverse effects. At present, various angiogenesis inhibitors are being studied extensively and undergoing clinical trials, and some have even been approved for use in cancer therapy (Table
(I) ANGIOSTATIN Angiostatin is an endogenous polypeptide inhibitor of angiogenesis and suppresses growth of small metastases. It is a 38-kDa internal fragment of plasminogen and consists of 4 Kringle domains, K1-4 (Calfa et al., 2006; Ruegg et al., 2006). Even though these K-domains are 50% homologous in structure, K1 is the most potent inhibitor, while K4 is virtually inactive in suppression of vascular endothelial cell growth (Ruegg et al., 2006). As a result, recombinant molecules K1-4 and K1-3 are used interchangeably in clinical testing, with the latter being more active, with a shorter half-life in vivo because of the smaller molecular size (Cao, 2004; Ruegg et al., 2006). Kringle-5 of plasminogen is also a potent inhibitor of angiogenesis (Cao et al., 1997). In vitro studies have suggested that these Kringle domain inhibition-potencies can be ranked in order: K-5 > K1-3 > K1-4 > K1 > K3 > K2 > K4. It is proposed that K-5 and K1-4 act via different pathways to inhibit angiogenesis, which results in their synergistic suppression of capillary endothelial cell growth when administered together (Cao, 2004). Angiostatin also specifically targets VECs to prevent their migration as well as proliferation during angiogenesis. This causes tumor regression via increased tumor cell apoptosis, even though tumor cells may still be proliferating (Holmgren et al., 1995).
There have been several proposals on the possible mechanisms by which angiostatin works. First, angiostatin causes direct apoptosis of VECs via increased tyrosine kinase activity of focal adhesion kinase (FAK), which results in disrupted turnover of focal adhesion contacts (Claesson-Welsh et al., 1998). Second, it can also inhibit bFGF and VEGF activation, specifically in the VECs of extracellular signal-regulated kinase (ERK)-1, ERK-2, and other phosphoproteins. Third, angiostatin was suggested to prevent vascular endothelial cell proliferation via mitosis arrest at the G2/M transition (Sim et al., 2000; Cao, 2004). Next, it has been suggested that angiostatin may inhibit cell invasion by binding tissue plasminogen activator to halt plasmin formation, which facilitates tumor invasion (Sim et al., 2000). Finally, some studies have discovered that the outer membrane of VECs possesses ATP synthase, which enables them to produce sufficient ATP in the absence of oxygen (Sim et al., 2000; Cao, 2004). One of angiostatin’s actions was to deprive these VECs of ATP by binding to the Angiostatin and angiostatin-related proteins have been tested for their safety and clinical usage. Recombinant human angiostatin (rhAngiostatin) K1-3 was observed, in a Phase I clinical trial, to be safe for prolonged treatment rather than interval treatment (Drixler et al., 2000). The maximum clinical dose was reported from an experiment on a group of 24 advanced cancer patients, who were given continuous subcutaneous injections of up to 4000 treatment days. Subcutaneous injection was the preferred means of administration over intravenous bolus daily. The safety of ’Angiostatin Cocktail’, a plasminogen activator that converts plasminogen to Angiostatin4.5 (Kringles 1–4 and 85% of Kringle-5) directly in vivo, has been successfully proven in Phase I clinical trials (Soff et al., 2005). Treatment combinations between angiostatin and cytotoxic agents have been shown to bring synergistic effects. In Lewis lung carcinoma, while radiation therapy or angiostatin therapy alone results in 62% and 31% reduction of tumor volume, respectively, combined treatment leads to significant tumor reduction of 89% compared with the 74% predicted outcome (Mauceri et al., 1998). Other studies on human tumor xenografts gave similar results. Although angiostatin is relatively safe for use, te Velde and colleagues (2002) observed reversible impairment upon anastomotic healing with mice that had undergone tumor surgical resection, which poses a risk when surgical intervention treatment is combined with angiostatin therapy. Therefore, angiostatins need to be further evaluated in Phases II/III clinical trials.
(II) ENDOSTATIN Endostatin inhibits migration and promotes apoptosis specifically in VECs via multiple pathways, a majority of which are yet unknown. Dhanabal et al.(1999) and Shichiri and Hirata (2001) showed that endostatin acts mainly by reducing expression of the anti-apoptotic proteins Bcl-2 and Bcl-XL as well as growth-associated factors, which results in a 15- to 30-fold increase in the apoptosis rate of VEC. In addition, endostatin also induces the activation of caspase-3, an intracellular protease that initiates cellular breakdown in mammalian apoptosis. In vivo, it also inhibits the phosphorylation of ERK-1 and -2 via the VEGF and bFGF pathways, specifically in VECs (Sim et al., 2000), and inhibits pigment epithelium-derived factor-mediated perivascular cell production in vivo (Skovseth et al., 2005). Furthermore, endostatin causes down-regulation of c-myc, a protein necessary in endothelial cell migration (Shichiri and Hirata, 2001), and inhibition of cyclin-D, resulting in G1 arrest in ECs (Hanai et al., 2002). Endostatin is the first endogenous inhibitor to enter clinical trials. Intravenous infusion of endostatin in patients has shown no significant toxicity (Eder et al., 2002; Herbst et al., 2002; Kulke et al., 2006). It shows linear pharmacokinetics in which an increasing dose results in proportionally increased bioavailability in plasma, with a mean half-life of 10.7 ± 4.1 hrs (Herbst et al., 2002). From recombinant human endostatin (rhEndostatin) treatment on 25 patients, significant tumor cell apoptosis as well as vascular endothelial cell apoptosis were reported (Herbst et al., 2002). Nevertheless, Eder et al.(2002) and Kulke et al.(2006) observed no significant tumor regression from clinical trials on three (Phase I) and 40 patients (Phase II), respectively, with pancreatic neuroendocrine tumors. There was a minor response to the treatment; however, based on the WHO anti-angiogenic agent criteria, it had no partial response (greater than 50% tumor size reduction). Interestingly, endostatin efficacy has recently been found to follow a biphasic dose-response curve (Celik et al., 2005; Tjin Tham Sjin, 2006) in in vitro experiments and in animal models. The U-shaped curve suggests that the maximum response can be achieved at either very low or very high doses, depending on tumor type. Thus, this finding can be important for future clinical trials in the adjustment of relevant doses according to tumor attributes. rhEndostatin is often combined with a second anti-cancer therapy. As with angiostatin, rhEndostatin has been found to have improved efficacy when combined with radiotherapy and chemotherapy. In mice implanted with Lewis lung carcinoma, a significant tumor growth delay was noted with treatment of radiotherapy combined with Endostatin, compared with growth in animals on radiotherapy alone (Luo et al., 2005). Furthermore, from in vivo and in vitro studies, rhEndostatin and adriamycin were found to have a synergistic effect on the inhibition of endothelial cell proliferation and differentiation in a dose-dependent manner (Plum et al., 2003). Although endostatin is a potential candidate among the endogenous angiogenesis inhibitors, and hence the target of many studies, further evaluation and advanced-phase clinical trials are still needed to be done before it can be of broader clinical use.
(III) MASPIN
The RSL (reactive serpin loop) is the primary functional domain of the serpin family, which accounts for serpin anti-protease activity (Zhang et al., 2000; Bailey et al., 2006). It was shown in vitro to inhibit non-endothelial cell migration, such as breast tumor cells, but has no inhibitory effects on endothelial cell migration and mitogenesis, as well as neovascularization in vivo (Zhang et al., 2000). In contrast, maspin with deleted N-terminal (maspin Both recombinant and secreted maspin act by blocking VEGF-/bFGF-induced vascular endothelial cell migration in a dose-dependent manner, with an observed ED50 of 0.2 to 0.3 µM. Thus, maspin seems to be more potent than small molecules such as captopril (10 µM), but less effective than other commonly known inhibitors like angiostatin (10 nM) (Zhang et al., 2000). Complete inhibition by maspin is achieved at concentrations of 0.5–1 µM, which coincides with the concentration range for the inhibition of tumor cell motility and invasion; however, the inhibition works via different pathways, and some are as yet undiscovered. Although the mechanisms of maspin inhibition of angiogenesis are not thoroughly understood, several studies have found that maspin nuclear expression is associated with improved cancer outcome. While expression of the oncogene c-erbB-2 correlates with higher tumor grade, increasing maspin expression was associated with decreased c-erbB-2 expression and lowered microvessel density in specimens from 69 patients with invasive ductal breast carcinoma (Sopel et al., 2005). In a study of 118 patients with high-grade advanced-stage ovarian serous carcinoma, tumors with increased nuclear maspin localization was found to have reduced pro-angiogenic factors VEGF and cyclooxygenase-2 expression, compared with tumors with cytoplasmic maspin localization (Solomon et al., 2006). In addition, median survival was significantly improved in patients with nuclear maspin expression, 1843 days compared with 1148 days in negative maspin tumors (Solomon et al., 2006). Nevertheless, cytoplasmic maspin expression results in median survival of only 637 days, which suggests further study to evaluate the implication of cytoplasmic maspin expression in tumor cells. Another report examined laryngeal carcinoma in 35 patients and reported that microvessel density was notably lower in patients with nuclear than cytoplasmic maspin localization (Marioni et al., 2006). The study also found that patients without carcinoma recurrence had higher mean maspin expression than those with recurrence. Ultimately, high nuclear maspin localization is important in limiting markers of angiogenesis, reduced microvessel density, prolonged survival, and reduced recurrence rates. Thus, a mechanism may be discovered if further in-depth studies show similar results.
(IV) PIGMENT EPITHELIUM-DERIVED FACTOR The human PEDF gene encodes for a 418-amino-acid protein with a hydrophobic signal characteristic of secreted proteins (Steele et al., 1993). PEDF has structural and sequence homology to members of the family of serine proteinase inhibitors (serpins) and contains a reactive center loop (RCL) typical of this family of proteins (Steele et al., 1993). A difference in the RCL sequence of PEDF is thought to disable it from being inhibitory toward proteases (Steele et al., 1993; Simonovic et al., 2001). Two functional epitopes have been identified on PEDF (Filleur et al., 2005). These include a 34-mer peptide (residues 24–57) and a 44-mer peptide (residues 58–101). The 34-mer peptide is responsible for anti-angiogenic action, possibly via a distinct receptor identified on endothelial cells, induces apoptosis, and blocks endothelial cell migration and corneal angiogenesis (Yamagishi et al., 2004). Filleur et al.(2005) demonstrated in vivo that overexpression of the 34-mer in the PC3 prostate adenocarcinoma cell line resulted in decreased tumor microvessel density and increased apoptosis, events not observed with 44-mer peptide overexpression.
An appealing aspect of PEDF’s activity is its selectivity, since it targets only de novo vessel formation and spares pre-established vasculature (Bouck, 2002). Although the mechanisms by which PEDF reduces neovascularization remain unknown, it now appears that it involves endothelial cell death, through the activation of the Fas/FasL death pathway (Volpert et al., 2002) and also via a disruption in the critical balance between pro- and anti-angiogenic factors, in particular VEGF. Cai et al.(2006) recently reported that PEDF has an inhibitory effect on VEGF-induced angiogenesis in bovine retinal microvascular endothelial cells via the enhancement of The expression patterns of VEGF, a potent pro-angiogenic factor, and PEDF have been well-characterized in the eye, and it is the balance of these opposing stimuli that prevents the development of choroidal neovascularization, which is involved in the diseases of macular degeneration and diabetic proliferative retinopathy (Ogata et al., 2001; Holekamp et al., 2002). In our laboratory, this inverse correlation was also seen in the epiphyseal growth plates of bone, where PEDF is highly expressed in the avascular resting and proliferative zones, whereas VEGF is more predominant in the lowermost layers of the hypertrophic zone (Quan et al., 2002–2003). It is by careful manipulation of this balance of angiogenesis, coupled with precise timing, that the growth plate microenvironment switches from an angiostatic to an angiogenic state during the physiological process of endochondral ossification. Quan et al. (2002–2003) postulated that it was the expression of such potent anti-angiogenic factors that significantly contributed to the inability of osteosarcoma to penetrate the avascular resting zones of the growth plate, a clinical phenomenon commonly seen in children and adolescents with metaphyseal tumors. Based on PEDF’s role as a potent endogenously produced anti-angiogenic factor, there have been several recent studies demonstrating that decreased PEDF expression is associated with a higher intratumoral microvessel density and a more metastatic phenotype in various tumors, such as those of the prostate, liver, gliomas, and lymphangiomas (reviewed in Ek et al., 2006b). Consequently, this has prompted further investigation into the effects of using PEDF for gene therapy of various tumors. Garcia et al.(2004) and Abe et al.(2004) both demonstrated, from in vivo studies, that overexpression of PEDF in human malignant melanoma cell lines by stable transfection with retrovirus and plasmids, respectively, significantly reduced intratumoral microvessel density as well as primary tumor growth and the development of metastasis. Moreover, Hase et al.(2005) and Streck et al.(2005) also reported similar results using virus-based expression vectors in neuroblastoma and human pancreatic cancer cell lines, respectively. Given that increased intratumoral microvessel density has been shown to be associated with a more aggressive and metastatic phenotype in the majority of cancers, reduction of tumor vascularity by PEDF may prove to be a promising avenue for targeted cancer therapy. We have shown a dramatic reduction in the establishment of pulmonary metastases when PEDF plasmid-expressing osteosarcoma cells were injected orthotopically into mice (Ek et al., 2007a). This result was confirmed when the recombinant protein for PEDF (Ek et al., 2007b) or two 25mer peptides (Ek et al., 2007c), based on the known active domains, were used in mice to curb osteosarcoma growth in the bone and metastasis to lungs. Furthermore, when the PEDF plasmid was encapsulated into chitosan nanoparticles (Dass et al., 2007), a similar result was noted, suggesting that this protein may indeed have efficacy against this debilitating, if not fatal, disease. Thus, PEDF has potent anti-angiogenic activity, which makes it a promising candidate for the anti-angiogenic therapy of cancer and ocular disorders plagued by incessant angiogenesis. Current attempts in various labs are now focused on developing delivery methods that enhance the efficacy of this promising protein. These methods include both protein and gene therapy.
(V) MONOCLONAL ANTIBODIES Ranibizumab (Lucentis) is a humanized monoclonal antibody fragment that binds with high specificity and affinity to VEGF-A (Narayanan et al., 2006). It is derived from bevacizumab, the same anti-VEGF antibody described above. Ranibizumab has been approved by the FDA (the US Food & Drug Administration) (FDA, 2004) to treat "wet" age-related macular degeneration (AMD). Inhibition of VEGF is designed to reduce angiogenesis and vascular permeability. Both of these processes play an important role in the etiology of neovascular "wet" age-related macular degeneration. Another treatment that has been approved by the FDA for "wet" age-related macular degeneration is pegaptanib. Pegaptanib is a pegylated aptamer, a single strand of nucleic acid that binds with a high degree of specificity to VEGF165 (Gragoudas et al., 2004). Three randomized clinical studies of ranibizumab and two of pegaptanib demonstrated significant clinical benefit for both pegaptanib and ranibizumab after 12 months (Takeda et al., 2007). Both pegaptanib and ranibizumab inhibited the progression of neovascular "wet" age-related macular degeneration.
First-line Treatment in Metastatic Colorectal Carcinoma Another Phase II trial, also done by Kabbinavar et al.(2005), involved 209 untreated patients at higher risk in two groups of patients: fluorouracil and leucovorin with placebo, or fluorouracil and leucovorin with bevacizumab 5 mg/kg every 2 wks. The groups were measured primarily by Mean Overall Survival in each group. Survival rates were 16.6 months for the bevacizumab group, compared with 5.5 months for the placebo group. Adverse effects were reported on the basis of the previous study; however, it is notable that venous thrombosis was not increased in the treated group. It was concluded that the use of bevacizumab is beneficial in addition to conventional chemotherapy. Furthermore, a double-blind, phase III clinical trial on 813 untreated metastatic colorectal cancer patients showed similar results (Hurwitz et al., 2004). Treatment included two groups: Saltz regimen (5-fluorouracil and folinic acid) alone as placebo, and Saltz regimen coupled with bevacizumab 5 mg/kg/fortnightly. Treatments were well-tolerated, with a significant increase in the ’bevacizumab combined’ group of overall survival (20.3 months vs. 15.6 months; p < 0.001) and in median progression-free survival (10.2 months vs. 6.2 months). Grade 3 hypertension occurred more frequently in the combined group, but was manageable by oral medications. In addition, bevacizumab was associated with gastrointestinal perforation, wound-healing complications, as well as arterial thrombo-embolic complications, especially in patients over 65 years of age. This study eventually led to FDA approval (in February, 2004) of bevacizumab in combination with 5FU chemotherapy as a first-line treatment for metastatic colorectal cancer. On June 20, 2006, bevacizumab was further approved by the FDA as a second-line treatment for metastatic carcinoma of the colon or rectum, in combination with FOLFOX4 (5-fluorouracil, leucovorin, and oxaliplatin (FDA Approval for Bevacizumab, 2004; accessed 10 Mar 2007). This approval was supported by an open-label, randomized control trial on 829 patients previously treated with 5FU and irinotecan-based therapy. The study was divided into three arms: bevacizumab alone (n = 244, 10 mg/kg every 2 wks); bevacizumab plus FOLFOX4 (n = 293, bevacizumab 10 mg/kg and FOLFOX4 followed the second day every 2 wks); and FOLFOX alone (n = 292, with intravenous infusion of oxaliplatin and leucovorin concurrently, whereas 5FU followed with a bolus dose then continuous dose for 2 days every 2 wks). The overall survival rate was significantly higher in patients receiving combination treatment as compared with those receiving FOLFOX4 alone (with median overall survival of 13.0 months and 10.8 months, respectively). In addition, patients on combination treatment were reported to have higher overall response rates and longer progression-free survival. Adverse effects were similar to those reported in previous studies: gastrointestinal perforation, wound-healing complications, hemorrhage, thrombo-embolism, nephritic syndrome, and congestive heart failure. Other, more common, side-effects include fatigue (19% vs. 13%), diarrhea (18% vs. 13%), sensory neuropathy (17% vs. 9%), nausea, vomiting, hypertension, abdominal pain, other neurologic toxicities, ileus, and headache.
First-line Treatment of Non-small-cell Lung Cancer
Phase II Trial, Metastatic Clear-cell Renal Carcinoma
(VI) OTHER MONOCLONAL ANTIBODIES
Another approach to the disruption of tumor vasculature is to inhibit adhesive interactions of endothelial cells with the surrounding extracellular matrix. The In contrast, a study done by Gutheil et al.(2000) revealed that eight of 14 patients experienced disease stabilization or a partial response, one of whom experienced prolonged survival of up to 22 months, and another had slight tumor shrinkage after only the first cycle of therapy. The study also reported that no significant toxicities were observed. Recently, McNeel and coworkers (2005) reported similar findings regarding adverse events in the use of Abegrin on 25 patients with metastatic solid tumors, including minor infusion-related reactions (rigors, flushing, fever, injection site reactions, and tachycardia), low-grade gastrointestinal symptoms, and asymptomatic hypophosphatemia. Three of the patients with renal cell cancer experienced prolonged stable disease (varying from 34 wks to 2 yrs), while there were no patients who showed complete or partial reversion. Despite the fact that various experiments with Abegrin in vitro, as well as with mice, had shown promising results, most phase I clinical trials have failed to demonstrate efficacious effects for Abegrin in humans. Nevertheless, Abegrin deserves further larger-scale studies for evaluation of its potential, particularly in treating renal cell carcinoma.
(VII) ZOLEDRONIC ACID In vivo experiments in animal models have shown promising results for bisphosphonates, particularly zoledronic acid (zoledronate, ZA). Croucher et al.(2003) conducted a study on the effect of ZA on mice implanted with myeloma bone tumors. Prior to treatment with ZA, the mice were recorded to have increased osteoclast formation, decreased cancellous bone volume, decreased total bone mineral density (BMD), and development of osteolytic bone lesions. Following injection of ZA, cancellous bone loss was completely prevented, BMD was stabilized, and myeloma bone tumor growth was inhibited, although there was no evidence of apoptosis. Also, there was some degree of reduction in tumor microvessel density. Notably, the median survival rate was significantly higher in the treated group compared with the control group (47 days vs. 35 days, respectively).
According to Luckman et al.(1998), nitrogen-containing bisphosphonates block the mevalonate biosynthetic pathway, which essentially reduces the substrates necessary to produce the proteins (Rab, Rac, Ras, and Rho) that are important in signaling pathways in endothelial migration. In addition, ZA can inhibit To determine the effects of ZA on cytokines and the growth factors involved in angiogenesis, Ferretti et al.(2005) conducted a study on 18 breast cancer patients with bone metastases. Patients not only underwent extensive screening to be considered, but also had not received any form of cancer therapy (radio-, chemo-, immunotherapy and growth factors) in the preceding 4 wks or steroids in the preceding 3 wks. A 4-mg dose of ZA in 100 mL of 0.9% saline was delivered intravenously over 15 min. Cytokine and angiogenic factor levels were measured before ZA infusion and again after 2 and 7 days. For the purpose of this review, only those levels related to angiogenesis will be discussed. Overall administration of ZA caused a transient significant decrease in serum levels of MMP-2, VEGF, and bFGF after 2 days, but an increase above the basal value after 7 days (with the exception of bFGF). Furthermore, Fournier et al.(2002) reported that ZA and other drugs (mentioned above) also reduced revascularization in testosterone-stimulated castrated rats. The rats were randomly treated with one drug/cohort at 20 µg/kg/day over 6 days. Ibandronate showed the highest reduction (51%), and ZA also showed a significant reduction of 47%, compared with the normal revascularization pattern. These results further highlighted not only the potential of ZA in treating osteolytic bone disease as a conventional bisphosphonate, but also its possible use in anti-angiogenic therapy, and prompted possible combined treatment to enhance the individual effects.
Thus, the field of angiogenesis is still hotly debated in cancer research and in other pathologies of the vasculature, such as ocular diseases. This is evidenced by the number of different agents that are currently being developed for clinical approval, plus the greater number constantly being discovered. The FDA approval of certain agents, such as Avastin®, again reassuringly attests to the emphasis being placed on finding potent anti-angiogenic compounds capable of disease modulation and with few side-effects. While many of the above treatments involve protein-based molecules that target specific aspects of an angiogenic pathway, other types of molecules are under investigation or have been approved that block multiple pathways. For example, Sunitinib (Sutent) is a small molecule that binds to and inhibits multiple receptor tyrosine kinases (George, 2007). Among the tyrosine kinases that it inhibits are all of the platelet-derived growth factor receptors and vascular endothelial growth factor receptors, the cytokine receptor CD117, colony-stimulating factor-1 receptor, the proto-oncogene RDT (re-arranged during transfection), and others. Thus, it blocks multiple receptors that can affect several phases of angiogenesis and is sufficiently effective that it has been approved by the FDA for the treatment of renal cell carcinoma and imatinib-resistant gastrointestinal stromal tumors. This review highlights many important molecules that have the potential to aid in the treatment of cancers and other pathologies where angiogenesis is detrimental. However, there is much more that can be and should be done, given the promising results achieved to date.
Received for publication April 30, 2007. Revision received July 23, 2007. Accepted for publication July 24, 2007.
Journal of Dental Research, Vol. 86, No. 10,
927-936 (2007)
|
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
Sign In to gain access to subscriptions and/or personal tools.
TOP
ABSTRACT
ANGIOGENESIS
and β subunits of ATP synthase. In spite of the volume of research on angiostatin hitherto, there are still many controversies over the actual mechanisms which give angiostatin its powerful anti-angiogenic property. 
N) shows no inhibitory activities. Studies involving mice treated with GST-maspin and control mice (GST only) reported significant reduction in microvessel density (n = 10) and improved percentages of complete inhibition in short-term as well as long-term treatment with maspin (
-secretase-dependent cleavage of the C-terminus of VEGFR-1, which consequently inhibits VEGFR-2-induced angiogenesis. Moreover, in a human osteosarcoma cell line,