Effect of ionizing radiation on thymidine uptake, differentiation, and VEGFR2 receptor expression in endothelial cells: the role of VEGF(165)

Intra- and Early Postoperative Evaluation of Malperfused Areas in an Irradiated Random Pattern Skin Flap Model Using Indocyanine Green Angiography and Near-Infrared Reflectance-Based Imaging and Infrared Thermography

Background: Assessment of tissue perfusion after irradiation of random pattern flaps still remains a challenge. Methods: Twenty-five rats received harvesting of bilateral random pattern fasciocutaneous flaps. Group 1 served as nonirradiated control group. The right flaps of the groups 2–5 were irradiated with 20 Gy postoperatively (group 2), 3 × 12 Gy postoperatively (group 3), 20 Gy preoperatively (group 4) and 3 × 12 Gy preoperatively (group 5). Imaging with infrared thermography, indocyanine green angiography and near-infrared reflectance-based imaging were performed to detect necrotic areas of the flaps. Results: Analysis of the percentage of the necrotic area of the irradiated flaps showed a statistically significant increase from day 1 to 14 only in group 5 (p < 0.05). Indocyanine green angiography showed no differences (p > 0.05) of the percentage of the nonperfused area between all days in group 1 and 3, but a decrease in group 2 in both the left and the right flaps. Infrared thermography and near-infrared reflectance-based imaging did not show evaluable differences. Conclusion: Indocyanine green angiography is more precise in prediction of necrotic areas in random pattern skin flaps when compared to hyperspectral imaging, thermography or clinical impression. Preoperative fractional irradiation with a lower individual dose but a higher total dose has a more negative impact on flap perfusion compared to higher single stage irradiation.

Gamma Low-Dose-Rate Ionizing Radiation Stimulates Adaptive Functional and Molecular Response in Human Aortic Endothelial Cells in a Threshold-, Dose-, and Dose Rate-Dependent Manner

A central question in radiation protection research is whether low-dose and low-dose-rate (LDR) exposures to ionizing radiation play a role in progression of cardiovascular disease. The response of endothelial cells to different LDR exposures may help estimate risk of cardiovascular disease by providing the biological mechanism involved. We investigated the effect of chronic LDR radiation on functional and molecular responses of human aorta endothelial cells (HAoECs). Human aorta endothelial cells were continuously irradiated at LDR (6 mGy/h) for 15 days and analyzed at time points when the cumulative dose reached 0.05, 0.5, 1.0, and 2.0 Gy. The same doses were administered acutely at high-dose rate (HDR; 1 Gy/min). The threshold for the loss of angiogenic capacity for both LDR and HDR radiations was between 0.5 and 1.0 Gy. At 2.0 Gy, angiogenic capacity returned to normal only for HAoEC exposed to LDR radiation, associated with increased expression of antioxidant and anti-inflammatory genes. Pre-LDR, but not pre-HDR, radiation, followed by a single acute 2.0 Gy challenge dose sustained the expression of antioxidant and anti-inflammatory genes and stimulated angiogenesis. Our results suggest that dose rate is important in cellular response and that a radioadaptive response is involved for a 2.0 Gy dose at LDR.

Pilot Evaluation of Angiogenesis Signaling Factor Response after Transcatheter Arterial Embolization for Hepatocellular Carcinoma

Purpose To identify changes in a broad panel of circulating angiogenesis factors after bland transcatheter arterial embolization (TAE), a purely ischemic treatment for hepatocellular carcinoma (HCC). Materials and Methods This prospective HIPAA-compliant study was approved by the institutional review board. Informed written consent was obtained from all participants prior to entry into the study. Twenty-five patients (21 men; mean age, 61 years; range, 30-81 years) with Liver Imaging Reporting and Data System category 5 or biopsy-proven HCC and who were undergoing TAE were enrolled from October 15, 2014, through December 2, 2015. Nineteen plasma angiogenesis factors (angiopoietin 2; hepatocyte growth factor; platelet-derived growth factor AA and BB; placental growth factor; vascular endothelial growth factor A and D; vascular endothelial growth factor receptor 1, 2, and 3; osteopontin; transforming growth factor β1 and β2; thrombospondin 2; intercellular adhesion molecule 1; interleukin 6 [IL-6]; stromal cell-derived factor 1; tissue inhibitor of metalloproteinases 1; and vascular cell adhesion molecule 1 [VCAM-1]) were measured by using enzyme-linked immunosorbent assays at 1 day, 2 weeks, and 5 weeks after TAE and were compared with baseline levels by using paired Wilcoxon tests. Tumor response was assessed according to modified Response Evaluation Criteria in Solid Tumors (mRECIST). Angiogenesis factor levels were compared between responders and nonresponders by mRECIST criteria by using unpaired Wilcoxon tests. Results All procedures were technically successful with no complications. Fourteen angiogenesis factors showed statistically significant changes following TAE, but most changes were transient. IL-6 was upregulated only 1 day after the procedure, but showed the largest increases of any factor. Osteopontin and VCAM-1 demonstrated sustained upregulation at all time points following TAE. At 3-month follow-up imaging, 11 patients had responses to TAE (complete response, n = 6; partial response, n = 5) and 11 patients were nonresponders (stable disease, n = 9; progressive disease, n = 2). In nonresponders, the percent change in IL-6 on the day after TAE (P = .033) and the mean percent change in osteopontin after TAE (P = .024) were significantly greater compared with those of responders. Conclusion Multiple angiogenesis factors demonstrated significant upregulation after TAE. VCAM-1 and osteopontin demonstrated sustained upregulation, whereas the rest were transient. IL-6 and osteopontin correlated significantly with radiologic response after TAE. ^© RSNA, 2017.

Exposure to ionizing radiation reveals global dose- and time-dependent changes in the urinary metabolome of rat

The potential for exposures to ionizing radiation has increased in recent years. Although advances have been made, understanding the global metabolic response as a function of both dose and exposure time is challenging considering the complexity of the responses. Herein we report our findings on the dose- and time-dependency of the urinary response to ionizing radiation in the male rat using radiation metabolomics. Urine samples were collected from adult male rats, exposed to 0.5 to 10 Gy γ-radiation, both before from 6 to 72 h following exposures. Samples were analyzed by liquid chromatography coupled with time-of-flight mass spectrometry, and deconvoluted mass chromatographic data were initially analyzed by principal component analysis. However, the breadth and complexity of the data necessitated the development of a novel approach to summarizing biofluid constituents after exposure, called Visual Analysis of Metabolomics Package (VAMP). VAMP revealed clear urine metabolite profile differences to as little as 0.5 Gy after 6 h exposure. Via VAMP, it was discovered that the response to radiation exposure found in rat urine is characterized by an overall net down-regulation of ion excretion with only a modest number of ions excreted in excess over pre-exposure levels. Our results show both similarities and differences with the published mouse urine response and a dose- and time-dependent net decrease in urine ion excretion associated with radiation exposure. These findings mark an important step in the development of minimally invasive radiation biodosimetry. VAMP should have general applicability in metabolomics to visualize overall differences and trends in many sample sets.

Inhibition of vascular endothelial growth factor A and hypoxia-inducible factor 1α maximizes the effects of radiation in sarcoma mouse models through destruction of tumor vasculature

PURPOSE

To examine the addition of genetic or pharmacologic inhibition of hypoxia-inducible factor 1α (HIF-1α) to radiation therapy (RT) and vascular endothelial growth factor A (VEGF-A) inhibition (ie trimodality therapy) for soft-tissue sarcoma.

METHODS AND MATERIALS

Hypoxia-inducible factor 1α was inhibited using short hairpin RNA or low metronomic doses of doxorubicin, which blocks HIF-1α binding to DNA. Trimodality therapy was examined in a mouse xenograft model and a genetically engineered mouse model of sarcoma, as well as in vitro in tumor endothelial cells (ECs) and 4 sarcoma cell lines.

RESULTS

In both mouse models, any monotherapy or bimodality therapy resulted in tumor growth beyond 250 mm(3) within the 12-day treatment period, but trimodality therapy with RT, VEGF-A inhibition, and HIF-1α inhibition kept tumors at <250 mm(3) for up to 30 days. Trimodality therapy on tumors reduced HIF-1α activity as measured by expression of nuclear HIF-1α by 87% to 95% compared with RT alone, and cytoplasmic carbonic anhydrase 9 by 79% to 82%. Trimodality therapy also increased EC-specific apoptosis 2- to 4-fold more than RT alone and reduced microvessel density by 75% to 82%. When tumor ECs were treated in vitro with trimodality therapy under hypoxia, there were significant decreases in proliferation and colony formation and increases in DNA damage (as measured by Comet assay and γH2AX expression) and apoptosis (as measured by cleaved caspase 3 expression). Trimodality therapy had much less pronounced effects when 4 sarcoma cell lines were examined in these same assays.

CONCLUSIONS

Inhibition of HIF-1α is highly effective when combined with RT and VEGF-A inhibition in blocking sarcoma growth by maximizing DNA damage and apoptosis in tumor ECs, leading to loss of tumor vasculature.

NEMO modulates radiation-induced endothelial senescence of human umbilical veins through NF-κB signal pathway

Recently several laboratories have reported that radiation induces senescence in endothelial cells. Senescent cells can secrete multiple growth-regulatory proteins, some of which affect tumor growth, survival, invasion or angiogenesis. The purpose of this study was to explore the mechanisms of radiation-induced senescence and its effects on angiogenesis in human umbilical vein endothelial cells (HUVECs). HUVECs were either pretreated with or without PS1145 prior to irradiation with 0-8 Gy. PS1145 is a novel, highly specific small-molecule inhibitor of nuclear factor kappa B essential modulator (NEMO). MTT assays showed that in HUVECs untreated with PS1145, there was an increase in the number of radiation-induced senescence-like endothelial cells 5 days after 8 Gy irradiation, while pretreatment with PS1145 significantly ameliorated the induction in senescence of HUVECs compared to the control group. Electrophoretic mobility shift assay (EMSA) showed that pretreatment with PS1145 inhibited the radiation-induced NF-κB activation, which regulates cell fate in response to genotoxic stress. In addition, Western blotting demonstrated less translocation of p65 from cytoplasm to nucleus. Furthermore, real-time polymerase chain reaction (PCR) showed that pretreatment with PS1145 inhibited the increase of mRNA expressions of interleukin-6 (IL-6) and p53-induced death domain (PIDD) protein, which have been show to play crucial roles in both senescence and apoptosis (P < 0.05). TUNEL staining revealed an increase in apoptotic HUVECs in the group pretreated with PS1145 after irradiation. The series of functional assays further showed that radiation-induced senescence-like HUVECs had malfunctions in migration, invasion and formation of capillary-like structures, compared with the sham-irradiated and untreated, irradiated groups. Taken together, these findings indicate that the angiogenic capacity of radiation-induced senescence-like HUVECs decreased, and that irradiation caused vascular endothelial cells to gain a senescence-like phenotype through the DSB/NEMO/NF-κB signal pathway. The data suggests that NEMO may be a critical switch that regulates cellular senescence and apoptosis caused by exposure to radiation, and provides new clues for the clinical potential of the combination of radiotherapy and angiogenesis inhibitors.

Supplemental oxygen reverses hypoxia-induced smooth muscle cell proliferation by modulating HIF-alpha and VEGF levels in a rabbit arteriovenous fistula model

BACKGROUND

Numerous mechanisms for the formation of intimal hyperplasia have been proposed but none have been proven or accepted. Our research focuses on the potential role of hypoxia-inducible factors (HIFs), vascular endothelial growth factor (VEGF), and platelet-derived growth factors as well as the extracellular signal-regulated kinase (ERK), phosphatidylinositide 3-kinase /protein Kinase B (PI3-K/AKT) pathway in hypoxia-mediated intimal hyperplasia processes. We hypothesize that HIF and VEGF will be downregulated with supplemental oxygen in our arteriovenous fistula rabbit model.

METHODS

Rabbits were randomized into different experimental groups with varying oxygen exposure (21% O2 or 30% O2) and receipt of surgery (surgery with fistula formation, no surgery, or sham operation with skin incision only). Plasma samples were collected at designated intervals in which cytokines and smooth muscle cell proliferation were measured. In addition, cell specimens were exposed to hyperoxic, normoxic, and hypoxic environments with cytokines measured at various time points.

RESULTS

Placement of an arteriovenous fistula resulted in hypoxia-induced HIF stabilization with a concurrent increase in VEGF levels. There was a 4.2-fold induction in HIF-1α levels in animals that were placed in normal air after surgery when compared with animals that were exposed to hyperoxic air. Also, VEGF level significantly increased after surgery in the normoxic group, reaching a maximum of 959 pg/mL. Plasma VEGF levels in the surgery and supplemental oxygen group were significantly lower than the normoxic surgery group with almost a 45% reduction in plasma VEGF levels (524 pg/mL). Activation of VEGF receptors on smooth muscle cells through ERK1 and AKT pathways resulted in significant smooth muscle cell proliferation and migration. These effects are dramatically reduced in animals that are exposed to a hyperoxic environment of 30% oxygen.

CONCLUSIONS

Our results suggest that short-term administration of supplemental oxygen inhibits HIFs and VEGF signaling to reduce smooth muscle proliferation in the local blood vessel. These results provide strong support for the therapeutic use of supplemental oxygen after arterial surgery to reduce intimal hyperplasia. These findings also provide a nidus for future clinical trials to determine whether this is clinically applicable in humans.

Antiangiogenic agents in the treatment of recurrent or newly diagnosed glioblastoma: analysis of single-agent and combined modality approaches

Surgical resection followed by radiotherapy and temozolomide in newly diagnosed glioblastoma can prolong survival, but it is not curative. For patients with disease progression after frontline therapy, there is no standard of care, although further surgery, chemotherapy, and radiotherapy may be used. Antiangiogenic therapies may be appropriate for treating glioblastomas because angiogenesis is critical to tumor growth. In a large, noncomparative phase II trial, bevacizumab was evaluated alone and with irinotecan in patients with recurrent glioblastoma; combination treatment was associated with an estimated 6-month progression-free survival (PFS) rate of 50.3%, a median overall survival of 8.9 months, and a response rate of 37.8%. Single-agent bevacizumab also exceeded the predetermined threshold of activity for salvage chemotherapy (6-month PFS rate, 15%), achieving a 6-month PFS rate of 42.6% (p < 0.0001). On the basis of these results and those from another phase II trial, the US Food and Drug Administration granted accelerated approval of single-agent bevacizumab for the treatment of glioblastoma that has progressed following prior therapy. Potential antiangiogenic agents-such as cilengitide and XL184-also show evidence of single-agent activity in recurrent glioblastoma. Moreover, the use of antiangiogenic agents with radiation at disease progression may improve the therapeutic ratio of single-modality approaches. Overall, these agents appear to be well tolerated, with adverse event profiles similar to those reported in studies of other solid tumors. Further research is needed to determine the role of antiangiogenic therapy in frontline treatment and to identify the optimal schedule and partnering agents for use in combination therapy.

Low-dose radiation augments vasculogenesis signaling through HIF-1-dependent and -independent SDF-1 induction

The inflammatory response to ionizing radiation (IR) includes a proangiogenic effect that could be counterproductive in cancer but can be exploited for treating impaired wound healing. We demonstrate for the first time that IR stimulates hypoxia-inducible factor-1α (HIF-1α) up-regulation in endothelial cells (ECs), a HIF-1α-independent up-regulation of stromal cell-derived factor-1 (SDF-1), as well as endothelial migration, all of which are essential for angiogenesis. 5 Gray IR-induced EC HIF-1α and SDF-1 expression was greater when combined with hypoxia suggesting an additive effect. While small interfering RNA silencing of HIF-1α mRNA and abolition of HIF-1α protein induction down-regulated SDF-1 induction by hypoxia alone, it had little effect on SDF-1 induction by IR, demonstrating an independent pathway. SDF-1-mediated EC migration in hypoxic and/or radiation-treated media showed IR induced strong SDF-1-dependent migration of ECs, augmented by hypoxia. IR activates a novel pathway stimulating EC migration directly through the expression of SDF-1 independent of HIF-1α induction. These observations might be exploited for stimulation of wound healing or controlling tumor angiogenesis.

BMS-690514, a VEGFR and EGFR tyrosine kinase inhibitor, shows anti-tumoural activity on non-small-cell lung cancer xenografts and induces sequence-dependent synergistic effect with radiation

Background:

Non-small-cell lung cancer (NSCLC) is an aggressive disease in which vascular endothelial growth factor (VEGF) and epidermal growth factor (EGF) are implicated in tumour growth, tumour resistance to radiation and chemotherapy, and disease relapse. We have investigated the anti-tumoural effects of BMS-690514, an inhibitor of both vascular endothelial growth factor receptor (VEGFR) and epidermal growth factor receptor (EGFR) signalling pathways, as a single agent and in combination with ionising radiation (IR) on several NSCLC cell lines.

Methods:

Radiosensitisation of several NSCLC cell lines by BMS-690514 was assessed in vitro using clonogenic assay and in vivo using nude mice.

Results:

In vitro studies showed that BMS-690514 alone decreases clonogenic survival of NSCLC cells lines but no potential enhancement of IR response was observed in the combination. In tumour-bearing mice, BMS-690514 alone inhibits the growth of NSCLC xenografts, including the T790M mutation-harbouring H1975 tumour. The concomitant combination of BMS-690514 and radiation did not increase mice survival in comparison with treatment with IR alone. In contrast, BMS-690514 markedly enhances the anti-tumour effect of radiation in a sequential manner on H1299 and H1975 xenografts. Immunohistochemistry revealed a qualitative reduction in vessel area after administrations of BMS-690514, compared with vehicle-treated controls, suggesting that revascularisation may explain the schedule dependency of the tumour-growth delay observed.

Conclusion:

The results of association with radiation show that BMS-690514 may be a successful adjuvant to clinical radiotherapy. These findings are of translational importance because the clinical benefits of anti-EGFR and anti-VEGFR therapy might be schedule dependent.

Enhanced adhesion of early endothelial progenitor cells to radiation-induced senescence-like vascular endothelial cells in vitro

The effects of ionizing radiation (IR) on tumor neovascularization are still unclear. We previously reported that vascular endothelial cells (ECs) expressing the IR-induced senescence-like (IRSL) phenotype exhibit a significant decrease in angiogenic activity in vitro. In this study, we examined the effects of the IRSL phenotype on adhesion to early endothelial progenitor cells (early EPCs). Adhesion of human peripheral blood-derived early EPCs to human umbilical vein endothelial cells (HUVECs) expressing the IRSL phenotype was evaluated by an adhesion assay under static conditions. It was revealed that the IRSL HUVECs supported significantly more adhesion of early EPCs than normal HUVECs. Expressions of ICAM-1, VCAM-1 and E-selectin were up-regulated in IRSL HUVECs. Pre-treatment of IRSL HUVECs with adhesion-blocking monoclonal antibodies against E-selectin and VCAM-1 significantly reduced early EPC adhesion to IRSL HUVECs, suggesting a potential role for the E-selectin and VCAM-1 in the adhesion between IRSL ECs and early EPCs. Therefore, the IRSL phenotype expressed in ECs may enhance neovascularization via increased homing of early EPCs. Our findings are first to implicate the complex effects of this phenotype on tumor neovascularization following irradiation.

Clinical biomarkers of angiogenesis inhibition

INTRODUCTION

An expanding understanding of the importance of angiogenesis in oncology and the development of numerous angiogenesis inhibitors are driving the search for biomarkers of angiogenesis. We review currently available candidate biomarkers and surrogate markers of anti-angiogenic agent effect.

DISCUSSION

A number of invasive, minimally invasive, and non-invasive tools are described with their potential benefits and limitations. Diverse markers can evaluate tumor tissue or biological fluids, or specialized imaging modalities.

CONCLUSIONS

The inclusion of these markers into clinical trials may provide insight into appropriate dosing for desired biological effects, appropriate timing of additional therapy, prediction of individual response to an agent, insight into the interaction of chemotherapy and radiation following exposure to these agents, and perhaps most importantly, a better understanding of the complex nature of angiogenesis in human tumors. While many markers have potential for clinical use, it is not yet clear which marker or combination of markers will prove most useful.

[Role of endothelium in radiation-induced normal tissue damages]

More than half of cancers are treated with radiation therapy alone or in combination with surgery and/or chemotherapy. The goal of radiation therapy is to deliver enough ionising radiation to destroy cancer cells, without exceeding the level that the surrounding healthy cells can tolerate. Unfortunately, radiation-induced normal tissue injury is still a dose limiting factor in the treatment of cancer with radiotherapy. Early and late side-effects not only limit radiation dose escalation, but might also affect the patient's quality of life. Vascular injury is one of the most common effects of radiotherapy on normal tissues. Radiation-induced fibrogenesis is characterized by an orchestrated pathological wound-healing response in which the radiation-induced endothelium dysfunction plays a critical role. Irradiated endothelial cells acquire a proinflammatory, procoagulant and prothrombotic phenotype. The knowledge of molecular mechanisms involved in endothelium dysfunction following radiation is needed to identify therapeutic targets and develop strategies to prevent and /or reduce side-effects of radiation therapy.

Proangiogenic effects of ionizing irradiation on squamous cell carcinoma of the hypopharynx

OBJECTIVE

There is experimental evidence that ionizing irradiation affects a proangiogenic response. However, the relevance of this effect on tumour growth in vivo is not in detail investigated yet. The present objectives were to examine the influence of ionizing radiation on the expression of the vascular endothelial growth factor (VEGF) and its receptors (flt-1 and flk-1), the microvessel density and the tumour proliferation, in head and neck squamous cell carcinoma (HNSCC).

METHODS

We used a HNSCC-cell line, derived from a hypopharyngeal tumour, for subcutaneous injection in 16 athymic nude mice. After reaching an average diameter of 12-14 mm the xenografts were randomised and 8 out of the 16 animals (therapy group) were irradiated with a single fraction of 6 Gy while the control group remained without any intervention. The irradiated and the respective control tumours were prepared after 7 (T7) and 70 days (T70) for immunohistochemical analysis. The expression of VEGF, its receptors flk-1 and flt-1, the vessel density (CD31) and the proliferation rate (Ki67) were quantified.

RESULTS

At the point of time T7 we observed a reduction of the tumour growth rate, of the proliferative activity and of the VEGF- as well as of the VEGF-R-expression. At the point of time T70 we found increased values for proliferation, microvessel density, VEGF- and flk-1 expression in the therapy group compared to the therapy group at T7 as well as to the control group at T70.

CONCLUSION

These changes might suggest a long-term proangiogenic effect of irradiation, which might result in growth promotion of the remaining tumour after the end of therapy.

Imaging of angiogenesis in cardiology

In the past decade, there have been major improvements in our understanding of angiogenesis at the genetic, molecular and cellular levels. Concentrated efforts in this area have led to new therapeutic approaches to ischaemic heart disease using angiogenic factors, gene therapy and progenitor cells. Despite very promising experimental results in animal studies, large clinical trials have failed to confirm the results in patients with coronary artery disease. Important questions such as selection of growth factors and donor cells, as well as the timing, dose and route of administration, have been raised and need to be answered. Molecular imaging approaches which may provide specific markers of the angiogenic process (e.g. integrin expression in endothelial cells) have been introduced and are expected to address some of these questions. Although few clinical imaging results are currently available, animal studies suggest the potential role of molecular imaging for characterisation of the angiogenetic process in vivo and for the monitoring of therapeutic effects.

Radiation-induced senescence-like phenotype in proliferating and plateau-phase vascular endothelial cells

The effects of ionizing radiation (IR) on tumor angiogenesis still remain largely unknown. In this study, we found that IR (8 Gy) induces a high-frequency (80-90%) senescence-like phenotype in vascular endothelial cells (ECs) undergoing exponential growth. This finding allowed us to characterize the IR-induced senescence-like (IRSL) phenotype by examining the gene expression profiles and in vitro angiogenic activities of these ECs. The expression levels of genes associated with cell cycle progression and DNA replication were remarkably reduced in the IRSL ECs. Additionally, the in vitro invasion and migration activities of these cells through Matrigel were significantly suppressed. We also found that confluent ECs exhibited a high-frequency IRSL phenotype when they were replated immediately after irradiation, whereas incubation in plateau-phase conditions reduced the induction of this phenotype and enhanced colony formation. The kinetics of DNA double-strand break repair, which showed a faster time course in confluent ECs than in growing ECs, may contribute to the protective mechanism associated with the IRSL phenotype. These results imply that the IRSL phenotype may be important for determining the angiogenic activity of ECs following irradiation. The present study should contribute to the understanding of the effects of IR on tumor angiogenesis.

Design of clinical trials of radiation combined with antiangiogenic therapy

Clinical trials showing longer survival when chemotherapy is combined with antiangiogenic agents (AAs) have led to growing interest in designing combined modality protocols that exploit abnormalities in tumor vasculature. Approved agents include bevacizumab, a recombinant monoclonal antibody that binds to vascular endothelial growth factor, and two small molecule multitargeted tyrosine kinase inhibitors of angiogenesis (SU11248 and BAY-43-9006) that have been approved for therapy of renal cancer. Targeting tumor vasculature has a strong biological rationale in radiation therapy, and preclinical studies consistently show an increase in radiosensitization with combined treatment. Preclinical studies indicate that excessive damage to tumor vasculature can result in radioresistance in some situations, and early clinical data suggest that treatment sequencing may be important when combining AAs with radiation. Radiation itself appears to antagonize any hypoxia that can be induced by long-term administration of AAs. The optimal biological doses of AAs with radiotherapy are unknown, and surrogate markers of efficacy remain to be validated. Early clinical trials should therefore include studies designed to identify mechanisms of interaction and increases in tumor hypoxia. This review highlights preclinical and early clinical data that are relevant for clinical trial design. Optimal radiation planning and delivery is required to minimize the volume of irradiated normal organs and to establish safe dose-volume parameters for phase II-III clinical trials.

Sulfoglycolipids as candidate antiangiogenic radiosensitizers

Angiogenesis is considered an essential process for the growth of solid tumors and, accordingly, angiogenesis has been a focus of attention for cancer therapy. Although various antiangiogenic agents have been developed, adverse effects and limitations associated with antitumor therapies have recently become apparent. To overcome these problems, combining such agents with chemotherapy or radiotherapy is now strongly recommended in clinical practice. Provided such combination treatment, from the onset of therapy, different strategies in developing antiangiogenic agents should be used to enhance any combinatory effects and reduce adverse effects. By applying the concept of radiosensitizers, a new class of antiangiogenic treatments should now be possible. We recently developed sulfoglycolipids that possess such properties. In this review, we discuss the properties of antiangiogenic radiosensitizers and their potential usefulness.

VEGF-associated tyrosine kinase inhibition increases the tumor response to single and fractionated dose radiotherapy

PURPOSE

In this study, the efficacy of combining ZD6474 (Zactima), a vascular endothelial growth factor (VEGF) receptor 2-associated tyrosine kinase inhibitor currently undergoing Phase II clinical trial evaluation, with single and fractionated dose radiation exposures was examined in a human colorectal carcinoma model (HT29).

METHODS AND MATERIALS

HT29 xenograft-bearing mice were treated with either single-dose (10 Gy) or multifraction (2 Gy/day for 2 weeks) radiotherapy alone or in conjunction with a 2-week course of ZD6474 (25 mg/kg). In the single-dose investigation, ZD6474 treatment followed radiotherapy, whereas in the fractionated dose studies the antiangiogenic therapy was given before, after, or concurrent with the radiation. Tumor response was determined by tumor growth delay.

RESULTS

ZD6474 increased the response of HT29 xenografts to both single and fractionated dose radiotherapy. In the fractionation studies sequencing of therapies had little impact on treatment outcomes; the time for the median tumors in each of the treatment groups to grow to five times the starting size was 53, 53.5, and 49 days, respectively.

CONCLUSIONS

These studies indicate that ZD6474, when used in conjunction with radiation therapy, has a clear therapeutic advantage, providing a rationale for considering the combination of this agent with radiotherapy in the clinic.

A quantitative study of the effects of ionizing radiation on endothelial cells and capillary-like network formation

The initial events of angiogenesis comprise endothelial cell activation, migration, and proliferation. The characteristics of retinal endothelial cells and capillaries are significantly altered in a number of diseases including cancer. Since radiation has been shown as a useful tool in radiotherapy by altering the proliferative changes, it is important to evaluate the responses of the endothelial cells and the capillary network to radiation. We quantified functional and kinetic responses of endothelial cells and capillaries to radiation in an in vitro model. An in vitro angiogenesis model was introduced in our study with endothelial cells cultured on an extracellular matrix gel in which hollow tube-like structures could be rapidly formed. Vessel formation was quantified using stereological techniques. The cell cycle kinetics of endothelial cells and accumulation of DNA damage after radiation were measured using laser scanning cytometry. To study the response of proliferative capillary-like structures to radiation, the vessel network was irradiated with 2 gray (Gy). To evaluate functional and kinetic responses and differentiation of endothelial cells to radiation, cells were irradiated with 2 and 6 Gy. Progressive time- and dose-dependent loss of endothelial cells occurred starting 24 hours after radiation. Vessel growth was significantly retarded at the higher dose. A significant percentage of DNA breaks were detected dose-dependently. A large percentage of G1 cells were measured in the irradiated endothelial cell population when compared to the respective sham-treated control population. These results indicate that radiation-induced endothelial cell injuries destroy the integrity of vascular structure. We postulated that apoptosis may represent a biologically relevant mechanism of radiation-induced endothelial cell damage.

Current status of angiogenesis inhibitors combined with radiation therapy

Angiogenesis inhibitors combined with cytotoxic chemotherapy have recently entered routine oncological practice. Several rationales exist for combining these agents with ionizing radiation, a primary curative cancer treatment, either in bimodal or trimodal fashion, i.e. with or without additional chemotherapy. More than 20 different anti-angiogenic agents have been studied in preclinical animal tumor models. This systematic review compares the results of preclinical studies published before February 2006. The combination of vascular endothelial growth factor (VEGF) inhibitors with irradiation consistently resulted in improved tumor growth delay (at least additive effects), despite different radiation schedules, drugs and doses, and combination regimens. Only two studies evaluated tumor control dose (TCD)50 as a measure of tumor cure (radiation dose yielding permanent local control in 50% of the tumors). While anti-VEGF receptor (VEGFR) antibody treatment improved the outcome, a VEGFR tyrosine kinase inhibitor showed negative results. For agents interfering with other pathways, the results are also not consistent, although most studies were positive. Trimodal approaches seem to improve tumor growth delay even further. Importantly, both radiotherapy schedule and sequence of the modalities in combined treatment may impact on the outcome. Hence, further preclinical studies examining these parameters need to be conducted. While preclinical research is ongoing, phase I and II clinical trials with bevacizumab, combretastatin A-4, thalidomide and different receptor tyrosine kinase inhibitors, usually combined with radio- and chemotherapy, have been designed. Early results suggest that acute toxicity is acceptable, planned surgery after such treatment is feasible, and that further evaluation of such combined modality treatment is warranted.

Rationale for combination therapies for choroidal neovascularization

PURPOSE

To provide a conceptual framework for the development and use of combination therapies for choroidal neovascularization secondary to age-related macular degeneration.

DESIGN

Literature review, integration of data, and creation of hypothesis.

METHODS

An assessment of angiogenesis, cancer therapy, and inflammation was performed as they may pertain to choroidal neovascularization. A conceptual framework was created in which therapies for choroidal neovascularization could be evaluated alone or in combination.

RESULTS

Angiogenesis occurs because cells produce angiogenic stimuli to encourage blood vessels to develop. This growth of vessels involves an orchestrated interaction among many mediators offering opportunity to modulate or inhibit the entire process. A two-component model for choroidal neovascularization is proposed. The vascular component of choroidal neovascularization is comprised of vascular endothelial cells, endothelial cell precursors, and pericytes. The extravascular component, which by histopathology appears to be both the source of angiogenic stimuli and often the largest component volumetrically, is comprised of inflammatory, glial and retinal pigment epithelial cells, and fibroblasts. Tissue damage can be caused by either component. Each component can be targeted through as variety of monotherapies. Combination therapies offer the possibility of attacking one component in more than one way or by attacking both components simultaneously.

CONCLUSIONS

The two-component model of choroidal neovascularization can be used to evaluate the mechanism of action and possible interactions of these agents in a conceptual framework. Extension of these ideas can help guide development of new treatment agents and approaches.

Low-dose irradiation promotes tissue revascularization through VEGF release from mast cells and MMP-9-mediated progenitor cell mobilization

Mast cells accumulate in tissues undergoing angiogenesis during tumor growth, wound healing, and tissue repair. Mast cells can secrete angiogenic factors such as vascular endothelial growth factor (VEGF). Ionizing irradiation has also been shown to have angiogenic potential in malignant and nonmalignant diseases. We observed that low-dose irradiation fosters mast cell–dependent vascular regeneration in a limb ischemia model. Irradiation promoted VEGF production by mast cells in a matrix metalloproteinase-9 (MMP-9)–dependent manner. Irradiation, through MMP-9 up-regulated by VEGF in stromal and endothelial cells, induced the release of Kit-ligand (KitL). Irradiation-induced VEGF promoted migration of mast cells from the bone marrow to the ischemic site. Irradiation-mediated release of KitL and VEGF was impaired in MMP-9–deficient mice, resulting in a reduced number of tissue mast cells and delayed vessel formation in the ischemic limb. Irradiation-induced vasculogenesis was abrogated in mice deficient in mast cells (steel mutant, Sl/Sl^d mice) and in mice in which the VEGF pathway was blocked. Irradiation did not induce progenitor mobilization in Sl/Sl^d mice. We conclude that increased recruitment and activation of mast cells following irradiation alters the ischemic microenvironment and promotes vascular regeneration in an ischemia model. These data show a novel mechanism of neovascularization and suggest that low-dose irradiation may be used for therapeutic angiogenesis to augment vasculogenesis in ischemic tissues.

Enhanced susceptibility of irradiated tumor vessels to vascular endothelial growth factor receptor tyrosine kinase inhibition

Previous experiments with PTK787/ZK222584, a specific inhibitor of vascular endothelial growth factor receptor (VEGFR) tyrosine kinases, using irradiated human FaDu squamous cell carcinoma in nude mice, suggested that radiation-damaged tumor vessels are more sensitive to VEGFR inhibition. To test this hypothesis, the tumor transplantation site (i.e., the right hind leg of nude mice) was irradiated 10 days before transplantation of FaDu to induce radiation damage in the host tissue. FaDu tumors vascularized by radiation-damaged blood vessels appeared later, grew at a slower rate, and showed more necrosis and a smaller vessel area per central tumor section than controls. PTK787/ZK222584 at a daily dose of 50 mg/kg body weight had no impact on growth of control tumors. In contrast, tumors vascularized by radiation-damaged vessels responded to PTK787/ZK222584 with longer latency and slower growth rate than controls, and a trend toward further increase in necrosis, indicating that irradiated tumor vessels are more susceptible to VEGFR inhibition than unirradiated vessels. Although not proving causality, expression analysis of VEGF and VEGFR2 shows that enhanced sensitivity of irradiated vessels to a specific inhibitor of VEGFR tyrosine kinases correlates with increased expression of the molecular target.

Angiogenesis and radiation response modulation after vascular endothelial growth factor receptor-2 (VEGFR2) blockade

The formation of new blood vessels (angiogenesis) represents a critical factor in the malignant growth of solid tumors and metastases. Vascular endothelial cell growth factor (VEGF) and its receptor VEGFR2 represent central molecular targets for antiangiogenic intervention, because of their integral involvement in endothelial cell proliferation and migration. In the current study, we investigated in vitro and in vivo effects of receptor blockade on various aspects of the angiogenic process using monoclonal antibodies against VEGFR2 (cp1C11, which is human specific, and DC101, which is mouse specific). Molecular blockade of VEGFR2 inhibited several critical steps involved in angiogenesis. VEGFR2 blockade in endothelial cells attenuated cellular proliferation, reduced cellular migration, and disrupted cellular differentiation and resultant formation of capillary-like networks. Further, VEGFR2 blockade significantly reduced the growth response of human squamous cell carcinoma xenografts in athymic mice. The growth-inhibitory effect of VEGFR2 blockade in tumor xenografts seems to reflect antiangiogenic influence as demonstrated by vascular growth inhibition in an in vivo angiogenesis assay incorporating tumor-bearing Matrigel plugs. Further, administration of VEGFR2-blocking antibodies in endothelial cell cultures, and in mouse xenograft models, increased their response to ionizing radiation, indicating an interactive cytotoxic effect of VEGFR2 blockade with radiation. These data suggest that molecular inhibition of VEGFR2 alone, and in combination with radiation, can enhance tumor response through molecular targeting of tumor vasculature.

Carboplatin selectively induces the VEGF stress response in endothelial cells: Potentiation of antitumor activity by combination treatment with antibody to VEGF

Vascular Endothelial Growth Factor (VEGF) functions as a key regulator in tumor angiogenesis. In addition, VEGF is an important survival factor for endothelial cells under chemical or physical stress. In our report, we show that treatment of endothelial cells with the chemotherapeutic agent carboplatin significantly increased the expression of VEGF. Furthermore, neutralization of secreted VEGF with specific polyclonal anti-VEGF antibodies or monoclonal antibody sensitized endothelial cells to carboplatin treatment and increased apoptosis several-fold. Interestingly, carboplatin treatment did not alter VEGF expression in tumor cells. Similarly, antibody to VEGF did not change the chemosensitivity of tumor cells to this drug. Most importantly, tumor-bearing animals treated with carboplatin showed an increase in VEGF immunoreactivity in the tumor vasculature, confirming the in vitro studies. Based on these observations, we determined whether neutralization of VEGF could enhance the anti-tumor activity of carboplatin in an in vivo ovarian cancer model system. A combination therapy consisting of a suboptimal dose of carboplatin (32.5 mg/kg/inj., q3d x 5; i.p.) and polyclonal anti-VEGF antibody (2 mg/inj., q3d x 10; i.p.) significantly enhanced solid tumor growth inhibition over individual monotherapies and included multiple complete responses. These findings suggest that VEGF is a critical endothelial cell specific survival factor that is induced by carboplatin and contributes to the protection of tumor vasculature during chemotherapy treatment. In addition, these results provide evidence for a potential mechanism that underlies enhanced anti-tumor activity achieved with chemotherapy and anti-VEGF antibody combination treatment regimens as recently reported in a number of clinical trials. We conclude that a similar type of combination therapy may be applicable to many types of malignancies since VEGF expression was differentially induced in the tumor host environment (i.e., tumor vasculature) and not in the tumor cells themselves; hence, this phenomenon may be independent of the type and origin of the primary cancer.

Improved free vascular graft survival in an irradiated surgical site following topical application of rVEGF

PURPOSE

Wound healing disorders following surgery in preirradiated tissue are clinically well known and may even become more crucial with the increasing use of neoadjuvant chemoradiation protocols. Both the expression of vascular endothelial growth factor (VEGF) and endoglin (CD105) play a key role in neovascularization and wound healing after soft tissue grafts in irradiated and nonirradiated tissue. Modulation of neovascularization through the application of recombinant VEGF (rVEGF) may be a therapeutic option to reduce wound healing disorders in irradiated tissue. An experimental in vivo model was used to study the possible role of rVEGF for reduction of wound healing disorders and the promotion of neovascularization.

METHODS AND MATERIALS

A free myocutaneous gracilis flap was transplanted from the groin into the neck region of Wistar rats (weight 300-500 g) with and without previous irradiation of the neck region with 40 Gy: Group 1 (n = 7) radiotherapy alone; Group 2 (n = 14) flap transplantation alone and rVEGF; Group 3 (n = 14) radiotherapy, transplantation, and rVEGF. Time interval between irradiation and grafting was 10 +/- 1 day. 1.0 micro g rVEGF/500 microL phosphate-buffered saline was applied s.c. intraoperatively and on Days 1 through 7. Neovascularization (CD105) and endogenous VEGF expression were analyzed by means of immunohistochemistry on Days 3, 5, 7, 14, and 28 postoperatively and quantified as labeling indices (LI).

RESULTS

After irradiation there was a continuous significant reduction of the cytoplasmic VEGF expression (MEAN LI: 0.018 +/- 0.048) compared with the nonirradiated control (mean LI: 0.042 +/- 0.006) (p < 0.001). VEGF expression after flap transplantation without irradiation after VEGF application was at a constantly higher level from Day 3 (mean LI: 0.044 +/- 0.01) to Day 28 postoperatively compared with the control group (Day 3, mean LI: 0.028 +/- 0.006) (p < 0.001). As an indication of increased neovascularization after the local application of rVEGF, a significantly increased expression of CD105 was found in the transition area and graft bed from Day 7 on (p < 0.001). After irradiation and grafting there was a significant overall increase in the VEGF- and CD105-expression throughout Day 28 after rVEGF in the transition area (p < 0.001).

CONCLUSION

Whereas irradiation alone led to a downregulation of the endogenous VEGF expression, rVEGF application resulted in an increased expression and in a CD105 associated neovascularization after soft tissue grafting in irradiated tissues. Application of rVEGF may enable modulation of wound healing by influencing neovascularization. This could indicate a possible clinical approach for reducing fibrosis and chronic wound healing disorders in irradiated tissues.