Differences in vascular response between primary and transplanted tumours

Establishing the Impact of Vascular Damage on Tumor Response to High-Dose Radiation Therapy

Approximately half of all patients with cancer receive radiotherapy, which is conventionally delivered in relatively small doses (1.8-2 Gy) per daily fraction over one to two months. Stereotactic body radiation therapy (SBRT), in which a high daily radiation dose is delivered in 1 to 5 fractions, has improved local control rates for several cancers. However, despite the widespread adoption of SBRT in the clinic, controversy surrounds the mechanism by which SBRT enhances local control. Some studies suggest that high doses of radiation (≥10 Gy) trigger tumor endothelial cell death, resulting in indirect killing of tumor cells through nutrient depletion. On the other hand, mathematical models predict that the high radiation dose per fraction used in SBRT increases direct tumor cell killing, suggesting that disruption of the tumor vasculature is not a critical mediator of tumor cure. Here, we review the application of genetically engineered mouse models to radiosensitize tumor cells or endothelial cells to dissect the role of these cellular targets in mediating the response of primary tumors to high-dose radiotherapy in vivo These studies demonstrate a role for endothelial cell death in mediating tumor growth delay, but not local control following SBRT.

A Bioluminescent and Fluorescent Orthotopic Syngeneic Murine Model of Androgen-dependent and Castration-resistant Prostate Cancer

Orthotopic tumor modeling is a valuable tool for pre-clinical prostate cancer research, as it has multiple advantages over both subcutaneous and transgenic genetically engineered mouse models. Unlike subcutaneous tumors, orthotopic tumors contain more clinically accurate vasculature, tumor microenvironment, and responses to multiple therapies. In contrast to genetically engineered mouse models, orthotopic models can be performed with lower cost and in less time, involve the use of highly complex and heterogeneous mouse or human cancer cell lines, rather that single genetic alterations, and these cell lines can be genetically modified, such as to express imaging agents. Here, we present a protocol to surgically injecting a luciferase- and mCherry-expressing murine prostate cancer cell line into the anterior prostate lobe of mice. These mice developed orthotopic tumors that were non-invasively monitored in vivo and further analyzed for tumor volume, weight, mouse survival, and immune infiltration. Further, orthotopic tumor-bearing mice were surgically castrated, leading to immediate tumor regression and subsequent recurrence, representing castration-resistant prostate cancer. Although technical skill is required to carry out this procedure, this syngeneic orthotopic model of both androgen-dependent and castration-resistant prostate cancer is of great use to all investigators in the field.

Patterns of Vasculature in Mouse Models of Lung Cancer Are Dependent on Location

PURPOSE

Preclinical studies of hypoxia are generally done using ectopic xenograft tumors, which behave differently from human tumors. Our previous findings have shown that subcutaneously implanted lung tumors exhibit more hypoxia than their orthotopic implanted or spontaneous K-ras-induced counterparts. We hypothesize that differences in hypoxia are due to site-specific differences in vascularity and perfusion.

PROCEDURES

To compare the presence and functionality of vessels in these tumor models, we studied vascular perfusion in vivo in real time.

RESULTS

Orthotopically implanted and spontaneous K-ras-induced lung tumors showed elevated perfusion, demonstrating vasculature functionality. Little contrast agent uptake was observed within the subcutaneously implanted tumors, indicating vascular dysfunction. These findings were corroborated at the microscopic level with Hoechst 33342 and Meca-32 staining.

CONCLUSIONS

From these observations, we concluded that differences in hypoxia in experimental models is related to vessel perfusion. Thus, appropriate selection of preclinical lung tumor models is essential for the study of hypoxia, angiogenesis and therapies targeting these phenomena.

Genetically engineered mouse models for studying radiation biology

Genetically engineered mouse models (GEMMs) are valuable research tools that have transformed our understanding of cancer. The first GEMMs generated in the 1980s and 1990s were knock-in and knock-out models of single oncogenes or tumor suppressors. The advances that made these models possible catalyzed both technological and conceptual shifts in the way cancer research was conducted. As a result, dozens of mouse models of cancer exist today, covering nearly every tissue type. The advantages inherent to GEMMs compared to in vitro and in vivo transplant models are compounded in preclinical radiobiology research for several reasons. First, they accurately and robustly recapitulate primary cancers anatomically, histopathologically, and genetically. Reliable models are a prerequisite for predictive preclinical studies. Second, they preserve the tumor microenvironment, including the immune, vascular, and stromal compartments, which enables the study of radiobiology at a systems biology level. Third, they provide exquisite control over the genetics and kinetics of tumor initiation, which enables the study of specific gene mutations on radiation response and functional genomics in vivo. Taken together, these facets allow researchers to utilize GEMMs for rigorous and reproducible preclinical research. In the three decades since the generation of the first GEMMs of cancer, advancements in modeling approaches have rapidly progressed and expanded the mouse modeling toolbox with techniques such as in vivo short hairpin RNA (shRNA) knockdown, inducible gene expression, site-specific recombinases, and dual recombinase systems. Our lab and many others have utilized these tools to study cancer and radiobiology. Recent advances in genome engineering with CRISPR/Cas9 technology have made GEMMs even more accessible to researchers. Here, we review current and future approaches to mouse modeling with a focus on applications in preclinical radiobiology research.

Tumor cells, but not endothelial cells, mediate eradication of primary sarcomas by stereotactic body radiation therapy

Cancer clinics currently use high-dose stereotactic body radiation therapy as a curative treatment for several kinds of cancers. However, the contribution of vascular endothelial cells to tumor response to radiation remains controversial. Using dual recombinase technology, we generated primary sarcomas in mice with targeted genetic mutations specifically in tumor cells or endothelial cells. We selectively mutated the proapoptotic gene Bax or the DNA damage response gene Atm to genetically manipulate the radiosensitivity of endothelial cells in primary soft tissue sarcomas. Bax deletion from endothelial cells did not affect radiation-induced cell death in tumor endothelial cells or sarcoma response to radiation therapy. Although Atm deletion increased endothelial cell death after radiation therapy, deletion of Atm from endothelial cells failed to enhance sarcoma eradication. In contrast, deletion of Atm from tumor cells increased sarcoma eradication by radiation therapy. These results demonstrate that tumor cells, rather than endothelial cells, are critical targets that regulate sarcoma eradication by radiation therapy. Treatment with BEZ235, a small-molecule protein kinase inhibitor, radiosensitized primary sarcomas more than the heart. These results suggest that inhibiting ATM kinase during radiation therapy is a viable strategy for radiosensitization of some tumors.

Atm deletion with dual recombinase technology preferentially radiosensitizes tumor endothelium

Cells isolated from patients with ataxia telangiectasia are exquisitely sensitive to ionizing radiation. Kinase inhibitors of ATM, the gene mutated in ataxia telangiectasia, can sensitize tumor cells to radiation therapy, but concern that inhibiting ATM in normal tissues will also increase normal tissue toxicity from radiation has limited their clinical application. Endothelial cell damage can contribute to the development of long-term side effects after radiation therapy, but the role of endothelial cell death in tumor response to radiation therapy remains controversial. Here, we developed dual recombinase technology using both FlpO and Cre recombinases to generate primary sarcomas in mice with endothelial cell-specific deletion of Atm to determine whether loss of Atm in endothelial cells sensitizes tumors and normal tissues to radiation. Although deletion of Atm in proliferating tumor endothelial cells enhanced the response of sarcomas to radiation, Atm deletion in quiescent endothelial cells of the heart did not sensitize mice to radiation-induced myocardial necrosis. Blocking cell cycle progression reversed the effect of Atm loss on tumor endothelial cell radiosensitivity. These results indicate that endothelial cells must progress through the cell cycle in order to be radiosensitized by Atm deletion.

Effects of HSP90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) on NEU/HER2 overexpressing mammary tumours in MMTV-NEU-NT mice monitored by Magnetic Resonance Spectroscopy

BACKGROUND

The importance of ERBB2/NEU/HER2 in the response of breast tumours to the heat shock protein 90 (HSP90) inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG; tanespimycin) has been demonstrated in the clinic. ERBB2 is an oncoprotein client that is highly dependent on HSP90. This and other oncogenic client proteins (e.g. B-RAF, C-RAF, ALK and CDK4) are depleted by 17-AAG in both animal tumours and patients. Here we investigate by Magnetic Resonance Spectroscopy (MRS) the metabolic response of 17-AAG in spontaneous, NEU/HER2 driven mammary tumours in transgenic MMTV-NEU-NT mice and in cells isolated and cultured from these tumours.

METHODS

Mammary tumours were monitored by 31P MRS in vivo and in tumour extracts, comparing control and 17-AAG treated mice. A cell line derived from NEU/HER2 mammary tumours was also cultured and the effect of 17-AAG was measured by 31P MRS in cell extracts. Molecular biomarkers were assessed by immunoblotting in extracts from cells and tumours. For comparison of tumour volume, metabolite concentrations and Western blot band intensities, two-tailed unpaired t-tests were used.

RESULTS

The NEU/HER2 mammary tumours were very sensitive to 17-AAG and responded in a dose-dependent manner to 3 daily doses of 20, 40 and 80mg/kg of 17-AAG, all of which caused significant regression. At the higher doses, 31P MRS of tumour extracts showed significant decreases in phosphocholine (PC) and phosphoethanolamine (PE) whereas no significant changes were seen at the 20mg/kg dose. Extracts of isolated cells cultured from the mammary carcinomas showed a significant decrease in viable cell number and total PME after 17-AAG treatment. Western blots confirmed the expected action of 17-AAG in inducing HSP72 and significantly depleting HSP90 client proteins, including NEU/HER2 both in tumours and in isolated cells.

CONCLUSIONS

The data demonstrate the high degree of sensitivity of this clinically relevant NEU/HER2-driven tumour model to HSP90 inhibition by 17-AAG, consistent with the clinical data, and suggest that the metabolic signature of choline phospholipids obtained by MRS could be useful both as a preclinical and clinical tool for investigating surrogate markers of response to treatment.

Effect of implantation site and growth of hepatocellular carcinoma on apparent diffusion coefficient of water and sodium MRI

Hepatocellular carcinoma (HCC) and liver metastases are an increasing problem worldwide. Non-invasive methods for the early detection of HCC and understanding of the tumor growth mechanisms are highly desirable.  Both the diffusion-weighted (1)H (DWI) and (23)Na MRI reflect alterations in tissue compartment volumes in tumors, as well as physiological and metabolic transformation in cells. Effects of untreated growth on apparent diffusion coefficient of water (ADC), single quantum (SQ) and triple quantum-filtered (TQF) (23)Na MRI were compared in intrahepatically and subcutaneously implanted HCCs in rats. Animals were examined weekly for 4 weeks after injection of N1S1 cells. ADC of intrahepatic HCC was 1.5-times higher compared to the nearby liver tissue, and with growth, the ADC did not increase. ADC of subcutaneous HCC was lower compared to intrahepatic HCC and it increased with growth. Untreated growth of both intrahepatic and subcutaneous HCCs was associated with an increase in SQ and TQF (23)Na signal intensity suggesting an increase in tissue Na(+) and intracellular Na(+) (Na(+)(i)), respectively, most likely due to an increase in relative extracellular space and Na(+)(i) concentration as a result of changes in tissue structure and cellular metabolism. Thus, SQ and TQF (23)Na MRI may be complementary to diffusion imaging in areas susceptible to motion for characterizing hepatic tumors and for other applications, such as, predicting and monitoring therapy response.

The use of xenograft models for the selection of cancer treatments with the EGFR as an example

Mouse models of cancer have consistently been used to qualify new anti-cancer drugs for development of human clinical trials. The most used models are xenografts of human tumors grown subcutaneously in immunodeficient mice such as athymic (nude) or severe combined immune deficient (SCID) mice. However, the number of anti-cancer agents that fail in the clinic far outweighs those considered effective, suggesting that the selection procedure for progression of molecules into the clinic requires improvement. This has provoked considerable skepticism about the value of using such preclinical models. As a result, a shift has occurred towards developing and using spontaneous mouse tumor arising in transgenic and/or knockout mice engineered to recapitulate various genetic alterations thought to be causative of specific types of human cancers. Alternatively, the option has been to improve human tumor xenograft models by using orthotopic transplantation and, therefore, promotion of metastatic spread of the resultant 'primary' tumors. Here we review the value and the limitations of xenograft models and their role in developing new anti-cancer treatments.

Prospects for bioreductive drug development

Bioreductive drugs are inactive prodrugs that are converted into potent cytotoxins under conditions of either low oxygen tension or in the presence of high levels of specific reductases. The biochemical basis for selectivity relies on the ability of oxygen to reverse the activation process and the presence of elevated reductase levels in some tumour types. Key criteria for an ideal bioreductive drug should include poor activity against aerobic cells, activation over a broad range of oxygen tensions and, penetration through the aerobic fraction of cells. In addition, the active drug should be capable of killing non-proliferating cells. Numerous compounds are currently at various stages of drug development but Mitomycin C, which is generally considered to be the prototype bioreductive drug, is the only one in clinical use today. Of the drugs currently being evaluated clinically, tirapazamine has definite clinical activity against a variety of solid tumours when used in combination with cisplatin. Other drugs, such as EO9 and various nitroimidazoles, have not been impressive in the clinic and further development is required to improve properties such as drug delivery in the case of indoloquinones. A novel approach to exploiting tumour hypoxia is the development of a gene-directed enzyme prodrug therapy (GDEPT) strategy, where a gene encoding for a prodrug activating enzyme has been placed under the control of a hypoxia responsive promoter sequence. It is generally recognised that bioreductive drugs must be directed towards patients whose tumours have hypoxic regions or have appropriate enzymological characteristics. In terms of identifying tumour hypoxia, there has been considerable progress in the development of nitroimidazole based hypoxia markers that can be detected either via non-invasive or invasive procedures. Another strategy currently undergoing preclinical evaluation is the use of agents that modulate tumour blood flow and synergistic effects have been reported between bioreductive drugs and photodynamic therapy or inhibitors of nitric oxide synthase for example. The development of clinically useful bioreductive drugs depends therefore on the expertise of scientists and clinicians with varying backgrounds. The purpose of this review is to describe and critically assess recent developments in this field, with particular emphasis being placed on drug development and strategies aimed at optimising bioreductive drug activity.

The C-neu mammary carcinoma in Oncomice; characterization and monitoring response to treatment with herceptin by magnetic resonance methods

To characterize spontaneously occurring c-neu/HER2 overexpressing tumours in oncomice and their response to herceptin by non-invasive magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI). Oncomice were monitored by localized 31P MRS during unperturbed growth and before and after treatment with 10 mg/kg herceptin (Hoffman La Roche) intraperitoneally for up to 21 days post-treatment. Vascular morphology and function was assessed by quantitation of tumour magnetic resonance (MR) relaxation rates R2* and R2 prior to and either during carbogen (95% O2/5% CO2) breathing or following administration of the blood-pool contrast agent NC100150 (Clariscan, Amersham Health). Immunohistochemistry showed strong membrane staining for HER2 protein overexpression. The 31P MRS showed only a significant (p<0.01) increase of phosphomonoester / total phosphate ratio over 21 days of growth. Herceptin increased the tumour volume doubling time compared to untreated tumours and significantly increased the phosphomonoester / beta-nucleoside triphosphate ratio 2 days after treatment (p=0.01). Tumours showed a highly heterogeneous yet significant (p<0.01) decrease or increase in R2* in response to carbogen or NC100150 respectively. The absence of a decline in tumour bioenergetics with growth, commonly seen in 31P MRS studies of transplanted rodent tumour models, coupled with the heterogeneous blood volume revealed by 1H MRI, suggest a metabolic and vascular phenotype similar to that found in human tumours.

Orthotopic models of cancer for preclinical drug evaluation: advantages and disadvantages

Considering the enormous effort that has taken place over the years to discover new chemotherapeutic drugs for treating the common cancers, the conventional murine and xenograft test systems used to test efficacy for drug development have identified only a limited number of useful agents that are active clinically at well tolerated doses. In recent years, considerable effort has been made to develop more clinically relevant models by the use of orthotopic transplantation of tumour material in rodents. It has been shown that it is now possible to transplant tumour material from a variety of tumour types into the appropriate anatomical site and often these tumours will metastasise in a similar manner and to similar locations as the same tumour type will in human cancer. As yet, although a body of literature has amassed on the technique itself and its implications for metastasis, there are relatively few laboratories using these test systems in drug development programmes. Nevertheless, given the expertise now being developed and some interesting observations being made on the role of the tumour site on response to therapeutic agents, it is likely that the use of orthotopic systems will strengthen our ability to select the most appropriate molecules for recommended use in clinical studies.

Metronomic scheduling: the future of chemotherapy?

Tumour endothelium is a new target for anticancer treatments. Proliferating endothelial cells from the tumour, even if qualitatively different from those of blood vessels in the normal tissue of origin, remain putatively normal and genetically stable cells. The results of recent experimental studies have suggested that frequent administration of certain cytotoxic agents at low doses (a tenth to a third of the maximum tolerated dose), known as 'metronomic' chemotherapy, increases the antiangiogenic activity of the drugs. The effects of these metronomic schedules of cytotoxic agents may be further enhanced by concurrent administration of novel, selective, treatments that inhibit, at a molecular level, the processes of tumour formation and growth eg angiogenesis, growth factor pathways, and other signal transduction cascades. The need to treat patients for long periods also supports the use of metronomic scheduling for chemotherapy, to minimise toxicity and to target both proliferating tumour cells and endothelial cells. This review describes the experimental studies involving metronomic schedules of chemotherapy, alone and in combination with angiogenesis inhibitors, and suggests a new therapeutic anticancer paradigm for controlling cancer by long-term therapy, based on the development of combinations of metronomic cytotoxic agents with individually tailored compounds designed to target specific molecules.

Tumor angiogenesis: past, present and the near future

The concept of treating solid tumors by inhibiting tumor angiogenesis was first articulated almost 30 years ago. For the next 10 years it attracted little scientific interest. This situation changed, relatively slowly, over the succeeding decade with the discovery of the first pro-angiogenic molecules such as basic fibroblast growth factor and vascular endothelial growth factor (VEGF), and the development of methods of successfully growing vascular endothelial cells in culture as well as in vivo assays of angiogenesis. However, the 1990s have witnessed a striking change in both attitude and interest in tumor angiogenesis and anti-angiogenic drug development, to the point where a remarkably diverse group of over 24 such drugs is currently undergoing evaluation in phase I, II or III clinical trials. In this review I will discuss the many reasons for this. These features, together with other recent discoveries have created intense interest in initiating and expanding anti-angiogenic drug discovery programs in both academia and industry, and the testing of such newly developed drugs, either alone, or in various combinations with conventional cytotoxic therapeutics. However, significant problems remain in the clinical application of angiogenesis inhibitors such as the need for surrogate markers to monitor the effects of such drugs when they do not cause tumor regressions, and the design of clinical trials. Also of concern is that the expected need to use anti-angiogenic drugs chronically will lead to delayed toxic side effects in humans, which do not appear in rodents, especially in short-term studies.

Intravital fluorescence videomicroscopy to study tumor angiogenesis and microcirculation

Angiogenesis and microcirculation play a central role in growth and metastasis of human neoplasms, and, thus, represent a major target for novel treatment strategies. Mechanistic analysis of processes involved in tumor vascularization, however, requires sophisticated in vivo experimental models and techniques. Intravital microscopy allows direct assessment of tumor angiogenesis, microcirculation and overall perfusion. Its application to the study of tumor-induced neovascularization further provides information on molecular transport and delivery, intra- and extravascular cell-to-cell and cell-to-matrix interaction, as well as tumor oxygenation and metabolism. With the recent advances in the field of bioluminescence and fluorescent reporter genes, appropriate for in vivo imaging, the intravital fluorescent microscopic approach has to be considered a powerful tool to study microvascular, cellular and molecular mechanisms of tumor growth.

A novel combretastatin A-4 derivative, AC-7700, shows marked antitumor activity against advanced solid tumors and orthotopically transplanted tumors

AC-7700, a novel combretastatin A-4 derivative, suppresses the growth of solid tumors by inhibiting tumor perfusion. We evaluated the antitumor activity of AC-7700 on solid tumors in two experimental models, an advanced tumor model (murine colon 26 (c26) adenocarcinoma, colon 38 (c38) adenocarcinoma, MethA fibrosarcoma, Sarcoma 180 (S180), Lewis lung carcinoma (3LL), human LS180 adenocarcinoma) and an orthotopically transplanted tumor model (c26), compared with that of cisplatin (CDDP). The maximum tolerable dose (MTD) of CDDP suppressed early-stage c26 and c38 tumor growth when treatment was started after the tumor volume (TV) reached 0.2-0.5 cm3, but it showed reduced activity against the same tumors at an advanced growth stage when TV exceeded 2 cm3. At its MTD, AC-7700 was active against all tumors tested except 3LL in both early and advanced growth stages, reducing the tumor mass and having a curative effect in advanced c38 tumors. AC-7700 was also effective on orthotopically transplanted c26 tumors, showing a comparable activity to that on subcutaneous tumors. Unlike flavon acetic acid, which damages tumor vasculature by inducing endogenous tumor necrosis factor-alpha production, AC-7700 potently suppressed the growth of advanced c26 tumors in athymic as well as euthymic mice. These results suggest that AC-7700 is a novel antivascular agent that may have potent activity against advanced-stage cancer in the clinical setting.

Assessment of induced rat mammary tumour response to chemotherapy using the apparent diffusion coefficient of tissue water as determined by diffusion-weighted 1H-NMR spectroscopy in vivo

Chemosensitivity of N-methyl-N-nitrosourea-induced rat mammary tumours treated with 5-fluorouracil at a dose of 100 mg kg(-1) i.p. was assessed by using diffusion-weighted 1H-MRS to measure the average diffusion coefficient (ADC) of water in the tumour tissue. ADC measurements prior to any therapy correlated positively with necrotic fraction. Tumours with low initial ADC (< 0.95 x 10(9) m2 s(-1)) showed an increase in ADC 7 days after treatment, whereas tumours with a high initial ADC (> 1.2 x 10(9) m2 s(-1)) showed a decrease. All tumours decreased significantly in volume (P < 0.05) 2, 5 and 7 days after treatment. At day 7 post-treatment, tumours with a high pre-treatment ADC started to regrow. The initial ADC value, as well as changes after treatment predict tumour chemosensitivity, which could be clinically relevant.

31P-magnetic resonance spectroscopy and 2H-magnetic resonance imaging studies of a panel of early-generation transplanted murine tumour models

The objective of this study was first to determine whether three slowly growing early-generation murine transplantable tumours, the T40 fibrosarcoma, T115 mammary carcinoma and T237 lung carcinoma, exhibit patterns of energetics and blood flow during growth that are different from those of the faster growing RIF-1 fibrosarcoma. Serial measurements were made with 31P-magnetic resonance spectroscopy (MRS), relating to nutritive blood flow and 2H-magnetic resonance imaging (MRI), which is sensitive to both nutritive and large-vessel (non-nutritive) flow. All four tumour lines showed a decrease in betaNTP/Pi and pH with growth; however, each line showed a different pattern of blood flow that did not correlate with the decrease in energetics. Qualitative histological analysis strongly correlated with the 2H-MRI. Second, their response to 5 mg kg(-1) hydralazine i.v. was monitored by 31P-MRS. A marked decrease in betaNTP/Pi and pH was observed in both the RIF-1 fibrosarcoma and the third-generation T115 mammary carcinoma after hydralazine challenge. In contrast, the fourth generation T40 fibrosarcoma and T237 lung carcinoma showed no change in 31P-MRS parameters. However, a fifth-generation T237 cohort, which grew approximately three times faster than fourth-generation T237 cohorts, exhibited a significant deterioration in betaNTP/Pi and pH in response to hydralazine. These data are consistent with a decoupling between large-vessel and nutritive blood flow and indicate that early-generation transplants that have a slow growth rate and vascular tone are more appropriate models of human tumour vasculature than more rapidly growing, repeatedly transplanted tumours.

Characterization of angiogenesis and microcirculation of high-grade glioma: an intravital multifluorescence microscopic approach in the athymic nude mouse

The current study follows angiogenesis and microcirculatory changes associated with malignant glioma growth by means of an intravital fluorescence microscopic approach, which allows for the direct and continuous visualization of the glioma microvasculature and its quantitative analysis. Fluorescently labeled C6 rat glioma cells (5 x 10(5)) were implanted into dorsal skinfold chamber preparations of athymic nude mice. Glioma growth, vascularization, microhemodynamics, vascular permeability, and leukocyte-endothelial cell interactions were simultaneously followed over a 22-day observation period using intravital epiillumination microscopy and a multifluorescent labeling technique. Analysis of the process of glioma vascularization revealed three stages with distinct microvascular characteristics: avascular stage (days 0 to 6), lag of glioma growth but initial glioma-induced angiogenesis within the host tissue in peritumoral areas; early vascular stage (days 6 to 14), glioma cell proliferation associated with a spatially homogeneous development of a glioma microvasculature; and late vascular stage (days 14 to 22), exponential tumor growth and expansion (> 400 mm3) with high vascular densities in the peritumoral region and reduced vascularization (microvascular perfusion) in the glioma center. Within the center, the functional vessel length per area correlated inversely with glioma size (P < 0.01). In the peritumoral region, functional vessel length per area was independent of glioma size, indicating persistent, high angiogenic activity throughout the observation period. Thus, the microvasculature of mature gliomas revealed a microvascular zonal division with a progressive reduction of the functional vessel length per area within the tumor center. The perfusion failure of individual microvessels within the glioma center was partly compensated by an increase of diameters (P < 0.05), and thus by an increase of blood flow in these functional microvessels (P < 0.05) over time. Histologic analysis demonstrated both expanding and infiltrating growth patterns, as well as focal necroses on day 22. These are the first data from repeated in vivo analysis of glioma growth, vascularization, and microcirculation.

Noninvasive monitoring of carbogen-induced changes in tumor blood flow and oxygenation by functional magnetic resonance imaging

PURPOSE

The response of tumors to radiotherapy can be enhanced if carbogen (95% O2, 5% CO2) is breathed. The timing of carbogen administration is critical, and a noninvasive method of monitoring the response of individual tumors would have obvious utility. Functional gradient recalled echo (GRE) magnetic resonance imaging (MRI) techniques are sensitive to changes in the concentrations of deoxyhemoglobin, which, thus, acts as an endogenous contrast agent for oxygenation status and blood flow.

METHODS AND MATERIALS

Subcutaneous GH3 prolactinomas in three rats were imaged at 4.7 Tesla with a GRE 1H sequence [echo time (TE) = 20 ms, repetition time (TR) = 80 ms, flip angle = 45 degrees, 1 mm slice, 256 phase encode steps, 4 cm field of view, in-plane resolution 0.08 x 0.08 mm, acquisition time = 4 min]. The rats breathed air or carbogen for four periods of 20 min; three control rats breathed only air.

RESULTS

Carbogen breathing caused increases of up to 100% in the GRE image intensity of the tumors. Reversion of air breathing caused the image intensity to fall; essentially the same response was observed with the second cycle of carbogen and air breathing. Control rat tumors showed no significant change.

CONCLUSIONS

The response of tumors to carbogen can be monitored noninvasively by GRE MRI. In principle, this could be due to an increase in oxygen content of the blood, a decrease in tumor cell oxygen consumption, or an increase in tumor blood flow. The very large changes in signal intensity suggest that a blood flow increase is the most probable explanation. If this technique can be successfully applied in man, it should be possible to optimize carbogen treatment for individual radiotherapy patients, and perhaps also to enhance tumor uptake of chemotherapeutic agents.

Effect of vasoactive drugs on tumour blood flow as determined by 2H nuclear magnetic resonance spectroscopy

A selective reduction in tumour blood flow (TBF) could enhance the effects of hyperthermia treatment and of drugs toxic to hypoxic cells. Vasodilator-induced changes in TBF were monitored in transplanted rat fibrosarcomas by non-invasively measuring the uptake of D2O using 2H nuclear magnetic resonance spectroscopy. Hydralazine (1 or 5 mg kg-1) caused a large (45%) reduction in mean arterial blood pressure (MABP) and a 40-60% reduction in TBF. Low-dose hydralazine (0.1 mg kg-1) caused a 20% reduction in MABP but no significant change in TBF. The doses of prazosin (1 mg kg-1) and calcitonin-gene related peptide (CGRP, 1 nmol kg-1) which caused a 20% reduction in MABP led to a 50-60% reduction in TBF. These results demonstrate the advantage of prazosin and CGRP over hydralazine for the reduction of TBF despite a small hypotensive effect. CGRP may be the most suitable of these agents for clinical use because of its short physiological half-life.

Hydralazine at thermoradiotherapy: tumor size and blood flow effects

PURPOSE

This study was aimed to assess the dependence on tumor size and blood flow of the efficacy of a vasoactive drug hydralazine with thermoradiotherapy.

METHODS AND MATERIALS

Experiments were performed on mice bearing SCC-VII tumors with volumes of about 85 and 340 mm3 (7-8 or 11-12 days after transplantation, respectively). Local hyperthermia (water bath, 43 degrees C, 0.5 h) was started 3 h after irradiation of tumors. Hydralazine (2.5 mg/kg, IP) was given 0.5 h before heating. Tumor blood flow was evaluated by laser Doppler flowmetry before, during and up to 2 days after the treatments.

RESULTS

It was shown that hydralazine and hyperthermia, even in combination with each other, had very weak anti-tumor effect, especially for 85 mm tumors. The agents also insignificantly enhanced the efficacy of radiotherapy excluding the case of polyradiomodification for 340 mm3 tumors when a dose modifying factor of about 2.0 was achieved. Thermometry showed only a small improvement by HDZ in heating patterns of tumors of both sizes. Meanwhile, the therapeutic efficacy of hydralazine and heat was correlated with the changes in tumor blood flow, first of all with the delayed effects. The radiomodifiers induced only minor and transient suppression of perfusion in the smaller tumors, and more markedly and for longer time decreased blood flow in the larger tumors. In the latter case, the inhibiting effect of the drug plus hyperthermia remained for at least 48 h after the treatment.

CONCLUSION

(a) The combined use of hydralazine and heat seems to be advisable only at radiotherapy of rather large advanced tumors; (b) the efficacy of such radiomodification is correlated with prolonged inhibition of tumor blood flow by these agents; and (c) hydralazine and hyperthermia are likely to kill selectively both acutely and chronically hypoxic radioresistant cancer cells.

Modification of metabolism of transplantable and spontaneous murine tumors by the nitric oxide synthase inhibitor, nitro-L-arginine

PURPOSE

To determine the effects of the nitric oxide synthase inhibitor, nitro-L-arginine on energy metabolism in transplantable and spontaneous murine tumors.

METHODS AND MATERIALS

The responses of the transplantable murine tumor SCCVII/Ha and a range of spontaneously arising murine mammary adenocarcinomas to 10 mg/kg IV nitro-L-arginine were examined using in vivo 31P magnetic resonance spectroscopy (MRS). The influence of Hypnorm/Hypnovel anesthesia on the response to nitro-L-arginine was also determined in the SCCVII/Ha tumors. Data were expressed as changes in the inorganic phosphate peak area relative to the sum of all peak areas from the 31P MR spectrum, or Pi/total.

RESULTS

Nitro-L-arginine at 10 mg/kg IV increased Pi/total 2-3-fold in the SCCVII/Ha tumors for at least 2 h after administration, in both anesthetized and nonanesthetized mice, consistent with increased tumor hypoxia. Similar increases in Pi/total were observed after 10 mg/kg IV nitro-L-arginine in 13 spontaneous murine tumors from three different mouse strains, where anesthetic was used.

CONCLUSION

The results indicate that tumor metabolism may be modified by an inhibitor of nitric oxide synthesis, that this modification occurs in both transplantable and spontaneous murine tumors and is not affected by anesthetic.

From hydralazine to CGRP to man?

Attempts to selectively reduce tumour blood flow have, in the past, concentrated on the use of hydralazine. However, although this vasodilator can be highly effective in experimental animals, it is only at such high concentration as to result in a severe and clinically unacceptable reduction in systemic blood pressure. At clinically acceptable levels, the drug appears to produce a small increase in tumour blood flow. We have used the techniques of magnetic resonance spectroscopy as indicators of metabolism and blood flow in a search for vasoactive drugs that would produce an effective reduction in tumour blood flow without causing severe hypotension or other serious side effects. Single injections of either prazosin or CGRP are shown to be substantially more effective than hydralazine in causing a reduction in tumour blood flow without massive reduction in blood pressure. Even more effective was CGRP given by continuous infusion. In this case a three-fold reduction in tumour blood flow could be obtained with a reduction of only 15-20% in systemic blood pressure. All these studies, however, have been made with transplanted animal tumours. Using high-dose hydralazine and primary tumours that were either radiation or chemically induced, we obtained a success rate of only about a 35% in causing selective reduction in blood flow. In contrast, in a transplanted tumour line derived from one of the non-responding radiation-induced primary lesions, the success rate was about 95%, consistent with the majority of animal studies using transplanted tumours.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo 31P MRS of experimental tumours

More than 50% of cancers fail to respond to any individual treatment and tumour follow-up after treatment plays a major role in routine therapy planning and pharmacological research. Today, MRS is the only technological approach providing non-invasive access to tumour biochemistry. Ten years ago, expectations were raised concerning 31P MRS as an exciting and promising technical approach to the study of tumours. However the expectations have not always come to fruition. How close are we now to seeing routine 31P NMR in clinical oncology? This review of the 127 published papers shows spectroscopy results in more than 150 experimental animal tumour models. These tumour/host/treatment systems provide us with a useful basis to evaluate the current state of the art, summarize the basic knowledge presently available, determine the key points underlying the present disappointment of some clinical oncologists and stimulate new basic research. The information collected concerns the discussion of the reliability of experimental models in oncology, the technical improvement of magnetic resonance technology and the monitoring of bioenergetic status, pH regulation and phospholipid metabolism in treated and untreated tumours. Recent advances (two-thirds of the papers have been published in the last 5 years) seem to provide more optimistic perspectives than those generally accepted a few years ago, in the depressing period following early pioneering work.

Growth and magnetic resonance characteristics of human squamous cell carcinoma xenografts implanted with cells suspended in Matrigel

Growth and magnetic resonance characteristics of a human squamous cell carcinoma SQ20B were studied in vivo as xenografts in nu/nu nude mice. Tumor cells injected subcutaneously in the flank using either Matrigel (MTG, an extract of basement membrane proteins) or growth medium (GM) as a vehicle were compared. Much higher tumor growth rates and cell density were observed with Matrigel than with GM implantation. Histology also showed that MTG implanted cells grew as vascularized solid tumors compared to GM tumors which formed cysts. As a result of increased cell density with the improved method, tumors as small as 0.3 cm3 provide high S/N magnetic resonance spectra which yield smaller standard deviations with fewer experiments.

Vinca alkaloids: anti-vascular effects in a murine tumour

We have investigated the blood flow modifying effects of the vinca alkaloids, vincristine and vinblastine in the murine carcinoma CaNT. Vinblastine at doses of 7.5 or 10 mg/kg induced profound and chronic reductions in tumour blood flow as measured by 86RbCl extraction. Following the maximum tolerated dose of 10 mg/kg, blood flow was reduced to 10% of pretreatment values after 2 h and remained below 20% of pretreatment values 24 h after drug administration. These findings are consistent with the early induction of necrosis by vinblastine and suggest that vascular-mediated cell death may account for a large part of the 11 day growth delay induced by this drug dose. In contrast to the large reductions in tumour blood flow, in skin, kidney, liver and muscle, blood flow reductions did not, at any time examined, exceed 40%. In all the normal tissues studied, blood flow had fully recovered by 6 h after vinblastine administration. Similar results, albeit less pronounced, have been obtained with vincristine at the maximum tolerated dose of 3 mg/kg. The results clearly show that both vinblastine and vincristine can induce, with some selectivity, a dramatic and prolonged reduction in tumour blood flow and that this may contribute to the anti-tumour effects against the CaNT tumour.

The influence of hydralazine on the vasculature, blood perfusion and chemosensitivity of MAC tumours

We have studied the influence of the peripheral vasodilator hydralazine (HDZ) on the vasculature and blood perfusion of two members of a series of subcutaneous murine adenocarcinomata of the colon (MAC tumours), and the influence of HDZ on the efficacy and/or toxicity of TCNU and melphalan. The fluorescent DNA stain Hoechst 33342, showed that HDZ caused a shutdown of tumour vasculature, related in magnitude to both dose and tumour differentiation state; 10 mg kg-1 caused an 80% vascular shutdown of well differentiated MAC 26 tumours, but only a 50% shutdown of the poorly differentiated MAC 15A tumours. 2.5 mg kg-1 was ineffective. The blood perfusion marker 99mTc-HMPAO showed that the normal perfusion of MAC tumours was consistently markedly less than that of lung, liver or kidneys (4-5% of lung perfusion). HDZ (10 mg kg-1) decreased MAC 26 perfusion by 63%, and that of MAC 15A by 20%. Again, 2.5 mg kg-1) was ineffective. Use of in vivo to in vitro clonogenic assays showed that HDZ (10 mg kg-1) potentiated the efficacy of melphalan (1-10 mg kg-1 i.p.) by a factor of 2.1, and increased the efficacy of TCNU (1-10 mg kg-1 i.v., factor = 1.7) when given 10 or 15 min respectively after dosing. However, the addition of HDZ increased the acute bone marrow toxicity of melphalan, but not that of TCNU. The clinical relevance of these results is discussed.

The response of spontaneous and transplantable murine tumors to vasoactive agents measured by 31P magnetic resonance spectroscopy

31P magnetic resonance spectroscopy has been used to compare the effects of the vasoactive agents hydralazine and flunarizine on the oxygenation of the transplantable tumors, SCCVII/Ha and 16C, and a range of spontaneous mammary tumors arising in the breeding stock in the Genetics Division at the Radiobiology Unit. The vasodilator hydralazine, previously shown to increase the radiobiological hypoxic fraction of transplantable murine tumors, increased inorganic phosphate to total phosphate (Pi/total) in SCCVII/Ha and 16C tumors. However, only two spontaneous tumors responded to this agent (2/12). The calcium antagonist flunarizine, which sensitizes the SCCVII tumor to X rays, consistent with a reduction in hypoxic fraction, reduced Pi/total in this and the 16C tumor. Further, most spontaneous tumors tested (8/10) responded to this agent, as measured by a reduction in Pi/total. These results point to fundamental differences between transplantable and spontaneously arising tumors in mice in their response to vasoactive agents.