Tumor physiologic response to combretastatin A4 phosphate assessed by MRI

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

Prognostic Value of Fluorine-19 MRI Oximetry Monitoring in cancer

Hypoxia is a key prognostic indicator in most solid tumors, as it is correlated to tumor angiogenesis, metastasis, recurrence, and response to therapy. Accurate measurement and mapping of tumor oxygenation profile and changes upon intervention could facilitate disease progression assessment and assist in treatment planning. Currently, no gold standard exists for non-invasive spatiotemporal measurement of hypoxia. Magnetic resonance imaging (MRI) represents an attractive option as it is a clinically available and non-ionizing imaging modality. Specifically, perfluorocarbon (PFC) beacons can be externally introduced into the tumor tissue and the linear dependence of their spin-lattice relaxation rate (R1) on the local partial pressure of oxygen (pO2) exploited for real-time tissue oxygenation monitoring in vivo. In this review, we will focus on early studies and recent developments of fluorine-19 MRI and spectroscopy (MRS) for evaluation of tumor oximetry and response to therapy.

Non-Invasive Evaluation of Acute Effects of Tubulin Binding Agents: A Review of Imaging Vascular Disruption in Tumors

Tumor vasculature proliferates rapidly, generally lacks pericyte coverage, and is uniquely fragile making it an attractive therapeutic target. A subset of small-molecule tubulin binding agents cause disaggregation of the endothelial cytoskeleton leading to enhanced vascular permeability generating increased interstitial pressure. The resulting vascular collapse and ischemia cause downstream hypoxia, ultimately leading to cell death and necrosis. Thus, local damage generates massive amplification and tumor destruction. The tumor vasculature is readily accessed and potentially a common target irrespective of disease site in the body. Development of a therapeutic approach and particularly next generation agents benefits from effective non-invasive assays. Imaging technologies offer varying degrees of sophistication and ease of implementation. This review considers technological strengths and weaknesses with examples from our own laboratory. Methods reveal vascular extent and patency, as well as insights into tissue viability, proliferation and necrosis. Spatiotemporal resolution ranges from cellular microscopy to single slice tomography and full three-dimensional views of whole tumors and measurements can be sufficiently rapid to reveal acute changes or long-term outcomes. Since imaging is non-invasive, each tumor may serve as its own control making investigations particularly efficient and rigorous. The concept of tumor vascular disruption was proposed over 30 years ago and it remains an active area of research.

Targeted microbubbles carrying lipid-oil-nanodroplets for ultrasound-triggered delivery of the hydrophobic drug, combretastatin A4

The hydrophobicity of a drug can be a major challenge in its development and prevents the clinical translation of highly potent anti-cancer agents. We have used a lipid-based nanoemulsion termed Lipid-Oil-Nanodroplets (LONDs) for the encapsulation and in vivo delivery of the poorly bioavailable combretastatin A4 (CA4). Drug delivery with CA4 LONDs was assessed in a xenograft model of colorectal cancer. LC-MS/MS analysis revealed that CA4 LONDs, administered at a drug dose four times lower than drug control, achieved equivalent concentrations of CA4 intratumorally. We then attached CA4 LONDs to microbubbles (MBs) and targeted this construct to VEGFR2. A reduction in tumor perfusion was observed in CA4 LONDs-MBs treated tumors. A combination study with irinotecan demonstrated a greater reduction in tumor growth and perfusion (P = 0.01) compared to irinotecan alone. This study suggests that LONDs, either alone or attached to targeted MBs, have the potential to significantly enhance tumor-specific hydrophobic drug delivery.

In vivo noninvasive preclinical tumor hypoxia imaging methods: a review

Tumors exhibit areas of decreased oxygenation due to malformed blood vessels. This low oxygen concentration decreases the effectiveness of radiation therapy, and the resulting poor perfusion can prevent drugs from reaching areas of the tumor. Tumor hypoxia is associated with poorer prognosis and disease progression, and is therefore of interest to preclinical researchers. Although there are multiple different ways to measure tumor hypoxia and related factors, there is no standard for quantifying spatial and temporal tumor hypoxia distributions in preclinical research or in the clinic. This review compares imaging methods utilized for the purpose of assessing spatio-temporal patterns of hypoxia in the preclinical setting. Imaging methods provide varying levels of spatial and temporal resolution regarding different aspects of hypoxia, and with varying advantages and disadvantages. The choice of modality requires consideration of the specific experimental model, the nature of the required characterization and the availability of complementary modalities as well as immunohistochemistry.

Oxygen-sensitive MRI assessment of tumor response to hypoxic gas breathing challenge

Oxygen-sensitive MRI has been extensively used to investigate tumor oxygenation based on the response (R₂ * and/or R₁ ) to a gas breathing challenge. Most studies have reported response to hyperoxic gas indicating potential biomarkers of hypoxia. Few studies have examined hypoxic gas breathing and we have now evaluated acute dynamic changes in rat breast tumors. Rats bearing syngeneic subcutaneous (n = 15) or orthotopic (n = 7) 13762NF breast tumors were exposed to a 16% O₂ gas breathing challenge and monitored using blood oxygen level dependent (BOLD) R₂ * and tissue oxygen level dependent (TOLD) T₁ -weighted measurements at 4.7 T. As a control, we used a traditional hyperoxic gas breathing challenge with 100% O₂ on a subset of the subcutaneous tumor bearing rats (n = 6). Tumor subregions identified as responsive on the basis of R₂ * dynamics coincided with the viable tumor area as judged by subsequent H&E staining. As expected, R₂ * decreased and T₁ -weighted signal increased in response to 100% O₂ breathing challenge. Meanwhile, 16% O₂ breathing elicited an increase in R₂ *, but divergent response (increase or decrease) in T₁ -weighted signal. The T₁ -weighted signal increase may signify a dominating BOLD effect triggered by 16% O₂ in the relatively more hypoxic tumors, whereby the influence of increased paramagnetic deoxyhemoglobin outweighs decreased pO₂ . The results emphasize the importance of combined BOLD and TOLD measurements for the correct interpretation of tumor oxygenation properties.

Structure Guided Design, Synthesis, and Biological Evaluation of Novel Benzosuberene Analogues as Inhibitors of Tubulin Polymerization

A promising design paradigm for small-molecule inhibitors of tubulin polymerization that bind to the colchicine site draws structural inspiration from the natural products colchicine and combretastatin A-4 (CA4). Our previous studies with benzocycloalkenyl and heteroaromatic ring systems yielded promising inhibitors with dihydronaphthalene and benzosuberene analogues featuring phenolic (KGP03 and KGP18) and aniline (KGP05 and KGP156) congeners emerging as lead agents. These molecules demonstrated dual mechanism of action, functioning both as potent vascular disrupting agents (VDAs) and as highly cytotoxic anticancer agents. A further series of analogues was designed to extend functional group diversity and investigate regioisomeric tolerance. Ten new molecules were effective inhibitors of tubulin polymerization (IC₅₀ < 5 μM) with seven of these exhibiting highly potent activity comparable to CA4, KGP18, and KGP03. For one of the most effective agents, dose-dependent vascular shutdown was demonstrated using dynamic bioluminescence imaging in a human prostate tumor xenograft growing in a rat.

Assessing Tumor Oxygenation for Predicting Outcome in Radiation Oncology: A Review of Studies Correlating Tumor Hypoxic Status and Outcome in the Preclinical and Clinical Settings

Tumor hypoxia is recognized as a limiting factor for the efficacy of radiotherapy, because it enhances tumor radioresistance. It is strongly suggested that assessing tumor oxygenation could help to predict the outcome of cancer patients undergoing radiation therapy. Strategies have also been developed to alleviate tumor hypoxia in order to radiosensitize tumors. In addition, oxygen mapping is critically needed for intensity modulated radiation therapy (IMRT), in which the most hypoxic regions require higher radiation doses and the most oxygenated regions require lower radiation doses. However, the assessment of tumor oxygenation is not yet included in day-to-day clinical practice. This is due to the lack of a method for the quantitative and non-invasive mapping of tumor oxygenation. To fully integrate tumor hypoxia parameters into effective improvements of the individually tailored radiation therapy protocols in cancer patients, methods allowing non-invasively repeated, safe, and robust mapping of changes in tissue oxygenation are required. In this review, non-invasive methods dedicated to assessing tumor oxygenation with the ultimate goal of predicting outcome in radiation oncology are presented, including positron emission tomography used with nitroimidazole tracers, magnetic resonance methods using endogenous contrasts (R₁ and R2*-based methods), and electron paramagnetic resonance oximetry; the goal is to highlight results of studies establishing correlations between tumor hypoxic status and patients’ outcome in the preclinical and clinical settings.

Combretastatin A-4 efficiently inhibits angiogenesis and induces neuronal apoptosis in zebrafish

Cis-stilbene combretastatin A-4 (CA-4) and a large group of its derivant compounds have been shown significant anti-angiogenesis activity. However the side effects even the toxicities of these chemicals were not evaluated adequately. The zebrafish model has become an important vertebrate model for evaluating drug effects. The testing of CA-4 on zebrafish is so far lacking and assessment of CA-4 on this model will provide with new insights of understanding the function of CA-4 on angiogenesis, the toxicities and side effects of CA-4. We discovered that 7-9 ng/ml CA-4 treatments resulted in developmental retardation and morphological malformation, and led to potent angiogenic defects in zebrafish embryos. Next, we demonstrated that intraperitoneal injection of 5, 10 and 20 mg/kg CA-4 obviously inhibited vessel plexus formation in regenerated pectoral fins of adult zebrafish. Interestingly, we proved that CA-4 treatment induced significant cell apoptosis in central nervous system of zebrafish embryos and adults. Furthermore, it was demonstrated that the neuronal apoptosis induced by CA-4 treatment was alleviated in p53 mutants. In addition, notch1a was up-regulated in CA-4 treated embryos, and inhibition of Notch signaling by DAPT partially rescued the apoptosis in zebrafish central nervous system caused by CA-4.

Readily accessible multifunctional fluorous emulsions

Mixtures of perfluorocarbon and water containing functionalized polymer surfactants and fluorous-tagged small molecules yield multifunctional emulsions with defined functionality on the inside and outside of the droplets.

Strategies for the facile fabrication of nanoscale materials and devices represent an increasingly important challenge for chemists. Here, we report a simple, one-pot procedure for the formation of perfluorocarbon emulsions with defined functionalization. The fluorous core allows for small molecules containing a fluorous tail to be stabilized inside the emulsions. The emulsions can be formed using a variety of hydrophilic polymers resulting in an array of sizes (90 nm to >1 micron) and surface charges (–95 mV to 65 mV) of fluid particles. The surface of the emulsions can be further functionalized, covalently or non-covalently, through in situ or post-emulsion modification. The total preparation time is 30 minutes or less from commercially available reagents without specialized equipment. We envision these emulsions to be applicable to both biological and materials systems.

Hypoxia as a Biomarker and for Personalized Radiation Oncology

Tumor hypoxia is a clinically relevant cause of radiation resistance. Direct measurements of tumor oxygenation have been performed predominantly with the Eppendorf histograph and these have defined the reduced prognosis after radiotherapy in poorly oxygenated tumors, especially head-and-neck cancer, cervix cancer and sarcoma. Exogenous markers have been used for immunohistochemical detection of hypoxic tumor areas (pimonidazole) or for positron-emission tomography (PET) imaging (misonidazole). Overexpression of hypoxia-related proteins such as hypoxia-inducible factor-1α (HIF-1α) has also been linked to poor prognosis after radiotherapy and such proteins are considered as potential endogenous hypoxia markers.

Combretastatins: more than just vascular targeting agents?

Several prodrugs of the naturally occurring combretastatins have undergone extensive clinical evaluation as vascular targeting agents (VTAs). Their increased selectivity toward endothelial cells together with their innate ability to rapidly induce vascular shutdown and inhibit tumor growth at doses up to 10-fold less than the maximum tolerated dose led to the clinical evaluation of combretastatins as VTAs. Tubulin is well established as the molecular target of the combretastatins and the vast majority of its synthetic derivatives. Furthermore, tubulin is a highly validated molecular target of many direct anticancer agents routinely used as front-line chemotherapeutics. The unique vascular targeting properties of the combretastatins have somewhat overshadowed their development as direct anticancer agents and the delineation of the various cell death pathways and anticancer properties associated with such chemotherapeutics. Moreover, the ongoing clinical trial of OXi4503 (combretastatin-A1 diphosphate) together with preliminary preclinical evaluation for the treatment of refractory acute myelogenous leukemia has successfully highlighted both the indirect and direct anticancer properties of combretastatins. In this review, we discuss the development of the combretastatins from nature to the clinic. The various mechanisms underlying combretastatin-induced cell cycle arrest, mitotic catastrophe, cell death, and survival are also reviewed in an attempt to further enhance the clinical prospects of this unique class of VTAs.

Metronomic vinorelbine: Anti-angiogenic activity in vitro in normoxic and severe hypoxic conditions, and severe hypoxia-induced resistance to its anti-proliferative effect with reversal by Akt inhibition

Metronomic chemotherapy is the protracted, dense administration of low sub-toxic doses of chemotherapy, to inhibit tumor angiogenesis. Vinorelbine is an orally bioavailable vinca alkaloid shown to be useable for metronomic administration. In clinical trials, metronomic vinorelbine has been demonstrated to generate sustainable antitumor efficacy at low nanomolar (nM) concentrations with negligible toxicity. We sought to determine whether the clinically relevant metronomic concentration of vinorelbine is anti-angiogenic in vitro and whether hypoxia, often induced by anti-angiogenic therapy, modifies its effectiveness. We found that the metronomic concentration of 10 nM vinorelbine inhibited human umbilical vein endothelial cell (HUVEC) proliferation, migration, tube formation and sprouting. Severe hypoxia, did not affect the inhibitory effect of metronomic vinorelbine on migration, tube formation and sprouting. However, severe hypoxia reduced its anti-proliferative effect by decreasing its ability to induce G2/M block as it shifted the cell population to the G1 phase and decreased the fraction of the cells in the DNA synthesis S phase. Furthermore, the pro-apoptotic effects of 10 nM vinorelbine were also decreased. Metronomic vinorelbine decreased the Bcl-2/Bax ratio in normoxia whereas the ratio was reduced in severe hypoxia but unaltered by vinorelbine treatment. Akt signals to an anti-apoptotic pathway and we demonstrated that the Akt inhibitor V reversed the protective effect of severe hypoxia. Thus, we provide evidence for the anti-angiogenic basis of metronomic vinorelbine and we show that severe hypoxia mediates resistance to its anti-proliferative effect on endothelial cells. Akt warrants further investigation as a potential target to circumvent this hypoxic resistance.

Modulation of the tumor vasculature and oxygenation to improve therapy

The tumor microenvironment is increasingly recognized as a major factor influencing the success of therapeutic treatments and has become a key focus for cancer research. The progressive growth of a tumor results in an inability of normal tissue blood vessels to oxygenate and provide sufficient nutritional support to tumor cells. As a consequence the expanding neoplastic cell population initiates its own vascular network which is both structurally and functionally abnormal. This aberrant vasculature impacts all aspects of the tumor microenvironment including the cells, extracellular matrix, and extracellular molecules which together are essential for the initiation, progression and spread of tumor cells. The physical conditions that arise are imposing and manifold, and include elevated interstitial pressure, localized extracellular acidity, and regions of oxygen and nutrient deprivation. No less important are the functional consequences experienced by the tumor cells residing in such environments: adaptation to hypoxia, cell quiescence, modulation of transporters and critical signaling molecules, immune escape, and enhanced metastatic potential. Together these factors lead to therapeutic barriers that create a significant hindrance to the control of cancers by conventional anticancer therapies. However, the aberrant nature of the tumor microenvironments also offers unique therapeutic opportunities. Particularly interventions that seek to improve tumor physiology and alleviate tumor hypoxia will selectively impair the neoplastic cell populations residing in these environments. Ultimately, by combining such therapeutic strategies with conventional anticancer treatments it may be possible to bring cancer growth, invasion, and metastasis to a halt.

Monitoring Combretastatin A4-induced tumor hypoxia and hemodynamic changes using endogenous MR contrast and DCE-MRI

PURPOSE

To benchmark MOBILE (Mapping of Oxygen By Imaging Lipid relaxation Enhancement), a recent noninvasive MR method of mapping changes in tumor hypoxia, electron paramagnetic resonance (EPR) oximetry, and dynamic contrast-enhanced MRI (DCE-MRI) as biomarkers of changes in tumor hemodynamics induced by the antivascular agent combretastatin A4 (CA4).

METHODS

NT2 and MDA-MB-231 mammary tumors were implanted subcutaneously in FVB/N and nude NMRI mice. Mice received 100 mg/kg of CA4 intraperitoneally 3 hr before imaging. The MOBILE sequence (assessing R1 of lipids) and the DCE sequence (assessing K(trans) hemodynamic parameter), were assessed on different cohorts. pO2 changes were confirmed on matching tumors using EPR oximetry consecutive to the MOBILE sequence. Changes in tumor vasculature were assessed using immunohistology consecutive to DCE-MRI studies.

RESULTS

Administration of CA4 induced a significant decrease in lipids R1 (P = 0.0273) on pooled tumor models and a reduction in tumor pO2 measured by EPR oximetry. DCE-MRI also exhibited a significant drop of K(trans) (P < 0.01) that was confirmed by immunohistology.

CONCLUSION

MOBILE was identified as a marker to follow a decrease in oxygenation induced by CA4. However, DCE-MRI showed a higher dynamic range to follow changes in tumor hemodynamics induced by CA4.

Magnetic resonance imaging of the tumor microenvironment in radiotherapy: perfusion, hypoxia, and metabolism

The tumor microenvironment is characterized by hypoxia, low pH, and high interstitial fluid pressure. Hypoxic regions in tumors with low partial pressure of oxygen (pO2) levels can result in resistance to radiotherapy, thus causing local failure. Therefore, it would be desirable to noninvasively measure pO2 levels in the tumor before, during, and after treatment to better customize therapy and follow treatment response. Several techniques used in preclinical and clinical studies to obtain the pO2 status of tissue, such as dynamic contrast-enhanced magnetic resonance imaging, blood oxygen level-dependent imaging, and electron paramagnetic resonance imaging, are reviewed. Furthermore, the ability to hyperpolarize specific metabolic substrates that are isotopically labeled with (13)C coupled with magnetic resonance spectroscopy enables noninvasive imaging of tissue metabolism, such as glycolysis.

Dynamic bioluminescence and fluorescence imaging of the effects of the antivascular agent Combretastatin-A4P (CA4P) on brain tumor xenografts

Combretastatin A-4 (CA4) is a natural product isolated from Combretum caffrum that inhibits tubulin polymerization by binding to the colchicine-binding site. A corresponding water soluble pro-drug (referred to as CA4P), has undergone extensive clinical trials and has been evaluated in pre-clinical studies using multiple modalities. We previously reported a novel assay based on dynamic bioluminescent imaging to assess tumor vascular disruption and now present its application to assessing multiple tumors simultaneously. The current study evaluated the vascular-disrupting activity of CA4P on subcutaneous 9L rat brain tumor xenografts in mice using dynamic bioluminescence imaging. A single dose of CA4P (120 mg/kg, intraperitoneally) induced rapid, temporary tumor vascular shutdown revealed by a rapid and reproducible decrease of light emission from luciferase-expressing 9L tumors following administration of luciferin as a substrate. A time-dependent reduction of tumor perfusion after CA4P treatment was confirmed by immunohistological assessment of the perfusion marker Hoechst 33342 and the tumor vasculature marker CD31. The vasculature showed distinct recovery within 24 h post therapy. Multiple tumors behaved similarly, although a size dependent vascular inhibition was observed. In conclusion, CA4P caused rapid, temporary tumor vascular shutdown and led to reduction of tumor perfusion in rat brain tumor xenografts and the multiple tumor approach should lead to more efficient studies requiring fewer animals and greater consistency.

Induction of hypoxia by vascular disrupting agents and the significance for their combination with radiation therapy

PURPOSE

This pre-clinical study was designed to investigate the effect of various vascular disrupting agents (VDAs) that have undergone or are in clinical evaluation, had on the oxygenation status of tumours and what effects that could have on the combination with radiation.

MATERIAL AND METHODS

The tumour model was a C3H mammary carcinoma grown in the right rear foot of female CDF1 mice and treated when at 200 mm(3) in size. The VDAs were the flavenoid compounds flavone acetic acid (FAA) and its more recent derivative 5,6-dimethylxanthenone-4-acetic acid (DMXAA), and the leading tubulin binding agent combretastatin A-4 phosphate (CA4P) and the A-1 analogue OXi4503. Oxygenation status was estimated using the Eppendorf oxygen electrode three hours after drug injection. Radiation response was determined following single or fractionated (10 fractions in 12 days) irradiations with a 240 kV x-ray machine using either a tumour re-growth or local tumour control assay.

RESULTS

All VDAs significantly reduced the oxygenation status of the tumours. They also influenced radiation response, but the affect was time and sequence dependent using single radiation schedules; an enhanced effect when the VDAs were injected at the same time or after irradiating, but no or even a reduced effect when given prior to irradiation. Only OXi4503 showed an increased response when given before the radiation. CA4P and OXi4503 also enhanced a fractionated radiation treatment if the drugs were administered after fractions 5 and 10.

CONCLUSIONS

VDAs clearly induced tumour hypoxia. This had the potential to decrease the efficacy of radiation. However, if the appropriate timing and scheduling were used an enhanced effect was observed using both single and fractionated radiation treatments.

Dynamic Contrast-Enhanced Magnetic Resonance Imaging (DCE-MRI) in Preclinical Studies of Antivascular Treatments

Antivascular treatments can either be antiangiogenic or targeting established tumour vasculature. These treatments affect the tumour microvasculature and microenvironment but may not change clinical measures like tumour volume and growth. In research on antivascular treatments, information on the tumour vasculature is therefore essential. Preclinical research is often used for optimization of antivascular drugs alone or in combined treatments. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is an in vivo imaging method providing vascular information, which has become an important tool in both preclinical and clinical research. This review discusses common DCE-MRI imaging protocols and analysis methods and provides an overview of preclinical research on antivascular treatments utilizing DCE-MRI.

Tubulin-destabilizing agent BPR0L075 induces vascular-disruption in human breast cancer mammary fat pad xenografts

BPR0L075, 6-methoxy-3-(3′,4′,5′-trimethoxy-benzoyl)-1H-indole, is a tubulin-binding agent that inhibits tubulin polymerization by binding to the colchicine-binding site. BPR0L075 has shown antimitotic and antiangiogenic activity in vitro. The current study evaluated the vascular-disrupting activity of BPR0L075 in human breast cancer mammary fat pad xenografts using dynamic bioluminescence imaging. A single dose of BPR0L075 (50 mg/kg, intraperitoneally (i.p.)) induced rapid, temporary tumor vascular shutdown (at 2, 4, and 6 hours); evidenced by rapid and reproducible decrease of light emission from luciferase-expressing orthotopic MCF7 and MDA-MB-231 breast tumors after administration of luciferin substrate. A time-dependent reduction of tumor perfusion after BPR0L075 treatment was confirmed by immunohistological staining of the perfusion marker Hoechst 33342 and tumor vasculature marker CD31. The vasculature showed distinct recovery within 24 hours post therapy. A single i.p. injection of 50 mg/kg of BPR0L075 initially produced plasma concentrations in the micromolar range within 6 hours, but subsequent drug distribution and elimination caused BPR0L075 plasma levels to drop rapidly into the nanomolar range within 24 h. Tests with human umbilical vein endothelial (HUVEC) cells and tumor cells in culture showed that BPR0L075 was cytotoxic to both tumor cells and proliferating endothelial cells, and disrupted pre-established vessels in vitro and ex vivo. In conclusion, BPR0L075 caused rapid, albeit, temporary tumor vascular shutdown and led to reduction of tumor perfusion in orthotopic human breast cancer xenografts, suggesting that this antimitotic agent may be useful as a vascular-disrupting cancer therapy.

Treatment with the vascular disrupting agent combretastatin is associated with impaired AQP2 trafficking and increased urine output

Combretastatin A-4 disodium phosphate (CA4P) is a vascular disrupting agent known to mediate its effects primarily on tumor blood vessels. CA4P has previously been shown to induce a significant increase in mean arterial blood pressure and in hemoglobin concentration in mice. In the present study, we examined whether this is associated with a general leakage of water into certain tissues or with changes in renal water handling. Munich-Wistar rats received either CA4P (30 mg/kg body wt) or saline intraperitoneally as a bolus injection. One hour later, hemoglobin concentration and mean blood pressure increased significantly. MRI showed no significant changes in tissue water content following CA4P administration. However, urine output and salt excretion increased 1 h after CA4P treatment, without changes in urinary and medullary osmolality. Aquaporin 2 (AQP2) mRNA levels in kidney inner medulla did not change 1 h after CA4P treatment, but semiquantitative confocal laser-scanning microscopy analysis demonstrated a decrease in phosphorylated AQP2 (pS256-AQP2) apical distribution within the collecting ducts of CA4P-treated rats compared with the characteristic apical localization in control rats. Furthermore, we demonstrated that CA4P cause disruption of microtubules and a weaker apical labeling of pS256-AQP2 in collecting duct principal cells within 1 h. In conclusion, our data indicate that water escapes from the vascular system after CA4P treatment, and it may take place primarily through a renal mechanism. The CA4P-mediated increase in urine output seems to be a local effect in the collecting ducts due to reduced AQP2 trafficking to the apical plasma membrane.

Hexafluorobenzene in comparison with perfluoro-15-crown-5-ether for repeated monitoring of oxygenation using 19F MRI in a mouse model

Hexafluorobenzene (HFB) and perfluoro-15-crown-5-ether (15C5) were compared as fluorine reporter probes of tissue oxygenation using (19)F MRI for dynamic assessment of muscle oxygenation, with special focus on muscle tissue toxicity of the probes, and consecutive alteration of animal behavior. The latter were also compared in terms of sensitivity to changes in oxygenation as well as of signal-to-noise ratio for accurate pO(2) measurements. For that purpose, mouse muscles were imaged at 11.7 T, at 2- and 36-h after intramuscular injection of HFB or 15C5. Histological analysis of the muscle tissue revealed a lack of toxicity for 15C5 from 2 up to 36-h postinjection, whereas HFB induced tissue necrosis, blood clots and thrombosis as soon as 24-h postinjection. This muscle toxicity led to a limitation in mice mobility 24-h after injection of HFB as evidenced by behavioral testing (open-field, grip strength, and catwalk tests), which was not the case after 15C5 intramuscular injection. Finally, pO(2) measurements assessed 2-h postinjection showed consistent values with both probes, evidencing cross-validation of the (19)F MRI oximetry technique for acute measurements. However, the measurement at 36-h was hampered for HFB, which showed significant lower values of muscle pO(2), whereas 15C5 was able to reliably assess muscle pO(2) at 36-h postinjection.

Phase Ib trial of radiotherapy in combination with combretastatin-A4-phosphate in patients with non-small-cell lung cancer, prostate adenocarcinoma, and squamous cell carcinoma of the head and neck

BACKGROUND

The vascular disrupting agent combretastatin-A4-phosphate (CA4P) demonstrated antitumour activity in preclinical studies when combined with radiation.

METHODS

Patients with non-small-cell lung cancer (NSCLC), prostate adenocarcinoma, and squamous cell carcinoma of the head and neck (SCCHN) received 27 Gy in 6 fractions treating twice weekly over 3 weeks, 55 Gy in 20 fractions over 4 weeks, and 66 Gy in 33 fractions over 6 weeks respectively. CA4P was escalated from 50 mg/m2 to 63 mg/m2. CA4P exposure was further increased from one to three to six doses. Patients with SCCHN received cetuximab in addition.

RESULTS

Thirty-nine patients received 121 doses of CA4P. Dose-limiting toxic effects (DLTs) of reversible ataxia and oculomotor nerve palsy occurred in two patients with prostate cancer receiving weekly CA4P at 63 mg/m2. DLT of cardiac ischaemia occurred in two patients with SCCHN at a weekly dose of 50 mg/m2 in combination with cetuximab. Three patients developed grade 3 hypertension. Responses were seen in 7 of 18 patients with NSCLC. At 3 years, 3 of 18 patients with prostate cancer had prostate-specific antigen relapse.

CONCLUSIONS

Radiotherapy with CA4P appears well tolerated in most patients. The combination of CA4P, cetuximab, and radiotherapy needs further scrutiny before it can be recommended for clinical studies.

In vivo mapping of tumor oxygen consumption using (19)F MRI relaxometry

Recently, we have developed a new electron paramagnetic resonance (EPR) protocol in order to estimate tissue oxygen consumption in vivo. Because it is crucial to probe the heterogeneity of response in tumors, the aim of this study was to apply our protocol, together with (19)F MRI relaxometry, to the mapping of the oxygen consumption in tumors. The protocol includes the continuous measurement of tumor po(2) during the following respiratory challenge: (i) basal values during air breathing; (ii) increasing po(2) values during carbogen breathing until saturation of tissue with oxygen; (iii) switching back to air breathing. We have demonstrated previously using EPR oximetry that the kinetics of return to the basal value after oxygen saturation are mainly governed by tissue oxygen consumption. This challenge was applied in hyperthyroid mice (generated by chronic treatment with L-thyroxine) and control mice, as hyperthyroidism is known to dramatically affect the oxygen consumption rate of tumor cells. Our recently developed snapshot inversion recovery MRI fluorocarbon oximetry technique allowed the po(2) return kinetics to be measured with a high temporal resolution. The kinetic constants (i.e. oxygen consumption rates) were higher for tumors from hyperthyroid mice than from control mice, data that are consistent with our previous EPR study. The corresponding histograms of the (19)F MRI data showed that the kinetic constants displayed a shift to the right for the hyperthyroid group, indicating a higher oxygen consumption in these tumors. The color maps showed a large heterogeneity in terms of oxygen consumption rate within a tumor. In conclusion, (19)F MRI relaxometry allows the noninvasive mapping of the oxygen consumption in tumors. The ability to assess the heterogeneity of tumor response is critical in order to identify potential tumor regions that might be resistant to treatment and therefore produce a poor response to therapy.

A perspective on vascular disrupting agents that interact with tubulin: preclinical tumor imaging and biological assessment

The tumor microenvironment provides a rich source of potential targets for selective therapeutic intervention with properly designed anticancer agents. Significant physiological differences exist between the microvessels that nourish tumors and those that supply healthy tissue. Selective drug-mediated damage of these tortuous and chaotic microvessels starves a tumor of necessary nutrients and oxygen and eventually leads to massive tumor necrosis. Vascular targeting strategies in oncology are divided into two separate groups: angiogenesis inhibiting agents (AIAs) and vascular disrupting agents (VDAs). The mechanisms of action between these two classes of compounds are profoundly distinct. The AIAs inhibit the actual formation of new vessels, while the VDAs damage and/or destroy existing tumor vasculature. One subset of small-molecule VDAs functions by inhibiting the assembly of tubulin into microtubules, thus causing morphology changes to the endothelial cells lining the tumor vasculature, triggered by a cascade of cell signaling events. Ultimately this results in catastrophic damage to the vessels feeding the tumor. The rapid emergence and subsequent development of the VDA field over the past decade has led to the establishment of a synergistic combination of preclinical state-of-the-art tumor imaging and biological evaluation strategies that are often indicative of future clinical efficacy for a given VDA. This review focuses on an integration of the appropriate biochemical and biological tools necessary to assess (preclinically) new small-molecule, tubulin active VDAs for their potential to be clinically effective anticancer agents.

In vivo near-infrared spectroscopy and magnetic resonance imaging monitoring of tumor response to combretastatin A-4-phosphate correlated with therapeutic outcome

PURPOSE

To develop a combination treatment consisting of combretastatin A-4-phosphate (CA4P) with radiation based on tumor oxygenation status.

METHODS AND MATERIALS

In vivo near-infrared spectroscopy (NIRS) and diffusion-weighted (DW) magnetic resonance imaging (MRI) were applied to noninvasively monitor changes in tumor blood oxygenation and necrosis induced by CA4P (30 mg/kg) in rat mammary 13762NF adenocarcinoma, and the evidence was used to optimize combinations of CA4P and radiation treatment (a single dose of 5 Gy).

RESULTS

NIRS showed decreasing concentrations of tumor vascular oxyhemoglobin and total hemoglobin during the first 2 h after CA4P treatment, indicating significant reductions in tumor blood oxygenation and perfusion levels (p < 0.001). Twenty-four hours later, in response to oxygen inhalation, significant recovery was observed in tumor vascular and tissue oxygenation according to NIRS and pimonidazole staining results, respectively (p < 0.05). DW MRI revealed significantly increased water diffusion in tumors measured by apparent diffusion coefficient at 24 h (p < 0.05), suggesting that CA4P-induced central necrosis. In concordance with the observed tumor oxygen dynamics, we found that treatment efficacy depended on the timing of the combined therapy. The most significant delay in tumor growth was seen in the group of tumors treated with radiation while the rats breathed oxygen 24 h after CA4P administration.

CONCLUSIONS

Noninvasive evaluation of tumor oxygen dynamics allowed us to rationally enhance the response of syngeneic rat breast tumors to combined treatment of CA4P with radiation.

Detection of tumor response to a vascular disrupting agent by hyperpolarized 13C magnetic resonance spectroscopy

Nuclear spin hyperpolarization can dramatically increase the sensitivity of the (13)C magnetic resonance experiment, allowing dynamic measurements of the metabolism of hyperpolarized (13)C-labeled substrates in vivo. Here, we report a preclinical study of the response of lymphoma tumors to the vascular disrupting agent (VDA), combretastatin-A4-phosphate (CA4P), as detected by measuring changes in tumor metabolism of hyperpolarized [1-(13)C]pyruvate and [1,4-(13)C(2)]fumarate. These measurements were compared with dynamic contrast agent-enhanced magnetic resonance imaging (DCE-MRI) measurements of tumor vascular function and diffusion-weighted MRI (DW-MRI) measurements of the tumor cell necrosis that resulted from subsequent loss of tumor perfusion. The rate constant describing flux of hyperpolarized (13)C label between [1-(13)C]pyruvate and lactate was decreased by 34% within 6 hours of CA4P treatment and remained at this lower level at 24 hours. The rate constant describing production of labeled malate from hyperpolarized [1,4-(13)C(2)]fumarate increased 1.6-fold and 2.5-fold at 6 and 24 hours after treatment, respectively, and correlated with the degree of necrosis detected in histologic sections. Although DCE-MRI measurements showed a substantial reduction in perfusion at 6 hours after treatment, which had recovered by 24 hours, DW-MRI showed no change in the apparent diffusion coefficient of tumor water at 6 hours after treatment, although there was a 32% increase at 24 hours (P < 0.02) when regions of extensive necrosis were observed by histology. Measurements of hyperpolarized [1-(13)C]pyruvate and [1,4-(13)C(2)]fumarate metabolism may provide, therefore, a more sustained and sensitive indicator of response to a VDA than DCE-MRI or DW-MRI, respectively.

Non-invasive imaging of combretastatin activity in two tumor models: Association with invasive estimates

INTRODUCTION

The efficacy of the vascular disrupting agent combretastatin A-4 phosphate (CA4P) depends on several factors including tumor size, nitric oxide level, interstitial fluid pressure, and vascular permeability. These factors vary among tumor types. The aim of this study was to investigate all these factors in two tumor models that respond differently to CA4P.

MATERIAL AND METHODS

Mice bearing C3H mammary carcinomas or KHT sarcomas (200 to 800 mm(3)) were intraperitoneally injected with CA4P (100 mg/kg). Tumor size and the effect of a nitric oxide inhibitor nitro-L-arginine (NLA) administered intravenously were evaluated by necrotic fraction histologically assessed at 24 hours. Interstitial fluid pressure (IFP) was measured using the wick-in-needle technique, and vascular characteristics were assessed with dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

RESULTS

Initial necrotic fraction was about 10% in both tumor models at 200 mm(3), but only increased significantly with tumor size in the C3H mammary carcinoma. In this tumor, CA4P significantly induced further necrosis by about 15% at all sizes, but in the KHT tumor, the induced necrotic fraction depended on tumor size. For both tumor types, NLA with CA4P significantly increased necrotic fraction above that for each drug alone. CA4P significantly decreased IFP in all tumors except in the 800 mm(3) C3H tumor, which had an initially non-significant lower value. Interstitial volume estimated by DCE-MRI increased in all groups, except the 800 mm(3) C3H tumors. DCE-MRI vascular parameters showed different initial characteristics and general significant reductions following CA4P treatment.

CONCLUSIONS

Both tumor models showed differences in all factors before treatment, and in their response to CA4P. Perfusion and permeability as estimated by DCE-MRI play a central role in the CA4P response, and interstitial volume and IFP seemed related. These factors may be of clinical value in the planning of CA4P treatments.

In vivo observation of intracellular oximetry in perfluorocarbon-labeled glioma cells and chemotherapeutic response in the CNS using fluorine-19 MRI

Preclinical development of therapeutic agents against cancer could greatly benefit from noninvasive markers of tumor killing. Potentially, the intracellular partial pressure of oxygen (pO(2) ) can be used as an early marker of antitumor efficacy. Here, the feasibility of measuring intracellular pO(2) of central nervous system glioma cells in vivo using (19) F magnetic resonance techniques is examined. Rat 9L glioma cells were labeled with perfluoro-15-crown-5-ether ex vivo and then implanted into the rat striatum. (19) F MRI was used to visualize tumor location in vivo. The mean (19) F T(1) of the implanted cells was measured using localized, single-voxel spectroscopy. The intracellular pO(2) in tumor cells was determined from an in vitro calibration curve. The basal pO(2) of 9L cells (day 3) was determined to be 45.3 ± 5 mmHg (n = 6). Rats were then treated with a 1 × LD10 dose of bischloroethylnitrosourea intravenously and changes in intracellular pO(2) were monitored. The pO(2) increased significantly (P = 0.042, paired T-test) to 141.8 ± 3 mmHg within 18 h after bischloroethylnitrosourea treatment (day 4) and remained elevated (165 ± 24 mmHg) for at least 72 h (day 6). Intracellular localization of the perfluoro-15-crown-5-ether emulsion in 9L cells before and after bischloroethylnitrosourea treatment was confirmed by histological examination and fluorescence microscopy. Overall, noninvasive (19) F magnetic resonance techniques may provide a valuable preclinical tool for monitoring therapeutic response against central nervous system or other deep-seated tumors.

Comparison of 1H blood oxygen level-dependent (BOLD) and 19F MRI to investigate tumor oxygenation

Fluorine-19 [(19)F] MRI oximetry and (1)H blood oxygen level-dependent (BOLD) MRI were used to investigate tumor oxygenation in rat breast 13762NF carcinomas, and correlations between the techniques were examined. A range of tissue oxygen partial pressure (pO(2)) values was found in the nine tumors while the anesthetized rats breathed air, with individual tumor pO(2) ranging from a mean of 1 to 36 torr and hypoxic fraction (HF10) (<10 torr) ranging from 0% to 75%, indicating a large intra- and intertumor heterogeneity. Breathing oxygen produced significant increase in tumor pO(2) (mean DeltapO(2) = 50 torr) and decrease in HF(10) (P < 0.01). (1)H BOLD MRI observed using a spin echo-planar imaging (EPI) sequence revealed a heterogeneous response and significant increase in mean tumor signal intensity (SI) (DeltaSI = 7%, P < 0.01). R(2)* measured by multigradient-echo (MGRE) MRI decreased significantly in response to oxygen (mean DeltaR(2)* = -4 s(-1); P < 0.05). A significant correlation was found between changes in mean tumor pO(2) and mean EPI BOLD DeltaSI accompanying oxygen breathing (r(2) > 0.7, P < 0.001). Our results suggest that BOLD MRI provides information about tumor oxygenation and may be useful to predict pO(2) changes accompanying interventions. Significantly, the magnitude of the BOLD response appears to be predictive for residual tumor HFs.

The Novel Vascular Disrupting Agent ANG501 Induces Cell Cycle Arrest and Enhances Endothelial Cell Sensitivity to Radiation

The efficacy of approaches in which vascular disrupting agents (VDA) are used in combination with conventional chemotherapy and/or radiation therapy in the treatment of cancer might be improved if there were a better understanding of the cellular and molecular changes induced in normal and malignant cells as a result of VD A exposure. Toward this goal, murine endothelial cells were treated in vitro with ANG501, a novel stilbene VDA developed in our laboratory, and alterations in gene expression determined by genome-wide microarray analysis and subsequently confirmed by Western blot analysis. Among the genes that were shown to be induced upon brief exposure to non-cytotoxic doses of ANG501 were several involved in the control of cell cycle progression and apoptosis, including p21Wafl and the heat shock/stress proteins hsp25, hsp70 and anti-B-crystallin. Reflecting such induction, functional studies confirmed that normal cell cycling is temporarily inhibited following treatment with ANG501 such that the majority of cells accumulate at the radiation-sensitive G2/M phase of the cell cycle at 6 hr. The effects were transient and by 24 hr normal cell cycling had largely resumed. Combination experiments confirmed that endothelial cells treated 6 hr previously with ANG501 were more readily killed by radiation. Importantly, significant effects were evident at clinically relevant radiation doses. Taken together these findings emphasize the need to consider the radiosensitizing activity of VD As when developing therapies in which these promising compounds are used in combination with radiation.

Sensitivity to tumor microvasculature without contrast agents in high spectral and spatial resolution MR images

Contrast-enhanced (CE)-MRI is sensitive to cancers but can produce adverse reactions and suffers from insufficient specificity and morphological detail. This research investigated whether high spectral and spatial resolution (HiSS) MRI detects tumor vasculature without contrast agents, based on the sensitivity of the water resonance line shape to tumor blood vessels. HiSS data from AT6.1 tumors inoculated in the hind legs of rats (N = 8) were collected pre- and post-blood pool contrast agent (iron-oxide particles) injection. The waterline in small voxels was significantly more asymmetric at the tumor rim compared to the tumor center and normal muscle (P < 0.003). Composite images were synthesized, with the intensity in each voxel determined by the Fourier component (FC) of the water resonance having the greatest relative image contrast at that position. We tested whether regions with high contrast in FC images (FCIs) contain vasculature by comparing FCIs with CE-MRI as the "gold standard" of vascular density. The FCIs had 75% +/- 13% sensitivity, 74% +/- 10% specificity, and 91% +/- 4% positive predictive value (PPV) for vasculature detection at the tumor rim. These results suggest that tumor microvasculature can be detected using HiSS imaging without the use of contrast agents.

The effect of combretastatin A4 disodium phosphate and 5,6-dimethylxanthenone-4-acetic acid on water diffusion and blood perfusion in tumours

PURPOSE

To evaluate the effect of the vascular disrupting drugs combretastatin A-4 disodium phosphate (CA4DP) and 5,6-dimethylxanthenone-4-acetic acid (DMXAA) on the intra/extracellular volume fraction of water and blood perfusion in tumours using MRI.

METHODS AND MATERIALS

Mice with C3H mammary carcinomas underwent repeated MRI T2-weighted imaging, water-diffusion and perfusion measurements before and up to 6-hours following CA4DP and DMXAA treatment.

RESULTS

CA4DP treatment caused an increase in water diffusion in those tumour areas that presented the lowest blood perfusion, however this appeared only after five hours. The blood perfusion in highly perfused tumour regions decreased immediately after administration of CA4DP. DMXAA treatment caused an early decrease in water diffusion in the low-perfused tumour segments and followed by a subsequent decrease in the remaining part of the tumour. The blood perfusion decreased early in all parts of the tumour.

CONCLUSION

The effect of CA4DP and DMXAA on tumour blood flow was comparable. The reduction in regional blood flow caused by CA4DP in the whole tumour segment occurred early, however, changes in ADC after DMXAA appeared more extended and earlier than after CA4DP treatment, especially in tumour areas already suffering from a low blood perfusion.

Molecular imaging of hypoxia

Hypoxia, a condition of insufficient O2 to support metabolism, occurs when the vascular supply is interrupted, as in stroke or myocardial infarction, or when a tumor outgrows its vascular supply. When otherwise healthy tissues lose their O2 supply acutely, the cells usually die, whereas when cells gradually become hypoxic, they adapt by up-regulating the production of numerous proteins that promote their survival. These proteins slow the rate of growth, switch the mitochondria to glycolysis, stimulate growth of new vasculature, inhibit apoptosis, and promote metastatic spread. The consequence of these changes is that patients with hypoxic tumors invariably experience poor outcome to treatment. This has led the molecular imaging community to develop assays for hypoxia in patients, including regional measurements from O2 electrodes placed under CT guidance, several nuclear medicine approaches with imaging agents that accumulate with an inverse relationship to O2, MRI methods that measure either oxygenation directly or lactate production as a consequence of hypoxia, and optical methods with NIR and bioluminescence. The advantages and disadvantages of these approaches are reviewed, along with the individual strategies for validating different imaging methods. Ultimately the proof of value is in the clinical performance to predict outcome, select an appropriate cohort of patients to benefit from a hypoxia-directed treatment, or plan radiation fields that result in better local control. Hypoxia imaging in support of molecular medicine has become an important success story over the last decade and provides a model and some important lessons for development of new molecular imaging probes or techniques.

Antivascular effects of combretastatin A4 phosphate in breast cancer xenograft assessed using dynamic bioluminescence imaging and confirmed by MRI

Bioluminescence imaging (BLI) has found significant use in evaluating long-term cancer therapy in small animals. We have now tested the feasibility of using BLI to assess acute effects of the vascular disrupting agent combretastatin A4 phosphate (CA4P) on luciferase-expressing MDA-MB-231 human breast tumor cells growing as xenografts in mice. Following administration of luciferin substrate, there is a rapid increase in light emission reaching a maximum after about 6 min, which gradually decreases over the following 20 min. The kinetics of light emission are highly reproducible; however, following i.p. administration of CA4P (120 mg/kg), the detected light emission was decreased between 50 and 90%, and time to maximum was significantly delayed. Twenty-four hours later, there was some recovery of light emission following further administration of luciferin substrate. Comparison with dynamic contrast-enhanced MRI based on the paramagnetic contrast agent Omniscan showed comparable changes in the tumors consistent with the previous literature. Histology also confirmed shutdown of tumor vascular perfusion. We believe this finding provides an important novel application for BLI that could have widespread application in screening novel therapeutics expected to cause acute vascular changes in tumors.

Preclinical studies to predict efficacy of vascular changes induced by combretastatin a-4 disodium phosphate in patients

PURPOSE

To determine how combretastatin A-4 disodium phosphate (CA4DP) dose-dependent changes in radiation response of a C3H mouse mammary carcinoma relate to measurements of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) parameters and how those mouse DCE-MRI results compare with published clinical DCE-MRI data.

METHODS AND MATERIALS

C3H mammary carcinomas grown in female CDF(1) mice were treated when at 200 mm(3) in size. Groups of mice were given graded radiation doses, either alone or followed 30 min later by an intraperitoneal injection of CA4DP, administered at doses of 10-250 mg/kg. The radiation dose producing local tumor control in 50% of treated animals at 90 days (TCD(50)) was calculated for each CA4DP dose. DCE-MRI was performed before and 3 h after CA4DP administration, and parameters describing vascularity and interstitial volume were estimated.

RESULTS

TCD(50) showed a dose-dependent decrease reaching significance at 25 mg/kg. At greater doses of 50 and 100 mg/kg, the TCD(50) increased slightly and was not significantly different from that of controls. TCD(50) significantly decreased again at 250 mg/kg. The drug dose-response curves for all post-treatment vascular DCE-MRI parameters showed a shape similar to that of the TCD(50) curve. A similar dose dependency was seen with previously published clinical data.

CONCLUSION

Our preclinical DCE-MRI data could predict the CA4DP enhancement of the tumor radiation response and suggest the clinical CA4DP doses necessary to improve the radiation response in patients.

Correlation of radiation response with tumor oxygenation in the Dunning prostate R3327-AT1 tumor

PURPOSE

To investigate the application of pretreatment oxygenation to the AT1 subline of the Dunning R3327 prostate tumor, which is more hypoxic and faster growing than the H1 subline previously studied.

METHODS AND MATERIALS

Dunning prostate R3327-AT1 tumors growing on Copenhagen rats were administered 30 Gy of X-ray radiation either with or without oxygen inhalation. Tumor oxygenation was sampled by (19)F nuclear magnetic resonance echo planar imaging relaxometry of the reporter molecule hexafluorobenzene, no more than 24 h before irradiation.

RESULTS

Large tumors (>3.0 cm(3)) exhibited significantly greater hypoxic fractions and lower mean partial pressure of oxygen (pO(2)) than their smaller counterparts (<1.5 cm(3)). However, unlike the R3327-HI subline, large AT1 tumors generally did not respond to oxygen inhalation in terms of altered hypoxic fraction or response to irradiation. Although the tumors did not respond to oxygen inhalation, each tumor had a different pO(2), and there was a clear trend between level of oxygenation at time of irradiation and tumor growth delay, with considerably better outcome when mean pO(2) > 10 mm Hg. The comparatively small baseline hypoxic fraction in the group of small tumors was virtually eliminated by breathing oxygen, and the growth rate was significantly reduced for tumors on rats breathing oxygen during irradiation.

CONCLUSIONS

These results further validate the usefulness of nuclear magnetic resonance oximetry as a predictor of response to radiation therapy.

Early effects of combretastatin-A4 disodium phosphate on tumor perfusion and interstitial fluid pressure

Combretastatin-A4 disodium phosphate (CA4DP) is a vascular-disruptive agent that causes an abrupt decrease in tumor blood flow. The direct actions of CA4DP include increases in vascular permeability and destabilization of the endothelial cytoskeleton, which are thought to contribute to occlusion of the tumor vasculature. It has been proposed that increased permeability causes a transient increase in interstitial fluid pressure (IFP), which in turn could collapse intratumoral blood vessels. We examined the immediate effects of CA4DP on tumor IFP in C3H mammary carcinoma. Mice were treated with 100 mg/kg CA4DP by intraperitoneal injection. Tumor perfusion was recorded by laser Doppler flowmetry at separate time points, and IFP was recorded continuously by the wick-in-needle method. In this study, we found that CA4DP treatment resulted in a rapid reduction in tumor perfusion, followed by a decrease in IFP; no increases in IFP were observed. This suggests that CA4DP-induced reductions in tumor perfusion are not dependent on increases in IFP.

Tumor Antivascular Effects of Radiotherapy Combined with Combretastatin A4 Phosphate in Human Non–Small-Cell Lung Cancer

PURPOSE

The tumor vascular effects of radiotherapy and subsequent administration of the vascular disrupting agent combretastatin A4 phosphate (CA4P) were studied in patients with advanced non-small-cell lung cancer using volumetric dynamic contrast-enhanced computed tomography (CT).

PATIENTS AND METHODS

Following ethical committee approval and informed consent, 8 patients receiving palliative radiotherapy (27 Gy in six fractions, twice weekly) also received CA4P (50 mg/m(2)) after the second fraction of radiotherapy. Changes in dynamic CT parameters of tumor blood volume (BV) and permeability surface area product (PS) were measured for the whole tumor volume, tumor rim, and center after radiotherapy alone and after radiotherapy in combination with CA4P.

RESULTS

After the second fraction of radiotherapy, 6 of the 8 patients showed increases in tumor PS (23.6%, p = 0.011). Four hours after CA4P, a reduction in tumor BV (22.9%, p < 0.001) was demonstrated in the same 6 patients. Increase in PS after radiotherapy correlated with reduction in BV after CA4P (r = 0.77, p = 0.026). At 72 h after CA4P, there was a sustained reduction in tumor BV of 29.4% (p < 0.001). Both increase in PS after radiotherapy and reduction in BV after CA4P were greater at the rim of the tumor. The BV reduction at the rim was sustained to 72 h (51.4%, p = 0.014).

CONCLUSION

Radiotherapy enhances the tumor antivascular activity of CA4P in human non-small-cell lung cancer, resulting in sustained tumor vascular shutdown.

Hypoxia: importance in tumor biology, noninvasive measurement by imaging, and value of its measurement in the management of cancer therapy

PURPOSE

The Cancer Imaging Program of the National Cancer Institute convened a workshop to assess the current status of hypoxia imaging, to assess what is known about the biology of hypoxia as it relates to cancer and cancer therapy, and to define clinical scenarios in which in vivo hypoxia imaging could prove valuable.

RESULTS

Hypoxia, or low oxygenation, has emerged as an important factor in tumor biology and response to cancer treatment. It has been correlated with angiogenesis, tumor aggressiveness, local recurrence, and metastasis, and it appears to be a prognostic factor for several cancers, including those of the cervix, head and neck, prostate, pancreas, and brain. The relationship between tumor oxygenation and response to radiation therapy has been well established, but hypoxia also affects and is affected by some chemotherapeutic agents. Although hypoxia is an important aspect of tumor physiology and response to treatment, the lack of simple and efficient methods to measure and image oxygenation hampers further understanding and limits their prognostic usefulness. There is no gold standard for measuring hypoxia; Eppendorf measurement of pO(2) has been used, but this method is invasive. Recent studies have focused on molecular markers of hypoxia, such as hypoxia inducible factor 1 (HIF-1) and carbonic anhydrase isozyme IX (CA-IX), and on developing noninvasive imaging techniques.

CONCLUSIONS

This workshop yielded recommendations on using hypoxia measurement to identify patients who would respond best to radiation therapy, which would improve treatment planning. This represents a narrow focus, as hypoxia measurement might also prove useful in drug development and in increasing our understanding of tumor biology.

Pathophysiologic Effects of Vascular-Targeting Agents and the Implications for Combination with Conventional Therapies

A functional vascular supply is critical for the continued growth and development of solid tumors. It also plays a major role in metastatic spread of tumor cells. This importance has led to the concept of targeting the vasculature of the tumor as a form of cancer therapy. Two major types of vascular-targeting agent (VTA) have now emerged: those that prevent the angiogenic development of the neovasculature of the tumor and those that specifically damage the already established tumor vascular supply. When used alone neither approach readily leads to tumor control, and so, for VTAs to be most successful in the clinic they will need to be combined with more conventional therapies. However, by affecting the tumor vascular supply, these VTAs should induce pathophysiologic changes in variables, such as blood flow, pH, and oxygenation. Such changes could have negative or positive influences on the tumor response to more conventional therapies. This review aims to discuss the pathophysiologic changes induced by VTAs and the implications of these effects on the potential use of VTAs in combined modality therapy.

Rat tumor response to the vascular-disrupting agent 5,6-dimethylxanthenone-4-acetic acid as measured by dynamic contrast-enhanced magnetic resonance imaging, plasma 5-hydroxyindoleacetic acid levels, and tumor necrosis

The dose-dependent effects of 5,6-dimethylxanthenone-4-acetic acid (DMXAA) on rat GH3 prolactinomas were investigated in vivo. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was used to assess tumor blood flow/permeability pretreatment and 24 hours posttreatment with 0, 100, 200, or 350 mg/kg DMXAA. DCE-MRI data were analyzed using K(trans) and the integrated area under the gadolinium time curve (IAUGC) as response biomarkers. High-performance liquid chromatography (HPLC) was used to determine the plasma concentration of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) following treatment to provide an index of increased vessel permeability and vascular damage. Finally, tumor necrosis was assessed by grading hematoxylin and eosin-stained sections cut from the same tumors investigated by MRI. Both tumor K(trans) and IAUGC were significantly reduced 24 hours posttreatment with 350 mg/kg DMXAA only, with no evidence of dose response. HPLC demonstrated a significant increase in plasma 5-HIAA 24 hours posttreatment with 200 and 350 mg/kg DMXAA. Histologic analysis revealed some evidence of tumor necrosis following treatment with 100 or 200 mg/kg DMXAA, reaching significance with 350 mg/kg DMXAA. The absence of any reduction in K(trans) or IAUGC following treatment with 200 mg/kg, despite a significant increase in 5-HIAA, raises concerns about the utility of established DCE-MRI biomarkers to assess tumor response to DMXAA.

Dependency of the effect of a vascular disrupting agent on sensitivity to tirapazamine and gamma-ray irradiation upon the timing of its administration and tumor size, with reference to the effect on intratumor quiescent cells

PURPOSE

The effect of vascular disrupting agent ZD6126 with time on the sensitivity to the hypoxic cytotoxin tirapazamine (TPZ) and gamma-rays was examined in large and small solid tumors.

METHODS

Mice bearing SCC VII tumors 1 or 1.5 cm in diameter received 5-bromo-2'-deoxyuridine (BrdU) continuously to label all proliferating (P) cells, followed by injection with or without ZD6126. In the absence of ZD6126, or 1 or 24 h following ZD6126 injection, the response to TPZ or gamma-ray irradiation in quiescent (Q) cells was assessed in terms of induced micronucleus (MN) frequency using immunofluorescence staining for BrdU. The MN frequency in the total cell population was determined from the tumors not pretreated with BrdU. Another group of tumor-bearing mice received a series of test doses of gamma-rays while alive or after tumor clamping to obtain hypoxic fractions (HFs) in the tumors.

RESULTS

One hour after ZD6126 injection, both small and large tumors showed lower and higher sensitivity, and 24 h after, higher and lower sensitivity, to gamma-rays and TPZ, respectively, than the tumors not treated with ZD6126. Further, they showed larger and smaller HFs 1 and 24 h after ZD6126 injection, respectively. Without ZD6126 and 1 h after injection, small tumors were more sensitive to gamma-rays and less sensitive to TPZ than large tumors, probably due to the smaller HFs than large tumors. In contrast, 24 h after the injection, these differences in sensitivity and the HF between small and large tumors were reversed. The changes in sensitivity and the size of the HF were more marked in the total cell population than in Q cells.

CONCLUSIONS

Following ZD6126 treatment, in terms of tumor control, especially large tumors and total tumor cell population, administering TPZ 1 h later and gamma-ray irradiation 24 h later were effective. Intratumor physiologic factors such as the size of the HF, depending on the time after ZD6126 injection, have to be taken into account when combining another treatment with ZD6126.