Magnetic resonance imaging and spectroscopy of combretastatin A4 prodrug-induced disruption of tumour perfusion and energetic status

The Use of Alternative Strategies for Enhanced Nanoparticle Delivery to Solid Tumors

Nanomaterial (NM) delivery to solid tumors has been the focus of intense research for over a decade. Classically, scientists have tried to improve NM delivery by employing passive or active targeting strategies, making use of the so-called enhanced permeability and retention (EPR) effect. This phenomenon is made possible due to the leaky tumor vasculature through which NMs can leave the bloodstream, traverse through the gaps in the endothelial lining of the vessels, and enter the tumor. Recent studies have shown that despite many efforts to employ the EPR effect, this process remains very poor. Furthermore, the role of the EPR effect has been called into question, where it has been suggested that NMs enter the tumor via active mechanisms and not through the endothelial gaps. In this review, we provide a short overview of the EPR and mechanisms to enhance it, after which we focus on alternative delivery strategies that do not solely rely on EPR in itself but can offer interesting pharmacological, physical, and biological solutions for enhanced delivery. We discuss the strengths and shortcomings of these different strategies and suggest combinatorial approaches as the ideal path forward.

Development of multifunctional Overhauser-enhanced magnetic resonance imaging for concurrent in vivo mapping of tumor interstitial oxygenation, acidosis and inorganic phosphate concentration

Tumor oxygenation (pO₂), acidosis (pH) and interstitial inorganic phosphate concentration (Pi) are important parameters of the malignant behavior of cancer. A noninvasive procedure that enables visualization of these parameters may provide unique information about mechanisms of tumor pathophysiology and provide clues to new treatment targets. In this research, we present a multiparametric imaging method allowing for concurrent mapping of pH, spin probe concentration, pO₂, and Pi using a single contrast agent and Overhauser-enhanced magnetic resonance imaging technique. The developed approach was applied to concurrent multifunctional imaging in phantom samples and in vivo in a mouse model of breast cancer. Tumor tissues showed higher heterogeneity of the distributions of the parameters compared with normal mammary gland and demonstrated the areas of significant acidosis, hypoxia, and elevated Pi content.

Power Doppler ultrasound and contrast-enhanced ultrasound demonstrate non-invasive tumour vascular response to anti-vascular therapy in canine cancer patients

Combretastatin A4-phosphate (CA4P) is an anti-vascular agent which selectively shuts down blood supply in tumours, resulting in extensive tumour necrosis. The aim of this study was to assess in vivo, non-invasive ultrasound techniques for the early evaluation of tumour perfusion following CA4P treatment of spontaneous tumours. Eight dogs that bore spontaneous tumours were enrolled and were subsequently treated with a single dose of intravenous CA4P. Perfusion of tumours was evaluated by power Doppler ultrasound (PDUS) pre-treatment (0 h), during the injection (10 min, 20 min, 30 min) and after CA4P infusion (24 and 72 h). Vascularity index (VI) of the tumour tissue was quantitatively analysed and accuracy was verified by correlation analysis with the results of immunohistochemical evaluation of microvessel density (MVD). Central and peripheral perfusion was evaluated by contrast-enhanced ultrasound (CEUS) pre-treatment and at 72 h post-treatment. Post-treatment, PDUS demonstrated a significant decrease in VI within 10 min of CA4P infusion. CEUS parameters demonstrated a significant decrease in blood velocity and volume in the central aspect of the tumour. Histology revealed a 4.4-fold reduction (p < 0.001, 95% CI [2.2,9.4]) in MVD and a 4.1-fold increase (p = 0.003, 95% CI [1.4,11.8]) in necrotic tumour tissue. A strong correlation between PDUS results and immunohistochemical results was found (Pearson R² = 0.957, p < 0.001). Furthermore, the findings of PDUS were supported by the objective results of the CEUS analyses. These data suggest a role for ultrasound in real-time, non-invasive monitoring of tumour vascular response as an early indicator of CA4P treatment efficacy.

A single dose of intravenous combretastatin A4-phosphate is reasonably well tolerated and significantly reduces tumour vascularization in canine spontaneous cancers

Combretastatin A4-phosphate (CA4P) is an anti-tumour vascular targeting agent which selectively blocks tumour blood flow. Research on CA4P in rodent tumour models is extensive; however, knowledge of its effect on spontaneous cancer is scarce. This study was conducted in canine patients with spontaneous solid tumours. The goal was to assess the toxicity and efficacy of CA4P in various spontaneous tumour types. Eight dogs with spontaneous tumours were enrolled and treated with a single dose of 75 mg m^-2 intravenous CA4P. The dogs were screened and monitored before and after injection. Pre- and post-treatment tumour blood flow was analysed in vivo by power Doppler ultrasound (PDUS) and contrast-enhanced ultrasound (CEUS). Vessel destruction and tumour necrosis were evaluated by histopathology. Clinically relevant toxicity was limited to one case of temporary tetraparesis; other adverse events were mild. Significant cardiovascular changes were mostly confined to changes in heart rate and cTnI levels. Macroscopic tumour size reduction was evident in 2 dogs. Based on PDUS and CEUS, CA4P induced a significant decrease in vascular index and tumour blood flow. Post-treatment, histopathology revealed a significant increase of necrotic tumoural tissue and a significant reduction in microvessel density in tumoural tissue. Anti-vascular and necrotizing effects of CA4P were documented in a variety of canine spontaneous cancers with only minimal side effects. This is the first study reporting the administration of CA4P to canine cancer patients with in vivo and ex vivo assessment, and a first step toward implementing CA4P in combination therapies in veterinary oncology patients. The use of CA4P in canine patients was approved and registered by the Belgian Federal Agency for Medicines and Health Products (FAMHP) (approval number 0002588, registration number 6518 ID 2F12).

Biodistribution and tolerance of intravenous iodine-131-labelled hypericin in healthy dogs

Hypericin (Hyp) is a necrosis-avid compound that can be efficiently labelled with radioiodine for both diagnostic and therapeutic purposes. Before ¹³¹ I-Hyp can be considered as a clinically useful drug in a combination therapy for canine cancer patients, evaluation of its toxicity is necessary. The aim of this study was to investigate the biodistribution and tolerance of a single dose administration of ¹³¹ I-Hyp. Three healthy dogs were included. ¹³¹ I-Hyp at a dose of 0.2 mg/kg and an activity of 185 MBq was intravenously injected. The effects on physical, haematological and biochemical parameters were characterized and the biodistribution and elimination pattern, the effective half-life and dose rate were assessed. Drug-related adverse events were limited to mild gastrointestinal signs, resolving within 48 hours. No significant differences were found in blood haematology and serum biochemistry before and after treatment. Following administration, highest percentage of injected dose (%ID ± SD) was found in the liver (5.5 ± 0.33), the lungs (4.17 ± 0.14) and the heart (3.11 ± 0.78). After 24 hours, highest %ID was found in colon (4.25 ± 1.45) and liver (3.45 ± 0.60). Clearance from all organs was effective within 7 days. Effective half-life was established at 80 hours, and the dose rate fell below <20 μSv/h at 1 m within 1 day. The current study reveals that single dose treatment with ¹³¹ I-Hyp at the described dose is well tolerated by healthy dogs and supports the use of radioiodinated hypericin in a combination therapy for canine cancer patients.

Pharmacological and physical vessel modulation strategies to improve EPR-mediated drug targeting to tumors

The performance of nanomedicine formulations depends on the Enhanced Permeability and Retention (EPR) effect. Prototypic nanomedicine-based drug delivery systems, such as liposomes, polymers and micelles, aim to exploit the EPR effect to accumulate at pathological sites, to thereby improve the balance between drug efficacy and toxicity. Thus far, however, tumor-targeted nanomedicines have not yet managed to achieve convincing therapeutic results, at least not in large cohorts of patients. This is likely mostly due to high inter- and intra-patient heterogeneity in EPR. Besides developing (imaging) biomarkers to monitor and predict EPR, another strategy to address this heterogeneity is the establishment of vessel modulation strategies to homogenize and improve EPR. Over the years, several pharmacological and physical co-treatments have been evaluated to improve EPR-mediated tumor targeting. These include pharmacological strategies, such as vessel permeabilization, normalization, disruption and promotion, as well as physical EPR enhancement via hyperthermia, radiotherapy, sonoporation and phototherapy. In the present manuscript, we summarize exemplary studies showing that pharmacological and physical vessel modulation strategies can be used to improve tumor-targeted drug delivery, and we discuss how these advanced combination regimens can be optimally employed to enhance the (pre-) clinical performance of tumor-targeted nanomedicines.

Technical Note: Quantitative dynamic contrast-enhanced MRI of a 3-dimensional artificial capillary network

PURPOSE

Variability across devices, patients, and time still hinders widespread recognition of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) as quantitative biomarker. The purpose of this work was to introduce and characterize a dedicated microchannel phantom as a model for quantitative DCE-MRI measurements.

METHODS

A perfusable, MR-compatible microchannel network was constructed on the basis of sacrificial melt-spun sugar fibers embedded in a block of epoxy resin. Structural analysis was performed on the basis of light microscopy images before DCE-MRI experiments. During dynamic acquisition the capillary network was perfused with a standard contrast agent injection system. Flow-dependency, as well as inter- and intrascanner reproducibility of the computed DCE parameters were evaluated using a 3.0 T whole-body MRI.

RESULTS

Semi-quantitative and quantitative flow-related parameters exhibited the expected proportionality to the set flow rate (mean Pearson correlation coefficient: 0.991, P < 2.5e-5). The volume fraction was approximately independent from changes of the applied flow rate through the phantom. Repeatability and reproducibility experiments yielded maximum intrascanner coefficients of variation (CV) of 4.6% for quantitative parameters. All evaluated parameters were well in the range of known in vivo results for the applied flow rates.

CONCLUSION

The constructed phantom enables reproducible, flow-dependent, contrast-enhanced MR measurements with the potential to facilitate standardization and comparability of DCE-MRI examinations.

High-field small animal magnetic resonance oncology studies

This review focuses on the applications of high magnetic field magnetic resonance imaging (MRI) and spectroscopy (MRS) to cancer studies in small animals. High-field MRI can provide information about tumor physiology, the microenvironment, metabolism, vascularity and cellularity. Such studies are invaluable for understanding tumor growth and proliferation, response to treatment and drug development. The MR techniques reviewed here include (1)H, (31)P, chemical exchange saturation transfer imaging and hyperpolarized (13)C MRS as well as diffusion-weighted, blood oxygen level dependent contrast imaging and dynamic contrast-enhanced MRI. These methods have been proven effective in animal studies and are highly relevant to human clinical studies.

Practical dynamic contrast enhanced MRI in small animal models of cancer: data acquisition, data analysis, and interpretation

Dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) consists of the continuous acquisition of images before, during, and after the injection of a contrast agent. DCE-MRI allows for noninvasive evaluation of tumor parameters related to vascular perfusion and permeability and tissue volume fractions, and is frequently employed in both preclinical and clinical investigations. However, the experimental and analytical subtleties of the technique are not frequently discussed in the literature, nor are its relationships to other commonly used quantitative imaging techniques. This review aims to provide practical information on the development, implementation, and validation of a DCE-MRI study in the context of a preclinical study (though we do frequently refer to clinical studies that are related to these topics).

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.

Magnetic resonance spectroscopy of cancer metabolism and response to therapy

Magnetic resonance spectroscopy allows noninvasive in vivo measurements of biochemical information from living systems, ranging from cultured cells through experimental animals to humans. Studies of biopsies or extracts offer deeper insights by detecting more metabolites and resolving metabolites that cannot be distinguished in vivo. The pharmacokinetics of certain drugs, especially fluorinated drugs, can be directly measured in vivo. This review briefly describes these methods and their applications to cancer metabolism, including glycolysis, hypoxia, bioenergetics, tumor pH, and tumor responses to radiotherapy and chemotherapy.

[Antitubulin agents]

Microtubules are dynamic filamentous cytoskeletal proteins that are an important therapeutic target in patients with tumors. Microtubule binding agents have been part of the pharmacopoeia of cancer for decades, and until the advent of targeted therapy microtubules represented the only alternative to DNA as a therapeutic target in cancer. There are currently a variety of available vinca alkaloids and taxanes and other agents, such as ixabepilone and eribulin, have also been approved. Maytansinoids have been used for the production of immunoconjugates, monoclonal antibodies covalently bound to antimitotic molecules. The screening of a variety of botanical species and marine organisms continues to yield promising new antitubulin agents with novel properties. Enhanced tumor specificity, reduced neurotoxicity, and insensitivity to chemoresistance mechanisms are the three main objectives in the current search for novel microtubule binding agents.

Vascular effects dominate solid tumor response to treatment with combretastatin A-4-phosphate

Vascular-targeted therapeutics are increasingly used in the clinic. However, less is known about the direct response of tumor cells to these agents. We have developed a combretastatin-A-4-phosphate (CA4P) resistant variant of SW1222 human colorectal carcinoma cells to examine the relative importance of vascular versus tumor cell targeting in the ultimate treatment response. SW1222(Res) cells were generated through exposure of wild-type cells (SW1222(WT) ) to increasing CA4P concentrations in vitro. Increased resistance was confirmed through analyses of cell viability, apoptosis and multidrug-resistance (MDR) protein expression. In vivo, comparative studies examined tumor cell necrosis, apoptosis, vessel morphology and functional vascular end-points following treatment with CA4P (single 100 mg/kg dose). Tumor response to repeated CA4P dosing (50 mg/kg/day, 5 days/week for 2 weeks) was examined through growth measurement, and ultimate tumor cell survival was studied by ex vivo clonogenic assay. In vitro, SW1222(Res) cells showed reduced CA4P sensitivity, enhanced MDR protein expression and a reduced apoptotic index. In vivo, CA4P induced significantly lower apoptotic cell death in SW1222(Res) versus SW1222(WT) tumors indicating maintenance of resistance characteristics. However, CA4P-induced tumor necrosis was equivalent in both lines. Similarly, rapid CA4P-mediated vessel disruption and blood flow shut-down were observed in both lines. Cell surviving fraction was comparable in the two tumor types following single dose CA4P and SW1222(Res) tumors were at least as sensitive as SW1222(WT) tumors to repeated dosing. Despite tumor cell resistance to CA4P, SW1222(Res) response in vivo was not impaired, strongly supporting the view that vascular damage dominates the therapeutic response to this agent.

Multiparametric MRI biomarkers for measuring vascular disrupting effect on cancer

Solid malignancies have to develop their own blood supply for their aggressive growth and metastasis; a process known as tumor angiogenesis. Angiogenesis is largely involved in tumor survival, progression and spread, which are known to be significantly attributed to treatment failures. Over the past decades, efforts have been made to understand the difference between normal and tumor vessels. It has been demonstrated that tumor vasculature is structurally immature with chaotic and leaky phenotypes, which provides opportunities for developing novel anticancer strategies. Targeting tumor vasculature is not only a unique therapeutic intervention to starve neoplastic cells, but also enhances the efficacy of conventional cancer treatments. Vascular disrupting agents (VDAs) have been developed to disrupt the already existing neovasculature in actively growing tumors, cause catastrophic vascular shutdown within short time, and induce secondary tumor necrosis. VDAs are cytostatic; they can only inhibit tumor growth, but not eradicate the tumor. This novel drug mechanism has urged us to develop multiparametric imaging biomarkers to monitor early hemodynamic alterations, cellular dysfunctions and metabolic impairments before tumor dimensional changes can be detected. In this article, we review the characteristics of tumor vessels, tubulin-destabilizing mechanisms of VDAs, and in vivo effects of the VDAs that have been mostly studied in preclinical studies and clinical trials. We also compare the different tumor models adopted in the preclinical studies on VDAs. Multiparametric imaging biomarkers, mainly diffusion-weighted imaging and dynamic contrast-enhanced imaging from magnetic resonance imaging, are evaluated for their potential as morphological and functional imaging biomarkers for monitoring therapeutic effects of VDAs.

Magnetic resonance imaging: a potential tool in assessing the addition of hyperthermia to neoadjuvant therapy in patients with locally advanced breast cancer

The poor overall survival for patients with locally advanced breast cancers has led over the past decade to the introduction of numerous neoadjuvant combined therapy regimens to down-stage the disease before surgery. At the same time, more evidence suggests the need for treatment individualisation with a wide variety of new targets for cancer therapeutics and also multi modality therapies. In this context, early determination of whether the patient will fail to respond can enable the use of alternative therapies that can be more beneficial. The purpose of this review is to examine the potential role of magnetic resonance imaging (MRI) in early prediction of treatment response and prognosis of overall survival in locally advanced breast cancer patients enrolled on multi modality therapy trials that include hyperthermia. The material is organised with a review of dynamic contrast (DCE)-MRI and diffusion weighted (DW)-MRI for characterisation of phenomenological parameters of tumour physiology and their potential role in estimating therapy response. Most of the work published in this field has focused on responses to neoadjuvant chemotherapy regimens alone, so the emphasis will be there, however the available data that involves the addition of hyperthermia to the regimen will be discussed The review will also include future directions that include the potential use of MRI imaging techniques in establishing the role of hyperthermia alone in modifying breast tumour microenvironment, together with specific challenges related to performing such studies.

Microtubule-binding agents: a dynamic field of cancer therapeutics

Microtubules are dynamic filamentous cytoskeletal proteins composed of tubulin and are an important therapeutic target in tumour cells. Agents that bind to microtubules have been part of the pharmacopoeia of anticancer therapy for decades and until the advent of targeted therapy, microtubules were the only alternative to DNA as a therapeutic target in cancer. The screening of a range of botanical species and marine organisms has yielded promising new antitubulin agents with novel properties. In the current search for novel microtubule-binding agents, enhanced tumour specificity, reduced neurotoxicity and insensitivity to chemoresistance mechanisms are the three main objectives.

A review and update of the current status of the vasculature-disabling agent combretastatin-A4 phosphate (CA4P)

Vascular-disrupting strategies impair a tumor's blood vessel network, which is essential for tumor progression and metastasis. Vascular-disrupting agents (VDAs) cause a rapid and selective vascular shutdown in tumors to produce extensive secondary neoplastic cell death due to ischemia. A lead agent in this therapeutic strategy is the tubulin depolymerizing agent combretastatin-A4 phosphate (CA4P). Used alone, CA4P induces extensive necrosis in a wide variety of preclinical cancer models and significant blood flow reductions in the patient tumors. Preclinical and clinical data further indicate that CA4P can effectively be combined with chemotherapy or radiotherapy. Finally, the potential of combining VDAs with antiangiogenic therapies has shown considerable promise in preclinical models and such combinations are now beginning to be evaluated in patients.

Technology insight: water diffusion MRI--a potential new biomarker of response to cancer therapy

Diffusion-weighted MRI (DW-MRI) is a functional imaging technique that displays information about the extent and direction of random water motion in tissues. Water movement in tissues is modified by interactions with hydrophobic cellular membranes, intracellular organelles and macromolecules. DW-MRI provides information on extracellular-space tortuosity, tissue cellularity and the integrity of cellular membranes. Images can be sensitive to large or small displacements of water, therefore, macroscopic water flows and microscopic water displacements in the extracellular space can be depicted. Preclinical and clinical data indicate a number of potential roles of DW-MRI in the characterization of malignancy, including determination of lesion aggressiveness and monitoring response to therapy. This Review outlines the biological basis of observations made on DW-MRI and describes how measurements are acquired and quantified, and discusses the interpretation of images and limitations of the technique. The strength of evidence for adoption of DW-MRI as a biomarker for the assessment of tumor response is presented.

Dose-response relationship of ultrasound contrast agent in an in vivo murine melanoma model

Many factors affect the sensitivity and reliability of tumor vasculature assessment at the small doses of contrast agent necessary for imaging mice. In this study we investigate the dose-response relationship of ultrasound contrast agent for a minimal exposure power Doppler technique (minexPD) in a murine melanoma model. K1735 murine melanomas grown in 25 C3H/HeN mice were imaged by power Doppler ultrasound using different doses of contrast agents, Optison(R) and Definity(R). Six mice were treated with an antivascular agent, combretastatin A4-phosphate (CA4P), and imaged before and after treatment. The color-weighted fractional area (CWFA) of the peak-enhanced image was measured to assess tumor perfusion on a relative scale of 0 to 100. CWFA increased logarithmically with dose (R(2)=0.97). Treatment with CA4P resulted in pronounced reduction in tumor perfusion 2 h after contrast injection, but perfusion recovered in the tumor periphery after 2 days. CWFA was significantly different between pre- and post-treatment for all doses at 2 h and 2 days (p < 0.05, respectively). There was no significant difference detectable between the two contrast agents, Optison(R) and Definity(R) (p = 0.46). In vivo tumor enhancement in mice increases as logarithmic function with dose. Although the extent of enhancement is dose dependent, the difference between pre- and post-therapy enhancement is relatively unchanged and uniform at varying doses. The two contrast agents tested in this study performed equally well. These results suggest that quantitative contrast-enhanced power Doppler imaging is an effective method for monitoring therapy response of tumors in mice.

Alpha-v integrins as therapeutic targets in oncology

Integrins are heterodimeric cell adhesion receptors that mediate intercellular communication through cell-extracellular matrix interactions and cell-cell interactions. Integrins have been demonstrated to play a direct role in cancer progression, specifically in tumor cell survival, tumor angiogenesis, and metastasis. Therefore, agents targeted against integrin function have potential as effective anticancer therapies. Numerous anti-integrin agents, including monoclonal antibodies and small-molecule inhibitors, are in clinical development for the treatment of solid and hematologic tumors. This review focuses on the role of alpha(v) integrins in cancer progression, the current status of integrin-targeted agents in development, and strategies for the clinical development of anti-integrin therapies.

Combination chemotherapy including combretastatin A4 phosphate and paclitaxel is effective against anaplastic thyroid cancer in a nude mouse xenograft model

CONTEXT

Anaplastic thyroid cancer (ATC) is extremely aggressive, and no effective treatment is available. Combretastatin A4 phosphate (CA4P), a vascular disrupting agent, has limited activity against ATC in a clinical trial, and so does paclitaxel.

OBJECTIVE

We hypothesized that a triple-drug combination including CA4P and paclitaxel would improve efficacy against ATC. Therefore, we evaluated two such combinations in vivo.

SETTING

We used a nude mouse xenograft model with ARO and KAT-4 cells.

INTERVENTIONS

The first combination consisted of CA4P, paclitaxel, and manumycin A (a farnesyltransferase inhibitor), and the second, CA4P, paclitaxel, and carboplatin.

MAIN OUTCOME MEASURES

Main outcome measures included tumor growth curves and tumor weights.

RESULTS

Tumor growth curve analysis (linear mixed models, P < 0.05) and xenograft weight analysis (Kruskal-Wallis one-way ANOVA on ranks, post hoc pairwise comparison, Dunn's test, P < 0.05) demonstrated that both triple-drug combinations were significantly better than placebo for both cell lines. Anti-bromodeoxyuridine immunostaining of xenograft sections from animals injected with bromodeoxyuridine before being killed showed that CA4P alone did not inhibit DNA synthesis, but manumycin A and paclitaxel did. CA4P decreased the depth of the viable outer rim of tumor cells on xenograft sections. Using electron microscopy, we found blebbing/budding of endothelial cells into capillary lumens and autophagy of tumor cells in CA4P-treated xenografts.

CONCLUSIONS

Both triple-drug combinations demonstrated excellent antineoplastic activity against ATC. The correlative findings in xenografts were consistent with vascular disruption but not direct inhibition of cell proliferation as the primary antineoplastic mechanism contributed by CA4P. These regimens warrant further investigation in clinical trials for ATC.

Correlation of MRI biomarkers with tumor necrosis in Hras5 tumor xenograft in athymic rats

Magnetic resonance imaging (MRI) can measure the effects of therapies targeting the tumor vasculature and has demonstrated that vascular-damaging agents (VDA) induce acute vascular shutdown in tumors in human and animal models. However, at subtherapeutic doses, blood flow may recover before the induction of significant levels of necrosis. We present the relationship between changes in MRI biomarkers and tumor necrosis. Multiple MRI measurements were taken at 4.7 T in athymic rats (n = 24) bearing 1.94 +/- 0.2-cm3 subcutaneous Hras5 tumors (ATCC 41000) before and 24 hours after clinically relevant doses of the VDA, ZD6126 (0-10 mg/kg, i.v.). We measured effective transverse relaxation rate (R2*), initial area under the gadolinium concentration-time curve (IAUGC(60/150)), equivalent enhancing fractions (EHF(60/150)), time constant (K(trans)), proportion of hypoperfused voxels as estimated from fit failures in K(trans) analysis, and signal intensity (SI) in T2-weighted MRI (T(2)W). ZD6126 treatment induced > 90% dose-dependent tumor necrosis at 10 mg/kg; correspondingly, SI changes were evident from T2W MRI. Although R2* did not correlate, other MRI biomarkers significantly correlated with necrosis at doses of > or = 5 mg/kg ZD6126. These data on Hras5 tumors suggest that the quantification of hypoperfused voxels might provide a useful biomarker of tumor necrosis.

Promise and Progress for Functional and Molecular Imaging of Response to Targeted Therapies

Biomarkers to predict or monitor therapy response are becoming essential components of drug developer's armamentaria. Molecular and functional imaging has particular promise as a biomarker for anticancer therapies because it is non-invasive, can be used longitudinally and provides information on the whole patient or tumor. Despite this promise, molecular or functional imaging endpoints are not routinely incorporated into clinical trial design. As the costs of clinical trials and drug development become prohibitively more expensive, the need for improved biomarkers has become imperative and thus, the relatively high cost of imaging is justified. Imaging endpoints, such as Diffusion-Weighted MRI, DCE-MRI and FDG-PET have the potential to make drug development more efficient at all phases, from discovery screening with in vivo pharmacodynamics in animal models through the phase III enrichment of the patient population for potential responders. This review focuses on the progress of imaging responses to new classes of anti-cancer therapies targeted against PI3 kinase/AKT, HIF-1alpha and VEGF. The ultimate promise of molecular and functional imaging is to theragnostically predict response prior to commencement of targeted therapy.

Vascular disrupting agents in clinical development

Growth of human tumours depends on the supply of oxygen and nutrients via the surrounding vasculature. Therefore tumour vasculature is an attractive target for anticancer therapy. Apart from angiogenesis inhibitors that compromise the formation of new blood vessels, a second class of specific anticancer drugs has been developed. These so-called vascular disrupting agents (VDAs) target the established tumour vasculature and cause an acute and pronounced shutdown of blood vessels resulting in an almost complete stop of blood flow, ultimately leading to selective tumour necrosis. As a number of VDAs are now being tested in clinical studies, we will discuss their mechanism of action and the results obtained in preclinical studies. Also data from clinical studies will be reviewed and some considerations with regard to the future development are given.

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.

The effects of the vascular disrupting agents combretastatin A-4 disodium phosphate, 5,6-dimethylxanthenone-4-acetic acid and ZD6126 in a murine tumour: a comparative assessment using MRI and MRS

The aim of this study was to use magnetic resonance (MR) techniques to non-invasively compare the effects of the three leading vascular disrupting agents, namely combretastatin A-4 disodium phosphate (CA4DP), 5,6-dimethylxanthenone-4-acetic acid (DMXAA) and ZD6126. A C3H mouse mammary carcinoma grown in the right rear foot of female CDF1 mice was used and treatments performed when tumours had reached 200 mm3 in volume. Drugs were prepared fresh before each experiment and intraperitoneally injected into restrained non-anaesthetised mice. Tumour response was evaluated using 31P-MR spectroscopy and T1- and T2- weighted imaging with a 7-Tesla, horizontal bore magnet, before and up to 24 hours after treatment. All three drugs significantly decreased bioenergetic status and pH, and did so in a time and dose dependent fashion, but there were differences; the decrease by CA4DP occurred more rapidly than for DMXAA or ZD6126, while DMXAA had a narrow window of activity compared to CA4DP and ZD6126. Changes in T1 weighted images for all three agents suggested a dose dependent increase in tumour oedema within three hours after treatment, consistent with an increase in vessel permeability. Using T2 weighted images there was some evidence of haemorrhagic necrosis by DMXAA, but such necrosis was limited following treatment with CA4DP or ZD6126.

Assessing responses to cancer therapy using molecular imaging

Tumor responses to therapy in the clinic are still evaluated primarily from non-invasive imaging measurements of reductions in tumor size. This approach, however, lacks sensitivity and can only give a delayed indication of a positive response to treatment. Major advances in our understanding of the molecular mechanisms responsible for cancer, combined with new targeted clinical imaging technologies designed to detect the molecular correlates of disease progression and response to treatment, are set to revolutionize our approach to the detection and treatment of the disease. We describe here the imaging technologies available to image tumor cell proliferation and migration, metabolism, receptor and gene expression, apoptosis and tumor angiogenesis and vascular function, and show how measurements of these parameters can be used to give early indications of positive responses to treatment or to detect drug resistance and/or disease recurrence. Special emphasis has been placed on those applications that are already used in the clinic and those that are likely to translate into clinical application in the near future or whose use in preclinical studies is likely to facilitate translation of new treatments into the clinic.

Effect of vascular targeting agent in rat tumor model: dynamic contrast-enhanced versus diffusion-weighted MR imaging

PURPOSE

To compare dynamic contrast material-enhanced magnetic resonance (MR) imaging and diffusion-weighted MR imaging for noninvasive evaluation of early and late effects of a vascular targeting agent in a rat tumor model.

MATERIALS AND METHODS

The study protocol was approved by the local ethics committee for animal care and use. Thirteen rats with one rhabdomyosarcoma in each flank (26 tumors) underwent dynamic contrast-enhanced imaging and diffusion-weighted echo-planar imaging in a 1.5-T MR unit before intraperitoneal injection of combretastatin A4 phosphate and at early (1 and 6 hours) and later (2 and 9 days) follow-up examinations after the injection. Histopathologic examination was performed at each time point. The apparent diffusion coefficient (ADC) of each tumor was calculated separately on the basis of diffusion-weighted images obtained with low b gradient values (ADC(low); b = 0, 50, and 100 sec/mm(2)) and high b gradient values (ADC(high); b = 500, 750, and 1000 sec/mm(2)). The difference between ADC(low) and ADC(high) was used as a surrogate measure of tissue perfusion (ADC(low) - ADC(high) = ADC(perf)). From the dynamic contrast-enhanced MR images, the volume transfer constant k and the initial slope of the contrast enhancement-time curve were calculated. For statistical analyses, a paired two-tailed Student t test and linear regression analysis were used.

RESULTS

Early after administration of combretastatin, all perfusion-related parameters (k, initial slope, and ADC(perf)) decreased significantly (P < .001); at 9 days after combretastatin administration, they increased significantly (P < .001). Changes in ADC(perf) were correlated with changes in k (R(2) = 0.46, P < .001) and the initial slope (R(2) = 0.67, P < .001).

CONCLUSION

Both dynamic contrast-enhanced MR imaging and diffusion-weighted MR imaging allow monitoring of perfusion changes induced by vascular targeting agents in tumors. Diffusion-weighted imaging provides additional information about intratumoral cell viability versus necrosis after administration of combretastatin.

Diffusion-weighted magnetic resonance imaging allows noninvasive in vivo monitoring of the effects of combretastatin a-4 phosphate after repeated administration

The noninvasive assessment of anticancer treatment efficacy is very important for the improvement of therapeutic window. The purpose of the present study was to evaluate the antitumoral effects of the vascular targeting agent, combretastatin A-4 phosphate (CA-4-P), at selected time points after repeated intraperitoneal drug administrations (25 mg/kg), using diffusion-weighted magnetic resonance imaging (DW-MRI). The experiments were performed during an overall follow-up period of 3 weeks on WAG/Rij rats with subcutaneously growing rhabdomyosarcomas. Each animal served as its own baseline. The DW-MRI studies were quantified by calculating the apparent diffusion coefficient (ADC) for different low and high b-values to separate the effects on tumor vasculature and cellular integrity. The changes in ADC as well as the extent of necrosis development (proportional to the tumor volume), measured on the MR images, were of comparable magnitude after each treatment. All ADC values showed a significant decrease at 6 hours, followed by a significant increase at 2 days for various CA-4-P administrations. DW-MRI allowed us to monitor both reduction in perfusion and changes in the extent of tumor necrosis after CA-4-P injection. Repeated CA-4-P administration retains efficacy in rat rhabdomyosarcomas, with similar findings after each drug administration.

Combretastatin A4 phosphate: background and current clinical status

Combretastatin A4 phosphate (CA4P) represents the lead compound in a group of novel tubulin depolymerising agents being developed as vascular targeting agents (VTAs). VTAs are drugs that induce rapid and selective vascular dysfunction in tumours. CA4P is a water-soluble prodrug of the cis-stilbene CA4 originally isolated from the tree Combretum caffrum. Preclinical studies have shown that CA4P induces blood flow reductions and subsequent tumour cell death in a variety of preclinical models. Moreover, this activity has been linked to its ability to rapidly alter the morphology of immature endothelial cells by disrupting their tubulin cytoskeleton. Phase I clinical trials have established a maximum tolerated dose in the range 60-68 mg/m2 and in addition have established that significant changes to tumour perfusion can be achieved across a wide range of doses. The dose-limiting toxicities include tumour pain, ataxia and cardiovascular changes. The maximum tolerated dose was independent of schedule, indicating the absence of cumulative toxicity. Although unexpected from preclinical studies, some evidence of clinical response was seen using CA4P as a single modality. Based on the Phase I data, combination studies of CA4P with established therapies are in progress and should determine whether the exciting preclinical data obtained when VTAs are used in combination with cytotoxic chemotherapy, radiation, radioimmunotherapy and even antiangiogenic agents, can be translated into man.

Molecular imaging of antiangiogenic agents

Many novel antiangiogenic agents are currently in various phases of clinical testing. These agents tend to be cytostatic, and therefore few responses are observed with conventional imaging by computerized tomography. Furthermore, toxicity with these agents is seen when the maximum-tolerated dose is combined with chemotherapy. Hence, there is a need to develop imaging strategies that can determine the minimum and optimum biologically active doses. There is increasing awareness of the need to obtain evidence of drug activity through the use of surrogate markers of the biologic mechanism of action during early clinical trials, in addition to determining the pharmacokinetics, toxicity profile, and maximum-tolerated dose. One of the major impediments to the rapid development of antiangiogenic agents in the past has been the lack of validated assays capable of measuring an antiangiogenic effect directly in patients. Recently, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) has emerged as a useful technique for noninvasive imaging of tumor vasculature in preclinical and clinical models. The problem of tumor heterogeneity remains to be addressed. The major challenge is the standardization of the technique worldwide for the purpose of early clinical studies that are likely to be multicenter. Convincing data on correlations between changes observed through molecular imaging and changes in tumor angiogenesis, and hence tumor biology, are still lacking. Whether this would translate into a survival advantage remains to be seen. The ultimate test of the surrogate biological end points determined by molecular imaging will occur in randomized phase III trials. Results of the first randomized trial that showed a survival advantage in favor of antiangiogenic agents were released at the American Society of Clinical Oncology meeting in 2003. There it was reported that the combination of 5-fluorouracil, leucovorin, and irinotecan (Camptosar; Pfizer Pharmaceuticals; New York, NY) with anti-vascular endothelial growth factor antibody (bevacizumab-Avastin; Genentech, Inc.; South San Francisco, CA) was superior to the chemotherapy regimen alone when used to treat patients with metastatic colorectal cancer. However, until further phase III clinical trials confirm these results, surrogate end points of clinical efficacy of the newer agents are urgently needed so that development of ineffective drugs can be halted early. This review briefly discusses the role of molecular imaging in general, and DCE-MRI in particular, in relation to treatment with antiangiogenic agents and highlights some of the difficulties encountered in this area.

Vascular disrupting agents: a new class of drug in cancer therapy

AIMS

To provide a comprehensive overview of the current state of development of a novel class of anti-cancer drugs, the vascular disrupting agents (VDAs), previously known as vascular targeting agents (VTAs).

MATERIALS AND METHODS

A comprehensive review, analysis and commentary of the published medical literature on VDAs was performed.

RESULTS

Tumour vascular targeting therapy exploits known differences between normal and tumour blood vessels. VDAs target the preexisting vessels of tumours (cf anti-angiogenics), and cause vascular shutdown leading to tumour cell death and rapid haemorrhagic necrosis within hours. It is becoming clear that VDAs have overlapping activity with anti-angiogenic drugs, which prevent the formation of new blood vessels. There are two types of VDA. First, biological or ligand-directed VDAs use antibodies, peptides or growth factors to target toxins or pro-coagulants to the tumour endothelium. In contrast, small molecule VDAs work either as tubulin-binding agents or through induction of local cytokine production. VDAs can kill tumour cells resistant to conventional chemotherapy and radiotherapy, and work best on cells in the poorly perfused hypoxic core of tumours, leaving a viable rim of well-perfused tumour tissue at the periphery, which rapidly regrows. Consequently, responses of tumours to VDAs given as single agents have been poor; however, combination therapy with cytotoxic chemotherapy, external-beam radiotherapy, and radioimmunotherapy, which target the peripheral tumour cells, has produced some excellent responses in animal tumours. VDAs are generally well tolerated with different side-effect profiles to current oncological therapies. Dynamic magnetic resonance imaging is most frequently used to obtain a pharmacodynamic end point to determine whether the VDA is acting on its intended target.

CONCLUSIONS

VDAs are a promising new class of drug, which offer the attractive possibility of inducing responses in all tumour types with combination therapy.

Disrupting tumour blood vessels

Low-molecular-weight vascular-disrupting agents (VDAs) cause a pronounced shutdown in blood flow to solid tumours, resulting in extensive tumour-cell necrosis, while they leave the blood flow in normal tissues relatively intact. The largest group of VDAs is the tubulin-binding combretastatins, several of which are now being tested in clinical trials. DMXAA (5,6-dimethylxanthenone-4-acetic acid) - one of a structurally distinct group of drugs - is also being tested in clinical trials. A full understanding of the action of these and other VDAs will provide insights into mechanisms that control tumour blood flow and will be the basis for the development of new therapeutic drugs for targeting the established tumour vasculature for therapy.

Diffusion-weighted MR imaging in monitoring the effect of a vascular targeting agent on rhabdomyosarcoma in rats

PURPOSE

To evaluate diffusion-weighted magnetic resonance (MR) imaging for monitoring tumor response in rats after administration of combretastatin A4 phosphate.

MATERIALS AND METHODS

Study protocol was approved by local ethical committee for animal care and use. Rhabdomyosarcomas implanted subcutaneously in both flanks of 17 rats were evaluated with 1.5-T MR unit by using four-channel wrist coil. Transverse T2-weighted fast spin-echo sequences, T1-weighted spin-echo sequences before and after gadodiamide administration, and transverse echo-planar diffusion-weighted MR examinations were performed before, 1 and 6 hours, and 2 and 9 days after intraperitoneal injection of vascular targeting agent (combretastatin A4 phosphate, 25 mg/kg). Apparent diffusion coefficient (ADC) was automatically calculated from diffusion-weighted MR imaging findings. These findings were compared with histopathologic results at each time point. For statistical analysis, paired Student t tests with Bonferroni correction for multiple testing were used.

RESULTS

T1-weighted images before combretastatin administration showed enhancement of solid tumor tissue but not of central necrosis. At 1 and 6 hours after combretastatin injection, enhancement of solid tissue disappeared almost completely, with exception of small peripheral rim. At 2 and 9 days after combretastatin injection, enhancement progressively reappeared in tumor periphery. ADC, however, showed decrease early after combretastatin injection ([1.26 +/- 0.16]x 10(-3) mm2/sec before, [1.18 +/- 0.17]x 10(-3) mm2/sec 1 hour after [P=.0005] and [1.08 +/- 0.14]x 10(-3) mm(2)/sec 6 hours after [P=.0007] combretastatin A4 phosphate injection), histologically corresponding to vessel congestion and vascular shutdown in periphery but no necrosis. An increase of ADC ([1.79 +/- 0.13]x 10(-3) mm2/sec) (P <.0001) 2 days after combretastatin A4 phosphate injection was paralleled by progressive histologic necrosis. A significant (P <.0001) decrease in ADC 9 days after treatment ([1.41 +/- 0.15]x 10(-3) mm2/sec) corresponded to tumor regrowth.

CONCLUSION

In addition to basic relaxation-weighted MR imaging and postgadolinium T1-weighted MR imaging to enable prompt detection of vascular shutdown, diffusion-weighted MR imaging was used to discriminate between nonperfused but viable and necrotic tumor tissues for early monitoring of therapeutic effects of vascular targeting agent.

Magnetic resonance imaging measurements of the response of murine and human tumors to the vascular-targeting agent ZD6126

PURPOSE

ZD6126 is a novel vascular targeting agent currently undergoing clinical evaluation. It acts by destabilizing the microtubulin of fragile and proliferating neoendothelial cells in tumors. The drug leads to blood vessel congestion, the selective destruction of the vasculature, and extensive necrosis in experimental tumors. The aim of the study reported here was to assess the ability of dynamic contrast enhanced magnetic resonance imaging (MRI) to measure the antivascular effects of ZD6126 in tumors.

EXPERIMENTAL DESIGN

The work was carried out in mice bearing C38 colon adenocarcinoma and in patients with advanced cancers. MRI was performed before and 6 h (human tumors) or 24 h (C38 tumors) after i.v. drug administration. Contrast agent (gadolinium diethylenetriaminepentaacetate) enhancement was characterized by the initial area under the gadolinium diethylenetriaminepentaacetate uptake versus time curve (IAUC). IAUC reflects blood flow, vascular permeability, and the fraction of interstitial space.

RESULTS

The median IAUC was reduced in all C38 tumors after ZD6126 administration [by 6-48% at 50 mg/kg (n = 3)], 58-91% at 100 mg/kg (n = 4), and 11-93% at 200 mg/kg (n = 6). In contrast, the administration of vehicle only led to no consistent change in median IAUC (n = 4). The ZD6126-induced changes in median IAUC appeared to be dose dependent (P = 0.045). No ZD6126-induced changes were apparent in murine muscle. Similar effects were seen in preliminary data from human tumors (11 tumors studied, 9 patients). At doses of 80 mg/m(2) and higher, the median IAUC post-ZD6126 treatment was reduced in all of the tumors studied (8 tumors, 6 patients) to 36-72% from the baseline value. There was a significant trend of increasing reductions with increasing exposure (P < 0.01). No drug-induced changes in human muscle or spleen IAUC were apparent. The reproducibility of the median IAUC parameter was investigated in patients. In 19 human tumors (measured in 19 patients) inter- and intratumor coefficients of variation were 64 and 18%.

CONCLUSIONS

The contrast enhanced-MRI measured median IAUC is a useful end point for quantifying ZD6126 antivascular effects in human tumors.

Molecular imaging using magnetic resonance: new tools for the development of tumour therapy

Molecular imaging - the exploitation of specific molecules as the source of image contrast - promises new insights into disease processes in the laboratory and since the imaging modalities employed are applicable clinically, can be used to translate this knowledge into new diagnostics and treatments in the clinic. This brief review focuses on the use of MR-based molecular imaging techniques for developing tumour therapy. As examples, methods for detecting drug-induced tumour cell apoptosis; the response of tumours and their susceptibilities to an antivascular drug; early signs of tumour immune rejection and methods for detecting immune cell infiltration of tumours are described.

Measuring tumour vascular response to antivascular and antiangiogenic drugs

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

Photodynamic therapy of established prostatic adenocarcinoma with TOOKAD: a biphasic apparent diffusion coefficient change as potential early MRI response marker

The goal of this study was to examine the use of diffusion-weighted magnetic resonance imaging (DW-MRI) for the assessment of early progression of photodamage induced by Pd-bacteriopheophorbide (TOOKAD)-based photodynamic therapy (PDT). TOOKAD is a novel second-generation photosensitizer for PDT of solid tumors developed in our laboratory and presently under clinical trials for prostate cancer (PC) therapy. Using the subcutaneous human prostate adenocarcinoma WISH-PC14 xenografts in nude mice as a model, a unique biphasic change in the apparent diffusion coefficient (ADC) was observed within the first 24 hours post-PDT, with initial decrease followed by an increase in ADC. Using DW-MRI, this phenomenon enables the detection of successful tumor response to PDT within 7 hours posttreatment. This process was validated by direct, histological, and immunohistochemical examinations and also by evaluation of serum prostate-specific antigen (PSA) levels that decreased significantly already 7 hours posttreatment. In vitro studies of multicellular cell spheroids confirmed a PDT-induced decrease in ADC, suggesting that lipid peroxidation (LPO) significantly contributes to ADC decline observed after PDT. These results demonstrate that TOOKAD-based PDT successfully eradicates prostate adenocarcinoma xenografts and suggests DW-MRI to be useful for the detection of early tumor response and treatment outcome in the clinical setting.

Vascular targeting agents as cancer therapeutics

Vascular targeting agents (VTAs) for the treatment of cancer are designed to cause a rapid and selective shutdown of the blood vessels of tumors. Unlike antiangiogenic drugs that inhibit the formation of new vessels, VTAs occlude the pre-existing blood vessels of tumors to cause tumor cell death from ischemia and extensive hemorrhagic necrosis. Tumor selectivity is conferred by differences in the pathophysiology of tumor versus normal tissue vessels (e.g., increased proliferation and fragility, and up-regulated proteins). VTAs can kill indirectly the tumor cells that are resistant to conventional antiproliferative cancer therapies, i.e., cells in areas distant from blood vessels where drug penetration is poor, and hypoxia can lead to radiation and drug resistance. VTAs are expected to show the greatest therapeutic benefit as part of combined modality regimens. Preclinical studies have shown VTA-induced enhancement of the effects of conventional chemotherapeutic agents, radiation, hyperthermia, radioimmunotherapy, and antiangiogenic agents. There are broadly two types of VTAs, small molecules and ligand-based, which are grouped together, because they both cause acute vascular shutdown in tumors leading to massive necrosis. The small molecules include the microtubulin destabilizing drugs, combretastatin A-4 disodium phosphate, ZD6126, AVE8062, and Oxi 4503, and the flavonoid, DMXAA. Ligand-based VTAs use antibodies, peptides, or growth factors that bind selectively to tumor versus normal vessels to target tumors with agents that occlude blood vessels. The ligand-based VTAs include fusion proteins (e.g., vascular endothelial growth factor linked to the plant toxin gelonin), immunotoxins (e.g., monoclonal antibodies to endoglin conjugated to ricin A), antibodies linked to cytokines, liposomally encapsulated drugs, and gene therapy approaches. Combretastatin A-4 disodium phosphate, ZD6126, AVE8062, and DMXAA are undergoing clinical evaluation. Phase I monotherapy studies have shown that the agents are tolerated with some demonstration of single agent efficacy. Because efficacy is expected when the agents are used with conventional chemotherapeutic drugs or radiation, the results of Phase II combination studies are eagerly awaited.

Combretastatin A4 phosphate

Combretastatin A4 phosphate (CA4P) is a water-soluble prodrug of combretastatin A4 (CA4). The vascular targeting agent CA4 is a microtubule depolymerizing agent. The mechanism of action of the drug is thought to involve the binding of CA4 to tubulin leading to cytoskeletal and then morphological changes in endothelial cells. These changes increase vascular permeability and disrupt tumor blood flow. In experimental tumors, anti-vascular effects are seen within minutes of drug administration and rapidly lead to extensive ischemic necrosis in areas that are often resistant to conventional anti-cancer treatments. Following single-dose administration a viable tumor rim typically remains from which tumor regrowth occurs. When given in combination with therapies targeted at the proliferating viable rim, enhanced tumor responses are seen and in some cases cures. Results from the first clinical trials have shown that CA4P monotherapy is safe and reduces tumor blood flow. There has been some promising demonstration of efficacy. CA4P in combination with cisplatin is also safe. Functional imaging studies have been used to aid the selection of doses for phase II trials. Both dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and positron emission tomography can measure the anti-vascular effects of CA4P in humans. This review describes the background to the development of CA4P, its proposed mechanism of action, the results from the first clinical trials with CA4P and the role of imaging techniques in its clinical development.

Microdensitometry of T2-weighted magnetic resonance (MR) images from patients with advanced neoplasias in a phase I clinical trial of an infrared pulsed laser device (IPLD)

BACKGROUND AND OBJECTIVE

The aim of this study was to determine whether an infrared pulsed laser device (IPLD)-induced pathophysiologic changes could be identified before measurable modifications in tumor volume.

STUDY DESIGN/PATIENTS AND METHODS

Pre-and post-IPLD treatment magnetic resonance (MR) images of tumor heterogeneities and peritumoral tissues were digitized and a linear transformation was performed to convert images to 256 intensity levels. Data were analyzed by using the Student's t-test and the Kolmogorov-Sminov test (alpha = 0.05).

RESULTS

The post-treatment mean intensity values of tumor heterogeneities increased significantly (P < 0.001) for all of the seven patients (n = 7) evaluated. For peritumoral tissues, a significant increase (P < 0.001) was measured in four patients (n = 4). The Kolmogorov-Sminov test showed significant values for the tumor tissue of six (n = 6) patients.

CONCLUSION

This is the first study of early evidence of anti-cancer activity of a novel IPLD showing a significant increase in the water content of tumor heterogeneities before measurable changes in tumor volume.

Structural, functional, and molecular MR imaging of the microvasculature

Magnetic resonance imaging (MRI) is widely applied for functional imaging of the microcirculation and for functional and structural studies of the microvasculature. The interest in the capabilities of MRI in noninvasively monitoring changes in vascular structure and function expanded over the past years, with specific efforts directed toward the development of novel imaging methods for quantification of angiogenesis. Molecular imaging approaches hold promise for further expansion of the ability to characterize the microvasculature. Exciting applications for MRI are emerging in the study of the biology of microvessels and in the evaluation of potential pharmaceutical modulators of vascular function and development, and preclinical MRI tools can serve for the design of mechanism-of-action-based noninvasive clinical methods for monitoring response to therapy. The aim of this review is to provide a current snapshot of recent developments in this rapidly evolving field.