Clinical aspects of a phase I trial of 5,6-dimethylxanthenone-4-acetic acid (DMXAA), a novel antivascular agent

Antibody drug-conjugates targeting the tumor vasculature: Current and future developments

Reducing the blood supply of tumors is one modality to combat cancer. Monoclonal antibodies are now established as a key therapeutic approach for a range of diseases. Owing to the ability of antibodies to selectively target endothelial cells within the tumor vasculature, vascular targeting programs have become a mainstay in oncology drug development. However, the antitumor activity of single agent administration of conventional anti-angiogenic compounds is limited and the improvements in patient survival are most prominent in combinations with chemotherapy. Furthermore, prolonged treatment with conventional anti-angiogenic drugs is associated with toxicity and drug resistance. These circumstances provide a strong rationale for novel approaches to enhance the efficacy of mAbs targeting tumor vasculature such as antibody-drug conjugates (ADCs).Here, we review trends in the development of ADCs targeting tumor vasculature with the aim of informing future research and development of this class of therapeutics.

Insights into the in vitro antitumor mechanism of action of a new pyranoxanthone

Naturally occurring xanthones have been documented as having antitumor properties, with some of them presently undergoing clinical trials. In an attempt to improve the biological activities of dihydroxyxanthones, prenylation and other molecular modifications were performed. All the compounds reduced viable cell number in a leukemia cell line K-562, with the fused xanthone 3,4-dihydro-12-hydroxy-2,2-dimethyl-2H,6H-pyrano[3,2-b]xanthen-6-one (5) being the most potent. The pyranoxanthone 5 was particularly effective in additional leukemia cell lines (HL-60 and BV-173). Furthermore, the pyranoxanthone 5 decreased cellular proliferation and induced an S-phase cell cycle arrest. In vitro, the pyranoxanthone 5 increased the percentage of apoptotic cells which was confirmed by an appropriate response at the protein level (e.g., PARP cleavage). Using a computer screening strategy based on the structure of several anti- and pro-apoptotic proteins, it was verified that the pyranoxanthone 5 may block the binding of anti-apoptotic Bcl-xL to pro-apoptotic Bad and Bim. The structure-based screening revealed the pyranoxanthone 5 as a new scaffold that may guide the design of small molecules with better affinity profile for Bcl-xL.

Collateral damage: toxic effects of targeted antiangiogenic therapies in ovarian cancer

First-line chemotherapy fails in more than 20% of patients with epithelial ovarian cancer and about 40-50% of women who respond to initial treatment relapse within 2 years. In the recurrent setting, second-line chemotherapeutic agents have a 15-20% response rate with no cures. Fortunately, clinical investigations that have assessed the efficacy of new, biologically targeted therapies have reinvigorated therapeutic options for patients living with ovarian and other malignancies. In view of the fact that ovarian cancer is one of the most angiogenic neoplasms, there is great hope that implementing targeted agents with antiangiogenic properties will improve outcomes. However, as experience grows with the antitumour activity of these drugs, new toxic effects are emerging. The effects of antiangiogenic agents on molecules and processes that also have physiologically important roles in healthy tissues are at the crux of these toxic effects, or "collateral damage". This review discusses the leading toxic effects encountered and anticipated in clinical investigation and practice with antiangiogenic agents in patients with ovarian cancer, with particular focus on potential management strategies.

The development of the tumor vascular-disrupting agent ASA404 (vadimezan, DMXAA): current status and future opportunities

IMPORTANCE OF THE FIELD

Targeting tumor vasculature with antiangiogenic agents improves outcomes achieved with chemotherapy in some cancers, but toxicity limits their applicability. Tumor vascular-disrupting agents (tumor-VDAs) induce an acute collapse in tumor vascular supply; ASA404 (vadimezan, 5,6-dimethylxanthenone-4-acetic acid [DMXAA]) is the tumor-VDA most advanced in clinical development. Recent randomized trials of ASA404 in combination with chemotherapy suggested a survival advantage in NSCLC comparable to that achieved with bevacizumab, but with little additional toxicity. Phase III trials in advanced NSCLC have completed accrual, and a review of this exciting agent is timely.

AREAS COVERED IN THIS REVIEW

This review focuses on the development of ASA404 to date, its mechanisms of action, the current body of clinical research and potential avenues for therapeutic use. It includes all completed clinical trials since it entered clinical testing in 1995 through to 2009.

WHAT THE READER WILL GAIN

This review will help the reader to understand why ASA404 is unique among tumor-VDAs; the clinical trial methodology required to evaluate such agents; and its remarkable potential clinical utility.

TAKE HOME MESSAGE

ASA404 is a tumor-VDA that offers considerable potential to improve outcomes in cancer patients in combination with existing treatments.

Phase II study of ASA404 (vadimezan, 5,6-dimethylxanthenone-4-acetic acid/DMXAA) 1800mg/m(2) combined with carboplatin and paclitaxel in previously untreated advanced non-small cell lung cancer

This single-arm phase II study evaluated the tumor-vascular disrupting agent ASA404 (vadimezan, 5,6-dimethylxanthenone-4-acetic acid/DMXAA) 1800mg/m(2) plus standard therapy of carboplatin and paclitaxel in patients with advanced non-small cell lung cancer (NSCLC). This ASA404 dose is 50% higher than that used in previous phase II studies. Thirty patients with histologically confirmed stage IIIb or IV NSCLC previously untreated with chemotherapy received carboplatin AUC 6mg/mlmin plus paclitaxel 175mg/m(2) plus ASA404 1800mg/m(2) every 21 days for up to six cycles. The addition of ASA404 1800mg/m(2) to standard therapy produced little change in the systemic exposure of either total or free carboplatin or paclitaxel, and was generally well-tolerated, with no cardiac serious adverse events or clinically relevant ophthalmic abnormalities. The best overall tumor response was partial response, which was seen in 37.9% of patients by independent assessment and in 46.7% by investigator assessment. Stable disease was seen in 48.3% of patients by independent assessment and in 43.3% by investigator assessment. Median time to tumor progression was 5.5 months by investigator assessment and median survival was 14.9 months. The data from this trial corroborate findings from a recent randomized phase II trial, which suggested improvements in efficacy variables, including survival, when ASA404 1200mg/m(2) was added to standard therapy for advanced NSCLC. The manageable safety profile, lack of adverse pharmacokinetic interactions and efficacy outcomes seen in this single-arm study suggest that ASA404 1800mg/m(2) is a viable dose for future combination studies.

Randomised phase II study of ASA404 combined with carboplatin and paclitaxel in previously untreated advanced non-small cell lung cancer

ASA404 (5,6-dimethylxanthenone-4-acetic acid or DMXAA) is a small-molecule tumour-vascular disrupting agent (Tumour-VDA). This randomised phase II study evaluated ASA404 plus standard therapy of carboplatin and paclitaxel in patients with histologically confirmed stage IIIb or IV non-small cell lung cancer (NSCLC) not previously treated with chemotherapy. Patients were randomised to receive </=6 cycles of carboplatin area under the plasma concentration-time curve 6 mg ml(-1) min and paclitaxel 175 mg m(-2) (CP, n=36) or standard therapy plus ASA404 1200 mg m(-2) (ASA404-CP, n=37). There was little change in the systemic exposure of either total or free carboplatin or paclitaxel on addition of ASA404. Safety profiles were similar and manageable in both groups, with most adverse effects attributed to standard therapy. Tumour response rate (31 vs 22%), median time to tumour progression (5.4 vs 4.4 months) and median survival (14.0 vs 8.8 months, hazard ratio 0.73, 95% CI 0.39, 1.38) were improved in the ASA404 combination group compared with the standard therapy group. In conclusion, this study establishes the feasibility of combining ASA404 with carboplatin and paclitaxel in patients with previously untreated, advanced NSCLC, demonstrating a manageable safety profile and lack of adverse pharmacokinetic interactions. The results indicate that there may be a benefit associated with ASA404, but this needs to be evaluated in a larger trial.

Selective targeting of the tumour vasculature

Selective targeting of the tumour vasculature in the treatment of solid organ malignancies is an alternative to conventional chemotherapy treatment. As the tumour progressively increases in size, angiogenesis or the formation of new vasculature is essential to maintain the tumour's continual growth and survival. Therefore disrupting this angiogenic process or targeting the neovasculature can potentially hinder or prevent further tumour expansion. Many anti angiogenic agents have been investigated with many currently in clinical trials and exhibiting varied results. Vascular disrupting agents such as the Combretastatins and OXi 4503 have shown promising preclinical results and are currently being examined in clinical trials.

The potential of DMXAA (ASA404) in combination with docetaxel in advanced prostate cancer

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is a vascular disrupting agent that has demonstrated efficacy in combination with taxane-based chemotherapy in patients with advanced cancer. Complementary modes of action, a lack of pharmacokinetic interaction and distinct adverse effect profiles provide a strong rationale for combining these anticancer agents. In a Phase II trial in men with hormone refractory prostate cancer, DMXAA (ASA404) in combination with docetaxel achieved a prostate-specific antigen response in more patients than docetaxel therapy alone, and was generally well tolerated. Further clinical evaluation of this combination in this patient population is warranted.

Phase I clinical evaluation of ZD6126, a novel vascular-targeting agent, in patients with solid tumors

BACKGROUND

ZD6126 is a novel vascular-targeting agent that disrupts the endothelial tubulin cytoskeleton causing selective occlusion of tumor vasculature and extensive tumor necrosis. This Phase I clinical study was conducted to evaluate the dose and administration schedule of ZD6126.

METHODS

Adult patients with solid tumors refractory to existing treatments received a 10-min, single-dose intravenous infusion of ZD6126 every 14 or 21 days. Subsequent dose escalation was performed, based on the incidence of adverse events (AEs) within the first cycle of drug administration. Blood samples were obtained for pharmacokinetic analysis, and the effects of ZD6126 on tumor vasculature were visualized using DCE-MRI technology.

RESULTS

Forty-four patients received ZD6126 (5-112 mg/m2 in the 21-day schedule, n=35; 40-80 mg/m2 in the 14-day schedule, n=9). Common AEs were similar in both groups and included abdominal pain, nausea and vomiting, which appeared to be dose related. The incidence of abdominal pain at 112 mg/m2 in the 21-day study prevented further dose escalation. Pharmacokinetic studies confirmed that ZD6126 is rapidly hydrolyzed to ZD6126 phenol. There was no difference in the pharmacokinetics of ZD6126 phenol upon repeat administration or between the two dosing regimens. DCE-MRI evaluation has demonstrated the antivascular effects of ZD6126.

CONCLUSIONS

This study identified that ZD6126 administered every 2 or 3 weeks at 80 mg/m2 was well tolerated, with mild but manageable gastrointestinal AEs. In approximately 11% (5 out of 44) of patients, ZD6126 was associated with cardiac events categorized as dose limiting toxicities (one patient with asymptomatic decreased left ventricular ejection fraction (LVEF), two with increased troponin concentrations, one with myocardial ischemia, and one with ECG signs of myocardial ischemia).

Early experience with novel immunomodulators for cancer treatment

Immunotherapy involves the treatment of cancer by modification of the host-tumour relationship. It is now known that this relationship is quite complex and only some of the interactions have been elucidated. Early attempts at immunotherapy, such as Coley's toxins, were undertaken without an understanding of the processes mediating the effects. With a better understanding of the immunology of this anticancer response, recent trials have focussed on certain aspects of the process to stimulate an antitumour response. In this review, the authors discuss a number of novel biological response modifiers that work as general stimulants of the immune system, through varied mechanisms including induction of stimulatory cytokines (such as IFN-alpha, TNF-alpha and IL-12) and activation of T cells and the antigen-presenting dendritic cells. These compounds include Toll-like receptor agonists, several of which are in clinical trials at present. In addition to immunomodulatory activity, some compounds such as 5,6-dimethylxanthenone-4-acetic acid (DMXAA) and thalidomide and its analogues also target existing or developing tumour vasculature. Some of these compounds have single-agent activity in clinical trials, while others such as DMXAA have shown promise in combination with chemotherapy without increasing toxicity. Lactoferrin is another compound that has shown clinical activity with low toxicity. At present, accepted indications for immunotherapy are limited to a few cancers such as renal cell carcinoma and melanoma. This paper looks at some of the reasons for the limited impact of immunotherapy so far and suggest possible avenues for further research with a greater likelihood of success.

The vascular disrupting agent, DMXAA, directly activates dendritic cells through a MyD88-independent mechanism and generates antitumor cytotoxic T lymphocytes

5,6-Di-methylxanthenone-4-acetic acid (DMXAA) is a small molecule in the flavanoid class that has antitumor activity. Although classified as a "vascular disrupting agent," we have recently conducted studies showing that DMXAA has remarkable efficacy in a range of tumors, working primarily as an immune modulator that activates tumor-associated macrophages and induces a subsequent CD8(+) T-cell-mediated response. To more completely analyze the effect of DMXAA on CD8(+) T-cell generation, we treated mice bearing tumors derived from EG7 thymoma cells that express the well-characterized chicken ovalbumin neotumor antigen. Treatment with DMXAA led to cytokine release, tumor cell necrosis, and ultimately reduction in tumor size that was lymphocyte dependent. Within 24 h of administration, we observed dendritic cell activation in tumor-draining lymph nodes (TDLN). This was followed by a rapid and marked increase in the number of tetramer-specific CD8(+) T cells in the spleens of treated animals. In contrast, the vascular disrupting agent combretastatin A4-phosphate, which caused a similar amount of immediate tumor necrosis, did not activate dendritic cells, nor induce an effective antitumor response. Using in vitro systems, we made the observation that DMXAA has the ability to directly activate mouse dendritic cells, as measured by increased expression of costimulatory molecules and proinflammatory cytokine release via a pathway that does not require the Toll-like receptor adaptor molecule MyD88. DMXAA thus has the ability to activate tumor-specific CD8(+) T cells through multiple pathways that include induction of tumor cell death, release of stimulatory cytokines, and direct activation of dendritic cells.

The chemotherapeutic agent DMXAA potently and specifically activates the TBK1-IRF-3 signaling axis

Vascular disrupting agents (VDAs) represent a novel approach to the treatment of cancer, resulting in the collapse of tumor vasculature and tumor death. 5,6-dimethylxanthenone-4-acetic acid (DMXAA) is a VDA currently in advanced phase II clinical trials, yet its precise mechanism of action is unknown despite extensive preclinical and clinical investigations. Our data demonstrate that DMXAA is a novel and specific activator of the TANK-binding kinase 1 (TBK1)–interferon (IFN) regulatory factor 3 (IRF-3) signaling pathway. DMXAA treatment of primary mouse macrophages resulted in robust IRF-3 activation and ∼750-fold increase in IFN-β mRNA, and in contrast to the potent Toll-like receptor 4 (TLR4) agonist lipopolysaccharide (LPS), signaling was independent of mitogen-activated protein kinase (MAPK) activation and elicited minimal nuclear factor κB–dependent gene expression. DMXAA-induced signaling was critically dependent on the IRF-3 kinase, TBK1, and IRF-3 but was myeloid differentiation factor 88–, Toll–interleukin 1 receptor domain–containing adaptor inducing IFN-β–, IFN promoter-stimulator 1–, and inhibitor of κB kinase–independent, thus excluding all known TLRs and cytosolic helicase receptors. DMXAA pretreatment of mouse macrophages induced a state of tolerance to LPS and vice versa. In contrast to LPS stimulation, DMXAA-induced IRF-3 dimerization and IFN-β expression were inhibited by salicylic acid. These findings detail a novel pathway for TBK1-mediated IRF-3 activation and provide new insights into the mechanism of this new class of chemotherapeutic drugs.

Development and validation of a liquid chromatography/tandem mass spectrometry method for the determination of DMXAA in human and mouse plasma

A rapid, sensitive, and specific LC/MS/MS-based method was developed for determining the concentration of DMXAA in human and mouse plasma. Sample preparation involved a single protein precipitation step using acetonitrile. Separation of DMXAA and 6-isopropoxy-9-oxoxanthene-2-carboxylic acid, the internal standard, was achieved on a Waters X-Terra C(18) (50 mm x 2.1mm i.d., 3.5 microm) analytical column using a mobile phase consisting of acetonitrile/10 mM ammonium acetate (55:45, v/v) containing 0.1% formic acid and isocratic flow at 0.2 mL/min for 3 min. The analytes were monitored by tandem mass spectrometry with electrospray positive ionization. Linear calibration curves were generated over the range of 5-3000 ng/mL. The values for precision and accuracy were <9.6%, except at the LLOQ (5 ng/mL) level, which was within 16.8%. Recovery of DMXAA in mouse plasma was >65%. DMXAA was stable through 2 freeze/thaw cycles, to 2h in mouse plasma or 50% acetonitrile, and on the autosampler to 5.1h. This method was subsequently used to measure concentrations of DMXAA in mice following intraperitoneal administration.

Consequences of increased vascular permeability induced by treatment of mice with 5,6-dimethylxanthenone-4-acetic acid (DMXAA) and thalidomide

PURPOSE

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) (AS1404), a small-molecule vascular disrupting agent currently in clinical trial, increases vascular permeability and decreases blood flow in both murine and human tumours. DMXAA induces tumour necrosis factor (TNF) in mice and the effects on vascular permeability are hypothesised to result from both direct (DMXAA) and indirect (TNF) effects. Skin temperature decreases in mice treated with high doses of DMXAA, raising the question of whether host toxicity is mediated by the induction of increased vascular permeability in normal tissue. Thalidomide is an anti-inflammatory agent that potentiates the anti-tumour activity of DMXAA but decreases induction of TNF in plasma. We wished to determine how it potentiated the effects of DMXAA.

METHODS

Vascular permeability was measured in Colon 38 tumour and liver tissue by uptake of Evans Blue dye. Blood haematocrit and body temperature were also measured.

RESULTS

Tumour vascular permeability was increased following administration of DMXAA (25 mg/kg i.p.), minimally affected following thalidomide (100 mg/kg i.p.) but strongly increased following co-administration of both drugs. In contrast, dye uptake into liver tissue was decreased following administration of DMXAA, thalidomide or both drugs. Administration of DMXAA at a potentially toxic dose (35 mg/kg i.p. or 50 mg/kg orally) was found to decrease body temperature and to increase the blood haematocrit, while administration of thalidomide alone (100 mg/kg i.p.) had no effect. Co-administration of thalidomide potentiated the effects of DMXAA on both body temperature and haematocrit but surprisingly did not increase toxicity.

CONCLUSIONS

The results are consistent with the hypothesis that the host toxicity of high-dose DMXAA is mediated by effects on host vasculature. Co-administration of thalidomide increases the effective dose of DMXAA by reducing clearance but also, by inhibiting production of circulating TNF, reduces the host toxicity of DMXAA.

Vascular damaging agents

To provide a comprehensive overview on vascular targeting agents and the application of radiobiological principles in pre-clinical and clinical studies, we completed a comprehensive review of published medical studies on vascular targeting agents using Pub Med. Vascular targeting agents are now divided into vascular disrupting agents (VDAs), which target the pre-existing tumour vasculature, and angiogenesis inhibitors (AIs), which prevent the formation of new blood vessels. Modest success has been seen when VDAs and AIs are used as single agents and therefore combination therapies that can work in a complimentary and synergistic manner, targeting both the tumour cells and endothelial cells, are needed. Radiobiological principles have been used to increase our understanding of these agents, and can explain the increased efficacy of combination treatments. In particular, the alteration of the tumour microenvironment by AIs and VDAs can lead to enhanced efficacy when combined with chemotherapy or radiotherapy, with phase II/III trials showing encouraging results. The optimal use and scheduling of AIs and VDAs remains to be determined. Further understanding of the mechanisms of action of these potentially very exciting anti-neoplastic agents is urgently required.

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.

Drug insight: vascular disrupting agents and angiogenesis--novel approaches for drug delivery

Vascular disrupting agents (VDAs), or endothelial disrupting agents, attempt to exploit the vascular endothelium that supplies rapidly dividing neoplasms. Unlike antiangiogenesis agents (e.g. the monoclonal antibody bevacizumab; and tyrosine kinase inhibitors sorafenib and sunitinib) that disrupt endothelial cell survival mechanisms and the development of a new tumor blood supply, VDAs are designed to disrupt the already established abnormal vasculature that supports tumors, by targeting their dysmorphic endothelial cells. Tumor vascular endothelium is characterized by its increased permeability, abnormal morphology, disorganized vascular networks, and variable density. VDAs induce rapid shutdown of tumor blood supply, causing subsequent tumor death from hypoxia and nutrient deprivation. The safety profile of this class of compounds is more indicative of agents that are indeed 'vascularly' active. For example, VDAs can cause: acute coronary and other thrombophlebitic syndromes; alterations in blood pressure, heart rate, and ventricular conduction; transient flush and hot flashes; neuropathy; and tumor pain. Despite these cardiovascular concerns some patients have benefited from VDAs in early clinical trials. Further drug development of VDAs must include the combination of these agents with other novel biological agents, cytotoxic chemotherapy, and radiotherapy. Close monitoring of patients receiving VDAs for any cardiovascular toxicity is imperative.

Evidence for the involvement of p38 MAP kinase in the action of the vascular disrupting agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA)

AIMS

DMXAA (AS1404), a small-molecule vascular disrupting agent that has now completed Phase II clinical trial, induces endothelial cell apoptosis, increased vascular permeability and decreased tumour blood flow in vivo. Its action is incompletely understood and we wished to develop an in vitro system to study its effects.

METHODS

Human tumour cell lines developed from aggressive tumours were grown on Matrigel to simulate a tumour microenvironment. Cells were analysed by light microscopy and by gene expression profiling.

RESULTS

Several cell lines formed networks when grown on Matrigel and the NZM7 melanoma cell line was chosen for further study. Addition of DMXAA at a clinically achievable concentration (30 microg/mL) prevented network formation, but co-addition of SB203580 (10 microM), a selective inhibitor of p38 MAP kinase, reversed the effect of DMXAA and restored network formation. Analysis of expression genes for endothelial and related functions showed that cells growing on Matrigel expressed a pattern similar to that of NZM7 cells growing as xenografts in vivo but different from that of cells grown on standard tissue culture plates. Addition of DMXAA resulted in the inhibition of expression of several genes including the transcriptional activator Ets1 and matrix metalloproteinase-2 (MMP2), but co-addition of SB203580 did not reverse these effects of DMXAA on gene expression.

CONCLUSION

The results suggest that p38 MAP kinase plays an important role in the action of DMXAA and that growth of tumour cells on Matrigel provides a promising model for further studies on the action of this drug.

5,6-Dimethylxanthenone-4-acetic acid in the treatment of refractory tumors: a phase I safety study of a vascular disrupting agent

This phase I safety study aimed to identify the optimal dose of the vascular disrupting agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA) for combination studies. Using a crossover design, 15 patients with refractory tumors were allocated randomly to receive six sequential doses of DMXAA (300, 600, 1,200, 1,800, 2,400, and 3,000 mg m(-2)), each given once-weekly as a 20-minute i.v. infusion. The drug was generally well tolerated. Transient, moderate increases in the heart rate-corrected cardiac QT interval occurred at the two highest doses. DMXAA produced transient dose-dependent increases in blood pressure. Transient, dose-related visual disturbances occurred at the two highest doses. No significant changes in K(trans) and k(ep) were observed but V(e), a secondary dynamic contrast-enhanced magnetic resonance imaging variable, increased significantly after giving DMXAA. At 1,200 mg m(-2), the Cmax and the area under the concentration-time curve over 24 hours for total and free DMXAA plasma concentrations were 315 +/- 25.8 microg/mL, 29 +/- 6.4 microg/mL x d, 8.0 +/- 1.77 microg/mL, and 0.43 +/- 0.07 microg/mL x d, respectively. Plasma levels of the vascular damage biomarker 5-hydroxyindoleacetic acid increased in the 4 hours after treatment in a dose-dependent fashion up to 1,200 mg m(-2), with a plateau thereafter. Doses in the range of 1,200 mg m(-2) have been selected for further studies (phase II combination studies with taxanes and platins are under way) because this dose produced no significant effect on heart rate-corrected cardiac QT interval, produced near maximum levels of 5-hydroxyindoleacetic acid, achieved DMXAA plasma concentrations within the preclinical therapeutic range, and was well tolerated.

Pharmacokinetics of 5,6-dimethylxanthenone-4-acetic acid (AS1404), a novel vascular disrupting agent, in phase I clinical trial

PURPOSE

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) (AS1404) is a novel antitumour agent that selectively disrupts tumour vasculature and induces cytokines. The purpose of this study was to determine the pharmacokinetics (PK) of DMXAA in cancer patients enrolled in a phase I clinical trial.

METHODS

DMXAA was administered as a 20-min i.v. infusion every 3 weeks and doses were escalated in cohorts of patients according to a predefined schema. PK samples were taken over the first 24 h of at least the first cycle.

RESULTS

DMXAA was administered to 63 patients at 19 dose levels from 6 to 4,900 mg m(-2), and 3,700 mg m(-2) was established as the maximum tolerated dose. The PK observed over the dose range showed a non-linear fall in clearance from 16.1 to 1.42 l h(-1) m(-2) and resultant increase in the area under the concentration-time curve (AUC) from 1.29 to 12,400 microM h. In contrast, the increase in peak plasma concentrations from 2.17 to 1,910 microM approximated linearity. DMXAA was highly protein-bound to albumin (>99%) until saturation occurred at higher doses, leading to a rapid increase in the free fraction (up to 20%) and greater concentrations of DMXAA bound to non-albumin proteins. However, the main determinant of the non-linearity of the PK appeared to be sequential saturation of elimination mechanisms, which include hydroxylation, glucuronidation and perhaps hepatic transport proteins. This resulted in an exaggerated non-linear increase in free DMXAA plasma concentrations and AUC compared to total drug.

CONCLUSIONS

The PK of DMXAA are well-defined, with a consistent degree of non-linearity across a very large dose range.

Antitumour action of 5,6-dimethylxanthenone-4-acetic acid in rats bearing chemically induced primary mammary tumours

PURPOSE

To evaluate the antitumour activity of 5,6-dimethylxanthenone-4-acetic acid (DMXAA), a vascular disrupting agent currently under phase II clinical trials in combination with cancer chemotherapy, in rats bearing chemically induced primary mammary tumours.

METHODS

Tumours were induced in female Wistar rats by injection of N-nitroso-N-methylurea at 100 mg/kg subcutaneously. A clinically relevant single dose of DMXAA (1,800 mg/m(2)) was given to animals when tumours were measurable. Tumour volume, extent of necrosis and cytokine profiles were measured.

RESULTS

Compared with the control group, DMXAA treatment significantly delayed tumour doubling time and extended the time from treatment to euthanasia. Four of five DMXAA-treated animals showed necrosis involving 3.7-41.2% of the area of the tumour section at 24 h compared with none of four control animals (P < 0.028, Chi-square test). Intratumoural levels of TNFalpha, IL-6, VEGF and IL-1alpha were increased 4 h after DMXAA treatment.

CONCLUSIONS

This study shows for the first time that DMXAA has significant in vivo antitumour activity against non-transplanted autochthonous tumours and in a host species other than the mouse.

Novel vascular targeting/disrupting agents: combretastatin A4 phosphate and related compounds

Novel anticancer compounds are being developed that attempt to exploit the unique properties of the vascular endothelium, which supplies rapidly dividing neoplasms. The goal of these vascular targeting agents (VTAs) or endothelial disrupting agents is to cause rapid shutdown of tumor blood supply with subsequent tumor death from hypoxia and nutrient deprivation. VTAs are classified into two broad categories: biologic therapies or small molecule compounds. A variety of VTAs are in early clinical development. These agents have demonstrated clinical activity in phase I trials and are being evaluated with cytotoxic chemotherapy and radiotherapy.

Activation of tumor-associated macrophages by the vascular disrupting agent 5,6-dimethylxanthenone-4-acetic acid induces an effective CD8+ T-cell-mediated antitumor immune response in murine models of lung cancer and mesothelioma

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is a small molecule in the flavanoid class that has antitumor activity thought to be due to ability to induce high local levels of tumor necrosis factor (TNF)-alpha that disrupt established blood vessels within tumors. The drug has completed phase 1 testing in humans and is currently in phase 2 trials in combination with chemotherapy. Although characterized as a "vascular disrupting agent," there are some studies suggesting that DMXAA also has effects on the immune system that are important for its efficacy. The goal of this study was to carefully define the immune effects of DMXAA in a series of murine lung cancer and mesothelioma cell lines with varying immunologic characteristics. We show that DMXAA efficiently activated tumor-associated macrophages to release a variety of immunostimulatory cytokines and chemokines, including TNF-alpha; IFN-inducible protein-10; interleukin-6; macrophage inflammatory protein-2; monocyte chemotactic protein-1; and regulated on activation, normal T-cell expressed, and secreted. DMXAA treatment was highly effective in both small and large flank tumors. Animals cured of tumors by DMXAA generated a systemic memory response and were resistant to tumor cell rechallenge. DMXAA treatment led to initial tumor infiltration with macrophages that was followed by an influx of CD8(+) T cells. These CD8(+) T cells were required for antitumor efficacy because tumor inhibitory activity was lost in nude mice, mice depleted of CD8(+) T cells, and perforin knockout mice, but not in CD4(+) T-cell-depleted mice. These data show that activation of tumor-associated macrophages by DMXAA is an efficient way to generate a CD8(+) T-cell-dependent antitumor immune response even in animals with relatively nonimmunogenic tumors. Given these properties, DMXAA might also be useful in boosting other forms of immunotherapy.

Vascular disrupting agents

It has been well established that a functioning vascular supply is essential for solid tumor growth and metastases. In the absence of a viable vascular network, tumors are unable to grow beyond a few millimeters and therefore remain dormant. Initiation of angiogenesis allows for continued tumor growth and progression. Targeting tumor vasculature, either by inhibiting growth of new tumor blood vessels (antiangiogenic agents) or by destroying the already present tumor vessels (vascular disrupting agents; VDA), is an area of extensive research in the development of new antitumor agents. These two groups differ in their direct physiological target, the type or extent of disease that is likely to be susceptible, and the treatment schedule. VDAs target the established tumor blood vessels, resulting in tumor ischemia and necrosis. These agents show more immediate effects compared to antiangiogenic agents and may have more efficacy against advanced bulky disease. VDAs can be divided into two groups--ligand-bound and small molecule agents. Both VDA groups have demonstrated antitumor effects and tumor core necrosis, but consistently leave a thin rim of viable tumor cells at the periphery of the tumor. More evidence suggests VDAs will have their greatest effect in combination with conventional chemotherapy or other modes of treatment that attack this outer rim of cells.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

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.

Maintaining the cold chain shipping environment for Phase I clinical trial distribution

The study aimed to demonstrate satisfactory inter-UK transit of cold storage clinical trial material. The product environment had to be maintained between 0 and 8 degrees C throughout transit until delivery. Straightforward, low cost and simplified shipping arrangements were sought that would be appropriate for small-scale Phase I clinical trial activities. A laboratory test defined an optimal three frozen gel pack configuration to maintain refrigerated environmental conditions for dummy product packs in a single type and size of insulated shipper. The internal environment was temperature monitored at 30-min intervals in all tests. Twelve Glasgow to London transits were then studied over 2 years to include all seasonal temperature variations. A configuration using three frozen gel packs and 4 h pre-chill of the transit container maintained the internal environment at 0-8 degrees C for up to 48 h during autumn, winter and spring. A modified four frozen gel pack configuration was suitable for summer transit. Thus cold shipment verification was successfully carried out for a small-scale distribution operation. It was proven that refrigerated shipping conditions could be maintained using a straightforward and cost effective 'passive' type system consisting of frozen gel packs and insulated transit containers.

Transport of the investigational anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid and its acyl glucuronide by human intestinal Caco-2 cells

5,6-Dimethylxanthenone-4-acetic acid (DMXAA), a potent cytokine inducer, exhibited marked antitumor activity when given as multiple oral doses in mice. The aim of this study was to examine the transport of DMXAA and its acyl glucuronide (DMXAA-G) using the human Caco-2 cells. DMXAA was minimally metabolized by Caco-2 cells and both DMXAA and DMXAA-G were taken up to a minor extent by the cells. The permeability coefficient (Papp) values of DMXAA over 10-500 microM were 4x10(-5) cm/s to 4.3x10(-5) cm/s for both apical (AP) to basolateral (BL) and BL-AP transport, while the Papp values for the BL to AP flux of DMXAA-G were significantly greater than those for the AP to BL flux, with Rnet values of 4.5-17.6 over 50-200 microM. The BL to AP active efflux of DMXAA-G followed Michaelis-Menten kinetics, with a Km of 83.5+/-5.5 microM, and Vmax of 0.022+/-0.001 nmol/min. The flux of DMXAA-G was energy and Na+-dependent and MK-571 significantly (P<0.05) inhibited its BL to AP flux, with an estimated Ki of 130 microM. These data indicate that the transport of DMXAA across Caco-2 monolayers was through a passive process, whereas the transport of DMXAA-G was mediated by MRP1/2.

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.

Tumor Dose Response to the Vascular Disrupting Agent, 5,6-Dimethylxanthenone-4-Acetic Acid, Using In vivo Magnetic Resonance Spectroscopy

PURPOSE

To use (31)P and (1)H magnetic resonance spectroscopy (MRS) to assess changes in tumor metabolic profile in vivo in response to 5,6-dimethylxanthenone-4-acetic acid (DMXAA) with a view to identifying biomarkers associated with tumor dose response.

EXPERIMENTAL DESIGN

In vivo (31)P and (1)H MRS measurements of (a) tumor bioenergetics [beta-nucleoside triphosphate/inorganic phosphate (beta-NTP/Pi)], (b) the membrane-associated phosphodiesters and phosphomonoesters (PDE/PME), (c) choline (mmol/L), and (d) lactate/water ratio were made on murine HT29 colon carcinoma xenografts pretreatment and 6 or 24 hours posttreatment with increasing doses of DMXAA. Following in vivo MRS, the tumors were excised and used for high-resolution (31)P and (1)H MRS of extracts to provide validation of the in vivo MRS data, histologic analysis of necrosis, and high-performance liquid chromatography.

RESULTS

Both beta-NTP/Pi and PDE/PME decreased in a dose-dependent manner 6 hours posttreatment with DMXAA, with significant decreases in beta-NTP/Pi with 15 mg/kg (P < 0.001) and 21 mg/kg (P < 0.01). A significant decrease in total choline in vivo was found 24 hours posttreatment with 21 mg/kg DMXAA (P < 0.05); this was associated with a significant reduction in the concentration of the membrane degradation products glycerophosphoethanolamine and glycerophosphocholine measured in tissue extracts (P < 0.05).

CONCLUSIONS

The reduction in tumor energetics and membrane turnover is consistent with the vascular-disrupting activity of DMXAA. (31)P MRS revealed tumor response to DMXAA at doses below the maximum tolerated dose for mice. Both (31)P and (1)H MRS provide biomarkers of tumor response to DMXAA that could be used in clinical trials.

Mechanisms of tumor vascular shutdown induced by 5,6-dimethylxanthenone-4-acetic acid (DMXAA): Increased tumor vascular permeability

The novel vascular targeting agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA) has completed phase 1 clinical trial and has shown tumor antivascular activity in both mice and humans. We have investigated its ability to change tumor vascular permeability, relating it to tumor vascular perfusion and other responses. The murine colon 38 adenocarcinoma was grown in C57Bl wild-type mice and mice lacking expression of either tumor necrosis factor receptor-1 (TNFR1(-/-)) or TNF (TNF-/-). Tumor vascular permeability, as measured by extravasation of albumin-Evans Blue complexes 4 hr after DMXAA treatment, was significantly increased in tumor tissue in C57Bl, TNFR1-/- and TNF-/- mice but not in normal (skin) tissue. Significant linear relationships were found between increased tumor vascular permeability, decreased functioning tumor blood vessels (measured by Hoechst 33342 staining at 4 hr), increased plasma 5-hydroxyindole-3-acetic acid concentrations (as a measure of serotonin release by platelets) and the degree of induced tumor hemorrhagic necrosis. The results support the hypothesis that DMXAA increases tumor vascular permeability both directly and through the induction of other vasoactive mediators, including TNF. DMXAA might be useful clinically to potentiate the vascular permeability of other anticancer modalities such as cytotoxic drugs, antibodies, drug conjugates and gene therapy.

Anti-vascular tumor therapy: recent advances, pitfalls and clinical perspectives

Anti-vascular tumor therapy represents a promising new strategy for cancer treatment. Anti-vascular treatment may be divided in anti-angiogenic and vascular targeting therapy. Whereas anti-angiogenic drugs aim on the inhibition of new vessel formation, vascular targeting compounds are designed to selectively destruct preexisting tumor blood vessels leading to secondary tumor cell death. Both anti-angiogenic drugs and vascular targeting agents have proven effective anti-tumoral activity in numerous preclinical studies over the last decade. In vivo, a combination with anti-vascular tumor therapy enhances the effects of other treatment modalities as chemo- and radiotherapy. Phase I clinical studies revealed a number of well-tolerated candidates. As monotherapy, however, anti-angiogenic treatment lacked efficacy in randomized clinical studies so far. In contrast, combination of anti-angiogenic therapy with chemotherapy was highly effective in an encouraging, large randomized phase III trial on metastatic colorectal cancer. This review will outline recent advances in the preclinical and clinical development of anti-vascular therapy with focus on vascular targeting. Conceptual differences between anti-angiogenic and vascular targeting therapies will be discussed with emphasis on specific problems and pitfalls in the conversion into the clinic.

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.

Determination of the investigational anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid and its acyl glucuronide in Caco-2 monolayers by liquid chromatography with fluorescence detection: application to transport studies

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is a potent cytokine inducer, with a bioavailability of >70% in the mouse. The aim of this study was to develop and validate HPLC methods for the determination of DMXAA and DMXAA acyl glucuronide (DMXAA-G) in the human intestinal cell line Caco-2 monolayers. The developed HPLC methods were sensitive and reliable, with acceptable accuracy (85-115% of true values) and precision (intra- and inter-assay CV < 15%). The total running time was within 6.8 min, with acceptable separation of the compounds of interest. The limit of quantitation (LOQ) values for DMXAA and DMXAA-G were 14.2 and 24 ng/ml, respectively. The validated HPLC methods were applied to examine the epithelial transport of DMXAA and DMXAA-G by Caco-2 monolayers. The permeability coefficient (Papp) values (overall mean +/- S.D., n = 3-9) of DMXAA over 10-500 microM were independent of concentration for both apical (AP) to basolateral (BL) (4.0 +/- 0.4 x 10(-5)cm/s) and BL-AP (4.3 +/- 0.5 x 10(-5)cm/s) transport, and of similar magnitude in either direction, with net efflux ratio (Rnet) values of 1-1.3. However, the Papp values for the BL to AP transport of DMXAA-G were significantly greater than those for the AP to BL transport, with Rnet values of 17.6, 6.7 and 4.5 at 50, 100 and 200 microM, respectively. Further studies showed that the transport of DMXAA-G was Na+- and energy-dependent, and inhibited by MK-571 [a multidrug resistance associated protein (MRP) 1/2 inhibitor], but not by verapamil and probenecid. These data indicate that the HPLC methods for the determination of DMXAA and DMXAA-G in the transport buffer were simple and reliable, and the methods have been applied to the transport study of both compounds by Caco-2 monolayers. DMXAA across Caco-2 monolayers was through a passive transcellular process, whereas the transport of DMXAA-G was mediated by MRP1/2.

Lack of neurotoxicity of the vascular targeting agent ZD6126 following repeated i.v. dosing in the rat

The vascular targeting agent ZD6126 is a water-soluble prodrug of N-acetylcolchinol that acts by disrupting the cytoskeleton of tumor endothelial cells. It is currently undergoing clinical evaluation in man. As peripheral neuropathy is a major dose-limiting toxicity associated with tubulin binding agents, the neurotoxic potential of ZD6126 was investigated in male and female Wistar rats. ZD6126 was administered i.v. at up to maximum tolerated doses using subacute (0 to 20 mg/kg/d for 5 days) and chronic (0 to 10 mg/kg/d for 5 days, repeated monthly for 6 months) dosing regimens. A separate study examined a combination of ZD6126 (three cycles of ZD6126 given as in the chronic dosing regimen) and paclitaxel (12 mg/kg/wk for 9 weeks) to assess whether coadministration of ZD6126 altered the time course or magnitude of a paclitaxel-induced neuropathy. Neurotoxic potential was examined using a comprehensive series of tests including a functional observation battery, measurements of muscle strength (forelimb and hind limb grip strength), nociception (tail flick test), locomotor activity, neuropathology, and whole nerve electrophysiology. There was no evidence that ZD6126 induced neurotoxicity in the rat following either subacute or chronic i.v. dosing. In a chronic electrophysiology study, ZD6126 produced a slight slowing of the maturational increase of caudal nerve amplitude, with some evidence of reversibility. However, this was not associated with any changes in caudal nerve conduction velocity, motor nerve conduction velocity or amplitude, functional observation battery behavioral and function parameters (including no effects on tail flick latency), and neuropathology. As expected, paclitaxel administration was associated with a significant decrease in caudal nerve conduction velocity (P = 0.0001). Coadministration of ZD6126 did not increase the neurotoxicity of paclitaxel. These studies suggest that ZD6126 should not induce the peripheral neuropathy associated with other antitubulin chemotherapeutic agents and that ZD6126 may not exacerbate the neurotoxicity of other agents with dose-limiting neuropathies.

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.

Vascular-targeting therapies for treatment of malignant disease

BACKGROUND

Tumor endothelium represents a valuable target for cancer therapy. The vasculature plays a critical role in the survival and continued growth of solid tumor masses; in addition, the inherent differences between tumor blood vessels and blood vessels associated with normal tissue make the tumor vasculature a unique target on which to base the design of novel therapeutics, which may allow highly selective treatment of malignant disease. Therapeutic strategies that target and disrupt the already formed vessel networks of growing tumors are actively being pursued. The goal of these approaches is to induce a rapid and catastrophic shutdown of the vascular function of the tumor so that blood flow is arrested and tumor cell death due to the resulting oxygen and nutrient deprivation and buildup of waste products occurs.

METHODS

Biologic approaches and small-molecule drugs that can be used to damage tumor vasculature have been identified. Physiologic, histologic/morphologic, and immunohistochemical assessments have demonstrated that profound disruption of the tumor vessel network can be observed minutes to hours after treatment. The small-molecule agents that have made the greatest advances in the clinical setting (5,6-dimethylxanthenone-4-acetic acid [DMXAA], combretastatin A4 disodium phosphate [CA4DP], and ZD6126) are the focus of the current review.

RESULTS

Loss of patent blood vessels, decreased tumor blood flow, extensive necrosis, and secondary ischemia-induced tumor cell death have been well documented in a variety of preclinical tumor models treated with agents such as DMXAA, CA4DP, and ZD6126. The use of such agents in conjunction with irradiation and other chemotherapeutic agents has led to improved treatment outcomes.

CONCLUSIONS

The targeting of tumors' supportive blood vessel networks could lead to improvements in cancer cure rates. It is likely that this approach will prove to be most efficacious when used in concert with conventional treatment strategies.

Vascular targeting agents

The role of the vascular network of a tumor has been the focus of much recent research. Angiogenesis, or the growth of new tumor blood vessels, was initially the main target in the development of novel antitumor agents. More recently, new therapeutic strategies have been designed to destroy established tumor blood vessels. These vascular targeting agents (VTAs) exert their action by producing a rapid shutdown of tumor blood flow, resulting in ischemia and tumor cell necrosis. VTAs can be broadly divided into biologic agents and small molecules. In contrast to the biologic agents, drug-based vascular targeting molecules have developed much further, with many clinical trials ongoing. Evidence suggests that VTAs may be useful as single agents but can be more effective when used in combination with other therapeutic regimens.

Relationship between tumour endothelial cell apoptosis and tumour blood flow shutdown following treatment with the antivascular agent DMXAA in mice

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) is currently undergoing clinical evaluation as an antivascular agent for the treatment of cancer. We have previously demonstrated that DMXAA induces apoptosis of vascular endothelial cells in murine tumour sections and in a breast carcinoma biopsy from one patient in a Phase I trial. We wished to determine the tissue selectivity of this effect and its relationship to induced blood flow changes. Mice with Colon 38 tumours were treated with DMXAA and tissues were examined for apoptosis by TdT-mediated dUTP nick-end labelling (TUNEL). Hoechst 33342 was used to stain functional vessels, with the loss of stained vessels used as a measure of tumour vascular collapse. Treatment with DMXAA at 25 mg kg⁻¹, its maximum tolerated dose (MTD), showed, after 3 h, a 12-fold increase in TUNEL staining of tumour vascular endothelial cells. In contrast, tissue from the heart, brain, liver and spleen showed no increase. Induction of apoptosis in tumour tissue was both dose-dependent, observable at doses as low as 5 mg kg⁻¹, and time-dependent. Apoptosis was significantly lower in Colon 38 tumours of mice, with a targeted disruption in the TNF gene (TNF^−/−), or in the TNF receptor 1 gene (TNFR^−/−), as compared with that in wild-type mice. Increasing the DMXAA dose to 50 mg kg⁻¹ in these knockout mice raised tumour apoptosis to a level comparable to that induced in wild-type mice given DMXAA at the MTD. For all the data, a significant correlation (r=0.94; P<0.001) was found between logarithmic percentage apoptosis induction and the logarithmic density of Hoechst-stained vessels. These results suggest that blood flow inhibition caused by DMXAA is tumour tissue-specific and is a consequence of induction of apoptosis in tumour vascular endothelial cells.

Induction of tumour necrosis factor and interferon-γ in cultured murine splenocytes by the antivascular agent DMXAA and its metabolites

The induction of haemorrhagic necrosis by 5,6-dimethylxanthenone-4-acetic acid (DMXAA) in transplantable murine tumours depends on the in situ synthesis of cytokines, particularly tumour necrosis factor (TNF). Since the in vivo action of DMXAA would be greatly clarified by the development of an in vitro model, we investigated whether DMXAA could induce cytokines in cultured murine splenocytes. DMXAA alone induced low amounts of TNF with an optimal concentration of 10 microg/mL and an optimal time of 4 hr. When combined with low concentrations of lipopolysaccharide, deactivated-lipopolysaccharide (dLPS) or phorbol-12-myristate-13-acetate that did not elicit TNF production alone, synergistic TNF production was obtained. DMXAA also induced interferon-gamma at an optimal dose of 300 microg/mL, but the addition of dLPS had no further effect. Decreasing culture pH, although not changing the optimal concentrations for stimulation, increased both TNF and interferon-gamma production in response to DMXAA. The major DMXAA metabolites, DMXAA-glucuronide and 6-hydroxy-5-methylxanthenone-4-acetic acid, did not induce either cytokine alone, in combination with dLPS or at low pH. The results indicate that DMXAA rather than a metabolite is responsible for cytokine induction and suggest that the microenvironment of the tumour may be responsible for the observed selective induction of cytokines in tumour tissue.