Measurement of plasma 5-hydroxyindoleacetic acid as a possible clinical surrogate marker for the action of antivascular agents

Serum and plasma 5-hydroxyindoleacetic acid as an alternative to 24-h urine 5-hydroxyindoleacetic acid measurement

BACKGROUND

Neuroendocrine tumours are slow growing tumours known to secrete a variety of vasoactive peptides which give rise to symptoms of the carcinoid syndrome. The diagnosis and monitoring of patients with neuroendocrine tumours is undertaken in many centres using 24 h urinary measurement of 5-hydroxyindoleacetic acid. However, 5-hydroxyindoleacetic acid can also be quantified in plasma and serum.

METHODS

We measured 5-hydroxyindoleacetic acid concentration in 134 paired EDTA plasma and urine samples from 108 patients with known neuroendocrine tumours and 26 healthy volunteers. We also compared 5-hydroxyindoleacetic acid concentrations in paired serum and plasma samples (n = 63), then analysed paired urine and serum samples (n = 97). Furthermore, we examined the impact of renal impairment on serum 5-hydroxyindoleacetic acid by analysing 5-hydroxyindoleacetic acid in patients without neuroendocrine tumours in different stages of chronic kidney disease, as indicated by the estimated glomerular filtration rate.

RESULTS

Plasma and urine 5-hydroxyindoleacetic acid had very similar diagnostic sensitivities and specificities, with areas under the curve on ROC analysis of 0.917 and 0.920, respectively. Serum and plasma 5-hydroxyindoleacetic acid values showed good correlation but serum results demonstrated a positive bias, indicating the necessity for different serum and plasma reference intervals. There was an inverse correlation between estimated glomerular filtration rate and serum 5-hydroxyindoleacetic acid concentration, with 5-hydroxyindoleacetic acid increasing once the estimated glomerular filtration rate falls below 60 mL/min/1.73 m(2).

CONCLUSION

The measurement of both serum and plasma 5-hydroxyindoleacetic acid can be used for the diagnosis and monitoring of patients with neuroendocrine tumours. Provided renal function is taken into consideration, either of these tests should be incorporated into standard practice as an alternative assay to urinary 5-hydroxyindoleacetic acid.

Preliminary Evidence That High-Dose Vitamin C has a Vascular Disrupting Action in Mice

High intravenous doses of vitamin C (ascorbic acid) have been reported to benefit cancer patients, but the data are controversial and there is incomplete knowledge of what physiological mechanisms might be involved in any response. Vitamin C is taken up efficiently by cells expressing SVCT2 transporters and since vascular endothelial cells express SVCT2, we explored the hypothesis that administration of high-dose vitamin C (up to 5 g/kg) to mice might affect vascular endothelial function. A single administration of vitamin C to mice induced time- and dose-dependent increases in plasma concentrations of the serotonin metabolite 5-hydroxyindole acetic acid (5-HIAA), a marker for vascular disrupting effects. Responses were comparable to those for the tumor vascular disrupting agents, vadimezan and fosbretabulin. High-dose vitamin C administration decreased tumor serotonin concentrations, consistent with the release of serotonin from platelets and its metabolism to 5-HIAA. High-dose vitamin C also significantly increased the degree of hemorrhagic necrosis in tumors removed after 24 h, and significantly decreased tumor volume after 2 days. However, the effect on tumor growth was temporary. The results support the concept that vitamin C at high dose increases endothelial permeability, allowing platelets to escape and release serotonin. Plasma 5-HIAA concentrations could provide a pharmacodynamic biomarker for vitamin C effects in clinical studies.

An update on the clinical development of drugs to disable tumor vasculature

Traditional methods of improving cancer therapy have focused primarily on achieving increased tumor cell kill. However, more recent strategies involve impairing the nutritional support system of the tumor by targeting the tumor vasculature. Rapid developments in this field in recent years have resulted in the identification of a variety of potential targets and a large number of investigational drugs, many of which are now in clinical development. In the following paper the authors review vascular disrupting therapies.

Phase I/II study of the tumour-targeting human monoclonal antibody-cytokine fusion protein L19-TNF in patients with advanced solid tumours

PURPOSE

L19-TNF is an armed antibody that selectively targets human TNF to extra domain B-fibronectin on tumour blood vessels. We performed a phase I/II first-in-man trial with L19-TNF monotherapy in metastatic solid cancer patients to study safety and signs of clinical activity.

METHODS

Six cohorts of patients were treated with increasing (1.3-13 μg/kg) doses of intravenous L19-TNF on day 1, 3, and 5 of repeated 3-weekly cycles, and 12 colorectal cancer patients were treated at 13 μg/kg. PK, antibody formation, changes in lymphocyte subsets, 5-HIAA plasma levels as well as safety and clinical activity were analysed.

RESULTS

Thirty-four patients received at least one L19-TNF dose. The serum half-life of L19-TNF at 13 μg/kg was 33.6 min, and maximum peak serum concentration was 73.14 μg/L. Mild chills, nausea and vomiting but no haemato- or unexpected toxicity were observed. Grade 3 lumbar pain in bone metastasis was the only dose-limiting toxicity found in one patient. Objective tumour responses were not detected. Transient stable disease occurred in 19 of 31 evaluable patients.

CONCLUSIONS

Intravenous L19-TNF on day 1, 3, and 5 of a 3-weekly schedule was safe up to 13 μg/kg, but did not result in objective tumour responses. The maximally tolerated dose (MTD) was not reached, allowing for further dose escalation of L19-TNF possibly in combination with chemotherapy.

Temporal aspects of the action of ASA404 (vadimezan; DMXAA)

IMPORTANCE OF THE FIELD

Tumor vascular disrupting agents (tumor VDAs) act by selective induction of tumor vascular failure. While their action is distinct from that of antiangiogenic agents, their clinical potential is likely to reside in improving the efficacy of combination therapy.

AREAS COVERED IN THIS REVIEW

This review describes the preclinical development, clinical trial and mode of action of ASA404, a flavonoid class tumor VDA. This class has a unique dual action, simultaneously disrupting vascular endothelial function and stimulating innate tumor immunity. This review covers the early development of ASA404, through to Phase III trial.

WHAT THE READER WILL GAIN

The reader will gain insight into the sequence of ASA404-induced changes in tumor tissue. Early events include increased vascular permeability, increased endothelial apoptosis and decreased blood flow, while later effects include the induction of serotonin, tumor necrosis factor, other cytokines and chemokines, and nitric oxide. This cascade of events induces sustained reduction of tumor blood flow, induction of tumor hypoxia and increased inflammatory responses. The reader will also gain an appreciation of how the potentiation of radiation and chemotherapeutic effects by ASA404 in murine tumors shaped the development of combination clinical trials.

TAKE HOME MESSAGE

Although there are species differences in ASA404 activity, many features of its action in mice translate to human studies. The future of ASA404 as an effective clinical agent will rely on the development of an appreciation of its ability to optimize the complex interaction between tumor vasculature and tumor immunity during therapy.

The unique characteristics of tumor vasculature and preclinical evidence for its selective disruption by Tumor-Vascular Disrupting Agents

The vasculature of solid tumors is fundamentally different from that of normal vasculature and offers a unique target for anti-cancer therapy. Direct vascular-targeting with Tumor-Vascular Disrupting Agents (Tumor-VDAs) is distinctly different from anti-angiogenic strategies, and offers a complementary approach to standard therapies. Tumor-VDAs therefore have significant potential when combined with chemotherapy, radiotherapy, and angiogenesis-inhibiting agents. Preclinical studies with the different Tumor-VDA classes have demonstrated key tumor-selective anti-vascular and anti-tumor effects.

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.

Vascular targeted therapies in oncology

Neovascularization is intimately involved in tumor survival, progression, and spread, factors known to contribute significantly to treatment failures. Thus, strategies targeting the tumor blood vessel support network may offer not only unique therapeutic opportunities in their own right, but also novel means of enhancing the efficacies of conventional anticancer treatments. This article reviews one such therapeutic approach directed at the tumor blood vessel support network. Vascular disrupting therapies seek the destruction of the established neovasculature of actively growing tumors. The goal of these therapies is to cause a rapid and catastrophic shutdown in the vascular function of the tumor in order to arrest the blood flow and produce tumor cell death as a result of oxygen and nutrient deprivation and the build up of waste products.

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.

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

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

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.

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.

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

The antitumour action of 5,6-dimethylxanthenone-4-acetic acid (DMXAA) is mediated through tumour-selective antivascular effects and cytokine induction. This clinical phase I trial was conducted to examine its toxicity, maximum tolerated dose, pharmacokinetics (PK) and pharmacodynamics (PD). A secondary objective was to assess its antitumour efficacy. DMXAA was administered every 3 weeks as a 20-min i.v. infusion. Dose escalation initially followed a modified Fibonacci schema but was also guided by PK and toxicity. A total of 63 patients received 161 courses of DMXAA over 19 dose levels ranging from 6 to 4900 mg m(-2). DMXAA was well tolerated at lower doses and no drug-related myelosuppression was seen. Rapidly reversible dose-limiting toxicities were observed at 4900 mg m(-2), including confusion, tremor, slurred speech, visual disturbance, anxiety, urinary incontinence and possible left ventricular failure. Transient prolongation of the corrected cardiac QT interval was seen in 13 patients evaluated at doses of 2000 mg m(-2) and above. A patient with metastatic cervical carcinoma achieved an unconfirmed partial response at 1100 mg m(-2), progressing after eight courses. The results of PK and PD studies are reported separately. DMXAA has antitumour activity at well-tolerated doses.

5,6-dimethylxanthenone-4-acetic acid (DMXAA), a novel antivascular agent: phase I clinical and pharmacokinetic study

The purpose of this phase I, dose-escalation study was to determine the toxicity, maximum tolerated dose, pharmacokinetics, and pharmacodynamic end points of 5,6-dimethylxanthenone acetic acid (DMXAA). In all, 46 patients received a total of 247 infusions of DMXAA over 15 dose levels ranging from 6 to 4900 mg x m(-2). The maximum tolerated dose was established at 3700 mg x m(-2); dose-limiting toxicities in the form of urinary incontinence, visual disturbance, and anxiety were observed at the highest dose level (4900 mg x m(-2)). The pharmacokinetics of DMXAA were dose dependent. Peak concentrations and area under the curve level increased from 4.8 microM and 3.2 microM h, respectively, at 6 mg x m(-2) to 1290 microM and 7600 microM h at 3700 mg x m(-2), while clearance declined from 7.4 to 1.7 l h(-1) x m(-2) over the same dose range. The terminal half-life was 8.1+/-4.3 h. More than 99% of the drug was protein bound at doses up to 320 mg x m(-2); at higher doses the percent free drug increased to a maximum of 6.9% at 4900 mg x m(-2). Dose-dependent increases in the serotonin metabolite 5-hydroxyindoleacetic acid were observed at dose levels of 650 mg x m(-2) and above. There was one unconfirmed partial response at 1300 mg x m(-2). In conclusion, DMXAA is a novel vascular targeting agent and is well tolerated.

Antivascular therapy of cancer: DMXAA

The vascular endothelium of tumour tissue, which differs in several ways from that of normal tissues, is a potential target for selective anticancer therapy. By contrast with antiangiogenic agents, antivascular agents target the endothelial cells of existing tumour blood vessels, causing distortion or damage and consequently decreasing tumour blood flow. DMXAA (5,6-dimethylxanthenone-4-acetic acid), a low-molecular-weight drug, has a striking antivascular and in some cases curative effect in experimental tumours. Its action on vascular endothelial cells seems to involve a cascade of events leading to induction of tumour haemorrhagic necrosis. These events include both direct and indirect effects, the latter involving the release of further vasoactive agents, such as serotonin, tumour necrosis factor, other cytokines, and nitric oxide from host cells. Phase I clinical trials of DMXAA have been completed and the next challenge to face is how the antivascular effect of this drug should be exploited for the treatment of human cancer.

DMXAA: an antivascular agent with multiple host responses

PURPOSE

To measure host responses to the antivascular agent DMXAA (5,6-dimethylxanthenone-4-acetic acid) and to compare them with those of other antivascular agents.

METHODS

Induction of tumor necrosis was measured in s.c. murine Colon 38 carcinomas growing in normal or tumor necrosis factor (TNF) receptor-1 knockout mice. Plasma and tumor tissue TNF concentrations were measured by ELISA. Plasma concentrations of 5-hydroxyindoleacetic acid (as a measure of serotonin release) and nitrite (as a measure of nitric oxide release) were measured by high-performance liquid chromatography.

RESULTS

Administration of DMXAA to tumor-bearing mice increased plasma and tumor tissue-associated TNF, in addition to increasing plasma nitric oxide, distinguishing its action from that of mitotic poisons that had an antivascular action. Results from TNF receptor-1 knockout mice showed that TNF played an important role in both its antitumor action and its host toxicity. Release of serotonin occurred in response to mitotic poisons, as well as to DMXAA. CONCLUCIONS: The antivascular action of DMXAA involves in situ production in tumor tissue of a cascade of vasoactive events, including a direct effect on vascular endothelial cells and indirect vascular effects involving TNF, other cytokines, serotonin, and nitric oxide. Now that Phase I clinical trials of DMXAA are completed, the optimization of this cascade in cancer patients is a major challenge. Plasma 5-hydroxyindoleacetic acid concentrations may provide a useful surrogate marker for the antivascular effects of DMXAA and other antivascular agents.

The antitumour activity of 5,6-dimethylxanthenone-4-acetic acid (DMXAA) in TNF receptor-1 knockout mice

5,6-dimethylxanthenone-4-acetic acid, a novel antivascular anticancer drug, has completed Phase I clinical trial. Its actions in mice include tumour necrosis factor induction, serotonin release, tumour blood flow inhibition, and the induction of tumour haemorrhagic necrosis and regression. We have used mice with a targeted disruption of the tumour necrosis factor receptor-1 gene as recipients for the colon 38 carcinoma to determine the role of tumour necrosis factor signalling in the action of 5,6-dimethylxanthenone-4-acetic acid. The pharmacokinetics of 5,6-dimethylxanthenone-4-acetic acid, as well as the degree of induced plasma and tissue tumour necrosis factor, were similar in tumour necrosis factor receptor-1(-/-) and wild-type mice. However, the maximum tolerated dose of 5,6-dimethylxanthenone-4-acetic acid was considerably higher in tumour necrosis factor receptor-1(-/-) mice (>100 mg kg(-1)) than in wild-type mice (27.5 mg kg(-1)). The antitumour activity of 5,6-dimethylxanthenone-4-acetic acid (25 mg kg(-1)) was strongly attenuated in tumour necrosis factor receptor-1(-/-) mice. However, the reduced toxicity in tumour necrosis factor receptor-1(-/-) mice allowed the demonstration that at a higher dose (50 mg kg(-1)), 5,6-dimethylxanthenone-4-acetic acid was curative and comparable in effect to that of a lower dose (25 mg kg(-1)) in wild-type mice. The 5,6-dimethylxanthenone-4-acetic acid -induced rise in plasma 5-hydroxyindoleacetic acid, used to reflect serotonin production in a vascular response, was larger in colon 38 tumour bearing than in non-tumour bearing tumour necrosis factor receptor-1(-/-) mice, but in each case the response was smaller than the corresponding response in wild-type mice. The results suggest an important role for tumour necrosis factor in mediating both the host toxicity and antitumour activity of 5,6-dimethylxanthenone-4-acetic acid, but also suggest that tumour necrosis factor can be replaced by other vasoactive factors in its antitumour action, an observation of relevance to current clinical studies.