| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Experimental Therapeutics |
Department of Biochemistry, University of Cambridge, Cambridge CB2 1GA [D. A. B., K. M. B.], and Gray Laboratory Cancer Research Trust, Mount Vernon Hospital, Northwood, Middlesex HA6 2JR [S. A. H., D. J. C.], United Kingdom
 |
ABSTRACT |
|---|
 Top ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
|
INTRODUCTION |
|---|
|
TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
20 min (2
, 5)
, and it appears unlikely that this is attributable either to apoptosis of endothelial cells or to antimitotic effects of CA4P, both of which would be likely to exert their effects over a longer timescale. CA4P induces a rapid change in shape in recently divided endothelial cells (2)
, and it may be this which triggers vascular disruption and hemorrhage leading to the death of tumor cells as a secondary event. Low levels of NO have been shown to potentiate the vascular damage induced in tumors by CA4P (6
, 7)
. A reversible increase in vascular resistance is seen in several tissues including spleen, heart, and kidney, after CA4P treatment, but the effects are more marked and, at sufficiently high doses, irreversible in tumors. This may be because tumor vessels are poorly developed and because the tumor interstitial fluid pressure is high (7
, 8) . However, even in those cases where vascular damage induces extensive necrosis, a rim of viable cells can remain at the tumor periphery, from which the tumor can continue to grow, with little effect on growth rate (9)
. For this reason, it has been suggested that antivascular therapies should be combined with conventional therapies, e.g., combination therapy of cisplatin with CA4P has been used to eliminate the viable tumor cells that remain at the tumor periphery following CA4P treatment (9)
. Because CA4P treatment has little effect on tumor growth rate, the action of the drug cannot be detected simply by monitoring tumor volume. For this reason, we implemented noninvasive MRI and MRS experiments to determine the effect of the drug on perfusion in a murine tumor model (5) . Similar MRI experiments are now being used to assess patient response in a Phase I clinical trial (10) . In our earlier study, dynamic inflow experiments with the contrast-enhancing agent GdDTPA indicated that response of the sarcoma F tumor to CA4P was spatially heterogeneous, with some regions showing very large decreases in perfusion. This heterogeneity may be explained by the angiogenic "hot spots" that exist around necrotic regions of tumors and which indicate a heterogeneous distribution of growth factors (11) and, hence, dividing endothelial cells, which may therefore be combretastatin sensitive. In many tumors, the main growth factor involved in stimulating division of endothelial cells is VEGF, which also causes increased vessel permeability (8) . MRI experiments with macromolecular contrast agents like BSA-GdDTPA have been used to identify regions of permeable vasculature within tumors, which may be used as a surrogate marker for VEGF expression (12, 13, 14, 15, 16, 17) . Noninvasive MRI and MRS experiments have been used here to map the permeability of vessels in five tumor models in mice and to determine whether there is a correlation with tumor susceptibility to CA4P.
|
MATERIALS AND METHODS |
|---|
|
TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
10-mm geometric mean diameter; this was 11–16 days (RIF-1 and SaF), 20–30 days (HT29), or 50–60 days (LoVo and SaS) after implantation. Studies were conducted in accordance with a project license issued under the United Kingdom Animals (Scientific Procedures) Act 1986.
Anesthetic, CA4P, and Contrast Agent Preparation.
Anesthesia was induced before beginning the experiments (3.9 mg/kg fentanyl citrate + 125 mg/kg fluanisone + 50 mg/kg midazolam, i.p.) and then maintained throughout (0.2 mg/kg fentanyl citrate + 6.7 mg/kg fluanisone, i.p. every 1.75 h). Animals were placed in a cradle, which held the tumor in the center of the surface coil used for tumor imaging and spectroscopy. A flow of warm air was used to maintain the body temperature. Combretastatin A4, as the disodium combretastatin A4 3-O-phosphate prodrug (kindly provided by G. R. Pettit, Arizona State University, Tempe, AZ; Ref. 20
), was dissolved in sterile saline (0.9% NaCl) and injected i.p. to give 100 mg/kg body weight. BSA-GdDTPA, prepared as described by Ogan et al. (21)
, was administered i.v. through a tail-vein catheter over a period of 30 s, to give 250 mg/kg body weight. GdDTPA (Magnevist, Schering, United Kingdom) was administered i.v. through a tail-vein catheter over a period of 30 s, to give 188 mg/kg body weight.
MRI and MRS.
Experiments were performed in a 9.4 T vertical-bore (8.9-cm diameter) superconducting magnet (Oxford), interfaced with a Varian UnityPlus spectrometer. An unshielded gradient set (Varian) was used with a probe incorporating a two-turn surface coil (20-mm diameter) tunable to frequencies of 400 MHz (1H imaging) and 162 MHz (31P spectroscopy). Data were processed using VNMR 5.3 and Image Browser 6.1 software (both Varian) and software written in C in this laboratory.
Contrast Agent Inflow Experiments.
BSA-GdDTPA or GdDTPA were introduced over 30 s as a bolus via a tail-vein catheter, and inflow into the tumor was monitored with a series of T1-weighted spin-echo images (TE = 12.3 ms, TR = 130 ms), which were collected for 15 min postinjection. For calculation of the paramagnetic contribution to the longitudinal relaxation rate (R1p) from the GdDTPA inflow experiments, an image with TR = 3 s was also acquired (22)
. Images were acquired from a 1-mm thick slice, and the field of view was 20 mm over a 256 x 128 (phase encode) matrix, which was zero-filled to 256 x 256 data points. BSA-GdDTPA inflow experiments were made before treatment with CA4P. Measurements of GdDPTA inflow and washout were carried out on the same animals before treatment with CA4P and 3 h after treatment. The animal was anesthetized throughout this period. Because BSA-GdDTPA concentration in the blood decreases monoexponentially with a half-life of 3 h (23
, 24)
, it was assumed that there were no changes in signal intensity because of changes in concentration of BSA-GdDTPA during the subsequent GdDTPA inflow experiments. The GdDTPA inflow experiment at 3 h was not compromised by the earlier experiment as image intensities returned to baseline levels within 2 h after the first GdDTPA injection. For the range of tissue concentrations of GdDTPA used in the study, the concentration was linearly proportional to relaxivity (data from phantom studies not shown).
Analysis of BSA-GdDTPA Inflow Data.
Vascular permeability was determined according to the pharmacokinetic model described by Su et al. (25)
, in which the signal intensity in each pixel depends on the concentration of contrast agent in the plasma (Cp) and tumor interstitial (Ci) volume fractions (Vp and Vi). The time dependencies of these concentrations are governed by transport constants from the plasma compartment to the interstitial compartment (K1) and vice versa (K2). We made several assumptions which simplified the analysis of our data. The initial concentration of contrast agent in the plasma was assumed to be reached at the time of injection. The concentration of contrast agent in the plasma, Cp, was assumed to remain constant during the course of the experiment, because the half-life for clearance is 3 h (23
, 24)
. As others have done (24)
, we assume that for experiments of this duration using BSA-GdDTPA, the transport constant from the extracellular compartment to plasma, K2, is zero. Indeed, we were not able to obtain values for K2 because the relatively low concentrations of BSA-GdDTPA achieved in the extracellular compartment did not permit an accurate determination. Experimental data were fit to a consequently simplified version of Su et al.’s model, shown in Eq. 1, where D0 = VpCp and D1 = ViCpK1; CT is the total concentration of contrast agent in a given pixel:
 |
A calibration curve relating signal intensity (S) under our experimental conditions to BSA-GdDTPA concentration was calculated using a phantom with properties similar to those of tumor tissue (25) made from Sephadex-100 with BSA-GdDTPA concentrations in the range 0–0.5 mg/ml, ln(S/S0) = a x ln([BSA-GdDTPA] + b) + c, with a = 0.46 ± 0.06, b = 0.024 ± 0.009, and c = 1.71 ± 0.06 (values ± SE; r2 = 0.995), where S0 is signal intensity from the phantom with no added BSA-GdDTPA, and [BSA-GdDTPA] is in mg/ml. A series of 20 spin-echo images acquired during 10 min after administration of BSA-GdDTPA were converted to BSA-GdDTPA concentration maps using this calibration curve. The series of maps were analyzed using the fitting routine within Image Browser 6.1 (Varian) to determine D0 and D1, according to Eq. 1. Poorly fit pixels (those with r2 < 0.6) were not included in the analysis. The GdDTPA inflow measurements showed that some tumors had poorly perfused centers, for which perfusion was not well characterized. Therefore, a 20-pixel-wide band in the periphery of all tumors was used in the analysis.
31P Spectra and Pi/NTP Ratios.
Localized 31P spectra of the tumor and underlying muscle tissue were obtained (5)
. Typically, 128 summed free induction decays, acquired into 11,968 data points, were collected from voxels of dimensions 6 x 12 x 12 mm. Spectra were referenced to the resonance of phosphocreatine at 0 ppm. Areas under peaks were integrated for calculation of Pi/NTP ratios.
Analysis of GdDTPA Inflow Data.
Initial-rate maps were calculated by subtraction of the sum of three images obtained immediately before injection of GdDTPA from the sum of three images collected immediately after injection of GdDTPA (at t = 15, 45, and 75 s). Alteration to perfusion was also assessed by integrating the curve obtained by plotting the mean signal intensity in a 20-pixel-wide band in the tumor periphery against time after injection of contrast agent, for the inflow experiments made before and 3 h after treatment with CA4P. Maps of Tu and Tc, time constants for uptake and clearance of GdDTPA as defined by Kennedy et al. (5
, 22)
, were generated from GdDTPA inflow experiments conducted before and 3 h after CA4P treatment. Briefly, changes in the paramagnetic contribution to the longitudinal relaxation rate (R1p), which is proportional to the concentration of the contrast agent, were fit, on a pixel-by-pixel basis, to Eq. 2
to give Tu and Tc. C0 is a constant (22)
:
 |
|
RESULTS |
|---|
|
TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
3 h after administration of CA4P (Fig. 1)
. The absence of a signal attributable to phosphocreatine in the spectra of RIF-1 tumors indicated that localized spectra were not contaminated with signal from underlying muscle. A significant decrease in levels of total NTP with a concomitant increase in Pi was seen during this period for two tumor lines, LoVo and RIF-1, which were consequently judged susceptible to CA4P (Fig. 1)
. A smaller, but significant, alteration in Pi/NTP levels was seen for the SaS and SaF tumors, which were considered to respond relatively poorly to CA4P, whereas no significant change in Pi/NTP level occurred during this period for the HT29 tumor (Fig. 1)
.
|
showed complete loss of perfusion in as much as 50% of the area of the slices observed in the SaF and RIF-1 tumors. The integral of the curve of mean peripheral pixel intensity after GdDTPA administration showed a significant decrease 3 h after CA4P treatment relative to pretreatment values for LoVo, RIF-1, and SaS but not for SaF and HT29 (P < 0.05, Mann-Whitney test; Table 1
). The kinetics of inflow and outflow of contrast agent were analyzed on a pixel-by-pixel basis to give values of the uptake (Tu) and clearance (Tc) time constants for each tumor type, before and 3 h after tumor treatment (Table 1)
. There was no change in Tu for HT29 tumors, but there was a trend of increased Tu for the other tumor types (the differences in Tu were not statistically significant). Tc was not calculated accurately because the values of the time constant for clearance were greater than the 10-min period of acquisition of images.
|
|
. There were also no changes in perfusion as determined from measurements of GdDTPA inflow (data not shown). This was confirmed for the other mouse strains and tumors used in this study, where a single animal was used in each case. The results for SCID mice (LoVo and HT29 tumors) indicate that if there are any changes in systemic blood pressure, brought about by CA4P administration, their effect on tumor perfusion and energetic status are relatively small when compared with the direct effect of the drug on tumor vasculature.
Vascular volume fraction and permeability, represented by the parameters D0 and D1, respectively (25)
, were obtained from analysis of BSA-GdDTPA inflow data. D1 values were significantly higher for LoVo and RIF-1 tumors than they were for SaS and HT29 tumors (P < 0.05, Mann-Whitney test; Table 1
). D0 values for HT29 tumors were significantly lower than those of LoVo and RIF-1 tumors (P < 0.05, Mann-Whitney test) but not of the other tumors. A correlation was found between vascular permeability, determined as D1, and the response of the five tumor models used in the study to CA4P, measured as Pi/NTP ratio 3 h after treatment (Fig. 3)
.
|
|
DISCUSSION |
|---|
|
TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Vascular permeability was assessed using the pharmacokinetic model of Su et al. (Ref. 25
; Table 1
). The larger D1 values for the LoVo and RIF-1 tumors indicated that the transport constant (K1) was greater in these tumors than in the SaS, SaF, and HT29 tumors, indicative of greater vessel permeability. The D0 value for the HT29 tumor was significantly lower than that of the LoVo and RIF-1 tumors, indicative of lower vascular volume fraction in this tumor, but was not significantly different from that of the SaS and SaF tumors. The variations in D1 and D0 did not correlate with tumor growth rate, as both RIF1 and LoVo tumors had relatively permeable vasculature and relatively high vascular volume fraction, yet grew at rates which were very different (geometric mean diameter of 10 mm was attained 11–16 and 50–60 days after implantation, respectively).
Responsiveness of tumors to treatment was assessed using two criteria. The most sensitive technique for assessing response was found to be 31P MRS. LoVo and RIF-1 were classed as responsive tumors because localized 31P spectroscopy showed a significant increase in Pi/NTP ratio within 3 h (Fig. 1)
, indicating that tumor cells were subject to hypoxia and/or hypoglycemia after systemic treatment with CA4P. A reduced response was seen for SaS and SaF, and no significant changes in Pi/NTP ratio were seen in the HT29 tumor.
The response of tumors to CA4P was also assessed on the basis of changes to perfusion 3 h after treatment. The clearest indicator of altered perfusion was the initial-rate map (Fig. 2)
, which highlighted the parts of the tumor in which perfusion was reduced. This was also evident when the integral of the curve obtained by plotting signal intensity against time during GdDTPA inflow was calculated. This may be taken as a measure of bulk perfusion, and significant decreases were seen for all tumor types, except for HT29. A more complete analysis of the rate constants of uptake and clearance of GdDTPA (22)
indicated that the rate of inflow of GdDTPA (Tu) into all tumors except for HT29 was decreased, 3 h after CA4P treatment (Table 1)
. However, the initial-rate maps illustrate that the vascular disruption caused by CA4P results primarily in loss of perfusion to regions of the tumor, rather than in a reduction of the overall rate of perfusion in the tumor. This was supported by the fact that the increase in Tu in the tumors was not statistically significant. As was the case for 31P MRS, these analyses indicated that LoVo and RIF-1 tumors respond more to CA4P treatment than do SaS, SaF, and HT29 tumors. Whereas the primary result of CA4P treatment is reduced perfusion, as determined using these MRI experiments, the secondary effect of damage to tumor cells is measured in the 31P MRS experiments. A direct relationship between the two assessments of tumor response to CA4P would not necessarily be expected. Furthermore, the 31P MRS experiment interrogates the whole of the tumor, whereas in the imaging experiments performed here, only the selected slice was observed. In the clinic, investigation of alterations to perfusion is likely to be routinely possible, whereas hardware for 31P MRS is less widespread.
The lack of response to CA4P treatment of the HT29 tumor could not be ascribed to properties of the SCID mouse host. Two other tumors were implanted into SCID mice, LoVo and RIF-1, both of which responded well to CA4P treatment. No significant differences were seen between RIF-1 tumors borne by SCID and C3H hosts in terms of perfusion, energetic status, or the responses of these to treatment with CA4P (data not shown).
The correlation between MRI measurements of vessel permeability, which has been shown to be a surrogate marker for VEGF expression (12, 13, 14, 15, 16, 17)
, and the susceptibility of the tumors to CA4P (Fig. 3)
accords with the expectation that angiogenic tumors, with higher levels of endothelial cell proliferation, will be more susceptible to the drug. This correlation is reinforced by the observation that there is also a correlation, albeit weaker, between drug susceptibility and vascular volume. The vascular volume of the unresponsive HT29 tumor was significantly lower then the vascular volumes of the LoVo and RIF-1 tumors, which responded well to the drug. Vascular volume, like permeability, is considered a reasonable surrogate marker of angiogenesis (27)
, and MRI measurements of vascular volume have been shown to correlate with VEGF-induced angiogenesis (14)
. However, the control of vessel permeability is complex, and it is possible that other growth factors may also play an important role (8
, 28) .
These experiments suggest that macromolecular contrast agents could be used in the clinic to assess permeability of tumor vasculature, as a possible indicator of the likelihood of response of tumors to CA4P. Macromolecular contrast agents other than those based on albumin-GdDTPA, e.g., the superparamagnetic iron oxide particle-containing agents (29) , may be suitable for such measurements.
|
ACKNOWLEDGMENTS |
|---|
|
FOOTNOTES |
|---|
1 Supported by the Cancer Research Campaign (Project Grant SP2350/0101). 
2 To whom requests for reprints should be addressed, at Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1GA, United Kingdom.
3 The abbreviations used are: CA4P, combretastatin A4 phosphate; MRI, magnetic resonance imaging; MRS, magnetic resonance spectroscopy; GdDTPA, gadolinium diethylenetriaminepentaacetate; NTP, nucleoside triphosphate; VEGF, vascular endothelial growth factor; SCID, severe combined immunodeficient.
Received 11/27/00. Accepted 7/18/01.
|
REFERENCES |
|---|
|
TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  G. M. Tozer, V. E. Prise, G. Lewis, S. Xie, I. Wilson, and S. A. Hill Nitric Oxide Synthase Inhibition Enhances the Tumor Vascular-Damaging Effects of Combretastatin A-4 3-O-Phosphate at Clinically Relevant Doses Clin. Cancer Res., June 1, 2009; 15(11): 3781 - 3790. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G M Tozer, C Kanthou, G Lewis, V E Prise, B Vojnovic, and S A Hill Tumour vascular disrupting agents: combating treatment resistance Br. J. Radiol., October 1, 2008; 81(Special_Issue_1): S12 - S20. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Strieth, M. E. Eichhorn, A. Werner, B. Sauer, M. Teifel, U. Michaelis, A. Berghaus, and M. Dellian Paclitaxel Encapsulated in Cationic Liposomes Increases Tumor Microvessel Leakiness and Improves Therapeutic Efficacy in Combination with Cisplatin Clin. Cancer Res., July 15, 2008; 14(14): 4603 - 4611. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. M. Tozer, S. Akerman, N. A. Cross, P. R. Barber, M. A. Bjorndahl, O. Greco, S. Harris, S. A. Hill, D. J. Honess, C. R. Ireson, et al. Blood Vessel Maturation and Response to Vascular-Disrupting Therapy in Single Vascular Endothelial Growth Factor-A Isoform-Producing Tumors Cancer Res., April 1, 2008; 68(7): 2301 - 2311. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. Hyodo, K.-i. Matsumoto, A. Matsumoto, J. B. Mitchell, and M. C. Krishna Probing the intracellular redox status of tumors with magnetic resonance imaging and redox-sensitive contrast agents. Cancer Res., October 15, 2006; 66(20): 9921 - 9928. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 F. Chen, X. Sun, F. De Keyzer, J. Yu, R. Peeters, W. Coudyzer, V. Vandecaveye, W. Landuyt, H. Bosmans, P. Van Hecke, et al. Liver Tumor Model with Implanted Rhabdomyosarcoma in Rats: MR Imaging, Microangiography, and Histopathologic Analysis Radiology, May 1, 2006; 239(2): 554 - 562. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. V. Skliarenko, S. J. Lunt, M. L. Gordon, A. Vitkin, M. Milosevic, and R. P. Hill Effects of the Vascular Disrupting Agent ZD6126 on Interstitial Fluid Pressure and Cell Survival in Tumors Cancer Res., February 15, 2006; 66(4): 2074 - 2080. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Seshadri, J. A. Spernyak, R. Mazurchuk, S. H. Camacho, A. R. Oseroff, R. T. Cheney, and D. A. Bellnier Tumor Vascular Response to Photodynamic Therapy and the Antivascular Agent 5,6-Dimethylxanthenone-4-Acetic Acid: Implications for Combination Therapy Clin. Cancer Res., June 1, 2005; 11(11): 4241 - 4250. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. C. Thoeny, F. De Keyzer, F. Chen, Y. Ni, W. Landuyt, E. K. Verbeken, H. Bosmans, G. Marchal, and R. Hermans Diffusion-weighted MR Imaging in Monitoring the Effect of a Vascular Targeting Agent on Rhabdomyosarcoma in Rats Radiology, March 1, 2005; 234(3): 756 - 764. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. L. Evelhoch, P. M. LoRusso, Z. He, Z. DelProposto, L. Polin, T. H. Corbett, P. Langmuir, C. Wheeler, A. Stone, J. Leadbetter, et al. Magnetic Resonance Imaging Measurements of the Response of Murine and Human Tumors to the Vascular-Targeting Agent ZD6126 Clin. Cancer Res., June 1, 2004; 10(11): 3650 - 3657. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. E. Thorpe Vascular Targeting Agents as Cancer Therapeutics Clin. Cancer Res., January 15, 2004; 10(2): 415 - 427. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K M Brindle Molecular imaging using magnetic resonance: new tools for the development of tumour therapy Br. J. Radiol., December 1, 2003; 76(suppl_2): S111 - S117. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Nambu, R. Nambu, M. Melia, and P. A. Campochiaro Combretastatin A-4 Phosphate Suppresses Development and Induces Regression of Choroidal Neovascularization Invest. Ophthalmol. Vis. Sci., August 1, 2003; 44(8): 3650 - 3655. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. L. Anderson, J. T. Yap, M. P. Miller, A. Robbins, T. Jones, and P. M. Price Assessment of Pharmacodynamic Vascular Response in a Phase I Trial of Combretastatin A4 Phosphate J. Clin. Oncol., August 1, 2003; 21(15): 2823 - 2830. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. M. Galbraith, R. J. Maxwell, M. A. Lodge, G. M. Tozer, J. Wilson, N. J. Taylor, J. J. Stirling, L. Sena, A. R. Padhani, and G. J.S. Rustin Combretastatin A4 Phosphate Has Tumor Antivascular Activity in Rat and Man as Demonstrated by Dynamic Magnetic Resonance Imaging J. Clin. Oncol., August 1, 2003; 21(15): 2831 - 2842. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Cancer Research | Clinical Cancer Research |
| Cancer Epidemiology Biomarkers & Prevention | Molecular Cancer Therapeutics |
| Molecular Cancer Research | Cancer Prevention Research |
| Cancer Prevention Journals Portal | Cancer Reviews Online |
| Annual Meeting Education Book | Meeting Abstracts Online |
, n = 1 control tumors except SaF where n = 6; error bars are ± 1 SD) for five tumor models. For the combretastatin-susceptible tumors, LoVo (i) and RIF-1 (ii), the signals from the 
-, ß-, and 
-phosphates of NTP fall, and the signal from Pi rises. Pi/NTP is not affected to the same extent in SaS (iii) and SaF (iv) tumors, and no significant change occurs for HT29 (v). No significant changes to Pi/NTP ratio are seen in control experiments. Significant differences between the ratio of Pi/NTP before treatment and after treatment: *, P < 0.05; Mann-Whitney test. PME = phosphate monoesters, predominantly phosphocholine and phosphoethanolamine 
