Characterisation of tumour blood flow using a 'tissue-isolated' preparation

Direct arterial injection of hyperpolarized 13 C-labeled substrates into rat tumors for rapid MR detection of metabolism with minimal substrate dilution

PURPOSE

A rat model was developed to enable direct administration of hyperpolarized 13 C-labeled molecules into a tumor-supplying artery for magnetic resonance spectroscopy (MRS) studies of tumor metabolism.

METHODS

Rat P22 sarcomas were implanted into the right inguinal fat pad of BDIX rats such that the developing tumors received their principle blood supply directly from the right superior epigastric artery. Hyperpolarized 13 C-molecules were either infused directly to the tumor through the epigastric artery or systemically through the contralateral femoral vein. Spectroscopic data were obtained on a 7 Tesla preclinical scanner.

RESULTS

Intra-arterial infusion of hyperpolarized 13 C-pyruvate increased the pyruvate tumor signal by a factor of 4.6, compared with intravenous infusion, despite an approximately 7 times smaller total dose to the rat. Hyperpolarized glucose signal was detected at near-physiological systemic blood concentration. Pyruvate to lactate but not glucose to lactate metabolism was detected in the tumor. Hyperpolarized 13 C-labeled combretastatin A1 diphosphate, a tumor vascular disrupting agent, showed an in vivo signal in the tumor.

CONCLUSIONS

The model maximizes tumor substrate/drug delivery and minimizes T1 relaxation signal losses in addition to systemic toxicity. Therefore, it permits metabolic studies of hyperpolarized substrates with relatively short T1 and opens up the possibility for preclinical studies of hyperpolarized drug molecules. Magn Reson Med 78:2116-2126, 2017. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Quantitative Estimation of Tissue Blood Flow Rate

The rate of blood flow through a tissue (F) is a critical parameter for assessing the functional efficiency of a blood vessel network following angiogenesis. This chapter aims to provide the principles behind the estimation of F, how F relates to other commonly used measures of tissue perfusion, and a practical approach for estimating F in laboratory animals, using small readily diffusible and metabolically inert radio-tracers. The methods described require relatively nonspecialized equipment. However, the analytical descriptions apply equally to complementary techniques involving more sophisticated noninvasive imaging.Two techniques are described for the quantitative estimation of F based on measuring the rate of tissue uptake following intravenous administration of radioactive iodo-antipyrine (or other suitable tracer). The Tissue Equilibration Technique is the classical approach and the Indicator Fractionation Technique, which is simpler to perform, is a practical alternative in many cases. The experimental procedures and analytical methods for both techniques are given, as well as guidelines for choosing the most appropriate method.

Overview of Methods for Overcoming Hindrance to Drug Delivery to Tumors, with Special Attention to Tumor Interstitial Fluid

Every drug used to treat cancer (chemotherapeutics, immunological, monoclonal antibodies, nanoparticles, radionuclides) must reach the targeted cells through the tumor environment at adequate concentrations, in order to exert their cell-killing effects. For any of these agents to reach the goal cells, they must overcome a number of impediments created by the tumor microenvironment (TME), beginning with tumor interstitial fluid pressure (TIFP), and a multifactorial increase in composition of the extracellular matrix (ECM). A primary modifier of TME is hypoxia, which increases the production of growth factors, such as vascular endothelial growth factor and platelet-derived growth factor. These growth factors released by both tumor cells and bone marrow recruited myeloid cells form abnormal vasculature characterized by vessels that are tortuous and more permeable. Increased leakiness combined with increased inflammatory byproducts accumulates fluid within the tumor mass (tumor interstitial fluid), ultimately creating an increased pressure (TIFP). Fibroblasts are also up-regulated by the TME, and deposit fibers that further augment the density of the ECM, thus, further worsening the TIFP. Increased TIFP with the ECM are the major obstacles to adequate drug delivery. By decreasing TIFP and ECM density, we can expect an associated rise in drug concentration within the tumor itself. In this overview, we will describe all the methods (drugs, nutraceuticals, and physical methods of treatment) able to lower TIFP and to modify ECM used for increasing drug concentration within the tumor tissue.

Nitric oxide synthase inhibition enhances the tumor vascular-damaging effects of combretastatin a-4 3-o-phosphate at clinically relevant doses

PURPOSE

The therapeutic potential of combining the prototype tumor vascular-disrupting agent combretastatin A-4 3-O-phosphate (CA-4-P) with systemic nitric oxide synthase (NOS) inhibition was investigated preclinically.

EXPERIMENTAL DESIGN

Vascular response (uptake of (125)I-labeled iodoantipyrine; laser Doppler flowmetry) and tumor response (histologic necrosis; cytotoxicity and growth delay) were determined.

RESULTS

Inducible NOS selective inhibitors had no effect on blood flow in the P22 rat sarcoma. In contrast, the non-isoform-specific NOS inhibitor N(omega)-nitro- l-arginine (l-NNA; 1 and 10 mg/kg i.v. or chronic 0.1 or 0.3 mg/mL in drinking water) decreased the P22 blood flow rate selectively down to 36% of control at 1 hour but did not induce tumor necrosis at 24 hours. CA-4-P, at clinically relevant doses, decreased the P22 blood flow rate down to 6% of control at 1 hour for 3 mg/kg but with no necrosis induction. However, l-NNA administration enhanced both CA-4-P-induced tumor vascular resistance at 1 hour (chronic l-NNA administration) and necrosis at 24 hours, with 45% or 80% necrosis for 3 and 10 mg/kg CA-4-P, respectively. Bolus l-NNA given 3 hours after CA-4-P was the most effective cytotoxic schedule in the CaNT mouse mammary carcinoma, implicating a particular enhancement by l-NNA of the downstream consequences of CA-4-P treatment. Repeated dosing of l-NNA with CA-4-P produced enhanced growth delay over either treatment alone in P22, CaNT, and spontaneous T138 mouse mammary tumors, which represented a true therapeutic enhancement.

CONCLUSIONS

The combination of NOS inhibition with CA-4-P is a promising approach for targeting tumor vasculature, with relevance for similar vascular-disrupting agents in development.

Quantitative estimation of tissue blood flow rate

Tissue blood flow rate (F) is a critical parameter for assessing functional efficiency of a blood vessel network following angiogenesis. This chapter aims to provide the principles behind estimation of F and a practical approach to its determination in laboratory animals using small, readily diffusible, and metabolically inert radiotracers. The methods described require relatively nonspecialized equipment. However, the analytical descriptions apply equally to complementary techniques involving sophisticated noninvasive imaging. Two techniques are described for the quantitative estimation of F using the tissue uptake following intravenous administration of radioactive iodoantipyrine (or other suitable radiotracer). The tissue equilibration technique is the classical approach, and the indicator fractionation technique, which is simpler to perform, is a practical alternative in many cases. The experimental procedures and analytical methods for both techniques are given, as well as guidelines for choosing the most appropriate method.

The endothelin B (ETB) receptor agonist IRL 1620 is highly vasoconstrictive in two syngeneic rat tumour lines: potential for selective tumour blood flow modification

The vascular effects of the endothelin B (ET_B) receptor agonist IRL 1620 were investigated in the rat P22 carcinosarcoma and a range of normal tissues in BDIX rats. Tissue blood flow rate was calculated from measurements of tissue uptake of radiolabelled iodoantipyrine. A comparison of vascular effects in the P22 tumour and the HSN sarcoma growing in CBH/CBi rats was made using laser Doppler flowmetry, showing similar effects of IRL 1620, with red cell flux rapidly decreasing by 50–60% and then returning to control levels within approximately 30 min. This corresponded to similar levels but different spatial organisation of ET_B binding sites in the two tumours, as measured by autoradiography. The decrease in tumour blood flow and an increase in vascular resistance suggest that the vascular component of ET_B receptors in the P22 tumour is localised on contractile elements rather than on endothelial cells. ET_A receptors were also identified. Vasoconstriction occurred uniformly throughout the P22 tumour mass, consistent with a measured homogeneous distribution of ET_B receptors. IRL 1620 caused vasoconstriction in normal skeletal muscle, kidney and small intestine of the BDIX rat as well as in tumour, but did not affect blood flow in other tissues. These effects could be useful for limiting toxicity of certain chemotherapeutic agents. Fully functional ET_B receptors are clearly expressed on tumour vasculature and IRL 1620 shows promise for short-term modification of tumour blood flow. Expression levels of ET_B receptors on the tumour vasculature could be useful for predicting which tumours are likely to respond to IRL 1620.

Evaluation of the anti-vascular effects of combretastatin in rodent tumours by dynamic contrast enhanced MRI

The anti-vascular effects of the tubulin binding agent, disodium combretastatin A-4 3-O-phosphate (CA-4-P), have been investigated in the rat P22 carcinosarcoma by measurements of radiolabelled iodoantipyrine uptake and dynamic contrast-enhanced MRI. The iodoantipyrine estimates of absolute tumour blood flow showed a reduction from 0.35 to 0.04 ml g(-1) min(-1) 6 h after 10 mg kg(-1) CA-4-P and to <0.01 ml g(-1) min(-1) after 100 mg kg(-1). Tumour blood flow recovered to control values 24 h after 10 mg kg(-1) CA-4-P, but there was no recovery by 24 h after the higher dose. Dynamic contrast-enhanced MR images were obtained at 4.7 T, following injection of 0.1 mmol kg(-1) Gd-DTPA and analysed assuming a model arterial input function. A parameter, K(trans), which is related to blood flow rate and permeability of the tumour vasculature to Gd-DTPA, was calculated from the uptake data. K(trans) showed a reduction from 0.34 to 0.11 min(-1) 6 h after 10 mg kg(-1) CA-4-P and to 0.07 min(-1) after 100 mg kg(-1). Although the magnitude of changes in K(trans) was smaller than that in tumour blood flow, the time course and dose-dependency patterns were very similar. The apparent extravascular extracellular volume fraction, nu(e), showed a four-fold reduction 6 h after 100 mg kg(-1) CA-4-P, possibly associated with vascular shutdown within large regions of the tumour. These results suggest that K(trans) values for Gd-DTPA uptake into tumours could be a useful non-invasive indicator of blood flow changes induced by anti-vascular agents such as combretastatin.

Modification of blood flow in the HSN tumour and normal tissues of the rat by the endothelin ET(B) receptor agonist, IRL 1620

Activation of endothelin receptors on the vasculature can produce a variety of responses from potent vasoconstriction to mild vasodilation, depending on the receptor complement within the tissue. To elucidate the potential role of endothelin analogues as tumour blood flow modifiers, we have evaluated the effect of the ET(B) receptor agonist, IRL 1620 ([Suc-(Glu9, Ala(11,15))-ET-1(8-21)]) in CBH/CBi rats bearing an HSN fibrosarcoma. Tissue blood flow and vascular resistance were determined, 20 min following administration of IRL 1620 (bolus intravenous), using the uptake of radiolabelled iodoantipyrine (125I-IAP). Blood flow was unchanged in most tissues. However, at doses > or = 1.0 nmol kg(-1) IRL 1620, blood flow in the brain and heart was increased, whereas in the small intestine it was reduced. Blood flow in the skeletal muscle was reduced at 1.0 nmol kg(-1) only. Tumour blood flow was significantly reduced at 3.0 and 5.0 nmol kg(-1). Vascular resistance was unchanged in most tissues although it was increased in the skeletal muscle at 1.0 nmol kg(-1), in the kidney at 1.0 and 3.0 nmol kg(-1) and in the brain and heart, it was reduced at 5.0 nmol kg(-1) IRL 1620. Vascular resistance was significantly increased in the tumour and the small intestine at doses > or = 1 nmol kg(-1) IRL 1620. Pretreatment of rats with BQ-788, an ET(B) receptor antagonist, selectively attenuated the tumour vascular response to 3 nmol kg(-1) IRL 1620 with no changes observed in the normal tissue responses. Our results demonstrate that the HSN tumour vasculature is selectively responsive to IRL 1620 at doses > 1 nmol kg(-1) compared with the majority of normal tissues with the exception of the small intestine, and that only the tumour response is highly sensitive to BQ-788 antagonism, under the experimental dosing regime investigated. These differences may be exploitable for therapeutic benefit.

The comparative effects of the NOS inhibitor, Nomega-nitro-L-arginine, and the haemoxygenase inhibitor, zinc protoporphyrin IX, on tumour blood flow

PURPOSE

To determine the relative effects of inhibiting nitric oxide synthase (NOS) and haemoxygenase (HO) on blood flow to the rat P22 carcinosarcoma.

METHODS AND MATERIALS

HO is the enzyme responsible for in vivo production of carbon monoxide (CO). The vascular effects of zinc protoporphyrin IX (ZnPP), a competitive inhibitor of HO, were compared with those of copper protoporphyrin IX (CuPP), a poor inhibitor of HO, in isolated ex vivo perfusions of the P22 tumour and in intact tumour-bearing rats. In ex vivo perfusions, tumour vascular resistance was calculated from measurements of perfusion pressure at a known flow rate. In intact animals, blood flow to tumour and normal tissues was calculated using a radiotracer uptake method. The effects of ZnPP were compared with those of the NOS inhibitor, N(omega)-nitro-L-arginine (L-NNA), and the combination of the two drugs.

RESULTS

HO activity in the P22 tumour was reduced by 50% following administration of either ZnPP or CuPP directly to ex vivo perfused tumours, suggesting an indirect effect on the enzyme. Enzyme inhibition was not associated with any significant vasoactive effect. Neither ZnPP nor CuPP, at a dose of 45 micromol x kg(-1) administered i.p., inhibited tumour HO in vivo. However, they did significantly decrease tumour blood flow to 60-70% of control, with similar effects in skin and brain. Skeletal muscle blood flow was increased to 150% of control. L-NNA decreased both tumour and skeletal muscle blood flow to around 40% of control. These differences suggest that the nonspecific effects of ZnPP and CuPP were not mediated by NOS inhibition. The combination of ZnPP and L-NNA improved the selective reduction in tumour blood flow achieved with either agent alone.

CONCLUSION

This suggests that the HO/CO pathway does not play a major vasodilatory role in this tumour. However, ZnPP and CuPP could be useful for inducing a relatively selective decrease in tumour blood flow via mechanisms unrelated to HO inhibition, especially when combined with NOS inhibition.

Vascular response of tumour and normal tissues to endothelin-1 following antagonism of ET(A) and ET(B) receptors in anaesthetised rats

Modification of blood flow by endothelin-1 (ET-1) was examined in the s.c. HSN fibrosarcoma and compared to normal tissues of anaesthetised CBH/CBi rats. The ET receptor subtypes involved in the response were investigated using the ET(A) and ET(B) receptor antagonists BQ-610 and BQ-788, respectively. Blood flow and vascular resistance were determined using the uptake of radiolabelled iodo-antipyrine (125I-IAP). BQ-610 or BQ-788 was infused for 30 min prior to blood flow determination. ET-1 was administered 15 min into the infusion time. BQ-610 and BQ-788 infused alone did not modify any vascular parameters. Tumour blood flow increased slightly following ET-1, contrasting with most normal tissues, in which blood flow was reduced. Vascular resistance increased in all tissues, including the tumour. Neither antagonist significantly modified the ET-1-induced changes in tumour blood flow or vascular resistance, whereas in the majority of normal tissues BQ-610 attenuated and BQ-788 potentiated the vascular resonse to ET-1. Our results show that the HSN tumour vasculature is only weakly responsive to ET- 1 and antagonism of its effects by BQ-610 and BQ-788. This contrasts with the majority of normal tissues, in which ET- 1 induces an intense vasoconstriction.

Reduced capacity of tumour blood vessels to produce endothelium-derived relaxing factor: significance for blood flow modification

The effect of nitric oxide-dependent vasodilators on vascular resistance of tumours and normal tissue was determined with the aim of modifying tumour blood flow for therapeutic benefit. Isolated preparations of the rat P22 tumour and normal rat hindlimb were perfused ex vivo. The effects on tissue vascular resistance of administration of sodium nitroprusside (SNP) and the diazeniumdiolate (or NONO-ate) NOC-7, vasodilators which act via direct release of nitric oxide (NO), were compared with the effects of acetylcholine (ACh), a vasodilator which acts primarily via receptor stimulation of endothelial cells to release NO in the form of endothelium-derived relaxing factor (EDRF). SNP and NOC-7 effectively dilated tumour blood vessels after preconstriction with phenylephrine (PE) or potassium chloride (KCl) as indicated by a decrease in vascular resistance. SNP also effectively dilated normal rat hindlimb vessels after PE/KCl constriction. Vasodilatation in the tumour preparations was accompanied by a significant rise in nitrite levels measured in the tumour effluent. ACh induced a significant vasodilation in the normal hindlimb but an anomalous vasoconstriction in the tumour. This result suggests that tumours, unlike normal tissues are incapable of releasing NO (EDRF) in response to ACh. Capacity for EDRF production may represent a difference between tumour and normal tissue blood vessels, which could be exploited for selective pharmacological manipulation of tumour blood flow.

A comparative study of tumour blood flow modification in two rat tumour systems using endothelin-1 and angiotensin II: influence of tumour size on angiotensin II response

Tumour blood flow modification following i.v. administration of angiotensin II (AT II, 0.19 nmol kg-1 min-1) or endothelin-1 (ET-1, 1 nmol kg-1) was compared in the P22 carcinosarcoma-bearing BD9 rat and the HSN fibrosarcoma-bearing CBH/CBi rat using the tissue uptake of radiolabelled iodoantipyrine. Results were compared with a range of normal tissues. HSN tumour blood flow was unmodified by either peptide, whereas P22 tumour blood flow was unmodified by ET-1 but was reduced to 80% of the control flow by AT II. Both peptides reduced absolute blood flow in the skin overlying the tumour, in contralateral skin, skeletal muscle, kidney and small intestine, whereas blood flow to the brain and heart was significantly increased by ET-1 and unmodified by AT II. Both peptides significantly increased vascular resistance (mean arterial blood pressure / tissue blood flow) in all normal tissues and both tumours, thus demonstrating the existence of vascular receptors for these 2 vasomodifiers, and the capacity of the vessels to respond to receptor activation. Dependency of response on tumour size was examined in the P22 tumour. In contrast to that in small P22 tumours (1.22 +/- 0.06 g), blood flow to large P22 tumours (7.18 +/- 0.25 g) was unmodified by AT II. Vascular resistance was equally increased in both tumour groups, thus illustrating little difference in the vascular response to AT II in the size range examined. Results show that the 2 rat tumours responded directly to ET-1 and AT II, but do not indicate any advantage of ET-1 over AT II in tumour blood flow modification. However, the existence of tumour vascular endothelin receptors suggests that the advent of less toxic and more controllable receptor ligands may make endothelin receptors of value in the modification of tumour blood flow.

Tumour blood flow modification by endothelin-related peptides in the rat HSN fibrosarcoma

Modification of tissue blood flow and tissue vascular resistance was examined in the female CBH rat, bearing a HSN fibrosarcoma, following bolus intravenous administration of 1 nM kg-1 endothelin-1 (ET-1) or 1 nM kg-1 sarafotoxin S6c (SX6c), selective agonists for endothelin A (ETA) and B (ETB) receptors respectively. Blood flow was measured 15 min after drug administration by the tissue uptake of 125I-labelled-iodoantipyrine. ET-1 and SX6c produced increases in mean arterial blood pressure (MABP) of 52 mmHg and 42 mmHg respectively. Blood flow to the tumour was unaffected by ET-1 treatment, whereas blood flow to normal tissues was reduced, the exception being the heart and the brain in which flow was increased. In contrast, tumour blood flow following SX6c was significantly increased, whereas blood flow in normal tissues was either unaltered or reduced. Vascular resistance was increased in all tissues and the tumour by ET-1 demonstrating that the tumour vasculature was constricting via ETA receptor activation. SX6c however, did not modify tumour vascular resistance, whereas it increased vascular resistance in all normal tissues, suggesting that the tumour lacks a functional population of ETB receptors. This discrepancy may provide a means for selectively modifying tumour blood flow.

Spatial heterogeneity of tumour blood flow modification induced by angiotensin II: relationship to receptor distribution

Angiotensin II (ATII) has potential for improving delivery of blood-borne anti-cancer agents to tumours by increasing tumour blood flow. However, ATII-induced hypertension is not always accompanied by an increase in tumour blood flow due to significant constriction of the tumour vasculature. Such unpredictability in tumour response to ATII limits the clinical usefulness of this approach. In this study, the potential of assessing numbers of binding sites for ATII as a predictor of tumour blood flow response to intravenous administration of ATII was investigated. The distribution of ATII receptors in the rat P22 carcinosarcoma was related to tumour blood flow distribution and blood flow response to ATII using an autoradiographic approach. ATII (0.2 microgram x kg-1 x min-1) increased mean arterial blood pressure of anaesthetized BD9 rats from 92.2 +/- 1.4 mmHG to 145.6+- 1.3 mmHg. Despite this increase in perfusion pressure, overall tumour blood flow to viable regions decreased by 20%, indicating significant constriction of tumour blood vessels. Autoradiographic localisation of tumour blood flow showed that the decrease in flow was confined to the tumour periphery, with no change at the tumour centre. This pattern was consistent with 10% more binding sites for ATII at the tumour periphery than at the tumour centre. Maximum number of binding sites (BLmax) for the P22 tumour was 0.38 +/- 0.09 fmol x mg-1, which is approximately a factor of 10 lower than published values for various normal tissues. The dissociation constant Kd was l.16 +/- 0.18 nM. These results encourage the development of techniques for analysis of receptor binding characteristics for predicting the response of individual tumours to blood flow manipulation using vasoactive agents.

Enhanced tumour specificity of an anti-carcinoembrionic antigen Fab' fragment by poly(ethylene glycol) (PEG) modification

Polyethylene glycol (PEG) modification of a chimeric Fab' fragment (F9) of A5B7 (alpha-CEA), using an improved coupling method, increases its specificity for subcutaneous LS174T tumours. PEGylation increased the area under the concentration-time curve (AUC0-144) in all tissues but there were significant differences (variance ratio test, F = 27.95, P < 0.001) between the proportional increases in AUC0-144, with the tumour showing the greatest increase. The increase in AUCtumour from F9 to PEG-F9 was similar to the reported increase from Fab' to F(ab')2 while the increase in AUCblood by PEGylation of F9 was only 21% of the reported increase from Fab' to whole IgG. A two sample t-test showed no significant differences between maximal tumour/tissue ratios for PEG-F9 and F9 while the tumour/tissue ratios for PEG-F9 remained high over a longer period, with tumour levels at least double those for F9. PEG-F9 emerges as a new generation antibody with potential advantages for both radioimmunotherapy and tumour imaging. Since there was a reduction in antigen binding, optimisation of PEGylation might further improve tumour specificity. The latter resulted from complex effects on both the entry into and exit rates from tumour and normal tissues in a tissue-specific fashion.

The response of tumour vasculature to angiotensin II revealed by its systemic and local administration to 'tissue-isolated' tumours

A tissue-isolated preparation of the P22 rat carcinosarcoma was used to investigate the tumour vascular response to angiotensin II (ATII). In particular, the relative importance of systemic and local tumour factors was assessed by comparing tumour vascular resistance during systemic administration of ATII and during administration directly into the tumour-supplying artery. The effect of hypervolaemia on tumour vascular resistance was determined as well as the effect of ATII on oxygen metabolism. Tumour vascular resistance was increased by ATII in a dose-dependent manner. The response was biphasic with an initial peak in resistance followed by a lower plateau phase. Systemic administration of ATII was more effective in increasing tumour vascular resistance than direct administration. This suggests that systemic administration is not causing any reopening of previously collapsed tumour blood vessels. Further evidence for this is that hypervolaemia caused no reduction in tumour vascular resistance and that there was no difference in oxygen extraction by tumours between groups treated with systemically and directly administered ATII. A heterogeneous distribution of ATII receptors in the P22 tumour is a more likely explanation for the known heterogeneity of blood flow response to ATII.

Nitric oxide in biological fluids: analysis of nitrite and nitrate by high-performance ion chromatography

The analysis of nitric oxide-derived nitrite and nitrate ions in biological fluids represents a proven strategy for determining nitric oxide participation in a diverse range of physiological and pathophysiological processes in vivo. In this article we describe a versatile method for the simultaneous measurement of NO2- and NO3- anions in both plasma and isolated tumour models based on anion-exchange chromatography with spectrophotometric detection (214 nm). This method compares well with the capillary electrophoresis technique, exhibiting an equivalent sensitivity for NO2-/NO3- anions and short run-times, i.e. not greater than 4 min. Comparisons are also made with two alternative but less satisfactory methods which employ ion-exchange or reversed-phase ion-pair chromatography with conductimetric as well as spectrophotometric detection. Technical problems associated with each method, particularly those arising from nitrate contamination, have been addressed.

The influence of nitric oxide on tumour vascular tone

Acetylcholine and sodium nitroprusside, which vasodilate via release of NO by endothelium-dependent and endothelium-independent mechanisms respectively, had little effect on tumour vascular resistance when administered to tissue-isolated tumours perfused in their normal state. However, under phenylephrine-induced vasoconstriction, sodium nitroprusside induced vasodilation whilst acetylcholine induced a small vasoconstriction. Phenylephrine itself induced an oscillatory change in tumour perfusion pressure. The nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine (L-NNA) caused a dose-dependent increase in vascular resistance in ex vivo perfused tumours which was greater than that in normal perfused hindlimbs. Systemic administration of L-NNA caused a 50% decrease in tumour blood flow which was a larger effect than in any of the normal tissues studied except spleen and skeletal muscle. Modification of NOS activity in tumours is a promising means for selective tumour blood flow modification. Investigation of endothelium-dependent versus endothelium-independent methods for modifying tumour blood flow may provide methods for further selectivity.