Pharmacological modification of tumor blood flow: lack of correlation between alteration of mean arterial blood pressure and changes in tumor perfusion

Tumorigenesis and Progression As A Consequence of Hypoxic TME：A Prospective View upon Breast Cancer Therapeutic Targets

Intratumoral hypoxia has a significant impact on the development and progression of breast cancer (BC). Rather than exerting limited regional impact, hypoxia create an aggressive macroenvironment for BC. Hypoxia-inducible factors-1(HIF-1) is extensively induced under hypoxia condition of BC, activating the transcription of multiple oncogenes. Thereinto, CD73 is the one which could be secreted into the microenvironment and is in favor of the growth, metastasis, resistance to therapies, as well as the stemness maintenance of BC. In this review, we address the significance of hypoxia/HIF-1/CD73 axis for BC, and provide a novel perspective into BC therapeutic strategies.

Breast Cancer Brain Metastasis Response to Radiation After Microbubble Oxygen Delivery in a Murine Model

OBJECTIVES

Hypoxic cancer cells have been shown to be more resistant to radiation therapy than normoxic cells. Hence, this study investigated whether ultrasound (US)-induced rupture of oxygen-carrying microbubbles (MBs) would enhance the response of breast cancer metastases to radiation.

METHODS

Nude mice (n = 15) received stereotactic injections of brain-seeking MDA-MB-231 breast cancer cells into the right hemisphere. Animals were randomly assigned into 1 of 5 treatment groups: no intervention, 10 Gy radiation using a small-animal radiation research platform, nitrogen-carrying MBs combined with US-mediated MB rupture immediately before 10 Gy radiation, oxygen-carrying MBs immediately before 10 Gy radiation, and oxygen-carrying MBs with US-mediated MB rupture immediately before 10 Gy radiation. Tumor progression was monitored with 3-dimensional US, and overall survival was noted.

RESULTS

All groups except those treated with oxygen-carrying MB rupture and radiation had continued rapid tumor growth after treatment. Tumors treated with radiation alone showed a mean increase in volume ± SD of 337% ± 214% during the week after treatment. Tumors treated with oxygen-carrying MBs and radiation without MB rupture showed an increase in volume of 383% ± 226%. Tumors treated with radiation immediately after rupture of oxygen-carrying MBs showed an increase in volume of only 41% ± 1% (P = 0.045), and this group also showed a 1 week increase in survival time.

CONCLUSIONS

Adding US-ruptured oxygen-carrying MBs to radiation therapy appears to delay tumor progression and improve survival in a murine model of metastatic breast cancer.

Sensitization of Hypoxic Tumors to Radiation Therapy Using Ultrasound-Sensitive Oxygen Microbubbles

PURPOSE

Much of the volume of solid tumors typically exists in a chronically hypoxic microenvironment that has been shown to result in both chemotherapy and radiation therapy resistance. The purpose of this study was to use localized microbubble delivery to overcome hypoxia prior to therapy.

MATERIALS AND METHODS

In this study, surfactant-shelled oxygen microbubbles were fabricated and injected intravenously to locally elevate tumor oxygen levels when triggered by noninvasive ultrasound in mice with human breast cancer tumors. Changes in oxygen and sensitivity to radiation therapy were then measured.

RESULTS

In this work, we show that oxygen-filled microbubbles successfully and consistently increase breast tumor oxygenation levels in a murine model by 20 mmHg, significantly more than control injections of saline solution or untriggered oxygen microbubbles (P < .001). Using photoacoustic imaging, we also show that oxygen delivery is independent of hemoglobin transport, enabling oxygen delivery to avascular regions of the tumor. Finally, we show that overcoming hypoxia by this method immediately prior to radiation therapy nearly triples radiosensitivity. This improvement in radiosensitivity results in roughly 30 days of improved tumor control, providing statistically significant improvements in tumor growth and animal survival (P < .03).

CONCLUSIONS

Our findings demonstrate the potential advantages of ultrasound-triggered oxygen delivery to solid tumors and warrant future efforts into clinical translation of the microbubble platform.

Non-invasive imaging of barriers to drug delivery in tumors

Solid tumors often develop high interstitial fluid pressure (IFP) as a result of increased water leakage and impaired lymphatic drainage, as well as changes in the extracellular matrix composition and elasticity. This high fluid pressure forms a barrier to drug delivery and hence, resistance to therapy. We have developed techniques based on contrast enhanced magnetic resonance imaging for mapping in tumors the vascular and transport parameters determining the delivery efficiency of blood borne substances. Sequential images are recorded during continuous infusion of a Gd-based contrast agent and analyzed according to a new physiological model, yielding maps of microvascular transfer constants, as well as outward convective interstitial transfer constants and steady state interstitial contrast agent concentrations both reflecting IFP distribution. We further demonstrated in non small cell human lung cancer xenografts the capability of our techniques to monitor in vivo collagenase induced increase in contrast agent delivery as a result of decreased IFP. These techniques can be applied to test drugs that affect angiogenesis and modulate interstitial fluid pressure and has the potential to be extended to cancer patients for assessing resistance to drug delivery.

Inhibition of cADP-ribose formation produces vasodilation in bovine coronary arteries

cADP-ribose (cADPR) induces the release of Ca(2+) from the intracellular stores of coronary artery smooth muscle cells. However, little is known about the role of cADPR-mediated intracellular Ca(2+) release in the control of vascular tone. The present study examined the effects of nicotinamide, a specific inhibitor of ADP-ribosylcyclase, on the vascular tone of bovine coronary arteries. A bovine coronary artery homogenate stimulated the conversion of nicotinamide guanine dinucleotide into cGDP-ribose, which is a measure of ADP-ribosylcyclase activity. Nicotinamide significantly inhibited the formation of cGDP-ribose in a concentration-dependent manner: at a concentration of 10 mmol/L, it reduced the conversion rate from 3.34+/-0.11 nmol. min(-1). mg(-1) of protein in control cells to 1.42+/-0.11 nmol. min(-1). mg(-1) of protein in treated cells, a 58% reduction. In U46619-precontracted coronary artery rings, nicotinamide produced concentration-dependent relaxation. Complete relaxation with nicotinamide occurred at a dose of 8 mmol/L; the median inhibitory concentration (IC(50)) was 1.7 mmol/L. In the presence of a cell membrane-permeant cADPR antagonist, 8-bromo-cADPR, nicotinamide-induced vasorelaxation was markedly attenuated. Pretreatment of the arterial rings with ryanodine (50 micromol/L) significantly blunted the vasorelaxation response to nicotinamide. However, iloprost- and adenosine-induced vasorelaxation was not altered by 8-bromo-cADPR. Moreover, nicotinamide significantly attenuated KCl- or Bay K8644-induced vasoconstriction by 60% and 70%, respectively. These results suggest that the inhibition of cADPR formation by nicotinamide produces vasorelaxation and blunts KCl- and Bay K8644-induced vasoconstriction in coronary arteries and that the cADPR-mediated Ca(2+) signaling pathway plays a role in the control of vascular tone in coronary circulation.

Enhancement of tumor perfusion and oxygenation by carbogen and nicotinamide during single- and multifraction irradiation

Numerous experimental and clinical studies have been completed regarding the effects of carbogen and nicotinamide on tumor oxygenation and radiosensitivity. The current study incorporates three physiological measurement techniques to further define spatial variations in oxygen availability and development of hypoxia after single- and multifraction irradiation in KHT murine fibrosarcomas. Distances to anatomical and perfused blood vessels were measured using immunohistochemical and fluorescent staining, intravascular oxygen levels were determined cryospectrophotometrically, and tumor hypoxia was quantified using uptake of EF5, a marker of hypoxia. Carbogen, nicotinamide, and the combination of both all increased intravascular oxygen availability compared to controls. While nicotinamide had no effect on the number of perfused blood vessels in nonirradiated tumors, carbogen produced a substantial closing of vessels. After a single dose of 4 Gy, only the combination of nicotinamide and carbogen produced significant improvements in oxygen availability, while numbers of perfused vessels were significantly increased for nicotinamide, unchanged for the combination of nicotinamide and carbogen, and significantly decreased for carbogen. After 4 x 4-Gy fractions, oxygen availability was increased substantially with the combination of nicotinamide and carbogen, somewhat with carbogen, and not at all with nicotinamide. Tumor oxygenation changes were estimated by EF5/Cy3 intensity distributions, which demonstrated that manipulative agents could produce disparate effects on tumor hypoxia when combined with either single- or multifraction irradiation.

Magnetic resonance imaging of perfusion in rat cerebral 9L tumor after nicotinamide administration

PURPOSE

To investigate the effect of nicotinamide on normal brain and 9L tumor blood flow in the rat using magnetic resonance imaging (MRI) and arterial spin tagging.

METHODS AND MATERIALS

Using MRI at 7 Tesla, measurements of blood perfusion were determined from two-dimensional maps of intracerebral 9L rat tumors and normal Fischer rat brains. The spatial and temporal influence of nicotinamide, 500 mg/kg i.p., on cerebral blood flow (CBF) was studied in normal brain and tumors between 5 and 21 days after tumor implantation. The MRI CBF measurements employed a variable tip-angle-gradient-recalled echo (VTA-GRE-CBF) readout of the magnetization of the tissue slice. The VTA-GRE-CBF required 8 minutes for a blood flow image with inplane resolution of 250 microm x 500 microm x 2 mm.

RESULTS

Normal brain blood flow decreased following the administration of nicotinamide. In contrast, tumor blood flow remained unaffected in the time following nicotinamide administration. Consequently, the blood flowing in the tumor relative to that in normal brain demonstrated a significant and selective increase in response to nicotinamide administration. Relative tumor blood flow increased at 10 minutes after nicotinamide injection compared with predrug levels and remained elevated for at least 1 hour.

CONCLUSION

The results suggest that nicotinamide will not enhance radiosensitivity of brain tumors. The results support the use of nicotinamide to improve delivery of anticancer therapeutics through its ability to selectively increase tumor blood flow relative to that in normal brain.

Oxygenation predicts radiation response and survival in patients with cervix cancer

BACKGROUND AND PURPOSE

Hypoxia appears to be an important factor in predicting tumor relapse following radiation therapy. This study measured oxygenation prior to treatment in patients with cervix cancer using a polarographic oxygen electrode to determine if oxygenation was an important prognostic factor with regard to tumor control and survival.

MATERIALS AND METHODS

Between May 1994 and June 1997, 74 eligible patients with cervix cancer were entered into an ongoing prospective study of tumor oxygenation prior to primary radiation therapy. All patients were evaluated with an Eppendorf oxygen electrode during examination under anesthesia. Oxygenation data are presented as the hypoxic proportion, defined as the percentage of pO2 readings of <5 mm Hg (abbreviated as HP5).

RESULTS

The HP5 ranged from 2 to 99% with a median of 52%. With a median follow-up of 1.2 years, the disease-free survival (DFS) rate was 69% for patients with HP5 of < or =50% compared with 34% for those with HP5 of >50% (log-rank P = 0.02). Tumor size above and below the median of 5 cm was also significantly related to DFS (P = 0.0003) and patients with bulky hypoxic tumors had a significantly lower DFS (12% at 2 years) than either bulky oxygenated or non-bulky oxygenated or hypoxic tumors (65%, P = 0.0001).

CONCLUSIONS

Hypoxia and tumor size are significant adverse prognostic factors in a univariate analysis of disease-free survival in patients with cervix cancer. A high risk group of patients with bulky hypoxic tumors have a significantly higher probability of relapse and death.

Nicotinamide reduces tumour interstitial fluid pressure in a dose- and time-dependent manner

Nicotinamide radiosensitizes a number of experimental tumours, and increases blood flow and mean pO2 in some tumours. It has been suggested that nicotinamide reduces tumour interstitial fluid pressure (IFP), thereby reducing transient vessel non-perfusion and acute hypoxia, and radiosensitizing tumours. To test this hypothesis, tumour IFP, transient vessel non-perfusion, and radiosensitivity after nicotinamide administration were examined in the murine carcinoma NT. Nicotinamide at doses of 500 and 1000 mg kg-1 significantly reduced tumour IFP within 20 min of administration, with recovery to control values by 60-80 min; 100 mg kg-1 had no effect. The percentage of previously non-perfused vessels that became perfused 20 min after administering 1000 mg kg-1 of nicotinamide significantly exceeded the percentage that became perfused within 20 min in the absence of nicotinamide. By 90 min after nicotinamide administration, this differential effect was abolished. The correlation in the time courses of reduced IFP and increased vessel perfusion after nicotinamide administration suggest that decreased IFP may accompany vessel reperfusion. However, 1000 mg kg-1 of nicotinamide radiosensitized the NT carcinoma 80 min after administration, whilst no radiosensitization was seen within 10 min. Thus it is unlikely that increased vessel perfusion is the sole mechanism of nicotinamide-induced radiosensitization in this tumour.

A comparison of the physiological effects of RSU1069 and RB6145 in the SCCVII murine tumour

The physiological and therapeutic effects of the bioreductive agent RSU1069 (80 mg/kg i.p.) and its prodrug RB6145 (240 mg/kg i.p.) were investigated in the SCCVII tumour. Using laser Doppler flowmetry it was found that RSU1069 produced a significant 30% reduction in tumour blood flow 30 min after administration, while RB6145 had no effect. Tumour oxygenation, measured with an Eppendorf oxygen electrode, was unchanged by either agent except for a reduction in values less than 2.5 mmHg at 30 min after injection. Neither agent significantly altered tumour energy metabolism, assessed by 31P magnetic resonance spectroscopy. Both agents significantly increased tumour glucose content by a factor of 1.6-1.7 at 30 min after injection, but had no effect on glucose-6-phosphate or lactate levels. Tumour growth was significantly delayed by heating (42.5 degrees C, 60 min), and although neither RSU1069 nor RB6145 alone had any effect on tumour growth they produced a similar enhancement of the tumour response to heat. The therapeutic effects are consistent with the known conversion in vivo of one third of the pro-drug RB6145 to its active product RSU1069, however the physiological effects of the two agents in the SCCVII tumour are not identical.

The effects of carbogen and nicotinamide on intravascular oxyhaemoglobin saturations in SCCVII and KHT murine tumours

Considerable effort has been focused on devising methods for manipulating tumour oxygenation and thereby improving tumour radiosensitivity. The combination of nicotinamide and carbogen has been proposed to oxygenate both chronically and acutely hypoxic cells in tumours. However, results have varied markedly with both tumour model and measurement technique. The current objectives were (1) to determine whether changes in radiosensitivity following oxygen manipulation correlated with changes in tumour oxygenation and (2) to assess whether oxygenation was preferentially improved in specific tumour micro-regions. Using two murine tumour lines, the SCCVII carcinoma and the KHT sarcoma, tumour intravascular HbO2 saturations were measured cryospectrophotometrically following nicotinamide, carbogen or the combination. Generally, nicotinamide had minor effects on oxygenation, arguing against a substantial effect on acute hypoxia, while carbogen and the combination produced marked and equivalent improvements in oxygen availability. These results demonstrate that changes in tumour radiosensitivity may not agree with corresponding changes in oxygenation, even within a given tumour model, and that the efficacy of a given manipulative agent may vary substantially with tumour line. One possible explanation for these findings is that different subpopulations of clonogenic vs non-clonogenic cells may be oxygenated by alternative treatments.

Nicotinamide and other benzamide analogs as agents for overcoming hypoxic cell radiation resistance in tumours. A review

Oxygen deficient hypoxic cells, which are resistant to sparsely ionising radiation, have now been identified in most animal and some human solid tumours and will influence the response of those tumours to radiation treatment. This hypoxia can be either chronic, arising from an oxygen diffusion limitation, or acute, resulting from transient stoppages in microregional blood flow. Although clinical attempts to overcome hypoxia have met with some success, the results have been far from satisfactory, and efforts are still being made to find better methods. Extensive experimental studies, especially in the last decade, have shown that nicotinamide and structurally related analogs can effectively sensitise murine tumours to both single and fractionated radiation treatments and that they do so in preference to the effects seen in mouse normal tissues. The earliest studies suggested that this enhancement of radiation damage was the result of an inhibition of the repair mechanisms, as was well documented in vitro. However, recent studies in mouse tumours have shown that the primary mode of action actually involves a reduction in tumour hypoxia. More specifically, these drugs prevent transient cessations in blood flow, thus inhibiting the development of acute hypoxia. This novel discovery led to the suggestion that the potential role of these agents as radiosensitizers would be when combined with treatments that overcame chronic hypoxia. The first attempt to demonstrate this combined nicotinamide with hyperthermia and found that the enhancement of radiation damage by both agents together was greater than that seen with each agent alone. Similar results were later seen for nicotinamide combined with a perfluorochemical emulsion, carbogen breathing, and pentoxifylline, and in all these studies the effects in tumours were always greater than those seen in appropriate normal tissues. Of all the analogs, it is nicotinamide itself which has been the most extensively studied as a radiosensitizer in vivo and the one that shows the greatest effect in animal tumours. It is also an agent that has been well established clinically for the treatment of a variety of disorders, with daily doses of up to 6 g being considered reasonably safe and associated with a low incidence of side effects. This human dose is equivalent to 100-200 mg/kg in mice and such doses will maximally sensitize murine tumours to radiation. These findings have now resulted in phase I/II clinical trials of nicotinamide, in combination with carbogen breathing, as a potential radiosensitizing treatment.

The modification of blood flow in tumours and their supplying arteries by nicotinamide

We have studied the ability of the radiosensitizer nicotinamide (NA) to alter the contractility of normal and tumour blood vessels using an ex vivo isolated artery perfusion system. NA at a concentration of 8.2 mM reduced the constrictions produced by phenylephrine (PE) by 2-fold in both normal epigastric arteries and those that had been supplying p22 tumours in BD9 rats. At that same concentration NA also attenuated the spontaneous, rhythmic contractions that were seen in many tumour arteries. When the tumour arteries were perfused together with the tumour they supplied NA had little effect on the flow resistance of the tumour vascular network but reduced the resistance by up to 30% when the arteries were preconstricted with phenylephrine. These direct effects on vascular resistance together with the reduction of interstitial fluid pressure could combined to improve the homogeneity of tumour perfusion.

The radiosensitizer nicotinamide inhibits arterial vasoconstriction

Nicotinamide (NA) is currently entering clinical trials as a radiosensitizer. A major component of its activity is the improvement of tumour oxygenation resulting from a reduction in microregional ischaemia. NA is known to reduce arterial blood pressure in rodents, suggesting a vascular component in its mechanism of action. We have used an ex vivo system to study the direct action of NA on the contractile properties of vascular smooth muscle. Isolated pieces of rat tail artery were internally perfused with Krebs' solution at a constant flow rate so that constriction of the arterial smooth muscle could be measured as an increase in perfusion pressure. Transient vasoconstrictor responses lasting up to 10 min were induced with bolus injections (10 microliters) of phenylephrine, at concentrations ranging from 10(-5) to 10(-2) M, into the internal perfusate whereas a constant increase in vasoconstrictor tone, giving perfusion pressures of 43-84 mmHg, was induced by constantly perfusing with PE (5 x 10(-6) M) or raising the K+ concentration of the Krebs' solution to 122 mM. The addition of NA to the perfusate significantly reduced the size of the transient vasoconstrictor responses in a dose-dependent manner and induced the precontracted vessels to relax. This action of NA could not be blocked either by N omega-nitro-L-arginine methyl ester (L-NAME), indomethacin or propranolol. We conclude that direct effects on supplying blood vessels probably contribute to the oxygenating action of NA in tumours, though the precise mechanism remains obscure.

Importance of nicotinamide dose on blood pressure changes in mice and humans

PURPOSE

The importance of nicotinamide dose on inducing blood pressure changes in mice and humans was investigated.

METHODS AND MATERIALS

Blood pressure measurements in human volunteers were made using an inflated cuff procedure after oral ingestion of 3 or 6 g nicotinamide. Animal blood pressure measurements were performed in fully awake nonanesthetized female CDF1 mice, 24 h after cannulation of the carotid artery.

RESULTS

In humans, the average (+/- 1 SE) resting systolic and diastolic pressures were 122.8 mmHg (+/- 2.5) and 80.6 mmHg (+/- 2.1), respectively. They were unchanged during the first 3 h after ingestion of either 3 g or 6 g nicotinamide. The resting value (+/- 1 SE) in mice was 115.1 mmHg (+/- 4.0) and this was significantly reduced following intraperitoneal injection of 400-1000 mg/kg nicotinamide. This decrease was maximal within 15-30 min after injection and was linearly dependent on drug dose. At doses of 200 mg/kg or less, no significant effect on blood pressure was observed.

CONCLUSION

Doses between 100-200 mg/kg in mice are known to be equivalent to 6 g in man and can also produce maximal radiosensitization in murine tumors. Our results, therefore, not only show that the mouse and human data are entirely consistent, but also suggest that nicotinamide-induced decreases in blood pressure are not necessary for radiosensitization.

Micro-regional mapping of HbO2 saturations and blood flow following nicotinamide administration

PURPOSE

Although nicotinamide administration has increased the radiosensitivity of experimental tumors, there is a scarcity of data detailing the underlying physiological mechanisms. The current study presents a method for quantifying both microregional distributions of intravascular HbO2 saturations and the presence or absence of blood flow in adjacent frozen tumor sections.

METHODS AND MATERIALS

Two murine tumor cell lines, KHT and SCCVII, were implanted and quick-frozen without the use of anesthetics. Nicotinamide was administered IP 1 h prior to freezing, and a fluorescent dye that preferentially stains cells adjacent to blood vessels was injected i.v. 1 min prior to freezing. To visualize the presence or absence of blood flow, six micron sections were first cut using a cryostat. The remaining frozen tumor block was then analyzed cryospectrophotometrically to determine intravascular HbO2 levels.

RESULTS

While KHT HbO2 levels increased somewhat predictably following nicotinamide, the response in SCCVII tumors varied with distance from the tumor surface. Near the periphery, SCCVII HbO2 levels increased, but nearer the tumor center, HbO2 levels actually decreased. Perfused blood vessels were uniformly distributed throughout the tumor volume except in regions of necrosis. Even vessels containing no measurable oxygen remained perfused, as evidenced by the presence of the fluorescent marker.

CONCLUSION

These results demonstrate that nicotinamide raises intravascular HbO2 saturations in both KHT and SCCVII tumors. This increase in oxygen delivery is not evenly distributed throughout the tumor volume in spite of a uniform distribution of perfused blood vessels. Blood flow in a substantial proportion of these vessels is most likely not sufficiently rapid to serve a functional purpose in terms of oxygen supply to the surrounding tumor tissue.

Bioreducible mustards: a paradigm for hypoxia-selective prodrugs of diffusible cytotoxins (HPDCs)

Existing hypoxia-selective cytotoxins (HSCs) are designed to kill only the hypoxic subpopulation in tumours, and to be used in conjunction with other therapies (e.g., radiation). A new class of drugs, hypoxia-activated prodrugs of diffusible cytotoxins (HPDCs) are proposed. These are designed to exploit, rather than merely deal with, tumour hypoxia, by releasing diffusible cytotoxins on bioreduction in hypoxic regions. Such diffusible cytotoxins are required to be much more cytotoxic than the parent prodrug, to be sufficiently stable (half lives from 0.1 to 10 min) to allow them to diffuse up to 200 microns from the hypoxic regions, and to be equally effective against all major tumour cell subpopulations, including non-cycling cells. Nitrogen mustards, which show little cell cycle specificity, which kill cells by a well-understood mechanism (DNA cross-links), and which have stabilities and reactivities able to be predictably controlled by structural variations, are proposed as suitable candidates fur such diffusible cytotoxins. Design parameters for two classes of potential HPDCs are discussed; nitro-deactivated aromatic mustards, and cobalt (III) complex-deactivated aliphatic mustards. Examples of both classes show greater cell-killing activity against intact compared with dissociated multi-cellular spheroids. This suggests they may indeed function as HPDCs, by penetrating to the hypoxic core of the spheroid and there releasing potent cytotoxins which diffuse out to kill surrounding cells at lower oxygen tensions.

Nicotinamide exerts different acute effects on microcirculatory function and tissue oxygenation in rat tumors

PURPOSE

Nicotinamide has been reported to preferentially radiosensitize tumor tissue, supposedly through a reduction in tumor hypoxia. This may occur as a result of nicotinamide-induced changes in tumor blood flow and therefore the present study was undertaken to evaluate the effect of nicotinamide on circulatory parameters in skeletal muscle and tumor tissue (subcutaneously-implanted DS-sarcomas) of the rat.

METHODS AND MATERIALS

Mean arterial blood pressure (measured in the common carotid artery using a pressure transducer) and red blood cell flux (as measured by laser Doppler flowmetry) were continuously monitored for 120 min following a single intraperitoneal application of nicotinamide (500 mg/kg). An arterial blood pressure/laser Doppler flux ratio was estimated for tumor and muscle tissue.

RESULTS

Nicotinamide significantly reduced the mean arterial blood pressure to a minimum value 25% below the pretreatment value 20 min after the commencement of drug administration, with partial recovery thereafter. Red blood cell flux through tumor tissue, following an initial rapid decrease, rose steadily to values 34% above those measured in control animals at t = 60 min, while the arterial blood pressure/laser Doppler flux ratio in tumor tissue fell to values 34% below those of control animals. In skeletal muscle similar trends were seen although the changes were not of the same extent as those seen in tumor tissue. Tumor pO2 was measured 60 min following i.p. application of nicotinamide using polarographic needle electrodes. Despite the significant increase in blood flow following nicotinamide, no significant difference was seen between pO2 histograms obtained in tumors in nicotinamide-treated and control animals.

CONCLUSION

These findings suggest that nicotinamide preferentially improves tumor microcirculatory function and effectuates a decrease in the arterial blood pressure/laser Doppler flux ratio within tumor tissue, effects which reach their maximum approximately 60 min following nicotinamide administration.