Selective drug-induced reduction of blood flow in tumor transplants

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Reproducibility of magnetic resonance perfusion imaging

Dynamic MR biomarkers (T2*-weighted or susceptibility-based and T1-weighted or relaxivity-enhanced) have been applied to assess tumor perfusion and its response to therapies. A significant challenge in the development of reliable biomarkers is a rigorous assessment and optimization of reproducibility. The purpose of this study was to determine the measurement reproducibility of T1-weighted dynamic contrast-enhanced (DCE)-MRI and T2*-weighted dynamic susceptibility contrast (DSC)-MRI with two contrast agents (CA) of different molecular weight (MW): gadopentetate (Gd-DTPA, 0.5 kDa) and Gadomelitol (P792, 6.5 kDa). Each contrast agent was tested with eight mice that had subcutaneous MDA-MB-231 breast xenograft tumors. Each mouse was imaged with a combined DSC-DCE protocol three times within one week to achieve measures of reproducibility. DSC-MRI results were evaluated with a contrast to noise ratio (CNR) efficiency threshold. There was a clear signal drop (>95% probability threshold) in the DSC of normal tissue, while signal changes were minimal or non-existent (<95% probability threshold) in tumors. Mean within-subject coefficient of variation (wCV) of relative blood volume (rBV) in normal tissue was 11.78% for Gd-DTPA and 6.64% for P792. The intra-class correlation coefficient (ICC) of rBV in normal tissue was 0.940 for Gd-DTPA and 0.978 for P792. The inter-subject correlation coefficient was 0.092. Calculated K^trans from DCE-MRI showed comparable reproducibility (mean wCV, 5.13% for Gd-DTPA, 8.06% for P792). ICC of K^trans showed high intra-subject reproducibility (ICC = 0.999/0.995) and inter-subject heterogeneity (ICC = 0.774). Histograms of K^trans distributions for three measurements had high degrees of overlap (sum of difference of the normalized histograms <0.01). These results represent homogeneous intra-subject measurement and heterogeneous inter-subject character of biological population, suggesting that perfusion MRI could be an imaging biomarker to monitor or predict response of disease.

A novel drug to reduce tumor perfusion: antitumor effect alone and with hyperthermia

We have investigated the feasibility of enhancing damage induced by hyperthermia in SCK murine tumors by reducing tumor blood perfusion using a new agent, KB-R8498. Within several minutes of an i.v. injection, the tumor perfusion was reduced to less than 20% of the control value, and it recovered to 40-70% of the control value by 1 h after injection. The perfusion in normal tissues decreased or increased soon after drug administration depending on the tissue type. However, by 1 h after drug treatment, perfusion in five of the seven tissues examined had returned to the control level. The tumor pH was also reduced after i.v. drug administration. Control tumors grew to four times the initial volume in 6 days. Tumors that were heated at 42.5 degrees C for 60 min were delayed in growth by 4 days compared to control tumors. There was a growth delay of 14 days when an i.v. injection of KB-R8498 was given and the tumors were heated at 42.5 degrees C either immediately or 1 h later. In drug-alone studies, the tumor growth was delayed by 4 days when the drug was infused continuously at a rate of 30-50 mg/kg day(-1) for 7 days or about 2 days when mice were treated with five daily injections of 30 mg/kg KB-R8498.

Noninvasive monitoring of carbogen-induced changes in tumor blood flow and oxygenation by functional magnetic resonance imaging

PURPOSE

The response of tumors to radiotherapy can be enhanced if carbogen (95% O2, 5% CO2) is breathed. The timing of carbogen administration is critical, and a noninvasive method of monitoring the response of individual tumors would have obvious utility. Functional gradient recalled echo (GRE) magnetic resonance imaging (MRI) techniques are sensitive to changes in the concentrations of deoxyhemoglobin, which, thus, acts as an endogenous contrast agent for oxygenation status and blood flow.

METHODS AND MATERIALS

Subcutaneous GH3 prolactinomas in three rats were imaged at 4.7 Tesla with a GRE 1H sequence [echo time (TE) = 20 ms, repetition time (TR) = 80 ms, flip angle = 45 degrees, 1 mm slice, 256 phase encode steps, 4 cm field of view, in-plane resolution 0.08 x 0.08 mm, acquisition time = 4 min]. The rats breathed air or carbogen for four periods of 20 min; three control rats breathed only air.

RESULTS

Carbogen breathing caused increases of up to 100% in the GRE image intensity of the tumors. Reversion of air breathing caused the image intensity to fall; essentially the same response was observed with the second cycle of carbogen and air breathing. Control rat tumors showed no significant change.

CONCLUSIONS

The response of tumors to carbogen can be monitored noninvasively by GRE MRI. In principle, this could be due to an increase in oxygen content of the blood, a decrease in tumor cell oxygen consumption, or an increase in tumor blood flow. The very large changes in signal intensity suggest that a blood flow increase is the most probable explanation. If this technique can be successfully applied in man, it should be possible to optimize carbogen treatment for individual radiotherapy patients, and perhaps also to enhance tumor uptake of chemotherapeutic agents.

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.

Effects of hyperoxia on T2* and resonance frequency weighted magnetic resonance images of rodent tumours

Experiments were performed to determine whether T2* and resonance frequency weighted MR images are sensitive to effects of hyperoxia on model tumors. Hyperoxia can increase tumor oxygen tension and thus affect T2* and/or the average resonance frequency within each image voxel due to the paramagnetism of oxygen itself or through modulation of the oxidation state of hemoglobin. Alternatively, changes in T2* during hyperoxia may reflect changes in tumor water content due to changes in systemic blood pressure. Mammary adenocarcinomas implanted in the flanks of rats were studied. Imaging sequences were preceded by two 90 degrees pulses separated by an evolution period of 50 or 75 ms and followed by a crusher gradient to eliminate transverse magnetization. This pulse sequence produced images which were sensitized to both T2* and the average resonance frequency of each voxel. Images were produced at 2 T using a gradient echo imaging method with a TR of 3 s. Images obtained during inhalation of air and 100% O2 were compared. Significant increases in image intensity were observed in most tumors during hyperoxia, particularly at the tumor center. The increase was accentuated when the evolution period was increased and greatly reduced when a 180 degrees refocusing pulse was placed at the center of the evolution period. These results suggest that hyperoxia reduces local magnetic susceptibility gradients leading to an increase in T2* or causes a shift in resonance frequency. The magnitude of this change may be a function of the rate at which oxygen is delivered to and metabolized by tumors and may also reflect tumor oxygen tension under normoxic conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnetic resonance imaging of rodent tumors using radiofrequency gradient echoes

This paper evaluates the use of radiofrequency (RF) magnetic field gradient echoes to provide contrast in magnetic resonance (MR) images of model tumors. Decay of RF gradient echoes as a function of evolution time was measured and sensitivity of the decay to changes in blood pressure was evaluated. Previous investigators have demonstrated that static field (B0) gradient echoes provide MR image contrast which is sensitive to the rate of self-diffusion of tissue water and may also be sensitive to the rate of tissue perfusion. Gradient echoes produced by RF magnetic field gradients provide a useful alternative to the conventional B0 methods. Unlike B0 gradient echoes RF gradient echoes are relatively insensitive to local magnetic susceptibility gradients and to magnetic field gradients produced by eddy currents. Differences between the two methods may be particularly significant for studies of tumors where large concentrations of deoxyhemoglobin and other paramagnetic substances may cause significant susceptibility gradients. Mammary adenocarcinomas subcutaneously implanted in the flanks of female Fisher rats were studied. Magnetic resonance experiments were performed at 2 T. A surface coil was used to provide an RF gradient and to excite and detect signals from the tumors. The decay of echo amplitude as a function of evolution time was measured and the decay at short and long evolution times was analyzed independently to calculate two apparent diffusion coefficients (ADCs). The preparation was extremely stable and the standard error for 10 consecutive measurements of gradient echo amplitude made over 30-60 min with an RF gradient strength of 50 kHz/cm, gradient duration of 1 ms (i.e., 50 cycles/cm), and echo evolution time (td) of 1 s was generally +/- 0.8%. The ADC calculated from the decay at short evolution times was approximately 3 x 10(-5) cm2/s. The ADC calculated from the decay at longer evolution times was approximately 0.5 x 10(-5) cm2/s. Both ADCs decreased immediately following sacrifice and administration of Hydralazine. The experiments demonstrate that measurements of RF gradient echo amplitudes in tumors can be made in vivo with a high degree of reproducibility and suggest that RF gradient echo amplitudes are sensitive to acute physiological changes in tumors. This method may be useful for characterization of tumors and prediction and monitoring of effects of therapeutic agents.

Modification of tumour blood flow--a review

Tumour vascularization is based on two types of vessels, incorporated normal host tissue vessels and newly formed vessels, and is characterized by a wide heterogeneity. No adrenergic innervation has been related to newly formed tumour vessels but may still be found in incorporated normal vessels. In most studies vasoconstricting drugs were found to decrease tumour blood flow, while vasodilating drugs had no significant influence on tumour blood flow. From this it was concluded that the tumour vascular bed is normally in a state close to maximal dilatation, and this is supported by observations of hypoxia and local acidosis in tumour tissue. Some conflicting results have been reported with an increased tumour blood flow after administration of, for example, calcium channel blockers. Tumour blood modification is of interest in radiotherapy but also in tumour hyperthermia, where circulatory disturbances are explained on a multifactorial basis. Based on conflicting observations, with results varying with tumour-host systems studied and techniques for flow recording, it seems reasonable to concentrate further on methodological studies to develop clinically relevant techniques for tumour blood flow recording.

Effect of hydralazine on the blood flow in tumors and normal tissues in rats

The effects of hydralazine on the blood flow in various normal tissues and R3230 Ac adenocarcinomas grown in different sites in rats were studied. Tumors were induced in the kidney, liver, and flank (s.c.) by injection of small pieces of tumors. Tumors were also induced in small intestine, cecum, and mesentery by intraperitoneal injection of finely minced tumor tissue. The blood flow and cardiac output were measured by radioactive microsphere method. An intra-arterial injection of hydralazine at 2.5-10.0 mg/kg significantly reduced the blood flow in most normal tissues, whereas it markedly increased the blood flow in muscle. The blood flow in the brain and testes remained unchanged. The blood flow in the tumors varied depending on the tumor site and was markedly smaller than the blood flow in the host normal tissues in which the tumors grew. The blood flow in tumors decreased significantly upon injection of hydralazine except in the tumors grown in the liver, where the blood flow remained unchanged. The hydralazine injection slightly increased cardiac output and markedly decreased blood pressure. It appeared that the decreases in blood flow by hydralazine in the tumors and most normal tissues were caused mainly by diversion of blood to muscle, resulting in a marked increase in the muscle blood flow without a similar concomitant increase in cardiac output. The results obtained in the present study indicate that hydralazine may be useful for improving the efficacy of hyperthermia or for enhancing the toxicity of hypoxic cell specific bioreductive drugs in treating the tumors grown in skeletal muscle. However, the significant decline in blood flow in most normal tissues may pose potential problems in the use of hydralazine to enhance the effect of hyperthermia or bioreductive drugs on tumors grown in normal tissues other than muscle.

The effect of hydralazine on blood flow and misonidazole toxicity in human tumour xenografts

The effect of post-irradiation hypoxia induced by 5 or 30 mg/kg hydralazine has been studied in three human tumour xenografts (two rectocolic adenocarcinomas and one melanoma) treated with two doses of misonidazole similar to those used in patients (0.1 and 0.2 mg/g). Only a small sensitization was detected using an in vitro colony assay. These results are in marked contrast to the results obtained with rodent tumours. This difference between human tumour xenografts and rodent tumours might be explained by differences in the reduction of tumour blood flow after hydralazine administration (5 and/or 10 mg/kg). Using the laser Doppler technique, the tumour blood flow reduction was 33% and 25% of the control for NA11 and HRT18 tumours, respectively. In contrast, hydralazine induced a 60-70% reduction in blood flow in the murine SCCVII tumour. Using the fluorescent marker Hoechst 33342, the reduction in perfusion was again more pronounced in the murine tumour as compared to the Na11 and HRT18 xenografts. The differences between human tumour xenografts and rodent tumours are not linked to the mouse strain used (nude versus C3H) nor to a tumour bed effect.

Drug induced perturbations in tumor blood flow: therapeutic potential and possible limitations

Chemical modulation of tumor blood flow has until recently received relatively little attention as a therapeutic tool. Developments in the last few years, both in technology and in drug development, have changed this perspective. Fluorescence activated cell sorting techniques have provided evidence for the existence of acutely hypoxic cells resulting from transient fluctuations in microregional tumor blood flow in experimental tumor systems. We have used such techniques to assess the effects of three systemically administered agents, nicotinamide, flunarazine and Flusol-DA, on the amount of acute hypoxia in the SCCVII tumor. The most effective agent identified in this study is the benzamide analog nicotinamide. We suggest that compounds which modulate such hypoxia could well have a role in radiation therapy, particularly when combined with techniques which increase the oxygen carrying capacity of the blood. The potential of tumor blood flow reduction to improve the effectiveness of bioreductive agents administered alone or in combination with radiation and/or hyperthermia, is well established in experimental systems. Further data are presented, which show that combining hydralazine and the beta-blocker propranolol can provide greater reduction in tumor blood flow than observed with hydralazine alone. Potential limitations of drug induced reduction in tumor blood flow are discussed including the possibility of inducing hypoxia in normal tissues.

The use of blood flow modifiers to improve the treatment response of solid tumors

There is now considerable interest in the possible use of agents which can change blood flow in solid tumors and thereby alter the response of tumors to different treatments. In the current presentation we briefly review the types of agents which have the potential to modify tumor blood flow, using nicotinamide. hydralazine, and pyrazinamide as examples and illustrate how they can be used to improve the response of murine tumors to radiation, hyperthermia and cyclophosphamide.

Potentiation of the tumor cytotoxicity of melphalan by vasodilating drugs

Previous studies have shown that several vasoactive drugs can selectively reduce blood flow and increase hypoxia in experimental tumor systems. Our studies with one such agent, the vasodilator hydralazine, have clearly demonstrated that it can increase the tumor cytotoxicity of drugs which are known to be more toxic under hypoxic conditions. We have now extended our investigations to determine whether such selective reductions in tumor blood flow induced by hydralazine can increase the tumor cytotoxicity of other classes of cancer chemotherapeutic drugs. Our initial studies have involved the alkylating agent melphalan. Administration of hydralazine (5 mg/kg IP) at various times before or after melphalan results in increased tumor cytotoxicity in the Lewis lung carcinoma. An enhancement factor of between 2 and 3 was obtained in this tumor system. Similar results are observed if the vasodilator cadralazine is used. In contrast to the enhancement of the tumor cytotoxicity of melphalan by hydralazine, systemic toxicity is only increased by a factor of 1.2. Therefore, therapeutic gain may accrue from the use of vasodilating agents in combination with melphalan. Studies using spheroids to establish the mechanism responsible for the enhanced tumor cytotoxicity indicate that both hypoxia and pH can influence melphalan toxicity.

Effects of hydralazine on in vivo tumor energy metabolism, hematopoietic radiation sensitivity, and cardiovascular parameters

Energy metabolism of murine FSaII foot tumors was studied by in vivo 31P-MRS in C3Hf/Sed mice. Spectroscopy was performed following exposure to escalating doses of hydralazine (HYD) ip. At 0.25 mg/kg, HYD caused a 20% increase in PCr/Pi and had no significant effect on mean arterial blood pressure. HYD doses greater than or equal to 2 mg/kg lead to hypotension which was associated with a decrease in PCr, NTP, pH, and an increase in Pi (p less than 0.01 for control vs 10 mg/kg HYD). When mice were given ip injections of HYD (0.25, 1, 2 and 10 mg/kg) 10 min prior to whole body irradiation, spleen stem cell survival after 6 Gy was increased (2.19 colonies in control animals vs 6.74 colonies per spleen in animals treated with greater than or equal to 2 mg/kg HYD), as was the LD50/30 dose (6.49 Gy [control] vs 9.00 Gy [10 mg/kg HYD]). The data provide evidence that PCr/Pi is a useful indicator of perfusion efficiency (and indirectly of hypoxic cell fraction) in FSaII tumors. These observations suggest that HYD may be a useful adjuvant for hyperthermic treatment of tumors and for potentiation of agents specifically toxic to hypoxic or nutrient-deprived cancer cells. HYD should be used with care in patients receiving radiation treatments or other therapies for which hypoxia can unfavorably affect treatment outcome.

Cinnarizine and flunarizine as radiation sensitisers in two murine tumours

The effect of the calcium antagonists, cinnarizine and flunarizine on the radiation sensitivity of two murine tumours, RIF-1 and SCCVII/St was investigated. Initial experiments giving the compounds at 50 mg kg-1 i.p. indicated that cinnarizine had no effect on cell survival after 20 Gy of X-rays in the RIF-1 sarcoma and only a small effect in the SCCVII/St carcinoma. However, flunarizine produced a small radiosensitisation in the RIF-1 tumour and a substantial sensitisation in the SCCVII/St tumour. Subsequent experiments in the SCCVII/St tumour indicated that the optimal radiosensitising dose of flunarizine was approximately 5 mg kg-1, although some sensitisation was apparent throughout the range of 0.05-500 mg kg-1. Flunarizine produced a parallel shift in the X-ray dose response curve, equivalent to a 5-fold reduction in hypoxic fraction. In a normal tissue study, 5 mg kg-1 flunarizine did not enhance the reduction in white cell counts produced by X-ray doses of 2-8 Gy. These data suggest that flunarizine may have some potential use as a radiosensitiser.

Mechanism of action of the selective tumor radiosensitizer nicotinamide

Nicotinamide has been shown to selectively enhance the radiation damage of tumors in preference to normal tissues. Our present study was an investigation into the mechanism responsible for this effect in the SCCVII/St tumor model grown on the backs of C3H/km mice. A large single injection of nicotinamide (1000 mg/kg), given intraperitoneally 60 minutes before whole body irradiation, significantly enhanced the radiation response of SCCVII tumors as measured by an in vivo/in vitro excision assay performed 24 hr following irradiation. It also gave rise to an almost 4-fold reduction in the binding of 14C-misonidazole, injected 1 hr after the nicotinamide and measured by scintillation counting of excised tumor material 24 hr later. This suggested that nicotinamide was decreasing the degree of tumor hypoxia. Attempts were made to correlate these results with nicotinamide-induced changes in tumor blood flow using the techniques of 133Xe clearance, 86RbCl extraction and Hoechst 33342 fluorescent labelling. Nicotinamide produced between a 30-40% increase in mean tumor cell fluorescence of Hoechst 33342, which was consistent with an increase in tumor blood flow. A similar response was obtained using the uptake of 86RbCl as the end point. However, no statistically significant difference was seen between the tumor blood flow of control and nicotinamide treated mice using the 133Xe clearance procedure. These results are discussed with respect to their clinical implications.

Potentiation of the anti-tumour effect of melphalan by the vasoactive agent, hydralazine

The vaso-active drug hydralazine causes a considerable increase in the cytotoxic effect of melphalan towards the KHT tumour in mice. The enhancement in response, measured as the concentration of melphalan required to achieve a given tumour response, is 3.0 and 2.35 when determined using the regrowth delay assay and the technique for determining surviving fraction in vitro following treatment in vivo respectively. In contrast, measurement of systemic toxicity shows that the addition of hydralazine only causes a small increase (ER = 1.15) in melphalan damage. This suggests that the drug combination may have some therapeutic benefit. The tumour specificity for the action of hydralazine is supported by the finding that binding of 3H-misonidazole is increased in tumours but not in other tissues when mice are treated with hydralazine. Increased binding of labelled misonidazole is associated with an increase in the level and duration of hypoxia, which will occur as a consequence of changes in tumour blood flow brought about by hydralazine. However, hypoxia per se is not responsible for the enhanced effect of melphalan, since the agent BW12C, which also induces substantial tumour hypoxia as a result of changing the O2 affinity of haemoglobin, has no effect on melphalan tumour cytotoxicity.

Chemical modification of tumour blood flow

The preferential modification of tumour blood flow with chemical agents has been extensively investigated for the purpose of improving the treatment of tumours with ionizing radiation, chemicals and hyperthermia. In this paper I have utilized representative vasoactive drugs to demonstrate the various mechanisms via which tumour haemodynamics can be modified selectively. Chemicals which alter tumour blood flow by changing blood viscosity are also discussed. Of particular interest are the calcium entry blockers, flunarizine and verapamil, which have been shown to increase tumour blood flow, tumour oxygenation, tumour radiation sensitivity and the cytotoxicity of chemotherapeutic agents. These encouraging research results provide compelling evidence that these two drugs may prove to be useful adjuvants for radiation therapy.

The effect of hydralazine on the tumor cytotoxicity of the hypoxic cell cytotoxin RSU-1069: evidence for therapeutic gain

The effect of the vasodilator hydralazine on both the tumor and systemic toxicity of RSU-1069 has been evaluated in C57B1 mice bearing Lewis lung tumors. The results obtained indicate that both hydralazine and RSU-1069 are cytotoxic to the Lewis lung tumor on their own. However, administration of hydralazine (5 mg/kg PO) at times up to either 3 hr before or 3 hr after RSU-1069 (0.1 mg/g IP) results in a level of cell killing greater than expected from additive effects. This potentiation by hydralazine was observed with doses of RSU-1069 from 0.01 to 0.1 mg/g. The results obtained using excision assays were confirmed using in situ growth delay as the endpoint. Growth delay (+/- s.e.m.) values for tumors to double in volume of 1.5 (+/- 1.2), 2.0 (+/- 1.3) and 6.0 (+/- 0.9) were obtained for hydralazine (5 mg/kg PO) alone, RSU-1069 (0.1 mg/g IP) alone and for hydralazine administered at the same time as RSU-1069 respectively. In contrast to the potentiating effect of hydralazine on the tumor cytotoxicity of RSU-1069, it had no significant effect on the systemic toxicity of RSU-1069 as measured by LD50/30d. No detailed studies to examine the mechanism responsible for the potentiation of tumor cytotoxicity have been performed in the present study. However, the results obtained would be consistent with previous reports that vasodilators such as hydralazine can selectively reduce tumor blood flow and thus oxygenation. Such reduced tumor oxygenation would increase the cytotoxic effects of RSU-1069 which is known to be more toxic to cells at reduced oxygen levels.

Potentiation of RSU-1069 tumour cytotoxicity by 5-hydroxytryptamine (5-HT)

It is known that many solid animal tumours have a lower oxygenation level than most normal tissues and, in addition, that this level of oxygenation can be further decreased by systemic administration of 5-hydroxytryptamine (5-HT). The present study has investigated if such selective decrease in tumour oxygenation can be exploited by using the hypoxic cell cytotoxin, RSU-1069. The results obtained show that 5-HT at a dose of 5 mg kg-1, although not cytotoxic alone, can potentiate the cytotoxic effects of RSU-1069 in the Lewis lung carcinoma over the dose range 0.01-0.15 mg g-1. Maximum potentiation occurs when 5-HT is administered after RSU-1069. Potentiation of RSU-1069 cytotoxicity was observed using both the soft agar excision assay as an endpoint as well as in situ growth delay. In addition, the study shows that potentiation of RSU-1069 (0.1 mg g-1) cytotoxicity can be seen with 5-HT doses as low as 0.5 mg kg-1. In contrast to the tumour cytotoxicity results, 5-HT at a dose of 5 mg kg-1 i.p. did not affect the systemic toxicity, as measured by LD50/7d of RSU-1069. Thus, these results indicate that 5-HT can increase the therapeutic efficiency of RSU-1069. Such a finding is consistent with the rationale that selective reduction in tumour blood flow and oxygenation induced by 5-HT can be exploited using the hypoxic cell cytotoxin RSU-1069.