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

Influence of 2-Nitroimidazoles in the Response of FaDu Cells to Ionizing Radiation and Hypoxia/Reoxygenation Stress

Cellular adaptations to hypoxia promote resistance to ionizing radiation (IR). This presents a challenge for treatment of head and neck cancer (HNC) that relies heavily on radiotherapy. Standard radiosensitizers often fail to reach diffusion-restricted hypoxic cells, whereas nitroimidazoles (NIs) [such as iodoazomycin arabinofuranoside (IAZA) and fluoroazomycin arabinofuranoside (FAZA)] can preferentially accumulate in hypoxic tumours. Here, we explored if the hypoxia-selective uptake of IAZA and FAZA could be harnessed to make HNC cells (FaDu) susceptible to radiation therapy. Cellular response to treatment was assessed through clonogenic survival assays and by monitoring DNA damage (immunofluorescence staining of DNA damage markers, γ-H2AX and p-53BP1, and by alkaline comet assay). The effects of reoxygenation were studied using the following assays: estimation of nucleoside incorporation to assess DNA synthesis rates, immunofluorescent imaging of chromatin-associated replication protein A as a marker of replication stress, and quantification of reactive oxygen species (ROS). Both IAZA and FAZA sensitized hypoxic HNC cells to IR, albeit the former is a better radiosensitizer. Radiosensitization by these compounds was restricted only to hypoxic cells, with no visible effects under normoxia. IAZA and FAZA impaired cellular adaptation to reoxygenation; high levels of ROS, reduced DNA synthesis capacity, and signs of replication stress were observed in reoxygenated cells. Overall, our data highlight the therapeutic potentials of IAZA and FAZA for targeting hypoxic HNC cells and provide rationale for future preclinical studies.

Defining normoxia, physoxia and hypoxia in tumours-implications for treatment response

Tumour hypoxia is increasingly recognized as a major deleterious factor in cancer therapies, as it compromises treatment and drives malignant progression. This review seeks to clarify the oxygen levels that are pertinent to this issue. It is argued that normoxia (20% oxygen) is an extremely poor comparator for "physoxia", i.e. the much lower levels of oxygen universally found in normal tissues, which averages about 5% oxygen, and ranges from about 3% to 7.4%. Importantly, it should be recognized that the median oxygenation in untreated tumours is significantly much lower, falling between approximately 0.3% and 4.2% oxygen, with most tumours exhibiting median oxygen levels <2%. This is partially dependent on the tissue of origin, and it is notable that many prostate and pancreatic tumours are profoundly hypoxic. In addition, therapy can induce even further, often unrecognized, changes in tumour oxygenation that may vary longitudinally, increasing or decreasing during treatment in ways that are not always predictable. Studies that fail to take cognizance of the actual physiological levels of oxygen in tissues (approximately 5%) and tumours (approximately 1%) may fail to identify the real circumstances driving tumour response to treatment and/or malignant progression. This can be of particular importance in genetic studies in vitro when comparison to human tumours is required.

Mechanisms of blood flow and hypoxia production in rat 9L-epigastric tumors

Classical descriptions of tumor physiology suggest two origins for tumor hypoxia; steady-state (diffusion-limited) hypoxia and cycling (perfusion-modulated) hypoxia. Both origins, primarily studied and characterized in murine models, predict relatively small, isolated foci or thin shells of hypoxic tissue interspersed with contrasting oxic tissue. These foci or shells would not be expected to scale with overall tumor size since the oxygen diffusion distance (determined by oxygen permeability and tissue oxygen consumption rate) is not known to vary dramatically from tumor to tumor. We have identified much larger (macroscopic) regions of hypoxia in rat gliosarcoma tumors and in larger human tumors (notably sarcomas and high-grade glial tumors), as indicated by biochemical binding of the hypoxia marker, EF5. Thus, we considered an alternative cause of tumor hypoxia related to a phenomenon first observed in window-chamber tumor models: namely longitudinal arteriole gradients. Although longitudinal arteriole gradients, as originally described, are also microscopic in nature, it is possible for them to scale with tumor size if tumor blood flow is organized in an appropriate manner. In this organization, inflowing blood would arise from relatively well-oxygenated sources and would branch and then coalesce to poorly-oxygenated outflowing blood over distances much larger than the length of conventional arterioles (multi-millimeter scale). This novel concept differs from the common characterization of tumor blood flow as disorganized and/or chaotic. The organization of blood flow to produce extended longitudinal gradients and macroscopic regional hypoxia has many important implications for the imaging, therapy and biological properties of tumors. Herein, we report the first experimental evidence for such blood flow, using rat 9L gliosarcoma tumors grown on the epigastric artery/vein pair.

Mathematical model of antibody targeting: important parameters defined using clinical data

Antibody-targeted therapy of cancer has shown benefits in the treatment of some cancers but selective delivery has not been optimized. Many parameters influence antibody targeting; some will have a greater effect than others and their effects will generally be interrelated. They include effects of blood flow and pressure, vascular permeability, venous and lymphatic drainage, permeation through extravascular spaces, antibody clearance, specificity, affinity and resistance to degradation. Quantitative data about the behaviour of targeting systems can be collected, and it is possible to describe the system in terms of compartments interconnected by equations defining the passage of targeting agents between them. A mathematical model of antibody targeting can thus be built. We have collected data on the time course of the distribution of four different antibody molecules of molecular weight 27, 100 and 150 kDa directed against carcinoembryonic antigen in patients with colorectal cancer. Laboratory data were used for parameters which could not be measured in patients. These data have been used to test the validity of the model for man and to develop it so that it is consistent with the diverse clinical data. The model is then used to understand the effects of changes to a parameter on tumour targeting efficiency and to select those parameters which have the greatest effect in therapy. Affinity of antibody, flow of antibody through the tumour and rate of elimination of antibody from the tumour were shown to be the most powerful parameters determining antibody localization. These concepts can be used to determine design parameters for antibody-targeted cancer therapy.

ARCON: accelerated radiotherapy with carbogen and nicotinamide in head and neck squamous cell carcinomas. The experience of the Co-operative group of radiotherapy of the european organization for research and treatment of cancer (EORTC)

BACKGROUND

Since there is increasing evidence that both acute (perfusion-limited) and chronic (diffusion-limited) hypoxia, and tumor repopulation may prejudice the outcome of radiotherapy, the combination of carbogen (95% oxygen-5% carbon dioxide) and nicotinamide with accelerated radiotherapy (ARCON) should reduce the impact of these factors of radioresistance.

AIM

This clinical study was aimed at determining the feasibility, as well as the qualitative and quantitative toxic effects of a therapeutic approach based on ARCON, and assessing the tumor response rates that can be achieved with this regime in patients with locally advanced tumors of the head and neck.

METHODS

A phase I/II study conducted between 1993 and 1996 by the Co-operative Group of Radiotherapy of the EORTC included three consecutive steps: accelerated fractionation (AF) combined with carbogen (11 analyzable patients), AF combined with the daily administration of nicotinamide (n=10), and AF with both carbogen and nicotinamide (n=17). Radiotherapy was based on an accelerated regime (72 Gy in 5.5 weeks). Nicotinamide was delivered 90 min before the first irradiation session, at a daily dose of 6 g. Carbogen breathing started 5 min before irradiation and lasted throughout the entire radiotherapy sessions.

RESULTS

No significant difference in loco-regional toxicity was found among the three study steps, when carbogen and nicotinamide, either alone or in combination, were combined with AF. The feasibility of the ARCON protocol, as proposed in the present EORTC study, appears to be significantly impaired when nicotinamide is added, at a daily dose of 6 g, to AF and carbogen, in an unselected group of patients. More than 20% of patients experienced grade 2 or 3 emesis. It also demonstrates, in unselected groups of patients, no significant difference in tumor response and local control when carbogen and nicotinamide, either alone or in combination, are added to accelerated radiotherapy. The percentages of objective response at 2 months were 81, 70 and 87%, respectively.

CONCLUSION

Future ARCON trials should target selected head and neck tumor localizations and stages, and a lower nicotinamide dose is needed to reduce severe upper gastro-intestinal toxicity.

In vivo and in vitro evaluation of combretastatin A-4 and its sodium phosphate prodrug

The anti-tumour effects and mechanism of action of combretastatin A-4 and its prodrug, combretastatin A-4 disodium phosphate, were examined in subcutaneous and orthotopically transplanted experimental colon tumour models. Additionally, the ability of these compounds to directly interfere with endothelial cell behaviour was also examined in HUVEC cultures. Combretastatin A-4 (150 mg kg(-1), intraperitoneally (i.p.)) and its water-soluble prodrug (100 mg kg(-1), i.p.) caused almost complete vascular shutdown (at 4 h), extensive haemorrhagic necrosis which started at 1 h after treatment and significant tumour growth delay in MAC 15A subcutaneous (s.c.) colon tumours. Similar vascular effects were obtained in MAC 15 orthotopic tumours and SW620 human colon tumour xenografts treated with the prodrug. More importantly, in the orthotopic models, necrosis was seen in vascularized metastatic deposits but not in avascular secondary deposits. The possible mechanism giving rise to these effects was examined in HUVEC cells. Here cellular networks formed in type I calf-skin collagen layers and these networks were completely disrupted when incubated with a non-cytotoxic concentration of combretastatin A-4 or its prodrug. This effect started at 4 h and was complete by 24 h. The same non-cytotoxic concentrations resulted in disorganization of F-actin and beta-tubulin at 1 h after treatment. In conclusion, combretastatin A-4 and its prodrug caused extensive necrosis in MAC 15A s.c. and orthotopic colon cancer and metastases, resulting in anti-tumour effects. Necrosis was not seen in avascular tumour nodules, suggesting a vascular mechanism of action.

RSR13, a synthetic allosteric modifier of hemoglobin, as an adjunct to radiotherapy: preliminary studies with EMT6 cells and tumors and normal tissues in mice

RSR13, 2[4-[[(3,5dimethylanilino)carbonyl]methyl]phenoxy]-2-methylpropion ic acid, a synthetic allosteric modifier of hemoglobin, reduces the affinity of hemoglobin for oxygen. The experiments reported here examined the effect of treatment with RSR13, combined with oxygen breathing, on the radiation response of EMT6 mammary tumors in BALB/c mice and of two normal tissues. RSR13 plus oxygen breathing increased the response of EMT6 tumors to irradiation. RSR13 had no discernible effects on tumors rendered maximally hypoxic by nitrogen asphyxiation, no discernible cytotoxic effects in EMT6 tumors, and no effect on the viability or radiation response of EMT6 cells in vitro under either aerobic or hypoxic conditions. The effects of RSR13 therefore reflect changes in tumor oxygenation, rather than a direct cytotoxic or radiosensitizing effect of the drug. RSR13 plus oxygen reduced the hypoxic fraction to 9% from the value of 24% found in both air-breathing and oxygen-breathing mice. Treatment with RSR13 plus oxygen did not alter the radiation response of the bone marrow progenitor cells (CFU-S) or acute radiation reactions in the skin. The improvement in tumor radiation response produced by treatment with RSR13 plus oxygen, combined with the absence of enhanced radiation reactions in the normal tissues, support further testing of RSR13 as an adjunct to radiotherapy.

Pharmacokinetics of nicotinamide in cancer patients treated with accelerated radiotherapy: the experience of the Co-operative Group of Radiotherapy of the European Organization for Research and Treatment of Cancer

BACKGROUND

The EORTC has initiated studies to combine nicotinamide with carbogen in accelerated fractionation schedules (ARCON), since for some tumour types, acute and chronic hypoxia as well as treatment protraction may prejudice the outcome of radiotherapy. The tolerable dose of nicotinamide and the optimal interval for administration need to be ascertained.

AIM

Full pharmacokinetic profiles of nicotinamide concentrations in plasma were analyzed repeatedly in 15 patients to determine the inter- and intra-patient variability in peak plasma concentrations and the optimum times for administering nicotinamide as a radiosensitizer.

METHODS

Nicotinamide (Nicobion) was administered in tablet form to patients with advanced head and neck and non-small cell lung carcinomas. A standard 6 g dose was given regardless of body weight after an overnight fast and at least 30 min before breakfast. In 15 patients, blood samples were taken prior to and 1, 2, 4, 6, 8, 12 and 24 h after administration of the drug. This full profile was determined on two to four occasions for the head and neck cancer patients and on two occasions for the lung cancer patients. For each profile, the maximum concentration of nicotinamide (Cmax), time to peak plasma concentration (Tmax), elimination half-lives (t1/2) and area under the curve (AUC) were determined. Compliance was recorded and nausea and vomiting were graded on a 0-3 scale. Complete profiles of the five major metabolites were also obtained.

RESULTS

In the 48 complete sets of blood samples, peak plasma concentrations ranged from 787 to 2312 nmol/ml with a median value of 1166 nmol/ml. The peak plasma concentration was achieved at 1 h in only 54% of the pharmacokinetic profiles, but at this time 92% of the profiles had already exceeded the target concentration of 700 nmol/ml, the level required in the mouse for tumour radiosensitization. The median t1/2 for all 15 cases was 9.3 h, with minimum and maximum values of 4.2 and 26.8 h. The highest concentrations of nicotinamide metabolites were found to be the N-oxide, 2-pyridone and 1-methylnicotinamide. The toxicity (nausea and vomiting) was scored and found not to be correlated with any of the pharmacokinetic parameters.

CONCLUSIONS

The plasma concentrations considered necessary to radiosensitize can easily be exceeded with a dose of 6 g taken as 12 x 500 mg in tablet form; 700 nmol/ml was achieved in all patients and apparently would have been achieved in most even with a considerable reduction in dose. An adequate time between administration and radiotherapy appeared to be 1 h with this drug formulation for 92% of the profiles.

Carbogen and nicotinamide combined with unconventional radiotherapy in glioblastoma multiforme: a new modality treatment

PURPOSE

A new radiotherapy schedule to treat glioblastoma multiforme after surgery, combining nicotinamide and carbogen.

METHODS AND MATERIALS

We analyzed 36 patients with glioblastoma multiforme treated after surgery with radiotherapy, Nicotinamide and Carbogen as follows: 7 patients were treated with accelerated fractionation: two fractions/day, 1.5 cGy/fraction, 6 h interval, 5 days/week, total dose 60 Gy in 4 weeks; 8 patients were treated with the same irradiation scheduling plus Nicotinamide at the dose of 4 g and 2 g in capsules, respectively, 1 h before the first and the second irradiation fraction; 21 patients were treated with accelerated radiotherapy, Nicotinamide, and Carbogen (inhaled 10 min before radiotherapy and during the whole course of irradiation). On the basis of surgical removal our patients were subdivided in three groups: totally resected, with residual tumor <50%, or >50%. Radiotherapy with accelerated fractionation was completed in the scheduled time without side effects on the whole group of patients and Carbogen inhalation did not cause significant change of cardiopulmonar parameters. The toxicity observed was predominant in the gastrointestinal tract and was related to Nicotinamide.

RESULTS

The median survival time (M.S.T.) was 10 months, as reported by others authors with conventional treatment, but in patients without surgical residual tumor and submitted to the complete treatment schedule, the survival at 35 months was around 25%.

CONCLUSIONS

We conclude that this method is feasible with acceptable toxicity; analyzing the survival curves appears to be a trend towards an improvement in survival in the subgroup of patients with gross total removal treated with the combination of Carbogen, Nicotinamide, and accelerated fractionation.

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.

Nicotinamide pharmacokinetics in patients

Pharmacokinetic analyses were performed on blood samples of 12 patients undergoing treatment with nicotinamide, hyperthermia and radiation therapy for a variety of recurrent/metastatic cancers. Escalating oral doses of 3, 4, 5, 6 and 10 g of nicotinamide showed a linear relationship between maximum recorded plasma concentrations and the dose in grams (correlation coefficient, r = 0.91). Maximum plasma levels were observed by 30 min in most patients ingesting up to 6 g of nicotinamide. In marked contrast, five out of six patients ingesting 10 g of nicotinamide demonstrated increasing plasma levels at least up to 3 h post-ingestion. Doses up to 6 g were well tolerated and resulted in average maximum recorded plasma levels (mean +/- 1 SEM) of 156.4 +/- 33.6 micrograms/ml. Doses of 10 g were generally not well tolerated, but a high plasma level was maintained on average for at least 4 h. Plasma concentrations of the above order have been previously associated with maximal enhancement of radiation damage in mouse tumor models. This suggests that radiosensitization can be expected to occur in human tumors following oral administration of a safe and well tolerated dose of 6 g. However, at higher doses (i.e., 10 g), the pharmacokinetics, and perhaps radiosensitization, may differ markedly.

Inhibition of the regulation of intracellular pH: potential of 5-(N,N-hexamethylene) amiloride in tumour-selective therapy

The viability of cells within the acidic microenvironment found in solid tumours is expected to depend on the regulation of intracellular pH (pHi). 5-(N,N-hexamethylene) amiloride (HMA) is a potent inhibitor of the Na+/H+ antiport, a major mechanism for the regulation of pHi. We have therefore studied the cytotoxicity of HMA in combination with nigericin, a cell-acidifying agent, for EMT-6 cells in monolayer cell culture, in spheroids and in a murine tumour model. The combination of nigericin and HMA was toxic to cells in tissue culture at extracellular pH (pHe) < or = 6.8 (as may be found in tumours) but not at pH 7.0 or above (as in most normal tissues). Compared with amiloride, the relative potency of HMA in causing in vitro cytotoxicity (approximately 100-fold) was similar to that for inhibition of the Na+/H+ antiport. The fluorescent probe Hoechst 33342 was used with flow cytometry to study the cytotoxicity of HMA and nigericin at different depths in multicellular tumour spheroids. Only small differences in the level of cell survival were observed, but higher concentrations of HMA were required as compared with those giving equal levels of survival in monolayer culture. The pharmacokinetics of HMA in mice was studied by using high-performance liquid chromatography: after intraperitoneal injection of 20 micrograms g-1, the plasma level of HMA peaked at 8 microM after about 15 min and decreased to 1 microM at 120 min; the half-life was 35 min. Nigericin and HMA, at doses of 1.25 micrograms g-1 and 10 micrograms g-1 respectively, failed to cause significant cell killing in the EMT-6 murine tumour, but the surviving fraction was reduced to approximately 0.004 when hydralazine was administered with nigericin and HMA. Local tumour irradiation (15 Gy), followed by treatment with these drugs, led to cell killing that was additive to the effects of drugs and radiation alone, so that hypoxic cells which survived radiation did not appear more sensitive to pH-dependent drug treatment. Acid-mediated therapy can lead to cell death in murine solid tumours, but further measures will be required before the strategy can be exploited clinically.

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.

Nicotinamide and carbogen: relationship between pO2 and radiosensitivity in three tumour lines

The effects of carbogen breathing, nicotinamide injection and their combination on tumour radiosensitivity were correlated with changes in tumour O2 tension to determine the relationship between radiosensitivity and measured pO2. The radiosensitivity (in vivo-in vitro colony assay) and O2 tension (computerized pO2 histograph KIMOC 6650) of two human xenografted tumours (HRT18 and Na11+) and one murine tumour (EMT6) were measured under similar experimental conditions. A single dose of radiation was delivered (8 Gy for HRT18 and Na11+, 12 Gy for EMT6). Carbogen breathing, nicotinamide injection, and their combination all significantly increased radiosensitivity in the three cell lines (p < 0.05); the most efficient treatment was carbogen plus nicotinamide. The radiosensitization was optimum for EMT6 and Na11+. Mean and median pO2 increased with all three treatments, except for carbogen breathing in EMT6. Carbogen breathing had little effect on the proportion of low pO2 values, but induced pO2 values > 30 mmHg in all three tumour lines. Nicotinamide decreased the proportion of low pO2 values. This effect is larger with the combination carbogen plus nicotinamide. Almost all pO2 values < 2.5 mmHg were eliminated for HRT18 and EMT6. The relationship between radiosensitization and pO2 was significant when pO2 was expressed as the percentage of values below 7, 8, 9 and 10 mmHg for the three lines.

Should carbogen and nicotinamide be given throughout the full course of fractionated radiotherapy regimens?

PURPOSE

Tumor radiosensitization with carbogen and nicotinamide (CON) was compared when both agents were given throughout fractionated radiotherapy with the sensitization observed when administered with only half of the fractions. The effect of overall treatment time on the local control of tumors irradiated in air or with CON was also investigated.

METHODS AND MATERIALS

Local tumor control of a rodent adenocarcinoma, CaNT, was studied using eight different 20-fraction x-ray regimens. An overall time of either 10 or 20 days was used and CON was given with all, the first half or last half of the treatment.

RESULTS

Relative to air, all six sensitizer combinations gave a large and significant increase in sensitization (p << 0.00001). Enhancement ratios were 1.9 and 2.1 when CON was given with all 20 fractions in either 10 or 20 days, respectively. For both overall times, enhancement ratios were reduced by 15-25% when CON was given with only half of the fractions. In air, reducing the treatment time from 20 to 10 days gave a small but significant decrease in the isoeffective doses. When CON was administered with either all or part of a schedule, varying the treatment time had little or no effect on local tumor control. No toxic side-effects were encountered when the sensitizers were administered 10 or 20 times, either once or twice per day.

CONCLUSION

CON is an effective and non-toxic tumor radiosensitizer. In CaNT tumors, a significantly greater effect is seen when CON is given with every fraction of the schedule. The sensitizers reduced or abolished the sparing effect of overall time.

Nicotinamide and carbogen: major effect on the radiosensitivity of EMT6 and HRT18 tumours

The effect of nicotinamide and/or carbogen breathing on the response of EMT6 and HRT18 tumours, irradiated in vivo, was evaluated using an in vitro colony assay. In the single dose schedules, the most efficient treatment was carbogen plus nicotinamide compared to air breathing mice. In a fractionated regime carbogen plus nicotinamide increased also response to radiation.

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.

Blood flow modification in the SCCVII tumor: effects of 5-hydroxytryptamine, hydralazine, and propranolol

The effects of hydralazine, 5-hydroxytryptamine (5-HT), and propranolol on blood flow in the SCCVII tumor were assessed using laser Doppler flowmetry. Both hydralazine and 5-HT, at doses of 1 and 5 mg/kg, reduced blood flow, as did propranolol at 10 mg/kg. Hydralazine and 5-HT at doses of 0.25 mg/kg slightly increased tumor blood flow, and a 10-20% increase in blood flow was also observed after 1 mg/kg of propranolol. However, propranolol at 1 mg/kg enhanced the blood flow reduction observed with 1 mg/kg of hydralazine. The concomitant administration of hydralazine and propranolol at these doses also translated into increased potentiation of the tumor cytotoxicity of the hypoxic cell cytotoxin RSU-1069.

Tumor blood flow changes induced by chemical modifiers of radiation response

This study was designed to investigate blood flow changes induced by chemical agents that are commonly used in combination with radiation therapy. The hypoxic cell cytotoxin RSU 1069 at a dose of 100 mg/kg was shown to reduce blood flow by 80% in both SCCVII and C3H mammary tumors. Blood flow reductions of 30-40% were also observed in both tumors following administration of the radiation sensitiser pimonidazole (Ro-03-8799) at a dose of 500 mg/kg. Moreover studies in the C3H mammary tumor indicated that this effect was tumor size-dependent, being more profound and of longer duration in 500 mg than in 100 mg tumors. Blood flow decreases were also observed in the SCCVII tumor after administration of adriamycin at a dose of 15 mg/kg. Blood flow decreased by over 80% immediately after drug administration but recovered to remain 20% below control values 1 hr after drug administration. In contrast to the other agents studied, cis platinum at a dose of 2 mg/kg produced a small 20-30% increase in blood flow which persisted over the 1 hr observation period. The potential implications of these findings are discussed.

Hydralazine does not increase hypoxia in tumors growing in preirradiated tissue

There is a growing interest in the exploitation of hypoxia in solid tumors for therapeutic gain by the use of hypoxic cytotoxins and other agents. Tumor hypoxia can be greatly increased in a number of animal tumor models with the vasoactive drug hydralazine (HDZ), and in some cases this potentiates the effect of drugs that are selectively toxic to hypoxic cells. Our interest was to determine if HDZ would also increase tumor hypoxia in tumors growing in previously irradiated normal tissue- a situation such as might be found in the clinic with regrowing solid tumors after radiotherapy. SCCVII tumors in untreated mice were compared with tumors growing in a previously irradiated tissue with respect to their level of hypoxia in response to HDZ. HDZ increased tumor hypoxia in the tumors from unirradiated mice as measured by 14C-misonidazole binding. However, HDZ had little or no effect on tumor hypoxia in tumors growing in previously irradiated sites. We also showed that the pre-HDZ extent of hypoxia was higher in tumors growing in a previously irradiated tissue. This may in part explain the lack of effect of HDZ in these tumors. The lack of response of the tumors growing in irradiated sites suggests a limitation on the use of HDZ in combination with specific hypoxic cytotoxins or other chemotherapeutic drugs in the treatment of recurrent solid tumors. The data also show that if such tumors have an elevated hypoxic fraction relative to their counterparts growing in untreated sites, these tumors might be intrinsically more resistant to conventional radiotherapy, but, on the other hand, might be sensitive to bioreductive drugs and more likely to be radiosensitized by a hypoxic cell sensitizer.