Sensitizers and radiation dose fractionation: results and interpretations

Design, Synthesis and Anticancer Evaluation of Nitroimidazole Radiosensitisers

The role of hypoxic tumour cells in resistance to radiotherapy, and in suppression of immune response, continues to endorse tumour hypoxia as a bona fide, yet largely untapped, drug target. Radiotherapy innovations such as stereotactic body radiotherapy herald new opportunities for classical oxygen-mimetic radiosensitisers. Only nimorazole is used clinically as a radiosensitiser, and there is a dearth of new radiosensitisers in development. In this report, we augment previous work to present new nitroimidazole alkylsulfonamides and we document their cytotoxicity and ability to radiosensitise anoxic tumour cells in vitro. We compare radiosensitisation with etanidazole and earlier nitroimidazole sulfonamide analogues and we identify 2-nitroimidazole and 5-nitroimidazole analogues with marked tumour radiosensitisation in ex vivo assays of surviving clonogens and with in vivo tumour growth inhibition.

Biomimetic nanoscale metal-organic framework harnesses hypoxia for effective cancer radiotherapy and immunotherapy

Tumor hypoxia presents a major impediment to effective cancer therapy with ionizing radiation and immune checkpoint inhibitors. Here we report the design of a biomimetic nanoscale metal–organic-framework (nMOF), Hf-DBP-Fe, with catalase-like activity to decompose elevated levels of H₂O₂ in hypoxic tumors to generate oxygen and hydroxyl radical. The generated oxygen attenuates hypoxia to enable radiodynamic therapy upon X-ray irradiation and fixes DNA damage while hydroxyl radical inflicts direct damage to tumor cells to afford chemodynamic therapy. Hf-DBP-Fe thus mediates effective local therapy of hypoxic cancer with low-dose X-ray irradiation, leading to highly immunogenic tumor microenvironments for synergistic combination with anti-PD-L1 immune checkpoint blockade. This combination treatment not only eradicates primary tumors but also rejects distant tumors through systemic anti-tumor immunity. We have thus advanced an nMOF-based strategy to harness hypoxic tumor microenvironments for highly effective cancer therapy using a synergistic combination of low dose radiation and immune checkpoint blockade.

Biomimetic Hf-DBP-Fe harnesses tumor hypoxia for cancer treatment via RT-RDT and CDT as well as synergistic combination with immune checkpoint blockade.

Overcoming Radioresistance: Small Molecule Radiosensitisers and Hypoxia-activated Prodrugs

The role of hypoxia in radiation resistance is well established and many approaches to overcome hypoxia in tumours have been explored, with variable success. Two small molecule strategies for targeting hypoxia have dominated preclinical and clinical efforts. One approach has been the use of electron-affinic nitroheterocycles as oxygen-mimetic sensitisers. These agents are best exemplified by the 5-nitroimidazole nimorazole, which has limited use in conjunction with radiotherapy in head and neck squamous cell carcinoma. The second approach seeks to leverage tumour hypoxia as a tumour-specific address for hypoxia-activated prodrugs. These prodrugs are selectively activated by reductases under hypoxia to release cytotoxins, which in some instances may diffuse to kill surrounding oxic tumour tissue. A number of these hypoxia-activated prodrugs have been examined in clinical trial and the merits and shortcomings of recent examples are discussed. There has been an evolution from delivering DNA-interactive cytotoxins to molecularly targeted agents. Efforts to implement these strategies clinically continue today, but success has been elusive. Several issues have been identified that compromised these clinical campaigns. A failure to consider the extravascular transport and the micropharmacokinetic properties of the prodrugs has reduced efficacy. One key element for these 'targeted' approaches is the need to co-develop biomarkers to identify appropriate patients. Hypoxia-activated prodrugs require biomarkers for hypoxia, but also for appropriate activating reductases in tumours, as well as markers of intrinsic sensitivity to the released drug. The field is still evolving and changes in radiation delivery and the impact of immune-oncology will provide fertile ground for future innovation.

The changing paradigm of tumour response to irradiation

Tumours contain multiple different cell populations, including cells derived from the bone marrow as well as cancer-associated fibroblasts and various stromal populations including the vasculature. The microenvironment of the tumour cells plays a significant role in the response of the tumour to radiation treatment. Low levels of oxygen (hypoxia) caused by the poorly organized vasculature in tumours have long been known to affect radiation response; however, other aspects of the microenvironment may also play important roles. This article reviews some of the old literature concerning tumour response to irradiation and relates this to current concepts about the role of the tumour microenvironment in tumour response to radiation treatment. Included in the discussion are the role of cancer stem cells, radiation damage to the vasculature and the potential for radiation to enhance immune activity against tumour cells. Radiation treatment can cause a significant influx of bone marrow-derived cell populations into both normal tissues and tumours. Potential roles of such cells may include enhancing vascular recovery as well as modulating immune reactivity.

Numerical chromosomal instability mediates susceptibility to radiation treatment

The exquisite sensitivity of mitotic cancer cells to ionizing radiation (IR) underlies an important rationale for the widely used fractionated radiation therapy. However, the mechanism for this cell cycle-dependent vulnerability is unknown. Here we show that treatment with IR leads to mitotic chromosome segregation errors in vivo and long-lasting aneuploidy in tumour-derived cell lines. These mitotic errors generate an abundance of micronuclei that predispose chromosomes to subsequent catastrophic pulverization thereby independently amplifying radiation-induced genome damage. Experimentally suppressing whole-chromosome missegregation reduces downstream chromosomal defects and significantly increases the viability of irradiated mitotic cells. Further, orthotopically transplanted human glioblastoma tumours in which chromosome missegregation rates have been reduced are rendered markedly more resistant to IR, exhibiting diminished markers of cell death in response to treatment. This work identifies a novel mitotic pathway for radiation-induced genome damage, which occurs outside of the primary nucleus and augments chromosomal breaks. This relationship between radiation treatment and whole-chromosome missegregation can be exploited to modulate therapeutic response in a clinically relevant manner.

Optimization of tumour control probability in hypoxic tumours by radiation dose redistribution: a modelling study

Tumour hypoxia is a known cause of clinical resistance to radiation therapy. The purpose of this work was to model the effects on tumour control probability (TCP) of selectively boosting the dose to hypoxic regions in a tumour, while keeping the mean tumour dose constant. A tumour model with a continuous oxygen distribution, incorporating pO(2) histograms published for head and neck patients, was developed. Temporal and spatial variations in the oxygen distribution, non-uniform cell density and cell proliferation during treatment were included in the tumour modelling. Non-uniform dose prescriptions were made based on a segmentation of the tumours into four compartments. The main findings were: (1) Dose redistribution considerably improved TCP for all tumours. (2) The effect on TCP depended on the degree of reoxygenation during treatment, with a maximum relative increase in TCP for tumours with poor or no reoxygenation. (3) Acute hypoxia reduced TCP moderately, while underdosing chronic hypoxic cells gave large reductions in TCP. (4) Restricted dose redistribution still gave a substantial increase in TCP as compared to uniform dose boosts. In conclusion, redistributing dose according to tumour oxygenation status might increase TCP when the tumour response to radiotherapy is limited by chronic hypoxia. This could potentially improve treatment outcome in a subpopulation of patients who respond poorly to conventional radiotherapy.

Combining bioreductive drugs and radiation for the treatment of solid tumors

Methods now exist for the identification of human tumors that contain significant numbers of hypoxic cells and are thereby suitable for treatment with bioreductive drugs to eliminate this refractory cell population. However, to fully exploit the potential of bioreductive drugs, they will need to be used in combination with other modalities likely to target the proliferating aerobic cells in the tumor. Radiation is the treatment that is most effective in killing aerobic cells; therefore, the present report reviews the available preclinical data on combined radiation/bioreductive drug treatments.

Alteration of tumour response to radiation by interleukin-2 gene transfer

We have previously shown that BALB/c-derived EMT6 mammary tumours transfected with interleukin (IL)-2 have decreased hypoxia compared to parental tumours, due to increased vascularization. Since hypoxia is a critical factor in the response of tumours to radiation treatment, we compared the radiation response of IL-2-transfected tumours to that of parental EMT6 tumours. Because the IL-2 tumours have an altered host cell composition, which could affect the interpretation of radiation sensitivity as measured by clonogenic cells, we employed flow cytometric analysis to determine the proportion of tumour cells vs host cells in each tumour type. Using this approach, we were able to correct the plating efficiency based on the number of actual tumour cells derived from tumours, making the comparison of the two types of tumours possible. We also excluded the possibility that cytotoxic T-cells present in EMT6/IL-2 tumours could influence the outcome of the clonogenic cell survival assay, by demonstrating that the plating efficiency of cells derived from EMT6/IL-2 tumours remained unchanged after depletion of Thy-1+ cells. The in vivo radiation response results demonstrated that IL-2-transfected tumours were more sensitive to radiation than parental EMT6 tumours. The hypoxic fraction of the EMT6/IL-2 tumours growing in vivo was markedly decreased relative to parental EMT6 tumours thus the increased sensitivity results from the increased vascularity we have previously observed in these tumours. These results indicate the potential therapeutic benefit of combining radiation and immunotherapy in the treatment of tumours.

Cervix cancer oxygenation measured following external radiation therapy

PURPOSE

Tumor hypoxia may be an important factor predicting relapse following radiation therapy. This study was designed to determine the relationship between the oxygenation parameters measured using a polarographic oxygen electrode, prior to and during treatment in patients with cervix cancer, and to assess these results with regard to patient survival.

MATERIALS AND METHODS

Forty-three patients had pretreatment oxygen assays performed and measurements repeated following external beam radiation to a median dose of 50 Gy (range 26-52 Gy). Stage distribution showed 15 patients in Stage IB, 17 in Stage II, and 11 in Stage III. The median tumor size was 5 cm (range 3-10 cm).

RESULTS

The median proportion of pO2 values <5 mm Hg (the HP5) was 41% following radiation, and the median pO2 was 12 mm Hg. These results were not significantly different from the pretreatment HP5 or pO2 of 37% and 12 mm Hg, respectively. Disease-free survival at 2 years was 50% in patients with posttreatment HP5 < or =50%, compared to 60% when posttreatment HP5 was >50% (p = 0.35).

CONCLUSIONS

Unlike pretreatment results, tumour oxygenation measured following external radiation does not appear to be a useful predictive assay in patients with cervical cancer.

Dynamics of tumor oxygenation, CD31 staining and transforming growth factor-beta levels after treatment with radiation or cyclophosphamide in the rat 13762 mammary carcinoma

PURPOSE

Tumors are dynamic tissues that undergo marked molecular, biochemical, and physiologic changes in response to cytotoxic anticancer therapies. Understanding the changes in tumor oxygenation and transforming growth factor-beta expression may allow improved treatment regimens to be developed.

METHODS AND MATERIALS

The effects of a single dose of radiation therapy (20 Gy) or a single dose of chemotherapy (cyclophosphamide, 250 mg/kg) on several molecular and physiologic parameters of the rat 13762 mammary carcinoma growing subcutaneously in female Fischer 344 rats were explored.

RESULTS

Treatment of the tumor-bearing animals with 20 Gy of radiation killed about two logs (99%) of the 13762 tumor cells, and treatment with cyclophosphamide (250 mg/kg) killed about 1.5 logs (95%) of the 13762 tumor cells. Hypoxia, as determined by a pO2 electrode, initially decreased in the tumors of treated animals until 6 h. posttreatment and then increased, so that 24 h. after administration of the radiation therapy or the chemotherapy the number of intratumoral vessels as determined by CD31 staining increased until about 24 h after cytotoxic therapy. Transforming growth factor-beta1, measured by radioimmunoassay, peaked in the serum between 6 h and 18 h and again between 72 h and 96 h after radiation therapy and peaked in the tumor at 24 h and again at 72 h after radiation therapy. The first serum peak after cyclophosphamide was 3 h after drug injection, with second peaks at 36 h and 48 h after drug administration. In the tumor, transforming growth factor-beta1 peaked between 6 h and 8 h after drug administration and again 36 h and 72 h after drug. Apoptosis was maximal 6 h after 20 Gy and 24 h after cyclophosphamide. Vascular endothelial growth factor was also increased in tumors after cytotoxic therapy.

CONCLUSIONS

These changes in the tumor physiologic status are sufficient to protect the tumor from a second cytotoxic insult administered days afterwards and to result in a restructuring of the tissue.

Comparison of in vivo efficacy of hypoxic cytotoxin tirapazamine and hypoxic cell radiosensitizer KU-2285 in combination with single and fractionated irradiation

Development of strategies to eradicate radioresistant hypoxic cells would be of great benefit for clinical radiotherapy. In the present study, the in vivo effects of a promising hypoxic cytotoxin, tirapazamine (3-amino-1,2,4-benzotriazine 1,4-di-N-oxide), were examined in comparison with those of KU-2285, one of the best hypoxic cell radiosensitizers, in combination with both single and fractionated irradiation. The tumor response was assessed by the standard in vivo-in vitro clonogenic assay using SCCVII tumors in C3H mice and EMT-6/KU tumors in Balb/c mice with different characteristics of tumor hypoxia. With single-dose irradiation (18 Gy), both tirapazamine and KU-2285 showed significant enhancement of cell killing in a dose-dependent manner, but tirapazamine was more effective for SCCVII tumors with acutely hypoxic cells, while KU-2285 was more effective for EMT-6/KU tumors predominantly with chronically hypoxic cells. In fractionated irradiation regimens (4 fractions of 5 Gy at 12 h intervals), tirapazamine showed more marked combined effects at 10 and 20 mg/kg than KU2285 at 100-200 mg/kg in both SCCVII and EMT-6/KU tumors. We concluded that the effectiveness of KU-2285 and tirapazamine was correlated with the nature of tumor hypoxia with single-dose irradiation, whereas tirapazamine appeared more potent than KU-2285 with fractionated irradiation. These findings suggest the potential usefulness of tirapazamine in clinical fractionated radiotherapy.

Hypoxia-activated prodrugs as antitumour agents: strategies for maximizing tumour cell killing

1. Hypoxia arises in solid tumour because of inefficient blood supply. While hypoxic cells are resistant to radiotherapy and probably to many chemotherapeutic drugs they can, in principle, be turned to advantage through the development of hypoxia-activated cytotoxic drugs (bioreductive drugs). 2. Three general approaches to exploiting tumour hypoxia are discussed. The first relies on fluctuating blood flow in tumours and the consequent cycling of cells through the hypoxic compartment. The second incorporates a prodrug approach in which drug activation gives rise to cytotoxic metabolites which diffuse out of hypoxic zones. The third utilizes selective inhibitors of tumour blood flow to induce additional hypoxia and thus enhance bioreductive drug activation. 3. The latter two approaches are illustrated by recent studies with the dinitrobenzamide nitrogen mustard class of bioreductive drugs and their combination with the tumour blood flow inhibitor 5,6-dimethylxanthenone-4-acetic acid.

Hypoxia and drug resistance

Biologically and therapeutically important hypoxia occurs in many solid tumor masses. Hypoxia can be a direct cause of therapeutic resistance because some drugs and radiation require oxygen to be maximally cytotoxic. Cellular metabolism is altered under hypoxic conditions. Hypoxia can result in drug resistance indirectly if under this condition cells more effectively detoxify the drug molecules. Finally, there is evidence that hypoxia can enhance genetic instability in tumor cells thus allowing more rapid development of drug resistance cells. The current review describes the effects of hypoxia on tumor response to a variety of anti-cancer agents and also describes progress toward therapeutically useful methods of delivering oxygen to tumors in an effort to overcome therapeutic resistance due to hypoxia. Finally, the use of hypoxic cell selective cytotoxic agents as a means of addressing hypoxic 'drug resistance' is discussed.

Middle dose rate irradiation in combination with carbogen inhalation selectively and more markedly increases the responses of SCCVII tumors

PURPOSE

Carbogen increases the radiation response of tumors and reduced dose rate irradiation spares the damage of normal tissues. The purpose in this paper is to investigate the possibility of selective radiosensitization of tumors by reduced dose rate irradiation in combination with carbogen inhalation.

METHODS AND MATERIALS

SCCVII tumors in C3H/He mice were irradiated at middle dose rate (0.1 Gy/min) or high dose rate irradiation (3.0 Gy/min) in combination with carbogen inhalation. The mice were enclosed in a box with carbogen flushing at 1.01/min. The tumor response was measured by a cytokinesis block micronucleus assay. The effects on intestinal crypt cells and bone marrow cells were investigated by microcolony assay or Hendry's method, respectively.

RESULTS

The anti-tumor effect of middle dose rate irradiation was equal to that of a high dose rate irradiation. Carbogen inhalation, more efficiently, increased the antitumor effect when combined with middle and high dose rate irradiation, and yielded enhancement ratios of 1.6 at around 2 Gy. Middle dose rate irradiation produced less damage on intestinal crypt cells and bone marrow cells in comparison with high dose rate irradiation, and carbogen inhalation never enhanced the responses of these normal tissues in combination with middle dose rate irradiation. Dose modifying factors were 1.3-2.0.

CONCLUSION

Since middle dose rate irradiation in combination with carbogen inhalation gave the therapeutic gain factors of 2.0-3.2, which were much larger than those obtained with any other radiosensitizers, this combination has a potential as a new modality for improving the results of cancer radiotherapy.

Magnetic resonance imaging of human melanoma xenografts in vivo: proton spin-lattice and spin-spin relaxation times versus fractional tumour water content and fraction of necrotic tumour tissue

Proton nuclear magnetic resonance (1H-nmr) imaging is used routinely in clinical oncology to provide macroscopic anatomical information, whereas its potential to provide physiological information about tumours is not well explored. To evaluate the potential usefulness of 1H-nmr imaging in the prediction of tumour treatment resistance caused by unfavourable microenvironmental conditions, possible correlations between proton spin-lattice and spin-spin relaxation times (T1 and T2) and physiological parameters of the tumour microenvironment were investigated. Tumours from six human melanoma xenograft lines were included in the study. 1H-nmr imaging was performed at 1.5 T using spin-echo pulse sequences. T1- and T2-distributions were generated from the images. Fractional tumour water content and the fraction of necrotic tumour tissue were measured immediately after 1H-nmr imaging. Significant correlations across tumour lines were found for T1 and T2 versus fractional tumour water content (p < 0.001) as well as for T1 and T2 versus fraction of necrotic tumour tissue (p < 0.05). Tumours with high fractional water contents had high values of T1 and T2, probably caused by free water in the tumour interstitium. Fractional water content is correlated to interstitial fluid pressure in tumours, high interstitial fluid pressure being indicative of high vascular resistance. Tumours with high fractional water contents are thus expected to show regions with radiobiologically hypoxic cells as well as poor intravascular and interstitial transport of many therapeutic agents. T1 and T2 decreased with increasing fraction of necrotic tumour tissue, perhaps because complexed paramagnetic ions were released during development of necrosis. Viable tumour cells adjacent to necrotic regions are usually chronically hypoxic. Tumours with high fractions of necrotic tissue are thus expected to contain significant proportions of radiobiologically hypoxic cells. Consequently, quantitative 1H-nmr imaging has the potential to be developed as an efficient clinical tool in prediction of tumour treatment resistance caused by hypoxia and/or transport barriers for therapeutic agents. However, much work remains to be done before this potential can be adequately evaluated. One problem is that high fractional tumour water contents result in longer T1 and T2 whereas high fractions of necrotic tumour tissue result in shorter T1 and T2; i.e. the two parameters which are indicative of treatment resistance contribute in opposite directions. Another problem is that the correlations for T1 and T2 versus fraction of necrotic tumour tissue are not particularly strong.

Combination of perfluorochemical emulsions and carbogen breathing with cancer chemotherapy

Over the past ten years several laboratories have explored the use of perfluorochemical emulsions (PFCE) and carbogen (95% O2/5% CO2; C) or oxygen breathing as an adjuvant to radiation therapy and/or chemotherapy in solid tumor model systems. The rationale for the use of PFCE and C or oxygen breathing in this therapeutic setting is that solid tumor masses contain areas of hypoxia which are therapeutically resistant. Since x-rays and many chemotherapeutic agents require oxygen to be maximally cytotoxic and most normal tissues are well-oxygenated, the additional oxygen put in circulation by the PFCE/C should not increase the normal tissue toxicities produced by the various therapies. Several anticancer agents are dependent on oxygen to be cytotoxic, these drugs such as the iron-chelating peptide bleomycin are enhanced in antitumor activity by the co-administration of a PFCE/C. The antitumor alkylating agents especially cyclophosphamide, BCNU and melphalan show increased tumor cell killing without a concomitant increase in bone marrow toxicity when administered with PFCE/C. Enhanced activity was also observed when topoisomerase II inhibitors such as adriamycin and etoposide were co-administered with PFCE/C. Positive effects, although smaller, were observed when antimetabolites such as 5-fluorouracil and methotrexate were co-administered with PFCE/C.

Tumour hypoxia: the picture has changed in the 1990s

Since the 1950s, the presence of hypoxic cells in human tumours has been widely regarded as a problem, and a variety of strategies have been developed and tested, both in experimental and clinical studies, to overcome this perceived problem. One of these strategies was the development of bioreductive cytotoxins--drugs which in themselves were relatively innocuous, but when metabolized under hypoxic conditions, became highly cytotoxic, thereby preferentially killing the hypoxic cells. Modelling studies and experimental data with newly developed hypoxic cytotoxins, such as SR 4233 (tirapazamine) and RSU 1069, have led to the realization not only that it is better to kill hypoxic cells in tumours than to radiosensitize or oxygenate them, but also that with these bioreductive cytotoxins hypoxic cells in tumours can be an advantage in cancer therapy. However, to realize the advantage of adding the drug with each radiation dose, the tumour must undergo a process analogous to reoxygenation, which we have termed 'rehypoxiation', by which hypoxic cells are regenerated after each dose of the hypoxic cytotoxin. In addition, we also discuss the fact that hypoxia is a cellular stress which activates many new genes. The activation of these genes will be a major focus for research in coming years and will undoubtedly lead to new approaches in cancer detection and treatment. In summary, the 1990s are bringing a fundamental change in our perception of tumour hypoxia, from a position of being a problem to that of being a solution in cancer treatment.

Pharmacokinetics of varying doses of nicotinamide and tumour radiosensitisation with carbogen and nicotinamide: clinical considerations

Plasma concentrations, after administration of varying doses of nicotinamide, were measured in CBA male mice using a newly-developed high performance liquid chromatography assay. In all dose groups, peak levels were observed within the first 15 min after an i.p. administration of 0.1, 0.2, 0.3 or 0.5 mg g-1 of nicotinamide. There was a clear dose-dependent increase in plasma concentration with increasing dose, with almost a five-fold lower concentration (1.0 vs 4.9 mumol ml-1) achieved with a dose of 0.1 mg g-1 compared with 0.5 mg g-1, respectively. The half-life of nicotinamide increased from 1.4 h to 2.2 h over the dose range (P < 0.01). Comparisons with previous pharmacokinetic data in humans show that clinically-relevant oral doses of 6 and 9 g in humans give plasma levels slightly higher than those achieved at 1 h with doses of 0.1 to 0.2 mg g-1 in mice. Tumour radiosensitisation with carbogen alone, and with carbogen combined with varying doses of nicotinamide (0.05 to 0.5 mg g-1), was investigated using a 10-fraction in 5 days X-ray schedule. Relative to air-breathing mice, a statistically significant increase in sensitisation was observed with both a local tumour control and with an in vivo/in vitro excision assay (P < or = 0.007). With the local control assay, a trend was observed towards lower enhancement ratios (ERs) with decreasing nicotinamide dose (from 1.85 to 1.55); carbogen alone was almost as effective as when combined with 0.1 mg g-1 of nicotinamide. With the excision assay, ERs for carbogen combined with nicotinamide increased with decreased levels of cell survival. At a surviving fraction of 0.02, enhancement ratios of 1.39-1.48 were obtained for carbogen plus 0.1 to 0.3 mg g-1 of nicotinamide. These were lower than those seen with the two higher doses of 0.4 to 0.5 mg g-1 (ERs = 1.63-1.69).

Radiosensitization of SCCVII tumours and normal tissues by nicotinamide and carbogen: analysis by micronucleus assay

The radiosensitizing effect on SCCVII tumours of carbogen (95% O2 + 5% CO2) combined with nicotinamide was investigated using the micronucleus assay and an in vivo/in vitro colony formation assay following single irradiation. The effects on intestinal crypt cells and bone marrow cells were also examined in mice. The frequency of micronuclei in tumours increased with an increase in the nicotinamide dose (administered 1 h before the irradiation of 5 Gy) from 0.1 to 1.0 mg/g, and showed a 1.3-fold increase at 1.0 mg/g when compared with radiation alone. The micronucleus frequency showed a more marked increase following irradiation combined with nicotinamide and carbogen inhalation starting 15 min before irradiation. The radiosensitizing effect reached a plateau at a nicotinamide dose of 0.1 mg/g in combination with carbogen, giving an enhancement ratio (ER) of 1.8 relative to radiation alone at 2 Gy. In the radiation dose range of 5-20 Gy, ERs of 1.8-1.9 and 1.5 (at a 10% cell-survival level) were obtained for the combination of 0.1 or 1.0 mg/g nicotinamide and carbogen and for carbogen alone, respectively. A slight increase in the regeneration response of the jejunum and the bone marrow was observed following irradiation combined with 0.1 mg/g nicotinamide and carbogen, yielding ERs of 1.07 for the jejunum and 1.15 for the bone marrow. Thus, the nicotinamide/carbogen combination, with its large therapeutic gain factor at low doses and proven low toxicity in humans, seems to improve the response of cancer to radiotherapy.

Enhancement of tumor radiosensitivity and reduced hypoxia-dependent binding of a 2-nitroimidazole with normobaric oxygen and carbogen: a therapeutic comparison with skin and kidneys

To evaluate the therapeutic potential of normobaric oxygen and carbogen as hypoxic-cell sensitizers, both radiosensitization in a mouse mammary carcinoma, mouse skin and kidneys, and the reduction in the proportion of hypoxic tumor cells were quantified in mice breathing air, oxygen, or carbogen. Local tumor control, acute skin reactions, reduced renal clearance, and hematocrit were used as assays. X rays as 10 fractions in 5 days were given to skin and tumors and 10F/12 days to kidneys. In the tumor study, the pre-irradiation breathing time was varied from 2 to 20 min. Hypoxic cells, before and during a 10F/5 day schedule, were quantified using a 2-nitroimidazole with a theophylline side chain. Bioreductively reduced metabolites of this probe were localized in hypoxic cells that were then stained using an immunofluorescent technique and analyzed by flow cytometry. The fraction of cells with high fluorescence intensity was 19% in air, 9% in oxygen, and 3% in carbogen-breathing mice. For all three gases, hypoxia-dependent binding was similar in non-irradiated tumors and those treated with four or nine fractions. Both gases significantly enhanced tumor radiosensitivity (ER = 1.3 to 1.6) and carbogen was slightly more effective than oxygen. With carbogen, maximum sensitization was observed with a 5 min pre-irradiation breathing interval. With oxygen, pre-irradiation breathing times of 2-20 min gave similar sensitization. In skin an enhancement ratio of 1.2 was observed, whereas enhancement ratios for both renal endpoints were significantly lower (1.0 to 1.07). Relative to both tissues, there was therefore a substantial therapeutic gain by irradiating CaNT tumors under both gases, especially with carbogen.

Enhancement of fractionated radiation therapy by an experimental concentrated perflubron emulsion (Oxygent) in the Lewis lung carcinoma

The concentrated Perflubron emulsion, Oxygent, in combination with breathing a 95% oxygen atmosphere, significantly enhanced the response of the murine Lewis lung carcinoma to fractionated radiation therapy. Administration of 4 to 12 g PFC/kg was most effective if administered prior to each x-ray fraction, however, alternated day and even once weekly administration of 8 or 12 PFC/kg and 95% oxygen breathing with each x-ray fraction resulted in significant improvements in tumor growth delay.

A therapeutic benefit from combining normobaric carbogen or oxygen with nicotinamide in fractionated X-ray treatments

The ability of normobaric oxygen and carbogen (95% O2 + 5% CO2) combined with nicotinamide to enhance the radiosensitivity of two rodent adenocarcinomas and of mouse skin and kidneys, using a 10 fraction radiation schedule, was compared with the effect of radiation in air with and without the drug. Tumour response was assayed using local control and regrowth delay, and compared with acute skin reactions, decreased renal 51Cr-EDTA clearance and reduction in haematocrit. Nicotinamide increased the radiation sensitivity of CaNT tumours under all three different oxygen concentrations tested (21, 95 and 100% oxygen). The effect was statistically significant for oxygen and carbogen but not for air; the combination of nicotinamide with carbogen gave the greatest increase in tumour radiosensitivity. Relative to treatments in air without the drug, the enhancement ratios (ER) at the TCD50 level were 1.17, 1.65 and 1.83 for CaNT tumours irradiated in air, oxygen or carbogen and injected with nicotinamide 1 h before each fraction. The ER in CaRH tumours irradiated in carbogen plus the drug was 1.83, which was greater, but statistically not significantly different, to that seen with carbogen alone (ER = 1.68). In skin, relative to air without the drug, the increase in radiosensitivity by nicotinamide was greater in oxygen and carbogen than in air (1.29, 1.36 and 1.08, respectively). The ERs for both assays of renal damage were similar and lower than those in skin: less than or equal to 1.07, less than or equal to 1.13 and less than or equal to 1.16 for irradiations done in air, oxygen and carbogen plus nicotinamide, relative to air alone. A comparison of these results in the tumours and normal tissues showed that a significant therapeutic benefit was obtained with normobaric oxygen and carbogen combined with nicotinamide. This benefit is greater than observed with other radiosensitizers tested so far. Toxic side effects of the treatment are unlikely in a clinical situation, since prolonged administration of nicotinamide is well tolerated in man. The combination of normobaric carbogen with nicotinamide could be an effective method of enhancing tumour radiosensitivity in clinical radiotherapy where hypoxia limits the outcome of treatment.

Variations of the hypoxic fraction in the SCC VII tumors after single dose and during fractionated radiation therapy: assessment without anesthesia or physical restraint of mice

Variations of the hypoxic fraction (HF) after single dose (13 Gy or 4 Gy) and during fractionated (5 fractions of 4 Gy, 1 or 2 fractions per day) radiation therapy were studied in SCC VII tumors implanted subcutaneously in the hind legs of C3H/He/Jms mice using the paired survival curve method. Whole-body irradiation was delivered to tumor-bearing mice without anesthesia or physical restraint, because both are known to increase the HF artificially. The HF decreased after a single 13 Gy dose in a biphasic fashion: extremely rapidly within 1 hr and comparatively slowly during the following 12-72 hr. On the other hand, nearly no fall of HF was observed in 24 hr following a single 4 Gy dose. Also, reoxygenation was found to occur more rapidly in the interfraction period as the number of fractions of 4 Gy increased irrespective of differences of interfraction time. However, the HF just before each radiation fraction was significantly higher than the pretreatment level for both fractionated regimens. Thus, the reoxygenation patterns observed after single low and high doses of irradiation were different from each other, and reoxygenation in each interfraction period did not always proceed in a similar manner to that after single low dose irradiation. Reoxygenation was facilitated as fractionated radiation therapy proceeded, but it was not sufficient for the HF to remain at a level comparable to that before irradiation.

Tumor hypoxia can be exploited to preferentially sensitize tumors to fractionated irradiation

The present study describes a new way in which tumors may be made more sensitive to fractionated irradiation without affecting the sensitivity of surrounding normal tissues. It involves exploiting the cycling or intermittent hypoxia that occurs in at least some solid tumors, but not in normal tissues, using a new drug SR 4233, a benzotriazine di-N-oxide, which is rapidly metabolized in hypoxic cells to a product that kills these cells. Using this approach with a rodent tumor in a fractionated x-ray treatment regimen similar to that used in human radiotherapy, the addition of SR 4233 produced a large enhancement of the radiation response of the tumor with no change in the sensitivity of normal mouse skin. Under identical circumstances, there was no effect of the hypoxic cell radiosensitizer SR 2508, showing that SR 4233 with intermittent hypoxia was superior to a protocol which sensitized the hypoxic cells to doses of 2.5 Gy per fraction.

SR 4233: a tumor specific radiosensitizer active in fractionated radiation regimes

The benzotriazine SR 4233, in addition to preferential killing of hypoxic cells both in vitro and in vivo, also radiosensitizes aerobic cells in vitro if the cells are exposed to the drug under hypoxic conditions, either before or after irradiation. We have attempted to exploit this aerobic radiosensitization in vivo, by giving SR 4233 with the hypoxia inducing agent, hydralazine, after each radiation dose in a 8 x 2.5 Gy fractionated regime. The results show greater than additive cytotoxicity using both cell survival and regrowth delay as the endpoints of radiation response, but no radiosensitization in parallel groups treated with the hypoxic cell radiosensitizer SR 2508. The data are, therefore, consistent with radiosensitization of the tumor aerobic cells by the SR 4233 treatments. Significantly, the effect occurred with equal magnitude with or without hydralazine. Further, there was no radiosensitization to radiation induced leg contraction in the thighs of mice, a late responding normal tissue endpoint. The results, therefore, demonstrate a selective radiosensitization of tumors to a multifraction regime and suggest that SR 4233, or a close analog, may be useful in radiation therapy.

Lymphocytic infiltration and cytotoxicity under hypoxic conditions in the EMT6 mouse mammary tumor

Infiltration of lymphocytes, neutrophils and macrophages was evaluated in hypoxic and well-oxygenated areas of the EMT6 mouse mammary adenocarcinoma, by in vivo staining with the fluorescent dye Hoechst 33342 followed by cell sorting on the basis of fluorescence intensity. Tumors were grouped by days post-injection (days 11-14, 15-17 and 20-27). As lymphocytes are the only host cell population in this tumor model to possess lytic activity against EMT6 tumor cells, the ability of sensitized T lymphocytes to lyse syngeneic EMT6 cells was examined under conditions of varying oxygen concentrations. Infiltrating lymphocytes were detected to the same extent in cell fractions from both areas in all tumors. In contrast, neutrophils were found in significantly higher percentages in the hypoxic population than in the well-oxygenated cell fraction of all but the largest tumors. Macrophages were present in significantly higher percentages in the well-oxygenated fraction than in the hypoxic fraction of day-11 to -14 tumors. Extreme radiobiological hypoxia (0% O2) resulted in a significant decrease in T-cell-mediated lysis of EMT6 tumor cells, compared to lysis in room air (20% O2), but lysis was not impaired under conditions of mild radiobiological hypoxia (1% O2). Our study indicates that host-cell infiltration into areas of differing oxygenation may be quantitated via in situ Hoechst staining followed by cell sorting; in the EMT6 tumor, lymphocytes appear to infiltrate hypoxic areas to the same extent as well-oxygenated areas, and T-lymphocyte killing of syngeneic tumor cells is significantly reduced, although still present, under these hypoxic conditions.

Effect of normobaric oxygen on tumor radiosensitivity: fractionated studies

The sensitizing ability of 100% normobaric oxygen was investigated in a mouse mammary carcinoma (CaNT) using a variety of fractionated regimens. Both regrowth delay and local control were used as assays of tumor response. With both assays, there was a similar and significant increase in radiosensitivity for all fractionated schedules. Enhancement ratios ranged from 1.24 to 1.45, the highest increase being observed with a 30 fraction schedule given in an overall time of 6 weeks. Thus, in CaNT tumors normobaric oxygen is a far more efficient radiosensitizer in fractionated treatments than the oxygen-mimetic compound misonidazole; an oxygen effect being observed at doses per fraction as low as 1.8 Gy. These results suggest strongly that normobaric gases could play an important role in the clinical management of tumors where hypoxia may limit the outcome of radiotherapy.

Radiation and chemotherapy sensitizers and protectors

Radiosensitizers and radioprotectors are part of the chemical modifier approach to cancer therapy whereby the state of the tumor cells and/or normal tissues are modified such that a therapeutic gain is achieved using conventional radiation or chemotherapy. Radiosensitization can be achieved by the use of oxygen-mimetic compounds, agents that alter DNA sensitivity to irradiation, maneuvers that alter DNA repair processes, and manipulation of tissue oxygenation. Standard chemotherapeutic agents such as cisplatin can be utilized in a manner that optimizes the radiosensitization properties. Protection and sensitization can occur by altering the thiol status of the cell. The chemical modifiers field is both developing novel approaches to cancer treatment and increasing the understanding of basic cancer biology.

Invasive cervical cancer in the 1990s

The death rate from invasive cervical cancer has decreased by 70% since the classic work by Papanicolaou and Traut in 1941 on the use of cytologic evaluation to detect cancer of the uterine cervix in the preinvasive in situ stage--a nearly 100% curable disease. Unfortunately, the survival stage for stage of invasive cervical cancer has remained static over the nearly 5 decades since their report. However, discoveries in the decade of the 1970s of the natural spread of cervical cancer, not only to the known pelvic lymph nodes, but the increasing incidence of paraaortic lymph node metastasis with advanced stages, the higher dose of radiation required to sterilize pelvic lymph node metastasis discovered at the time of radical hysterectomy or for locally advanced cervical cancer treated solely by radiation therapy, and the use of radiation potentiators, such as hydroxyurea, should lead to the significant reduction in the annual 7,000 deaths from this disease in the decade of the 1990s.

Hypoxia and reoxygenation in human melanoma xenografts

Tumor hypoxia and reoxygenation pattern following single dose (10.0 Gy) and fractionated (7 fractions of 2.0 Gy, 1 fraction per day) irradiation were studied in five human melanoma xenograft lines using the paired survival curve method. The hypoxic fractions differed significantly among the melanoma lines; they were found to be 6 +/- 3% (E.E.), 22 +/- 8% (E.F.), 31 +/- 11% (G.E.), 45 +/- 17% (M.F.), and 15 +/- 5% (V.N.). There were no clear correlations between hypoxic fraction and tumor volume-doubling time or vascular density, suggesting that intrinsic cellular characteristics, for example, rate of oxygen consumption and ability to retain clonogenicity under hypoxic stress, also may play an important role for the magnitude of the hypoxic fractions in the melanomas. Reoxygenation was rapid and extensive in all melanoma lines; 12-24 hr after the single dose irradiation or the last fraction of the fractionated irradiation, the hypoxic fractions were similar to those in untreated tumors and stayed at that level up to at least 10 days after irradiation. The hypoxic fractions 1-10 days after irradiation tended to be higher after fractionated than after single dose irradiation, but the differences were not statistically significant. There was a positive correlation between the hypoxic fractions in untreated tumors and the hypoxic fractions after irradiation and reoxygenation, suggesting that it may be possible to predict radiation resistance caused by hypoxia from the hypoxic fractions in tumors before start of radiation therapy. However, hypoxia is probably not a major cause of failure in the radiation therapy of malignant melanoma.

Positive clinical experience with misonidazole in brachytherapy and external radiotherapy

We performed a clinical evaluation of Misonidazole (MISO) radiosensitization in brachytherapy and two schedules of hypofractionated external radiotherapy in 3 non randomized studies. MISO (1 g/m2/d) was administered to patients with ENT tumors treated by brachytherapy, two applications of 35 Gy each with an interval of 1 month. For 46 patients with tumor responses less than 50% (in the largest dimension) at time of second application, 21 received MISO and 25 did not. For these poorly radiosensitive tumors, the addition of MISO significantly increased the rate of complete remission from 9/25 (36%) in controls to 14/21 (67%) (p less than 0.05). We studied MISO with radiation hypofractionation for conservative breast cancer with 4 fractions over 17 days (5 Gy on days 1, 3 and 6.5 Gy on days 15 and 17). Brachytherapy alone was delivered three weeks later. MISO (1 g/m2/d) was given to 38 patients with 87 acting as controls. Radiosensitization was measured by mean tumor diameter at brachytherapy, which showed a residual mass of 33% in the group without MISO and only 17% in the group with MISO (p less than 0.05). We also studied MISO with radiation hypofractionation for large ENT tumors with 14 fractions over 45 days, 2 sessions with a 4 hour interval per day for totals of 6 Gy on days 1 and 3; 8 Gy on days 15, 17, 29, 31; and 6 Gy on day 45. MISO (1 g/m2/d) was given to 49 patients with 21 acting as controls. MISO increased the rate of complete remission from 7/21 (33%) in controls to 32/49 (65%) (p less than 0.02).

Normobaric oxygen as a sensitizer of hypoxic tumor cells

A series of experiments have been performed to determine the relative radiosensitivities of a mouse mammary tumor (CA NT) irradiated in 100% normobaric oxygen or in air, using clinically relevant dose-fractionation schedules. The results demonstrate that normobaric oxygen is a potent radiosensitizer, its effect being more marked with smaller size fractions. Enhancement ratios (ER) of 1.23 to 1.45 were obtained. By contrast, no significant benefit was seen with single doses (ER = 1.05 +/- 0.14). The degree of radiosensitization with oxygen, in these fractionated regimens, is greater than that reported for the chemical radiosensitizer, misonidazole. It therefore seems appropriate to re-examine the clinical potential of normobaric oxygen and to consider how the conditions of normobaric radiotherapy achieved in the laboratory can be translated to man.

The proportion of stem cells in murine tumors

Considerable evidence suggests that tumors contain only a minority of cells which are capable of regrowing the tumor (ie. tumor stem cells). Since all tumor stem cells must be killed if treatment is to be successful, the number of stem cells in a tumor can be expected to be an important determinant of curability. We have attempted to examine the proportion of stem cells in a variety of murine tumors by making measurements of three different parameters which might be expected to be related to stem cell content: (a) the radiation dose required to control the tumor (TCD50); (b) the number of cells required to transplant the tumor (TD50) and (c) the in vitro plating efficiency. An inverse correlation has been demonstrated between measured TCD50 and TD50 values for two independent groups of murine tumors of varying histopathological type. An inverse correlation was also obtained between the TD50 value and in vitro plating efficiency for a group of spontaneous murine mammary tumors. These correlations most likely reflect underlying differences in the stem cell content of the tumors, and indicate that there is a wide range (2-3 orders of magnitude) of stem cell proportions in different murine tumors, even those which have been transplanted a number of times.

A phase I/II study of the hypoxic cell sensitizer misonidazole as an adjunct to high fractional dose radiotherapy in patients with unresectable squamous cell carcinoma of the head and neck: a RTOG randomized study (#79-04)

A randomized prospective trial was performed to study the toxicity and efficacy of the hypoxic cell sensitizer, misonidazole (MISO), used as an adjunct to high fractional dose radiotherapy in the management of unresectable Stage III and IV squamous cell carcinomas of the oral cavity, oropharynx and hypopharynx. From June 1979 to February 1983, 42 patients were randomized with 40 patients available for analysis. In the radiotherapy (RT) only group, 19 patients received a short course of high fractional dose radiotherapy with 400 rad per day, 5 days per week, to a total of 4400 to 5200 rad. In the radiotherapy plus misonidazole group (RT + MISO) 21 patients received the same radiotherapy plus 1.5 gm/m2 of misonidazole 3 times a week for a total of 7 doses. The observed side effects associated with misonidazole were: persistent numbness and paresthesia (1 patient), transient peripheral nerve paresis and persistent paresthesia (1 patient), and nausea and vomiting (2 patients). The treatment related morbidities were similar in both groups. Acute mucositis was seen in 4 of 19 patients in the RT group and 3 of 21 patients in the RT + MISO group. Acute airway obstruction requiring tracheotomy was seen in 2 patients with massive tumor in the base of tongue (1 in each group). Severe dysphagia requiring NG tube feeding was seen in 3 patients in the RT + MISO group and 3 patients in the RT group. The initial complete response rate in the RT group was 53%, versus 48% in the RT + MISO group. The estimated 2-year loco-regional control rates were 10% for RT alone and 17% for RT + MISO (no significancy). These results indicate that the addition of misonidazole does not improve the efficacy of high fractional dose radiotherapy for management of unresectable head and neck carcinomas. However, high fractional dose radiotherapy can be administered for the management of advanced head and neck carcinomas with acceptable morbidity and thus, is a useful regimen for future clinical trials of hyperbaric oxygen or new hypoxic cell sensitizers.

Intermittent use of a perfluorochemical emulsion (Fluosol-DA 20%) and carbogen breathing with fractionated irradiation

Oxygen carrying perfluorochemical emulsions have been shown to enhance the response of animal tumors to large single doses of radiation. Clinically, however, perfluorochemical emulsions are being used with only some fractions in multiple fraction radiation courses. To test the efficacy of a perfluorochemical emulsion under conditions that are closer to those used in current clinical trials, BA1112 rat sarcomas were treated with three fractions per week of 6 Gy per fraction. Once a week, animals were infused i.v. with a perfluorochemical emulsion at 15 ml/kg, and allowed to breathe carbogen (95% O2:5% CO2) for 30 minutes prior to and during irradiation. The 50% tumor control dose was 93.1 (87.9-103.1) Gy in the control arm compared to 74.3 (64.9-83.9) Gy for the animals given the perfluorochemical infusion plus carbogen breathing. The dose modification factor for intermittent perfluorochemical infusion and carbogen breathing was 1.26 (1.15-1.40). In a similar fractionated schedule, the hypoxic cell radiosensitizer, misonidazole, given once per week at 600 mg/kg, gave a dose modification factor of 1.22 (1.14-1.32).

Applicability of animal tumor data to cancer therapy in humans

The problem of applying experimental tumor studies to clinical cancer therapy is a complex one. The radiotherapy literature contains many examples of premature efforts to apply laboratory observations to the clinic, and many examples of failures to adequately consider animal tumor observations in the design of clinical studies. This review covers three areas: tumor hypoxia, where clinical trials based on animal tumor data have been conducted with radiosensitizers, hyperbaric oxygen, and systemic oxygen carriers; dose fractionation, where current trials of hyperfractionation are based in part on animal tumor studies; and chemo-radiotherapy, where clinical trials are only beginning to exploit concepts developed in animal tumor systems. The use of animal tumor systems extends past the screening of new agents. Animal tumor models can be used in biological, physiological, and pharmacological studies to elucidate the biological factors influencing the efficacy of therapeutic agents. Tumor studies can be combined with studies of normal tissues to predict the toxicities to be anticipated in clinical trials, and to assess the potential for therapeutic gain. Animal studies can also provide data which are useful in designing optimal clinical trials of new agents and maximizing the potential for successful clinical application of new approaches. In general, it is not possible to apply specific laboratory data directly to man. To translate, rather than transpose, information from the laboratory to the clinic, the model studies must be directed at evaluating principles, rather than merely quantifying results. Only through studies of mechanisms, by designing experiments to test or refute a hypothesis, will it be possible to apply model studies to man.

Misonidazole in fractionated radiotherapy of a murine mammary carcinoma: comparison of tumor and normal tissue response

The potential therapeutic benefit of misonidazole was tested in radiotherapy with 1, 2, 5, and 10 equal fractions, using as endpoints local tumor control (TCD50) of murine mammary carcinoma MDAH-MCa-4 and leg contracture at the TCD50, measured 120 days after irradiation. In controls and misonidazole-treated mice, the TCD50 increased with the number of fractions, from 66.7 to 114.6 Gy in controls, and from 43.3 to 75.7 Gy with misonidazole. At doses of greater than or equal to 0.1 mg/g body weight, misonidazole reduced the TCD50 in all fractionation schedules; however, because of toxicity, 1.0 and 0.6 mg/g could be given with only 1 or 2 fractions. Leg contracture at the TCD50 was greatest (14.5 mm) in control mice treated with a single dose of radiation, and was least (7.2 to 7.4 mm) in those treated with a single dose of radiation preceded by 1.0 or 0.6 mg misonidazole/g body weight. With 0.1 mg misonidazole/g, the leg contracture at the TCD50 was less (9.8 to 12.2 mm with the various schedules) than in controls (12.0 to 14.5 mm) for 1, 5, or 10 fractions. Therefore, a therapeutic gain could be obtained by using misonidazole with 1, 2, 5, or 10 fractions, but the greatest gain occurred with 1 fraction, with high doses of misonidazole, that is, 0.6 to 1.0 mg/g.