Modification of tumour radiation response in vivo by the benzamide analogue pyrazinamide

Therapeutic Modification of Hypoxia

Regions of reduced oxygenation (hypoxia) are a characteristic feature of virtually all animal and human solid tumours. Numerous preclinical studies, both in vitro and in vivo, have shown that decreasing oxygen concentration induces resistance to radiation. Importantly, hypoxia in human tumours is a negative indicator of radiotherapy outcome. Hypoxia also contributes to resistance to other cancer therapeutics, including immunotherapy, and increases malignant progression as well as cancer cell dissemination. Consequently, substantial effort has been made to detect hypoxia in human tumours and identify realistic approaches to overcome hypoxia and improve cancer therapy outcomes. Hypoxia-targeting strategies include improving oxygen availability, sensitising hypoxic cells to radiation, preferentially killing these cells, locating the hypoxic regions in tumours and increasing the radiation dose to those areas, or applying high energy transfer radiation, which is less affected by hypoxia. Despite numerous clinical studies with each of these hypoxia-modifying approaches, many of which improved both local tumour control and overall survival, hypoxic modification has not been established in routine clinical practice. Here we review the background and significance of hypoxia, how it can be imaged clinically and focus on the various hypoxia-modifying techniques that have undergone, or are currently in, clinical evaluation.

PET hypoxia imaging with FAZA: reproducibility at baseline and during fractionated radiotherapy in tumour-bearing mice

PURPOSE

Tumour hypoxia is linked to treatment resistance. Positron emission tomography (PET) using hypoxia tracers such as fluoroazomycin arabinoside (FAZA) may allow identification of patients with hypoxic tumours and the monitoring of the efficacy of hypoxia-targeting treatment. Since hypoxia PET is characterized by poor image contrast, and tumour hypoxia undergoes spontaneous changes and is affected by therapy, it remains unclear to what extent PET scans are reproducible. Tumour-bearing mice are valuable in the validation of hypoxia PET, but identification of a reliable reference tissue value (blood sample or image-derived muscle value) for repeated scans may be difficult due to the small size of the animal or absence of anatomical information (pure PET). Here tumour hypoxia was monitored over time using repeated PET scans in individual tumour-bearing mice before and during fractionated radiotherapy.

METHODS

Mice bearing human SiHa cervix tumour xenografts underwent a PET scan 3 h following injection of FAZA on two consecutive days before initiation of treatment (baseline) and again following irradiation with four and ten fractions of 2.5 Gy. On the last scan day, mice were given an intraperitoneal injection of pimonidazole (hypoxia marker), tumours were collected and the intratumoral distribution of FAZA (autoradiography) and hypoxia (pimonidazole immunohistology) were determined in cryosections.

RESULTS

Tissue section analysis revealed that the intratumoral distribution of FAZA was strongly correlated with the regional density of hypoxic (pimonidazole-positive) cells, even when necrosis was present, suggesting that FAZA PET provides a reliable measure of tumour hypoxia at the time of the scan. PET-based quantification of tumour tracer uptake relative to injected dose showed excellent reproducibility at baseline, whereas normalization using an image-derived nonhypoxic reference tissue (muscle) proved highly unreliable since a valid and reliable reference value could not be determined. The intratumoral distribution of tracer was stable at baseline as shown by a voxel-by-voxel comparison of the two scans (R = 0.82, range 0.72-0.90). During treatment, overall tracer retention changed in individual mice, but there was no evidence of general reoxygenation.

CONCLUSION

Hypoxia PET scans are quantitatively correct and highly reproducible in tumour-bearing mice. Preclinical hypoxia PET is therefore a valuable and reliable tool for the development of strategies that target or modify hypoxia.

Lectin intravital perfusion studies in tumor-bearing mice: micrometer-resolution, wide-area mapping of microvascular labeling, distinguishing efficiently and inefficiently perfused microregions in the tumor

Intravital lectin perfusion was combined with computer-guided scanning digital microscopy to map the perfused elements of the vasculature in tumor-bearing mice. High-precision composite images (spatial precision 1.3 micron and optical resolution 1.5 micron) were generated to permit exact positioning, reconstruction, analysis, and mapping of entire tumor cross-sections (c. 1 cm in diameter). Collation of these mosaics with nuclear magnetic resonance maps in the same tumor plane identified sites of rapid contrast medium uptake as tumor blood vessels. Digitized imaging after intravital double labeling allowed polychromatic visualization of two different types of mismatched staining. First, simultaneous application of two lectins, each bearing a different fluorochrome, revealed organ-specific differential processing in the microvascular wall. Second, sequential application of two boluses of one lectin, bearing different fluorochromes successively, distinguished between double-labeled microvessels, representing efficiently perfused vascular segments, and single-labeled microvessels, with inefficient or intermittent perfusion. Intravital lectin perfusion images of blood vessels in the vital functional state thus highlighted biologically significant differences in vessel function and served as high-resolution adjuncts to MR imaging.

DNA damage and repair in tumour and non-tumour tissues of mice induced by nicotinamide

In vivo DNA damage and repair was induced by nicotinamide (NAM) in adenotype 12 virus-induced mouse sarcoma A12B3 and sarcoma F inoculated into CBA mice. DNA damage, NAM and NAD concentrations were measured after in vivo exposure to NAM, in tumours and spleens by alkaline elution and by HPLC analysis. Our results indicate that NAM between 100-1000 mg kg-1 causes a high level of in vivo DNA strand breaks in tumours and normal tissues in mice bearing the immunogenic sarcoma A12B3 but not in the non-immunogenic sarcoma F. The repair process was also delayed by the NAM treatment probably owing to inhibition of the DNA repair enzyme, poly(ADP-ribose)polymerase, as evidenced by accumulation of NAM and NAD. These data are consistent with NAM having a mechanism of action as a radiosensitiser at least in part by DNA repair inhibition. In addition, it should also be considered that high doses of NAM might cause considerable complications to normal tissue in tumour-bearing individuals.

A phase I/II evaluation of metoclopramide as a radiosensitiser in patients with inoperable squamous cell carcinoma of the lung

The feasibility of administering metoclopramide (MCA) as a radiosensitizer has been evaluated in 23 patients with a pathological or cytological diagnosis of a squamous cell carcinoma of the lung, clinically evaluated as inoperable. All patients received 40-60 Gy radiotherapy fractionated into 1.8 Gy fractions 5 times per week (Monday-Friday). Two MCA treatment regimens were used: (i) MCA at 2 mg/kg administered by intravenous-infusion 1-2 h prior to radiotherapy 3 times per week (Monday, Wednesday, Friday); and (ii) MCA at 1 mg/kg administered by intravenous infusion 1-2 h prior to radiotherapy 5 times per week (Monday-Friday). 11 of the 23 patients treated with radiotherapy and MCA had none to mild pneumonitis or fibrosis and another 8 of the 23 had moderate levels. No patient had their therapy interrupted due to radiation-related side-effects. The MCA-related side-effects were as expected, i.e. 78% of the patients experienced sedation/tiredness and 48% expressed restlessness/anxiety symptoms. Both the total dose and serum levels of MCA were significantly associated to the MCA side-effect profile. Tumour response, duration of tumour response and survival were significantly positively correlated to the total and weekly doses of MCA administered to the patients during their radiotherapy treatment. These favourable phase II data have justified the initiation of a phase II/III randomised multicentred trial being carried out in Europe to evaluate MCA as a radiosensitiser.

Transient perfusion in human melanoma xenografts

Studies of transplantable rodent tumours have suggested that malignant tissue might experience transient perfusion at the microvascular level. The purpose of the work reported here was to investigate whether transient perfusion can be demonstrated in xenografted human tumours. Tumours of four melanoma lines (A-07, D-12, R-18, U-25), grown orthotopically in Balb/c nu/nu mice, were included in the study. Transient perfusion was studied by using the double-fluorescent staining technique. Hoechst 33342 and DiOC7(3) were either administered simultaneously or Hoechst 33342 was administered 20 min before DiOC7(3). Detection of transient perfusion by this method requires that vessels are non-functional for at least 5 min owing to the distribution half-lives of the dyes in the blood. Usable combinations of dye concentrations were found by varying the concentrations of Hoechst 33342 and DiOC7(3) systematically. The level of perfusion mismatch following simultaneous administration of the dyes ranged from approximately 1.5% for U-25 tumours to approximately 3.0% for R-18 tumours at these combinations. Moreover, the fraction of vessels stained only with Hoechst 33342 and the fraction of vessels stained only with DiOC7(3) were not significantly different whether the dyes were administered simultaneously or sequentially. Transient perfusion could not be demonstrated in any of the tumour lines. Thus, the fraction of vessels stained only with Hoechst 33342 and the fraction of vessels stained only with DiOC7(3) were not significantly higher after sequential than after simultaneous administration of the dyes. Moreover, the vessels stained only with Hoechst 33342 and the vessels stained only with DiOC7(3) were randomly distributed within the tumours whether the dyes were administered simultaneously or sequentially. Consequently, acute hypoxia caused by transient perfusion is probably a less pronounced phenomenon in malignant tissue than previous studies of rodent tumours have suggested.

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

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

The effect of nicotinamide on microregional blood flow within tumours assessed using laser Doppler probes

Laser Doppler probes have been used to provide real-time spatial flow mapping of microregional erythrocyte flux within the murine Sarcoma F (SaF). The results demonstrate that fluctuations in microregional red blood cell flux are a common feature of SaF tumours, with approximately 50% of regions demonstrating a change of at least a factor of 2 over a one-hour sampling period and 16% of regions showing a greater than 5-fold change. Administration of 250 mg/kg nicotinamide induced a small reduction in the number of microregions showing fluctuations, 9% of regions demonstrated large fluctuations (i.e., > or = 5) relative to 16% in control tumours. At a dose of 500 mg/kg, the number of changes in microregional erythrocyte flux was increased. However, unlike either untreated or nicotinamide 250 mg/kg pretreated animals, these changes were mostly increases in microregional flow. Indeed, 12 of the 13 changes by a factor of 5 or greater, observed following nicotinamide at 500 mg/kg, were increases. This effect on microregional flow translated into only a 25% increase in macroregional flow determined by averaging all the regions sampled.

The role of inhibitors of poly(ADP-ribose) polymerase as resistance-modifying agents in cancer therapy

Poly(ADP-ribose) polymerase (PARP) plays an important role in a number of cellular processes including DNA repair. Since poly(ADP-ribosyl)ation occurs in response to radiation- or drug-induced DNA damage, inhibitors of the enzyme may enhance the antitumour activity of radiotherapy or cytotoxic drug treatment. In this review the development of PARP inhibitors is discussed, and structure-activity relationships amongst inhibitors of the enzyme are presented. Studies to date regarding the in vitro and in vivo activity of PARP inhibitors, as resistance modifying agents in cancer therapy, are also overviewed.

Antibody directed enzyme prodrug therapy (ADEPT). A review of some theoretical, experimental and clinical aspects

The concept of generating cytotoxic agents from non-toxic prodrugs at tumour sites by antibody vectored enzyme introduces a wide range of opportunities. Various prodrug-enzyme combinations have been described and encouraging results reported in xenograft models. Whilst the mouse model is a valuable tool in this approach translation to the human patient may expose more complex issues. The objective of restricting drug action to tumour sites and thus allowing greatly increased cytotoxic action requires more precise restriction of enzyme activity to tumour sites than has been achieved with an antibody vector and natural clearance alone. Assisted clearance mechanisms have been found effective. Alternatively, or additionally, the difference between prodrug and active drug creates the opportunity to degrade active drug selectively in blood and thus protect normal tissues. In order to give more than one cycle of treatment it will be necessary for the antibody-enzyme conjugate to be nonimmunogenic or for the concurrent administration of immunosuppressive agents. A pilot scale clinical trial with a prototype prodrug indicated the feasibility of antibody directed enzyme prodrug therapy (ADEPT).

Enhancement of cyclophosphamide cytotoxicity in vivo by the benzamide analogue pyrazinamide

The ability of pyrazinamide to enhance the in vivo cytotoxicity of cyclophosphamide in Lewis lung and RIF-1 tumours was investigated. Using an in vivo/in vitro excision assay a large single dose of pyrazinamide (500 mg kg-1 i.p.) was shown to enhance the tumour cell killing by cyclophosphamide. This enhancement was greatest when pyrazinamide was administered before the alkylating agent and had a dose-modifying effect on all cyclophosphamide doses tested, giving rise to a mean (+/- 1 s.e.) enhancement ratio (ER) of 1.54 (+/- 0.15) for the Lewis lung and 1.24 (+/- 0.08) for the RIF-1 tumour. Pyrazinamide also increased the cytotoxic action of cyclophosphamide in a normal tissue, namely white blood cell counts. However, the ER was only 1.14 (+/- 0.08), which although not significantly different from the value seen in RIF-1 was significantly less than the ER obtained with Lewis lung, suggesting the possibility of a therapeutic gain. This benzamide analogue did not appear to inhibit recovery from cyclophosphamide-induced potentially lethal damage in tumours, nor did it alter the bioactivation of cyclophosphamide or the subsequent clearance of the cytotoxic species from the plasma, so the mechanism for this chemosensitisation remains unclear.

Further evaluation of nicotinamide and carbogen as a strategy to reoxygenate hypoxic cells in vivo: importance of nicotinamide dose and pre-irradiation breathing time

The combination of nicotinamide and carbogen breathing is awaiting clinical evaluation as a strategy to overcome tumour hypoxia and thus enhance radiation response. We have continued our evaluation of this approach in the murine SCCVII tumour with the aim of determining the importance of nicotinamide dose and the pre-irradiation breathing time (PIBT) for carbogen. For carbogen breathing alone maximal enhancement of radiation response was observed with PIBT's of between 5 and 30 min. When nicotinamide (1,000 mg kg-1 IP) was administered 60 min prior to irradiation little or no variation in radiation response was observed for all the PIBT's examined (5-90 min). Indeed at all PIBT's the cell survival obtained for the carbogen nicotinamide and radiation combination was indistinguishable from that expected for a fully aerobic response. For PIBT's of 15 and 60 min we examined the influence of nicotinamide doses between 50 and 1,000 mg kg-1. Significant radiosensitizing effects were observed for all nicotinamide doses tested above 50 mg kg-1. Moreover for doses of 250 mg kg-1 and above the cell survival data was consistent with that expected for a fully aerobic response. No additional benefit accrued from raising the nicotinamide dose above 250 mg kg-1. These results indicate that significant radiosensitization may be expected even with clinically achievable nicotinamide doses when it is combined with carbogen breathing. Furthermore, the use of nicotinamide may reduce the critical importance of PIBT on the radiosensitization observed with carbogen.

Normal tissue reactions in mice after combined treatment with metoclopramide and ionizing radiation

We have previously shown that metoclopramide potentiates the effect of ionizing radiation and cisplatin treatment of human squamous cell carcinomas from the head and neck region xenografted to nude mice. In the present tumor study, the dose scheduling of metoclopramide in combination with radiation was evaluated, and metoclopramide was shown to be most effective in potentiating the cytotoxic effect of radiation when administered one hour before radiation. The effect of radiation in combination with metoclopramide on normal tissue was also studied in two well-established models. Acute skin reactions to radiation exposure were studied in 129-type mice, and metoclopramide did not enhance the acute skin reaction in this in vivo model. Survival after whole body irradiation was studied in heterozygote Balb/c nu/+ mice as a measure of bone marrow toxicity. Metoclopramide was not found to affect the LD50/30 in this in vivo model. The absence of potentiation of radiation damage to normal tissue in these animal studies, makes metoclopramide an interesting possibility for future clinical evaluation.