Radiosensitization in vivo: a study with an homologous series of 2-nitroimidazoles

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.

The influence of tumour temperature on ischemia-induced cell death: potential implications for the evaluation of vascular mediated therapies

We have evaluated the influence of tumour temperature on the kinetics and extent of tumour cell death following induction of ischemia. Induction of ischemia in SCCVII tumours implanted subcutaneously in the back of syngeneic C3H mice results in a rapid cooling of the tumour from a resting value of 35.2 degrees C, towards ambient temperature. In the SCCVII tumour no significant cell kill is detected in the first hour of ischemia. At longer times cell kill was detected and a survival level of 2 x 10(-2) was attained after 6 h of ischemia. However, if the tumours were maintained at a temperature of 37 degrees C following induction of ischemia a more rapid and dramatic reduction in cell survival is observed with a survival level of approximately 10(-5) being attained after 4 h of ischemia. Qualitatively similar results are obtained using the Lewis lung tumour implanted in C57BL mice. Similar rapid cooling following occlusion of the blood supply is observed in the C3H/TIF tumour implanted in the foot. For the C3H/TIF induction of ischemia for up to 6 h resulted in no significant growth delay provided the tumours were kept at room temperature. However, if the tumours were maintained at a temperature of 37 degrees C during the ischemic insult significant growth delay was observed for all clamping times exceeding 1 h. These results show the importance of tumour temperature on the kinetics and extent of tumour cell kill during ischemia. This finding has particular relevance for studies in which agents known to reduce tumour blood flow are used in animals bearing superficially located tumours.

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.

Nicotinamide, Fluosol DA and Carbogen: a strategy to reoxygenate acutely and chronically hypoxic cells in vivo

The effect of Nicotinamide and/or treatment with Fluosol DA and Carbogen breathing on the radiation response of 500-750 mg SCCVII and KHT tumours has been evaluated. Pretreatment with Fluosol DA/Carbogen or Nicotinamide resulted in relatively modest enhancements of radiation damage with enhancement factors of 1.1 and 1.3 being observed using an in vivo/in vitro clonogenic end-point. A combination of Nicotinamide and Fluosol DA/Carbogen resulted in a larger enhancement factor of 1.6 over the radiation dose ranges studied. These modification factors reflect a value close to that expected for a fully aerobic response in this survival range. Growth delay studies in the SCCVII tumour provided similar results. Using a recently developed fluorescence activated cell sorting technique, which utilizes the in vivo pharmacokinetic and DNA binding properties of the bisbenzamide stain Hoechst 33342, the effect of Nicotinamide and/or Fluosol DA/Carbogen schedules on the occurrence of acute hypoxia was assessed. The results clearly show that Nicotinamide significantly reduces the amount of 'acute hypoxia', but has a lesser effect on 'chronic' hypoxic cells. However, combinations of Nicotinamide and Fluosol DA/Carbogen significantly increase the response of both 'acutely' and 'chronically hypoxic' cells. The results provide evidence that a combination of Nicotinamide and Fluosol DA/Carbogen can provide an effective way of reoxygenating both acutely and chronically hypoxic cells.

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

Pyrazinamide, the pyrazine analogue of nicotinamide, has been evaluated for its ability to modify the radiation response of hypoxic cells both in vivo and in vitro. Results obtained with three different murine tumour systems EMT6, LLC and SCCVII showed that pyrazinamide at a dose of 0.5 mg g-1 i.p. resulted in enhanced radiation response. Dose modification factors of between 1.3 and 1.6 were observed using in vivo/in vitro clonogenic assays. This enhancement was greater than that obtained in mouse intestine using crypt cell survival as an endpoint (DMF 1.1). In contrast to the tumour data in vivo, the in vitro results indicate that pyrazinamide displays little radiosensitising or toxic properties towards hypoxic CHO cells in culture. These results suggest that pyrazinamide exerts its effects in vivo either by directly perturbing tumour physiology or by being converted to an active metabolite. Blood flow studies performed using laser Doppler flowmetry indicate that pyrazinamide produces a small (32%) increase in overall tumour blood flow in the SCCVII tumour. Based on this finding, additional studies on tumour perfusion at the microregional level were performed in the SCCVII tumour using a histological technique involving injection of fluorescent stains which demarcate functional vasculature. The data show that when compared to saline injected controls, pyrazinamide reduced the number of vessels opening and closing over a 20 min period from 10.2% to 3.8%. This finding suggests that pyrazinamide may exert its effects at least in part by reducing the occurrence of acute hypoxia resulting from dynamic changes in microregional perfusion.

Acute hypoxia in tumors: implications for modifiers of radiation effects

Radioresistant hypoxic cells have been shown to occur in nearly all the animal tumors studied to date. Furthermore, there is a large amount of evidence, albeit indirect, that hypoxic cells exist and impair the effectiveness of radiation therapy in some human cancers. Surprisingly little is known, however, about the natural history of such hypoxic cells. Recently in our laboratories, we have developed methods which enable us to select and analyse cells from tumors as a function of their distance from the tumor blood supply. Utilizing this technique, we have been able to demonstrate using SCCVII tumors greater than or equal to 500 mg that even cells close to the blood supply may become hypoxic at any particular time. This information provides direct evidence that, at least for that tumor, hypoxia can result from transient fluctuations in blood perfusion. The existence of acutely, as well as, chronically hypoxic cells within tumors has several implications for treatment strategies. Treatments designed to increase oxygen content in the blood may not be particularly effective in sensitizing acutely hypoxic cells. However small, freely diffusable radiosensitizers would distribute throughout the tumor, and should be equally effective in sensitizing both acutely and chronically hypoxic cells to radiation. Acute hypoxia may thus be the best possible indicator for the use of chemical radiosensitizers in radiation therapy.

Cell selection from a murine tumour using the fluorescent probe Hoechst 33342

Images

RSU 1069, a 2-nitroimidazole containing an alkylating group: high efficiency as a radio- and chemosensitizer in vitro and in vivo

Electron affinity as measured by the one-electron reduction potentian, E7(1), is the major factor influencing radiosensitizing efficiency in vitro. RSU 1069 has an electron affinity (E7(1) = 398 mV) similar to misonidazole; however, the ability of this compound to sensitize hypoxic cells is considerably greater in vitro than that of misonidazole, e.g., 0.2 mM RSU 1069 gives an enhancement ratio of greater than 2.0 compared to 1.4 for the same concentration of misonidazole. Radiosensitization studies with the MT tumor in vivo also showed RSU 1069 to be a more efficient sensitizer than misonidazole. An administered dose of 0.08 mg/g RSU 1069 yielded an enhancement of 1.8 to 1.9 using tumor cell survival and tumor cure as end-points. At least a 10-fold higher dose of misonidazole is required for a similar degree of sensitization. Low doses of RSU 1069 also radiosensitize the Lewis lung and B16 experimental tumors. The ability of RSU 1069 to potentiate the cytotoxic action of melphalan and other cytotoxic drugs towards the MT tumor was also examined. RSU 1069 (0.08 mg/g) given to mice 1 hour before melphalan gave an enhancement of 2.8.

Potentiation of cyclophosphamide cytotoxicity in vivo: a study with misonidazole and fifteen other 1-substituted 2-nitroimidazoles

It has recently been reported that compounds more lipophilic than misonidazole are better potentiators of CCNU tumor cytotoxicity in vivo. There is now a need to extend these studies to include other tumors and cytotoxic drugs. In the present study we have shown that misonidazole (MISO) can potentiate cyclophosphamide cytotoxicity in the Lewis lung carcinoma but not in the B16 melanoma. Further studies in the Lewis lung carcinoma, using 15 1-substituted 2-nitroimidazoles possessing a range of octanol:water partition coefficient (P) (from 0.18- greater than 100) have shown that potentiation increases with increasing lipophilicity. The most efficient compounds at administered dose levels of 1.0 and 2.0 mumol/g were Ro-07-1902, benznidazole (Ro-07-1051) and RSU 1050 which possess octanol:water partition coefficients in the range of 2.5-10. However, on the basis of an equitoxic administered dose (1/2LD50/2d), little difference in potentiation is seen over the range of P studied.

Intestinal microflora as potential modifiers of sensitizer activity in vivo

Treatment of mice (some bearing Lewis Lung tumors), with penicillin (PEN) at 500 mg/l drinking water for one week prior to treatment with misonidazole (MIS), resulted in: the elimination of their anaerobic cecal flora; a decrease in MIS-induced neurotoxicity; an increase in pharmacological exposure to MIS; a decrease in MIS chemopotentiation; a probable increase in MIS radiosensitization; an increase in MIS induced hypothermia. Assuming no chemical interaction between PEN and MIS, these observations indicate that the intestinal microflora can influence the activity of MIS in vivo. Therefore their influence should be considered in all sensitizer-related studies in vivo. The observed reduction in the neurotoxic but not the radiosensitizing potential of MIS following PEN treatment indicates a therapeutic benefit.