Initial report of the phase I trial of the hypoxic cell radiosensitizer SR-2508

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.

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.

Nitroimidazoles with a halogen-containing side-chain

Syntheses are reported for: nine 2-nitroimidazoles, the abbreviated names all beginning with E, based on derivation from Etanidazole); five 2-methyl-5-nitroimidazoles (M compounds, derived from Metronidazole); and five 2-methyl-4-nitroimidazoles (labelled 2M4N compounds). The nitroimidazoles all have an amide side-chain at the N1 atom of the imidazole, with 17 of them containing one to five halogen atoms. The aim is to study compounds for comparison with EF5 (the number showing the presence of five F-atoms), a previously reported, pentafluoropropylacetamide derivative of 2-nitroimidazole that is currently used as a hypoxia marker drug to detect cancerous tumours. The new compounds are characterized by standard methods, including X-ray structural data for the fluorinated MF5, 2M4NF5, and 2M4NF1(−1) species, with the “–1” indicating two C-atoms in an alkylamide chain rather than the three C-atoms in the propylacetamide of EF5. Intra- and inter-molecular H-bonding is seen in the solid state structures, likely an important property in biological use; another key property of the nitroimidazoles is their reduction potentials, and the measured CV data confirm that 2NO2Im compounds with longer side-chains and more F-atoms (like EF5) are worth investigating for possible activity as hypoxia-selective, bioreductive agents.

Next-Generation Hypoxic Cell Radiosensitizers: Nitroimidazole Alkylsulfonamides

Innovations in the field of radiotherapy such as stereotactic body radiotherapy, along with the advent of radio-immuno-oncology, herald new opportunities for classical oxygen-mimetic radiosensitizers. The role of hypoxic tumor cells in resistance to radiotherapy and in suppression of immune response continues to endorse tumor hypoxia as a bona fide, yet largely untapped, drug target. Only nimorazole is used clinically as a radiosensitizer, and there is a dearth of new radiosensitizers in development. Here we present a survey of novel nitroimidazole alkylsulfonamides and document their cytotoxicity and ability to radiosensitize anoxic tumor cells in vitro. We use a phosphate prodrug approach to increase aqueous solubility and to improve tumor drug delivery. A 2-nitroimidazole and a 5-nitroimidazole analogue demonstrated marked tumor radiosensitization in either ex vivo assays of surviving clonogens or tumor regrowth delay.

Biodistribution and dosimetry of (18)F-EF5 in cancer patients with preliminary comparison of (18)F-EF5 uptake versus EF5 binding in human glioblastoma

PURPOSE

The primary purpose of this study was to assess the biodistribution and radiation dose resulting from administration of (18)F-EF5, a lipophilic 2-nitroimidazole hypoxia marker in ten cancer patients. For three of these patients (with glioblastoma) unlabeled EF5 was additionally administered to allow the comparative assessment of (18)F-EF5 tumor uptake with EF5 binding, the latter measured in tumor biopsies by fluorescent anti-EF5 monoclonal antibodies.

METHODS

(18)F-EF5 was synthesized by electrophilic addition of (18)F(2) gas, made by deuteron bombardment of a neon/fluorine mixture in a high-pressure gas target, to an allyl precursor in trifluoroacetic acid at 0° then purified and administered by intravenous bolus. Three whole-body images were collected for each of ten patients using an Allegro (Philips) scanner. Gamma counts were determined in blood, drawn during each image, and urine, pooled as a single sample. PET images were analyzed to determine radiotracer uptake in several tissues and the resulting radiation dose calculated using OLINDA software and standard phantom. For three patients, 21 mg/kg unlabeled EF5 was administered after the PET scans, and tissue samples obtained the next day at surgery to determine EF5 binding using immunohistochemistry techniques (IHC).

RESULTS

EF5 distributes evenly throughout soft tissue within minutes of injection. Its concentration in blood over the typical time frame of the study (∼3.5 h) was nearly constant, consistent with a previously determined EF5 plasma half-life of ∼13 h. Elimination was primarily via urine and bile. Radiation exposure from labeled EF5 is similar to other (18)F-labeled imaging agents (e.g., FDG and FMISO). In a de novo glioblastoma multiforme patient, focal uptake of (18)F-EF5 was confirmed by IHC.

CONCLUSION

These results confirm predictions of biodistribution and safety based on EF5's characteristics (high biological stability, high lipophilicity). EF5 is a novel hypoxia marker with unique pharmacological characteristics allowing both noninvasive and invasive measurements.

The radiation response of cells from 9L gliosarcoma tumours is correlated with [F18]-EF5 uptake

PURPOSE

Tumour hypoxia affects cancer biology and therapy-resistance in both animals and humans. The purpose of this study was to determine whether EF5 ([2-(2-nitro-1-H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide]) binding and/or radioactive drug uptake correlated with single-dose radiation response in 9L gliosarcoma tumours.

MATERIALS AND METHODS

Twenty-two 9L tumours were grown in male Fischer rats. Rats were administered low specific activity (18)F-EF5 and their tumours irradiated and assessed for cell survival and hypoxia. Hypoxia assays included EF5 binding measured by antibodies against bound-drug adducts and gamma counts of (18)F-EF5 tumour uptake compared with uptake by normal muscle and blood. These assays were compared with cellular radiation response (in vivo to in vitro assay). In six cases, uptake of tumour versus muscle was also assayed using images from a PET (positron emission tomography) camera (PENN G-PET).

RESULTS

The intertumoural variation in radiation response of 9L tumour-cells was significantly correlated with uptake of (18)F-labelled EF5 (i.e., including both bound and non-bound drug) using either tumour to muscle or tumour to blood gamma count ratios. In the tumours where imaging was performed, there was a significant correlation between the image analysis and gamma count analysis. Intertumoural variation in cellular radiation response of the same 22 tumours was also correlated with mean flow cytometry signal due to EF5 binding.

CONCLUSION

To our knowledge, this is the first animal model/drug combination demonstrating a correlation of radioresponse for tumour-cells from individual tumours with drug metabolism using either immunohistochemical or non-invasive techniques.

Microenvironmental adaptation of experimental tumours to chronic vs acute hypoxia

This study investigated long-term microenvironmental responses (oxygenation, perfusion, metabolic status, proliferation, vascular endothelial growth factor (VEGF) expression and vascularisation) to chronic hypoxia in experimental tumours. Experiments were performed using s.c.-implanted DS-sarcomas in rats. In order to induce more pronounced tumour hypoxia, one group of animals was housed in a hypoxic atmosphere (8% O₂) for the whole period of tumour growth (chronic hypoxia). A second group was acutely exposed to inspiratory hypoxia for only 20 min prior to the measurements (acute hypoxia), whereas animals housed under normal atmospheric conditions served as controls. Acute hypoxia reduced the median oxygen partial pressure (pO₂) dramatically (1 vs 10 mmHg in controls), whereas in chronically hypoxic tumours the pO₂ was significantly improved (median pO₂=4 mmHg), however not reaching the control level. These findings reflect the changes in tumour perfusion where acutely hypoxic tumours show a dramatic reduction of perfused tumour vessels (maybe the result of a simultaneous reduction in arterial blood pressure). In animals under chronic inspiratory hypoxia, the number of perfused vessels increased (compared to acute hypoxia), although the perfusion pattern found in control tumours was not reached. In the chronically hypoxic animals, tumour cell proliferation and tumour growth were significantly reduced, whereas no differences in VEGF expression and vascular density between these groups were observed. These results suggest that long-term adaptation of tumours to chronic hypoxia in vivo, while not affecting vascularity, does influence the functional status of the microvessels in favour of a more homogeneous perfusion.

In vitro activity of Etanidazole against the protozoan parasite Trypanosoma cruzi

We investigated the in vitro action of an hydrosoluble 2-nitroimidazole, Etanidazole (EZL), against Trypanosoma cruzi, the etiologic agent of Chagas disease. EZL displayed lethal activity against isolated trypomastigotes as well as amastigotes of T. cruzi (RA strain) growing in Vero cells or J774 macrophages, without affecting host cell viability. Although not completely equivalent to Benznidazole (BZL), the reference drug for Chagas chemotherapy, EZL takes advantage in exerting its anti-T. cruzi activity for longer periods without serious toxic side effects, as those recorded in BZL-treated patients. Our present results encourage further experiments to study in depth the trypanocidal properties of this drug already licensed for use in human cancers.

Simulation of intratumoral release of Etanidazole: effects of the size of surgical opening

The efficacy of radiotherapy can be enhanced by the delivery of radiosensitizer (Etanidazole) to brain tumor from biodegradable polymer implants. This process is investigated by simulation carried out at a cut section of tumor with polymeric wafers of Etanidazole loading implanted in the resected cavity. The coupled mass and momentum equations are solved to obtain the transient solution of the drug distribution in the tumor. The polymeric delivery shows high therapeutic index, indicating the wafers' success in delivering more drugs to the tumor rather than to the tissue. The penetration distance of Etanidazole was found to decrease from 14 mm (at 5th/40th day after implantation) to 6.5 mm (at 30th/75th day), suggesting an initial high burst of drug release which cause nearby tissue toxicity and a low effective drug delivery towards the later stages. The short penetration depth is due to Etanidazole having low interstitial Peclet number and high elimination/diffusion modulus. Edema causes the interstitial pressure, velocity, and concentration to increase in all domains, and leads to enhanced convection and a lowering of therapeutic index. Simulations on the open tumor geometry show significantly lower efficacy of the drug delivery due to the uneven distribution of drug in the tumor zone.

Results of a phase II trial of external beam radiation with etanidazole (SR 2508) for the treatment of locally advanced prostate cancer (RTOG Protocol 90-20)

PURPOSE

RTOG Protocol 90-20 was designed to evaluate the effect of the hypoxic cell sensitizer Etanidazole (SR-2508) on locally advanced adenocarcinoma of the prostate treated with concurrent external beam irradiation.

METHODS AND MATERIALS

Patients with biopsy-proven adenocarcinoma of the prostate with locally advanced T2b, T3, and T4 tumors were eligible for this study. No patients with disease beyond the pelvis were eligible. Serum prostate specific antigen (PSA) was mandatory. All patients received definitive external beam irradiation using standard four-field whole pelvis treatment to 45-50 Gy, followed by a cone down with a minimum total dose to the prostate of 66 Gy at 1.8-2.0 Gy/fraction over 6.5-7.5 weeks. Etanidazole was delivered 1.8 g/m2 given 3 times a week to a total of 34.2 g/m2 or 19 doses.

RESULTS

Thirty-nine patients were entered onto the study. Three patients refused treatment; therefore, 36 patients were eligible for further evaluation. Median follow-up was 36.9 months from treatment end. All patients had elevated initial PSA levels, and 18 patients had PSAs of > 20 ng/ml. Tumor classification was T2, 12 patients (33.3%); T3, 22 patients (61.1%); and T4, 2 patients (5.6%). Complete clinical response, defined as PSA < 4 ng/ml and complete clinical disappearance, was attained in 17.9% of (5/28 pts) with information at 90 days and 56% of patients by 12 months following treatment. Relapse-free survival was 13% at 3 years with PSA < 4 ng/ml. There were no Grade 4 or 5 toxicities, either acute (during treatment) or in follow-up.

CONCLUSIONS

Results of this trial regarding PSA response and clinical disappearance of disease are similar to historical controls and do not warrant further investigation of etanidazole as was done in this trial. Drug toxicity that, in the past, has been unacceptably high with other hypoxic cell sensitizers does not appear to be a significant problem with this drug.

Solubility behavior, phase transition, and structure-based nucleation inhibition of etanidazole in aqueous solutions

PURPOSE

The solubility behavior, phase transition and inhibition of the nucleation process of etanidazole were characterized.

METHODS

Solubility measurements as a function of time permitted characterization of the solubility behavior and phase transition. The precipitate from saturated solutions was isolated and characterized by differential scanning calorimetry, polarized light microscopy, x-ray powder diffraction and coulometric analysis. The physical stability of metastable systems was examined in the presence of various structure-based nucleation inhibitors.

RESULTS

Etanidazole is soluble in water with an equilibrium solubility of 68.1 mg/mL, pH 6.5 with changes in pH having virtually no effect on the solubility. Etanidazole reaches concentrations in excess of 150 mg/mL within one hour. Etanidazole solutions prepared at 150 mg/ mL contained crystals after rotating for 24 hours. The crystals were isolated and characterized as etanidazole monohydrate. the solubility of etanidazole monohydrate in water increased with time reaching an equilibrium solubility of 68 mg/mL after 24 hours. Therefore, the solubility studies were actually determining the solubility of the more stable monohydrate from of etanidazole. Etanidazole solutions at concentrations of 50, 100 and 150 mg/mL were stabilized to varying degrees with structure-based nucleation inhibitors (imidazole, ethanolamine or diethanolamine).

CONCLUSIONS

Anhydrous etanidazole undergoes a transition in aqueous solutions to the more stable monohydrate when the solubility of the monohydrate is exceeded. The physical stability of etanidazole solutions at 4 degrees C is improved following autoclaving. The addition of structure-based nucleation inhibitors effectively stabilized the metastable systems.

Pilot study of local hyperthermia, radiation therapy, etanidazole, and cisplatin for advanced superficial tumours

Five patients (six hyperthermia sites) with advanced superficial tumours were treated with combined etanidazole, cisplatin, local hyperthermia, and radiation therapy as part of a Phase I pilot study. Treatment was given once weekly and consisted of etanidazole 3 gm/m2 IV bolus, cisplatin 50 mg/m2 IV bolus, hyperthermia for 60 min with a target temperature of 43 degrees C, and radiation therapy 500 cGy/fraction (median total dose 3000 cGy) for a total of six weeks. Blood levels of etanidazole were taken during treatment at week 1 and week 4. Etanidazole drug exposure was calculated using the trapezoidal rule and expressed as the area under the curve (AUC) of plasma concentration x time. Five of six treatment sites had received prior irradiation. Prior chemotherapy had been given in three patients and tamoxifen therapy given in the other two patients. The median follow-up time is 34 months; 3/5 patients have died of disease. The most significant toxicity was grade I or II nausea and vomiting associated with 19/32 treatments (59%) and a second degree burn in 2/6 fields. None of the five patients experienced peripheral neuropathy, skin ulceration, or needed surgical repair. In addition, there was mild renal toxicity; pharmacokinetic analysis showed a 28-75% increase in the week 1 to week 4 AUC in three patients, all of whom had a decrease in creatinine clearance over the same time of 15-47%. This pilot study suggests this combined modality therapy can be delivered without major complications and that renal function, determined by creatinine clearance, affects clearance of etanidazole and alters the AUC. Therefore, monitoring renal function is important in patients receiving etanidazole in addition to other nephrotoxic agents such as cisplatin. The impact of etanidazole on the therapeutic index of hyperthermia, radiation therapy and cisplatin may be worth of study, especially since a positive interaction between these modalities is found in laboratory models.

Combined-modality therapy of esophageal cancer with radiotherapy, etanidazole, and cisplatin-fluorouracil, with or without surgery: neurotoxicity, other toxicities and outcome

PURPOSE

To investigate whether etanidazole and cisplatin can be given safely together and to evaluate the relationship between incidence of peripheral neuropathy and cumulative exposure to etanidazole and cisplatin, as well as other toxicities and treatment outcome.

METHODS AND MATERIALS

Thirty-two previously untreated patients with locally advanced esophageal cancer were entered on a Phase I study of etanidazole combined with radiation therapy and chemotherapy. Cisplatin/5-FU (two cycles, weeks 1 and 4) and etanidazole (weeks 2, 3 and 5) were given concurrently with radiation therapy. Eligible patients then underwent surgical resection. All patients were scheduled to receive two additional cycles of cisplatin/5-FU chemotherapy after completion of radiation therapy (definitive arm) or surgery (preoperative arm).

RESULTS

Of 19 fully evaluable patients, nine (47%) developed peripheral neuropathy. Six of six patients, 65 years or older, experienced peripheral neuropathy, compared with three of 13 patients less than 65 years old (p = .003). For patients younger than 65 years, two of the two patients with single dose area under the curve (AUC) > 4.0 mMhr experienced peripheral neuropathy, compared with one of 11 patients with single-dose AUC < 4.0 mMhr (p = .03). Grade 4 toxicity included neutropenia (23%) and thrombocytopenia (26%). No other Grade 4 toxicity was observed. The pathologic complete response rate in patients who underwent surgical resection was 29%.

CONCLUSION

This regime of chemotherapy, radiotherapy, and etanidazole had acceptable toxicity. However, combining etanidazole and cisplatin appears to increase the risk of developing peripheral neuropathy for at least some categories of patients. Further studies of these interactions are needed.

Use of the hypoxic cell sensitizer etanidazole (SR-2508) with intravenous melphalan and prednisone in the treatment of multiple myeloma: a pharmacokinetic study

PURPOSE

A study was undertaken adding the alkylating agent sensitizer etanidazole to intravenous melphalan and oral prednisone for patients with multiple myeloma. This study explored the toxicity profile of these agents when given together and assessed the ability to attain adequate serum levels of etanidazole to permit sensitization to occur.

METHODS AND MATERIALS

Etanidazole was administered intravenously in two doses of 3 g/m2 and 5 g/m2 90 min apart immediately prior to the administration of intravenous melphalan and oral prednisone for three consecutive cycles (total dose 24 g/m2). Patients received three additional cycles without etanidazole, allowing a comparison of hematologic toxicity from melphalan with and without etanidazole.

RESULTS

Hematologic toxicity was moderate (Grade 3 or 4), but severity was similar during cycles with and without etanidazole. Only one patient developed a Grade 1 peripheral neuropathy questionably related to etanidazole. Most patients had etanidazole levels of > or = 70 ug/ml for 7 h, a level felt to be necessary for sensitization to occur.

CONCLUSION

Etanidazole, administered as described, results in adequate serum levels for potential alkylating agent sensitization, without significant toxicity.

Phase I pharmacokinetic study of the hypoxic cell sensitizer etanidazole with carboplatin and cyclophosphamide in the treatment of advanced ovarian cancer

PURPOSE

A Phase I study was undertaken to determine the maximum tolerated dose of the hypoxic cell sensitizer etanidazole which could be administered with carboplatin and cyclophosphamide, to determine whether adequate serum levels of etanidazole were achieved to allow for alkylating agent sensitization, and whether pretreatment with etanidazole altered carboplatin pharmacokinetics.

METHODS AND MATERIALS

Patients received 2 g/m2 of intravenous etanidazole followed by a second dose of 4 g/m2 90 min later, followed by intravenous carboplatin (300 mg/m2) and cyclophosphamide (600 mg/m2) for four treatment cycles. Patients received an additional two cycles of carboplatin and cyclophosphamide without etanidazole.

RESULTS

Two patients who received a total of 24 g/m2 of etanidazole developed Grade 1 neurotoxicity, and therefore etanidazole doses were not escalated further. The grade of granulocytopenia was worse after cycles with etanidazole than after those without (p = 0.03), but clinical outcome was not different. Etanidazole levels were adequate for alkylating agent sensitization (> 70 ug/ml) in all patients for the majority of the 7 h of testing. Pharmacokinetic data suggested t1/2 alpha and t1/2 beta for carboplatin were prolonged after pretreatment with etanidazole.

CONCLUSION

Etanidazole, 2 g/m2 followed by 4 g/m2 90 min later, is safe and results in adequate serum levels for alkylating agent sensitization. Neurotoxicity appears to prevent dose escalation of etanidazole, and an interaction between etanidazole and carboplatin may have enhanced neurotoxicity in these patients.

Phase I/pharmacokinetic/biochemical study of the nitroimadazole hypoxic cell sensitiser SR2508 (etanidazole) in combination with cyclophosphamide

SR2508 sensitises certain hypoxic tumor cells in vitro and in vivo to the cytotoxic action of radiation and alkylating agents. The mechanism of sensitisation may derive in part from depletion of glutathione (GSH) and possibly inhibition of GSH-dependent enzymes in target cells. We treated 46 evaluable patients with cyclophosphamide 750-1000 mg m-2 followed by SR2508 at eight dose levels ranging from 2.5 to 15.0 g m-2. Each patient received SR2508 as a single agent initially, followed a week later by the combination of cyclophosphamide and SR2508. Initially, myelosuppression was the major toxicity; potentiation of cyclophosphamide-induced leukopenia by SR2508 required a dose reduction of cyclophosphamide to 750 mg m-2 at SR2508 doses above 7.2 g m-2. At doses above 9.4 g m-2 an acute syndrome of muscle pains and painful paresthesias of the extremities lasting 12-24 h was observed to occur with increasing severity. This side-effect was intolerable in two of three patients treated at 15.0 g m-2. The only other reproducible side-effect was nausea and vomiting which was controllable with antiemetics. Plasma and urine SR2508 concentrations were measured by HPLC in 45 patients. Plasma elimination curves fit a 2-compartment model. The mean terminal half-life at each dose level ranged from 5.1-5.8 h. The mean area under the plasma concentration-time curve was linearly related to dose, and mean total body clearance ranged from 46.6-94.0 ml-1 min-1 m-2; renal clearance accounted for 65.7-79.3%. Pretreatment with cyclophosphamide did not influence the kinetics of SR2508 in individual patients. Examination of the glutathione content of peripheral mononuclear cells and tumour samples showed that depletion to below 50% of control occurred in the majority of patients. GSH transferase inhibition occurred with a similar time-course, but to a lesser extent. These data suggest that the further evaluation of this regimen should be conducted with SR2508 administration preceding that of cyclophosphamide and that its evaluation in cyclophosphamide-sensitive tumours is warranted.

The Janeway Lecture 1992. Nine decades of radiobiology: is radiation therapy any the better for it?

Nine decades have elapsed since Pierre Curie performed the first radiobiologic experiment when he used a radium tube to produce an ulcer on his arm and charted its progress and ultimate healing. A wide range of topics have been investigated in experimental radiation biology from chromosomal aberrations to fractionation effects in normal tissues to the use of neutrons and bioreductive drugs. Many of the strategies used in clinical radiation therapy, including hyperfractionation and accelerated treatment, are firmly based on laboratory experiments conducted in the past. Much current research is focused on understanding the molecular genetics of cancer to identify the genes that are activated or deleted in cells exposed to radiation. Radiobiology has played a key role in shaping radiation therapy into the vigorous, scientifically based, and highly quantitative branch of medicine that it is currently. In addition, research is preparing the field for the future when treatment protocols must be based on molecular rather than cellular biology.

Relationship between intracellular concentration and radiosensitizing effect of pimonidazole and etanidazole on two human melanoma cell lines

A study on the uptake and radiosensitizing properties of pimonidazole (PIMO) and etanidazole (ETA) was made in vitro with two melanoma cell lines: Na11+ and Na11-. The amelanotic Na11- was derived from the pigmented Na11+ cell line and was characterized by the complete absence of pigmentation in vitro. Plateau-phase cells were heavily pigmented for the Na11+ line whereas exponential cells showed a lower melanin content. Radiosensitivity was studied using an in vitro colony assay; intracellular drug concentration was determined by HPLC. For both cell lines the uptake of PIMO was always higher than the uptake of ETA, and slightly higher in hypoxia than in air. The uptake of PIMO was exceptionally high in plateau-phase cells of the pigmented Na11+ cell line. The radiosensitizing effect of ETA did not differ greatly as a function of the cell kinetics in both cell lines, whereas the radiosensitizing effect of PIMO was cell line-dependent. Sensitization of exponential cells by PIMO was similar in both cell lines but significantly less in plateau-phase cells, with the heavily pigmented Na11+ cell line being least affected, despite a three-fold increase in PIMO uptake observed in plateau cells relative to exponentially growing Na11+ cells. The uptake of PIMO may be related to the high melanin content of plateau-phase Na11+ cells and therefore this in vitro model may be useful for predicting the effects of agents such as PIMO towards melanotic melanomas in vivo.

Nitroimidazole adducts as markers for tissue hypoxia: mechanistic studies in aerobic normal tissues and tumour cells

Two aspects of the aerobic metabolism of nitroimidazole markers for hypoxia were investigated. Several normal murine tissues which are likely to be well oxygenated bind misonidazole at rates comparable to those of hypoxic regions in tumours. The possibility that this aerobic activation occurs via an oxygen independent process such as an initial two electron reduction was studied. Binding to the oesophageal mucosa of mice which occurred under hypoxia in vitro was inhibited by at least 95% in the presence of 10% oxygen. Dicoumarol, an inhibitor of DT-diaphorase, was shown to cause only small reductions in misonidazole binding to oesophageal epithelium and smooth muscle in vitro and to EMT6 tumours, liver, oesophageal and tracheal epithelium, parotid gland and smooth muscle in vivo. Thus an oxygen-insensitive process is not a major cause of the high binding rate in oesophageal mucosa, and may not contribute significantly to the observed binding in other normal tissues. It has been suggested that metabolism of nitroimidazoles by aerobic cells in tumours might be sufficient to minimise access of these compounds to hypoxic regions, particularly at the micromolar concentrations currently in use clinically. The uptake of 125I-iodoazomycin arabinoside by RIF-1 and EMT6 tumours was found to be directly proportional to injected dose over concentrations between 0.5 and 50 microM. Labelling of hypoxic regions in EMT6 tumours by high specific activity 3H-misonidazole at 1 microM was found to be similar to that obtained at 50 microM.

Assessment of a range of novel nitro-aromatic radiosensitizers and bioreductive drugs

In a directed search for the best compounds for clinical evaluation, some 150 selected nitroaromatic compounds, representing 6 distinct types, namely, furans, thiophenes, imidazoles, pyrazoles, pyrroles, and triazoles, have been synthesized and tested as hypoxic cell radiosensitisers and bioreductive drugs. These compounds have a wide range of one-electron redox potentials, ranging from -700 mV for 3-nitropyrroles to -250 mV for 5-nitrofurans. Within each series, those agents bearing alkylating moieties on the side chain are generally the more effective radiosensitisers in vitro. Studies in vivo demonstrated that the bifunctional nitroimidazoles were superior to the other nitroarenes tested. In terms of bioreductive cell killing, the best differential between oxic and hypoxic cell toxicity was shown for the bifunctional 2-nitroimidazoles, which had values greater than 20. In contrast, the other classes of nitroarines generally showed differential toxicities of less than 10.

Logistics in designing clinical trials for etanidazole (SR 2508): an RTOG experience

In a Phase II study of etanidazole (SR 2508), the dose of 17 x 2 g/m2 (total drug dose: 34 g/m2) was tested in 33 patients and the toxicity was deemed acceptable. A Phase III trial is now in progress comparing conventional radiotherapy with conventional radiotherapy plus etandizole (2 g/m2 i.v. 30 to 60 min before radiotherapy each Monday, Wednesday, and Friday to 34 g/m2 in 17 doses) in patients with unresectable head and neck carcinomas. A recent analysis showed only 14.7% grade 1 and 3.9% Grade 2 peripheral neuropathy. In the initial study design, 133 evaluable patients per treatment arm could achieve an 80% level of power of detecting a 15% difference in local-regional control rates between the radiotherapy arm (25% local-regional control at 2 years) and the radiotherapy plus etanidazole arm (assuming a 40% rate). Allowing for 20 ineligible cases in each arm, a total number of 306 was required. An interim analysis showed that 27% of the patients assigned to radiotherapy plus etanidazole are receiving less than 14 doses of the drug. It is assumed that less than 14 drug doses will not produce any therapeutic gain, therefore, a true 40% local-regional control rate in the radiotherapy plus etanidazole arm will be observed as a 36% rate when analyzed by assigned treatment. Using this information, the study was modified to have an 80% level of power in detecting a difference between a 25% local-regional control rate in the radiotherapy group and a 36% rate in the radiotherapy plus etanidazole group. Allowing for a 10% patient ineligibility rate, 518 patients are required. With 12 patients entered per month, it is estimated that patient accrual to this study will continue through October 1991.

Pharmacokinetics of intratumoral RK-28, a new hypoxic radiosensitizer

RK-28 is one of the new hypoxic cell radiosensitizers being developed in Japan and has been tested clinically. To reduce its toxicity and increase its sensitizing activity, intratumoral injection of RK-28 was performed during intraoperative radiation therapy for pancreatic cancer. This report presents the results of pharmacokinetic studies performed in 10 of the 17 patients who were administrated intravenous or intratumoral RK-28 during intraoperative radiation therapy. No adverse effects were noted following intravenous or intratumoral injection of the drug. Pharmacokinetic studies demonstrated several metabolites of RK-28 in both serum and tumor tissues. After intratumoral injection, the tumor drug concentration ranged from 123 micrograms/g to 9,292 micrograms/g just after intraoperative radiation therapy (30-50 min after injection of the compound), while the serum concentration ranged from 4.1 to 9.8 micrograms/ml. The tumor drug concentration was 23.3 micrograms/g at 45 min after intravenous injection of RK-28. Thus, intratumoral injection of RK-28 was superior to intravenous administration in this pharmacokinetic study. The combination of intraoperative radiation therapy and intratumoral injection of RK-28 appears to be a feasible treatment method.

Oncogenic transforming potential of etanidazole

The 2-nitroimidazole etanidazole is in Phase III randomized trials as an adjunct to radiotherapy in both the United States and Europe. Paradoxically, it is one of the more oncogenic radiosensitizers examined with the in vitro oncogenic transformation assay based on C3H 10T1/2 cells. To obtain an estimate of the possible number of second malignancies that might be induced in patients receiving the drug, the in vitro transformation assay was used to compare etanidazole with gamma-rays, for which cancer risk estimates are available from the Japanese survivors. For the clinical protocol used in RTw] studies, which results in a cumulative dose of 36 mM hr, the drug-induced cancer incidence was estimated to be 2.8 to 14%.

In vivo radiosensitizing activity of a new fluorinated hypoxic cell radiosensitizer, KU-2285, in combination with radiation dose fractionation

Since most clinical radiotherapy is given as multiple small irradiation fractions, the present study was undertaken to test the in vivo radiosensitizing activity of a new hypoxic cell radiosensitizer, KU-2285, in combination with radiation dose fractionation. Radiosensitizing activity was measured by a growth delay assay using a transplanted mammary tumor in C3H/He mice, and by an in vivo-in vitro assay using the SCC VII tumor. KU-2285 was injected intraperitoneally 30 min before irradiation in all experiments. The in vivo-in vitro assay using SCC VII tumors showed that 12.5 micrograms/g of KU-2285 sensitized the tumors to irradiation (5 Gy/fr x 5 fr/48 hr or 6 Gy/fr x 3 fr/48 hr). KU-2285 also sensitized the transplanted mammary tumors to fractionated irradiation. We concluded that KU-2285 was able to sensitize two different murine tumors when given in combination with radiation dose fractionation.

A fluorinated 2-nitroimidazole, KU-2285, as a new hypoxic cell radiosensitizer

To develop new hypoxic cell radiosensitizers, we incorporated fluorine atoms into the side chain of the 2-nitroimidazole. Of the resulting compounds, KU-2285 (a 2-nitroimidazole with an N1-substituent of CH2CF2CONHCH2-CH2OH) was considered the most useful as a hypoxic cell radiosensitizer. In this study, its in vivo radiosensitizing activity and acute toxicity were compared with those of etanidazole. The reduction potentials of KU-2285 and etanidazole were -0.96 V and -1.05 V vs Ag/Ag+ in N,N-dimethylformamide, respectively, and their respective octanol/water partition coefficients were 0.25 and 0.040. The in vivo radiosensitizing activity of KU-2285 was found to be similar to that of etanidazole at the same administration dose when assayed by an in vivo-in vitro assay, a growth delay assay, and a tumor control assay using SCC VII tumor or transplanted mammary tumor in C3H/He mice. Although the radiosensitizing activity of etanidazole was reduced when it was administered orally, there was no significant difference in the radiosensitizing activity of KU-2285 whether it was administered intravenously, intraperitoneally, or orally. The acute toxicity measured as the LD50/7 in 8-week-old female C3H/HeJ mice was found to be 2.4 g/kg (intravenously), 2.1 g/kg (intraperitonealy), and 4.25 g/kg (orally) for KU-2285, whereas it was 4.75 g/kg (intravenously) for etanidazole.

A comparison of radiosensitization by etanidazole and pimonidazole in mouse tumors

Radiosensitization by pimonidazole (Ro 03-8799) was tested in three murine tumors, EMT6/SF using the excision assay, SCC-VII/SF using the excision and growth delay assays, and MDAH-MCa-4 using TCD50 assays with both single doses and 6 fractions of radiation with a 24-hr interfraction interval. Results were compared with those using etanidazole (SR-2508), both at equitoxic doses and at doses giving tumor concentrations similar to those achievable in the clinic. In excision assays with EMT6/SF and SCC-VII/SF tumors, pimonidazole and etanidazole gave similar radiosensitization at similar concentrations in the tumors. Pimonidazole, however, did not demonstrate radiosensitization in SCC-VII/SF tumors in the growth delay assay, despite tumor concentrations that gave maximum sensitization in the excision assay. Furthermore, pimonidazole gave less than expected sensitization in single dose and 6-fraction TCD50 assays with MDAH-MCa-4 tumors, and less sensitization than comparable levels of etanidazole in this tumor line. When the concentration of pimonidazole in the tumors was approximately 0.36 mumoles/g the dose modification factor (DMF = dose without sensitizer/dose with sensitizer to give an isoeffect) was 1.56 (1.40-1.74, 95% c.l.) in single dose TCD50 assays. Etanidazole, however, gave a DMF of 1.92 (1.59-2.32) with a tumor concentration of approximately 0.32 mumoles/g and 1.69 (1.46-1.96) with a tumor concentration of approximately 0.21 mumoles/g. Thus, etanidazole gave more consistent sensitization for different tumors and different endpoints than did pimonidazole. The results appear to confirm the disappointing performance of pimonidazole in the clinic.

Effect of pH, oxygenation, and temperature on the cytotoxicity and radiosensitization by etanidazole

The effect of etanidazole was examined in vitro and in vivo in the FSaIIC tumor system. At pH 7.40 and 37 degrees C, etanidazole at 5-500 microM for 1 hr was minimally cytotoxic. At 42 degrees C and 43 degrees C, however, the cytotoxicity of etanidazole increased. Etanidazole was more cytotoxic at pH 6.45 and 37 degrees than at pH 7.40 by about 1 log. Increasing the temperature to 42 degrees C or 43 degrees C at pH 6.45 during drug exposure, however, caused little increase in drug killing above the lethality of hyperthermia. When the radiosensitizing abilities of etanidazole were tested in vitro, there was a radiation dose modifying factor of 2.40 at pH 7.40, but only 1.70 at pH 6.45. In vivo, etanidazole (1 g/kg) produced a radiation dose modifying factor of 1.47, whereas 43 degrees C for 30 min produced a radiation dose modifying factor of 1.38. The combination resulted in a radiation dose modifying factor of 2.29. When the cytotoxicities of hyperthermia (43 degrees C x 30 min), etanidazole (500 mg/kg or 1 mg/kg), and radiation (10 Gy) combinations were assayed by Hoechst 33342 dye selected tumor subpopulations, 43 degrees C x 30 min increased the killing of irradiated dim cells by approximately 9.2-fold but by only 2.9-fold in bright cells. Etanidazole (1 g/kg) increased radiation killing of bright cells by about 3-fold and dim cells by about 4.3-fold. The combination of hyperthermia and etanidazole increased the killing of both dim and bright cells exposed to radiation by approximately 10-fold versus 10 Gy alone.

Clinical trials with etanidazole (SR-2508) by the Radiation Therapy Oncology Group (RTOG)

Following the completion of a phase I study of etanidazole (SR 2508), a new hypoxic cell sensitizer, the RTOG, began a phase II/III trial. The objectives of the study were to determine the toxicity and efficacy of SR 2508, combined with conventional radiotherapy for the management of unresectable stage III and IV head and neck squamous carcinomas. During the first step (or the Phase II portion) of the study, 33 patients received radiotherapy plus SR 2508 (RT + SR 2508). The incidence of drug toxicities was modest; including 24% grade I peripheral neuropathy (PN), 6% grade II PN, 27% grade I or II nausea and vomiting, 9% allergy and 15% reversible neutropenia. Because observed toxicities were deemed acceptable, the second step (or phase III portion) was then activated. Patients were randomized to receive either RT or RT + SR 2508. As of November 20, 1989, a total of 242 patients have been entered onto the Phase III portion of the study. One hundred twenty-two patients were randomized to the RT + SR 2508 arm and 120 patients were randomized to the RT alone arm. The analyses presented in this report are based on data available. The incidence of drug toxicities has been low, with 18% grade I or II PN, 26% nausea and vomiting (including one grade III), 14% allergy (including one grade III) and 13% reversible neutropenia.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of 2-nitroimidazole hypoxic cell radiosensitizers in rodent peripheral nervous tissue

The concentrations of seven 2-nitroimidazoles--including misonidazole, etanidazole (SR-2508), pimonidazole (Ro 03-8799), desmethylmisonidazole (Ro 05-9963), RK28, RP170, and KU2285--were measured in the sciatic nerves of C3H/He mice using reverse-phase high-performance liquid chromatography. Drug exposure to the peripheral nervous system was highest for misonidazole, followed by desmethylmisonidazole, etanidazole, pimonidazole and RK28. The lower drug exposure of pimonidazole and RK28 seemed to be related to their lower hydrophilicity. The apparent biological half-lives of the compounds in the peripheral nerves were correlated to their hydrophilicity: the more hydrophilic the compound, the longer the apparent biological half-life in the peripheral nervous tissue of the mice. Measurement of drug exposure in the rodent peripheral nervous system, rather than in the brain, was a better indicator for estimating the occurrence of clinical peripheral neuropathy by 2-nitroimidazoles.

KIH-802, an acetohydroxamic acid derivative of 2-nitroimidazole, as a new potent hypoxic cell radiosensitizer: radiosensitizing activity, acute toxicity, and pharmacokinetics

The radiosensitizing activity, acute toxicity, and pharmacokinetics of a new hypoxic cell radiosensitizer, potassium 2-nitroimidazole-1-acetohydroxamate (KIH-802), were compared with those of misonidazole (MISO) and etanidazole (SR-2508). The radiosensitizing activity of KIH-802 was slightly higher than that of MISO and SR-2508 in vitro and was similar to or slightly higher than that of MISO or SR-2508 in vivo. The acute toxicity of KIH-802 was slightly higher than that of MISO. The concentrations of KIH-802 in the brains and peripheral nerves of mice were as low as those of SR-2508 and lower than those of MISO.

Final report of the phase I trial of the hypoxic cell radiosensitizer SR 2508 (etanidazole) Radiation Therapy Oncology Group 83-03

In a Phase I trial SR 2508 was administered by rapid intravenous infusion to 102 patients receiving radiation therapy. The dose-limiting toxicity was peripheral sensory neuropathy (PN) which was related to the cumulative dose administered. The highest single daily dose, 3.7 g/m2, was tolerated without toxicity. The lowest cumulative toxic dose was 21.6 g/m2, and the highest non-toxic dose was 40.8 g/m2. Grade 1 neuropathies were mild and self-limited; grade 2 neuropathies were long-lasting and debilitating. In a retrospective analysis, the risk of developing neurotoxicity was related to the cumulative drug exposure calculated by the area-under-the-curve (AUC) of plasma concentration versus time. There was an increased incidence of neuropathy in patients with a cumulative AUC of greater than or equal to 36 mM-hr. At a total dose of 34 g/m2 over 6 weeks, the incidence of Grade 1 neuropathy was approximately 30%; no grade 2 neuropathy occurred at this dose and schedule. Additional toxicities observed included nausea and vomiting (6%), skin rash (4%), and transient arthralgias (3%). One patient had transient abnormalities in liver function tests of unknown etiology. (In a more recent Phase II trial neutropenia has been observed which may be related to SR2508). Approximately three times more SR 2508 is tolerable compared to misonidazole, and it appears that severe neuropathy can be avoided by monitoring individual patient pharmacokinetic parameters. Evaluation of the efficacy of this hypoxic cell sensitizer is in progress.

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.

A new, potent 2-nitroimidazole nucleoside hypoxic cell radiosensitizer, RP170

The radiosensitizing activity, acute toxicity and pharmacokinetics of RP170, a new hypoxic cell radiosensitizer, were compared with those of misonidazole (MISO) and SR2508. RP170 belongs to the group of 2-nitroimidazole nucleosides, which are designed to be selectively excluded from the neural tissue. The reduction potential of RP170 was similar to that of MISO and SR2508. The partition coefficients in octanol/water of RP170, MISO, and SR2508 were 0.094, 0.35, and 0.021, respectively. The radiosensitizing activity of RP170 was similar to that of MISO and SR2508 in vitro and in vivo. There was no significant difference in the radiosensitizing activity of RP170 in vivo between intravenous and intraperitoneal administration. The acute toxicity of RP170 was the same as that of SR2508. Pharmacokinetic evaluation showed that the concentration of RP170 in the brain was as low as that of SR2508. RP170 is expected to have the same radiosensitizing effects as MISO and SR2508, and to be less neurotoxic than MISO.

Estimation of plasma area under the curve for etanidazole (SR 2508) in toxicity prediction and dose adjustment

The hydrophilic 2-nitroimidazole radiosensitizer etanidazole is currently undergoing clinical evaluation. Although considerably less neurotoxic than misonidazole because of its rapid renal clearance and partial exclusion from the nervous system, total dose is limited by peripheral neuropathy. Monitoring plasma etanidazole concentration in patients to determine the area under the curve (AUC0-infinity) has been proposed as a method of predicting patients at risk, and of providing a quantitative basis for dose reduction in such patients. Successful application of this policy requires accurate assessment of AUC0-infinity. We have analyzed plasma data for 18 patients receiving 2 g/m2 etanidazole to determine the errors introduced in the estimation of AUC0-infinity caused by omitting selected time points from the analysis. A 'baseline' AUC0-infinity value was calculated by integration of the rate equation for the 2-compartment model using data points at 0, 15, and 30 min and 1, 2, 4, 8, 12, and 24 hr after the end of infusion. The mean +/- SD area for AUC0-infinity was 502 +/- 152 micrograms ml-1 h (2.35 +/- 0.71 mM.h). Omitting the zero or the 24 hr time point, the average errors were quite small (2.5% in both cases), but errors of up to 16.4 and 7.3%, respectively, were seen for individual patients. Leaving out both the 8 hr and 12 hr points at the same time gave a similar low average error of 2.9%, with a highest error of 7.3%. Omitting all data points after 4 hr, the mean error was 24.7% and 15 of 18 patients had errors in excess of 10%. In addition, failure to correct for infusion time results in an underestimation of AUC0-infinity averaging 4.5% (range 1.9-8.7%). The choice of sampling times for toxicological monitoring will depend upon the accuracy with which the AUC0-infinity must be known. Including all data points between 0 and 24 hr will minimize errors. Considering the general similarity in the errors introduced by omitting the 8 hr and 12 hr points together compared to those seen with exclusion of the single 24 hr point, the choice between these truncated sampling options would be expected to lie in the relative inconvenience caused to patients and medical staff for the particular dose schedule used. The short sampling schedule (0-4 hr) should not be used.

Micropharmacokinetics of chemical modifiers

Classical pharmacokinetic analysis of plasma, urine and tissue specimens continues to be of major value to the rational development of chemical modifiers of cancer treatment. However, in addition, increasingly sophisticated analytical techniques are becoming available which allow the pathways of microdistribution and micrometabolism of drugs to be traced down to the cellular and molecular level. New developments described here include flow cytometry, magnetic resonance spectroscopy, and molecular enzymology. These are predicted to have a major impact on the optimization of chemical modification.

Ro 03-8799: preferential relative uptake in human tumor xenografts compared to a murine tumor: comparison with SR-2508

The pharmacokinetics of Ro 03-8799 and SR-2508 were studied in 6 tumor cell lines (5 human, 1 murine) transplanted into athymic nude mice. The human tumors were rectocolic adenocarcinomas (HRT18, HT29) and melanomas (Be11, Na11+, Na11-), the rodent tumor was a mammary tumor (EMT6). The concentrations of drugs in tumor, blood, plasma, and red cells were measured by HPLC 15, 30, and 45 minutes after the simultaneous i.v. injection of 0.1 mg/g of each compound. Little or no difference was found between the concentrations in the plasma, blood, and red cells for either drug; SR-2508 concentration was higher than that of Ro 03-8799. Both compounds were concentrated in all the tumors, the concentration increases or decreases as a function of the time, depending on the cell line; there was more Ro 03-8799 in the pigmented melanomas than in the other tumors. The relative uptake of Ro 03-8799 and SR-2508 in the tumor was evaluated by the ratio of drug concentrations in the tumor and in the blood. The results suggest that Ro 03-8799 and SR-2508 accumulated in all the tumors, the relative uptake of Ro 03-8799 was higher than that of SR-2508. For human tumors, the ratios increased between 15 and 45 minutes. They ranged from 1.2 to 3.6 for SR-2508 and from 3.1 to 14.6 for Ro 03-8799. For Ro 03-8799, the highest ratios were found for the melanomas; the uptake was higher in the pigmented Na11+ tumors than in the non-pigmented Na11-. The ratios for EMT6 were about 2.3 for SR-2508 and 4.7 for Ro 03-8799; these ratios varied slightly between 15 and 45 minutes.

Initial results of a phase I trial of continuous infusion SR 2508 (etanidazole): a radiation therapy oncology group study

To exploit both the oxygen-mimetic and "pre-incubation" or continuous exposure effects of the 2-nitroimidazole radiosensitizers, we are conducting a Phase I trial of continuous infusion SR 2508 for patients receiving brachytherapy. Following the administration of a loading dose of 2 g/m2, SR 2508 is administered by continuous infusion for 48 hr. Twenty-one patients have completed treatment. The initial total dose was 8 g/m2 with patients currently receiving 15 g/m2. No toxicity has been observed. At the higher doses the steady-state plasma concentrations have been between 50 and 70 micrograms/ml. It is not yet known whether or not hypoxic sensitizers will be of benefit clinically, and if so, when during a course of treatment is the optimal time to use them. Given the lack of toxicity and plasma concentrations achievable with continuous infusion, future studies will be conducted using SR 2508 during both the external beam and brachytherapy aspects of treatment.

Induction of tumour hypoxia post-irradiation: a method for increasing the sensitizing efficiency of misonidazole and RSU 1069 in vivo

It is known that hydralazine can decrease blood flow to experimental murine tumours. A consequence of this, in the KHT sarcoma, is the induction of close to 100 per cent radiobiological hypoxia, which lasts for nearly 2 h following i.v. injection of 5 mg/kg hydralazine to the mouse. This phenomenon is exploitable in order to increase the apparent sensitizing efficiency of the nitroheterocyclic radiosensitizers, misonidazole and RSU 1069, and is demonstrated using the treatment schedule: sensitizer----60 min----X-rays----1 min----hydralazine. Such a strategy will first take advantage of the radiosensitizing properties of the nitroimidazole, then after irradiation the hydralazine should allow expression of the differential toxicity towards hypoxic cells known to occur with misonidazole and RSU 1069. Misonidazole gives an enhancement ratio (ER) of 1.3 at 100 mg/kg, rising to 2.0 at 1000 mg/kg. Where hydralazine is given after irradiation, no additional cell kill is observed with 1000 mg/kg. In contrast, at lower doses of misonidazole, hydralazine induces a substantial increase in cell killing such that the ER obtained with 100 mg/kg is the same as that achieved with 1000 mg/kg misonidazole when used alone with radiation. Similarly, 20 mg/kg RSU 1069 with radiation followed by hydralazine is equivalent to the radiosensitizing effect of 80 mg/kg RSU 1069 without hydralazine. In addition, doses of RSU 1069 that normally give no radiosensitization (5 or 10 mg/kg) produce substantial increases in cell killing when combined with hydralazine.

Additivity of radiosensitization by the combination of SR 2508 (etanidazole) and Ro 03-8799 (pimonidazole) in a murine tumor system

The nitroimidazole radiosensitizers SR 2508 and Ro 03-8799 have different dose-limiting toxicities in man and hence can be used in combination. Such therapy will be beneficial only if their radiosensitizing properties are additive, which this study sought to determine using clinically relevant radiosensitizer concentrations in the EMT6 tumor in the flanks of BALB/c mice. 240 mg/kg of each drug gave tumor concentrations (+/- 2 se) 55 min after i.v. administration of the combination of 50.4 +/- 10.6 micrograms/g (236 +/- 50 nmol/g) for SR 2508 and 39.7 +/- 9.0 micrograms/g (137 +/- 31 nmol/g) for Ro 03-8799. The radiosensitization by both agents administered both singly and in combination at 240 mg/kg and singly at 480 mg/kg was measured, giving sensitizers 30 min before 20 Gy of 250 kV X rays. Tumor response was assayed by clonogenic cell survival. SER values (with 95% confidence limits) were 1.28 (1.20-1.37) for 240 mg/kg SR 2508, 1.20 (1.10-1.30) for 240 mg/kg Ro 03-8799, 1.46 (1.33-1.59) for 240 mg/kg of both drugs in combination, 1.46 (1.38-1.55) for 480 mg/kg SR 2508 and 1.46 (1.31-1.62) for 480 mg/kg Ro 03-8799. These data confirm the additivity of radiosensitization by the two drugs administered in combination.

Failure of misonidazole-sensitized radiotherapy to impact upon outcome among stage III-IV squamous cancers of the head and neck

As part of the RTOG research effort in the treatment of advanced, inoperable squamous cancer of the head and neck region, the hypoxic cell sensitizer, misonidazole, was selected for investigation as an adjuvant to definitive irradiation. Based upon a pilot experience (78-02) showing a 67% complete response rate among 36 AJC Stage III-IV patients receiving full-dose irradiation and 6 weekly p.o. doses of misonidazole, a phase III trial was carried out from '79-'83. Three hundred and six patients were entered, 42% of whom had oropharyngeal primaries and with 78% of all cases representing T3 or T4 (inoperable) lesions. Only 16% of the entire series presented with N0 necks. Fractionation was altered among the misonidazole-receiving patients, in contrast to "standard" 5 treatments per week among "control" patients, such that 2 separate treatments were given on each day of p.o. misonidazole administration (2.0 gm/m2/wk X 6 doses, 2.5 Gy in a.m., 2.1 Gy in p.m.). Total tumor doses were identical among the two treatment arms except that a limitation of 40.0 Gy to spinal cord was specified for sensitized radiotherapy vs. 45.0 Gy for "control" patients. Primary tumor clearance was observed to be 55-60%, with minor variations according to tumor stage and site. The local regional control rate among radiotherapy-alone patients was 26% at 2 years compared to 22% (2 years) within the misonidazole-receiving group. Analysis of survival revealed no advantage to the sensitized patients, with 55 +/- 2% surviving 1 year and 22 +/- 1% living 3 years following treatment in both treatment categories. Distant metastases as first site of failure (12-13%) and the local failure among initial complete responders (46%) showed no advantage to the misonidazole group. Although a misonidazole dosage of 2.0 gm/m2/wk X 6 (12 gm/m2 total) is well tolerated, no clinical benefit was demonstrated in this randomized trial. Other nitroimidazole analogs (e.g. SR-2508) are now being investigated.

Induction of DNA strand breaks by reduced nitroimidazoles. Implications for DNA base damage

Radiation-reduced 2-nitroimidazoles (misonidazole, RSU-1137, Ro.03-8799 and Ro.03-8800) incubated in air with plasmid DNA (pH 7.0, 310K) induce DNA strand breakage, as revealed following subsequent heat or alkali treatment. Only RSU-1137 resulted in the binding of a [2-14C] fragment and significant yields of heat-labile strand breaks (greater than 20% loss of type-I DNA after 48 hr incubation). RSU-1137 was shown to be greater than 6 times more effective than misonidazole at producing alkali-labile breaks. In fact, the efficiency of alkali-induced strand break production is in the order: misonidazole less than Ro.03-8799 approximately Ro.03-8800 less than RSU-1137. Reaction of these reduced 2-nitroimidazoles with 2'-deoxyguanosine (dG) also results in the formation of a common glyoxal-dG product, with its yield and rate of production being dependent upon the 2-nitroimidazole used. It has been demonstrated that these variations are influenced by the N-1 side-chain of the 2-nitroimidazole. Product yields are approximately 5-6 times greater with misonidazole than with RSU-1137. From the evidence presented, it is apparent that formation of glyoxal (or a glyoxal-like product) is not responsible for the DNA strand breakage seen. It is inferred that these breaks are induced by a nitro-reduction product(s) which remains unidentified.

The radiosensitizing effects of misonidazole (MISO) in combination with diethyl maleate (DEM) in mouse mammary tumors

Large radiosensitization of C3H/He mouse mammary tumors was obtained with the combination of a non-protein sulfhydryl (NPSH) depletor, diethyl maleate (DEM), and misonidazole (MISO), compared with MISO alone over a range of MISO dose. The difference in enhancement ratios (ER's) for these two treatments was especially prominent at small MISO doses. ER's of 2.06 and 1.44 were obtained, respectively, by combined treatment with DEM (760 mg/kg) and MISO (100 mg/kg) or treatment with MISO alone. Radiosensitization of tumors by DEM alone was observed for doses over 600 mg/kg. When DEM was combined with MISO (100 mg/kg), ER's of the combination were larger than that of MISO alone, for doses over 400 mg/kg of DEM. Similarly, in case of DEM plus MISO (300 mg/kg), the ER's became larger than MISO alone, for doses over 200 mg/kg of DEM. The NPSH content in untreated tumors was 1.08 mmole/kg on the average and no changes in NPSH content was observed after MISO treatment. DEM treatment markedly reduced the NPSH content of tumors as a function of DEM dose and this decrease in NPSH was not significantly affected by MISO treatment. Tumor NPSH was reduced to 24% or less of control by administration of 760 mg/kg of DEM with or without MISO. These results are consistent with competition theory of NPSH and electron affinic radiosensitizers.

Sensitization by SR-2508 plus Ro 03-8799

The primary toxicity of Ro 03-8799 is a central nervous system toxicity, whereas that of SR-2508 is a peripheral neuropathy. The feasibility of reducing overall toxicity while maintaining maximal radiosensitization by using the two sensitizers together was tested. The LD50/2 of Ro 03-8799 was 0.68 mg/g body wt (mg/gbw) after intravenous (i.v.) administration, and that of SR-2508 was 4.4 mg/gbw after i.v. administration. When both drugs were given together in equitoxic proportions, the LD50/2 was 0.45 mg of Ro 03-8799 plus 2.9 mg of SR-2508/gbw. These doses are 66% of the respective LD50/2 values of the drugs when given separately. Radiosensitization was evaluated using in vivo-in vitro assays with EMT6/SF tumors in BALB/c mice. At drug doses between 10 and 60% of the LD50/2, sensitization was generally maximal and similar to that from misonidazole, but there was less sensitization below this dose, both with the drugs given separately and together. If chronic toxicities of these drugs overlap as do the acute toxicities there will be little or no additional benefit from using these drugs in combination, compared to using them separately.