Phase I trial of the hypoxic cell radiosensitizer SR-2508: the results of the five to six week drug schedule

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

Improved radiosensitizing effect of the combination of etanidazole and paclitaxel for hepatocellular carcinoma in vivo

Hepatocellular carcinoma (HCC) is one of the most critical global health issues. Potential curative therapies, including surgical resection, are offered to only a limited number of patients. Therefore, new and effective treatment strategies are required. Recently, radiotherapy with hypoxic radiosensitizers has shown promise in cancer therapy. Our previous study demonstrated that radiosensitization produced by etanidazole and paclitaxel was additive in vitro. This study was carried out to determine the synergistic effect of the two drugs in murine HCC H22 cell xenograft-bearing BALB/c mice in vivo. The morphology of the transplanted tumors was observed. The drug content in the blood and tumors of mice was measured by high-performance liquid chromatography. The radiosensitizing effect on H22 cell xenograft-bearing mice was evaluated in terms of tumor growth inhibition and survival. Expression of hypoxia inducible factor-1α (HIF-1α) was studied using immunohistochemistry. The morphological consequences on the H22 xenografts were consistent with the pathological characteristics of HCC. There was no significant difference in drug content in the blood and tumors between single drug and combination administration. The combination of the two drugs improved the radiosensitizing effect in vivo compared to single drug administration in an animal model. The changes in HIF-1α expression indirectly verified the above-mentioned results. This study may provide a new combination of radiosensitizers for HCC radiotherapy.

New developments in radiation therapy for head and neck cancer: intensity-modulated radiation therapy and hypoxia targeting

Intensity-modulated radiation therapy (IMRT) has revolutionized radiation treatment for head and neck cancers (HNCs). When compared to the traditional techniques, IMRT has the unique ability to minimize the dose delivered to normal tissues without compromising tumor coverage. As a result, side effects from high-dose radiation have decreased and patient quality of life has improved. In addition to toxicity reduction, excellent clinical outcomes have been reported for IMRT. The first part of this review will focus on clinical results of IMRT for HNC. Tumor hypoxia, or the condition of low oxygen, is a key factor for tumor progression and treatment resistance. Hypoxia develops in solid tumors due to aberrant blood vessel formation, fluctuation in blood flow, and increasing oxygen demands for tumor growth. Because hypoxic tumor cells are more resistant to ionizing radiation, hypoxia has been a focus of clinical research in radiation therapy for half a decade. Interest for targeting tumor hypoxia has waxed and waned as promising treatments emerged from the laboratory, only to fail in the clinics. However, with the development of new technologies, the prospect of targeting tumor hypoxia is more tangible. The second half of the review will focus on approaches for assessing tumor hypoxia and on the strategies for targeting this important microenvironmental factor in HNC.

New developments in radiation therapy for head and neck cancer: intensity-modulated radiation therapy and hypoxia targeting

Intensity-modulated radiation therapy (IMRT) has revolutionized radiation treatment for head and neck cancers (HNCs). When compared to the traditional techniques, IMRT has the unique ability to minimize the dose delivered to normal tissues without compromising tumor coverage. As a result, side effects from high-dose radiation have decreased and patient quality of life has improved. In addition to toxicity reduction, excellent clinical outcomes have been reported for IMRT. The first part of this review will focus on clinical results of IMRT for HNC. Tumor hypoxia, or the condition of low oxygen, is a key factor for tumor progression and treatment resistance. Hypoxia develops in solid tumors due to aberrant blood vessel formation, fluctuation in blood flow, and increasing oxygen demands for tumor growth. Because hypoxic tumor cells are more resistant to ionizing radiation, hypoxia has been a focus of clinical research in radiation therapy for half a decade. Interest for targeting tumor hypoxia has waxed and waned as promising treatments emerged from the laboratory, only to fail in the clinics. However, with the development of new technologies, the prospect of targeting tumor hypoxia is more tangible. The second half of the review will focus on approaches for assessing tumor hypoxia and on the strategies for targeting this important microenvironmental factor in HNC.

Radiosensitization by the combination of SR-2508 and paclitaxel in hypoxic human tumor cells in vitro

The two radiosensitizers SR-2508 (etanidazole) and paclitaxel (taxol) have different dose-limiting toxicities in humans. Combination of the two radiosensitizers may increase radiosensitization without increasing toxicity. This study was carried out to determine the synergistic radiosensitizing effect of combination of SR-2508 and paclitaxel in two hypoxic human tumor cell lines: a breast carcinoma (MCF-7) and a carcinoma cervicis (HeLa). The 3-(4,5 dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) assay was used to determine the number of surviving cells. Cell cycle was evaluated by flow cytometry. Cell viability was measured by the ability of single cells to form colonies in vitro. Our data demonstrated that the radiosensitization produced by the two radiosensitizers was additive in hypoxic HeLa cells while held in the G(1) phase of the cell cycle. On the other hand, there was no synergistic radiosensitizing effect in hypoxic MCF-7 cells by combination of the two drugs. Our results suggested that the synergistic radiosensitizing effect of SR-2508 and paclitaxel may be tumor-dependent and that breast cancer may not be a good candidate. This study may provide a new combination of radiosensitizers in radiotherapy for cervical carcinoma.

A phase I trial of etanidazole and hyperfractionated radiotherapy in children with diffuse brainstem glioma

PURPOSE

To determine the toxicity and maximum tolerated dose of etanidazole administered concurrently with hyperfractionated radiation therapy (HRT) for children with brainstem glioma.

METHODS AND MATERIALS

Eighteen patients with brainstem glioma were treated with etanidazole and HRT on a dose escalation protocol (Phase I trial) between 1990 and 1996. All patients had MRI confirmation of diffuse pontine glioma and signs/symptoms of cranial nerve deficit, ataxia, or long tract signs of <6 months' duration. Cervicomedullary tumors were excluded. Patients (median age: 8.5 years; 11 males, 7 females) received HRT to the tumor volume plus a 2-cm margin with parallel-opposed 6-15-MV photons. The total dose was 66 Gy in 44 fractions (1.5 Gy b.i.d., with at least 6 h between fractions) for the first 3 patients and 63 Gy in 42 fractions for the subsequent 15 patients. Etanidazole was administered as a rapid i.v. infusion 30 min before the morning fraction of HRT. Planned doses of etanidazole were 1.8 g/m(2) x 17 doses (30.6 g/m(2)) at Step 1 to a maximum of 2.4 g/m(2) x 21 doses (50.4 g/m(2)) at Step 8. Dose escalation was planned with 3 patients at each of the 8 levels.

RESULTS

Three patients were treated at each dose level except Level 2, on which only 1 patient was treated. The highest dose level achieved was Level 7, which delivered a total etanidazole dose of 46.2 g/m(2). Two patients were treated at this level, and both patients experienced Grade 3 toxicity in the form of a diffuse cutaneous rash. Three patients received a lower dose of 42 g/m(2) (dose Level 6) without significant toxicity, and this represents the maximum tolerated dose (MTD). There were 23 cases of Grade 1 toxicity (10 vomiting, 5 peripheral neuropathy, 2 rash, 2 constipation, 1 weight loss, 3 others), 11 cases of Grade 2 toxicity (4 vomiting, 2 skin erythema, 2 constipation, 1 arthralgia, 1 urinary retention, 1 hematologic), and 4 Grade 3 toxicities (2 rash, 1 vomiting, 1 skin desquamation). Grade 2 or 3 peripheral neuropathy was not seen at any dose level. The median survival from the start of treatment was 8.5 months (range: 3-58 months).

CONCLUSION

The MTD of etanidazole in children receiving HRT for brainstem glioma is 42 g/m(2), with cutaneous rash as the dose-limiting toxicity. This is in contrast to the adult experience, which demonstrates a 24% lower MTD of 34 g/m(2) limited by peripheral neuropathy.

Concurrent chemoradiation in the treatment of head and neck cancer

The concurrent use of chemotherapy with radiation in the management of head and neck cancer has been shown in several randomized studies and two recent meta-analyses to result in statistically and clinically significant gains in locoregional control and overall survival. This article highlights and summarizes the results of selected randomized studies. Concurrent single-agent chemotherapy trials, hypoxic cell sensitizer trials, and multiagent chemotherapy trials are presented. Trials employing concurrent chemoradiation in the postoperative setting, trials employing concurrent chemoradiation with an altered fractionation scheme, and promising results using concurrent chemoradiation in the previously radiated patient are presented. Current ongoing trials and future directions in concurrent chemoradiation are discussed.

Measuring hypoxia and predicting tumor radioresistance with nuclear medicine assays

Tumor cells at low oxygen tension are relatively radioresistant. The hypoxic fraction of individual tumors before, during and after radiotherapy is likely to have prognostic value but its diagnosis still awaits an accurate and acceptable assay. The recent indications that hypoxia can also induce the expression of specific genes and promote a more aggressive tumor phenotype makes its diagnosis even more important. Over 15 years ago, misonidazole, an azomycin-based hypoxic cell radiosensitizer, was found to link covalently to cellular molecules at rates inversely proportional to intracellular oxygen concentration. The use of bioreducible markers to positively label zones of viable hypoxic cells within solid tumors and to predict for tumor radioresistance was proposed. Several hypoxic markers have now been identified and their selective binding within tumors has been measured by both invasive and non-invasive assays. Research from our laboratory has emphasized both mechanistic and preclinical studies associated with nuclear medicine procedures for measuring tumor hypoxia and predicting tumor radioresistance. This report updates radiation oncologists about the status of nuclear medicine hypoxic marker research and development as of mid-1997. While several potential imaging agents have been identified, their testing and validation in appropriate human tumors will require focused research efforts by individual academic departments and, possibly, by clinical trials performed through cooperative groups. Since the prediction of hypoxia in individual tumors could strongly impact radiotherapy treatment planning, the radiation oncology research community is best positioned to execute the validation studies associated with these markers.

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.

Results of an RTOG phase III trial (RTOG 85-27) comparing radiotherapy plus etanidazole with radiotherapy alone for locally advanced head and neck carcinomas

PURPOSE

The objectives of this study were to determine the efficacy and toxicity of Etanidazole (ETA), a hypoxic cell sensitizer, when combined with conventional radiotherapy (RT) in the management of advanced head and neck carcinomas.

METHODS AND MATERIALS

From March 1988 to September 1991, 521 patients who had Stage III or IV head and neck carcinomas were randomized to receive conventional RT alone (66 Gy in 33 fractions to 74 Gy in 37 fractions, 5 fractions per week) or RT+ETA (2.0 g/m2 thrice weekly for 17 doses), of whom 504 were eligible and analyzable. Treatment assignments were stratified before randomization according to the primary site (oral cavity + hypopharynx vs. supraglottic larynx + oropharynx + nasopharynx), T-stage (T1-3 vs. T4), and N-stage (N0-2 vs. N3). Pretreatment characteristics were balanced. In the RT-alone arm, 39% of patients had T3 and 34% had T4 disease, whereas in the RT+ETA arm, 42% of patients had T3 and 33% had T4 disease. Thirty-eight percent of the RT-alone patients and 37% of the RT+ETA patients had N3 disease. The median follow-up of surviving patients was 3.38 years, with a range between 0.96 and 5.63 years.

RESULTS

One hundred and ninety-four of the 252 (77%) RT+ETA patients received at least 14 doses of the drug. Overall RT protocol compliance rate was 82% in the RT-alone arm and 86% in the RT+ETA arm. No Grade 3 or 4 central nervous system or peripheral neuropathy was observed in the RT+ETA arm. Eighteen percent of the patients developed Grade 1 and 5% developed Grade 2 peripheral neuropathy. Other drug related toxicities included nausea/vomiting (27%), low blood counts (15%), and allergy (9%). Most of these toxicities were Grade 1 and 2. The incidence of severe acute and late radiation effects were similar between the two arms. The 2-year actuarial local-regional control rate (LCR) was 40% for the RT-alone arm and 40% for the RT+ETA arm. Two-year actuarial survival was 41% for the RT-alone arm and 43% for the RT+ETA arm (p = 0.65). Multivariate analyses were performed to investigate the influence of covariates on treatment effects. A strong treatment interaction with N-stage was revealed: LCR (50% vs. 40% at 2 years), RT+ETA improved for patients with N0-2 disease but not for N3 patients (22% for RT+ETA and 40% for RT). Further analyses showed that RT+ETA was more advantageous in N0-1 patients, with a 2-year LCR of 55% for RT+ETA vs. 37% for RT only (p = 0.03). A similar phenomenon was observed when using survival as the end point.

CONCLUSION

The results showed that adding Etanidazole to conventional RT produced no global benefit for patients who had advanced head and neck carcinomas. There was a suggested benefit for patients who had N0-1 disease, and that needs to be confirmed by another study.

Phase II trial of external beam radiation with etanidazole (SR 2508) for the treatment of locally advanced prostate cancer

PURPOSE

To evaluate the efficacy and toxicity of the addition of etanidazole (ETA) to external beam radiation.

METHODS AND MATERIALS

Fifty eight previously untreated patients with locally advanced adenocarcinoma of the prostate were entered on a Phase II trial of etanidazole (ETA) combined with standard external beam radiation therapy. ETA was given concurrently with irradiation. Four patients received less than 25% of the intended dose of ETA and were ineligible for further analysis. The stage of the remaining patients were T2c-11, T3-39, T4-1, bulky local recurrence after prostatectomy-1, and T3, N1-2.

RESULTS

Forty-five of 54 patients (83.3%) achieved a clinical complete response (CCR) in the prostate and seminal vesicles as judged by digital rectal exam (DRE). Responses were rapid with a median time to CCR of 3.4 months, range 0-22.8 months. Local control was maintained in 82% of the patients who achieved a CCR. Fifteen of 32 eligible patients with a normal DRE underwent prostate biopsies from 12-20 months after treatment, seven had negative biopsies (46.6%). Distant metastases occurred in 18 patients (33.3%). Pretreatment prostatic specific antigen (PSA), Gleason score, and stage were not associated with treatment outcome in a univariate analysis.

CONCLUSION

While ETA plus radiation was associated with a rapid CCR, the overall treatment outcome of these patients appeared to be similar to published reports of patients receiving RT alone. The rapid response rate may imply biologic activity of the ETA. In future trials, it may be reasonable to focus on patients at lower risk for the subsequent development of distant disease.

Radiosensitizing activity and pharmacokinetics of multiple dose administered KU-2285 in peripheral nerve tissue in mice

PURPOSE

In a clinical trial in which a 2-nitroimidazole radiosensitizer was administered repeatedly, the dose-limiting toxicity was found to be peripheral neuropathy. In the present study, the in vivo radiosensitizing activity of KU-2285 in combination with radiation dose fractionation, and the pharmacokinetics of cumulative dosing of KU-2285 in the peripheral nerves were examined.

METHODS AND MATERIALS

The ability of three nitroimidazoles, misonidazole (MISO), etanidazole (SR-2508) and KU-2285, to sensitize SCCVII tumors to radiation treatment has been compared for drug doses in the range 0-200 mg/kg. Single radiation doses or two different fractionation schedules (6 Gy/fractions x three fractions/48 h or 5 Gy/fractions x five fractions/48 h) were used; the tumor cell survival was determined using an in vivo/in vitro colony assay. The pharmacokinetics in the sciatic nerves were undertaken, when KU-2285 or etanidazole were injected at a dose of 200 mg/kg intravenously one, two, three or four times at 2-h intervals.

RESULTS

At less than 100 mg/kg, KU-2285 sensitized SCCVII tumors more than MISO and SR-2508 by fractionated irradiation. Evaluation of pharmacokinetics in the peripheral nerves showed that the apparent biological half-life of SR-2508 increased with the increases in the number of administrations, whereas that of KU-2285 became shorter.

CONCLUSION

Since most clinical radiotherapy is given in small multiple fractions, KU-2285 appears to be a hypoxic cell radiosensitizer that could be useful in such regimens, and that poses no risk of chronic peripheral neurotoxicity.

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.

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.

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.

4-Fluorobenzylamine and phenylalanine methyl ester conjugates of 2-nitroimidazole: evaluation as hypoxic cell radiosensitizers

We have synthesized two 2-nitroimidazole derivatives and evaluated their hypoxic radiosensitization properties. The first, a 4-fluorobenzylamine conjugate of 2-nitroimidazole (PK-110), was designed so that it could also be labeled with the F-18 and used for positron emission tomographic imaging of hypoxia. The second, the L-phenylalanine methyl ester conjugate of 2-nitroimidazole (PK-130), was designed in an attempt to exploit amino acid transport channels to enhance drug transport into the tumor. The effects of these drugs (and SR-2508, for comparison) in vitro on the aerobic and hypoxic radiosensitivity of Chinese hamster V79 cells were evaluated using clonogenic assays. PK-130 and PK-110 at 0.1 and 1.0 mM were more efficient hypoxic cell radiosensitizers than obtained with 1.0 mM SR-2508. Marginal aerobic radiosensitization was observed for 1.0 mM treatment with PK-130 and PK-110, however, no aerobic radiosensitization was observed at 0.1 mM. Glutathione (GSH) depletion (less than 5% of control levels) by L-buthionine sulfoximine (BSO) further enhanced the SER for both PK-130 and PK-110 at 0.1 mM to 3.2 +/- 0.63 and 2.4 +/- 0.16, respectively. The results of this study encourage the in vivo tumor radiosensitization evaluation of PK-130 and PK-110.

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.

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.

Oral (po) dosing with RSU 1069 or RB 6145 maintains their potency as hypoxic cell radiosensitizers and cytotoxins but reduces systemic toxicity compared with parenteral (ip) administration in mice

RB 6145 is a pro-drug of the hypoxic cell radiosensitizer RSU 1069 with reduced systemic toxicity. The maximum tolerated dose (MTD) of RSU 1069 for C3H/He mice was 80 mg/kg (0.38 mmol/kg) ip but 320 mg/kg (1.5 mmol/kg) following po administration. The MTD values of RB 6145 were 350 mg/kg (0.94 mmol/kg) ip and 1 g/kg (2.67 mmol/kg) po. Toxicity of RSU 1069 toward bone marrow stem cells was also less after po administration than after ip administration; 0.1 mmol/kg ip RSU 1069 and 0.38 mmol/kg po RSU 1069 both reduced the surviving fraction of clonogenic CFU-A cells by 50%. Oral administration of RSU 1069 resulted in lower spermatogenic toxicity. No loss of intestinal crypts was detected after ip or po administration of RSU 1069. Some nephrotoxicity was observed in half of the mice given the highest po dose of 1.5 mmol/kg of RSU 1069; this was not observed following the highest ip dose of drug. For RSU 1069 and RB 6145, administered by either route, the maximum hypoxic cell radiosensitization in murine KHT sarcomas, occurred when the drugs were given 45-60 min before 10 Gy of X rays. The degree of radiosensitization produced by a particular dose of either compound was largely independent of the route of administration. Preliminary pharmacokinetic studies, using 3H-RSU 1069, suggested that anti-tumor efficacy correlated with peak blood level of label and concentration in the tumor at the time of irradiation, which were not reduced by po compared with ip administration. Normal tissue toxicity tended to correlate with total exposure over time, which was reduced approximately two-fold by po administration. Oral administration of RSU 1069 or RB 6145, as well as being convenient, may give therapeutic benefit since dose-limiting toxicity in mice was reduced compared with parenteral administration, whereas radiosensitizing activity was less affected.

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.

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)

Abnormal clinical pharmacokinetics of the developmental radiosensitizers pimonidazole (Ro 03-8799) and etanidazole (SR 2508)

The hypoxic cell radiosensitizers Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole) are under evaluation as single agents (Phase III) and in combination (Phase I). Ro 03-8799 produces an acute, transient central nervous system syndrome, whereas SR 2508 causes cumulative, peripheral neurotoxicity; both effects are dose-limiting. Pharmacokinetic studies have shown the importance of area under the plasma drug concentration versus time curve (AUC) in predicting the risk of peripheral neuropathy. Most patients have very similar pharmacokinetic parameters. This study reports 2/25 patients receiving 0.75 g/m2 Ro 03-8799 plus 2.0 g/m2 SR 2508 who showed significant discrepancies in drug handling. One patient exhibited a markedly elevated AUC and prolonged t1/2 beta for SR 2508 and this was associated with an unusually rapid onset of peripheral neuropathy. A second patient showed normal handling of SR 2508 but prolonged values for both t1/2 alpha and t1/2 beta for Ro 03-8799 and unusually low levels of its N-oxide metabolite. In addition a low peak Ro 03-8799 concentration combined with a very high volume of distribution was found in this patient, leading to a normal AUC value and toxicity profile. Both patients exhibited a relatively low creatinine clearance. The mechanisms which may underlie these findings are discussed, and the importance of pharmacokinetic monitoring in the use of these agents is emphasized.

Critical appraisal of experimental radiation modalities for malignant astrocytomas

The management of patients with supratentorial malignant astrocytomas has remained a major problem. Patients continue to die from a lack of local control in 90% of cases despite an improvement of median survival seen with the use of postoperative radiation therapy. Because of this, there has been considerable interest in exploring novel ways of possibly improving results. This paper reviews the rationale and clinical results with the use of altered fractionation schemes, brachytherapy, radiation sensitizers, hyperthermia, particle therapy, and radiosurgery in the treatment of these patients. Currently, there is no demonstrated advantage with the use of these experimental modalities in the initial management of patients. There would appear to be some benefit for selected patients who are treated with brachytherapy at recurrence, but its efficacy as part of initial management remains to be determined determined in ongoing randomized prospective trials.

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.

Radiosensitization by a new nucleoside analogue: 1-[2-hydroxy-1-(hydroxymethyl)ethoxy]methyl-2-nitroimidazole (RP-170)

A new potent hypoxic cell sensitizer, a 2-nitroimidazole nucleoside analogue having methoxyglycerol as a sugar moiety at the N-1 position of the imidazole ring (RP-170), has been synthesized. Its radiosensitizing activities in vitro and in vivo were investigated and compared with those of misonidazole (MISO) and etanidazole (SR-2508). As might be expected from the almost identical electron affinities of the three compounds, they were equally effective against hypoxic EMT6 cells in vitro. The in vivo-in vitro excision analysis showed that RP-170 was also as effective as MISO and etanidazole to radiosensitize solid tumor cells in vivo. An intraperitoneal administration of 200 mg/kg of RP-170 and an intravenous administration of the same dose of etanidazole showed an equal sensitizer-enhancement ratio of 1.51 to solid EMT6/KU tumors. Its effectiveness was also demonstrated by growth delay assay using solid SCCVII tumors. As predicted from the low partition coefficient, RP-170 and etanidazole showed apparently lower toxicity in vivo than MISO; their LD50/14 were 4.3, 4.8, and 1.8 g/kg in our experiment, respectively. Moreover, RP-170 showed fast clearance from serum in mice (t1/2 = 10.24 min) and poor penetration into neural tissues. Although RP-170 does not show any advantages over etanidazole in terms of sensitization or toxicity, RP-170 might be preferable under certain circumstances because it can be given orally.

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.

Keynote address: cellular reduction of nitroimidazole drugs: potential for selective chemotherapy and diagnosis of hypoxic cells

Nitroimidazole drugs were initially developed as selective radiosensitizers of hypoxic cells and, consequently, as adjuvants to improve the local control probabilities of current radiotherapies. Misonidazole (MISO), the prototype radiosensitizing drug, was found in Phase I clinical studies to cause dose-limiting neurotoxicities (mainly peripheral neuropathies). MISO was also found to be cytotoxic in the absence of radiation and to covalently bind to cellular molecules, both processes demonstrating rates much higher in hypoxic compared with oxygenated cells. It is likely that neurotoxicity, cellular cytotoxicity and adduct formation results from reactions between reduction intermediates of MISO and cellular target molecules. Spin-offs from radiosensitizer research include the synthesis and characterization of more potent hypoxic cytotoxins and the exploitation of sensitizer-adducts as probes for measuring cellular and tissue oxygen levels. Current developments in hypoxic cell cytotoxin and hypoxic cell marker research are reviewed with specific examples from studies which characterize the cellular reduction of TF-MISO, (1-(2-nitro-1-imidazolyl)-3[2,2,2-trifluoroethoxy]-2-propanol).

Characteristics of fluorinated nitroazoles as hypoxic cell radiosensitizers

Types of 2-nitroimidazoles and 3-nitro-1,2,4-triazoles bearing one or two fluorine atoms on their side chains were synthesized to evaluate their physicochemical properties, radiosensitizing effects, and toxicity. The reduction potential of the compounds containing one fluorine was similar to that of misonidazole (MISO), whereas that of the difluorinated compounds was slightly higher. Both mono- and difluorinated compounds had an in vitro sensitizing activity comparable to or slightly higher than that of MISO. The fluorinated 3-nitrotriazoles were almost as efficient as the 2-nitroimidazoles with the same substituent. In vivo, some of the compounds were up to twice more efficient than MISO, whereas others were as efficient as MISO. Toxicity in terms of LD50/7 in mice was quite variable depending on the side-chain structure; the amide derivatives were less toxic than MISO, whereas the alcohol and ether derivatives were more toxic. In view of the radiosensitizing effect and toxicity in vivo, at least one compound, KU-2285 (a 2-nitroimidazole with an N1-substituent of: CH2CF2CONHCH2CH2OH) has been found to be as useful a hypoxic cell sensitizer as SR-2508.

Pharmacokinetics of etanidazole (SR-2508) in bladder and cervical cancer: evidence of diffusion from urine

Following an IV infusion of 2.0 g/m2 of Etanidazole, the mean tumor concentration 40 min after injection was 126 micrograms/g in bladder cancer and 65 micrograms/g in cervical cancer. The tumor/plasma concentration ratio was 1.88 in bladder and 0.85 in cervical cancer. This high tumor concentration in bladder cancer could be accounted for by diffusion from a highly concentrated urine. This renders bladder cancer a suitable clinical model for testing this sensitizer.

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.

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

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

The multi-dose clinical tolerance and pharmacokinetics of the combined radiosensitizers, Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole)

The hypoxic cell radiosensitizers Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole) have differing physico-chemical properties and clinical toxicities. The former is basic, lipophilic and produces an acute but transient central nervous system syndrome; the latter is neutral, hydrophilic and causes cumulative peripheral neuropathy. We therefore investigated the possibility of combining these agents to achieve additive radiosensitization with no enhancement of toxicity, as demonstrated in a rodent tumor model. Following a single dose study which showed a lack of interaction with respect to both toxicity and pharmacokinetics, twenty-one patients have now completed simultaneous drug administration on an escalating, multiple dose schedule. There has been no adverse acute interaction up to 0.75 g/m2 Ro 03-8799 with 2 g/m2 SR 2508 for 15 doses. At this dose-level, however, all patients experienced peripheral neuropathy. There was no adverse pharmacokinetic interaction, or perturbation of plasma pharmacokinetics between initial and final infusions. Tumor concentrations were determined in 48 biopsy samples 0-60 min after administration. Mean values normalized to a dose of 0.75 g/m2 Ro 03-8799 plus 2 g/m2 SR 2508 were 33 micrograms/g Ro 03-8799 and 74 micrograms/g SR 2508. These would be expected to produce a single-dose sensitizer enhancement ratio of around 1.5. The combination is predicted to be around 6.8 times more active than misonidazole, and superior to any single agent tested to date. The current schedules are reaching the limits of clinical tolerance, and an attempt is now being made to define the optimal regimen for use in a randomized clinical trial of the combination.

Hypoxic cell radiosensitizers in the treatment of high grade gliomas: a new direction using combined Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole)

The hypoxic cell radiosensitizers Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole) have been evaluated for their simultaneous penetration into human brain tumors and surrounding normal tissue. Thirteen patients received a dose of 1 g of each agent, infused over a 10 minute period during neurosurgery. Samples of glioma (20), brain (10) and cerebrospinal fluid (1) were obtained at a mean time (+/- SD) of 31 +/- 18 min from the end of infusion. A 24 hr plasma time course was measured in six patients. Nitroimidazole concentrations were determined by HPLC. For a mean dose of 0.55 g/m2 of each agent, the mean tumor concentrations (+/- SD) were 17.0 +/- 12.0 micrograms/g for Ro 03-8799 and 13.5 +/- 10.9 micrograms/g for SR 2508. The tumor/plasma ratios were 279 +/- 230% and 47 +/- 34% respectively. For adjacent 'normal' brain tissue, the radiosensitizer concentrations were 29.9 +/- 13.1 micrograms/g for Ro 03-8799, and 4.0 +/- 1.7 micrograms/g for SR 2508, and the brain/plasma ratios were 430 +/- 29% and 14 +/- 8% respectively. There was a significant trend towards increasing accumulation of both agents with time, in both tumor and normal brain. Concentrations in cerebrospinal fluid were very low. Plasma pharmacokinetics for Ro 03-8799 were similar to previous experience, but for SR 2508 the terminal half-life was greater in this series by a factor of 1.3. The results confirm that Ro 03-8799 is distributed widely in the central nervous system, and demonstrate that SR 2508 can achieve high tumor concentrations when the blood-brain barrier is compromised. The concentrations achieved with the combination are indicative of a significant advantage over metronidazole, misonidazole, or either agent alone, and normalized to the therapeutic dose of 0.75 g/m2 plus 2.0 g/m2 SR 2508 are consistent with those giving additive sensitization in an in vivo mouse tumor model.

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.