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

Tirapazamine plus cisplatin and irradiation in a mouse model: improved tumor control at the cost of increased toxicity

PURPOSE

Tirapazamine (TPZ) reportedly enhances the tumor cell killing effect of cisplatin up to fivefold and it is an attractive drug for combination with radiotherapy. We evaluated the toxicity of a fractionated combined treatment.

METHODS

Murine RIF-1 fibrosarcomas growing on the right hind foot of C3-H mice were used. Within 2 weeks, animals were treated with six i.p. injections of TPZ (43.2-172.8 mg/kg total), and/or cisplatin (24 mg/kg total) and ten fractions of 2 Gy to the tumor. All treatments were carried out under anesthesia. Maximum follow-up was 35 days. The local tumor control was determined by calculating the tumor doubling time t (2vo). In addition to standard toxicity assessment, the major inner organs were examined histologically.

RESULTS

The administration of low TPZ doses to the cisplatin/radiotherapy treatment caused only little changes in tumor doubling time (t (2vo)) and led to a lethality rate of 15-30%. Higher TPZ doses caused an increase in t (2vo), but also a further increase in lethality and toxicity in particular to the heart, liver, kidney and stomach. Cisplatin/radiotherapy treatment without TPZ produced no severe toxicity.

CONCLUSIONS

This is a detailed study of both the acute and delayed toxicities of combined TPZ treatment in a mouse model. In our study the addition of TPZ to the cisplatin/radiotherapy treatment caused a significant increase in toxicity with only moderate effect on the tumor.

Tirapazamine causes vascular dysfunction in HCT-116 tumour xenografts

BACKGROUND AND PURPOSE

Tirapazamine is a hypoxic cytotoxin currently undergoing Phase II/III clinical evaluation in combination with radiation and chemotherapeutics for the treatment of non-hematological cancers. Tissue penetration studies using multicellular models have suggested that tirapazamine exposure may be limited to cells close to blood vessels. However, animal studies show tirapazamine enhances the anti-tumour activity of radiation and chemotherapy and clinical studies with tirapazamine, so far, are promising. To investigate this apparent paradox we examined the microregional effects of tirapazamine in vivo by mapping drug effects with respect to the position of blood vessels in tumour cryosections.

PATIENTS AND METHODS

Tirapazamine was administered i.p. to mice bearing HCT-116 tumours, which were excised at various times after treatment. Images of multiple-stained cryosections were overlaid to provide microregional information on the relative position of proliferating cells, hypoxia, perfusion and vasculature.

RESULTS

We observed extensive and permanent vascular dysfunction in a large proportion of tumours from mice treated with tirapazamine. In the affected tumours, blood flow ceased in the centrally located tumour vessels, leaving a rim of functional vessels around the periphery of the tumour. This vascular dysfunction commenced within 24 h after tirapazamine administration and the areas affected appeared to be replaced by necrosis over the following 24-48 h.

CONCLUSIONS

Because the majority of hypoxic cells are located in the center of tumours we propose that the activity of tirapazamine in vivo may be related to its effects on tumour vasculature and that its activity against hypoxic cells located distal to functional blood vessels may not be as important as previously believed.

Tirapazamine plus cisplatin in advanced or recurrent carcinoma of the uterine cervix: a Southwest Oncology Group study

The objective of this study was to determine objective response and overall survival (OS) and progression-free survival (PFS) following cisplatin plus tirapazamine treatment in eligible consenting patients with metastatic or recurrent squamous or adenosquamous carcinoma of the cervix. Treatment consisted of intravenous tirapazamine, 260 mg/m(2), followed by cisplatin, 75 mg/m(2), every 21 days for six cycles. Of 56 registered cases, 52 were evaluable for toxicity. There were six grade 4 toxicities (anemia [three], dyspnea [one], neutropenia/granulocytopenia [one], and dehydration [one]). Fifty-three patients were evaluable for response, OS, and PFS. The 6-month OS rate was 56.6% (95% CI 43.3-69.9%). The objective response rate was 32.1% (4 complete [2 confirmed and 2 unconfirmed] and 13 partial [8 confirmed and 5 unconfirmed]). Higher response rates (16/34 [47.1%] vs 1/19 [5.3%], P= 0.0018) were observed in patients who had not previously received radiation-sensitizing chemotherapy, as were OS and PFS (13.9 vs 4.0 months, P < 0.0001; 5.3 vs 1.8 months, P= 0.01). The OS was considered too low to warrant further testing in this disease setting. Despite this, tirapazamine plus cisplatin was active in patients who had not received cisplatin previously. Prior use of radiosensitizing chemotherapy impacted response and survival significantly and should be considered in future clinical trials.

Prospects for bioreductive drug development

Bioreductive drugs are inactive prodrugs that are converted into potent cytotoxins under conditions of either low oxygen tension or in the presence of high levels of specific reductases. The biochemical basis for selectivity relies on the ability of oxygen to reverse the activation process and the presence of elevated reductase levels in some tumour types. Key criteria for an ideal bioreductive drug should include poor activity against aerobic cells, activation over a broad range of oxygen tensions and, penetration through the aerobic fraction of cells. In addition, the active drug should be capable of killing non-proliferating cells. Numerous compounds are currently at various stages of drug development but Mitomycin C, which is generally considered to be the prototype bioreductive drug, is the only one in clinical use today. Of the drugs currently being evaluated clinically, tirapazamine has definite clinical activity against a variety of solid tumours when used in combination with cisplatin. Other drugs, such as EO9 and various nitroimidazoles, have not been impressive in the clinic and further development is required to improve properties such as drug delivery in the case of indoloquinones. A novel approach to exploiting tumour hypoxia is the development of a gene-directed enzyme prodrug therapy (GDEPT) strategy, where a gene encoding for a prodrug activating enzyme has been placed under the control of a hypoxia responsive promoter sequence. It is generally recognised that bioreductive drugs must be directed towards patients whose tumours have hypoxic regions or have appropriate enzymological characteristics. In terms of identifying tumour hypoxia, there has been considerable progress in the development of nitroimidazole based hypoxia markers that can be detected either via non-invasive or invasive procedures. Another strategy currently undergoing preclinical evaluation is the use of agents that modulate tumour blood flow and synergistic effects have been reported between bioreductive drugs and photodynamic therapy or inhibitors of nitric oxide synthase for example. The development of clinically useful bioreductive drugs depends therefore on the expertise of scientists and clinicians with varying backgrounds. The purpose of this review is to describe and critically assess recent developments in this field, with particular emphasis being placed on drug development and strategies aimed at optimising bioreductive drug activity.

A phase I/II evaluation of tirapazamine administered intravenously concurrent with cisplatin and radiotherapy in women with locally advanced cervical cancer

PURPOSE

This is a Phase I/II dose escalation study to determine the tolerable dose of tirapazamine (TPZ), and the toxicity of a regimen using TPZ with cisplatin, and radiotherapy in women with locally advanced cervical cancer.

METHODS AND MATERIALS

Eligible women for this study were those with a diagnosis of locally advanced cervix cancer, who were less than 75 years of age, having provided informed consent, and who had undergone the necessary prestudy investigations. External-beam radiotherapy (RT) was given to a minimum dose of 4500 cGy in 25 fractions (Day 1-35), and brachytherapy then delivered to bring the total dose at point A to 8500 cGy. The first dose level of the study used TPZ 190 mg/m(2) and cisplatin 75 mg/m(2) on Days 1, 15, and 29 of RT. TPZ 160 mg/m(2) alone was used on Days 8, 10, 12 and 22, 24, 26 of RT. A conventional dose-escalation step method was then used to determine the maximum tolerated dose (MTD) of TPZ.

RESULTS

Four patients were treated at Level 1, 6 at Level 2, and 5 at Level 3. Only 1 patient experienced a dose-limiting toxicity (DLT) at Level 2, but 2 of the 5 patients at Level 3 incurred DLTs. Level 2 was declared the MDT (TPZ 290 mg/m(2) on Days 1, 15, 29 and 220 mg/m(2) on Days 8, 10, 12 and 22, 24, 26). At 6 months, 13 of 15 patients had complete pelvic control of disease.

CONCLUSION

Level 2 of this regime was identified as the MDT. The use of TPZ with concurrent cisplatin and pelvic radiotherapy has acceptable toxicity and should be considered for further Phase 2 testing in view of the promising responses noted.

Single-arm, open-label phase II study of intravenously administered tirapazamine and radiation therapy for glioblastoma multiforme

PURPOSE

This phase II study tested the efficacy and safety of tirapazamine (Sanofi Synthelabo Research, Malvern, PA), a bioreductive agent, in glioblastoma multiforme (GBM) patients. The patients were staged according to a model constructed by a recursive partitioning analysis (RPA) of glioma patients in prior Radiation Therapy Oncology Group (RTOG) trials and compared with a matched standard population, as predicted by the model.

PATIENTS AND METHODS

A total of 124 patients diagnosed with a GBM were treated with radiation therapy and intravenous tirapazamine between January 27,1995, and April 25,1997. All patients received 60 Gy in 2-Gy fractions. Tirapazamine was delivered three times a week for 12 treatments during radiotherapy. Fifty-five patients received tirapazamine at 159 mg/m(2). A second dose level, 260 mg/m(2), was opened, and 69 patients were entered.

RESULTS

There was no significant survival advantage to the drug in any RPA class at either dose level. The median survival time was 10.8 months for the patient population treated with the 159-mg/m(2) dose of tirapazamine and 9.5 months for the group treated with the 260-mg/m (2) dose of tirapazamine. Survival times by RPA class for patients receiving tirapazamine at 159 mg/m(2) were 27.4 months (class III), 10.8 months (class IV), 7.9 months (class V), and 3.8 months (class VI). Survival times by RPA class for patients receiving tirapazamine at 260 mg/m(2) were 16.2 months (class III), 10.3 months (class IV), 5. 1 months (class V), and 1.3 months (class VI). Patients in RPA class III treated in the 159 mg/m(2) dose arm had a notably longer survival than patients in the RTOG database RPA class III, but the difference did not reach statistical significance. There were no fatal toxicities. Grade 3/4 toxicities were more frequent at the higher dose level.

CONCLUSION

Survival in the population treated with radiation and tirapazamine was equivalent to the control population. Patients in RPA class III treated with radiation and tirapazamine at the 159-mg/m(2) dose had a longer survival when compared with the historical controls. The improvement in survival did not reach statistical significance. Toxicity was acceptable in both treatment arms, but grade 3/4 toxicities were more frequent in the higher dose regimen.

DNA damage measured by the comet assay in head and neck cancer patients treated with tirapazamine

Tirapazamine (TPZ) [3-amino-1,2,4-benzotriazine 1,4-dioxide, SR4233, WIN 59075, and Tirazone] is a novel anticancer drug that is selectively activated by the low oxygen environment in solid tumors. By killing the radioresistant hypoxic cells, TPZ potentiates the antitumor efficacy of fractionated irradiation of transplanted tumors in mice. As this cell kill is closely correlated with TPZ-induced DNA damage, we investigated whether human head and neck cancers would show DNA damage similar to that seen in mouse tumors following TPZ administration. TPZ-induced DNA damage in both transplanted tumors in mice and in neck nodes of 13 patients with head and neck cancer was assessed using the alkaline comet assay on cells obtained from fine-needle aspirates. The oxygen levels of the patients' tumors were also measured using a polarographic oxygen electrode. Cells from the patients' tumors showed DNA damage immediately following TPZ administration that was comparable to, or greater than, that seen with transplanted mouse tumors. The heterogeneity of DNA damage in the patients' tumors was greater than that of individual mouse tumors and correlated with tumor hypoxia. The similarity of TPZ-induced DNA damage in human and rodent tumors suggests that tirapazamine should be effective when added to radiotherapy or to cisplatin-based chemotherapy in head and neck cancers.

Implantable polymers for tirapazamine treatments of experimental intracranial malignant glioma

Malignant gliomas remain refractory to intensive radiotherapy and cellular hypoxia enhances clinical radioresistance. Under hypoxic conditions, the benzotriazine di-N-oxide (3-amino-1,2,4-benzotriazine 1,4-dioxide) (tirapazamine) is reduced to yield a free-radical intermediate that results in DNA damage and cellular death. For extracranial xenografts, tirapazamine treatments have shown promise. We therefore incorporated tirapazamine into the synthetic, biodegradable polymer, measured the release, and tested the efficacy both alone and in combination with external beam radiotherapy in the treatment of experimental intracranial human malignant glioma xenografts. The [(poly(bis(p-carboxyphenoxy)-propane) (PCPP):sebacic acid (SA) (PCPP:SA ratio 20:80)] polymer was synthesized. The PCPP:SA polymer and solid tirapazamine were combined to yield proportions of 20% or 30% (wt/wt). Polymer discs (3 x 2 mm) (10 mg) were incubated (PBS, 37 degrees C), and the proportion of the drug released vs. time was recorded. Male nu/nu nude mice were anesthetized and received intracranial injections of 2 x 10(5) U251 human malignant glioma cells. For single intraperitoneal (i.p.) drug and/or external radiation treatments, groups of mice had i.p. 0.3 mmol/kg tirapazamine, 5 Gy cranial irradiation, or combined treatments on day 8 after inoculation. For fractionated drug and radiation treatments, mice had i.p. 0.15 mmol/kg tirapazamine, 5 Gy radiation, or combined treatments on days 8 and 9 after inoculation. For intracranial (i.c.) polymer treatments, mice had craniectomies and intracranial placement of polymer discs at the site of cellular inoculation. The maximally tolerated percentage loading of tirapazamine in the polymer.disc was determined. On day 7 after inoculation, groups of mice had i.c. empty or 3% tirapazamine alone or combined with radiation (5 Gy x 2 doses) or combined with i.p. drug (0.15 mmol/kg x 2 doses on days 8 and 9). Survival was recorded. Polymers showed controlled, protracted in vitro release for over 100 days. The 5 Gy x 1 treatment resulted in improved survival; 28.5 +/- 3.7 days (P = 0.01 vs. controls), while the single i.p. 0.3 mmol/kg tirapazamine treatment, 17.5 +/- 1.9 days (P = NS) and combined treatments; 21.5 +/- 5.0 days (P = NS) were not different. The fractionated treatments: 5 Gy x 2, i.p. 0.15 mmol/kg tirapazamine x 2 and the combined treatments resulted in improved survival: 44.5 +/- 3.9 (P < 0.001), 24.5 +/- 2.3 (P = 0.05) and 50.0 +/- 6.0 (P < 0.001), respectively. Survival after intracranial empty polymer was 16.5 +/- 3.0 days and increased to 31.0 +/- 3.0 (P = 0.003) days when combined with the 5 Gy x 2 treatment. The survival after the polymer bearing 3% tirapazamine alone vs. combined with radiation was not different. The combined 3% tirapazamine polymer, i.p. tirapazamine, and radiation treatments resulted in both early deaths and the highest long-term survivorship. The basis for potential toxicity is discussed. We conclude that implantable biodegradable polymers provide controlled intracranial release for treatment of experimental glioma. For treatment of malignant gliomas, the combination of continuous polymer-mediated delivery and fractionated systemic delivery of tirapazamine with external beam radiotherapy warrants further exploration.

Comparison of the effectiveness of tirapazamine and carbogen with nicotinamide in enhancing the response of a human tumor xenograft to fractionated irradiation

The goal of this study was to compare, with a human tumor xenograft, two different strategies for increasing tumor response to fractionated irradiation, namely, oxygenating the hypoxic tumor cells with carbogen and nicotinamide, or killing these cells with the hypoxic cytotoxin, tirapazamine (TPZ). We used the human hypopharyngeal squamous cell carcinoma cell line FaDu implanted in immune-deficient SCID mice and assessed its response to radiation by cell survival and by growth delay. The tumors were irradiated either once or twice daily with 2 or 2.5 Gy/fraction with either TPZ (0.08 mmol/kg) or nicotinamide (1,000 mg/kg) with carbogen breathing. We also tested the effect of giving TPZ on alternate days, or daily during the first half of the course, the second half, or for the whole course of radiation. We found that adding TPZ or nicotinamide with carbogen to the fractionated radiation regimen enhanced the response of the human xenograft. The enhancement was somewhat greater (though not significantly so) for TPZ, especially when given with each radiation dose. In conclusion, adding TPZ, or nicotinamide plus carbogen, to fractionated irradiation enhanced the response of this human tumor xenograft to fractionated irradiation. Consistent with theoretical modeling, there was a greater enhancement of the radiation response of the tumor when TPZ was given with each radiation dose than when given with only half of the radiation doses.

Effect of interstitial and/or systemic delivery of tirapazamine on the radiosensitivity of human glioblastoma multiforme in nude mice

The purpose of this study was to investigate the feasibility and the efficacy of administering tirapazamine by a slow-releasing polymer disc that was implanted interstitially into a U251 (human glioblastoma multiforme) tumor grown in nude mice. Tumor-bearing animals, with a tumor nodule 0.8 cm3 in size, were distributed to groups receiving combinations of empty or drug-containing polymer implants in the tumor or contralateral leg, intraperitoneal (i.p.) drug, and/or irradiation. The drug (i.p.) alone (14 mg/kg x6) or in combination with tumor drug implant (2 mg) did not significantly increase the tumor volume doubling time compared to that of control animals. Given with 12 Gy of irradiation in twice a day 2-Gy fractions, combined i.p. drug and tumor drug implant significantly delayed tumor growth compared to irradiation alone, which was not achieved with either drug treatment alone added to irradiation. Toxicity, as manifested by transient weight loss, was primarily seen in animals receiving radiation and i.p. tirapazamine. These results indicated that a slow-releasing tirapazamine disc can be produced and the addition of an interstitially implanted tirapazamine disc further increased the effectiveness of i.p. tirapazamine.

Intervention with the hypoxic tumor cell sensitizer etanidazole in the combined modality treatment of limited stage small-cell lung cancer. A one-institution study

PURPOSE

We report the toxicity, patterns of failure and survival of a cohort of patients with limited disease (LD) small-cell lung cancer (SCLC) treated with combined radiation and chemotherapy. During the course of thoracic irradiation, we added intravenous (i.v.) etanidazole (SR-2508, a third-generation 2-nitroimidazole) as a hypoxic cell sensitizer in an attempt to reduce the primary local failure rate and improve survival.

METHODS AND MATERIALS

Between July 1988 and August 1990, 30 consecutive patients with limited disease SCLC were enrolled and treated on a Phase II protocol receiving a standard combination chemotherapy regimen utilizing i.v. cisplatin 25 mg/m2/day x 3 days, i.v. etoposide 100 mg/m2/day x 3 days alternating with intravenous cyclophosphamide 1000 mg/m2/day, intravenous doxorubicin 15 mg/m2, and intravenous vincristine 2 mg (CAV) to a total of six cycles every 3 weeks. Radiotherapy and etanidazole were started after the first cycle of chemotherapy. Etanidazole was administered intravenously at a dose of 2 g/m2 three times per week for a total of 30 g/m2 during the course of thoracic radiation that delivered 50.00 Gy tumor dose in 25 fractions in an overall time of 6 weeks.

RESULTS

The overall response rate of the primary lesion in the thorax was 96% (CR + PR), with 64% complete responses. The median time to treatment failure was 18 months. Of the patients that have relapsed, only 18% failed in the thorax (alone or concomitant with other sites). This is a marked improvement compared to the 40-50% rate reported in the literature. The 2-year crude survival was 46%. The 3- and 5-year crude survival rate with no evidence of disease was 33 and 30%, respectively. We have observed a 10% increase in the incidence of transient etanidazole related peripheral neuropathies compared to previous etanidazole studies not utilizing systemic chemotherapy. There was no increased incidence of radiation esophagitis, pulmonary toxicity, or nephro- or myelotoxicity over and above what has been routinely observed with this radio/chemotherapy regimen. There were no treatment related deaths.

CONCLUSION

The moderate increase in etanidazole-related transient peripheral neuropathies could have been related to the concomitant use of etanidazole with vincristine and cisplatin. Although the almost 50% improvement in the incidence of tumor failure rate in the thorax in this small group of patients did not correlate with an equal marked improvement in their survival, the 5-year survival outcome in our series is at least equal or better than the best reports in the literature of larger clinical trials. We believe there is sufficient data from this study, particularly the improvement of local tumor control, to warrant a large randomized controlled clinical trial, using the most current systemic chemotherapy with concomitant thoracic irradiation with or without the most effective available hypoxic cell cytotoxic/sensitizer.

Efficacy of agents counteracting hypoxia in fractionated radiation regimes

BACKGROUND AND PURPOSE

Solid tumours contain hypoxic cells which are resistant to radiotherapy. This study compares the efficacy of several strategies to counteract diffusion-limited hypoxia, or intermittent hypoxia in a fractionated regimen of 1 to 6 x 2 Gy.

MATERIALS AND METHODS

Nicotinamide (250 mg/kg), perflubron emulsion (Oxygent) (4 ml/kg), tirapazamine (SR4233) (0.10 mmol/kg) and carbogen breathing, administered alone or in combination, were investigated on two tumour cell lines: EMT6 (a rodent mammary carcinoma) and HRT18 (a human rectal adenocarcinoma) using a clonogenic assay. The radiosensitizing effect of the agents was assessed after 1 and 6 x 2 Gy for drugs used alone, and 1, 2, 4, 6 x 2 Gy for drugs used in combination.

RESULTS

At the end of the fractionated radiation regimen, the combination of nicotinamide + carbogen induced the greatest radiosensitization for EMT6 tumours, while greatest radiosensitization of HRT18 was obtained with nicotinamide + carbogen + tirapazamine.

CONCLUSION

The efficacy of the strategies for overcoming hypoxia using a fractionated regimen depends on the tumour cell line. These differences could be linked to differences in the initial percentages of acute and chronic hypoxic cells, and to changes in the two types of hypoxia during treatment.

Gadolinium(III) texaphyrin: a tumor selective radiation sensitizer that is detectable by MRI

Gadolinium(III) texaphyrin (Gd-tex2+) is representative of a new class of radiation sensitizers detectable by magnetic resonance imaging (MRI). This porphyrin-like complex has a high electron affinity [E1/2 (red.) approximately = -0.08 V versus normal hydrogen electrode] and forms a long-lived pi-radical cation upon exposure to hydrated electrons, reducing ketyl radicals, or superoxide ions. Consistent with these chemical findings, Gd-tex2+ was found to be an efficient radiation sensitizer in studies carried out with HT29 cells in in vitro as well as in in vivo single and multifraction irradiation studies with a murine mammary carcinoma model. Selective localization of Gd-tex2+ in tumors was confirmed by MRI scanning.

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

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

Does tirapazamine (SR-4233) have any cytotoxic or sensitizing effect on three human tumour cell lines at clinically relevant partial oxygen pressure?

Solid human tumours contain areas with low oxygen tension (pO2). For bioreductive drugs it is important to define the cytotoxic effect according to drug concentration and to clinically relevant pO2. In this study, the pO2 dependence of the survival of three human cell lines (HRT 18, Na11 +, and MEWO), exposed to tirapazamine (SR-4233) alone or combined with ionizing radiation, was studied in vitro. Gas changes were made to obtain five different oxygen concentrations: air (20.9% O2), 10, 2, 0.2 and 0.02% O2 (hypoxia). Tirapazamine below a concentration of 100 microM was not cytotoxic in air or at 10% O2. At 100 microM tirapazamine was toxic in 2% O2, and at 50 microM in 0.2% O2. For pO2 < 0.2% O2, there was a marked increase in cell killing when 10 microM tirapazamine was combined with 2 Gy, compared with either 10 microM or 2 Gy given alone (p < 0.03). The cytotoxic effect of tirapazamine on human tumour cells in vitro is highly dependent on clinically relevant pO2's. The activation of tirapazamine at a low concentration and at a pO2 found mainly in tumours could yield a very beneficial therapeutic ratio.

In vitro cytotoxicity and chemosensitizing activity of the dual function nitroimidazole RB 6145

PURPOSE

To determine the cytotoxicity and chemosensitizing potential of RG 6145 in mouse KHT/iv and human A549 tumor cells.

METHODS AND MATERIALS

RSU 1069, the lead compound in a series of nitroimidazoles containing an alkylating aziridine function, has been shown to possess a high degree of selective cytotoxicity for hypoxic cells in addition to being a potent sensitizer of radiation and chemotherapy. Unfortunately, preliminary clinical studies have revealed a dose-limiting gastrointestinal toxicity for RSU 1069. Recently RB 6145, the ring-opened analogue of RSU 1069, has been found to be less emetic than RSU 1069. In the present studies, we assessed both the differential hypoxic cell cytotoxicity of RB 6145 and its chemosensitizing potential when combined concomitantly with variable doses of the activated form of cyclophosphamide (4-hydroperoxy-cyclophosphamide, 4-OOH-CP) or the nitrosourea CCNU.

RESULTS

As we had observed previously for RSU 1069, RB 6145 was found to be less cytotoxic to human than rodent tumor cells. In addition, the degree of selective cytotoxicity toward hypoxic cells was (a) less in A549 than in KHT/iv cells (factor of 9 vs. 80) but (b) comparable to that seen with RSU 1069. For both cell lines, inclusion of the sensitizer enhanced the cell killing of the chemotherapeutic agent 4-OOH-CP by a factor of approximately 1.5-1.7-fold. When combined with CCNU, RB 6145 increased the killing of A549 cells approximately 1.8-fold. Similar hypoxic cell preferential cytotoxicity and enhancement in anti-tumor treatment efficacy were seen when A549 cells were exposed to the R enantiomer of RB 6145 (PD 144872) either alone or in combination with CCNU.

CONCLUSION

These data support the notion that further consideration should be given to the clinical application of these bioreductive agents.

Intrinsic and extrinsic characteristics of human tumors relevant to radiosurgery: comparative cellular radiosensitivity and hypoxic percentages

We have collected the in vitro x-ray radiation survival characteristics of 181 lines from 12 different classes of exponentially growing human tumor cells (sarcomas, lung cancers, colo-rectal cancers, medulloblastomas, melanoma, breast cancers, prostate cancers, renal cell cancers, grades III and IV brain tumors, ovarian, and head and neck cancers). This information was used to intercompare survival after single high doses of 20-40 Gy for each tumor line. Radiosensitivities could roughly be divided into two groups. The more radiosensitive group included: sarcoma, small-cell lung cancer, non-small cell lung cancer, colorectal cancer, medulloblastoma and melanoma. The more radioresistant group included breast, prostate, renal cell, primary brain tumors, ovarian tumors, and head and neck cancers. Using a model of a 3 cm diameter brain lesion containing about 1.4 x 10(9) oxic cells, the single doses calculated to reduce survival to 1 cell were: sarcoma and small cell lung cancers-22-23 Gy; melanoma-25 Gy; non-small cell lung and colorectal cancer-26 Gy; medullo-blastoma-28 Gy; breast, prostate, renal cell, primary brain tumors, ovarian tumors, and head and neck cancers-30-36 Gy. If, however, tumors contained on average 20 percent hypoxic cells, the dose needed for equivalent cell killing increased by about a factor of 2.6-2.8. Also, there was no correlation between the ranking of relative radiosensitivities of the various classes of tumor cells at high doses (as in radiosurgery) to the sensitivity at low doses (as in conventional fractionated radiotherapy).

CONCLUSION

available information on the intrinsic radiosensitivity of human tumor cells indicates that meaningful differences exist among different histological classes of neoplasm that are relevant to the single high doses used in radioneurosurgery, and which could constitute a basis for "tailoring" the administered dose to the particular neoplasm. However, if intracerebral lesions contain a large number of hypoxic cells (e.g., 20%), this may constitute a significant problem.

Manipulation and exploitation of the tumour environment for therapeutic benefit

We describe aspects of the tumour microenvironment that are available as targets for manipulation. In particular, the question asked is whether hypoxia in tumours is a problem to be overcome, or a physiological abnormality to be exploited? Bioreductive drugs require metabolic reduction to generate cytotoxic metabolites. This process is facilitated by appropriate reductases and the lower oxygen conditions present in solid tumours compared with normal tissues. Because of their specificity, bioreductive drugs are used to help answer this question. Other aspects of tumour physiology and biochemistry that may be exploited include tissue dependent reductase expression, pH and angiogenesis.

Comparison of the enhancement of tumor responses to fractionated irradiation by SR 4233 (tirapazamine) and by nicotinamide with carbogen

PURPOSE

This study was undertaken to compare in a fractionated regimen, with clinically relevant radiation doses, two radiation response modifiers that function by different mechanisms: SR 4233, a bioreductive agent toxic to hypoxic cells, and nicotinamide with carbogen, a combination that has been shown to improve tumor oxygenation.

METHODS AND MATERIALS

Cell survival assays were used to examine the response of three different tumors: KHT, RIF-1 and SCCVII/St in C3H/Km mice. Regrowth delay studies were also performed with the RIF-1 tumor. A fractionated irradiation schedule, consisting of twice daily 2.5 Gy treatments was investigated with and without drug pretreatment. SR 4233 was given IP at 0.12 mmol/kg one half hour before each irradiation. Nicotinamide (250, 500, 1000 mg/kg) was given IP 1 h before each irradiation with carbogen exposure 5 min prior to and during the irradiation.

RESULTS

Both treatment strategies enhanced the response of all three tumors to the fractionated radiation regimen. However, for two of the tumors (KHT and SCCVII), SR 4233 produced a significantly greater enhancement than did the combination of nicotinamide + carbogen. For the RIF-1 tumor (which has the lowest hypoxic fraction of the three), the response was comparable for the two modalities. For nicotinamide + carbogen, there was no significant change in the radiation enhancement at nicotinamide doses between 250 and 1000 mg/kg.

CONCLUSION

Adding the bioreductive cytotoxin SR 4233 or nicotinamide + carbogen to fractionated irradiation enhances the response of the three transplanted tumors used in this study to fractionated irradiation. The radiation enhancement was significantly greater, however, for SR 4233 for two of the tumors with comparable results in the third. The data are consistent with the prediction that killing tumor hypoxic cells can produce a similar or greater enhancement of the efficacy of fractionated radiation in enhancing tumor response than either oxygenating or radiosensitizing these cells.

Interaction of the bioreductive drug SR 4233 and photodynamic therapy using photofrin in a mouse tumor model

PURPOSE

Combining the bioreductive drug SR 4233 with interstitial photodynamic therapy to improve efficacy.

METHODS AND MATERIALS

RIF1 tumors were implanted subcutaneously in mice and treated with interstitial photodynamic therapy. The bioreductive drug SR 4233 (a benzotriazine which exhibits preferential cell killing under hypoxic conditions) was combined with photodynamic therapy to exploit the induced hypoxia. SR 4233 was given to mice prior to or just after illumination. The effect of multiple SR 4233 injections given over the first 3 days after treatment was also evaluated.

RESULTS

The results from experiments with a 24 hr interval between Photofrin and illumination showed that SR 4233 produced only a small additional growth delay compared with photodynamic therapy alone (light doses of 300 or 400 J/cm, combined with 6 x 15 mg/kg SR 4233). Some cures (6/60), however, were found in groups treated with 200 to 400 J/cm with SR 4233, whereas only two cures (2/77) occurred at light doses up to 400 J/cm after photodynamic therapy alone. Reducing the interval between Photofrin injection and illumination increased the number of cures in the combination group, although this was associated with a marked increase in toxicity. A small increase in cure rate was observed for the combination of photodynamic therapy (6 hr interval) and SR 4233, although this was not significant due to the limited number of mice that survived treatment.

CONCLUSION

Only a limited effect of combining SR 4233 and interstitial photodynamic therapy was observed in this tumor model. A possible explanation could be the rapid conversion of SR 4233 into inactive metabolites.

SR 4233 (tirapazamine): a new anticancer drug exploiting hypoxia in solid tumours

SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide, WIN 59075, tirapazamine) is the lead compound in a new class of bioreductive anticancer drugs, the benzotriazine di-N-oxides. It is currently undergoing Phase I clinical testing. The preferential tumour cell killing of SR 4233 is a result of its high specific toxicity to cells at low oxygen tensions. Such hypoxic cells are a common feature of solid tumours, but not normal tissues, and are resistant to cancer therapies including radiation and some anticancer drugs. The killing of these tumour cells by SR 4233, particularly when given on multiple occasions, can increase total tumour cell killing by fractionated irradiation by several orders of magnitude without increasing toxicity to surrounding normal tissues. Topics covered in this review include the rationale for developing a hypoxic cytotoxic agent, the cytotoxicity of SR 4233 as a function of oxygen concentration, the mechanism of action of the drug and its intracellular target and the in vivo evidence that the drug may be useful as an adjunct both to radiotherapy and chemotherapy. Finally, the major unanswered questions on the drug are outlined.

The experimental development of bioreductive drugs and their role in cancer therapy

Bioreductive drugs undergo metabolic reduction to generate cytotoxic metabolites. This process is facilitated by bioreductive enzymes and the lower oxygen conditions present in solid tumours compared to normal tissues. Because of this specificity, bioreductive drugs have enormous potential to contribute to modern cancer therapy. Examples undergoing clinical trials include N-oxides such as tirapazamine, aziridinylnitroimidazoles RSU 1069/RBU 6145 and quinones such as indoloquinone EO9. Other novel structures are also under study. Here we review the experimental development of bioreductive drugs and their role in cancer therapy.

Toxicology of daily administration to mice of the radiation potentiator SR 4233 (WIN 59075)

We have investigated the feasibility of administration of an effective dose of the hypoxic cytotoxin, SR 4233, Monday-Friday daily for 6 weeks. From a thorough hematological, histopathological and clinical chemistry evaluation throughout the course and during a 3-week recovery period, we conclude that daily administration of a radiopotentiating dose of SR 4233 in mice is well tolerated and that bone marrow suppression is likely to be the dose-limiting toxicity.

In vitro activity of the benzotriazine dioxide SR 4233 against human tumour colony-forming units

SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide) is a novel bioreductive agent selectively toxic to hypoxic cells. It is active as a radiation sensitiser in vitro. Using a human tumour cloning system we have studied the effects of SR 4233 against freshly explanted human tumour specimens under hypoxic and non-hypoxic culture conditions. For hypoxic conditions, final concentrations of SR 4233 of 10.0-500 mumol/l were used in short-term (1 h) exposure experiments. Final concentrations in non-hypoxic experiments ranged from 10 to 1350 mumol/l. 25 tumour specimens were tested under each culture condition. Of those, 14 (56%) were evaluable. The most common tumour types recruited included ovarian, non-small cell lung, and breast cancer. A moderate concentration-dependent increase in the frequency of inhibited tumour specimens under non-hypoxic conditions was observed with zero out of 10 sensitive specimens at 10 mumol/l as compared with five out of 14 (36%) sensitive specimens at 500 mumol/l (P < 0.02). However, when hypoxic conditions were used SR 4233 had a profound antitumour activity, (two out of 14 specimens sensitive at 10 mumol/l compared with 10 out of 10 specimens sensitive at 500 mumol/l, P < 0.00005). We conclude that SR 4233 is active against tumour colony-forming units in vitro and that its antitumour activity is greatly increased against hypoxic tumour cells.

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.

Characterization of a CHO cell line resistant to killing by the hypoxic cell cytotoxin SR 4233

One approach to understanding the mechanism of selective hypoxic cell killing by the benzotriazine-di-N-oxide, SR 4233, is to characterize cell lines that exhibit increased resistance to killing by this drug. The Chinese Hamster Ovary cell line BL-10 was originally isolated on the basis of its hypersensitivity to killing by bleomycin. It is 2.7-fold more resistant to hypoxic cell killing by SR 4233 than wild-type CHO on comparison of D0's. However, both BL-10 and CHO possess the same sensitivity to killing by SR 4233 under aerobic conditions. We have excluded the explanation that differential metabolism of SR 4233 is responsible for its increased survival as both BL-10 and CHO produce the two-electron product SR 4317 at the same rate (3 nmoles/hr/10(6) cells). Analysis of free radical production, DNA double-strand break induction, and glutathione (GSH) levels suggested that the resistance of BL-10 to killing by SR 4233 might result from increased intracellular radical scavenger pathways. Using buthionine sulfoximine (BSO) to decrease cellular GSH levels, we found a marked increase in the sensitivity of BL-10 cells to SR 4233 killing under hypoxia, but a much smaller increase in the sensitivity of CHO cells. Taken together, these data imply that the high GSH levels in BL-10 cells is responsible for its resistance to SR 4233 cytotoxicity.