Modification of chemotherapy by nitroimidazoles

Synthesis of a novel nitroimidazole-spermidine derivative as a tumor-targeted hypoxia-selective cytotoxin

A four-step synthesis of (R,S)-N(4)-[3-(2-nitro-1-imidazolyl)-2-hydroxypropyl]-spermidine trihydrochloride (4) is described and the utilization of the polyamine active transport system for the uptake of this compound in cells is demonstrated. Thus, V79 cells pretreated with an inhibitor of spermidine biosynthesis, alpha-difluoromethylornithine (DFMO), are ca. 2-fold more sensitive to 4 under hypoxic conditions, compared to untreated cells. Similarly, radiosensitization of hypoxic V79 cells by 4 is improved in DFMO-pretreated cells.

Phase I trial of high dose paracetamol and carmustine in patients with metastatic melanoma

Reduced glutathione (GSH) production by tumour cells has been proposed as a mechanism for resistance to alkylating agents. High levels of paracetamol can deplete intracellular GSH. We conducted a phase I trial of high dose paracetamol and carmustine (BCNU) in patients with advanced malignant melanoma to determine the optimal biological dose and the maximum tolerated dose (MTD) with the goal of increasing sensitivity to BCNU by GSH depletion. Groups of three to five patients received escalating doses of paracetamol (10, 15 or 20 g/m(2)) every 3 weeks. Every other cycle, BCNU (10 mg/m(2)) was given 6.5 h after administration of paracetamol and 45 min before a 20 h infusion of N-acetylcysteine. Once the MTD for paracetamol had been determined, the dose of BCNU was sequentially escalated in subsequent cohorts to 150 mg/m(2). GSH levels were measured in peripheral blood mononuclear cells (PBMCs) and, when available, in tumour biopsies. The MTD of paracetamol was 15 g/m(2). The dose of BCNU was safely escalated to 150 mg/m(2). The most common toxicity was grade II nausea/vomiting. At 15 g/m(2), peak paracetamol levels (median 253 microg/ml) were reached between 1 and 4 h. No changes in GSH levels in PBMCs were seen. There were two partial responses, including a dramatic decrease in hepatic metastases. Treatment of melanoma patients with paracetamol (15 g/m(2)) every 3 weeks and BCNU (150 mg/m(2)) every 6 weeks is safe. The observation of two partial responses has led to a phase II study to evaluate treatment with high dose paracetamol alone or in combination with BCNU.

Animal models for studying lung cancer and evaluating novel intervention strategies

The pathogenesis of lung cancer progression, invasion and metastasis remains undefined. Clinically relevant laboratory models of the disease could greatly facilitate its clarification. Model systems of lung cancer that accurately reflect different biologic properties and disease stages are necessary to ensure proper experimental design of studies aimed at increasing our understanding of the disease. Such models are also essential tools to accelerate development of new therapies for lung cancer. In this review we summarize the available lung cancer model systems in use today and define both their utility and limitations.

Enhancement of the antitumor effect of cyclophosphamide with the hypoxia-selective cytotoxin NLCQ-1 against murine tumors and human xenografts

The antitumor effect of cyclophosphamide (CPM) was investigated against SCCVII murine tumors and PC-3 human xenografts in combination with the hypoxia-selective cytotoxin 4-[3-(2-nitro-1-imidazolyl)-propylamino]-7-chloroquinoline hydrochloride (NLCQ-1). The in vivo-in vitro and the tumor regrowth assays were used, respectively, as end points. In certain cases the hypoxia-selective cytotoxin tirapazamine (TPZ) was included for comparison purposes. In the SCCVII/C3H model, bone marrow toxicity studies were performed in parallel by using a modified CFU-GM assay. In the SCCVII/C3H model, when NLCQ-1 (10 mg/kg i.p.) was given 1 h before cyclophosphamide (CPM; 75-200 mg/kg i.p.), dose-modification factors (DMFs) of 1.9 and 1.0 were achieved for the antitumor effect and bone marrow toxicity, respectively. The corresponding DMF values obtained with TPZ (23 mg/kg) given 2.5 h (optimal time) before CPM were 1.3 and 1.0, respectively. Thus, therapeutic indices (T.I.) of 1.9 and 1.3 were achieved with NLCQ-1 and TPZ, respectively. In the PC-3/athymic nude mouse model, NLCQ-1 (10 mg/kg) given 90 min before CPM (36 mg/kg), qd x 4, increased tumor regrowth delay by 8.7 days compared to CPM alone, at 16-fold the original tumor size. The corresponding log cell kill was 0.86 and -0.03 for NLCQ-1 + CPM and CPM alone, respectively. In general, NLCQ-1 in combination with nontoxic but inactive CPM doses (36 or 54 mg/kg, qd x 4) elicited good antitumor activity without subsequent additive systemic toxicity, whereas NLCQ-1 had minimal effect in combination with the active but toxic (> 10% mean net weight loss) CPM dose of 80 mg/kg. These results suggest a potential use of NLCQ-1 in the clinic as an adjuvant to chemotherapy with CPM.

Targeting tumor blood vessels: an adjuvant strategy for radiation therapy

BACKGROUND AND PURPOSE

The neovascularization of tumor cells is a prerequisite if a clinically relevant tumor size is to be reached. A continuously expanding vessel network supplying nutritional requirements and removing waste products is essential for continued tumor development, growth and survival.

RESULTS

In many tumors, the growing endothelium is unable to fully support the demands of the neoplastic cell population. As a consequence of the inadequacies of the resulting aberrant vasculature, microenvironmental conditions develop in tumors which are not only detrimental to the response of tumors to conventional anticancer treatments, but may lead to or predispose cells to genetic modifications resulting in more aggressive phenotypes and higher metastatic potential. Yet the utter dependence of the tumor on its induced vessel formation for growth, survival and spread has also created a great deal of enthusiasm for developing therapeutic approaches to specifically targeting the tumor microcirculation.

CONCLUSIONS

The application of such strategies as adjuvants to conventional radiation treatments offers unique opportunities to develop more effective cancer therapies.

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.

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.

NLCQ-1, a novel hypoxic cytotoxin: potentiation of melphalan, cisDDP and cyclophosphamide in vivo

PURPOSE

To investigate in vivo interactions between the recently developed bioreductive agent 4-[3-(2-nitroimidazolyl)-propylamino]-7-chloroquinoline hydrochloride (NLCQ-1) and the chemotherapeutic agents melphalan (L-PAM), cis-platin (cisDDP) and cyclophosphamide (CPM).

METHODS AND MATERIALS

EMT6 and FSaIIC tumor cells were inoculated (subcutaneously) into the leg(s) of female Balb/c and male C3H mice, respectively. Treatment was initiated at 10 mm (EMT6) and 5 mm (FSaIIC) mean tumor diameter. The in vivo-in vitro and tumor regrowth assays were used, respectively, as endpoints. Bone marrow toxicity studies were also performed when the in vivo-in vitro assay was used. Drugs were given by i.p. injection. Tumors were excised 18-h after chemotherapeutic drug administration (Balb/c mice) or measured daily until three times their original size (C3H mice). The optimum administration schedule for potentiation between NLCQ-1 and each chemotherapeutic drug, as well as dose modification factors (DMF) at the optimum time, were determined with the in vivo-in vitro assay. When the tumor regrowth assay was used, each chemotherapeutic agent was given either as a single dose or as a split dose over two consecutive days at the optimum administration time after a 10 mg/kg NLCQ-1 i.p. injection.

RESULTS

NLCQ-1 (at 0.33 times MTD), strongly potentiated the antitumor effect of L-PAM, cisDDP and CPM without concurrent enhancement in bone marrow toxicity. Potentiation was strictly schedule dependent and the optimum effect (1.5 to 2 logs killing beyond additivity) was observed when NLCQ-1 was given 60-, 45-, and 110-min before L-PAM, cisDDP, and CPM, respectively. The DMF values at 30% survival were 2.5, 1.9, and 3.8 for L-PAM, cisDDP, and CPM, respectively. DMF values for bone marrow toxicity at 50% survival were ca. 1 for all chemotherapeutic drugs. Pretreatment with NLCQ-1 resulted in 4-12 days extra delay in the regrowth of FSaIIC tumors.

CONCLUSIONS

These results support the clinical investigation of NLCQ-1 as a chemosensitizer.

Potentiation of cisplatin activity by the bioreductive agent tirapazamine

PURPOSE

The chemosensitizing potential of the benzotriazine-N-oxide tirapazamine was determined in rodent mammary tumor cells grown as solid tumors.

MATERIALS AND METHODS

C3H/HeJ mice bearing i.m. transplanted 16C mammary carcinomas were treated with varying doses of either cisplatin alone or cisplatin in combination with a 0.27 mmol/kg dose of tirapazamine. Tumor response to single agent or combination therapy was assessed using an in situ tumor growth delay assay. Normal tissue toxicity resulting from the tirapazamine, cisplatin, or tirapazamine plus cisplatin was determined by measuring bone marrow stem cell (CFU-GM) toxicities and blood urea nitrogen (BUN) levels.

RESULTS

Tirapazamine itself had no measurable effect on the growth of this tumor. However, when administered from 3 h before to simultaneously with a single dose of cisplatin, the resultant tumor growth delay was significantly increased as compared to that seen with cisplatin alone. The administration of tirapazamine 3 h prior to a range of doses of cisplatin was found to result in a dose modifying factor (DMF) of approximately 1.7 in tumor response compared to cisplatin alone. Tirapazamine did demonstrate some hematologic toxicity on its own but it did not potentiate the toxicity of cisplatin when the two agents were administered in combination. BUN analysis showed that tirapazamine had little effect on BUN levels but did suppress the BUN values of mice treated with the combination of tirapazamine and 15 mg/kg cisplatin as compared to cisplatin alone.

CONCLUSIONS

The present findings suggest that the addition of tirapazamine to cisplatin therapy may lead to a therapeutic benefit.

Cytotoxicity of antitumor platinum complexes with L-buthionine-(R,S)-sulfoximine and/or etanidazole in human carcinoma cell lines sensitive and resistant to cisplatin

Human 2008 ovarian carcinoma cells and the C13 CDDP-resistant subline and human MCF-7 breast carcinoma cells and the MCF-7/CDDP CDDP-resistant subline were exposed to L-buthionine-(S,R)-sulfoximine (50 microM) for 48 h prior to and during exposure for 1 h to the antitumor platinum complexes, cis-diamminedichloroplatinum(II), carboplatin or D,L-tetraplatin and/or to etanidazole (1 mM) for 2 h prior to and during exposure for 1 to the antitumor platinum complexes. These modulators alone did not significantly alter the cytotoxicity of CDDP toward either parental line. A twofold enhancement in cytotoxicity was observed with carboplatin in the 2008 cells and with D,L-tetraplatin in both parental lines with the single modulators. The modulator combination (buthionine sulfoximine/etanidazole) was very effective along with D,L-tetraplatin in both the MCF-7 parent and MCF-7/CDDP cell lines where at the higher platinum complex concentrations there was 1.5 to 3 logs increased killing of cells by the drug plus the modulators compared with the drug alone. Similarly, when C13 cells were exposed to CDDP (100 microM) or D,L-tetraplatin (100 microM) along with buthionine sulfoximine and etanidazole there was a 2-log increase in cell killing compared with exposure to the platinum complex alone. Treatment of each of the four cell lines with buthionine sulfoximine decreased both the non-protein and total sulfhydryl content of the cells. Treatment with the combination of modulators did not produce a further decrease in cellular sulfhydryl content compared with buthionine sulfoximine alone. The total sulfhydryl content in MCF-7 cells and 2008 cells exposed to buthionine sulfoximine and etanidazole was 58% and 31% of normal and the total sulfhydryl content of MCF-7/CDDP cells and C13 cells treated the same way was 54% and 23% of normal, respectively. DNA alkaline elution was used to assess the impact of exposure to the modulators, buthionine sulfoximine and etanidazole, alone and in combination on the cross linking of DNA by the antitumor platinum complexes in the MCF-7 and MCF-7/CDDP cell lines. Overall, the increases in DNA cross linking factors were greater in the MCF-7 cells than in the MCF-7/CDDP cells. These results indicate a possible clinical potential for this modulator combination.

Chemosensitization of CCNU in KHT murine tumor cells in vivo and in vitro by the agent RB 6145 and its isomer PD 144872

The cytotoxicity and chemosensitizing potential of the nitrohetercyclic agent RB 6145 and its R enantiomer PD 144872 were determined in rodent tumor cells grown in tissue culture or as solid tumors. Using a clonogenic cell survival assay the degree of selective cytotoxicity of these bioreductive drugs was first determined in KHT/iv cells. Cells treated under hypoxic conditions were observed to be approximately 50-80-fold more susceptible to the action of RB 6145 or PD 144872 than were cells exposed under air. To assess the in vitro chemosensitizing potential of RB 6145 and PD 144872, doses of these agents which led to survival values between 0.5 and 1.0 under hypoxic conditions were administered, and were then combined concomitantly with variable doses of the nitrosourea CCNU. Exposure to the nitrosourea was for 1 h. The results showed that inclusion of either sensitizer enhanced the cell killing of the chemotherapeutic agent 2.4-2.6-fold. Subsequent experiments evaluated the therapeutic benefit of combining these bioreductive agents with CCNU in KHT sarcoma-bearing C3H/HeJ mice. When given at times ranging from 90 min before to 60 min after CCNU exposure, these bioreductive drugs increased the tumoricidal activity of the chemotherapeutic agent. Complete dose response curves combining RB 6145 and PD 144872 and a range of CCNU doses also were evaluated. The sensitizers (0.75 mmol/kg) were administered 30 min prior to the chemotherapeutic agent and survival of clonogenic tumor cells 22-24 h after treatment was used to assay tumor response.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoxia and drug resistance

Biologically and therapeutically important hypoxia occurs in many solid tumor masses. Hypoxia can be a direct cause of therapeutic resistance because some drugs and radiation require oxygen to be maximally cytotoxic. Cellular metabolism is altered under hypoxic conditions. Hypoxia can result in drug resistance indirectly if under this condition cells more effectively detoxify the drug molecules. Finally, there is evidence that hypoxia can enhance genetic instability in tumor cells thus allowing more rapid development of drug resistance cells. The current review describes the effects of hypoxia on tumor response to a variety of anti-cancer agents and also describes progress toward therapeutically useful methods of delivering oxygen to tumors in an effort to overcome therapeutic resistance due to hypoxia. Finally, the use of hypoxic cell selective cytotoxic agents as a means of addressing hypoxic 'drug resistance' is discussed.

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.

Synthesis and radioiodination of a nido-1,2-carboranyl derivative of 2-nitroimidazole

The synthesis of a nido-carboranyl congener of misonidazole, 1-(3'-nido-carboranyl-2'-hydroxy)propyl-2-nitroimidazole, has been carried out. Alternative methods of preparations were conducted to optimize the chemical yield, with a five step synthesis giving an overall yield (from 1,2-carborane) of 36%. A diastereomeric pair of nido-carboranyl compounds was obtained. The diastereomeric nido-carboranyl misonidazole congeners were (radio)iodinated to yield (> 90%) a mixture of diastereomeric compounds in which the iodine had bonded to a boron atom on the nido-carborane moiety. These compounds will be investigated for their application to boron neutron capture therapy (BNCT) and hypoxia imaging of cancer.

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.

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.

Potentiation of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea's toxicity in vitro by two new bioreductive agents

Two new bioreductive compounds, 9-[3-(2-nitro-1-imidazolyl)propylamino]acridine hydrochloride (NLA-1) and 9-[2-(2-nitro-1-imidazolyl)ethylamino]acridine hydrochloride (NLA-2), which behave as hypoxic cytotoxins and radiosensitizers, have been investigated for potentiation of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea's (CCNU) cytotoxic activity in vitro using V-79 cells. The preincubation effect as well as conditions of coadministration of CCNU with each sensitizer have been examined. In this latter case, the median-effect analysis was applied to evaluate whether the phenomenon was additive or synergistic. A clonogenic assay was used to score survival. Both bioreductive compounds, even at very low concentrations, significantly enhance the cytotoxic activity of CCNU under conditions of hypoxic preincubation. The enhancement of CCNU cytotoxicity is dependent upon preincubation time and the concentrations of both CCNU and the specific bioreductive agent. Coincubation of cells under hypoxia with CCNU and each bioreductive agent led to some potentiation, but only at lower survival levels. No chemosensitization was observed under aerobic conditions with either sensitizer.

Potentiation of alkylating chemotherapy by dual function nitrofurans in multi-cell spheroids and solid tumors

The cytotoxicity and chemosensitizing potential of four dual function nitrofurans was determined in human HT-29 multi-cell spheroids and rodent KHT sarcoma solid tumors. Spheroids were treated with a range of doses of the bioreductive drugs for a period of up to 48 h and the extent of cell kill was assessed at various times after treatment. Cytotoxicity was determined using a clonogenic cell-survival assay. The results demonstrated that two of the nitrofurans were even more toxic to spheroid cells than was the potent bioreductive nitroimidazole aziridine RSU 1069. The dose of the nitrofuran which, after a 24-h exposure, led to a survival value between 0.5 and 1.0, then was chosen for subsequent studies aimed at assessing the ability of these agents to potentiate the efficacy of the nitrosourea CCNU. Exposure to this chemotherapeutic agent was for a period of 1 h. The results indicated that all four dual function nitrofurans enhanced the cell killing of the conventional chemotherapeutic agent by factors ranging from 1.1 to 1.7. Subsequent studies evaluated the therapeutic benefit of combining these bioreductive agents and CCNU in KHT sarcoma-bearing C3H/HeJ mice. The nitrofurans were administered i.p. 0.5 h prior to the chemotherapy and tumor response was assessed by measuring the survival of clonogenic KHT cells 22-24 h after treatment. Normal tissue toxicity was determined using a bone marrow stem cell (CFU-GM) assay. Combining these bioreductive agents with CCNU increased the tumor cell kill by factors of 1.2 to 1.7.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytotoxicity of dual function nitrofurans in rodent and human tumor cells

The efficacy and selective hypoxic cell cytotoxicity of four dual function nitrofurans and two nitroimidazole-aziridines was determined in human (A549, HT-29) and rodent (KHT/iv) tumor cells. All bioreductive compounds were found to be less effective at killing human than mouse tumor cells (approximately 2-6-fold). This reduced cytotoxicity in the human tumor cells occurred irrespective of the state of oxygenation. In addition, the degree of selective toxicity toward hypoxic cells or the cytotoxicity factor (CF), defined as the ratio of the surviving fraction in air to that under hypoxic conditions, was (a) greater for the nitroimidazole-aziridines than for the nitrofurans and (b) less in the human than the rodent tumor cell lines investigated. For example, CF values in A549 or HT-29 cells typically were 2-4-fold lower than those determined in KHT/iv cells. This reduction in the CF in the human cells resulted from a greater loss in the hypoxic toxicity than in the aerobic toxicity when compared with the rodent cells.

Chemical radiosensitizers in cancer therapy

The development of effective low-LET radiation therapy for cancer has been hindered by the lack of consistent differential responses to radiation between tumor and normal tissues. One major difference between many solid tumors and the surrounding normal stroma is the presence of hypoxic foci in solid tumors due to the inadequate supply of nutritional needs as a result of the breakdown of microvasculature. Consequently, failure of conventional radiotherapy and local recurrences are in part attributed to the radioresistant hypoxic cell populations, present in the tumor. Local cure/control rates of a tumor can be increased only by an effective increase in the radiation dose. At the same time, an increase in such a dose would damage the oxic normal stroma, more than the hypoxic tumor cells. Hence, specific modification of tumor radiosensitivity by the use of chemical radiosensitizers, in combination with conventional radiotherapy, is an attractive alternative. Many clinicians and radiotherapists are skeptical about the outcome of using radiosensitizers in patients. Nevertheless, a vast amount of information is currently available regarding the first- and second-generation radiosensitizers both in murine and in human tumors. As a result, it is hoped that eventually a radiosensitizing drug would be discovered/synthesized that will overcome the drawbacks so far encountered in their use in the clinic. In this article, the development of chemical radiosensitizers since the early sixties, the basis for their selection, their mechanism(s) of action, and the results obtained with the various groups of radiosensitizers are reviewed.

Interaction of misonidazole, hyperthermia, and irradiation in a C3H mammary carcinoma and its surrounding skin in vivo

The interaction of irradiation, Misonidazole (MISO), and hyperthermia was studied in a C3H mouse mammary carcinoma and its surrounding skin in vivo. MISO (0.5-1.0 mg/g) was injected 30 min before irradiation. Hyperthermia (41.5 degrees-43.5 degrees C for 60 min) was given either simultaneously, 0.5 hr, or 4 hr after X rays. The results were evaluated as the radiation dose to achieve tumor control (TCD50) or moist desquamation of the skin (DD50) in half of the treated animals. A therapeutic gain was found when the enhancement in tumors were greater than that found in skin. The combination of simultaneous heat and irradiation caused great enhancement in radiation response, but with no therapeutic gain. A slightly lower enhancement of the damage in both tissues was found with a 30 min interval between irradiation and hyperthermia, whereas heat 4 hr after X rays gave a small, but significant therapeutic gain. MISO significantly enhanced the response in tumors but not in skin. Combined trimodality treatment with MISO, irradiation, and hyperthermia resulted in enhancement ratios up to 15, dependent on temperature, radiation-heat interval, and to a lesser extent the MISO dose. The enhancement was for all schedules most pronounced in the tumors, resulting in an improved therapeutic effect. The combination of MISO and hyperthermia may be a valuable addition to radiotherapy, especially if heat and irradiation can be applied with close interval and with one of the modalities given selectively to the tumor.

2-Nitroimidazole potentiation of nitrosourea induced cytotoxicity in subcutaneous implants of rat 9L brain tumor cells

To determine if the 2-nitroimidazole (2-NI) and the nitrosourea (NU) in a brain tumor chemopotentiation trial should be selected on the basis of known structure-activity relationships (electron affinity, lipophilicity, alkylating activity, carbamoylating activity), s.c. implants of rat 9L brain tumor cells were treated with combinations of misonidazole (MISO) or etanidazole (SR-2508) administered under oxic and hypoxic conditions, and BCNU, CCNU or chlorozotocin (CLZ) administered under oxic conditions. Cell kill was assessed by an in vivo to in vitro colony formation assay. To mimic the 'preincubation effect', the 2-NI was injected i.p., and 30 min later the tumor was clamped. After 2 hr, the clamp was released, and the NU administered immediately. MISO (2.5 mmole/kg) and SR-2508 (3.75 mmole/kg) reached the same peak tumor concentration in 30 min. Both 2-NIs were metabolized at the same rate in the clamped tumors; however, metabolism of the 2-NIs by hypoxic cells over the 2 hr clamping period did not produce any measurable s.c. 9L cell kill. The relative effectiveness of the NUs for killing oxic s.c. 9L tumor cells was: BCNU greater than CCNU greater than CLZ. Clamping the tumor prior to NU administration did not change the NU cytotoxicity. No potentiation of the NU cytotoxicity by the 2-NIs was observed in oxic tumors. Although metabolism of MISO by hypoxic cells did not result in potentiation of CLZ cytotoxicity at any dose, it resulted in potentiation of BCNU cytotoxicity at all doses and CCNU cytotoxicity at high doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Combination of etanidazole with cyclophosphamide and platinum complexes

In an effort to improve the therapeutic efficacy and selectivity of cyclophosphamide (CTX), cis-diamminedichloroplatinum(II) (CDDP), and carboplatin (Carbo), these antitumor alkylating agents were combined with the 2-nitroimidazole drug etanidazole (ETA). As revealed by tumor-cell survival assay in the FSaIIC murine fibrosarcoma, the addition of ETA (1 g/kg, i.p.) just prior to the i.p. injection of various doses of the alkylating agents resulted in increases in the tumor-cell kill produced by each drug (CTX, 10-fold; CDDP, 20-fold; and Carbo, 5- to 15-fold), whereas toxicity to bone marrow granulocyte-macrophage colony-forming units (CFU-GM) increased only about 0- to 3-fold. When CTX was combined with either CDDP or Carbo, striking increases in tumor-cell killing were observed (20- to 100-fold across the CDDP dose range and 5- to 20-fold across the dose range of Carbo), which were supra-additive for CDDP and additive for Carbo as revealed by isobologram analysis. The addition of ETA to these alkylating-agent combinations produced a further approx. 20-fold increase in tumor-cell kill for both CTX/CDDP and CTX/Carbo. This effect was greatest at the lowest dose of the platinum drug tested and was supra-additive in the case of CDDP and additive for Carbo. Following treatment with ETA/CTX/CDDP, bone marrow CFU-GM toxicity increased only about 5-fold over that of CTX/CDDP alone, but the injection of ETA/CTX/Carbo resulted in a 10- to 20-fold increase in bone marrow toxicity as compared with that obtained using CTX/Carbo alone. Tumor growth-delay studies revealed significant increases in the antitumor effect of the alkylating agents when these were given in combination with ETA. Both the ETA/CTX/CDDP and the ETA/CTX/Carbo combinations produced tumor growth delays of 23 days, which represented approx. 1.6-fold increases over those obtained using the alkylating-agent combinations alone. These results suggest that ETA could significantly improve the therapeutic efficacy of these alkylating agents, whether they are given individually or in combination.

Effects of oxygenation and pH on tumor cell response to alkylating chemotherapy

In the present investigations we evaluated the consequence of changing the cellular microenvironment on the treatment efficacy of the alkylating chemotherapeutic agent melphalan. Human A549 adenocarcinoma and mouse KHT/iv sarcoma cells were treated with melphalan under aerobic or hypoxic conditions at pH 6.6 or 7.4. Both low oxygenation and acidic pH individually were found to increase tumor cell killing by this chemotherapeutic agent. However, the magnitude of the enhanced toxic effect was greatest when hypoxic conditions and acidic pH were combined during treatment. For example, A549 cells treated with melphalan under hypoxic conditions at pH 6.6 were approximately 3 times more sensitive to this anticancer drug than were cells exposed in air at pH 7.4. Conditions of low oxygen and pH also increased the chemosensitization potential of the nitroimidazole misonidazole (MISO) when combined with this chemotherapeutic agent. Thus, when KHT/iv cells were treated with the combination of melphalan plus MISO, the resulting enhancement ratio increased from 1.8 to 2.5, when the pH maintained during the treatment was changed from physiologic (7.4) to acidic (6.6).

Enhancement of melphalan (L-PAM) toxicity by reductive metabolites of 1-methyl-2-nitroimidazole, a model nitroimidazole chemosensitizing agent

Chemosensitization of bifunctional alkylators by misonidazole (MISO) and related nitroimidazoles in vitro has been shown to require hypoxic exposures. Presumably, reductive metabolism of the nitroimidazole under hypoxic conditions results in generation of a chemosensitizing intermediate(s) in a manner analogous to that described for the hypoxic toxicity of these compounds. In an attempt to identify these intermediates, we examined the ability of reductive metabolites of a model 2-nitroimidazole compound, 1-methyl-2-nitroimidazole (INO2), to enhance the toxicity of melphalan (t-PAM) in HT-29 human colon cancer cells. INO2 was a modest chemosensitizing agent, enhancing L-PAM only under hypoxic conditions. The 2-electron reduction product, 1-methyl-2-nitrosoimidazole (INO), was a potent chemosensitizer, enhancing L-PAM toxicity at micromolar concentrations under either aerobic or hypoxic conditions. In contrast, the 4- and 6-electron reduction products, 1-methyl-2-[hydroxylamino]imidazole and 1-methyl-2-aminoimidazole, respectively, failed to modify cell kill by L-PAM even at millimolar concentration. These results suggest that nitrosoimidazoles may be the active chemosensitizing species generated upon the reductive metabolism of nitroimidazoles.

Enhancement of chemotherapy and nitroimidazole-induced chemopotentiation by the vasoactive agent hydralazine

Nitroimidazoles have been shown to be potent sensitisers of certain clinically active chemotherapeutic agents. This process of chemopotentiation has been shown to be hypoxia-mediated. The present studies evaluated whether increasing the level of hypoxia in the tumour tissue, by treatment with the vasoactive agent hydralazine, could modify the chemosensitising ability of nitroheterocyclics. Administering either misonidazole or RSU 1164 before, or hydralazine after, the chemotherapeutic agents melphalan, cyclophosphamide or the nitrosourea CCNU, increased the extent of cell kill in both the KHT sarcoma and RIF-1 tumour. However, even greater enhancements could be achieved when hydralazine was used in treatment protocols in which a nitroimidazole was combined with chemotherapy. For example, a 5.0 mg kg-1 dose of hydralazine given 30 min after melphalan, or a 2.5 mmol kg-1 dose of misonidazole administered 30 min before melphalan, increased, compared to melphalan alone, the resultant tumour cell kill by factors of approximately 1.9 and approximately 1.3, respectively. By comparison, when hydralazine was given after the melphalan plus misonidazole combination, treatment efficacy was enhanced approximately 3-fold compared to melphalan alone. Yet in contrast to the results of the tumour response studies, the inclusion of hydralazine did not increase the bone marrow toxicity associated with the chemotherapeutic agent when used alone or in conjunction with a nitroimidazole. The results, therefore, imply that the addition of hydralazine to the chemotherapy, or chemotherapy-sensitiser protocol, led to a therapeutic advantage.

Chemosensitization of the nitrosoureas by 2-nitroimidazoles in the subcutaneous 9L tumor model: pharmacokinetic and structure-activity considerations

Alterations of the pharmacokinetics and cytotoxic effects of the nitrosoureas, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and 1-(2-chloroethyl)-3-(cyclohexyl)-1-nitrosourea (CCNU) by the 2-nitroimidazoles, misonidazole (MISO) and SR-2508 were investigated using the subcutaneous (sc) 9L tumor model in male Fisher 344 rats. When 50 mg/kg of CCNU was given i.p., the peak plasma concentration of CCNU was about 3 micrograms/ml. CCNU was eliminated with biphasic kinetics that had a terminal half-time (T1/2) of approximately 47 min. When 2.5 mmole/kg of MISO was given i.p. 150 min before CCNU, the peak plasma concentration of CCNU was increased by approximately 63% with no change in the elimination kinetics. Clamping did not change the pharmacokinetics of CCNU in either plasma or tumors. MISO pretreatment increased the peak CCNU concentration in unclamped tumors by 3-fold, but had no effect on the CCNU pharmacokinetics in clamped tumors. With the exception of a decrease in the peak BCNU concentration in tumors similar to that observed with MISO, SR-2508 (3.75 mmole/kg, i.p.) did not change the pharmacokinetics of BCNU or CCNU in plasma and tumors. CCNU had no effect on the MISO concentration in plasma and unclamped tumors. However, in the clamped tumors, CCNU delayed the return of the MISO concentration to the unclamped tumor level by about an additional 60 min after the clamp was released. SR-2508 was eliminated from the plasma with biphasic kinetics having an initial and terminal T1/2 of approximately 11 and approximately 76 min, respectively. SR-2508 reached a peak tumor concentration of about 500 micrograms/ml in 30 min. The elimination T1/2 for SR-2508 in unclamped and clamped tumors was approximately 81 and approximately 42 min, respectively. When the clamp was released, the SR-2508 concentration returned to the level found in the unclamped tumors approximately 90 min after it reached its nadir; BCNU and CCNU had no effect on the kinetics of this process. MISO significantly potentiated the cytotoxicity of BCNU in clamped tumors at surviving fractions less than or equal to 0.5. MISO did not potentiate the cytotoxicity of CCNU until the surviving fraction reached 0.05. SR-2508 did not potentiate the cytotoxicity of either BCNU or CCNU.(ABSTRACT TRUNCATED AT 400 WORDS)

The in vivo response of a C3H mammary carcinoma to treatment with misonidazole, cyclophosphamide and radiation

The potential chemosensitizing effect of the nitroaromatic radiosensitizer misonidazole (MISO) on the alkylating agent cyclophosphamide (CTX), and the interactions of these agents with radiation, have been investigated in a C3H mammary carcinoma in CDF1 mice. MISO at 1,000 mg/kg caused a small increase in tumour growth time (TGT; time to reach 3 times treatment volume) from 3.6 days to 4.5 days. CTX (100 mg/kg) increased the TGT to 15.7 days. The combined treatment of MISO and CTX given with intervals of either 15 min or 4 h increased the TGT to 23.3 and 23.8 days respectively. The radiation enhancement ratio (ER) was found to be 2.13 and 1.10 for MISO administered before or after x-rays respectively. The corresponding ERs for CTX were 1.16 and 1.22. The two drugs given in combination resulted in significant radiation ERs of 2.68 (both drugs given within 30 min before x-rays), 3.00 (MISO 30 min before and CTX 3 1/2 h after x-rays) and 1.40 (both drugs given after x-rays). In contrast to what has previously been reported, and in contrast to the tumour regrowth delay data, the results of the tumour control experiments were found to reflect no more than an additive action of the two drugs when used together with radiation in vivo.

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.

Metabolism and action of amino acid analog anti-cancer agents

The preclinical pharmacology, antitumor activity and toxicity of seven of the more important amino acid analogs, with antineoplastic activity, is discussed in this review. Three of these compounds are antagonists of L-glutamine: acivicin, DON and azaserine; and two are analogs of L-aspartic acid: PALA and L-alanosine. All five of these antimetabolites interrupt cellular nucleotide synthesis and thereby halt the formation of DNA and/or RNA in the tumor cell. The remaining two compounds, buthionine sulfoximine and difluoromethylornithine, are inhibitors of glutathione and polyamine synthesis, respectively, with limited intrinsic antitumor activity; however, because of their powerful biochemical actions and their low systemic toxicities, they are being evaluated as chemotherapeutic adjuncts to or modulators of other more toxic antineoplastic agents.

Cytotoxic effect of misonidazole and cyclophosphamide on aerobic and hypoxic cells in a C3H mammary carcinoma in vivo

The chemosensitising effect of the nitroaromatic radiosensitiser misonidazole (MISO) on the alkylating agent cyclophosphamide (CTX) has been investigated in a C3H mammary carcinoma in CDF1 mice. The selective cytotoxicity against aerobic and hypoxic cells was measured indirectly, using a local tumour control (TCD50) assay. The hypoxic fraction was calculated from the dose difference between the TCD50S for tumours irradiated either in air or under clamped conditions. The relative survival of tumour cells after drug therapy was expressed as a surviving fraction (SF). CTX (100 mg kg-1) was found to be considerably more toxic towards hypoxic than aerobic cells (SF 4% versus 52%). MISO (1000 mg kg-1) was almost exclusively toxic to hypoxic cells (SF 22%). When MISO and CTX were administered simultaneously a decrease in the surviving fraction was observed. The effect on aerated cells was found to be 10-fold more than expected from addition of toxicities, suggesting a chemosensitising effect on these cells by MISO when used in combination with CTX. No synergistic effect was found on radiobiologically hypoxic cells. The exact role of hypoxia for the development of chemosensitisation seems to be complex and requires additional research in the future.

Biodistribution of misonidazole and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in rats bearing unclamped and clamped 9L subcutaneous tumors

The biodistribution of misonidazole (MISO) and 1,3bis(2-chloroethyl)-1-nitrosourea (BCNU) was studied using the subcutaneous (s.c.) 9L tumor model in male Fisher 344 rats. A transient hypoxia in these tumors was created by clamping the blood supply to the tumor. Reoxygenation occurred upon release of the clamp. The plasma and tumor concentrations of MISO and BCNU were quantitated by high pressure liquid chromatography. When 12 mg/kg of BCNU was given i.p. without MISO, the peak plasma concentration was about 6 micrograms/ml, and the elimination half-time was about 16 min. When 2.5 mmole/kg of MISO was given i.p. 150 min before the BCNU, the peak plasma concentration of BCNU increased by approximately 33%, and the plasma elimination half-time increased by approximately 57%. Clamping the tumor for 120 min did not significantly change the BCNU concentration in plasma, but in tumors the time to reach the peak level was delayed slightly, and the peak concentration was reduced when compared to that in the unclamped tumors. MISO pretreatment decreased the BCNU peak concentration in both unclamped and clamped tumors, but the decrease was more pronounced in the unclamped tumors. In both unclamped and clamped tumors, the BCNU concentration and its rate of disappearance were identical about 30 min after BCNU administration, with or without MISO pretreatment. The elimination half-time of MISO from the plasma (approximately 142 min) was identical for rats with unclamped or clamped tumors. The half-time for the disappearance of MISO from unclamped tumors was about 98 min. BCNU had no effect on the MISO concentration in plasma and unclamped tumors. MISO disappeared in the clamped tumors with a half-time of about 40 min. When the clamp was released, the MISO concentration returned to the level in the unclamped tumors after about 45 min. BCNU delayed the return of the MISO concentration to the unclamped tumor level by about 60 min. Two conclusions can be drawn from this study. First, the pharmacokinetics of each drug changed when the two drugs were combined. Second, the data indicate that alterations in the tumor BCNU pharmacokinetics are not the major mechanism responsible for the chemopotentiation previously measured in s.c. 9L tumors.

Radiation therapy alone or combined with misonidazole in the treatment of locally advanced non-oat cell lung cancer: report of an RTOG prospective randomized trial

From September 1979 to February 1983, 268 patients with unresectable, locally advanced (RTOG Stage III), non-small cell lung cancer were randomized to receive radiation therapy alone (RT) (50 Gy large field and 10 Gy boost), or combined with misonidazole (400 mg/m2 2-4 hr prior to RT daily for 5-6 weeks to a maximum dose of 12 g/m2 or until tumor progression). One hundred twenty-three patients who received irradiation alone and 116 given RT + misonidazole were evaluable for toxicity, time to tumor progression, and survival as of April 1987. The distribution of patient characteristics was similar in both treatment groups; 59% of the patients had a Karnofsky score of 90 or better, 53% had adenocarcinoma or large cell tumors, and 47% had Stage T3 tumors. Complete tumor regression was reported for 33 (27%) patients treated with radiation therapy alone and 24 (21%) who received misonidazole + RT. Median survival was 8 months with RT alone and 7.4 months with misonidazole + RT. Ninety-five percent of the patients have died. Seventy percent of the patients treated with radiation alone and 77% of those treated with misonidazole + RT died of progressive disease. Three patients treated with radiation alone and two with RT + misonidazole died subsequent to radiotherapy-related pneumonitis or pulmonary fibrosis. There was no significant improvement in response rates, local control, or survival for patients who received daily misonidazole along with irradiation compared with patients treated by irradiation alone.

Misonidazole chemosensitization of EMT6 spheroids to melphalan

Nitroaromatic radiosensitizers are effective chemosensitizers in vitro and in vivo. We have used EMT6 tumour cells grown as multicellular spheroids to try and further understand the role that hypoxia plays in this process. Our results show that the cell killing produced in whole spheroids following a 1-h exposure to melphalan (L-PAM) was enhanced by a 3-h pre-exposure to 5 mM misonidazole (MISO), an enhancement ratio of 1.3-1.7 being obtained. Sequentially disaggregating spheroids we also found that both L-PAM toxicity and MISO chemosensitization were relatively constant as a function of depth within the spheroid. The binding of 14C-MISO to spheroid cells, measured by scintillation counting of disaggregated cells and by autoradiography analysis of sectioned spheroids, demonstrated that binding increased with depth. However, cells in the outer layers of the spheroid bound more 14C-MISO than expected with fully aerobic cells, while in the innermost viable cells the binding was less than that measured in cells which were fully radiobiologically hypoxic. This suggests that the majority of viable spheroid cells were at oxygen tensions intermediate between those found in either fully aerobic or radiobiologically hypoxic cells, yet their levels of oxygenation were sufficiently low for MISO chemosensitization to occur.

The chemosensitizing and cytotoxic effects of RSU 1164 and RSU 1165 in a murine tumor model

RSU 1069, the lead compound in a series of nitroimidazoles containing an alkylating aziridine function, has been shown to be a potent radiosensitizer and chemopotentiator both in vitro and in vivo. However, this agent also demonstrates significant in situ toxicity. Recently it has been shown that less toxic analogues of RSU 1069 can be produced by the introduction of alkyl substituents to moderate the reactivity of the aziridine function. The present investigations were undertaken to evaluate the in vivo cytotoxicity and chemosensitizing efficacy of two such analogues, RSU 1164 and RSU 1165. All experiments were performed with KHT sarcomas grown intra-muscularly. In the cytotoxicity studies, a range of sensitizer doses was utilized whereas in the chemopotentiation investigations a fixed sensitizer exposure was combined simultaneously with a range of doses of the nitrosourea CCNU. In both studies, tumor cell survival was determined 22-24 hr after treatment using a soft agar clonogenic assay. Normal tissue toxicity in the chemopotentiation studies was assessed by bone marrow CFU-S assay. Both analogues were found to be significantly less cytotoxic to KHT sarcoma cells than RSU 1169 (a factor of 4-6 in dose at 50% cell survival). Combining a 1.0 to 2.0 mmol/kg dose of RSU 1164 or RSU 1165 with a range of doses of CCNU increased tumor cell killing by a factor of 1.5-1.6 but did not enhance bone marrow stem cell toxicity. The addition of either sensitizer to CCNU treatment therefore led to a significant therapeutic benefit.

Metabolic binding of misonidazole to mouse tissues. Comparison between labels on the ring and side chain, and the production of tritiated water

The 2-nitroimidazole, misonidazole, is of current interest as an imaging agent for hypoxic regions in tumors and in vascular disease such as stroke. The basis of this technique is the reductive activation and binding of nitroheterocycles which is much more efficient in the absence of oxygen. The appropriate molecular location for an active isotope on the nitroheterocyclic probe depends on the nature of the metabolites retained in tissues after the parent drug has been cleared. Previous studies with tumor cells in vitro indicated that a ring label (2-14C) and a side-chain label (3H) were retained equally efficiently in the acid-insoluble fraction, whereas 1.5 to 3 times more side-chain label was retained in the total pool (acid soluble plus acid insoluble) of metabolites in several normal murine tissues. We show here that the excess side-chain label in six normal tissues, plasma and EMT6 tumors was found entirely in the acid-soluble fraction as a volatile component. This volatile component was tentatively identified as tritiated water. It appeared that, in general, molecular products of misonidazole metabolism were retained in mouse tissues, with the exceptions that a small excess of ring label was found in liver and heart and that tritiated water appeared in the acid-soluble fraction of all tissues. Tritiated water would not be important in imaging studies but could be a factor in studies in which scintillation counting of tritiated nitroheterocyles is used.

Characterization of radiation resistant hypoxic cell subpopulations in KHT sarcomas. (II). Cell sorting

Hypoxic cells in KHT sarcomas were characterized using fluorescence activated cell sorting based on the diffusion properties of the fluorochrome Hoechst 33342. Tumour-bearing female C3H/HeJ mice were injected i.v. with 10 micrograms g-1 Hoechst 33342 and the cells derived from the tumours sorted on the basis of their staining intensities. For each sorted fraction the DNA histogram was evaluated using FCM analysis. The results indicated that the bright and dim cells were not equally distributed about the cell cycle. For example, a greater proportion of S phase cells were in the bright subpopulations whereas the dim subpopulations contained an increased proportion of cells in G1. When the tumours were irradiated with a single dose of radiation prior to cell sorting, the dim cells survived preferentially. Dose response curves for the 20% most dim and 20% most bright cells, sorted on the basis of fluorescence intensity, then were determined. The survival curves of the dim and bright cells were found to have slopes similar to those of KHT cells irradiated in situ in dead animals or in vitro under fully oxic conditions, respectively. In addition, when KHT sarcoma-bearing mice were given a 2.5 mmol kg-1 dose of misonidazole (MISO) prior to irradiation and cell sorting, the dim subpopulation was sensitized whereas the bright subpopulation was not. These findings suggest that (i) compared to well-oxygenated areas, hypoxic regions of KHT tumours contain a smaller percentage of cells actively proliferating and (ii) Hoechst 33342 sorting may allow the detailed in situ evaluation of agents acting directly against hypoxic cells in solid tumours.

Potentiation of the anti-tumour effect of melphalan by the vasoactive agent, hydralazine

The vaso-active drug hydralazine causes a considerable increase in the cytotoxic effect of melphalan towards the KHT tumour in mice. The enhancement in response, measured as the concentration of melphalan required to achieve a given tumour response, is 3.0 and 2.35 when determined using the regrowth delay assay and the technique for determining surviving fraction in vitro following treatment in vivo respectively. In contrast, measurement of systemic toxicity shows that the addition of hydralazine only causes a small increase (ER = 1.15) in melphalan damage. This suggests that the drug combination may have some therapeutic benefit. The tumour specificity for the action of hydralazine is supported by the finding that binding of 3H-misonidazole is increased in tumours but not in other tissues when mice are treated with hydralazine. Increased binding of labelled misonidazole is associated with an increase in the level and duration of hypoxia, which will occur as a consequence of changes in tumour blood flow brought about by hydralazine. However, hypoxia per se is not responsible for the enhanced effect of melphalan, since the agent BW12C, which also induces substantial tumour hypoxia as a result of changing the O2 affinity of haemoglobin, has no effect on melphalan tumour cytotoxicity.

Potentiation of combination chemotherapy by nitroheterocyclics

The effect of including a nitroimidazole in a treatment regimen combining two alkylating chemotherapeutic agents was evaluated in a mouse tumor model. KHT sarcoma-bearing female C3H/HeJ mice were treated with a single dose of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) either alone or in combination with the radiosensitizer misonidazole (MISO) prior to a range of cyclophosphamide (CY) doses. CCNU (5 mg/kg) preceded CY treatment by 3 hr. MISO (1 mmol/kg) was given 1 hr after CCNU (2 hr prior to CY). Tumor response was assessed using either an in vitro to in vivo clonogenic cell survival assay 22-24 hr after treatment or an in situ delay of tumor regrowth assay. The results demonstrated that the inclusion of MISO at a dose of 1.0 mmol/kg increased tumor cell kill resulting from the combination of CCNU-CY by a factor of 1.4-1.5 compared to that seen in the absence of the nitroimidazole. Normal tissue toxicity resulting from the CCNU-CY or CCNU-MISO-CY combinations was determined by measuring bone marrow stem cell (CFU-S or CFU-GM) toxicity. Compared to CCNU-CY alone, the addition of MISO did not enhance this normal tissue toxicity. These findings indicate that the inclusion of MISO in a combination chemotherapy protocol may yield a significant therapeutic benefit.

Applicability of animal tumor data to cancer therapy in humans

The problem of applying experimental tumor studies to clinical cancer therapy is a complex one. The radiotherapy literature contains many examples of premature efforts to apply laboratory observations to the clinic, and many examples of failures to adequately consider animal tumor observations in the design of clinical studies. This review covers three areas: tumor hypoxia, where clinical trials based on animal tumor data have been conducted with radiosensitizers, hyperbaric oxygen, and systemic oxygen carriers; dose fractionation, where current trials of hyperfractionation are based in part on animal tumor studies; and chemo-radiotherapy, where clinical trials are only beginning to exploit concepts developed in animal tumor systems. The use of animal tumor systems extends past the screening of new agents. Animal tumor models can be used in biological, physiological, and pharmacological studies to elucidate the biological factors influencing the efficacy of therapeutic agents. Tumor studies can be combined with studies of normal tissues to predict the toxicities to be anticipated in clinical trials, and to assess the potential for therapeutic gain. Animal studies can also provide data which are useful in designing optimal clinical trials of new agents and maximizing the potential for successful clinical application of new approaches. In general, it is not possible to apply specific laboratory data directly to man. To translate, rather than transpose, information from the laboratory to the clinic, the model studies must be directed at evaluating principles, rather than merely quantifying results. Only through studies of mechanisms, by designing experiments to test or refute a hypothesis, will it be possible to apply model studies to man.