tirapazamine and 1-(2-nitro-1-imidazolyl)-3-aziridino-2-propanol

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.

Topics: Animals; Antineoplastic Agents; Aziridines; Combined Modality Therapy; DNA Damage; Humans; Indolequinones; Indoles; Misonidazole; Neoplasms; Radiation-Sensitizing Agents; Tirapazamine; Triazines

Topics: Animals; Antineoplastic Agents; Antioxidants; Cell Hypoxia; Humans; Misonidazole; Neoplasms; Nitroimidazoles; Prodrugs; Radiation-Sensitizing Agents; Tirapazamine; Triazines

Hypoxic cells exist in solid tumors in regions of poor vascularity and are likely to be exposed to insufficient concentrations of chemotherapeutic agents. Furthermore, because these cells may be cycling very slowly or may be quiescent, they may not be sensitive to agents which are most active in proliferating cells. Under conditions of reoxygenation, hypoxic cells which have survived therapy may re-enter the cell cycle and repopulate a tumor which had shown responsiveness. Three classes of agents have recently been shown to be selectively toxic to hypoxic cells in vitro. For some of these agents, combinations of hypoxic cell-selective drugs and agents with selectivity for well oxygenated cells have demonstrated improved tumor cell kill in solid transplantable rodent tumor systems. The selectivity of these classes of drugs apparently stems from enhanced activation of the drug under hypoxic conditions. Although all of these drugs exhibit selectivity, the wide divergence in the therapeutic ratios for the individual agents suggests that it may be possible to develop newer agents that are highly toxic to hypoxic cells with little toxicity for normal tissues.

Topics: Animals; Antineoplastic Agents; DNA Damage; DNA Repair; Humans; Misonidazole; Mitomycin; Mitomycins; Oxygen; Tirapazamine; Triazines

Bioreductively activated drugs have been used as antimicrobials, chemotherapeutic agents, and radiation sensitizers. The present paper is an overview of their mechanism of action and application in the treatment of cancer.. Drugs such as nitroimidazoles, mitomycins, and benzotriazine di-N-oxides were a focus of this research. Studies have ranged from the chemistry of the reductive process of activation to in vitro and in vivo studies in rodent and human cells, through to clinical testing. The variety of techniques and test systems brought to bear on these compounds is a strength of this field of research.. A detailed chemical understanding of the mechanism of action of a variety of bioreductives is now available. The enzymatic processes by which these drugs are activated and the cofactors involved in this activation are becoming well understood. Recent advances have been made in the design and use of dual-function bioreductives, bioreductive triggers of drug activation, and DNA-targeted bioreductives. Significant success has been demonstrated clinically with bioreductive drugs, used in combination with radiation and front-line chemotherapeutic agents. The areas of antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT) are identified as new directions for bioreductive therapy.. The use of bioreductively-activated drugs for the treatment of cancer has made steady progress. The success obtained clinically and the new molecular approaches currently being implemented promise significant advances in the future.

Topics: Animals; Antineoplastic Agents; Aziridines; Cell Hypoxia; DNA, Neoplasm; Forecasting; Humans; Indolequinones; Indoles; Misonidazole; Mitomycin; Nitrofurans; Nitroimidazoles; Oxidation-Reduction; Prodrugs; Radiation-Sensitizing Agents; Tirapazamine; Triazines

P450 reductase (NADPH: cytochrome c (P450) reductase, EC 1.6.2.4) plays an important role in the reductive activation of the bioreductive drug tirapazamine (SR4233). Thus, in a panel of human breast cancer cell lines, expression of P450 reductase correlated with both the hypoxic toxicity and the metabolism of tirapazamine [Patterson et al (1995) Br J Cancer 72: 1144-1150]. To examine this dependence in more detail, the MDA231 cell line, which has the lowest activity of P450 reductase in our breast cell line panel, was transfected with the human P450 reductase cDNA. Isolated clones expressed a 78-kDa protein, which was detected with anti-P450 reductase antibody, and were shown to have up to a 53-fold increase in activity of the enzyme. Using six stable transfected clones covering the 53-fold range of activity of P450 reductase, it was shown that the enzyme activity correlated directly with both hypoxic and aerobic toxicity of tirapazamine, and metabolism of the drug under hypoxic conditions. No metabolism was detected under aerobic conditions. For RSU1069, toxicity was also correlated with P450 reductase activity, but only under hypoxic conditions. Measurable activity of P450 reductase was found in a selection of 14 primary human breast tumours. Activity covered an 18-fold range, which was generally higher than that seen in cell lines but within the range of activity measured in the transfected clones. These results suggest that if breast tumours have significant areas of low oxygen tension, then they are likely to be highly sensitive to the cytotoxic action of tirapazamine and RSU 1069.

Topics: Antineoplastic Agents; Breast Neoplasms; Female; Humans; Misonidazole; NADPH-Ferrihemoprotein Reductase; Radiation-Sensitizing Agents; Tirapazamine; Transfection; Triazines; Tumor Cells, Cultured

The alkaline comet assay was applied to individual cells from mice exposed to two bioreductive drugs, tirapazamine and RSU 1069, with the goal of comparing DNA damage to tumours and normal tissues. More DNA single-strand breaks (SSBs) and a greater heterogeneity in DNA damage were observed in tumour cells than in spleen and marrow cells of mice exposed to 10-100 mg/kg tirapazamine, consistent with the presence of hypoxic cells and the greater bioreductive capacity of tumours. In mice injected with 25-200 mg/kg RSU 1069, aerobic cells exhibited large numbers of SSBs while toxic DNA interstrand crosslinks were produced only in hypoxic cells. Cells from bone marrow and spleen showed extensive numbers of SSBs, but minimal crosslinking compared to tumours where 10-20% of cells were heavily crosslinked. DNA damage produced by these two bioreductive drugs may be useful in estimating the range of individual cell oxygen contents within tumours and normal tissues.

Topics: Aerobiosis; Animals; Antineoplastic Agents; Bone Marrow; Carcinoma, Squamous Cell; Cell Hypoxia; DNA Damage; DNA, Single-Stranded; Dose-Response Relationship, Radiation; Electrophoresis, Agar Gel; Hydrogen-Ion Concentration; Male; Mice; Mice, Inbred C3H; Misonidazole; Organ Specificity; Oxidation-Reduction; Radiation-Sensitizing Agents; Spleen; Tirapazamine; Triazines

Toxicity from drugs activated by bioreductive metabolism has been suggested as a means to eliminate the treatment resistance caused by hypoxic tumor cells. In general, drugs have been selected to maximize the hypoxic cytotoxicity ratio [exposure (drug concentration x time) in air:exposure in nitrogen] to cause equal toxicity. On this basis, two recently developed drugs have very similar characteristics; an aziridine derivative of misonidazole (RSU1069) and a benzotriazine di-N-oxide (SR4233). The oxygen dependence of the toxic response has not previously been characterized. This report shows that the toxicity from SR4233 extends over a much greater range of oxygen concentrations than does that of RSU1069. Furthermore, unlike all previous drugs studied, the toxicity of SR4233 does not level off at high oxygen concentrations, but continues to decrease as the oxygen concentration increases. For 1 mM oxygen (the solubility of oxygen in medium at 37 degrees C equilibrated with 100% oxygen and water vapor) the toxicity from SR4233 is at least 2000-fold less than that for hypoxia. Modeling the effect of oxygen on combined radiation and toxicity shows that radiation plus SR4233 should be much more effective in eliminating hypoxic cells than radiation plus RSU1069. The unusual oxygen dependence of toxicity by SR4233 may indicate a unique biochemical activation process.

Topics: Animals; Cell Hypoxia; Cell Line; Cricetinae; Cricetulus; Fibroblasts; Misonidazole; Oxygen; Radiation-Sensitizing Agents; Tirapazamine; Triazines

Topics: Antineoplastic Agents; Cell Hypoxia; Humans; Misonidazole; Nitroimidazoles; Oxidation-Reduction; Prodrugs; Radiation-Sensitizing Agents; Tirapazamine; Triazines

The therapeutic potential of a variety of bioreductive agents, including misonidazole, RSU-1069, NFVO, mitomycin C, porfiromycin, and SR-4233 was evaluated using Chinese hamster V79 multicell spheroids in vitro. Fluorescence-activated cell sorting techniques were used to selectively recover cells from various depths within the spheroids to measure the differential cytotoxicity in the cells near the hypoxic core of the spheroid relative to the well oxygenated peripheral cells. At the high cell density found in spheroids (as in tissues in vivo) the differential toxicity observed was typically much less than expected, based on data from single cell systems. In some cases, this was due to lack of sufficient hypoxia in the spheroids; in other cases, drug treatment itself produced reoxygenation through metabolic or toxic effects during treatment. An unexpected observation of considerable concern was rapid bioreduction of the more active agents; this sometimes occurred at rates that exceeded drug delivery, resulting in considerably less efficacy when large hypoxic fractions were present (e.g. mitomycin C, NFVO, and SR-4233). This suggests that induction of hypoxia prior to bioreductive agent therapy may not be the most productive approach. Though none of the agents showed "ideal" properties, porfiromycin was judged to give the best combination of differential toxicity, longevity in situ, and ability to reach the entire hypoxic cell subpopulation.

Topics: Animals; Antineoplastic Agents; Cell Aggregation; Cell Hypoxia; Cell Survival; Cricetinae; Evaluation Studies as Topic; In Vitro Techniques; Misonidazole; Mitomycin; Models, Biological; Nitrofurans; Porfiromycin; Prodrugs; Tirapazamine; Triazines

Various bioreductive drugs that are potent hypoxic cell cytotoxins can also function as effective potentiators of radiation action when administered in vivo post irradiation. There is evidence that a contributory mechanism to this potentiation is enhanced sensitivity to the bioreductive drugs exhibited by cells that are damaged sublethally by radiation.

Topics: Animals; Antineoplastic Agents; Aziridines; Cell Hypoxia; Cell Survival; Combined Modality Therapy; Dose-Response Relationship, Drug; Indolequinones; Indoles; Mice; Mice, Inbred C3H; Misonidazole; Neoplasm Transplantation; Neoplasms, Experimental; Nitroimidazoles; Prodrugs; Tirapazamine; Triazines

The oncogenic transforming potential of a series of bifunctional bioreductive drugs were examined under either aerated or hypoxic conditions to assess the contribution of side chains or nitroreduced products toward their carcinogenic mechanisms. Both the cytotoxicity and transforming effects of these drugs increased as a function of dose under hypoxia. In air and at doses that resulted in comparable cell killing, RSU-1069 and RB-88716 were substantially more oncogenic than RSU-1164 or SR-4233. In nitrogen, the oncogenicity of SR-4233 as a function of survival increased, whereas the transforming effect for the aziridine-containing drugs, RSU-1969 and RB-88716, decreased. These data suggest that, among the drugs examined, the transforming moiety in air is largely a function of the alkylating aziridine group. In hypoxia, the reduction of the nitro-moiety to the corresponding active metabolites may be responsible for much of the transformation observed.

Topics: Animals; Antineoplastic Agents; Aziridines; Cell Hypoxia; Cell Line; Cell Transformation, Neoplastic; Mice; Misonidazole; Neoplasms, Second Primary; Nitrofurans; Prodrugs; Radiation-Sensitizing Agents; Risk; Tirapazamine; Triazines

Hypobaric hypoxia has been used to induce tumor hypoxia for in vivo comparison of the anti-tumor effects of the bioreductive agents SR 4233 (3-amino-1,2,4-benzotriazine-1,4-dioxide), RSU 1069 (1(2-nitro-1-imidazolyl)-3-aziridino-2-propanol), and Nitromin (methylbis(2-chloroethyl)amine N-oxide). BDF mice bearing the T50/80 mammary carcinoma were treated with these agents over a range of doses under normobaric (oxic) and hypobaric (hypoxic) conditions. The time taken for the tumor to double treatment volume (volume doubling time) was used as a measure of anti-tumor effect. Volume doubling time was plotted against log dose and dose response curves were fitted. A dose enhancement ratio (the ratio of drug doses required to give an equivalent anti-tumor effect under oxic and hypoxic conditions) was determined. The dose enhancement ratios for SR 4233 and RSU 1069 were 8.8 and 8.5, respectively, showing that these agents had an equivalent and substantial enhancement of their cytotoxicity when combined with hypobaric hypoxia. For Nitromin, no significant dose response effect was obtained under oxic conditions precluding the calculation of the dose enhancement ratio. SR 4233 was found to have increased systemic toxicity when combined with hypobaric hypoxia, suggesting that it is more readily activated than the other drugs tested. This in vivo test system will allow determination of the dose enhancement ratio for novel bioreductive agents and facilitate their comparison.

Topics: Animals; Biotransformation; Cell Hypoxia; Dose-Response Relationship, Drug; Mammary Neoplasms, Experimental; Mechlorethamine; Mice; Mice, Inbred Strains; Misonidazole; Radiation-Sensitizing Agents; Tirapazamine; Triazines

In this work tumour hypoxia is induced by physically occluding the tumour vascular supply by clamping, or by giving mice 5 mg kg-1 hydralazine. These methods have previously been shown to increase the radiobiological hypoxic fraction in tumours close to 100%. Their effectiveness in potentiating the bioreductive toxicity of: misonidazole (800 mg kg-1), RSU1069 (80 mg kg-1), mitomycin C (5 mg kg-1) and SR4233 (50 mg kg-1) is assessed in the RIF-1 and KHT tumours using regrowth delay as an assay. Clamping alone for 120 min gives little or no response, but when RSU1069 is administered 15 min before clamping, large growth delays result. RIF-1 tumours clamped for 90 or 120 min with RSU1069 give cure rates of 12.5% and 37.5% respectively. Less effect with clamping is seen for the other bioreductive agents. The effect of hydralazine with RSU1069 although significant in the RIF-1 tumour, is modest compared to that for clamping. Small enhancements of toxicity are seen with hydralazine in combination with misonidazole in the RIF-1 tumour and mitomycin C in both tumours. The varying effectiveness of these treatments is attributed to several factors which include the level and duration of hypoxia, concentration and contact time of the bioreductive drugs, the microenvironment of the tumour and the nature of the reductive metabolic pathways available in the different tumour cell types.

Topics: Animals; Antineoplastic Agents; Cell Hypoxia; Constriction; Hydralazine; Mice; Mice, Inbred C3H; Misonidazole; Mitomycin; Mitomycins; Radiation-Sensitizing Agents; Sarcoma, Experimental; Tirapazamine; Triazines

We have investigated the cross-sensitivity of a number of cell lines to three different classes of bioreductive drugs under both aerobic and hypoxic conditions. The cell lines used were selected for their sensitivity to DNA-damaging agents and fall into two groups. One group, MMC cells derived from CHO-K1 cells (Robson et al., 1985), show a range of sensitivities to mitomycin C in air. The second group, irs cells were cloned from V79 Chinese hamster fibroblasts (Jones et al., 1987) and exhibit sensitivity to ionising radiation. The sensitivity of both groups of cells to mitomycin C (MMC), RSU-1069 and SR4233 was assessed under aerobic and hypoxic conditions. No difference in aerobic or hypoxic toxicity of MMC was observed for CHO-K1 or MMC sensitive cell lines (MMC-2 and MMC-3). However, the MMC-resistant cell line (MMCr) was 10 times more sensitive under hypoxic than aerobic conditions. This suggests that MMCr cells lack or are deficient in the enzymes responsible for activating MMC under aerobic conditions compared to other MMC cells. In contrast, differential toxicities of between 3 and 30 have been observed for all CHO cells treated with RSU-1069 and SR4233. Treatment of V79 and irs cells with RSU-1069 and SR4233 also resulted in selective toxicity towards hypoxic cells. Differential toxicities between 50 and 100 were observed for V79 cells. For both RSU-1069 and SR4233, the hypoxic toxicities were similar in V79 and irs cells but in air, the radiation sensitive cells were up to 10 times more sensitive than wild type cells.

Topics: Animals; Antineoplastic Agents; Cell Hypoxia; Cell Line; Cell Survival; Misonidazole; Mitomycin; Mitomycins; Radiation Tolerance; Radiation-Sensitizing Agents; Tirapazamine; Triazines