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Biomedical subjects

S Rockwell

Publications and source records attributed to S Rockwell.

At least 109 records · Page 6Linked to original sources
PubMed1988

Preclinical studies of porfiromycin as an adjunct to radiotherapy.

The bioreductive alkylating agent porfiromycin (POR) is more toxic to EMT6 cells that are hypoxic at the time of treatment than to aerobic cells. The toxicity of POR to hypoxic EMT6 cells in vitro was similar to that of mitomycin C (MC): the aerobic toxicity of POR was considerably less than that of MC. Treatment of cells in vitro with POR before and during irradiation did not sensitize either hypoxic or aerobic cells to X rays; instead, only additive cytotoxicity was produced. In contrast, treatment of solid EMT6 tumors in vivo with POR plus radiation produced supra-additive cytotoxicity, as assessed by analyses of the complete dose-response curves for the killing of tumor cells by radiation alone or by POR alone. The supra-additivity of the combination regimens appeared to reflect the preferential killing by each agent of those tumor cells which were in an environment conferring resistance to the other agent. In contrast, combinations of POR and X rays produced only additive cytotoxicities to marrow CFU-GM. Supra-additive antineoplastic effects were obtained at doses of POR which produced little hematologic or other host toxicity. The complementary cytotoxicities of radiation and POR to cells in different microenvironments in solid tumors and the absence of a similar effect in normal tissue make optimized regimens combining radiotherapy and POR unusually promising for the treatment of solid tumors.

Animals↗
PubMed1987

Correlation between drug uptake and selective toxicity of porfiromycin to hypoxic EMT6 cells.

Mitomycin C and its methylated analogue porfiromycin (Por) have significant potential as adjuncts to regimens presently used for treating solid tumors because of their preferential toxicity to cells existing in an hypoxic environment. An understanding of the factors producing the differential activity of these drugs under aerobic and hypoxic conditions would facilitate the development of new agents of this class. Previous studies have focused on the enzymes that reductively activate the mitomycins and on the interaction of these drugs with DNA; none of these studies has fully explained the differences in cytotoxicity observed under hypoxic and aerobic conditions. The present investigation demonstrates that the rate of Por uptake is directly correlated with cytotoxicity under both aerobic and hypoxic conditions. Uptake of Por into hypoxic cells is more rapid than into aerobic cells at equal drug concentrations. Hypoxic cells also accumulate drug in concentrations well in excess of those in the extracellular medium; this is apparently a reflection of drug sequestration in these cells. This sequestration of Por, which affects the rate and extent of uptake in hypoxic cells, does not take place in aerobic cells. The failure of aerobic cells to sequester drug is evidenced by the very rapid efflux of Por from these cells upon removal of extracellular Por and by the fact that aerobic cells attain a state of equilibrium between the intracellular and extracellular drug concentrations. The findings demonstrate that differences in the uptake and retention of Por are associated with the preferential toxicity of Por to hypoxic cells.

Animals↗
PubMed1987

Tumor hypoxia: its impact on cancer therapy.

The presence of radiation resistant cells in solid human tumors is believed to be a major reason why radiotherapy fails to eradicate some such neoplasms. The presence of unperfused regions containing hypoxic cells may also contribute to resistance to some chemotherapeutic agents. This paper reviews the evidence that radiation resistant hypoxic cells exist in solid tumors, the assumptions and results of the methods used to detect hypoxic cells, and the causes and nature of tumor hypoxia. Evidence that radiation resistant hypoxic cells exist in the vast majority of transplanted rodent tumors and xenografted human tumors is direct and convincing, but problems with the current methodology make quantitative statements about the magnitude of the hypoxic fractions problematic. Evidence that radiation resistant hypoxic cells exist in human tumors is considerably more indirect than the evidence for their existence in transplanted tumors, but it is convincing. However, evidence that hypoxic cells are a significant cause of local failure after optimal clinical radiotherapy or chemotherapy regimens is limited and less definitive. The nature and causes of tumor hypoxia are not definitively known. In particular, it is not certain whether hypoxia is a chronic or a transient state, whether hypoxic cells are proliferating or quiescent, or whether hypoxic cells have the same repair capacity as aerobic cells. A number of new methods for assessing hypoxia are reviewed. While there are still problems with all of the new techniques, some of them have the potential of allowing the assessment of hypoxia in individual human tumors.

Animals↗
PubMed1987

Enhancement of IUdR radiosensitization by low energy photons.

The effect of the photon energy on the radiosensitization produced by iododeoxyuridine (IUdR) was examined using Chinese hamster cells in vitro. Radiosensitization by IUdR was considerably higher for 60 keV photons from 241Am sources than for the 860 keV photons (average energy) from 226Ra sources, under continuous low dose rate conditions applicable to intracavitary brachytherapy (a dose rate of 0.57 Gy/hr). Also, IUdR radiosensitization was higher for 250 kV X rays than for 4 MV X rays under the acute exposure conditions used in external beam radiation therapy (dose rates of 1 to 2 Gy/min). These data support the hypothesis that photons with energies just greater than 32.2 keV, the K-absorption edge of iodine, are more effective in causing cell damage than are photons of other energies, because their absorption results in the production of Auger electron cascades and therefore in the production of high linear energy transfer (LET) radiations.

Americium↗
PubMed1987

Enhancement of tumor radiation response by the combination of a perfluorochemical emulsion and hyperbaric oxygen.

The presence of radioresistant hypoxic cells in tumors is believed to be one of the limiting factors in achieving local tumor control by radiotherapy. Treatment with hyperbaric oxygen during irradiation has been shown to improve the radiation response of many solid tumors in rodents and of some tumors in patients. Intravenous administration of perfluorochemical emulsions combined with oxygen breathing at atmospheric pressure has also been shown to improve the radiation response of several rodent tumors. Theoretical considerations suggest that the combination of a perfluorochemical emulsion and hyperbaric oxygen should be significantly more effective than either agent alone. This hypothesis was tested by examining the radiation response of BA1112 rhabdomyosarcomas growing in WAG/rij-Y rats. Treatment with a perfluorochemical emulsion, Fluosol-DA, plus hyperbaric oxygen (3 Atmospheres O2) significantly increased the radiation response of the malignant cells in these solid tumors. The observed changes in the tumor cell survival curve suggest that the combination of Fluosol-DA and HBO decreases the proportion of severely hypoxic cells in the tumor to less than 1.5% of the original value. The effect of the Fluosol-DA dose and the duration of pretreatment with HBO are described.

Animals↗
PubMed1987

Development of an 241Am applicator for continuous low-dose-rate irradiation of the rat sarcoma BA1112.

An 241Am applicator for continuous low-dose-rate irradiation of the rat sarcoma BA1112 has been developed. The irradiator consists of two disc sources, each containing 800 mCi of 241Am, an isotope which emits primarily 60 keV photons. The disc sources are held in a specially-designed light-weight helmet which surrounds the tumor on the head of the rat. Dose distributions produced by these sources have been measured using an ionization chamber, thermoluminescent dosimeters and Fricke dosimeter. A computerized treatment planning system has been modified to compute dose distributions from 241Am sources, to optimize the design of this applicator. Computed and measured dose distributions for several values of separation between the 241Am discs are presented. A survival curve for cells from tumors irradiated in vivo with this applicator has been determined by an in vitro colony formation technique. The mean lethal dose DO was found to be 720 cGy for an average tumor dose rate of 95 +/- 7 cGy/hr. The major advantages of the 241Am applicator in comparison with the 192Ir applicator used previously for continuous low-dose-rate studies are: a considerably smaller half value layer thickness and the longer half life of the radionuclide. These features make it more suitable for long-term tumor cure studies because of the lower whole body dose to the animal, the availability of relatively constant dose-rate irradiators for many years, the decreased shielding requirements for the animal care facility and the diminished exposure to laboratory personnel involved with the implants on the animals.

Americium↗
PubMed1987

Effect of a perfluorochemical emulsion on the radiation response of BA1112 rhabdomyosarcomas.

The effect of treatment with a perfluorochemical emulsion (Fluosol DA, 20%), carbogen, or the combination of these two agents on the radiation response of BA1112 tumors in WAG/rij rats was examined. Fluosol and carbogen as single agents had only small effects on the tumor cell survival curve. The combination of Fluosol plus carbogen had a larger effect on tumor cell survival, reducing the hypoxic fraction of the tumor from 23 to 1.6%. The amount of sensitization was a function of the Fluosol dose, with maximal augmentation of the radiation response obtained at doses of 7.5-15 ml/kg. Carbogen pretreatments ranging from 5 to 60 min in duration all had similar effects on tumor radiosensitivity. The effect of the perfluorochemical emulsion plus carbogen on the survival of irradiated tumor cells appears to reflect changes in tumor oxygenation, rather than cytotoxic or immunological effects, since the perfluorochemical emulsion (with or without carbogen) had no effect on the viability of cells in unirradiated tumors. These experiments extend previous studies by ourselves and others using mouse tumors to show that the combination of a perfluorochemical emulsion and carbogen breathing can also increase the radiation response of a nonimmunogenic rat tumor.

Administration, Inhalation↗
PubMed1987

Effects of the mode of administration of mitomycin on tumor and marrow response and on the therapeutic ratio.

The antineoplastic and toxic effects of mitomycin (MC) administered as a single pulse injection were compared with the effects of MC given as infusions over 2, 5, or 24 hours and with the effects of the drug administered in fractionated regimens incorporating 2-20 injections. The antineoplastic effects of MC were assessed using EMT6 mouse mammary tumors implanted in BALB/c mice; tumor response was assessed using both cell survival and tumor growth assays. The host toxicity of MC was assessed by measuring the survival of bone marrow stem cells (CFU-S) and by determining LD50/30's (which reflect primarily marrow injury). The survival curve for cells from tumors treated with varying doses of MC was exponential, with no shoulder which might suggest accumulation of sublethal lesions. No evidence was found that the tumor cells repaired MC damage when left in situ for up to 24 hours after treatment (repair of potentially lethal damage) or that the tumor repaired sublethal damage during times of up to 24 hours between drug treatments or during drug infusions. The responses of tumors treated with single injections of MC, with infusions over 2, 5, and 24 hours, with two injections separated by 5 or 24 hours, and with five daily injections were all similar. The survival curve for CFU-S from mice treated with varying doses of MC was exponential, with no shoulder which might suggest accumulation of sublethal lesions. LD50/30's were the same whether MC was administered as single injections, as infusions over 2, 5 or 24 hours, or as five daily fractions. Longer fractionated regimens, in which mice received daily injections, 5 days per week, for up to 4 weeks, produced less host toxicity but also less antineoplastic effect; the therapeutic ratio for the most protracted regimen tested was lower than that for single injections. The therapeutic ratios achieved with infusions and fractionated regimens were never greater than that obtained with a large single dose, confirming clinical impressions that infrequent, large doses of MC are an optimal method of treatment with this drug.

Animals↗
PubMed1986

Effect of some proliferative and environmental factors on the toxicity of mitomycin C to tumor cells in vitro.

Mitomycin C (MC) was more toxic to EMT6 mouse mammary tumor cells in vitro under hypoxia than under aerobic conditions. This was evidenced both in survival curves defining the toxicity of treatment with a constant dose of MC given for different periods of time and in survival curves defining the effect of a 1-hr treatment with different concentrations of MC. Cultures preincubated for different periods from 1 to 6 hr in hypoxia all had similar MC sensitivities. In contrast, cultures preincubated in hypoxia for 4 hr then "reoxygenated" before MC treatment had the same sensitivities as cultures which had never been hypoxic. This shows that the oxygenation at the time of MC exposure, rather than the preincubation protocol, was the parameter determining cell sensitivity. Exponentially-growing (proliferating) and plateau-phase (quiescent) cultures had similar MC survival curves; neither type of culture repaired potentially lethal damage when held for up to 24 hr in HBSS after MC treatment. The sensitivity of the cells increased as the pH of the culture medium was decreased to low values (pH 7.0-6.0), but extracellular pH had no significant effect on the cytotoxicity of MC over the physiologic range (7.0-7.4). These data suggest that differences in the metabolic characteristics of cells that are hypoxic and aerobic during MC treatment, rather than perturbations of the cell proliferation patterns, the PLDR capacity of the cells, or the extracellular pH during the hypoxic incubation, are responsible for the different sensitivities of aerobic and hypoxic cells to MC in vitro.

Animals↗
PubMed1986

Reactions of tumors and normal tissues in mice to irradiation in the presence and absence of a perfluorochemical emulsion.

The effect of pre-treatment with the perfluorochemical emulsion, Fluosol-DA, on the radiation response of normal tissues and EMT6 mammary tumors in BALB/c mice was examined. Pre-treating tumor-bearing mice with .015 ml/g of Fluosol and 30 min of carbogen (95% O2/5% CO2) increased the number of tumor cells killed by irradiation with doses of 2.5-20 Gy; the change in the radiation dose-response curve was consistent with a reduction in the hypoxic fraction. Fluosol did not alter the response of tumors in air-breathing or N2-asphyxiated mice and carbogen alone did not alter the radiation response of this tumor significantly. Carbogen treatments 5-60 min in duration produced similar enhancements of tumor radiosensitivity in Fluosol-treated animals. Pre-treatment with Fluosol plus carbogen also increased the number of tumor cells killed by a fractionated regimen of four 2.5 Gy fractions given over 2 days. Pre-treatment with Fluosol-DA plus carbogen, therefore, increased the antineoplastic effects of radiotherapy in both single-dose and multi-fraction radiation regimens. In contrast, Fluosol did not increase the effect of radiation on the partially committed (CFU-GM) or pluripotent (CFU-S) stem cells of the bone marrow or on the CFU-GM of the spleen. The radiation response of the skin was only slightly enhanced by pre-treatment with Fluosol plus carbogen. These data show that treatment of mice with perfluorochemical emulsions plus carbogen can produce therapeutic gain by improving the radiation response of solid tumors, without producing an equivalent increase in the radiation response of potentially dose-limiting normal tissues. These findings encourage further evaluations of these agents as adjuncts to clinical radiotherapy.

Animals↗
PubMed1986

Perfluorochemicals and hyperbaric oxygen in radiation therapy.

It has been postulated that tumors contain hypoxic cells of decreased radiation sensitivity, which limit curability with radiation therapy. Hyperbaric oxygen has been used in an attempt to improve tumor oxygenation. The nature of the oxygen concentration-radiation sensitivity relationship (oxygen increases the slope of the radiation cell survival curve) suggests that a small number of hypoxic cells, as few as one in one million, would limit tumor curability. Oxygen moves by diffusion from the capillary into the tumor. An increase in partial pressure in the capillary will increase the effective diffusion distance. To improve tissue oxygenation effectively the partial pressure of oxygen in blood must be significantly increased throughout the length of the capillary, in particular at the venous end. Theoretical considerations indicate that hyperbaric oxygen as presently used in radiation therapy, 3 ATA, would lead to only marginal improvement. PartO2 may be as much as 0.8 atm below that of the inspired gas; this plus the consumption of oxygen along the length of the capillary lead to predictions of values for PEnd CapO2 of less than twice normal. Such considerations explain the rather limited success of hyperbaric oxygen with radiation therapy. Thus it is unnecessary to postulate an absence of hypoxic cells to explain this clinical observation. In the presence of perfluorocarbon micelles the non-hemoglobin-bound oxygen carrying capacity of blood is significantly increased. Theoretical considerations predict that the difference between PartO2 and PO2 of the inspired gas should be decreased. Furthermore, the nonhemoglobin-bound oxygen carrying capacity should be adequate to satisfy tissue consumption requirements without unloading hemoglobin, thereby avoiding the "PO2 buffering effect of hemoglobin" and permitting a significant increase in PO2 throughout the capillary length. This effect has been demonstrated using a rodent tumor model.

Animals↗
PubMed1986

Effectiveness and biological effects of techniques used to induce hypoxia in solid tumors.

Acute hypoxia is often induced in rodent tumors during studies of the oxygenation or the therapeutic responses of the tumors, either by clamping the blood supply to the tumors or by asphyxiating the hosts with nitrogen. Analyses of data from such experiments generally assume that these processes have no effects other than the induction of hypoxia and that uniform, severe hypoxia is produced throughout the tumors. The studies described in this report test several aspects of these assumptions using BA1112 rat rhabdomyosarcomas and EMT6 mouse mammary tumors. Both clamping and asphyxiation appear to be effective in producing hypoxia in the tumors. Induction of artificial hypoxia for the times necessary for irradiation was not toxic to the tumor cells and generally did not alter the growth of unirradiated tumors. However, the techniques are not without significant effects. Prolonged clamping produced extensive cytotoxicity. Clamping BA1112 tumors for 30 min and removing the clamp just before irradiation altered the tumor cell survival curve and the TCD50. Furthermore, anesthesia and/or restraint (necessary during clamping) have significant effects on tumors and hosts. The data show that the assumptions underlying the use of clamping and N2-asphyxiation to produce hypoxia for short periods in vivo are generally reasonable for BA1112 and EMT6 tumors, but illustrate the necessity for carefully examining the effects and efficacies of the procedures in each tumor/host system.

Animals↗
PubMed1986

Absence of contact effects in irradiated EMT6-Rw tumors.

Some cells have been reported to show greater resistance to drugs or radiation when growing with close intercellular contacts in spheroids or in solid tumors than when growing with few intercellular contacts in sparse cultures. In some cases this increased resistance reflects an increased capacity of cells in close contact to repair cytotoxic damage. However, not all tumors show contact effects, and in some tumors and spheroids the increased resistance appears to be produced by environmental factors, such as hypoxia, rather than by changes in the repair capacity of the cells. To assess whether EMT6-Rw cells showed increased intrinsic radioresistance when grown as solid tumors, we compared survival curves for cells in exponentially growing monolayers and in solid tumors in BALB/c mice. To avoid complications arising from regional heterogeneity in oxygenation within solid tumors, these irradiations were performed under conditions of uniform, maximal hypoxia. The two survival curves were indistinguishable. Moreover, survival curves for cells suspended from solid tumors, plated at low densities and irradiated immediately, after 5 h of incubation or after 24 h of incubation, were indistinguishable from one another and were indistinguishable from survival curves for cells suspended from exponentially growing monolayers and irradiated immediately using an identical protocol. It therefore appears that contact effects are insignificant for irradiated EMT6-Rw tumors and that the intrinsic radiosensitivity of these cells is similar in culture and in solid tumors.

Animals↗
PubMed1986

Radiosensitization, pharmacokinetics, and toxicity of a 2-nitroimidazole nucleoside (RA-263).

A 2-nitroimidazole nucleoside, 1-(2',3'-dideoxy-alpha-D-erythro-hex-2'-enopyranosyl)-2-nitroimida zole (RA-263), has been investigated for its radiosensitization, pharmacokinetics, and toxicity properties. The in vitro radiosensitization tests against hypoxic Chinese hamster (V-79) cells demonstrated that RA-263 was a more potent radiosensitizer than misonidazole and at 2 mM concentration approached the oxic curve. Significant in vitro radiosensitization activity was also observed in EMT6 mammary tumor cells. The in vitro cytotoxicity data suggested that RA-263 is considerably more toxic to hypoxic cells than misonidazole. The increased cytotoxicity may be related to its higher depletion of nonprotein thiols (NPSH) than misonidazole. The combined effects of radiosensitization and hypoxic cell toxicity were measured by preincubation of the V-79 cells for 4 h under hypoxic conditions before irradiation. The results demonstrated a synergistic response by causing a significant decrease in the extrapolation number with loss of shoulder of the radiation survival curves. The in vivo radiosensitization experiments conducted by the in vivo-in vitro cloning assay with the EMT6 mammary tumor indicate that RA-263 is an effective sensitizer. Pharmacokinetic data suggested that RA-263 was eliminated from plasma by a rapid alpha phase and a slower beta phase with T 1/2 of 36 and 72 min, respectively. The concentration in the brain was approximately one-sixth of tumor concentration, suggesting that RA-263 is excluded from the CNS. Moreover, RA-263 was two times less toxic than misonidazole on equimolar basis by acute LD50 tests. This agent was also significantly less mutagenic than misonidazole in a strain of Escherichia coli.

Animals↗
PubMed1986

Effect of 1-methyl-5-sulfonamide-4-nitroimidazole and 1-methyl-5-bromide-4-nitroimidazole on the radiosensitivity of EMT6 tumor cells.

The compounds 1-methyl-5-sulfonamide-4-nitroimidazole and 1-methyl-5-bromide-4-nitroimidazole are extremely effective radiosensitizers of hypoxic cells in vitro. Preincubation of hypoxic cells with these and related drugs produces greater sensitization than is obtained with short incubations or is expected from the electron affinities of the compounds. The experiments reported here were undertaken to determine whether these agents were effective in sensitizing the naturally occurring hypoxic cells of solid tumors in mice. Neither of the 4-nitroimidazoles produced significant radiosensitization of the hypoxic cells of EMT6 mouse mammary tumors in vivo with any of the single- or multiple-injection regimens tested.

Animals↗
PubMed1985

Chemotherapeutic attack of hypoxic tumor cells by the bioreductive alkylating agent mitomycin C.

Since the cure of solid tumors is limited by the presence of cells with low oxygen contents, we have approached the development of treatment regimens and of new drugs for these tumors by investigating agents which are preferentially bioactivated under hypoxia. Major emphasis has been directed at studying the mode of action of the mitomycin antibiotics, as bioreductive alkylating agents. Using primarily the EMT6 mouse mammary carcinoma as a solid tumor model, we have found that mitomycin C and porfiromycin are preferentially toxic to cells with low oxygen contents. The mitomycin analog BMY-25282 is more toxic to hypoxic cells than are mitomycin C and porfiromycin; however, unlike these antibiotics, BMY-25282 is preferentially toxic to well-oxygenated cells. With these three mitomycins, we have observed a correlation between cytotoxicity to hypoxic cells, the rate of generation of reactive products, and the redox potentials of the drugs. Investigations of the enzymes in EMT6 cells that could possibly activate mitomycin C have revealed that cytochrome P-450 and xanthine oxidase are not present in measurable quantities and therefore are not responsible for activation of mitomycin C. Activities representative of NADPH-cytochrome c reductase and DT-diaphorase are present in these neoplastic cells. Comparison of these enzymatic activities in EMT6, CHO, and V79 cells with the rate of generation of reactive products under hypoxia shows a direct correlation between these two parameters, but there is no quantitative correlation between these two parameters and the amount of cytotoxicity. Use of purified NADPH-cytochrome c reductase and inhibitors of this enzyme demonstrated that NADPH-cytochrome c reductase can activate mitomycin C, but that it is probably not the only enzyme participating in this bioactivation in EMT6 cells. The DT-diaphorase inhibitor dicoumarol was employed to show that this enzyme is not involved in the activation of mitomycin C to a cytotoxic agent. Instead, DT-diaphorase appears to metabolize mitomycin C to a nontoxic product. This property has been exploited to develop a new treatment regimen for solid tumors. Using X-rays to eliminate well oxygenated cells of a solid tumor implant of the EMT6 carcinoma, we have found that the combination of dicoumarol plus mitomycin C is more toxic to hypoxic tumor cells in vivo than mitomycin C alone. Furthermore, knowledge of the biochemical mechanism of mitomycin C activation permits a prediction of which tumors can best be treated with this combination of drugs by measuring enzymatic activities in biopsy specimens.

Animals↗