Biomedical subjects

M A Baker

Publications and source records attributed to M A Baker.

At least 55 records · Page 3Linked to original sources

PubMed1991

Elaboration of cellular DNA breaks by hydroperoxides.

Cellular damage produced by ionizing radiation and peroxides, hydrogen peroxide (HOOH) and the organic peroxides tert-butyl (tBuOOH) or cumene hydroperoxide (CuOOH) were compared. DNA breaks, toxicity, malondialdehyde production, and the rate of peroxide disappearance were measured in a human adenocarcinoma cell line (A549). The alkaline and neutral filter elution assays were used to quantitate the kinetics of single and double strand break formation and repair (SSB and DSB), respectively. Peroxides, at 0.01-1.0 mM, produce multiphasic dose response curves for both toxicity and DNA SSBs. Radiation, 1-6 Gy, produced a shouldered survival curve, and both DNA SSB and DSBs produced in cells x-rayed on ice were nearly linear with dose. The peroxides produced more SSBs than radiation at equitoxic doses. X-ray induced DNA single strand breaks were rejoined rapidly by cells at 37 degrees C with approximately 80% of initial damage repaired in 20 min. Peroxide induced SSBs were maximal after 15 min at 37 degrees C. Rejoining proceeded thereafter, but at a rate less than for x-ray induced strand breaks. Significant DNA DSBs could not be achieved by peroxides even at concentrations 50-fold higher than required to produce SSBs. HOOH treatment of DNA on filters following cell lysis and proteolysis produced SSBs. CuOOH and tBuOOH produced no SSBs in lysed cell DNA. None of the peroxides produced DSBs when incubated with lysed cell DNA. Malondialdehyde was released from cells incubated with organic hydroperoxides, but not HOOH, nor up to 40 Gy of x-rays. HOOH was metabolized three times faster than the organic peroxides. The overall results demonstrate the necessity for a metabolically active cell environment to elaborate maximal DNA strand breaks and cell death at hydroperoxide concentrations of 10(-4) or greater, but prevent strand breaks and stimulate cell growth at 10(-5) M.

Adenocarcinoma↗

PubMed1991

Dexamethasone/ifosfamide/cisplatin/etoposide (DICE) as therapy for patients with advanced refractory non-Hodgkin's lymphoma: preliminary report of a phase II study.

Twenty-two patients with refractory intermediate- or high-grade non-Hodgkin's lymphoma were treated with dexamethasone 10 mg every six hours and ifosfamide 1 g/m2, cisplatin 25 mg/m2, etoposide 100 mg/m2 (DICE), and mesna uroprotection daily x4 every 3 to 4 weeks. Pretreatment with prochlorperazine and metoclopramide was given to prevent nausea and vomiting. Eighteen men and four women, aged 21 to 74 years (median age, 65) have received a total of 64 cycles. Seventeen patients had stage IV, one had stage III, and four patients had stage II disease. Seven patients had B symptoms and 11 had marrow involvement. Only two patients had had more than one previous chemotherapy regimen. Median time from last chemotherapy to DICE was 7 months (range 1 to 41). Two patients who suffered early treatment-related deaths (from sepsis) were classified as nonresponders. Six of 22 patients (27%) achieved complete remission (2 to 22+ months), and 11 (50%) had partial remissions (1 to 8+ months) for an overall response rate of 77%. Median survival has not been reached yet, and 12 patients are alive 1 to 22 months from the start of treatment. Nine patients had nadir granulocyte counts less than 0.5 x 10(9)/L; six required RBC transfusions and five, platelet transfusions. The platelet nadir was less than 50 x 10(9)/L in 13 patients. Four patients had microscopic hematuria, two had grade 3 gastrointestinal toxicity, and one had a transient episode of delirium and blurred vision.

Adult↗

PubMed1991

Unusual findings in two patients with acquired deletions of the long arm of chromosome 5 (5q-).

Two patients with acquired deletions of the long arm of chromosome 5 (5q-) are presented and discussed. For both, clinical and other laboratory tests are highly atypical of patients with haematological malignancy characterized by 5q-. These unusual presentations of the 5q- anomaly further emphasize the heterogeneity of clinical presentations associated with this acquired chromosomal abnormality.

Aged↗

PubMed1991

Influence of hydration state on hormonal responses to exercise in dogs.

The purpose of these studies was to determine how plasma levels of arginine vasopressin (pAVP) are related to workload, plasma osmolality (pOsm), blood volume (BV) and plasma angiotensin II (pAII) in exercising dogs. Measurements were made in dogs running on a treadmill at 7.5 km. hr-1 at slopes of 0, 10% and 20% when they were hydrated ad lib and when they had been deprived of drinking water and also in dogs running on a 20% slope after an IV infusion of hypertonic NaCl. Dehydration increased pOsm by 6.6% and reduced BV by 10% in resting dogs. In dehydrated animals, pAVP, pAII and pOsm were elevated above hydrated levels at rest and during exercise at all three workloads. In hydrated dogs, pOsm rose during exercise at 10% and 20% slopes but pAVP rose above resting levels only at the highest workload and pAII was not affected by exercise. In dehydrated dogs, pOsm and pAVP rose during exercise at 10% and 20% slopes and pAII was elevated at the 20% slope. BV decreased during exercise at the highest workload in both hydrated and dehydrated animals. After hypertonic NaCl, pAVP rose during exercise but pOsm and pAII did not. The results suggest that both osmotic and nonosmotic factors contribute to the release of AVP in exercising dogs and that exercise leads to a leftward shift in the relationship of pAVP to pOSM which could be a result of reduced blood volume.

Angiotensin II↗

PubMed1991

GM-CSF therapy for delayed engraftment after autologous bone marrow transplantation.

Based on previous observations that granulocyte-macrophage colony-stimulating factor (GM-CSF) promotes granulocyte recovery following chemotherapy, we evaluated the effect of recombinant human GM-CSF on hematopoietic progenitors and clinical outcome in six patients with delayed engraftment (greater than 55 days) after high-dose therapy and autologous bone marrow transplantation (ABMT). Three patients responded to a 14-day course of GM-CSF (10 micrograms/kg body weight/day) with at least a sevenfold rise in circulating granulocytes and a corresponding increase in granulopoietic activity in the bone marrow. A fourth patient died of infection on the 8th day of GM-CSF therapy with no evidence of response, and the remaining two, one of whom received a lower dose of GM-CSF (5 micrograms/kg/day), did not respond. There was no change in platelet or red cell transfusion requirements in any patient during the treatment. In two of the three responders, the granulocyte counts returned to pretreatment levels by 4 and 7 weeks after stopping the drug, respectively. We observed a marked increase in marrow-derived as well as in circulating granulocyte-macrophage progenitors (granulocyte-macrophage colony-forming units, CFU-GM) by the end of the 14-day course of GM-CSF in the three responders. There was no change in the frequency of circulating or marrow-derived erythroid (erythroid burst-forming units, BFU-E) or multilineage (multilineage colony-forming units, CFU-GEMM) progenitors. The results indicate that GM-CSF therapy in patients with markedly delayed engraftment after ABMT may stimulate granulopoiesis, but the effect is transient in some patients.

Acute Disease↗

PubMed1990

Microtiter plate assay for the measurement of glutathione and glutathione disulfide in large numbers of biological samples.

By combining the least complicated and expedient methods of sample handling with the sensitivity and specificity of the GSH assay by enzymatic recycling and the small volumes and software capabilities of microtiter plate technology we have devised a rapid, sensitive, and easy assay for GSH and GSSG in biological samples. The assay is sensitive to 5 pmol in sample volumes of 50 microliters, although other volumes could be used. The use of a computer-driven microplate with software capable of linear kinetic data storage and analysis on each well, Maxline series microplate readers and Softmax software, enables the user not only to assay large numbers of samples per day but also to have immediate calculated results. We suggest by examples that measurements of total GSH as well as changes in GSH:GSSG in vitro and in vivo are feasible with this technology.

Animals↗

PubMed1990

Human leukemic myeloblasts and myeloblastoid cells contain the enzyme cytidine 5'-monophosphate-N-acetylneuraminic acid:Gal beta 1-3GalNAc alpha (2-3)-sialyltransferase.

We have examined the role of CMP-NeuAc:Gal beta 1-3GalNAc-R alpha(2-3)-sialyltransferase in fresh leukemia cells and leukemia-derived cell lines. Enzyme activity in normal granulocytes using Gal beta 1-3GalNAc alpha-o-nitrophenyl as substrate was 1.5 +/- 0.7 nmol/mg/h whereas activity in morphologically mature granulocytes from 6 patients with chronic myelogenous leukemia (CML) was 4.2 +/- 1.6 nmol/mg/h (P less than 0.05). Myeloblasts from 5 patients with CML in blast crisis showed enzyme activity levels of 6.5 +/- 2.5 nmol/mg/h. From 2 patients with CML, both blasts and granulocytes were obtained, with higher enzyme activity in the patients' blasts (7.1 nmol/mg/h) than in their granulocytes (4.9 nmol/mg/h) in both cases, suggesting that the increase in enzyme activity is related to the differentiation or proliferation status of the CML cells. However, similarly high enzyme levels were also seen in myeloblasts from acute myeloblastic leukemia patients (5.6 +/- 1.4 nmol/mg/h) and in some acute myeloblastic leukemia-derived cell lines (KG1a and HL60), suggesting that increased levels of this enzyme are not directly correlated with the presence of the Ph1 chromosome. This alpha(2-3)-sialyltransferase activity can also be detected in normal peripheral blood lymphocytes and exhibits increased activity in chronic lymphocytic leukemia cells and acute lymphoblastic leukemia. These data suggest that the level of enzyme activity may vary with growth rate and maturation status in myeloid and lymphoid hemopoietic cells. Finally, we have identified a glycoprotein in acute myeloblastic leukemia cells that serves as a substrate for the alpha(2-3)-sialyltransferase. The desialylated form of the glycoprotein was resialylated in vitro by the purified placental form of this alpha(2-3)-sialyltransferase and exhibits a molecular weight of about 150,000.

Blast Crisis↗

PubMed1990

Activated protein kinase C directly phosphorylates the CD34 antigen on hematopoietic cells.

The CD34 antigen is a human leukocyte membrane protein expressed specifically by lymphohematopoietic progenitor cells. We found that CD34 is a phosphoprotein and therefore examined the regulation of its phosphorylation. Activation of protein kinase C (PKC) enhanced CD34 phosphorylation. The PKC activators, 12-O-tetradecanoylphorbol-13-acetate and bryostatin-1, induced rapid, stoichiometric hyperphosphorylation of CD34 protein in cells, resulting in a 5-fold increase in CD34 phosphorylation. In vitro kinase studies revealed that purified PKC could directly phosphorylate purified CD34. Only serine phosphorylation was detected in the CD34 molecule. Two-dimensional phosphopeptide mapping experiments indicated that PKC induces the phosphorylation of identical serine residue(s) in vitro and in vivo (in KG1 cells). These are newly phosphorylated serine residue(s), which are not detectably phosphorylated in CD34 from exponentially growing KG1 cells. These data indicate that the developmental stage-specific molecule, CD34, is a phosphorylation target for activated PKC. Furthermore, these findings raise the possibility that PKC activation and phosphorylation of the CD34 molecule may play a role in signal transduction during early lymphohematopoiesis.

Antigens, CD↗

PubMed1990

Expression of the CD34 gene in vascular endothelial cells.

All seven of a set of CD34 monoclonal antibodies that recognize epitopes on an approximately 110 Kd glycoprotein on human hemopoietic progenitor cells also bind to vascular endothelium. Capillaries of most tissues are CD34 positive, as are umbilical artery and, to a lesser extent, vein, but the endothelium of most large vessels and the endothelium of placental sinuses are not. Angioblastoma cells and parafollicular mesenchymal cells in fetal skin are also CD34 positive, as are some stromal elements. An approximately 110 Kd protein can be identified by Western blot analysis with CD34 antibodies in detergent extracts of freshly isolated umbilical vessel endothelial cells, and CD34 mRNA is present in cultured umbilical vein cells as well as other tissues rich in vascular endothelium (breast, placenta). These data indicate that the binding of CD34 antibodies to vascular endothelium is to the CD34 gene product, and not to crossreactive epitopes. Despite the presence of CD34 mRNA in cultured, proliferating endothelial cells, the latter do not bind CD34 antibodies. In addition, CD34 antigen cannot be upregulated by growth factors. We conclude that under these conditions, CD34 protein is downregulated or processed into another form that is unreactive with CD34 antibodies. Electron microscopy of umbilical artery, breast, and kidney capillary vessels reveals that in all three sites, CD34 molecules are concentrated on membrane processes, many of which interdigitate between adjacent endothelial cells. However, well-established endothelial cell contacts with tight junctions are CD34 negative. CD34 may function as an adhesion molecule on both endothelial cells and hematopoietic progenitors.

Antibodies, Monoclonal↗

PubMed1990

Diamide induced shift in protein and glutathione thiol: disulfide status delays DNA rejoining after X-irradiation of human cancer cells.

By treating a human tumor cell line with various concentrations of diamide, we explored the relationship between extent and duration of protein and nonprotein thiol oxidation, initiation of DNA double-strand break rejoining after X-rays, and the degree of radiosensitization. We also examined the relationship between protein thiol status and the non-protein thiol, glutathione (GSH). A549 cells were irradiated and incubated postirradiation with 0, 100, 300 or 500 microM diamide for 1 h. The dose of radiation required to give 10% survival decreased from 4.8 Gy to 3.2 Gy with 300 microM and to 2.7 Gy with 500 microM diamide (enhancement ratios of 1.5 and 1.8, respectively) but was not significantly affected by 100 microM diamide. The time of initiation of double-stranded DNA rejoining after X-irradiation (DNA repair) was delayed by 300 and 500 microM diamide. Furthermore, DNA rejoining began only after total cellular protein thiol content recovered to 55% of pretreatment levels for both concentrations. Intracellular GSH/GSSG ratios decreased immediately after diamide addition to less than 1. Large decreases in GSH/GSSG ratio preceded significant loss of protein thiols, but protein-glutathione mixed disulfides accounted for a minor percentage of the total protein thiol oxidized (up to 20%). We believe that diamide-induced protein thiol loss, and not GSH oxidation, is related to the cessation of DNA strand rejoining after X-irradiation, thereby affecting survival.

Azo Compounds↗

PubMed1990

Chronic myeloid leukemia arising in a progenitor common to T cells and myeloid cells.

Until recently, T cells were believed not to be involved in chronic myeloid leukemia. We describe an example of CML in T lymphoblastic crisis with massive generalized lymphadenopathy in which the blasts were CD2(+), CD5(+), and CD7(+), variably CD1(+) and CD3(+), and both responded to and could be induced to produce the T cell growth factor, interleukin-2. Additionally, the blasts were shown to contain the CML-related tyrosine kinase P210bcr-abl rather than the smaller kinase associated with Ph1(+) ALL. Finally, the participation of the T lymphoid lineage in the CML clone was proven by the presence of the same BCR rearrangement in blasts as in granulocytes, suggesting the existence of a bone marrow progenitor common to the T cell and myeloid lineages.

Adult↗

PubMed1989

Intracellular distribution and pharmacokinetics of daunorubicin in anthracycline-sensitive and -resistant HL-60 cells.

Anthracycline-sensitive (HL-60) and -resistant (HL-60/AR) cells, which do not overexpress the P-glycoprotein, each transport and distribute daunorubicin (DNR) into distinct intracellular locations, as visualized by digitized video fluorescence microscopy. At pH 7.4, the fluorescence of DNR in HL-60 cells appears distributed diffusely in both the nucleus and cytoplasm. In contrast, HL-60/AR cells show much less fluorescence in the nucleus and cytoplasm; most of the fluorescence localizes first to the Golgi apparatus and is then gradually shifted to the lysosomes and/or mitochondria. In pharmacokinetic studies, HL-60/AR cells exposed to different extracellular concentrations of [14C]DNR consistently accumulated less radioactive drug than the parent HL-60 cells. Incubation of HL-60/AR cells with sodium azide and deoxyglucose blocked the efflux of [14C]DNR and also prevented the shift of DNR fluorescence from the Golgi apparatus to the lysosomes/mitochondria. The efflux and the intracellular shift of DNR could also be inhibited by lowering the temperature to 18 degrees C, which stops endosomal membrane fusion. When DNR was allowed to accumulate in HL-60 or HL-60/AR cells at pH 5 there was an increase in the proportion of drug fluorescence in the membranes of both HL-60 and HL-60/AR cells; a decrease in the amount of drug retained by HL-60, but not by HL-60/AR cells; and a decrease in the cytostatic effects of DNR on both HL-60 and HL-60/AR cells. These data suggest that DNR resistance is associated with a failure of DNR to pass through membranes and to bind to cytoplasmic and nuclear structures. Instead, most of the drug is taken up by the Golgi apparatus from which it is then shifted to the lysosomes or to mitochondria, or out of the cell.

Antibiotics, Antineoplastic↗

PubMed1989

Role of glutathione and dependent enzymes in anthracycline-resistant HL60/AR cells.

We studied the cellular enzymatic defenses against anthracycline-induced free radical damage in the HL60 human myelogenous leukemia cell line and in its anthracycline-resistant subline, HL60/AR. Intracellular glutathione (GSH) levels and gamma-glutamyl transpeptidase activity were lower in HL60/AR than in HL60 cells. Glutathione-S-transferase (GST) and glutathione peroxidase activities were similar in both cell lines. The intracellular distribution of GSH/GST was visualized by digitized video fluorescence microscopy, utilizing the fluorescent probe monochlorobimane fluorescence microscopy, utilizing the fluorescent probe monochlorobimane (MBCl), which is specifically conjugated to GSH by GST. In HL60 cells stained with the MBCl probe, a bright diffuse cytoplasmic and nuclear fluorescence pattern was observed, whereas in HL60/AR cells, the fluorescence was mostly localized to the Golgi apparatus with a lesser component of diffuse cytoplasmic and nuclear fluorescence. Pretreatment of HL60/AR cells with buthionine sulfoximine (BSO) partially reversed resistance to daunorubicin. This effect of BSO on resistance was associated not only with the abolition of localized MBCl fluorescence to the Golgi apparatus but also with increased intracellular accumulation and retention of daunorubicin. The results of our studies demonstrate that inhibition of GSH synthesis in HL60/AR cells results in significant sensitization to daunorubicin and suggest that changes in the intracellular distribution of GSH/GST and/or increased drug retention may be involved in mediating this effect.

Antibiotics, Antineoplastic↗

PubMed1989

In vitro hepatotoxicity of SR 4233 (3-amino-1,2,4-benzotriazine-1,4-dioxide), a hypoxic cytotoxin and potential antitumor agent.

SR 4233 (3-amino-1,2,4-benzotriazine-1,4-dioxide) is presently undergoing investigation as an antitumor agent because of its high selective toxicity for hypoxic cells in vitro and in vivo. It has been found to be 15 to 200 times more toxic to hypoxic rodent and human cell lines than their normoxic counterparts. We investigated the toxicity of SR 4233 in primary cultures of hepatocytes under various oxygen tensions, ranging from 1% to 20% oxygen. The 50% lethal dose of SR 4233 was found to be 50 times lower in hepatocyte monolayers at 1% O2 versus 20% O2. Even at 4% O2, a concentration that prevails in the pericentral area of the liver under conditions of normal blood flow, SR 4233 was an order of magnitude more toxic than at 20% O2. All samples were analyzed for metabolites, and metabolism was found to be dependent on both the SR 4233 concentration and the oxygen tension. Formation of the major metabolite SR 4317 occurred to the greatest extent at the lowest oxygen concentration and the highest SR 4233 concentration. Very little metabolism occurred at 10 to 20% O2, which is in agreement with data in Chinese hamster ovary cells under aerobic conditions.

Animals↗

PubMed1989

Thermal panting in dehydrated dogs: effects of plasma volume expansion and drinking.

Dehydrated mammals reduce thermoregulatory evaporation and regulate deep body temperature (Tb) at elevated levels when they are exposed to heat. These experiments were designed to study the effects of plasma volume (PV) replacement and the effects of drinking H2O or 0.9% NaCl on thermal panting in dehydrated dogs resting at Ta 40-41 degrees C. Five dogs were studied when they were hydrated (control) and in 4 different experiments when they had been dehydrated (D) by removal of drinking water: (1) D, no treatment; (2) D + intravenous dextran infusion; (3) D + drinking H2O; (4) D + drinking NaCl. Compared to controls, dehydrated dogs had lower respiratory frequency (f), higher Tb, lower PV and blood volume (BV) and higher plasma osmolality (pOsmol). Intravenous dextran infusion restored BV to the control level without reducing pOsmol; but f and Tb remained at dehydrated levels after the infusion. When the dogs drank either H2O or NaCl,f increased significantly during the first minute after drinking. pOsmol decreased by 6 min after drinking H2O but did not change after drinking NaCl. BV measured 5 min after the end of drinking H2O was not different from dehydrated BV. It is concluded that (1) Restoration of BV to hydrated levels in dehydrated, heat-exposed dogs with elevated pOsmol does not restore f or Tb to hydrated levels; (2) There is a rapid recovery of thermal panting after drinking which is not dependent on changes in pOsmol or in BV.

Animals↗

PubMed1989

Structure-activity relationships for benzotriazine di-N-oxides.

SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide) is a bioreductive agent that selectively kills and radiosensitizes hypoxic mammalian cells in vitro and murine tumors in vivo. In an attempt to better understand the mechanism of action of the drug, and to determine whether a superior analog may exist, 15 benzotriazine-di-N-oxide analogs of SR 4233 have been evaluated to date for the following properties: hypoxic and aerobic toxicity toward CHO cells in vitro, drug-induced stimulation of oxygen consumption by incubation with respiration-inhibited cells, and acute LD50 evaluated in BALB/c mice. We noted several correlations between these biological properties of the drugs and some of their physicochemical characteristics. Both the hypoxic cytotoxicity and stimulation of oxygen consumption by respiration-inhibited cells were positively correlated with E1/2, the polarographic half-wave reduction potential, and a measure of electron affinity. The air-to-nitrogen differential cytotoxicity reached a maximum (corresponding to SR 4233) and then declined with increasing E1/2. The acute LD50 of each analog in mice decreased with increasing E1/2. One new compound, SR 4482, was found to be more toxic to hypoxic cells in vitro, but less toxic to mice, than SR 4233. It is similar in structure to SR 4233, but lacks any substituent in the 3-position of the triazine ring. This promising drug may represent a member of a new subseries of 1,2,4-benzotriazines with different structure-activity relationships.

Animals↗

PubMed1989

Effects of dehydration and rehydration on thermoregulatory sweating in goats.

1. Measurement of rectal temperature (Tr), sweat rate, respiratory frequency (f) and respiratory evaporation (Eresp) were made in one Nubian and four Alpine-Toggenberg goats while they stood for 90 min in a climate chamber at 40 degrees C ambient temperature (Ta). The animals were studied when they were hydrated, when they had been dehydrated by 48 h water deprivation, and when they were rehydrated by voluntary drinking of water or saline or by intraruminal water administration. Plasma osmolality (Posm), plasma protein concentration (PP) and haematocrit (Hct) were measured before every experiment and before and after voluntary drinking. 2. Hydrated animals increased evaporation by panting and sweating during heat exposure and Tr rose about 1 degree C. The rate of sweating was as high or higher than Eresp. Dehydrated animals had lower sweat rates and higher Tr than hydrated animals, but f and Eresp were the same in hydrated and dehydrated animals. 3. When dehydrated goats were allowed to drink after 60 min of heat exposure, sweating began abruptly within 3 min of the start of drinking in every animal whether water or saline was drunk. Sweat rate returned to hydrated levels or higher before any change occurred in Posm, PP or Hct. Respiratory frequency was higher after drinking than in dehydrated animals which were not allowed to drink. 4. When water was administered by rumen tube after 60 min of heat exposure, sweating in the Nubian occurred with a short latency, similar to the onset after drinking. In the other four animals, sweating onset occurred on average at 13 min 42 s after intraruminal water administration. 5. It is concluded that sweating is a significant avenue of evaporative heat loss in these goats when they are hydrated and exposed to high Ta. Sweat rate is markedly reduced after water deprivation but returns to hydrated levels within 3 min after the start of drinking. The rapid recovery of sweating after voluntary drinking is not initiated by changes in Posm or in blood volume and does not appear to depend upon osmoreceptors in the mouth or gastrointestinal tract since it occurs after drinking either water or saline. The arrival of water in the rumen may be sufficient to initiate immediate sweating in some goats, but the act of drinking is necessary in others.

Animals↗

PubMed1988

Metabolism of SR 4233 by Chinese hamster ovary cells: basis of selective hypoxic cytotoxicity.

The metabolism of SR 4233 (3-amino-1,2,4-bentotriazine-1,4-dioxide), recently reported as highly toxic to hypoxic cells in vitro, was studied by using suspensions of Chinese hamster ovary cells. The rates of formation of two known reduction products, the 1-oxide and the unoxygenated 3-aminobenzotriazine, were measured in aerobic and hypoxic cell suspensions for drug treatments producing both hypoxic and aerobic cytotoxicity. Formation of the 1-oxide and a small amount of the 3-aminobenzotriazine occurred preferentially in hypoxic suspensions. These metabolites were relatively nontoxic to either aerobic or hypoxic cells, implying another mechanism of toxicity. The activation of SR 4233 by single electron transfer, hypothetically forming a toxic drug radical, was explored. Aerobic stimulation of oxygen consumption in respiration-inhibited cells and malondialdehyde release from aerobic cells in the presence of SR 4233 indicated the formation of active oxygen species during drug activation. Increased malondialdehyde release in hypoxic cells and its attenuation by the hydrogen donor, dimethylthiourea, implied the presence of an oxidizing radical. Unlike the nitroimidazole, misonidazole, hypoxic metabolism of SR 4233 did not deplete intracellular glutathione or result in increased binding of drug metabolites to cellular macromolecules. These results are consistent with macromolecular damage caused by an oxygen sensitive, nonbinding, drug-free radical intermediate with oxidizing properties as the mechanism of selective hypoxic toxicity of SR 4233.

Animals↗