Biomedical subjects

P Workman

Publications and source records attributed to P Workman.

At least 145 records · Page 8Linked to original sources

PubMed1990

Doxorubicin-3'-NH-oestrone-17-oxime-ethyl-carbonyl, a doxorubicin-oestrone conjugate that does not redox cycle in rat liver microsomes.

In summary doxorubicin-3'-NH-oestrone-17-oximethyl-carbonyl (Dox-Oes) is a covalent adduct of the anthracycline antitumor agent doxorubicin and oestrogen. Dox-Oes does not generate free radicals in rat liver microsomes as detected by electron spin resonance spectroscopy or redox cycle as shown by lack of superoxide anion formation and NADPH oxidation. Furthermore Dox-Oes actually inhibits free radical formation by doxorubicin used in equimolar amounts. The lack of free radical formation by doxorubicin when covalently linked to oestrone supports the development of Dox-Oes as a non-cardiotoxic derivative whilst potentially improving its targeting to oestrogen positive breast tumour cells.

Animals↗

PubMed1990

Decreased NADPH:cytochrome P-450 reductase activity and impaired drug activation in a mammalian cell line resistant to mitomycin C under aerobic but not hypoxic conditions.

Mitomycin C (MMC) is regarded as the prototype bioreductive alkylating agent in clinical use. To elucidate the biochemical basis of MMC resistance, we isolated a drug resistant derivative (designated CHO-MMC) of a Chinese hamster ovary cell line (CHO-K1) by exposure to progressively higher concentrations of MMC. CHO-MMC cells exhibited a 17-fold increase in resistance to MMC and were 33-fold cross-resistant to the monofunctional derivative, decarbamoyl mitomycin C. In contrast, CHO-MMC cells showed only a 2-fold level of resistance to BMY 25282, a more easily activated analogue of MMC, and exhibited parental sensitivity to MMC under radiobiologically hypoxic conditions. CHO-MMC cells showed no increased resistance to a range of DNA damaging agents including several other alkylating agents (e.g., melphalan and methyl methanesulfonate). Cross-resistance to drugs associated with the multidrug resistant phenotype (e.g., Adriamycin and vincristine) was present only at very low levels. Using a specific high performance liquid chromatography technique, we examined the rates of reduction of MMC and BMY 25282 in cell extracts from CHO-K1 and CHO-MMC cells under both aerobic (air) and hypoxic (N2) conditions. Reduction rates for both drugs were at least 30-fold faster under nitrogen than in air. Metabolism of MMC was undetectable in air but was readily detectable under nitrogen and was 2- 3-fold slower in CHO-MMC cell extracts than in CHO-K1 cell extracts. Although BMY 25282 was more readily reduced under nitrogen, no difference was detected between extracts from CHO-K1 or CHO-MMC cells in the rate of reduction of BMY 25282 under either air or nitrogen. The activity of NADPH:cytochrome P-450 (cytochrome c) reductase, an enzyme implicated in the bioreductive activation of MMC, was 3-4-fold lower in CHO-MMC cells than in the parental line. These findings suggest that the resistance of CHO-MMC cells to MMC under aerobic conditions may be due to impaired metabolic activation of the drug as a result of a decrease in NADPH:cytochrome P-450 reductase activity. This supports the view that decreased bioreductive enzyme activity may be a significant mechanism for acquired resistance to MMC in tumor cells in vivo and that more readily activated analogues may be potentially useful in overcoming this specific form of resistance.

Aerobiosis↗

PubMed1990

Enzymology of the reductive bioactivation of SR 4233. A novel benzotriazine di-N-oxide hypoxic cell cytotoxin.

SR 4233 (3-amino-1,2,4-benzotriazine-1,4-dioxide) is a novel benzotriazine di-N-oxide which shows unusually high selective toxicity towards hypoxic cells, probably as a result of reductive bioactivation. Using an HPLC assay for the parent drug and its 2- and 4-electron reduction products (SR 4317 and SR 4330, respectively), we have examined the enzymology of SR 4233 reductive metabolism in vitro using a variety of different enzyme preparations. SR 4233 was converted extremely rapidly to SR 4317 under N2 by mouse liver microsomes, and showed a marked preference for NADPH over NADH as a reduced cofactor. The reaction was inhibited completely in air and boiled preparations. It was also inhibited by 78-86% in carbon monoxide (CO), implicating cytochrome P-450 as the major microsomal SR 4233 reductase. The kinetics of reductive metabolism of SR 4233 to SR 4317 by mouse liver microsomes conformed to Michaelis-Menten kinetics, with a Km of 1.4 mM and a Vmax of 950 nmol/min/mg protein. SR 4233 reduction was also catalysed by mouse liver cytosol under N2. However, rates were markedly slower than for microsomes and showed an equal dependency on NADH and NADPH. The cytosolic enzymes aldehyde oxidase and xanthine oxidase both catalysed SR 4233 reduction to SR 4317 under N2. Purified buttermilk xanthine oxidase also catalysed this reaction. In contrast to other enzyme preparations, DT-diaphorase from Walker 256 tumour cells reduced SR 4233 predominantly to SR 4330, and this reaction occurred under aerobic conditions. These data illustrate that SR 4233 is reduced rapidly by a wide variety of reductases. We propose that the therapeutic selectivity of SR 4233 will be controlled by the relative expression of reductases in tumour versus normal tissues, and in particular by the differential participation of putative activating versus detoxifying enzymes.

Aldehyde Oxidase↗

PubMed1990

Multiparametric analysis of cell membrane permeability by two colour flow cytometry with complementary fluorescent probes.

We describe an improved twin-probe multiparameter flow cytometric technique to examine cell membrane permeability. Ability to retain preloaded intracellular bis-carboxyethyl carboxy fluorescein (BCECF, green fluorescence) and to exclude extracellular propidium (red fluorescence) is measured, simultaneously with forward and right-angle scatter. This has significant advantages over an earlier method using fluorescein together with ethidium. In addition to the two expected cell populations which were stained green positive, red negative (by convention membrane "intact" and "viable," Region 1) and green negative, red positive ("membrane-damaged" and "non-viable," Region 3), a third population was seen which fluoresced neither green nor red and displayed intermediate light scatter characteristics (Region 2). This was true for each of 9 cell types in vitro. For EMT6 mouse mammary tumour cells held under sub-optimal conditions or treated with membrane-active drugs, progression from Region 1 to Region 2 was observed, followed by further progression from Region 2 to Region 3. Cells eventually accumulated in Region 3. These results suggest that sequential changes in membrane structure lead to increased permeability, first with respect to intracellular BCECF and in turn to extracellular propidium.

Animals↗

PubMed1990

9-Alkyl, morpholinyl anthracyclines in the circumvention of multidrug resistance.

The intramolecular combination of 9-alkyl substitution in the anthracycline A-ring plus incorporation of the amino group of the daunosamine sugar within a morpholinyl ring led to the retention of almost complete activity against P-glycoprotein positive, multidrug resistant variants of a mouse mammary tumour line and a human small cell lung cancer line. Resistance factors were close to unity. These structural elements may prevent efflux by the P-glycoprotein multidrug transporter. The use of 9-alkyl, morpholinyl anthracyclines with resistance circumvention properties may have clinical application.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1990

Abnormal clinical pharmacokinetics of the developmental radiosensitizers pimonidazole (Ro 03-8799) and etanidazole (SR 2508).

The hypoxic cell radiosensitizers Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole) are under evaluation as single agents (Phase III) and in combination (Phase I). Ro 03-8799 produces an acute, transient central nervous system syndrome, whereas SR 2508 causes cumulative, peripheral neurotoxicity; both effects are dose-limiting. Pharmacokinetic studies have shown the importance of area under the plasma drug concentration versus time curve (AUC) in predicting the risk of peripheral neuropathy. Most patients have very similar pharmacokinetic parameters. This study reports 2/25 patients receiving 0.75 g/m2 Ro 03-8799 plus 2.0 g/m2 SR 2508 who showed significant discrepancies in drug handling. One patient exhibited a markedly elevated AUC and prolonged t1/2 beta for SR 2508 and this was associated with an unusually rapid onset of peripheral neuropathy. A second patient showed normal handling of SR 2508 but prolonged values for both t1/2 alpha and t1/2 beta for Ro 03-8799 and unusually low levels of its N-oxide metabolite. In addition a low peak Ro 03-8799 concentration combined with a very high volume of distribution was found in this patient, leading to a normal AUC value and toxicity profile. Both patients exhibited a relatively low creatinine clearance. The mechanisms which may underlie these findings are discussed, and the importance of pharmacokinetic monitoring in the use of these agents is emphasized.

Aged↗

PubMed1990

The cell membrane and cell signals: new targets for novel anticancer drugs.

In the concluding Discussion session, emphasis focussed on the potential for interfering selectively with cell membranes and cell signalling in tumour as against normal tissues. There could be no doubt that tremendous advances are being made in our understanding of the molecular changes associated with malignancy and that the information available for the rational design of inhibitors of particular signalling pathways is increasingly sophisticated. There was a consensus that we need more information on the qualitative and quantitative differences in the structure and function of membranes and the signalling machinery in various normal tissues as compared to their cancerous counterparts. Ideally we will develop drug against, for example, specific forms of, let us say, protein kinase C or tyrosine kinase which are found to be predominantly active in neoplastic cells. This may well prove possible, at least in some instances, in which case a safe therapeutic margin will be assured. But differences may in other situations turn out to be in the level of expression rather than purely qualitative in nature, and the scale of the disparate expression may not always be great. Even in such situations, adequate therapeutic selectivity may still be achieved. This may derive from a "damping down" of signalling in the hyperactive tumour. Although there are legitimate concerns regarding the possible toxic effects of administering signal-wrecking molecules in man, we should not be pessimistic as there are clear precedents elsewhere in medicine for drugs acting on membrane signals proving to be safe and effective against expectation informed by hindsight. There may also be concerns about new forms of drug resistance. But this will be so for any new agent or novel target. And with mechanism of action clearly to the fore we should be able to predict resistance pathways in advance and devise appropriate circumvention strategies or targeted second line therapies. There was a palpable buzz at the meeting that this is a valid, different and above all rational approach. Not only that, but the new therapeutic molecules which we discover will themselves prove to be valuable tools with which to probe further into the mechanisms of malignancy and signal transduction. We had expected to see a bewildering amount of new information from the basic sciences of molecularbiology and cell physiology, and we got it. But it was also impressive to witness the number of new compounds coming through which look like real drugs or at least exciting lead compounds. The membrane-active ether lipids are in clinical trial. Bryostatin 1 will shortly join them.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1989

Improved cellular accumulation is characteristic of anthracyclines which retain high activity in multidrug resistant cell lines, alone or in combination with verapamil or cyclosporin A.

We have examined the cellular accumulation of anthracycline compounds, alone or in conjunction with resistance modifiers, in an attempt to identify mechanisms by which multidrug resistance (MDR) can be circumvented. This was facilitated by using the EMT6 mouse mammary tumour cell line EMT6/P and its MDR subline EMT6/AR1.0 with 30-fold resistance to Adriamycin (ADM), and the human small cell lung cancer line H69/P together with its MDR subline H69/LX4 with 100-fold resistance to ADM. Both MDR lines hyperexpress membrane P-170 glycoprotein. The accumulation of ADM was compared to that seen for the anthracycline analogues aclacinomycin A (ACL), Ro 31-1215 and 4'-deoxy-4'-iodo-Adriamycin (iodo-ADM). These analogues were selected because of their high activity against MDR sublines, including H69/LX4 and EMT6/AR1.0. Both MDR cell lines exhibited a deficiency in ADM accumulation compared to the parent lines. Smaller differentials were seen using Ro 31-1215 or iodo-ADM. Both resistant sublines were able to accumulate ACL in identical amounts to their respective parental sublines. Improved drug accumulation is likely to contribute to the improved activity of the analogues against MDR cell lines. However, the relative accumulation defects in the resistant lines did not correlate exactly with the degree of resistance to a particular compound. Cyclosporin A (5 micrograms/ml) or verapamil (3.3 micrograms/ml) caused a preferential increase in uptake in both MDR sublines, with a small or negligible effect for the parental line. A smaller effect was observed with iodo-ADM and Ro 31-1215, and levels of ACL were unchanged in the MDR lines in the presence of either resistance modifier. These results indicate two mechanisms for circumventing drug resistance due to reduced drug accumulation. Structurally modified derivatives can partially or completely eliminate uptake differentials between parent and drug resistant cell lines. Any residual uptake can be eliminated using resistance modifiers. The two mechanisms may both operate via inhibition or circumvention of P-170 mediated efflux. The situation is complex, however, and this study indicates the possible involvement of additional resistance mechanisms.

Aclarubicin↗

PubMed1989

Stimulation by localized tumor hyperthermia of reductive bioactivation of 2-nitroimidazole benznidazole in mice.

We have investigated the effects of localized tumor hyperthermia (LTH; 43.5 degrees C x 30 min) on the reductive bioactivation of the 2-nitroimidazole benznidazole in C3H mouse normal tissues and KHT tumors. Mice were allocated to one of three treatment groups: (a) unrestrained controls, (b) sham tumor treatment, and (c) LTH. Concentrations of benznidazole and its amine metabolite were determined by high-performance liquid chromatography. Conscious mice were given LTH or sham treatment 2.5 h after 2.5 mmol/kg benznidazole i.p. This gave steady-state plasma benznidazole concentrations of 120-170 micrograms/ml at 2-5 h in all three groups. Plasma amine concentrations were very low at 0.1-1 micrograms/ml in all cases. Liver benznidazole concentrations were similar to plasma but amine concentrations were 30-40-fold greater at 20-40 micrograms/g in all three groups, implicating the liver as a major site of reductive metabolism. Benznidazole concentrations in tumors from unrestrained mice were comparable to those in plasma and liver, with tumor/plasma ratios of 85-113%. Tumor amine concentrations were intermediate at about 2-3 micrograms/g, indicating reductive bioactivation had occurred. Sham treatment decreased tumor benznidazole concentrations by 25-50%, particularly at later times, and amine concentrations were correspondingly increased. This may be a result of sham tumor treatment at 37 degrees C, a temperature 3-4 degrees C higher than in unrestrained controls. More importantly, LTH further decreased tumor benznidazole concentrations over sham treatment, e.g., by 59% from 114 to 47 micrograms/g (P less than 0.01) immediately after heating. Amine concentrations were correspondingly elevated, e.g., by 40% from 5.1 to 8.4 micrograms/g (P less than 0.01). These results clearly show that LTH can selectively enhance the reductive bioactivation of benznidazole in KHT tumors in mice, and support a particular role for the use of bioreductive agents with heat.

Amines↗

PubMed1989

Effects of elevated temperature on misonidazole O-demethylation by mouse liver microsomes: kinetic and stability studies of a model mixed-function oxidase reaction.

We investigated the effects of a range of temperatures (33 degrees-44 degrees C) on the stability and kinetics of C3H mouse liver microsomal misonidazole (MISO) O-demethylase in vitro. Microsomal O-demethylase activity was stable for 60 min at 37 degrees C and for 30 min at 41 degrees C but was steadily inactivated with longer incubation times. Inactivation at 44 degrees and 47 degrees C was exponential, with half-lives of 41 and 11 min, respectively. MISO O-demethylation followed Michaelis-Menten kinetics from 33 degrees to 44 degrees C. The apparent Vmax for desmethylmisonidazole (Ro 05-9963) formation was decreased by 32% (from 2.14 to 1.47 nmol min-1 mg-1 protein) with a 4 degrees decrease from 37 degrees to 33 degrees C. An increase of 4 degrees from 37 degrees to 41 degrees C enhanced the Vmax by 47%, but there was only an additional 9% increase for a further 3 degrees rise to 44 degrees C. Apparent Km values were unaltered at about 1.6 mM. These results show that elevated temperatures in the clinically relevant hyperthermia range (41 degrees-44 degrees C) can enhance a model mixed-function oxidase reaction in vitro. Such effects may be important for the metabolism, activity and toxicity of anticancer drugs combined with hyperthermia in vivo.

Animals↗

PubMed1989

Identification of anthracyclines and related agents that retain preferential activity over adriamycin in multidrug-resistant cell lines, and further resistance modification by verapamil and cyclosporin A.

A range of anthracyclines and related compounds were evaluated for activity against murine and human cell lines exhibiting multidrug resistance (MDR). Cell lines used were the NCI-H69 human small-cell lung cancer line and the EMT6 murine mammary tumour line, together with their multidrug-resistant counterparts produced by in vitro exposure to Adriamycin (ADM). Chemosensitivity testing was carried out using the tetrazolium (MTT) dye assay. Results were expressed as the ratio of the ID50 for the resistant line to that obtained in the parent, i.e. the resistance factor (RF). Compounds exhibiting much lower RF values than ADM in both resistant cell lines were identified as those anthracyclines with 9-alkyl substituents and those with certain changes to the amino sugar residue at position 3' and 4', together with the anthracenedione mitoxantrone (MIT). In a further attempt to overcome resistance, we used four of these compounds, Ro 31-1215, 4'-deoxy-4'-iodo-ADM (iodo-ADM), aclacinomycin A (ACL) and MIT (all yielding low RF values), in combination with the resistance modifiers verapamil (VRP) and cyclosporin A (CYA). Additional enhancement of chemosensitivity was achieved in the ADM-resistant sublines, as shown by the further decrease in RF values. At the concentrations used, the largest effects were generally seen with CYA, and the combination of this modifier with ACL and MIT was particularly effective. For the H69/LX4 resistant line, the latter combinations gave RF values approaching unity. These findings point to the use of analogues with the 9-alkyl substituent and/or specific changes to the sugar residue in combination with resistance modifiers as a therapeutic strategy for circumvention of the MDR phenotype.

Animals↗

PubMed1989

Inhibition of cellular esterases by the antitumour imidazotetrazines mitozolomide and temozolomide: demonstration by flow cytometry and conventional spectrofluorimetry.

Using flow cytometry and conventional spectrofluorimetry we have previously shown that chloroethylnitrosoureas (CNUs) can exhibit marked inhibition of cellular enzymes catalysing hydrolysis of fluorescein diacetate (FDA). More potent inhibition was seen for the carbamoylating CNUs, whereas alkylating agents were largely inactive. We now report results obtained with the developmental imidazotetrazines mitozolomide and temozolomide in comparison with BCNU, the novel alkylating agents clomesome and cyclodisone, and the active mitozolomide metabonate MCTIC. Inhibition of EMT6 mouse mammary-tumour esterases was seen for mitozolomide and temozolomide, and activity against purified porcine carboxylesterase was demonstrated. Flow cytometric analysis showed that inhibition occurred across the entire EMT6 cell population, with no evidence of a subpopulation resistant to enzyme inhibition. Inhibitory potency for the imidazotetrazines was much weaker than for BCNU. With EMT6 cells, I50 values from flow cytometry were 9.7 x 10(-3) M and 1.5 x 10(-3) M for mitozolomide and temozolomide compared with 3.7 x 10(-4) M for BCNU. These were higher than the ID50 values for in vitro antitumour activity (MTT assay), 8.5 x 10(-6) M in the case of mitozolomide and 1.2 x 10(-5) M for BCNU, but similar to that of 5.6 x 10(-4) M for the less toxic temozolomide. MCTIC and cyclodisone showed very low activity, but significant inhibition was seen for clomesome. The results are consistent with the view that the imidazotetrazines do not exhibit major carbamoylating ability, although significant effects are seen at cytotoxic concentrations of temozolomide. In addition, the potential for the generation of carbamoylating species at the enzyme active site cannot be ruled out.

Animals↗

PubMed1989

Estimation of plasma area under the curve for etanidazole (SR 2508) in toxicity prediction and dose adjustment.

The hydrophilic 2-nitroimidazole radiosensitizer etanidazole is currently undergoing clinical evaluation. Although considerably less neurotoxic than misonidazole because of its rapid renal clearance and partial exclusion from the nervous system, total dose is limited by peripheral neuropathy. Monitoring plasma etanidazole concentration in patients to determine the area under the curve (AUC0-infinity) has been proposed as a method of predicting patients at risk, and of providing a quantitative basis for dose reduction in such patients. Successful application of this policy requires accurate assessment of AUC0-infinity. We have analyzed plasma data for 18 patients receiving 2 g/m2 etanidazole to determine the errors introduced in the estimation of AUC0-infinity caused by omitting selected time points from the analysis. A 'baseline' AUC0-infinity value was calculated by integration of the rate equation for the 2-compartment model using data points at 0, 15, and 30 min and 1, 2, 4, 8, 12, and 24 hr after the end of infusion. The mean +/- SD area for AUC0-infinity was 502 +/- 152 micrograms ml-1 h (2.35 +/- 0.71 mM.h). Omitting the zero or the 24 hr time point, the average errors were quite small (2.5% in both cases), but errors of up to 16.4 and 7.3%, respectively, were seen for individual patients. Leaving out both the 8 hr and 12 hr points at the same time gave a similar low average error of 2.9%, with a highest error of 7.3%. Omitting all data points after 4 hr, the mean error was 24.7% and 15 of 18 patients had errors in excess of 10%. In addition, failure to correct for infusion time results in an underestimation of AUC0-infinity averaging 4.5% (range 1.9-8.7%). The choice of sampling times for toxicological monitoring will depend upon the accuracy with which the AUC0-infinity must be known. Including all data points between 0 and 24 hr will minimize errors. Considering the general similarity in the errors introduced by omitting the 8 hr and 12 hr points together compared to those seen with exclusion of the single 24 hr point, the choice between these truncated sampling options would be expected to lie in the relative inconvenience caused to patients and medical staff for the particular dose schedule used. The short sampling schedule (0-4 hr) should not be used.

Etanidazole↗

PubMed1989

Micropharmacokinetics of chemical modifiers.

Classical pharmacokinetic analysis of plasma, urine and tissue specimens continues to be of major value to the rational development of chemical modifiers of cancer treatment. However, in addition, increasingly sophisticated analytical techniques are becoming available which allow the pathways of microdistribution and micrometabolism of drugs to be traced down to the cellular and molecular level. New developments described here include flow cytometry, magnetic resonance spectroscopy, and molecular enzymology. These are predicted to have a major impact on the optimization of chemical modification.

Animals↗

PubMed1989

Demonstration of tumor-selective retention of fluorinated nitroimidazole probes by 19F magnetic resonance spectroscopy in vivo.

We have evaluated two fluorinated misonidazole analogues, Ro 07-0741 and CCI-103F, as potential probes for the non-invasive identification of hypoxic tumor cells by 19F magnetic resonance spectroscopy (MRS) in vivo. The equipment used was a 1.9 T Oxford Research Systems TMR-32 spectrometer, fitted with a 15 mm diameter surface coil. Signal was readily detectable, with similar intensity from EMT6 tumor, liver, and brain at early times (1-2 hr) after i.v. injection in BALB/c mice, indicative of an initial uniform biodistribution of parent probes. At later times (5-10 hr) there was a progressive reduction in signal intensity from brain and liver, but tumor levels remained constant or declined more slowly. This is illustrated by tumor/brain ratios at 6-7 hr of 2.9 (Ro 07-0741) and 4.2 (CCI-103F). In 4/5 mice analyzed at 20-24 hr after Ro 07-0741, and 1/2 following CCI-103F, tumor signal remained detectable. This occurred in the absence of parent probe as measured by HPLC, suggesting the involvement of a product of nitroreductive bioactivation. Studies with KHT and RIF-1 tumors in C3H/He mice showed a similar trend but retention in RIF-1 was less dramatic, and this was consistent with the known hypoxic fractions and comparative in vivo nitroreductase activities. These promising results support the continuing development of 19F nitroimidazole probes for non-invasive identification of hypoxic cells in vivo.

Animals↗

PubMed1989

Molecular enzymology of the reductive bioactivation of hypoxic cell cytotoxins.

The hypoxic cell cytotoxins SR 4233, benznidazole (Benzo), and CB 1954 were readily reduced by anaerobic mouse liver microsomes in vitro to their respective amino or single N-oxide derivatives. The reactions were inhibited in air and required reduced cofactors, particularly NADPH. The rates of reductive bioactivation were markedly different for each drug, with SR 4233 much greater than CB 1954 greater than Benzo. Using purified cytochrome P-450 reductase (P-450 reductase) and an inhibitory antibody to this enzyme, we demonstrated that P-450 reductase was involved in the reductive bioactivation of all 3 compounds. It had a minor role in SR 4233 reduction, but a more important involvement in CB 1954 metabolism to its 4-amino metabolite. Using carbon monoxide, a specific inhibitor of cytochrome P-450 (P-450), we demonstrated that P-450 was involved in both SR 4233 and Benzo reduction. P-450 had a major role both in SR 4233 conversion to SR 4317 and in the latter steps of Benzo amine formation. Purified xanthine oxidase was shown to reduce SR 4233 and Benzo in vitro, but cytosolic aldehyde oxidase activity was only detectable with Benzo as substrate. Characterizing the relative participation of the various reductases in tumor versus normal tissues may allow a more rational selection and application of hypoxic cell cytotoxins in cancer therapy.

Animals↗

PubMed1989

Accelerated elimination of pimonidazole following microsomal enzyme induction in mice: a possible approach to reduced neurotoxicity of the pimonidazole-etanidazole combination.

The pimonidazole-etanidazole combination shows promise for improved radiosensitization, although pimonidazole may increase the dose-limiting peripheral neurotoxicity of etanidazole in man. Induction of liver microsomal drug metabolizing enzymes by phenobarbitone resulted in reduced exposure to pimonidazole in plasma, brain, and tumor in mice. Peak tumor concentrations were lowered, but to a lesser extent. Phenobarbitone induction caused no change in the urinary ratio of pimonidazole to its N-oxide metabolite, and in fact, markedly reduced the circulating metabolite concentrations in plasma. There was no effect of phenobarbitone on plasma or tissue pharmacokinetics of etanidazole, which is eliminated by renal clearance. The results suggest that hepatic microsomal enzyme induction may be a possible approach to reducing the toxicity of the pimonidazole-etanidazole combination, and may also provide valuable information on the enzymology of pimonidazole metabolism.

Animals↗

PubMed1989

A multiple dose study of the combined radiosensitizers Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole).

The hypoxic cell radiosensitizers Ro 03-8799 and SR 2508 have different clinical toxicities. The former produces an acute but transient central nervous system syndrome, whereas the latter produces cumulative peripheral neuropathy. Following single dose studies, an escalating multiple dose schedule using both drugs in combination showed no unexpected adverse reactions at lower doses. This study identifies the clinical tolerance and pharmacokinetics when doses in the region of the maximal tolerated dose are given to 26 patients receiving infusions of 0.75 g/m2 Ro 03-8799 and 2 g/m2 SR 2508 three times per week. At 15 doses, 3/4 patients experienced WHO grade 2 peripheral neuropathy, whereas at 12 doses 1/9 developed grade 2 and 6/9 developed grade 1 neuropathies. This represents a lower dose of SR 2508 than can be given alone suggesting that some interaction between the two drugs does exist in terms of chronic peripheral neurotoxicity. Pharmacokinetic studies show no adverse interactions between the two drugs and minimal inter-patient variation. From bivariate analysis, cumulative AUC for Ro 03-8799 has the most significant correlation with the development of peripheral neuropathy. Tumor drug concentrations normalized to the administered dose show mean values of 34 micrograms/g Ro 03-8799 and 76 micrograms/g SR 2508 30 minutes after infusion. These could be expected to produce a single dose sensitizer enhancement ratio of 1.5. The combination of the two sensitizers at the maximum tolerable dose may be expected to give an increased therapeutic efficacy over either drug alone.

Drug Combinations↗