Evaluation of the cancer chemopreventive efficacy of rice bran in genetic mouse models of breast, prostate and intestinal carcinogenesis.

Brown rice is a staple dietary constituent in Asia, whereas rice consumed in the Western world is generally white, obtained from brown rice by removal of the bran. We tested the hypothesis that rice bran interferes with development of tumours in TAg, TRansgenic Adenocarcinoma of the Mouse Prostate (TRAMP) or Apc(Min) mice, genetic models of mammary, prostate and intestinal carcinogenesis, respectively. Mice received rice bran (30%) in AIN-93G diet throughout their post-weaning lifespan. In TAg and TRAMP mice, rice bran did not affect carcinoma development. In TRAMP or wild-type C57Bl6/J mice, dietary rice bran increased kidney weight by 18 and 20%, respectively. Consumption of rice bran reduced numbers of intestinal adenomas in Apc(Min) mice by 51% (P<0.01), compared to mice on control diet. In parallel, dietary rice bran decreased intestinal haemorrhage in these mice, as reflected by increased haematocrit. At 10% in the diet, rice bran did not significantly retard Apc(Min) adenoma development. Likewise, low-fibre rice bran (30% in the diet) did not affect intestinal carcinogenesis, suggesting that the fibrous constituents of the bran mediate chemopreventive efficacy. The results suggest that rice bran might be beneficially evaluated as a putative chemopreventive intervention in humans with intestinal polyps.

Growth-inhibitory and cell cycle-arresting properties of the rice bran constituent tricin in human-derived breast cancer cells in vitro and in nude mice in vivo.

Tricin, a flavone found in rice bran, inhibits the growth of human-derived malignant MDA-MB-468 breast tumour cells at submicromolar concentrations. As part of the exploration of tricin as a potential cancer chemopreventive agent, we investigated the duration and cell cycle specificity of growth inhibition elicited by tricin in vitro and the effect of tricin on the development of MDA-MB-468 tumours grown in immune-compromised MF-1 mice in vivo. Preincubation of MDA-MB-468 cells with tricin (1-40 microM) for 72 h compromised cell growth after tricin removal, and such irreversibility was not observed in human breast-derived nonmalignant HBL-100 cells. Tricin (>/=5 microM) arrested MDA-MB-468 cells in the G2/M phase of the cell cycle without inducing apoptosis as adjudged by annexin V staining. In nude mice consumption of tricin with the diet (0.2%, w w(-1)) from 1 week prior to MDA-MB-468 cell implantation failed to impede tumour development. Steady-state levels of tricin in plasma, breast tumour tissue and intestinal mucosa, as measured by HPLC, were 0.13 microM and 0.11 and 63 nmol g(-1), respectively. Cells were exposed to tricin (0.11, 1.1 or 11 microM) in vitro for 72 h and then implanted into mice. The volume of tumours in animals bearing cells pre-exposed to 11 microM tricin was less than a third of that in mice with control cells, while tumours from cells incubated with 0.1 or 1.1 microM tricin were indistinguishable from controls. These results suggest that the potent breast tumour cell growth-inhibitory activity of tricin in vitro does not directly translate into activity in the nude mouse bearing the MDA MB-468 tumour. While the results do not support the notion that tricin is a promising candidate for breast cancer chemoprevention, its high levels in the gastrointestinal tract after dietary intake render exploration of its ability to prevent colorectal carcinogenesis propitious.

Characterisation of metabolites of the putative cancer chemopreventive agent quercetin and their effect on cyclo-oxygenase activity.

Quercetin (3,5,7,3',4'-pentahydroxyflavone) is a flavone with putative ability to prevent cancer and cardiovascular diseases. Its metabolism was evaluated in rats and human. Rats received quercetin via the intravenous (i.v.) route and metabolites were isolated from the plasma, urine and bile. Analysis was by high-performance liquid chromatography and confirmation of species identity was achieved by mass spectrometry. Quercetin and isorhamnetin, the 3'-O-methyl analogue, were found in both the plasma and urine. In addition, several polar peaks were characterised as sulphated and glucuronidated conjugates of quercetin and isorhamnetin. Extension of the metabolism studies to a cancer patient who had received quercetin as an i.v. bolus showed that (Quercetin removed) isorhamnetin and quercetin 3'-O-sulphate were major plasma metabolites. As a catechol, quercetin can potentially be converted to a quinone and subsequently conjugated with glutathione (GSH). Oxidation of quercetin with mushroom tyrosinase in the presence of GSH furnished GSH conjugates of quercetin, two mono- and one bis-substituted conjugates. However, these species were not found in biomatrices in rats treated with quercetin. As cyclo-oxygenase-2 (COX-2) expression is mechanistically linked to carcinogenesis, we examined whether quercetin and its metabolites can inhibit COX-2 in a human colorectal cancer cell line (HCA-7). Isorhamnetin and its 4'-isomer tamarixetin were potent inhibitors, reflected in a 90% decrease in prostaglandin E-2 (PGE-2) levels, a marker of COX-2 activity. Quercetin was less effective, with a 50% decline. Quercetin 3- and 7-O-sulphate had no effect on PGE-2. The results indicate that quercetin may exert its pharmacological effects, at least in part, via its metabolites.

Pharmacokinetics in mice and growth-inhibitory properties of the putative cancer chemopreventive agent resveratrol and the synthetic analogue trans 3,4,5,4'-tetramethoxystilbene.

Resveratrol (trans-3,5,4'-trihydroxystilbene) is a naturally occurring polyphenol with cancer chemopreventive properties in preclinical models of carcinogenesis, including those of colorectal cancer. Recently, a variety of analogues of resveratrol have been synthesised and investigated in in vitro assays. One analogue, 3,4,5,4'-tetramethoxystilbene (DMU 212), showed preferential growth-inhibitory and proapoptotic properties in transformed cells, when compared with their untransformed counterparts. As part of a chemoprevention drug development programme, the pharmacokinetic properties of DMU 212 were compared with those of resveratrol in the plasma, liver, kidney, lung, heart, brain and small intestinal and colonic mucosa of mice. DMU 212 or resveratrol (240 mg kg(-1)) were administered intragastrically, and drug concentrations were measured by HPLC. Metabolites were characterised by cochromatography with authentic reference compounds and were identified by mass spectrometry. The ratios of area of plasma or tissue concentration vs time curves of resveratrol over DMU 212 (AUC(res)/AUC(DMU212)) for the plasma, liver, small intestinal and colonic mucosa were 3.5, 5, 0.1 and 0.15, respectively. Thus, resveratrol afforded significantly higher levels than DMU 212 in the plasma and liver, while DMU 212 exhibited superior availability compared to resveratrol in the small intestine and colon. Resveratrol was metabolised to its sulphate or glucuronate conjugates, while DMU 212 underwent metabolic hydroxylation or single and double O-demethylation. DMU 212 and resveratrol inhibited the growth of human-derived colon cancer cells HCA-7 and HT-29 in vitro with IC(50) values of between 6 and 26 microM. In the light of the superior levels achieved in the gastrointestinal tract after the administration of DMU 212, when compared to resveratrol, the results provide a good rationale to evaluate DMU 212 as a colorectal cancer chemopreventive agent.

Effects of dietary curcumin on glutathione S-transferase and malondialdehyde-DNA adducts in rat liver and colon mucosa: relationship with drug levels.

Curcumin prevents colon cancer in rodent models. It inhibits lipid peroxidation and cyclooxygenase-2 (COX-2) expression and induces glutathione S-transferase (GST) enzymes. We tested the hypothesis that 14 days of dietary curcumin (2%) affects biomarkers relevant to cancer chemoprevention in the rat. Levels of inducible COX-2, as reflected by prostaglandin E(2) production by blood leukocytes, were measured ex vivo. Total GST activity and adducts of malondialdehyde with DNA (M(1)G), which reflect endogenous lipid peroxidation, were measured in colon mucosa, liver, and blood leukocytes. Curcumin and its metabolites were analyzed by high-performance liquid chromatography in plasma, and its pharmacokinetics were compared following a diet containing 2% curcumin versus intragastric (i.g.) administration of curcumin suspended in an amphiphilic solvent. The curcumin diet did not alter any of the markers in the blood but increased hepatic GST by 16% and decreased colon M(1)G levels by 36% when compared with controls. Administration of carbon tetrachloride during the treatment period increased colon M(1)G levels, and this increase was prevented by dietary curcumin. Dietary curcumin yielded low drug levels in the plasma, between 0 and 12 nM, whereas tissue concentrations of curcumin in liver and colon mucosa were 0.1--0.9 nmol/g and 0.2--1.8 micromol/g, respectively. In comparison with dietary administration, suspended curcumin given i.g. resulted in more curcumin in the plasma but much less in the colon mucosa. The results show that curcumin mixed with the diet achieves drug levels in the colon and liver sufficient to explain the pharmacological activities observed and suggest that this mode of administration may be preferable for the chemoprevention of colon cancer.

Cell-specific loss of cytochrome P450 2B1 in rat lung following treatment with pneumotoxic and non-pneumotoxic trialkylphosphorothioates.

This study was designed to test the hypothesis that the reduction in cytochrome P450 (CYP) 2B1 content and activity of rat lung microsomes, following dosing with pneumotoxic trimethylphosphorothioates, results from damage to specific cell types. Of the lung cells exhibiting immunolabelling for CYP2B1, only type I cells showed signs of susceptibility to the pneumotoxins O,O.S-trimethylphosphorothioate and O,S,S-trimethylphosphorodithioate. While most type I cells became necrotic, type II and Clara cells showed no signs of injury, despite their gradual loss of CYP2B1, as detected by immunogold labelling. This loss of labelling was accompanied by a 75% reduction in the immunoreactive CYP2B1 content and an 85% reduction in pentoxyresorufin O-dealkylase activity in lung microsomes. In contrast, the non-pneumotoxic analogue O,O,S-trimethylphosphorodithioate, differing from O,O,S-trimethylphosphorothioate by only the presence of a P = S rather than a P = O moiety, caused an even more rapid fall in pulmonary pentoxyresorufin O-dealkylase activity, but only a slight reduction in the microsomal content of CYP2B1. The recovery of this activity began within 12 hr of dosing. O,O,S-Trimethylphosphorodithioate, which acts as a suicidal inhibitor of pulmonary CYP2B1, did not cause any detectable lung injury or increase in cell division. These results are consistent with the initial reduction in both enzyme content and activity caused by the P = O - containing pneumotoxins resulting, almost entirely, from death of type I cells. Subsequent reductions that occur long after clearance of the toxin may be exacerbated by the onset of mitosis in Clara and type II cells.

Pulmonary availability of isotretinoin in rats after inhalation of a powder aerosol.

Repeated oral administration of chemopreventive retinoids such as isotretinoin over extended periods of time is associated with intolerable systemic toxicity. Here isotretinoin was formulated as a powder aerosol, and its delivery to the lungs of rats was studied with the aim to explore the possibility of minimizing adverse effects associated with its oral administration. Rats received isotretinoin orally (0.5, 1 or 10 mg kg(-1)) or by inhalation (theoretical dose approximately 1 or approximately 10 mg kg(-1)) in a nose-only inhalation chamber. Isotretinoin was quantitated by high-pressure liquid chromatography in plasma and lung tissue. The ratios of mean area of concentration-vs-time curve (AUC) values in the lungs over mean AUCs in the plasma for isotretinoin following single or repeated aerosol exposure surpassed those determined for the oral route by factors of between two (single low-dose) and five (single high-dose). Similarly, the equivalent ratios for the maximal peak concentrations in lungs and plasma obtained after aerosol exposure consistently exceeded those seen after oral administration, suggesting that lungs were exposed to higher isotretinoin concentrations after aerosol inhalation than after oral administration of similar doses. Repeated high doses of isotretinoin by inhalation resulted in moderate loss of body weight, but microscopic investigation of ten tissues including lung and oesophagus did not detect any significant aerosol-induced damage. The results suggest that administration of isotretinoin via powder aerosol inhalation is probably superior to its application via the oral route in terms of achieving efficacious drug concentrations in the lungs.

Association of tamoxifen biliary excretion rate with prior tamoxifen exposure and increased mdr1b expression.

ATPase transporter proteins are commonly found in the hepatocyte canalicular membrane. Some of these, in particular the multidrug resistance (mdr1b) gene, have been previously demonstrated to be inducible genes. In this study, we found that tamoxifen induced expression of the mdr1b gene in the liver up to 40-fold after 14 days' exposure to tamoxifen in the diet at a concentration of 420 ppm. As tamoxifen and its metabolites are primarily excreted into the bile, we investigated if the increased expression of mdr1b in the liver following tamoxifen exposure had any effect on its excretion in rats. We found that the excretion of tamoxifen and its metabolites into bile was increased from 8 +/- 1% to 51 +/- 18% (mean +/- SD) of an administered dose of 180 nmol/kg over a collection period of 3 hr in rats that had received tamoxifen (35 mg/kg) orally for 12 days (plus a 3-day rest) prior to the experiment. These data suggest that prolonged treatment with tamoxifen may result in lower serum and tumour concentrations, due to a self-mediated enhancement of excretion via mdr1b gene-encoded P-glycoprotein. This may have implications for other drugs sharing the same route of excretion and co-administered with tamoxifen.

Chemoprevention of aflatoxin B1-induced carcinogenesis by indole-3-carbinol in rat liver--predicting the outcome using early biomarkers.

Indole-3-carbinol (I3C) was examined for its ability to inhibit aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in male Fischer rats when administered either before or after the carcinogen. After 13 weeks, animals pretreated with I3C (0.5% in the diet) for 2 weeks prior to administration of AFB1 and with continuing treatment during exposure to the carcinogen were protected from development of preneoplastic lesions, as determined by the classical markers gamma-glutamyltranspeptidase (GGT) and glutathione S-transferase (GST) P. In animals receiving AFB1 for 6 weeks before treatment with I3C, there was no obvious protective effect at 13 weeks compared with animals receiving only AFB1. Using cytokeratin 18 expression as a marker, animals fed AFB1 alone had a small number of positive foci at 13 weeks. However, no cytokeratin-positive foci were visible in the majority of livers from either group receiving I3C in combination with AFB1 and after 43 weeks all animals in these groups were protected from liver tumour formation. These results suggest that expression of cytokeratin 18, a later phenotypic change in foci than induction of GST-P and GGT, correlates more closely with tumour outcome in this model. I3C appeared to retard progression of AFB1-induced carcinogenesis at both the initiation and promotion stages. Continuous treatment with I3C for 13 weeks caused significant induction of CYP1A1, 1A2, 3A and 2B1/2, GST Yc2, aflatoxin B1 aldehyde reductase and quinone reductase. Such alteration of the drug metabolizing capacity of the liver by I3C contributes to blocking of initiation, while the observed inhibition of ornithine decarboxylase, a rate limiting enzyme in polyamine biosynthesis, and of tyrosine kinase activity may contribute to the suppressive effect of I3C.

Selective inhibition and induction of CYP activity discriminates between the isoforms responsible for the activation of butylated hydroxytoluene and naphthalene in mouse lung.

1. Selective induction and inhibition experiments have been used to identify the cytochrome P450 (CYP) isoforms responsible for butylated hydroxytoluene (BHT) bioactivation in mouse lung. 2. Pre-treatment of BALB/c mice with O,O,O-trimethylphosphorothioate (OOOMeP(S)), which prevented all the signs of toxicity observed following BHT treatment, inhibited the pulmonary activity of pentoxyresorufin O-dealkylase (PROD) and coumarin hydroxylase but not 4-nitrophenol hydroxylase. 3. Pulmonary coumarin hydroxylase activity was greater in DBA than in BALB/c mice but the severity of BHT-induced lung injury was similar. 4. Pre-treatment with pyrazole, which exacerbated BHT-induced lung injury, did not affect pulmonary coumarin hydroxylase or 4-nitrophenol hydroxylase activity but increased that of PROD. 5. Pre-treatment with OOOMeP(S) prevented the lethargy and weight-loss associated with naphthalene poisoning but not the pulmonary injury. Pre-treatment with pyrazole did not exacerbate naphthalene-induced injury. 6. Members of both CYP2F and 2B sub-families have been shown to exhibit PROD activity and 2F2 activates naphthalene in mouse lung. The current studies, however, indicate that 2F2 is unlikely to be a significant component of PROD activity in mouse lung. 2F2, like coumarin hydroxylase (2A5) and 4-nitrophenol hydroxylase (2E1), is not responsible for the pulmonary activation of BHT, which is largely attributable to an isoform of 2B, probably 2B10.

A comparison of the acute pathology induced by 3-phenylamino-1,2-propanediol (PAP) and its mono-oleoyl ester in rodents with the toxic oil syndrome in man.

Phenylamino-1,2 propanediol (PAP) and its mono-oleoyl ester have been identified in samples of the cooking oil thought to be responsible for the Toxic Oil Syndrome (TOS) which occurred in Spain in 1981. The acute toxicity of PAP and its mono-oleoyl ester have been examined in rats and mice, after daily administration for periods of up to 14 days to determine whether these compounds could produce any of the pathologies of TOS. Even at the highest dose, the 1-mono-oleoyl ester of 3-phenylamino-1,2 propanediol did not cause any toxicity in rats or mice when given intraperitoneally. 3-Phenylamino-1,2 propanediol, however, was toxic when administered to rats by this route. After 6-10 consecutive daily doses of PAP, at the highest dose administered (350 mg kg-1), all of the rats became unwell. Postmortem examination showed that the major pathology present was massive pulmonary thromboembolism. Further investigations of the toxicity of PAP after intravenous administration showed that it was not directly vasotoxic. The pulmonary thromboembolism seen with intraperitoneally administered PAP was due to the toxic effect of PAP on the mesenteric tissue and blood vessels, causing thrombosis which subsequently embolised the blood vessels in the lung. Intra-gastric administration of PAP caused no toxicity in rats. Comparatively, the pathology seen after intraperitoneal administration of PAP was not thought to be representative of the pathology of the toxic oil syndrome in man.

The comparative toxicity of chlorambucil and chlorambucil-spermidine conjugate to BALB/c mice.

The acute intraperitoneal toxicities of chlorambucil and chlorambucil-spermidine conjugate have been compared, in mice. Both compounds were neurotoxic and also caused a prolonged fall in bodyweight and a depletion of lymphocyte numbers associated with a fall in the total leukocyte count and loss of spleen and thymus weight. Alanine aminotransferase and aspartate aminotransferase activities and blood urea nitrogen concentration were increased at 24 h after conjugate administration, but had returned to normal at 72 h. Chlorambucil significantly decreased blood urea nitrogen concentration for 72 h, but did not affect transferase activity. Tissue concentrations of conjugate were measurable in liver and kidney for 12 days and lung for 5 days after dosing. The toxicity of both compounds was cumulative. In mol/kg, the chlorambucil-spermidine conjugate was 10-fold more toxic than chlorambucil, on the basis of their neurotoxicity, but only 2- to 3-fold more toxic on the basis of their effects on lymphocyte depression. The increased toxicity of the conjugate does not improve its therapeutic index relative to chlorambucil.

CYP4B1 activates 4-ipomeanol in rat lung.

Inhibition of pulmonary CYP4B1 activity by pretreatment of rats with p-xylene decreased the ability of lung microsomes to N-hydroxylate 2-aminofluorene and prevented the lung damage normally seen after dosing with ipomeanol. The toxicity of ipomeanol, as assessed by acute lethality, was decreased by a factor of eight. In contrast, induction of CYP1A1 by Aroclor or beta-naphthoflavone, or inhibition of CYP2B1 by O,O,S-trimethyl-phosphorodithioate, as assessed by measurement of lung microsomal dealkylation of ethoxyresorufin or pentoxyresorufin, did not change ipomeanol toxicity. A polyclonal antibody raised against CYP4B1 prevented the covalent binding of [14C]-ipomeanol to lung microsomal protein in vitro. Antibodies raised against the other major P450 isozymes of rat lung, CYP2B1 and CYP1A1, had no effect on this binding. Aroclor, beta-naphthoflavone, and O,O,S-trimethylphosphorodithioate failed to affect binding of radiolabeled ipomeanol in vivo, but pretreatment with p-xylene resulted in a significant reduction in this binding. The CYP4B1 substrate 2-aminofluorene, when dosed to rats, caused a sixfold decrease in ipomeanol toxicity. These results indicate that in the rat, unlike the rabbit, pulmonary bioactivation of ipomeanol is predominantly dependent upon CYP4B1.

1-Nitronaphthalene toxicity in rat lung and liver: effects of inhibiting and inducing cytochrome P450 activity.

In rats, 1-nitronaphthalene (1-NN) causes both pulmonary and hepatic toxicity. Pulmonary toxicity is evident as bronchiolar damage, with necrosis of Clara cells and ciliated cells, whereas hepatic injury involves vacuolation of centrilobular hepatocytes. Pretreatment with O,O,S-trimethylphosphorodithioate [OOS-MeP(S)] or p-xylene gave three- to fourfold protection against 1-NN toxicity. These pretreatments also prevented both the increase in lung weight and the rise in gamma-glutamyltranspeptidase and alkaline phosphatase activity in bronchoalveolar lavage fluid normally associated with 1-NN toxicity. Pretreatment with Aroclor 1254 or beta-naphthoflavone (beta-NF) did not alter the LD50 of 1-NN. Aroclor or beta-NF pretreatment did, however, prevent morphological signs of lung injury and any increase in either lung weight or enzyme activity in bronchoalveolar lavage fluid. Liver damage was not prevented by these treatments; indeed, injury was exacerbated and was transferred from centrilobular to periportal areas. In control rats the covalent binding of [1-14C]NN to liver microsomes was eight times greater than to lung microsomes. Pretreatment with OOS-MeP(S) decreased covalent binding to lung microsomes, without affecting binding to liver microsomes. Conversely, both Aroclor and beta-NF slightly increased covalent binding in lung, but increased liver binding by 250-300%. Phenobarbitone also increased binding to liver microsomes by 250-300%, but failed to increase, or alter, the distribution of liver damage. The reported effects of these pretreatments indicate that the toxicity of 1-NN is probably activated by isoenzyme CYP2B1 in lung, but by isoenzymes CYP1A1 or CYP1A2 in the liver.

Cytochrome P450 2B isoenzymes are responsible for the pulmonary bioactivation and toxicity of butylated hydroxytoluene, O,O,S-trimethylphosphorothioate and methylcyclopentadienyl manganese tricarbonyl.

O,O,S-Trimethylphosphorothioate and methylcyclopentadienyl manganese tricarbonyl damage the type I pneumocytes of the alveolar epithelium in rats. Butylated hydroxytoluene causes similar damage in mice. The toxicity of these compounds is dependent on their bioactivation by the cytochrome P-450 (CYP) system. A range of compounds, that modifies the activity of specific CYP isoenzymes, has been used to establish those particular isoenzymes involved in bioactivation. Pulmonary toxicity was assessed by measurement of lung weight and changes in the activity of gamma-glutamyltranspeptidase and alkaline phosphatase in bronchoalveolar lavage fluid. O,O,S-Trimethylphosphorodithioate, bromophos, p-xylene and 2,4-dichloro-(6-phenyl-phenoxy)ethylamine all inhibited the dealkylation of pentoxyresorufin, an indicator of CYP2B1 activity, and also prevented pulmonary toxicity. There was a significant negative correlation between the level of pulmonary pentoxyresorufin dealkylation after pretreatment with O,O,S-trimethylphosphorodithioate and the severity of lung injury. This pretreatment also reduced the toxicity of butylated hydroxytoluene by a factor of 20 and methylcyclopentadienyl manganese tricarbonyl by a factor of 10. Modification of the activity of CYP1A1, CYP2E1 and CYP4B1 did not alter the toxicity of these compounds. These results indicate that pulmonary CYP2B1 is responsible for the bioactivation and toxicity of O,O,S-trimethylphosphorothioate and methylcyclopentadienyl manganese tricarbonyl in rats and the orthologous 2B isoenzyme in mice activates butylated hydroxytoluene.

Pulmonary cytochrome P450 in rats exposed to formaldehyde vapor.

The lungs of rats exposed to formaldehyde vapor, for 6 hr/day over 4 consecutive days, were examined for signs of injury and for changes in the level, or activity, of cytochrome P450. The animals were supplied with 10 ppm formaldehyde vapor generated, in two separate experiments, either from an aqueous solution of formaldehyde or from heated paraformaldehyde. All rats were exposed for 6 hr, on each of 4 consecutive days, and killed 1 day after the onset of the fourth period of exposure. The lung weights and gains in body weight of exposed animals were indistinguishable from those of their controls. Lungs from the formaldehyde-exposed animals did not show any signs of injury, even at the ultrastructural level. Bronchoalveolar lavage samples from exposed animals showed no increase in alkaline phosphatase or gamma-glutamyl transpeptidase activity. The total concentration of cytochrome P450 in the lungs of exposed animals was similar to that found in their controls. The P450 activity of pulmonary microsomes from exposed animals was not significantly different from that obtained with samples from the control animals. These results indicate that repeated exposure to 10 ppm formaldehyde vapor does not injure the deep lung of rats and has no effect on the level of lung P450 or on its activity against substrates for the most common pulmonary forms of this enzyme.

Inhibition and induction of cytochrome P450 isoenzymes in rat lung.

The O-dealkylation of pentoxyresorufin, a substrate for P450 2B1, was decreased in lung microsomes from rats dosed with O,O,S-trimethylphosphorodithioate, O,O,O-trimethylphosphorothioate, bromophos, fenitrothion, p-xylene and 2,4-dichloro-(6-phenylphonoxy)ethylamine. This activity was decreased by antibodies to P450 2B1 but unaffected by antibodies to P450 1A1 or 4B1. This reduction reflected both inactivation and destruction of P450 2B1; destruction of this protein was particularly marked after bromophos and fenitrothion. Pyrazole was the only compound in this study to induce the O-dealkylation of pentoxyresorufin. None of these compounds altered the rate of ethoxyresorufin O-dealkylation, an indicator of P450 1A1 activity, but this activity was induced greatly by both Aroclor and beta-naphthoflavone, p-Xylene was the only compound to decrease P450 4B1 activity, as determined by the N-hydroxylation of 2-aminofluorene. In the liver, bromophos, fenitrothion, p-xylene and 2,4-dichloro-(6-phenylphonoxy)ethylamine all had marked effects on the O-dealkylation of ethoxyresorufin and pentoxyresorufin but, at the dose used, O,O,O-trimethylphosphorothioate and O,O,S-trimethylphosphorodithioate had minimal effects in this tissue. Thus, both O,O,O-trimethylphosphorothioate and O,O,S-trimethylphosphorodithioate are exquisitely selective inhibitors of pulmonary P450 2B1 activity. Their use, together with pyrazole, will facilitate future studies of the pulmonary activation of toxins by P450 2B1.

A comparison of the acute toxicity, neuropathology, and electrophysiology of N,N-diethyl-m-toluamide and N,N-dimethyl-2,2-diphenylacetamide in rats.

The insect repellent DEET and the structurally related herbicide diphenamid both cause ataxia associated with a spongiform myelinopathy largely confined to the cerebellar roof nuclei. This local myelinopathy was accompanied by the formation of neuronal cytoplasmic clefts and was produced by a single dose of 1 to 3 g/kg N,N-diethyl-m-toluamide (DEET). These dose levels also produced a severe and often fatal prostration and clear electrophysiological signs of prolonged suppressed seizure activity. Diphenamid produced an identical myelinopathy after doses of 0.8 to 1.5 g/kg but without the severe prostration, suppressed seizures, or neuronal clefts. The effects of diphenamid were shown to be reversible over 3 to 7 days by neuropathological, motor, and auditory evoked response indices. Both compounds caused characteristic changes in auditory evoked response which may be useful in clinical diagnosis. Six other alkyl amides, two of which produce signs of CNS excitation, failed to produce myelinopathy at the maximum tolerated doses. Our findings show close parallels with a number of human cases of DEET poisoning and indicate that other amides, like diphenamid, also pose a potential hazard.

Protection against chemical-induced lung injury by inhibition of pulmonary cytochrome P-450.

Protection afforded by trialkyl phosphorothionates against the lung injury caused by trialkyl phosphorothiolates probably results from the inhibition by the P = S moiety of the thionates, of one or more pulmonary cytochrome P-450 isozymes. The aromatic hydrocarbons p-xylene and pseudocumene also protect against this injury and inhibit some P-450 isozymes, but by a different mechanism. OOS-Trimethylphosphorothionate and p-xylene were compared as protective agents against the effect of OOS-trimethylphosphorothiolate and two other lung toxins ipomeanol and 1-nitronaphthalene that are known to be activated by cytochrome P-450. The effects of these protective compounds, in vivo, on pulmonary cytochrome P-450 activity were also determined. Both compounds inhibited pentoxyresorufin O-deethylase activity, but not ethoxyresorufin O-deethylase. The phosphorothionate was most effective against lung injury caused by the phosphorothiolates and 1-nitronaphthalene, whereas p-xylene was much more effective against ipomeanol. beta-Naphthoflavone, which induces pulmonary ethoxyresorufin O-deethylase activity, did not protect against phosphorothiolate or 1-nitronaphthalene injury, and it was only marginally effective in decreasing the toxicity of ipomeanol.

Ultrastructural effects of phosphorothionates and other inhibitors of lung monooxygenases: protection against trialkylphosphorothiolate-induced lung injury.

An oral dose (25 mg/kg) of O,O,S-triethylphosphorothiolate (OOSEtO) to rats results in selective injury of type I pneumocytes, degranulation of Clara cells, and pronounced increase in lung weight. A dose (12.5 mg/kg) of the related compound O,O,S-trimethylphosphorothionate (OOSMeS) causes neither injury nor degranulation but, when administered 2 h before OOSEtO (25 mg/kg), protects against all the signs of lung injury that would otherwise result from this dose of the compound. The administration of OOSMeS also results in the formation of large, electron-lucent granules within the apical cytoplasm of the Clara cells. The granules are not birefringent, and histochemical procedures indicate that they do not contain carbohydrate but may consist of lipid accumulated around a proteinaceous core. Similar granules are also observed after administration of p-xylene, pseudocumene, and the pesticide bromophos. These compounds, like OOSMeS, inhibit 7-ethoxycoumarin O-deethylase activity in the lung and are capable of protecting against trialkylphosphorothiolate toxicity. This inhibition of 7-ethoxycoumarin O-deethylase activity suggests loss of pulmonary cytochrome P-450. This loss may account for both the protective action of these compounds and the formation of abnormal granules within Clara cells.