Styrylquinoline derivatives: a new class of potent HIV-1 integrase inhibitors that block HIV-1 replication in CEM cells

On the basis of the fact that several polynucleotidyl transferases, related to HIV integrase, contain in their active site two divalent metal cations, separated by ca. 4 A, new potential HIV integrase inhibitors were designed, in which a quinoline substructure is linked to an aryl nucleus possessing various hydroxy substitution patterns, by means of an ethylenic spacer. Although the most active compounds contain the catechol structure, this group is not essential for the activity, since compound 21 that lacks such a moiety is a potent drug, implicating the presence of a different pharmacophore. The most promising styrylquinolines thus synthesized inhibit HIV-1 integrase in vitro at micromolar or submicromolar concentrations and block HIV replication in CEM cells, with no significant cellular toxicity in a 5-day period assay. These inhibitors are active against integrase core domain-mediated disintegration, suggesting that fragment 50-212 is their actual target. These new styrylquinolines may provide lead compounds for the development of novel antiretroviral agents for AIDS therapeutics, based upon inhibition of HIV integrase. They might also be used in the elucidation of the mechanism of inhibition of this enzyme; e.g., they could serve as candidates for cocrystallization studies with HIV integrase.

Review of the toxicology of styrene

Styrene is used in the production of plastics and resins, which include polystyrene resins, acrylonitrile-butadiene-styrene resins, styrene-acrylonitrile resins, styrene-butadiene copolymer resins, styrene-butadiene rubber, and unsaturated polyester resins. In 1985, styrene ranked in the top ten of synthetic organic chemicals produced in the U.S. This review focuses on various aspects of styrene toxicology including acute and chronic toxicity, carcinogenicity, genotoxicity, pharmacokinetics, effects on hepatic and extrahepatic xenobiotic-metabolizing enzymes, pharmacokinetic modeling, and covalent interactions with macromolecules. There appear to be many similarities between the toxicity and metabolism of styrene in rodents and humans. Needed areas of future research on styrene include studies on the molecular dosimetry of styrene in terms of both hemoglobin and DNA adducts. The results of such research should improve our ability to assess the relationship between exposure to styrene and surrogate measures of "effective dose", thereby improving our ability to estimate the effects of low-level human exposures.

Metabolism and toxicity of styrene

The absorption, blood levels, distribution, excretion, and biotransformation of styrene in man and experimental animals are briefly reviewed. The acute toxicity of styrene appears to be unrelated to its biotransformation. Reports of organ toxicity upon chronic exposure to styrene are rare; however, since the chief intermediate in styrene metabolism is an epoxide, hepatotoxicity due to covalent binding at the site of formation appears to be a possibility.

Leukemia and cumulative exposure to butadiene, styrene and benzene among workers in the synthetic rubber industry

Retrospective, quantitative estimates of exposure to 1,3-butadiene, styrene and benzene were developed for a follow-up study of leukemia mortality among 16610 subjects employed at six North American styrene-butadiene rubber manufacturing plants (418846 person-years, 58 leukemia deaths). The estimation procedure entailed identifying work areas within each manufacturing process, historical changes in exposure potential and specific tasks involving exposure, and using mathematical models to calculate job- and time-period-specific average exposures. The resulting estimates were linked with the subjects' work histories to obtain cumulative exposure estimates, which were employed in stratified and Poisson regression analyses of mortality rates. Mantel-Haenszel rate ratios adjusted by race, age, and cumulative styrene exposure increase with cumulative butadiene exposure from 1 in the nonexposed category to 4.5 in the category of 80 ppm-years or more (P = 0.01). The risk pattern is less clear and statistically nonsignificant for styrene exposure. A trend of increasing risk with butadiene exposure is still present after exclusion of the nonexposed category (P = 0.03). A parsimonious interpretation of the findings presented here, in light of previous epidemiologic studies, is that exposure to butadiene in the synthetic rubber industry produces a dose-related increase in the occurrence of leukemia.

Solvent-induced ototoxicity in rats: an atypical selective mid-frequency hearing deficit

Most previous reports of ototoxicity following exposure to several volatile organic solvents have restricted testing to the low- and mid-frequencies (2-20 kHz) of the hearing range in the rat (0.25-80 kHz). We report here that inhalation exposure to styrene, mixed xylene, toluene, and 1,1,2-trichloroethylene resulted in hearing dysfunction only in the mid-frequency range and spared function at lower and higher frequencies. Adult male Long Evans rats were exposed via inhalation (whole body) in flow-through chambers. The following exposures were used: styrene, 1600 ppm; 1,1,2-trichloroethylene, 3500 ppm; toluene, 2500 ppm; mixed xylenes, 1800 ppm (N = 7-8 per group, 8 h/day for 5 days), and n-butanol, 4000 ppm (N = 10/group, 6 h/day for 5 days). Testing of auditory function was conducted 5 to 8 weeks after exposure using reflex modification audiometry (RMA). RMA thresholds were determined for frequencies from 0.5 to 40 kHz. Results indicated increased RMA thresholds for the mid-frequency tones (e.g., 8 and 16 kHz), but not higher or lower tones, for all solvents except n-butanol. Toluene and xylene also increased thresholds at 24 kHz. These data indicate that for those solvents reported thus far to cause hearing loss, the deficit is restricted to mid-frequencies in rats.

Hearing loss in rats caused by inhalation of mixed xylenes and styrene

We have reported that inhalation exposure of rats to toluene causes permanent hearing loss, e.g. Pryor et al. Neurobehav. Toxicol. Teratol. 5, 53-62 (1983). Therefore, it was of considerable interest to examine the ototoxic potential of two structurally related solvents--mixed xylenes and styrene--compared with that of toluene. Male, weanling Fischer-344 rats were exposed to clean air or solvents in four identical 62.5 l Plexiglas chambers. Exposures to 800, 1000, and 1200 ppm were daily for 14 hours/day and lasted 6 weeks for mixed xylenes, 3 weeks for styrene. An additional experiment with xylenes examined the effect of exposure for only 4 (1700 ppm) or 8 (1450 ppm) hours or for 8 (1450 ppm) hours on three consecutive days to compare the results with those obtained with toluene under comparable exposure schedules. Both xylenes and styrene caused marked hearing loss as assessed by behavioral (conditioned avoidance) and electrophysiologic (brainstem auditory-evoked response) methods. Moreover, both solvents appeared to be more potent ototoxicants than toluene, as indicated by effective concentration, effective durations of exposure, and the range of frequencies affected.

The neurotoxicity of industrial solvents: a review of the literature

Organic solvents, particularly stryrene, are used widely in boatbuilding. They may be absorbed by workers either through the respiratory tract or the skin. Uptake is influenced by level and duration of exposure, work load, and specific physiochemical features of each solvent, as well as by work practices and use of protective equipment. Kinetics of metabolism and excretion kinetics are highly variable among compounds. Metabolites can be measured in blood, urine, or exhaled breath and may serve as indirect indices of absorption. Acute high-dose exposure to organic solvents can produce a transient narcotic effect on the central nervous system. This effect occurs in proportion to brain dose, which in turn is determined by intensity and duration of exposure. Additionally, chronic exposures to organic solvents have been reported to produce an increased frequency of neurologic signs and symptoms. These findings include peripheral neuropathies and toxic encephalopathies. The latter are characterized by alterations in affect, memory loss, and impaired cognition. Concern exists that prolonged excessive exposure to organic solvents may lead to premature and persistent dementia in certain workers.

Risk assessment extrapolations and physiological modeling

The process of assessing the risk associated with human exposure to environmental chemicals inevitably relies on a number of assumptions, estimates and rationalizations. One of the more challenging aspects of risk assessment involves the need to extrapolate beyond the range of conditions used in experimental animal studies to predict anticipated human risks. The most obvious extrapolation required is that from the tested animal species to humans; but others are also generally required, including extrapolating from high dose to low dose, from one route of exposure to another and from one exposure timeframe to another. Several avenues are available for attempting these extrapolations, ranging from the assumption of strict correspondence of dose to the use of statistical correlations. One promising alternative for conducting more scientifically sound extrapolations is that of using physiologically based pharmacokinetic models that contain sufficient biological detail to allow pharmacokinetic behavior to be predicted for widely different exposure scenarios. In recent years, successful physiological models have been developed for a variety of volatile and nonvolatile chemicals, and their ability to perform the extrapolations needed in risk assessment has been demonstrated. Techniques for determining the necessary biochemical parameters are readily available, and the computational requirements are now within the scope of even a personal computer. In addition to providing a sound framework for extrapolation, the predictive power of a physiologically based pharmacokinetic model makes it a useful tool for more reliable dose selection before beginning large-scale studies, as well as for the retrospective analysis of experimental results.

A study on the mutagenic activity of styrene and styrene oxide

Styrene oxide is multagenic, without metabolic activation, to S. typhimurium strains TA 1535 and TA 100, which have been devised to detect mutagens causing base-pair substitutions. Styrene seems to be mutagenic toward the same strains, but only after metabolic activation. The toxicity of both styrene and styrene oxide make the construction of reliable dose-response curves rather difficult. Diethylmaleate and 3,3,3-trichloropropene oxide enhanced the mutagenicity of styrene oxide in the presence of homogenate; this result indicates the participation of epoxide hydratase and glutathione S-oxide transferase in the metabolism of styrene oxide. These two chemicals did not influence the mutagenic activity of styrene. Styrene glycol and 4-tert-butyl-brenzcatechin were not mutagenic to any of the strains studied. Results show that further, more detailed experimental and, possibly, epidemiologic studies are warranted.

Early indicators of renal damage in workers exposed to organic solvents

In order to investigate the renal function, a cross-sectional study was carried out on four groups of workers significantly exposed to a mixture of alicyclic and aliphatic C5-C7 hydrocarbons, to styrene, to a mixture mostly composed of toluene and xylenes and to chlorinated hydrocarbons, respectively. The study involved 438 workers. Exposure was characterized by means of urinary metabolites, or by means of environmental measures, when biological indicators were not available. The renal function impairment indicators included total proteinuria, albuminuria and urinary excretion of muramidase (E.C. 3.2.1.17) and beta-glucuronidase (E.C. 3.2.1.31). The trend of these parameters provides some evidence of renal damage due to occupational exposure to organic solvents and suggests that the lesions are mild and tubular rather than glomerular.

Human peripheral lymphocytes as indicators of microsomal epoxide hydrolase activity in liver and lung

In this study, we have applied an improved assay for the determination of microsomal epoxide hydrolase activity to assess enzymatic levels in human lung, liver, and blood lymphocytes. The assay is fluorescence-based and monitors the epoxide hydrolase-mediated conversion of (+/-)-benzo[a]pyrene-4,5-epoxide to (+/-)-trans-benzo[a]pyrene-4,5-dihydrodiol, using a high pressure liquid chromatography separation system. Approximately a 40-fold range in microsomal epoxide hydrolase activities was detected in blood lymphocytes collected from 70 individual donors. In 38 individuals who were sampled twice after a 3-month interval, the repeatability of an individual's lymphocyte epoxide hydrolase activity was highly correlated (r = 0.80, p < 0.02). In addition, within the same individual there appeared to be a strong correlation between lymphocyte and liver epoxide hydrolase activity (r = 0.92, p = 0.02), and some correlation between liver and lung activity (r = 0.58, p = 0.05). Activities were assessed in lymphocytes from a styrene-exposed worker population but no significant associations between blood concentrations of styrene and epoxide hydrolase activity levels were observed. Neither were any correlations detected in these workers between epoxide hydrolase activities and age, years on the job, alcohol consumption, sex, or smoking status. The results of our study suggest that blood lymphocytes are a useful sentinel cell for epoxide hydrolase activity determinations in individuals, as these measures are relatively stable over time and appear to reflect activity levels in other target organs.

The Effects of GSTM1 and GSTT1 Polymorphisms on Micronucleus Frequencies in Human Lymphocytes In vivo

The influence of genetic polymorphisms in GSTM1 and GSTT1 genes on micronucleus frequencies in human peripheral blood lymphocytes was assessed through a pooled analysis of data from seven laboratories that did biomonitoring studies using the in vivo cytokinesis-block micronucleus assay. A total of 301 nonoccupationally exposed individuals (207 males and 94 females) and 343 workers (237 males and 106 females) occupationally exposed to known or suspected genotoxic substances were analyzed by Poisson regression. The results of the pooled analysis indicate that the GSTT1 null subjects had lower micronucleus frequencies than their positive counterparts in the total population (frequency ratio, 0.55; 95% confidence interval, 0.33-0.89). The protective effect of this genotype is reversed with increasing age, with a frequency ratio of 1.33 (95% confidence interval, 1.06-1.68) in subjects aged 60 years. A significant overall increase in micronucleus frequency with age and gender (P < 0.001 and P = 0.024, respectively) was observed, females having higher micronucleus frequencies than males, when occupationally exposed (P = 0.002). Nonoccupationally exposed smokers had lower micronucleus frequencies than nonsmokers (P = 0.001), whereas no significant difference in micronucleus level was observed between smokers and nonsmokers in the occupationally exposed group (P = 0.79). This study confirms that pooled analyses, by increasing the statistical power, are adequate for assessing the involvement of genetic variants on genome stability and for resolving discrepancies among individual studies.

The effects of occupational exposure to styrene on high-frequency hearing thresholds

Subchronic exposure to styrene has been reported to produce high-frequency hearing loss in rats. In humans, hearing thresholds for higher frequencies (greater than 8 kHz) are also more vulnerable to ototoxic drugs than those at lower frequencies. Since hearing loss at frequencies above 8 kHz does not seem to play a role in speech processing, hearing loss at frequencies above 8 kHz in workers exposed to styrene or other solvents might easily escape detection. Therefore, hearing thresholds were evaluated at frequencies up to 16 kHz in workers exposed to styrene and compared to those of a control group of unexposed workers. The airborne concentrations of styrene typically did not exceed 150 mg/m3 although individual exposures did, at times, reach higher values (up to 700 mg/m3). In accordance with the literature, an age-dependent increase in hearing thresholds at high frequencies was found. Compared to controls, workers exposed to styrene did not appear to demonstrate an aggravated age-dependent decrease in hearing high frequencies. A comparison, however, within the experimental group between the least exposed and the most exposed workers revealed a statistically significant difference on hearing thresholds at high frequencies.

Pneumotoxicity and hepatotoxicity of styrene and styrene oxide

The purpose of this study was to investigate the toxicity of styrene and styrene oxide in the lung in comparison to the toxicity in the liver. Pneumotoxicity caused by styrene or styrene oxide was measured by elevations in the release of gamma-glutamyltranspeptidase (GGT) and lactate dehydrogenase (LDH) into bronchoalveolar lavage fluid (BALF), while hepatotoxicity was measured by increases in serum sorbitol dehydrogenase (SDH) in non-Swiss Albino (Hsd:NSA) mice. Intraperitoneal administration of styrene at doses of 500-1000 mg/kg caused consistent dose-dependent increases in both sets of biomarkers with the hepatic effect appearing earlier than the pulmonary effect. Pyridine, phenobarbital, and beta-naphthoflavone, inducers of CYP2E1, CYP2B, and CYP1A, respectively, increased the toxicity of styrene. Pyridine and phenobarbital treatments increased mortality due to styrene. Styrene oxide exists in two enantiomeric forms: (R)- and (S)-styrene oxide, and the differential toxicities of the two enantiomers and racemic styrene oxide were compared. In all studies, (R)-styrene oxide caused greater toxicity than the (S) enantiomer, especially in the liver. Trichloropropene oxide, an epoxide hydrolase inhibitor, was used to inhibit styrene oxide detoxification and increased its hepatotoxicity, while buthionine sulfoxamine, a glutathione depletor, did not. These results demonstrated the greater role of epoxide hydrolase in styrene oxide detoxification.

Genotoxicity of organic chemicals frequently found in the air of mobile homes

The 19 chemicals most commonly detected in a study of mobile homes in Texas were tested for mutagenicity using a battery of bacterial test strains; the literature was searched to obtain additional information concerning the mutagenicity and carcinogenicity of these chemicals. Formaldehyde was found to be present in 100% of the mobile homes and at the highest mean concentration (167 ppb). The remaining organic chemicals were all present at much lower mean concentrations (less than 10 ppb) and at varying frequencies (2-95%). Of the 19 chemicals tested for mutagenicity, only formaldehyde gave a positive response. A review of the literature revealed that 4 of the chemicals tested, formaldehyde, styrene, tetrachloroethylene and benzene, have been shown to be animal and/or human carcinogens. Thus, formaldehyde is not the only genotoxin present in the air of mobile homes but because it was present in the air of all mobile homes tested at much higher concentrations than the other organic chemicals, formaldehyde should be considered one of the major potential genotoxic hazards present in the air of mobile homes.

Subchronic inhalation studies of styrene in CD rats and CD-1 mice

Groups of 10 male and 10 female Charles River (CRL) CD (Sprague-Dawley-derived) rats were exposed to styrene vapor at 0, 200, 500, 1000, or 1500 ppm 6 hr per day 5 days per week for 13 weeks. Styrene had no effect on survival, hematology, or clinical chemistry. Males at 1500 ppm weighed 10% less after 13 weeks and males and females at 1000 and 1500 ppm consumed more water than controls. Histopathologic changes were confined to the olfactory epithelium of the nasal mucosa. Groups of 20 male and 20 female CRL CD-1 and B6C3F1 mice were exposed to styrene vapor at 0, 15, 60, 250, or 500 ppm 6 hr per day 5 days per week for 2 weeks. Mortality was observed in both CD-1 and B6C3F1 mice exposed to 250 or 500 ppm; more female mice, but not males, died from exposure to 250 ppm than from 500 ppm. Groups of 10 male and 10 female CRL CD-1 mice were exposed to styrene vapors at 0, 50, 100, 150, or 200 ppm 6 hr per day 5 days per week for 13 weeks. Two females exposed to 200 ppm died during the first week. Liver toxicity was evident in the decedents and in some female survivors at 200 ppm. Changes were observed in the lungs of mice exposed to 100, 150, or 200 ppm and in the nasal passages of all treatment groups, those exposed to 50 ppm being less affected. Satellite groups of 15 male rats and 30 male mice were exposed as described above for 2, 5, or 13 weeks for measurement of cell proliferation (BrdU labeling). No increase in cell proliferation was found in liver of rats or mice or in cells of the bronchiolar or alveolar region of the lung of rats. No increase in labeling index of type II pneumocytes was seen in mouse lungs, while at 150 and 200 ppm, an increased labeling index of Clara cells was seen after 2 weeks and in occasional mice after 5 weeks. Large variations in the labeling index among animals emphasize the need for large group sizes. For nasal tract effects, a NOAEL was not found in CD-1 mice, but in CD rats, the NOAEL was 200 ppm. For other effects, the NOAEL was 500 ppm in rats and 50 ppm in mice.

Evidence for DNA and protein binding by styrene and styrene oxide

Styrene is metabolized to styrene oxide, a direct-acting mutagen and carcinogen. Styrene oxide reacts with DNA mainly at the N-7 position in guanine, but also at other sites and with other bases. Substitution occurs at both the alpha- and beta-positions of the styrene molecule. Experiments with radiolabeled styrene and styrene oxide demonstrate that both have a low level of DNA binding activity in experimental animals. 32P-Postlabeling studies have demonstrated the potential of the technique to detect styrene-DNA adducts. Styrene oxide alkylates several nucleophilic sites in proteins, particularly cysteine sulfydryl, histidine imidazole, lysine amino, aspartic, and glutamic carboxylic groups, and the N-terminal position. In experimental animals, styrene oxide treatment results in cysteine adducts in hemoglobin and albumin, valine adducts in hemoglobin, and carboxylic acid adducts in hemoglobin. The extent of alkylation is low compared with that produced by ethylene oxide. The available evidence indicates, therefore, that styrene and styrene oxide have low DNA and protein binding activities in vivo. There is preliminary evidence for the presence of DNA adducts and for adducts in hemoglobin and albumin in blood cells of styrene-exposed workers. Nevertheless, the applicability and sensitivity of DNA and protein adduct detection methods for monitoring human exposure to styrene remain to be determined.

Biological monitoring for mutagenic effects of occupational exposure to butadiene

The use of biological markers in the evaluation of human exposure to hazardous agents has increased rapidly in recent years. Because 1,3-butadiene is a mutagenic carcinogen, existing occupational levels of exposure may be appropriately evaluated using somatic cell mutation as a biomarker. Previously, we have described a biomarker study of workers in a butadiene monomer plant (Ward et al., 1994). We now report results from a second study of the same group of workers, conducted after plant modernization, and present preliminary results from a study of exposures in a styrene butadiene rubber (SBR) plant. Air levels of butadiene were determined using either charcoal tubes with air pumps or passive badge dosimeters. The quantity of a butadiene metabolite in the urine was used as a biomarker of exposure and the mutagenic effects of exposure were measured using the autoradiographic hprt mutant lymphocyte assay. In all three studies, the frequencies of hprt mutants were significantly elevated in workers from the areas of highest exposure when compared to workers from lower exposure areas or non-exposed subjects. The concentration of the urinary metabolite was significantly increased in high-exposed workers in the first study of monomer plant workers but not in the second. In the first monomer plant study, historical air concentrations of butadiene were higher in the production units than in the central control unit. While concurrent determined air concentrations were not elevated in the second monomer plant study, they were elevated in high exposure areas in the SBR plant study. Mutant frequencies in the lower-exposure and the non-exposed groups were consistent with historical values for non-smoking individuals who were not exposed to known mutagens. The use of biomarkers, including the hprt mutant lymphocyte assay, may be of great value in determining an appropriate occupational exposure limit for butadiene.

Styrene Metabolism, Genotoxicity, and Potential Carcinogenicity

This report reviews styrene biotransformation, including minor metabolic routes, and relates metabolism to the genotoxic effects and possible styrene-related carcinogenicity. Styrene is shown to require metabolic activation in order to become notably genotoxic and styrene 7,8-oxide is shown to contribute quantitatively by far the most (in humans more than 95%) to the genotoxicity of styrene, while minor ring oxidation products are also shown to contribute to local toxicities, especially in the respiratory system. Individual susceptibility depending on metabolism polymorphisms and individual DNA repair capacity as well as the dependence of the nonlinearity of the dose-response relationships in the species in question and the consequences for risk evaluation are analyzd.

Biomonitoring human exposure to environmental carcinogenic chemicals

A coordinated study was carried out on the development, evaluation and application of biomonitoring procedures for populations exposed to environmental genotoxic pollutants. The procedures used involved both direct measurement of DNA or protein damage (adducts) and assessment of second biological effects (mutation and cytogenetic damage). Adduct detection at the level of DNA or protein (haemoglobin) was carried out by 32P-postlabelling, immunochemical, HPLC or mass spectrometric methods. Urinary excretion products resulting from DNA damage were also estimated (immunochemical assay, mass spectrometry). The measurement of adducts was focused on those from genotoxicants that result from petrochemical combustion or processing, e.g. low-molecular-weight alkylating agents, PAHs and compounds that cause oxidative DNA damage. Cytogenetic analysis of lymphocytes was undertaken (micronuclei, chromosome aberrations and sister chromatid exchanges) and mutation frequency was estimated at a number of loci including the hprt gene and genes involving in cancer development. Blood and urine samples from individuals exposed to urban pollution were collected. Populations exposed through occupational or medical sources to larger amounts of some of the genotoxic compounds present in the environmental samples were used as positive controls for the environmentally exposed population. Samples from rural areas were used as negative controls. The project has led to new, more sensitive and more selective approaches for detecting carcinogen-induced damage to DNA and proteins, and subsequent biological effects. These methods were validated with the occupational exposures, which showed evidence of DNA and/or protein and/or chromosome damage in workers in a coke oven plant, garage workers exposed to diesel exhaust and workers exposed to ethylene oxide in a sterilization plant. Dose reponse and adduct repair were studied for methylated adducts in patients treated with methylating cytostatic drugs. The biomonitoring methods have also demonstrated their potential for detecting environmental exposure to genotoxic compounds in nine groups of non-smoking individuals, 32P-postlabelling of DNA adducts being shown to have the greatest sensitivity.

Brain dopamine as a target for solvent toxicity: effects of some monocyclic aromatic hydrocarbons

Adult male rabbits were exposed to toluene, xylene, styrene, ethylbenzene, vinyltoluene or were dosed with hippuric, methylhippuric, mandelic, phenylglyoxylic, and 7-methyl-mandelic acids. Styrene, vinyltoluene and ethylbenzene caused a marked depletion of striatal and tubero-infundibular dopamine. Such an effect was also caused by treatment with mandelic and phenylglyoxylic acids. These results indicate that dopamine is a target for some solvents of their metabolites, the presence of a lateral vinyl- or ethyl-chain which may be biotransformed into alpha-keto acids being crucial for the effect. Experiments in vitro suggest that dopamine condenses non-enzymatically with reactive carbonylic groups of such and other alpha-keto acids, thus becoming ineffective as neurotransmitter. This mechanism might account for the neurobehavioral and neuroendocrine changes which have been reported in workers occupationally exposed to styrene and to some solvent mixtures.