Personal breathing zone exposures among hot-mix asphalt paving workers; preliminary analysis for trends and analysis of work practices that resulted in the highest exposure concentrations

An exposure assessment of hot-mix asphalt (HMA) paving workers was conducted to determine which of four exposure scenarios impacted worker exposure and dose. Goals of this report are to present the personal-breathing zone (PBZ) data, discuss the impact of substituting the releasing/cleaning agent, and discuss work practices that resulted in the highest exposure concentration for each analyte. One-hundred-seven PBZ samples were collected from HMA paving workers on days when diesel oil was used as a releasing/cleaning agent. An additional 36 PBZ samples were collected on days when B-100 (100% biodiesel, containing no petroleum-derived products) was used as a substitute releasing/cleaning agent. Twenty-four PBZ samples were collected from a reference group of concrete workers, who also worked in outdoor construction but had no exposure to asphalt emissions. Background and field blank samples were also collected daily. Total particulates and the benzene soluble fraction were determined gravimetrically. Total organic matter was determined using gas chromatography (GC) with flame ionization detection and provided qualitative information about other exposure sources contributing to worker exposure besides asphalt emissions. Thirty-three individual polycyclic aromatic compounds (PACs) were determined using GC with time-of-flight mass spectrometry; results were presented as either the concentration of an individual PAC or a summation of the individual PACs containing either 2- to 3-rings or 4- to 6-rings. Samples were also screened for PACs containing 4- to 6-rings using fluorescence spectroscopy. Arithmetic means, medians, and box plots of the PBZ data were used to evaluate trends in the data. Box plots illustrating the diesel oil results were more variable than the B-100. Also, the highest diesel oil results were much higher in concentration than the highest B-100 results. An analysis of the highest exposure results and field notes revealed a probable association between these exposures and the use of diesel oil, use of a diesel-powered screed, elevated HMA paving application temperatures, lubricating and working on broken-down equipment, and operation of a broom machine.

Using urinary biomarkers of polycyclic aromatic compound exposure to guide exposure-reduction strategies among asphalt paving workers

INTRODUCTION

Paving workers are exposed to polycyclic aromatic compounds (PACs) while working with hot-mix asphalt (HMA). Further characterization of the source and route of these exposures is necessary to guide exposure-reduction strategies.

METHODS

Personal air (n=144), hand-wash (n=144), and urine (n=480) samples were collected from 12 paving workers over 3 workdays during 4 workweeks. Urine samples were collected at preshift, postshift, and bedtime and analyzed for 10 hydroxylated PACs (1-OH-pyrene; 1-, 2-, 3-, 4-OH-phenanthrene; 1-, 2-OH-naphthalene; 2-, 3-, 9-OH-fluorene) by an immunochemical quantification of PACs (I-PACs). The air and hand-wash samples were analyzed for the parent compounds corresponding to the urinary analytes. Using a crossover study design, each of the 4 weeks represented a different exposure scenario: a baseline week (normal conditions), a dermal protection week (protective clothing), a powered air-purifying respirator (PAPR) week, and a biodiesel substitution week (100% biodiesel provided to replace the diesel oil normally used by workers to clean tools and equipment). The urinary analytes were analyzed using linear mixed-effects models.

RESULTS

Postshift and bedtime concentrations were significantly higher than preshift concentrations for most urinary biomarkers. Compared with baseline, urinary analytes were reduced during the dermal protection (29% for 1-OH-pyrene, 15% for I-PACs), the PAPR (24% for 1-OH-pyrene, 15% for I-PACs), and the biodiesel substitution (15% for 1-OH-pyrene) weeks. The effect of PACs in air was different by exposure scenario (biodiesel substitution>dermal protection>PAPR and baseline) and was still a significant predictor of most urinary analytes during the week of PAPR use, suggesting that PACs in air were dermally absorbed. The application temperature of HMA was positively associated with urinary measures, such that an increase from the lowest application temperature (121°C) to the highest (154°C) was associated with a 72% increase in ΣOH-fluorene and 1-OH-pyrene and an 82% increase in ΣOH-phenanthrene. Though PACs in hand-wash samples were not predictors of urinary analytes, the effects observed during the PAPR scenario and the week of increased dermal protection provide evidence of dermal absorption.

CONCLUSIONS

Our results provide evidence that PACs in air are dermally absorbed. Reducing the application temperature of asphalt mix appears to be a promising strategy for reducing PAC exposure among paving workers. Additional reductions may be achieved by requiring increased dermal coverage of workers and by substituting biodiesel for diesel oil as a cleaning agent.

Predictors of dermal exposures to polycyclic aromatic compounds among hot-mix asphalt paving workers

OBJECTIVES

The primary objective of this study was to identify the source and work practices that affect dermal exposure to polycyclic aromatic compounds (PACs) among hot-mix asphalt (HMA) paving workers.

METHODS

Four workers were recruited from each of three asphalt paving crews (12 workers) and were monitored for three consecutive days over 4 weeks for a total of 12 sampling days per worker (144 worker days). Two sampling weeks were conducted under standard conditions for dermal exposures. The third week included the substitution of biodiesel for diesel oil used to clean tools and equipment and the fourth week included dermal protection through the use of gloves, hat and neck cloth, clean pants, and long-sleeved shirts. Dermal exposure to PACs was quantified using two methods: a passive organic dermal (POD) sampler specifically developed for this study and a sunflower oil hand wash technique. Linear mixed-effects models were used to evaluate predictors of PAC exposures.

RESULTS

Dermal exposures measured under all conditions via POD and hand wash were low with most samples for each analyte being below the limit of the detection with the exception of phenanthrene and pyrene. The geometric mean (GM) concentrations of phenanthrene were 0.69 ng cm(-2) on the polypropylene layer of the POD sampler and 1.37 ng cm(-2) in the hand wash sample. The GM concentrations of pyrene were 0.30 ng cm(-2) on the polypropylene layer of the POD sampler and 0.29 ng cm(-2) in the hand wash sample. Both the biodiesel substitution and dermal protection scenarios were effective in reducing dermal exposures. Based on the results of multivariate linear mixed-effects models, increasing frequency of glove use was associated with significant (P < 0.0001) reductions for hand wash and POD phenanthrene and pyrene concentrations; percent reductions ranged from 40 to 90%. Similar reductions in hand wash concentrations of phenanthrene (P = 0.01) and pyrene (P = 0.003) were observed when biodiesel was substituted for diesel oil as a cleaning agent, although reductions were not significant for the POD sampler data. Although task was not a predictor of dermal exposure, job site characteristics such as HMA application temperature, asphalt grade, and asphalt application rate (tons per hour) were found to significantly affect exposure. Predictive models suggest that the combined effect of substituting biodiesel for diesel oil as a cleaning agent, frequent glove use, and reducing the HMA application temperature from 149°C (300°F) to 127°C (260°F) may reduce dermal exposures by 76-86%, varying by analyte and assessment method.

CONCLUSIONS

Promising strategies for reducing dermal exposure to PACs among asphalt paving workers include requiring the use of dermal coverage (e.g. wearing gloves and/or long sleeves), substituting biodiesel for diesel oil as a cleaning agent, and decreasing the HMA application temperature.

Acute symptoms associated with asphalt fume exposure among road pavers

BACKGROUND

Although asphalt fume is a recognized irritant, previous studies of acute symptoms during asphalt paving have produced inconsistent results. Between 1994 and 1997, the National Institute for Occupational Safety and Health (NIOSH) evaluated workers at seven sites in six states.

METHODS

NIOSH (a) measured exposures of asphalt paving workers to total (TP) and benzene-soluble particulate (BSP), polycyclic aromatic compounds, and other substances; (b) administered symptom questionnaires pre-shift, every 2 hr during the shift, and post-shift to asphalt exposed and nonexposed workers; and (c) measured peak expiratory flow rate (PEFR) of asphalt paving workers when they completed a symptom questionnaire.

RESULTS

Full-shift time-weighted average exposures to TP and BSP ranged from 0.01 to 1.30 mg/m(3) and 0.01 to 0.82 mg/m(3), respectively. Most BSP concentrations were <0.50 mg/m(3). Asphalt workers had a higher occurrence rate of throat irritation than nonexposed workers [13% vs. 4%, odds ratio (OR) = 4.0, 95% confidence interval (CI): 1.2-13]. TP, as a continuous variable, was associated with eye (OR = 1.34, 95% CI: 1.12-1.60) and throat (OR = 1.40, 95% CI: 1.06-1.85) symptoms. With TP dichotomous at 0.5 mg/m(3), the ORs and 95% CIs for eye and throat symptoms were 7.5 (1.1-50) and 15 (2.3-103), respectively. BSP, dichotomous at 0.3 mg/m(3), was associated with irritant (eye, nose, or throat) symptoms (OR = 11, 95% CI: 1.5-84). One worker, a smoker, had PEFR-defined bronchial lability, which did not coincide with respiratory symptoms.

CONCLUSIONS

Irritant symptoms were associated with TP and BSP concentrations at or below 0.5 mg/m(3).

Development of a flow-injection fluorescence method for estimation of total polycyclic aromatic compounds in asphalt fumes

Traditionally, measurements of specific polycyclic aromatic compounds (PACs) have been attempted as an estimate of asphalt fume exposure. However, asphalt fumes contain numerous alkyl substituted PACs, including PACs containing heteroatoms of nitrogen, oxygen, and sulfur. Many of these compounds coelute precluding the resolution of the individual compounds resulting in ambiguous data. Moreover, many researchers believe that some observed health hazards are associated with PACs overall and not just a few select PACs. Therefore, NIOSH method 5800 was developed to evaluate total PACs as a chemical class in asphalt fumes. Asphalt fume samples were collected on a poly(tetrafluoroethylene) filter backed by an XAD-2 sorbent tube. The samples were extracted with hexane; then, a cyano-solid-phase-extraction column was used to remove the polar compounds while the aliphatic and aromatic compounds were eluted with hexane. An equal volume of dimethyl sulfoxide (DMSO) was added to the hexane extract, causing the aromatic compounds to partition into the DMSO, thus isolating the PACs. The PACs were then analyzed for fluorescence using a flow-injection method with two fluorescence detectors. Wavelength settings for the first detector (254-nm excitation, 370-nm emission) emphasized the 2- to 4-ring PACs that may cause eye and respiratory tract irritation. Wavelength settings of the second detector (254-nm excitation, 400-nm emission) emphasized the 4- and higher-ring PACs that are often mutagenic and possibly carcinogenic.

DNA strand breaks, oxidative damage, and 1-OH pyrene in roofers with coal-tar pitch dust and/or asphalt fume exposure

OBJECTIVE

To determine the potential for asphalt fume exposure to increase DNA damage, we conducted a cross-sectional study of roofers involved in the application of roofing asphalt.

METHODS

DNA strand breaks and the ratio of 8-hydroxydeoxyguanosine (8-OHdG) to 2-deoxyguanosine (dG) were measured in peripheral blood leukocytes of roofers. In addition, urinary excretion of 8-OHdG and 8-epi-prostaglandin F2alpha (8-epi-PGF) was also measured. The study population consisted of 26 roofers exposed to roofing asphalt and 15 construction workers not exposed to asphalt during the past 5 years. A subset of asphalt roofers (n = 19) was exposed to coal-tar pitch dust (coal tar) during removal of existing roofs prior to applying hot asphalt. Personal air monitoring was performed for one work-week to measure exposure to total particulates, benzene-soluble fraction of total particulates, and polycyclic aromatic compounds (PACs). Urinary 1-OH-pyrene levels were measured as an internal biomarker of PAC exposure.

RESULTS

Full-shift breathing zone measurements for total particulates, benzene-solubles and PACs were significantly higher for coal-tar exposed workers than for roofers not exposed to coal tar. Similarly, urinary 1-OH-pyrene levels were higher in coal-tar exposed roofers than roofers not exposed to coal tar. Total particulates or benzene-soluble fractions were not associated with urinary 1-OH-pyrene, but PAC exposure was highly correlated with urinary 1-OH-pyrene. When stratified by 1-OH-pyrene excretion, DNA strand breaks increased in a dose-dependent manner, and leukocyte 8-OHdG/dG decreased in a dose-dependent manner. Significant changes in DNA damage appeared to be linked to PACs from coal-tar exposure, although asphalt fume alone was associated with a small but significant increase in urinary 1-OH-pyrene and DNA strand breaks.

CONCLUSIONS

Results are consistent with previous reports that asphalt or coal-tar exposure can cause DNA damage. Urinary 8-epi-PGF remained relatively constant during the week for virtually all subjects, regardless of exposure indicating that neither asphalt nor coal-tar exposure induces an overt oxidative stress. A small, but statistically significant increase in 8OHdG was evident in end-of-week urine samples compared with start-of-week urine samples in roofers exposed to coal-tar. The increase in urinary 8OHdG coupled with the decrease in leukocyte 8-OHdG/dG, suggests that coal-tar exposure induces protective or repair mechanisms that result in reduced levels of steady-state oxidative-DNA damage.

Determination of total sulfur compounds and benzothiazole in asphalt fume samples by gas chromatography with sulfur chemiluminescence detection

As part of a collaborative project between the National Institute for Occupational Safety and Health and the Federal Highway Administration to evaluate asphalt pavers' exposures to asphalt fume and their potential health effects, a method was developed for the determination of total sulfur compounds and benzothiazole in asphalt fume samples. Asphalt fume samples were collected from asphalt mixtures with and without the addition of ground-up rubber tires. The asphalt fume samples were collected with sampling trains that consisted of a Teflon membrane filter and an XAD-2 adsorbent tube. Filter and sampling tube media were extracted with hexane and subsequently analyzed by gas chromatography with a sulfur chemiluminescence detector. Separation was achieved with a 100 percent dimethyl polysiloxane fused silica column. Typical calibration curves had linear correlation coefficients of 0.99 or better with a relative standard deviation (RSD) of 5 percent. Benzothiazole desorption efficiency (DE) determined using spiked sampling tubes ranged from 96.5 percent at 5.0 micrograms to 89.4 percent at 40 micrograms with RSD values from 0.9 to 4.0 percent. Benzothiazole storage recovery determined using sampling tubes spiked at 20 micrograms and refrigerated for 30 days at 4 degrees C was 89.8 percent when corrected for the DE with an RSD of 1.1 percent. The limit of detection for the method determined using spiked sampling tubes was 0.30 microgram. Quantitation for total sulfur compounds and benzothiazole was against benzothiazole standards in hexane. Because of detector selectivity, sample preparation consisted of a simple hexane extraction even when samples had a high background due to hydrocarbon overload. Detector sensitivity provided quantitation in the sub-microgram region. Because of the sample preparation step and because benzothiazole was determined during the same analysis run, this method is straightforward and analytically efficient. The method has been used to analyze asphalt fume samples collected at several asphalt paving and roof operations.

Dip5p mediates high-affinity and high-capacity transport of L-glutamate and L-aspartate in Saccharomyces cerevisiae

Genes encoding homologues of known amino-acid permeases were deleted in a strain also deficient in the general amino-acid permease. The uptake capacity of the mutants was investigated for several L-alpha-amino acids. Deletion of a gene denoted DIP5 results in the loss of L-aspartate and L-glutamate uptake. The dip5 mutation caused a several hundred-fold reduction of uptake of the two amino acids, both in cells grown on proline as a nitrogen source and in cells grown on ammonium. DIP5-dependent uptake of L-aspartate and L-glutamate was somewhat lower in ammonium-grown cells than in proline-grown cells. Transcriptional regulation is at least partially responsible for this difference, as shown by assaying the DIP5 promoter fused to lacZ. This suggests that the promoter is subject to nitrogen catabolite repression. Transport of a few other amino acids was moderately affected by dip5 but was not competed by L-aspartate in the DIP5 parental strain; transport of these amino acids is therefore unlikely to be mediated by Dip5p. Our results suggest that DIP5 encodes an amino-acid permease with a high transport capacity and a high affinity for L-glutamate and L-aspartate, with a Kt of about 50 microM for both.

Stereoselective S-oxygenation of 2-aryl-1,3-dithiolanes by the flavin-containing and cytochrome P-450 monooxygenases

The reaction of NalO4, highly purified flavin-containing monooxygenase (EC 1.14.13.8), and microsomes from hog liver with 2-aryl-1,3-dithiolanes and 2-aryl-1,3-dithiolane S-oxides was investigated. The initial rates determined for the microsome- and purified flavin-containing monooxygenase-catalyzed rate of S-oxidation of para-substituted 2-aryl-1,3-dithiolanes were similar, demonstrating that S-oxidation of these substrates occurred with similar velocities at saturating concentrations of substrate and, at least for the first S-oxidation, the reaction was insensitive to the nature of the para-substituent. The diastereoselectivity of S-oxygenation of 2-aryl-1,3-dithiolanes was determined and, in general, a marked preference for addition of oxygen to the sulfide sulfur atom was observed to occur trans to the aryl groups. In all cases examined, enantioselective enzymatic S-oxidation was observed. For S-oxide formation in microsomes, the data provided evidence for a minor role of cytochrome P-450 in S-oxide formation, but the flavin-containing monooxygenase was mainly responsible for production of S-oxide. In contrast to previous reports, the enantioselectivity of S-oxidation catalyzed by highly purified cytochrome P-450IIB-1 and cytochrome P-450IIB-10 was not always opposite to that catalyzed by hog liver flavin-containing monooxygenase activity. 2-Aryl-1,3-dithiolane S-oxides were also oxidized a second time by NalO4, microsomes, or highly purified flavin-containing monooxygenase from hog liver but not cytochrome P-450IIB-1 or P-450IIB-10. The rate of the second oxidation was 10-15-fold slower than the corresponding first S-oxidation and S,S'-dioxide formation was markedly dependent on the electronic nature of the para-substituent (Hammett correlation rho value of -1.3 and -1.1 for microsomes and highly purified flavin-containing monooxygenase from hog liver, respectively). The large dependence of the rate of S,S'-dioxide formation on the nature of the para-substituent demonstrates that velocity values at saturating concentrations of S-oxide were not the same for all 2-aryl-1,3-dithiolane S-oxides and suggests that the chemical nature of the 2-aryl-1,3-dithiolane S-oxide contributes to the rate-determining step of this enzymatic reaction.

S-oxygenation of thiobencarb (Bolero) in hepatic preparations from striped bass (Morone saxatilis) and mammalian systems

The in vitro S-oxygenation of thiobencarb (Bolero; p-chlorobenzyl N,N-diethylthiocarbamate) in the presence of hepatic microsomes from freshwater- and seawater-adapted striped bass was investigated. Thiobencarb S-oxide was the principal metabolite and accounted for 98% of the total thiobencarb metabolized by striped bass liver microsomes. Studies on the biochemical mechanisms for striped bass hepatic S-oxygenation suggest that this reaction is catalyzed largely by the flavin-containing monooxygenase and to a lesser extent by cytochromes P-450. Following the short incubation period used, no thiobencarb sulfone was detected and no evidence was found for a contribution of cooxidation in the S-oxidation of thiobencarb. This conclusion was supported by studies with microsomes and purified mammalian monooxygenases which also metabolized thiobencarb without cooxidizing factors. Highly purified cytochrome P-450IIB-1 S-oxygenated thiobencarb more efficiently than highly purified hog liver flavin-containing monoxygenase. Thiobencarb S-oxide and thiobencarb sulfone were efficient carbamylating agents and reacted with thiol and amine nucleophiles, whereas thiobencarb itself was relatively stable to transthiocarbamylation. Monooxygenase-catalyzed S-oxygenation of thiobencarb by striped bass liver microsomes may represent a bioactivation process which could explain the known toxicity of thiobencarb in fish.

Enantioselective S-oxygenation by flavin-containing and cytochrome P-450 monooxygenases

The reaction of the modified Sharpless reagent, as well as microsomes and highly purified flavin-containing monooxygenase from hog liver, and cytochrome P-450IIB-1 from rat liver efficiently S-oxygenates 2-aryl-1,3-oxathiolanes with significant diastereoselectivity and enantioselectivity. The absolute configuration of the synthetic S-oxides and the enzyme-derived S-oxides was correlated by NMR analysis and by the sign of the Cotton effects obtained from circular dichroism studies. of the sulfides studied, the trans-S-oxide was the major diastereomer produced from monooxygenase-catalyzed biotransformations. In all cases examined, enantioselective S-oxygenation was observed although enantiomeric excess varied from 7 to 100%. In contrast to previous reports, the enantioselectivity of S-oxygenation catalyzed by cytochrome P-450IIB-1 was not always opposite to that of hog liver flavin-containing monooxygenase activity. The presence of the minor S-oxide diastereomers in each case was due to incomplete chiral processing by each monooxygenase and not to a competing achiral nonenzymatic process. The results suggest that the active site of hog liver flavin-containing monooxygenase places greater constraints than that of cytochrome P-450IIB-1 on substrate orientation, but in both cases trans-S-oxide formation is strongly preferred possibly due to steric interactions of the substrate and the active site.

Conjugate addition ligands of opioid antagonists. Methacrylate esters and ethers of 6 alpha- and 6 beta-naltrexol

Alpha- and beta-naltrexol derived esters 9 and 10 and ethers 11 and 12, each containing the alpha, beta-unsaturated ester functionality, were prepared as conformationally more flexible analogues of spiro-alpha-methylene-gamma-lactones 5 and 6. All were active in the opioid radioreceptor binding assay against [3H]bremazocine and more active against [3H]DAGO, indicating mu-subtype selectivity, but only ether 12 showed significant irreversible activity. We conclude that small structural changes, made in very closely related electrophilic opioids, lead to changes in receptor binding. All four compounds were long-acting antagonists to morphine in mice, with ester 10 being approximately equipotent with naltrexone.

Enantioselective S-oxygenation of para-methoxyphenyl-1,3-dithiolane by various tissue preparations: effect of estradiol

Liver, kidney, and lung microsomes prepared from nonpretreated female Sprague-Dawley rats catalyze the NADPH- and oxygen-dependent S-oxygenation of para-methoxyphenyl-1,3-dithiolane. Studies on the biochemical mechanism of dithiolane S-oxygenation in liver, kidney, and lung microsomes suggest that this reaction is catalyzed in a diastereoselective and enantioselective fashion by the flavin-containing monooxygenase and, to a lesser extent, the cytochromes P-450. This conclusion is based on results examining the effects of selective cytochrome P-450 inhibitors and positive effectors, microsome heat-inactivation treatment, and alternate substrates for the flavin-containing monooxygenase. Liver and kidney microsomes prepared from ovarectomized female rats tended to have decreased S-oxygenase activity, compared with nonpretreated female rats, whereas ovarectomized rats pretreated with estradiol had markedly lower S-oxygenase activity. In contrast, lung microsomal S-oxygenase activity, which is low in pulmonary microsomes from nonpretreated female rats, increases 2-4-fold after ovariectomization and estradiol pretreatment. In female Sprague-Dawley rats, estradiol pretreatment is mainly responsible for the large decrease (or increase) in S-oxygenase activity observed in the tissues examined, although it is unlikely that estradiol alone controls flavin-containing monooxygenase S-oxygenase activity.

S-oxygenation of eptam in hepatic microsomes from fresh- and saltwater striped bass (Morone saxatilis)

The in vitro liver microsomal oxidation of eptam (ethyl N,N-dipropylthiocarbamate) in the presence of freshwater and salt water adapted striped bass liver microsomes was investigated. In freshwater hepatic microsomes from striped bass, eptam is S-oxygenated in a process consistent with the involvement of monooxygenase activity. In contrast, both eptam S-oxide and eptam sulfone are formed in microsomes from salt water adapted striped bass microsomes in a process that is independent of monooxygenase activity and consistent with a role of cooxidation by hydroperoxy fatty acids. The mechanism of oxidation of eptam by hydroperoxy fatty acids may involve radical species. Both eptam S-oxide and eptam sulfone are efficient carbamylating agents toward thiol nucleophiles and react with substituted thiophenols to produce thiocarbamates while eptam itself is relatively stable to trans thiocarbamylation. Monooxygenase-catalyzed S-oxygenation of eptam in freshwater striped bass hepatic microsomes may represent a bioactivation route, which may explain the toxicity of thiocarbamate herbicides such as eptam toward freshwater fish.

Regiochemistry and enantioselectivity in the oxidative N-dealkylation of verapamil

The oxidative N-dealkylation of verapamil (1), a calcium channel antagonist, was examined in the presence of rat and human liver microsomes by using GC-MS methodology and synthesized regio-isomeric standards. All three possible secondary amine metabolites, N-methyl-4-(3,4-dimethoxyphenyl)-4-cyano-5-methylhexylamine (5), norverapamil (4), and N-methyl-2-(3,4-dimethoxyphenyl)ethylamine (3), were formed as microsomal metabolites. Compound 5 and norverapamil (4) were major products. Substrate stereoselectivity for the N-dealkylation process was determined when pseudoracemic verapamil[equimolar (S)-(-)-verapamil-d6 and (R)-(+):verapamil-d0] was used as substrate. In the presence of rat liver microsomes, a slight enantiomeric preference for the metabolism of (R)-verapamil to secondary amines 3 and 5 (S/R ratio = 0.88 and 0.78, respectively) was observed. In contrast, (S)-verapamil was preferentially metabolized to norverapamil (4) and primary amine 9 (S/R ratio = 1.20 for both). The enantioselectivity for the N-dealkylation process in the presence of human liver microsomes was slight and variable (six samples). Quantitatively, the major N-dealkylation routes in both microsomal systems yielded norverapamil (4) and secondary amine 5. Greater substrate enantioselectivity was observed for the N-dealkylation process in rat liver microsomes than in human liver microsomes. In rat liver microsomal studies, two aliphatic aldehydes (2 and 6) were successfully trapped as their O-methyloximes (7 and 11, respectively) by using methoxylamine. In addition, the alcohols formed from reduction of these aldehydes were observed, due in part to a direct reduction by NADPH.(ABSTRACT TRUNCATED AT 250 WORDS)

Regiochemistry and substrate stereoselectivity of O-demethylation of verapamil in the presence of the microsomal fraction from rat and human liver

The oxidative O-demethylation of verapamil (1), a calcium channel antagonist, in the presence of rat and human liver microsomes was examined. By using GC/MS methodology and synthesized regioisomeric standards, we showed that three of the four possible monophenolic metabolites, alpha-[3-([2-(3,4-dimethoxyphenyl)ethyl]methyl-amino) propyl]-3-methoxy-4-hydroxy-alpha-(1-methylethyl)phenyl-acetonitrile (2), alpha-[3-([2-(3,4-methoxyphenyl)ethyl]methyl-amino) propyl]-3-hydroxy-4-methoxy-alpha-(1-methylethyl)phenylaceto nitrile (3), and alpha-[3-([2-(3-methoxy-4-hydroxyphenyl)ethyl]methylamino) propyl]-3,4-dimethoxy-alpha-(1-methylethyl)phenylacetonitrile (4) were formed. The other possible regioisomeric monophenolic metabolite 5 was not observed. Substrate stereoselectivity for the O-demethylation process was determined when pseudoracemic verapamil [equimolar (S)-(-)-verapamil-d6 and (R)-(+)-verapamil-d0] was used as substrate. In the presence of rat liver microsomes, significant substrate stereoselectivity was observed for formation of 4 (S/R ratio 2.28), whereas marginal substrate stereoselectivity was observed in the formation of both 3 and 2 (S/R ratio approximately 0.8). Substrate stereoselectivity for the O-demethylation process in the presence of human liver microsomes was slight and variable (six samples). Quantitatively, the ratio of O-demethylation products obtained (4:2:3) was similar in the presence of rat and human liver microsomes. In both systems, more than one-half of the total O-demethylation occurred in the aromatic ring of the phenethylamine moiety, and of the total O-demethylation process, more para- than meta-O-demethylation was observed. The similarity of regioselectivity for O-demethylation in the presence of rat and human liver microsomes suggests a similar cytochrome P-450 isozyme or set of isozymes may be responsible for the O-demethylation process.