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Research Article

Materials and Methods Top

Study subjects. Twenty adults living in the Raleigh-Durham area of North Carolina volunteered for the study in response to an announcement that included a nominal payment in return for time spent for the experiment. None were affiliated with the National Institute of Environmental Health Sciences (NIEHS). The study protocol, including informed consent procedure, was approved by the appropriate institutional review boards for the protection of human subjects at the National Institutes of Health.

Volunteers were screened on the basis of a standard occupational/environmental medical history from Duke Medical Center (Durham, NC) (Duke Occupational Health Service 1966). Pregnant women and persons with a diagnosed illness were excluded, as were persons taking any medicine for treatment of illness, with the exception of aspirin, acetaminophen, allergy medications, and oral contraceptives. Five male and four female volunteers met the inclusion criteria and agreed to participate.

Feeding study. The volunteers were instructed to fast from midnight until reporting to the NIEHS health station for the study at 0730 hr. Each person arrived with a completed food survey listing all foods consumed after dinner the previous evening. There were no dietary restrictions. Height, weight, temperature, and blood pressure were recorded, followed by an initial blood draw.

Volunteers were then served a breakfast known to contain methyleugenol, consisting of the brand of gingersnaps found on initial analysis to contain approximately 3.3 µg methyleugenol per gram, or 18 µg/cookie. The gingersnaps were purchased locally from a large supermarket chain. Orange juice was provided as a beverage. Each subject's meal consisted of 12 gingersnaps, containing approximately 216 µg methyleugenol. This was the largest practical dosing felt to be representative of a moderate dietary intake in average adult volunteers ingesting commercial food containing methyleugenol.

Specimen collection. Blood was collected at 15, 30, 60, and 120 min after the meal of gingersnaps. The times were based on a model of the anticipated rates of absorption, distribution, and elimination of methyleugenol in humans developed before contracting this study (Blumenthal G, personal communication). Blood samples were centrifuged and the cell layer discarded. Serum for future study were frozen after collection and sent to CDC frozen on dry ice.

Methyleugenol analysis. Methyleugenol analyses were performed on a 4-mL aliquot of serum spiked with a ¹³C₃-labeled methyleugenol internal standard (Barr et al. 2000). The serum was filtered to 0.2 µL and denatured with 50% formic acid, and then aspirated through a C₁₈ solid-phase extraction cartridge. The cartridge was washed with water and eluted with methylene chloride. The eluate was cleaned up on a silica cartridge with 1 g anhydrous sodium sulfate and then concentrated to 10 µL after addition of a keeper solvent and recovery standard. A 2-µL sample was analyzed by gas chromatography-high-resolution mass spectrometry in the single-ion monitoring mode at 10,000 resolution. Two ions were monitored for methyleugenol (m/z 178.0994, which is M⁺, and m/z 163.0759, which is M-CH₃⁺) and one ion each for the labeled internal standard (m/z 181.1094) and recovery standard [m/z 164.0473; 3´,4´-(methylenedioxy)-acetophenone]. The 178.0994 ion was used for quantification and the 163.0759 for confirmation. The limit of detection of the method was 3.1 pg/g, and the relative SD (RSD) ranged from 8.9% to 21%. The higher RSDs were seen at the low concentration end of the linear range. Extensive checks for quality control and quality assurance were incorporated into the method (Barr et al 2000).

Results Top

Subjects. Age, sex, height, and weight of subjects are shown in Table 1. All were healthy residents of North Carolina living in or near Durham, Cary, or Research Triangle Park.

Serum levels. Measured levels of methyleugenol as picograms per gram (parts per trillion) in serum are presented in Table 2 and as nanograms per gram (parts per billion), lipid basis, in Table 3. The time course of median wet weight serum levels is represented graphically in Figure 1. The median fasting level of methyleugenol in serum was 13 pg/g wet weight (range, < 3.1-37 pg/g). The median serum level at 15 min peaked at 54 pg/g wet weight (range, 25-100 pg/g). The median level declined to 40 pg/g (range, 26-99 pg/g) at 30 min, to 29 pg/g (range, 13-94 pg/g) at 1 hr, and to 20 pg/g (range, 15-61 pg/g) at 2 hr.

Table 1.

Table 2.

Table 3.

Figure 1.

Median of methyleugenol serum concentrations for nine subjects [pg/g (ppt) wet weight]. Error bars indicate SE.

Discussion Top

Results for methyleugenol levels in serum from the human clinical study, in which subjects ate gingersnaps, were consistent with expectations (based on elimination rates in male and female F344/N rats and B6C3F₁ mice) that methyleugenol blood levels in human adults would peak rapidly after a meal and have a half-life of elimination of about 90 min. The mean ± SD methyleugenol plasma concentrations (wet weight) at the lowest dose (37 mg/kg) 15 min after gavage exposure were 573 ± 228 ng/g (ppb) for male rats and 652 ng/g (two samples, no SD) for female rats. In mice, the mean ± SD concentrations in plasma (wet weight) for the lowest exposure group (37 mg/kg) 10 min after exposure were 417 ± 128 ng/g for males and 681 ± 50 ng/g for females (NTP 1998). At these doses, the increased liver cancer risks were 17.5%, 17.1%, 34%, and 52.2% in male and female rats and male and female mice, respectively. The human plasma concentrations were roughly 10,000 times smaller, with an average concentration of 0.0539 ± 0.0083 ng/g. The average weight for humans was 68.3 kg yielding a dosage of 3.16 µg/kg, which is approximately 10,000 times smaller than the dose given the rodents.

For humans, the estimated half-life of methyleugenol was approximately 100 min (95% confidence interval, 1-6 hr).

In a concurrent study of 213 nonfasting subjects in the third National Health and Nutrition Examination Survey (NHANES III, 1988-1994), the range of serum levels of methyleugenol was < 3.1-390 pg/g, with a median of 16 pg/g (National Center for Health Statistics 1994). In 16 subjects, serum levels of methyleugenol exceeded 54 pg/g, the median peak level observed in the clinical study. Four NHANES subjects had serum levels exceeding 100 pg/g, the highest peak level measured in this clinical experiment. The maximum concentration found, 390 pg/g, is nearly four times the highest concentration found in this clinical study but still nearly 2,000 times less than peak levels observed at the lowest dose used in the NTP rodent studies (Barr et al. 2000).

Methyleugenol has been proposed to be metabolized extensively by various cytochrome P450 isozymes, with metabolism rates by human liver microsomal preparations varying more than 37-fold (Gardner et al. 1997). Therefore, this broad range of concentrations found in the NHANES sample (from below the limit of detection to 300 pg/g) may represent a combination of differences in metabolic rates, time since last meal, and food consumed at last meal.

The finding of methyleugenol in the blood of the general U.S. population at levels higher than the peak levels obtained in the clinical study was unexpected, and the significance of these levels with respect to health consequences remains to be determined. The information from these studies has been shared with the U.S. Food and Drug Administration. The reasons for an elevated level in a substantial number of adults in the general U.S. population require further investigation, as does the risk of cancer and other health effects associated with background levels of exposure to methyleugenol.

References Top

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