Elevated HPRT mutation frequencies in aflatoxin-exposed residents of daxin, Qidong county, People's Republic of China

Molecular biomarkers are becoming increasingly important tools to identify people who are at highest risk of developing cancer. For many years we have been studying residents of Qidong County, People's Republic of China, to examine the combined impact of aflatoxin exposure with other risk factors as contributors to the high liver cancer incidence rates in this region. This study was conducted to determine the effects of aflatoxin exposure, as measured by serum aflatoxin-albumin adduct levels, on somatic mutation frequency in the human hypoxanthine guanine phosphoribosyl transferase gene (HPRT). Subjects were assigned as low or high according to a dichotomization around the population mean of aflatoxin-albumin adducts. HPRT mutant frequency was determined in individuals by a T cell clonal assay and the samples were categorized as low or high according to mean values. Separate analyses were also conducted for the small set of hepatitis B virus surface antigen (HBsAg)-positive and the larger set of HBsAg-negative individuals, known risk factors for liver cancer. An odds ratio of 19.3 (95% confidence interval 2.0, 183) was demonstrated for a high HPRT mutation frequency in individuals with high aflatoxin exposure compared with those with low aflatoxin exposure. This association indicates that aflatoxin-induced DNA damage in T lymphocytes, assessed using the validated surrogate albumin adduct markers, leads to increased mutations reflected as elevated HPRT gene mutations. This cross-sectional study suggests the potential use of mutation frequency of the HPRT gene as a long-term biomarker of aflatoxin exposure in high risk populations.

Protective alterations in phase 1 and 2 metabolism of aflatoxin B1 by oltipraz in residents of Qidong, People's Republic of China

BACKGROUND

Residents of Qidong, People's Republic of China, are at high risk for development of hepatocellular carcinoma, in part due to consumption of foods contaminated with aflatoxins, which require metabolic activation to become carcinogenic. In a randomized, placebo-controlled, double-blind phase IIa chemoprevention trial, we tested oltipraz, an antischistosomal drug that has been shown to be a potent and effective inhibitor of aflatoxin-induced hepatocarcinogenesis in animal models.

METHODS

In 1995, 234 adults from Qidong were enrolled. Healthy eligible individuals were randomly assigned to receive by mouth 125 mg oltipraz daily, 500 mg oltipraz weekly, or a placebo. Sequential immunoaffinity chromatography and liquid chromatography coupled to mass spectrometry or to fluorescence detection were used to identify and quantify phase 1 and phase 2 metabolites of aflatoxin B1 in the urine of study participants. Reported P values are two-sided.

RESULTS

One month of weekly administration of 500 mg oltipraz led to a 51% decrease in median levels of the phase 1 metabolite aflatoxin M1 excreted in urine compared with administration of a placebo (P = .030), but it had no effect on levels of a phase 2 metabolite, aflatoxin-mercapturic acid (P = .871). By contrast, daily intervention with 125 mg oltipraz led to a 2.6-fold increase in median aflatoxin-mercapturic acid excretion (P = .017) but had no effect on excreted aflatoxin M1 levels (P = .682).

CONCLUSIONS

Intermittent, high-dose oltipraz inhibited phase 1 activation of aflatoxins, and sustained low-dose oltipraz increased phase 2 conjugation of aflatoxin, yielding higher levels of aflatoxin-mercapturic acid. While both mechanisms can contribute to protection, this study highlights the feasibility of inducing phase 2 enzymes as a chemopreventive strategy in humans.

Oltipraz chemoprevention trial in Qidong, Jiangsu Province, People's Republic of China

Oltipraz has been used clinically in many regions of the world as an antischistosomal agent and is an effective inhibitor of aflatoxin hepatocarcinogenesis in rats. This chemopreventive action of oltipraz results primarily from an altered balance in aflatoxin metabolic activation and detoxication. In 1995, a randomized, placebo-controlled, double-blind intervention was conducted in residents of Qidong, People's Republic of China, who are at high risk for exposure to aflatoxin and development of hepatocellular carcinoma. The major study objectives were to define a dose and schedule for oltipraz that would reduce levels of aflatoxin biomarkers in biofluids of the participants, and to further characterize dose-limiting side effects. Two hundred thirty-four healthy eligible individuals, including those infected with HBV, were randomized to receive either 125 mg oltipraz daily, 500 mg oltipraz weekly, or placebo. Blood and urine specimens were collected to monitor potential toxicities and evaluate biomarkers over the 8-week intervention and subsequent 8-week follow-up periods. Overall, compliance in the intervention was excellent; approximately 85% of the participants completed the study. Objective evaluation of adverse events was greatly facilitated by inclusion of a placebo arm in the study design. A syndrome involving numbness, tingling, and pain in the fingertips was the only event that occurred more frequently among the active groups (18 and 14% of the daily 125 mg and weekly 500 mg arms, respectively) compared to placebo (3%). These symptoms were reversible and could be relieved with non-steroidal antiinflammatory agents. A more complete understanding of the chemopreventive utility of oltipraz awaits completion of an assessment of the efficacy of oltipraz in modulating levels of aflatoxin biomarkers.

Oltipraz chemoprevention trial in Qidong, People's Republic of China: modulation of serum aflatoxin albumin adduct biomarkers

In 1995, 234 adults from Qidong, People's Republic of China, were enrolled and followed in a Phase IIa 4-methyl-5-(N-2-pyrazinyl)-1,2-dithiole-3-thione (oltipraz) chemoprevention trial. Residents of this area are at high risk for development of hepatocellular carcinoma, in part due to consumption of aflatoxin-contaminated foods. The intervention was a randomized, placebo-controlled, double-blind study. Elements of the study design and clinical outcomes have been recently published (Jacobson et al, Cancer Epidemiol. Biomark. Prev., 6: 257-265, 1997). The primary objective was to conduct a preliminary assessment of the ability of oltipraz to modulate levels of a validated biomarker of aflatoxin exposure and of the risk of hepatocellular carcinoma by determining levels of aflatoxin-albumin adducts in sera. Healthy eligible individuals were randomized into three arms to receive p.o. 125 mg of oltipraz daily, 500 mg of oltipraz weekly, or placebo for 8 weeks. There were no consistent changes in biomarker levels in the placebo arm over the 16-week observation period, nor was any apparent effect observed in the arm receiving 125 mg of oltipraz each day. However, individuals receiving 500 mg of oltipraz once a week for 8 weeks showed a triphasic response to oltipraz. No effect was observed during the 1st month of the intervention, whereas a significant (P = 0.001) diminution in adduct levels was observed during the 2nd month of active intervention and during the lst month of follow-up. A partial rebound in adduct levels toward baseline values was observed during the 2nd month postintervention. Linear regression models up to week 13 confirmed a significant (P = 0.008) weekly decline of biomarker levels in the group receiving 500 mg of oltipraz once a week. However, despite these effects relative to baseline values within the 500-mg weekly arm, there were no statistically significant differences in biomarker trajectories between treatment arms. The genotype for glutathione S-transferase M1, an oltipraz-inducible isoform involved in the detoxification of aflatoxin B1, did not appear to affect either baseline levels or rates of decline in the biomarker. A follow-up Phase IIb trial with a longer intervention period will be necessary to determine the full extent to which aflatoxin biomarker burden can be reduced and whether diminution of biomarkers can be sustained over the long term.

Oltipraz chemoprevention trial in Qidong, People's Republic of China: study design and clinical outcomes

In 1995, 234 adults from Qidong, Jiangsu Province, People's Republic of China, where hepatocellular carcinoma is the leading cause of cancer deaths and exposure to dietary aflatoxins is widespread, were enrolled and followed in a Phase II chemoprevention trial. The goals of the study were to define a dose and schedule of oltipraz for reducing levels of validated aflatoxin biomarkers and to characterize dose-limiting toxicities. Healthy eligible individuals, including those infected with hepatitis B virus, were randomized to receive either 125 mg of oltipraz daily, 500 mg of oltipraz weekly, or placebo. Blood and urine specimens were collected to monitor toxicities and evaluate biomarkers over the 8-week intervention period and subsequent 8-week follow-up period. Unique trial aspects included a synchronous follow-up schedule, daily observed administration of all medications, timely international data transference, and use of biomarkers as outcomes. One hundred thirty-two participants took their medications without interruptions, approximately 77% contributed all nine urine samples, and 78% contributed all seven blood samples. Fifty-one participants (21.8%) reported clinical adverse events. An extremity syndrome, developing soon after the start of treatment, was the only event that occurred more frequently (P = 0.002) among the active groups (18.4 and 14.1% of the daily 125 and weekly 500 mg arms, respectively) compared with placebo (2.5%). The oltipraz arms did not differ in symptom type or severity, and there were no indications of exacerbated drug intolerance among the few participants infected with hepatitis B virus. The good compliance with an intense follow-up schedule shows that chemoprevention trials with biomarker end points may be conducted in such populations.

Temporal patterns of aflatoxin-albumin adducts in hepatitis B surface antigen-positive and antigen-negative residents of Daxin, Qidong County, People's Republic of China

Molecular epidemiological studies of populations at high risk for liver cancer have shown that hepatitis B virus (HBV) and aflatoxin B1 (AFB1) exposure are two major risk factors for this disease. These etiological agents, combined with nutritional deficiencies, are important for the initiation and promotion of liver cancer in various parts of the world. In Qidong, People's Republic of China, liver cancer accounts for 10% of all adult deaths, and both HBV and AFB1 exposures are common. To study temporal and possible chemical-viral interactions in people, serum samples were collected during a longitudinal study designed to measure aflatoxin molecular biomarkers in residents of Daxin Township, Qidong City, People's Republic of China. In this study, the temporal modulation of aflatoxin adduct formation with albumin over multiple lifetimes of serum albumin was examined in both HBV-positive and HBV-negative people in two periods: September-December 1993 (wave 1) and June-September 1994 (wave 2). During the 12-week monitoring period of wave 1, 120 individuals (balanced by gender and HBV status) provided a total of 792 blood samples. AFB1-albumin adducts were detected in all but one of the serum samples. The range of binding detected by RIA in the Daxin population was 0.17-4.39 pmol AFB11/mg albumin with an overall mean +/- SD of 1.51 +/- 0.21 pmol AFB11/mg albumin. The mean +/- SD for weeks 0, 2, 4, 6, 8, 10 and 12 of wave 1 were 1.21 +/- 0.41, 1.58 +/- 0.70, 1.36 +/- 0.52, 1.71 +/- 0.44, 1.18 +/- 0.60, 2.00 +/- 0.59, and 1.68 +/- 0.34 pmol AFB1/mg albumin, respectively. During wave 2, 103 individuals from wave 1 provided a total of 396 blood samples collected monthly over wave 2, with mean +/- SD aflatoxin-albumin adduct levels of 1.19 +/- 0.37, 0.85 +/- 0.45, 0.89 +/- 0.28, and 0.61 +/- 0.15 pmol AFB1/mg albumin. Using linear regression models, the mean aflatoxin-albumin adduct levels increased (P < 0.05) during the 12 weeks of wave 1 and decreased (P < 0.05) over the 4 months of wave 2. Neither HBV surface antigen status nor gender modified either the baseline mean or the temporal trend. High-performance liquid chromatography confirmation was done on a subset of serum samples, and the results show an excellent association between the immunoassay data and high-performance liquid chromatography. Taken together, these data demonstrate that AFB1-albumin is a sensitive and specific biomarker for assessing exposure to this carcinogen in the population in Qidong.

Respiratory aflatoxicosis: suppression of pulmonary and systemic host defenses in rats and mice

Dietary aflatoxin B1 (AFB1) exposure impairs innate and acquired host defenses resulting in increased susceptibility to infections in domesticated animals. Experimental studies have confirmed this observation by demonstrating the immunosuppressive effects of AFB1 ingestion. In addition to being present in dietary components, AFB1 is also found in significant amounts in respirable particles of grain dust. To determine the effect of respiratory tract exposure to AFB1 on host defenses, rats and mice were exposed either by aerosol inhalation or intratracheal instillation to AFB1. Nose-only inhalation exposure of rats to AFB1 aerosols suppressed alveolar macrophage (AM) phagocytosis at an estimated dose of 16.8 micrograms/kg with the effect persisting for approximately 2 weeks. To determine whether another mode of respiratory tract exposure, intratracheal instillation, reflected inhalation exposure, animals were treated with increasing concentrations of AFB1 which also suppressed AM phagocytosis in a dose-related manner albeit at doses at least an order of magnitude more than that obtained by aerosol inhalation. Intratracheal administration of AFB1 also suppressed the release of tumor necrosis factor-alpha from AMs and impaired systemic innate and acquired immune defenses as shown, respectively, by suppression of peritoneal macrophage phagocytosis and the primary splenic antibody response. These findings demonstrate that experimental respiratory tract exposure to AFB1 suppresses pulmonary and systemic host defenses and indicates that inhalation exposure to AFB1 is an occupational hazard where exposure to AFB1-laden dust is common.

Urinary 6 beta-hydroxycortisol/cortisol ratios measured by high-performance liquid chromatography for use as a biomarker for the human cytochrome P-450 3A4

Cortisol is metabolized to 6 beta-hydroxycortisol by the human cytochrome P-450 3A4, an enzyme implicated in the critical epoxidation reactions of aflatoxin B1 and certain polycyclic aromatic hydrocarbons. A method has been developed to measure the 6 beta-hydroxycortisol:cortisol ratio in human urine by reverse-phase high-performance liquid chromatography using diode-array detection. This method permits measurement of both cortisols in a single chromatographic run and has the potential to measure other metabolites useful for the determination of other human cytochrome P-450 activities. Using the 6 beta-hydroxycortisol:cortisol ratio as a marker of cytochrome P-450 3A4 activity adjusts for the circadian fluctuation in cortisol levels, providing a determination that is consistent over a 24-h period, obviating the need for 24-h urine collections. This high-performance liquid chromatography technique was used for the analysis of 14 different human urines and a range of ratios of 1.6 to 21.7 (mean +/- SE, 6.2 +/- 1.6) was found. These values are consistent with those reported in the literature. This method should be useful in molecular epidemiological studies investigating relationships between the susceptibility from environmental carcinogen exposure and cancer risk.

Aflatoxin B1--DNA adduct formation in rat liver following exposure by aerosol inhalation

There is growing concern that human exposure to respirable grain dust contaminated with aflatoxin B1 (AFB1), a potent hepatocarcinogen, may be a risk factor for a number of human diseases. The objective of this study was to determine if liver DNA adduct formation occurs in rats following either intratracheal injection or nose-only aerosol inhalation exposure to AFB1. Male Fischer 344 rats were exposed by both routes of administration, and in preliminary data using intratracheal instillation, up to 2% of the administered dose became bound to liver DNA. In the nose-only aerosol inhalation experiments, rats were exposed for up to 120 min. Immediately after exposure, four animals were killed at each time point and their livers removed, DNA isolated and purified and analyzed for aflatoxin-DNA adducts by HPLC. A linear dose-response relationship was observed with a correlation coefficient of 0.96 between increasing length of exposure, and the amount of aflatoxin-N7-guanine adducts formed per mg DNA, the mean values and standard errors were 4.2 +/- 0.18, 15.3 +/- 4.3, 21.6 +/- 2.8 and 56.8 +/- 4.6 pmol aflatoxin-DNA adducts per mg DNA for the 20, 40, 60 and 120 min exposures respectively. The amounts of aflatoxin-DNA adducts formed were statistically significantly different (P less than 0.01) among the treated groups. These results indicate that aerosol inhalation is an effective route of exposure to AFB1 in rats that results in genotoxic damage in the liver.

Aflatoxin M1 in human breast milk from The Gambia, west Africa, quantified by combined monoclonal antibody immunoaffinity chromatography and HPLC

Maternal to child exposure of aflatoxin M1 in breast milk is an underevaluated risk factor from dietary exposure to aflatoxin B1. A molecular dosimetry study in The Gambia, West Africa, was initiated to explore the relationships between dietary intake of aflatoxins during a 1 week period and a number of aflatoxin biomarkers including aflatoxin metabolite excretion into breast milk. For the breast milk study, five lactating women were identified and milk samples were collected by hand expression once a day during days 3-7 for three women and during days 3-6 for the two other women. Aflatoxin M1 (AFM1) in human milk was measured in all five subjects by a preparative monoclonal antibody immunoaffinity column/HPLC method. In three of the five women, aflatoxin G1 was found. Estimates of the percentage of aflatoxin in the diet excreted as AFM1 in milk ranged from 0.09 to 0.43%. Thus, these data indicate that a rapid methodology exists to assess the levels of AFM1 excretion in human milk and to use this approach as a biomarker for exposure of children to this carcinogen.

Aflatoxins isolated by immunoaffinity chromatography from foods consumed in the Gambia, West Africa

An aflatoxin-specific, monoclonal antibody-based immunoaffinity chromatography method has been developed for the rapid isolation of aflatoxins from human foods. Aflatoxins were isolated by immunoaffinity chromatography from a variety of cooked foods, including maize, rice, millets, groundnut sauces, and leaf sauces, collected in The Gambia, West Africa. The aflatoxins were measured by direct fluorescence or high-pressure liquid chromatography. The highest levels were found for groundnut sauces, mean 162 ppb (range 18 to 943 ppb) for 18 positive samples, but aflatoxins were found in other foods; e.g., maize, mean 9.7 ppb (range 2 to 35 ppb) for nine positive samples. The food analysis results were used with records of the amounts of cooked food to estimate a mean daily intake for an individual of the order of 3.5 micrograms of aflatoxins per day. This approach for exposure assessment is considered in relation to other biomarkers of aflatoxin exposure using biological fluids.