In vivo human metabolism of [2-14C]2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)

Human in Vivo Pharmacokinetics of [(14)C]Dibenzo[def,p]chrysene by Accelerator Mass Spectrometry Following Oral Microdosing

Dibenzo(def,p)chrysene (DBC), (also known as dibenzo[a,l]pyrene), is a high molecular weight polycyclic aromatic hydrocarbon (PAH) found in the environment, including food, produced by the incomplete combustion of hydrocarbons. DBC, classified by IARC as a 2A probable human carcinogen, has a relative potency factor (RPF) in animal cancer models 30-fold higher than benzo[a]pyrene. No data are available describing the disposition of high molecular weight (>4 rings) PAHs in humans to compare to animal studies. Pharmacokinetics of DBC was determined in 3 female and 6 male human volunteers following oral microdosing (29 ng, 5 nCi) of [(14)C]-DBC. This study was made possible with highly sensitive accelerator mass spectrometry (AMS), capable of detecting [(14)C]-DBC equivalents in plasma and urine following a dose considered of de minimus risk to human health. Plasma and urine were collected over 72 h. The plasma Cmax was 68.8 ± 44.3 fg·mL(-1) with a Tmax of 2.25 ± 1.04 h. Elimination occurred in two distinct phases: a rapid (α)-phase, with a T1/2 of 5.8 ± 3.4 h and an apparent elimination rate constant (Kel) of 0.17 ± 0.12 fg·h(-1), followed by a slower (β)-phase, with a T1/2 of 41.3 ± 29.8 h and an apparent Kel of 0.03 ± 0.02 fg·h(-1). In spite of the high degree of hydrophobicity (log Kow of 7.4), DBC was eliminated rapidly in humans, as are most PAHs in animals, compared to other hydrophobic persistent organic pollutants such as, DDT, PCBs and TCDD. Preliminary examination utilizing a new UHPLC-AMS interface, suggests the presence of polar metabolites in plasma as early as 45 min following dosing. This is the first in vivo data set describing pharmacokinetics in humans of a high molecular weight PAH and should be a valuable addition to risk assessment paradigms.

Accelerator mass spectrometry-enabled studies: current status and future prospects

Accelerator mass spectrometry is a detection platform with exceptional sensitivity compared with other bioanalytical platforms. Accelerator mass spectrometry (AMS) is widely used in archeology for radiocarbon dating applications. Early exploration of the biological and pharmaceutical applications of AMS began in the early 1990s. AMS has since demonstrated unique problem-solving ability in nutrition science, toxicology and pharmacology. AMS has also enabled the development of new applications, such as Phase 0 microdosing. Recent development of AMS-enabled applications has transformed this novelty research instrument to a valuable tool within the pharmaceutical industry. Although there is now greater awareness of AMS technology, recognition and appreciation of the range of AMS-enabled applications is still lacking, including study-design strategies. This review aims to provide further insight into the wide range of AMS-enabled applications. Examples of studies conducted over the past two decades will be presented, as well as prospects for the future of AMS.

Salvage of oxidized guanine derivatives in the (2'-deoxy)ribonucleotide pool as source of mutations in DNA

Recent evidence suggests that salvage of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-oxo-7,8-dihydro-guanine (8-oxoGua) can contribute substantially to levels of 8-oxoGua in DNA and RNA. However, it remains to be determined if this mechanism contributes to mutagenesis and disease. This review covers the predominant methods for detecting 8-oxoGua and its derivatives, summarizes some of the relevant recent DNA repair studies and discusses the mechanisms for metabolism of oxidized guanine derivatives in the (2'-deoxy)ribonucleoside and (2'-deoxy)ribonucleotide pools.

Quantitative metabolism using AMS: Choosing a labeled precursor

Biological radioisotope studies suffer from a lack of sensitive measurement techniques and therefore traditionally require large amounts of labeled material to produce a measurable signal. Such quantities of materials are often significantly higher than naturally-occurring levels preventing these studies from replicating physiological conditions. AMS affords the sensitivity necessary to perform biological radioisotope studies with low levels of labeled material that preserve physiological conditions. The choice of labeled material can substantially affect the ease of interpretation and comprehensiveness of these studies. Here, the benefits and limitations of whole-cell labeling with (14)C-glucose and targeted pathway labeling with (14)C-nicotinic acid are discussed and compared.

Recent advances in biomedical applications of accelerator mass spectrometry

The use of radioisotopes has a long history in biomedical science, and the technique of accelerator mass spectrometry (AMS), an extremely sensitive nuclear physics technique for detection of very low-abundant, stable and long-lived isotopes, has now revolutionized high-sensitivity isotope detection in biomedical research, because it allows the direct determination of the amount of isotope in a sample rather than measuring its decay, and thus the quantitative analysis of the fate of the radiolabeled probes under the given conditions. Since AMS was first used in the early 90's for the analysis of biological samples containing enriched ¹⁴C for toxicology and cancer research, the biomedical applications of AMS to date range from in vitro to in vivo studies, including the studies of 1) toxicant and drug metabolism, 2) neuroscience, 3) pharmacokinetics, and 4) nutrition and metabolism of endogenous molecules such as vitamins. In addition, a new drug development concept that relies on the ultrasensitivity of AMS, known as human microdosing, is being used to obtain early human metabolism information of candidate drugs. These various aspects of AMS are reviewed and a perspective on future applications of AMS to biomedical research is provided.

The Urinary Metabolite Profile of the Dietary Carcinogen 2-Amino-1-Methyl-6-Phenylimidazo[4,5-b]Pyridine Is Predictive of Colon DNA Adducts after a Low-Dose Exposure in Humans

Epidemiologic evidence indicates that exposure to heterocyclic amines in the diet is an important risk factor for the development of colon cancer. Well-done cooked meats contain significant levels of heterocyclic amines, which have been shown to cause cancer in laboratory animals. To better understand the mechanisms of heterocyclic amine bioactivation in humans, the most mass abundant heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), was used to assess the relationship between PhIP metabolism and DNA adduct formation. Ten human volunteers where administered a dietary relevant dose of [(14)C]PhIP 48 to 72 hours before surgery to remove colon tumors. Urine was collected for 24 hours after dosing for metabolite analysis, and DNA was extracted from colon tissue and analyzed by accelerator mass spectrometry for DNA adducts. All 10 subjects were phenotyped for cytochrome P4501A2 (CYP1A2), N-acetyltransferase 2, and sulfotransferase 1A1 enzyme activity. Twelve PhIP metabolites were detected in the urine samples. The most abundant metabolite in all volunteers was N-hydroxy-PhIP-N(2)-glucuronide. Metabolite levels varied significantly between the volunteers. Interindividual differences in colon DNA adducts levels were observed between each individual. The data showed that individuals with a rapid CYP1A2 phenotype and high levels of urinary N-hydroxy-PhIP-N(2)-glucuronide had the lowest level of colon PhIP-DNA adducts. This suggests that glucuronidation plays a significant role in detoxifying N-hydroxy-PhIP. The levels of urinary N-hydroxy-PhIP-N(2)-glucuronide were negatively correlated to colon DNA adduct levels. Although it is difficult to make definite conclusions from a small data set, the results from this pilot study have encouraged further investigations using a much larger study group.

SULT1A1 genotype and susceptibility to colorectal cancer

Several procarcinogens that are present in cooked red meat and tobacco smoke are substrates for sulfotransferase 1A1 (SULT1A1). The association between environmental exposures and colorectal cancer risk may be modified by individual differences in the metabolism. Thus, we investigated the effect of a common polymorphism in the SULT1A1 gene associated with decreased enzyme activity on the susceptibility to colorectal cancer in a population-based case-control study. Patients (505) and 604 age- and sex-matched controls provided detailed risk factor information and were genotyped for SULT1A1 G638A using a fluorescence-based melting curve analysis method. Multivariate logistic regression analysis was used to estimate colorectal cancer risk associated with environmental exposures by SULT1A1 genotype. SULT1A1 genotype was not an independent risk factor for colorectal cancer. Risk of colorectal cancer associated with frequent consumption of red meat was significantly elevated among carriers of the SULT1A1*2 allele but not increased among subjects with the SULT1A1*1/*1 genotype (odds ratio (OR) 2.1, 95% confidence interval (CI) 1.1-4.1 and OR 1.0, 95% CI 0.5-2.1, respectively). Colorectal cancer risk associated with 30+ pack-years of active smoking was higher among carriers of the SULT1A1*2 allele (OR 1.7, 95% CI 1.0-3.2) than among individuals with the SULT1A1*1/*1 genotype (OR 1.1, 95% CI 0.6-2.1). Our results do not support a main effect of SULT1A1 genotype with regard to colorectal cancer but suggest that individuals with the low activity SULT1A1*2 allele may be at higher risk following carcinogen exposure than those with the SULT1A1*1/*1 genotype.

UGT1A1 polymorphisms are important determinants of dietary carcinogen detoxification in the liver

PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-f]pyridine), the most abundant heterocyclic amine in diet, is involved in the etiology of cancer. PhIP and its carcinogenic metabolite N-hydroxy-PhIP (N-OH-PhIP) are extensively conjugated by UDP-glucuronosyltransferase (UGTs) with wide variability. This study aimed to determine the genetic influence of UGTs on the hepatic detoxification of this carcinogen. The formation of N-OH-PhIP glucuronides was studied in 48 human liver samples by mass spectrometry. Liver samples were genotyped for common polymorphisms and correlated with UGT protein levels and N-OH-PhIP glucuronidation activities. The formation of four different N-OH-PhIP glucuronide metabolites was observed in all livers. The major metabolite was N-OH-PhIP-N(2)-glucuronide (N(2)G), which is the primary metabolite found in human urine, and showed a high interindividual variability (up to 28-fold). Using an heterologous expression system, the bilirubin-conjugating UGT1A1 enzyme was identified among all known UGTs (n = 16) as the predominant enzyme involved. The significant correlation between UGT1A1 protein content and formation of N(2)G (Rs = 0.87; P < .0001) suggests a critical role for UGT1A1 in the hepatic metabolism of this carcinogen. UGT1A1 expression was strongly determined by the presence of the common promoter polymorphisms, UGT1A1*28 (TATA box polymorphism) (P = .0031), -3156G/A (P = .0006) and -3279G/T (P = .0017), and rates of N(2)G were indeed correlated with these polymorphisms (P < .05), whether analyzed individually or in combination (haplotypes). In conclusion, UGT1A1 polymorphisms modulate the hepatic metabolism of the carcinogenic intermediate of PhIP and may determine the level of its exposure and potentially influence the risk of cancer through dietary exposure to HCAs.

PhIP metabolites in human urine after consumption of well-cooked chicken

We devised an assay to quantify the metabolites of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in human urine following a single exposure to well-cooked meat. Our method uses LC/MS/MS to detect four metabolites and four deuterated internal standard peaks in a single chromatographic run. N2-OH-PhIP-N2-glucuronide was the most abundant urinary metabolite excreted by the 12 individuals who participated in our study. N2-PhIP glucuronide was the second most abundant metabolite for 8 of the 12 volunteers. The stability of PhIP metabolism over time was studied in three of the volunteers who repeated the assay eight times over a 2.5 year-period. PhIP metabolite excretion varied in each subject over time, although the rate of excretion was more constant. Our results suggest that quantifying PhIP metabolites should make future studies of individual susceptibility and dietary interventions possible.

Metabolites of 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP) in human urine after consumption of charbroiled or fried beef

Heterocyclic amines (HAs) are carcinogenic combustion products formed during the cooking of meat at moderate to high temperatures. PhIP is the most common HA formed in fried, grilled or broiled meat, and is a colon, breast, and prostate carcinogen in rodents. The major metabolites of PhIP detected in human urine are N(2)-OH-PhIP-N(2)-glucuronide, PhIP-N(2)-glucuronide, N(2)-OH-PhIP-N(3)-glucuronide, and 4'-PhIP-sulphate. We have measured the time course of PhIP in untreated and acid- or alkali-hydrolyzed urines from 10 healthy non-smoking subjects ingesting identical amounts of char-broiled beef (containing both HAs and PAHs) for 5 days. The morning after the first day of broiled beef consumption (containing 7.7 micro g PhIP), urinary concentration of PhIP increased 14- to 38-fold above mean prefeed concentration. Following cessation of broiled meat consumption, urinary PhIP declined to near prefeed levels within 48-72 h. The ratio of alkali-labile PhIP metabolites to unmetabolized PhIP varied by 2.7-fold among subjects, ranging from 18:1 to 48:1. In a subsequent study we measured PhIP in acid-hydrolyzed urine from 66 subjects ingesting beef pan-fried at high temperature. A significant correlation (r=0.61, P<0.0001) was observed between the amount of fried meat ingested and concentration of PhIP in urines collected between 0 and 12h after feeding. Other investigators have identified 2-OH-PhIP in acid-hydrolyzed urine from these subjects, and also observed a significant correlation (r=0.52, P<0.0001) with the amount of fried meat ingested. Additional studies have measured PhIP metabolites in subjects consuming their normal (unrestricted) diet. PhIP was detected in acid-hydrolyzed urine from 20 to 50% of these subjects, depending on ethnic group. Taken together, these studies indicate that significant amounts of PhIP are bioavailable from ingestion of fried or char-broiled meats, and that urinary PhIP metabolites reflect recent (12-24h) ingestion. Furthermore, significant interindividual differences in the amounts of urinary PhIP metabolite excreted are observed following ingestion of similar amounts of PhIP. These differences do not correlate with interindividual differences in excretion of urinary pyrene metabolites in the same individuals after ingestion of char-broiled beef, indicating that levels of PhIP and pyrene metabolites in human urine are mediated by compound-specific metabolic factors.

Factors affecting human heterocyclic amine intake and the metabolism of PhIP

We are working to understand possible human health effects from exposure to heterocyclic amines that are formed in meat during cooking. Laboratory-cooked beef, pork, and chicken are capable of producing tens of nanograms of MeIQx, IFP, and PhIP per gram of meat and smaller amounts of other heteroyclic amines. Well-done restaurant-cooked beef, pork, and chicken may contain PhIP and IFP at concentrations as high as tens of nanograms per gram and MeIQx at levels up to 3 ng/g. Although well-done chicken breast prepared in the laboratory may contain large amounts of PhIP, a survey of flame-grilled meat samples cooked in private homes showed PhIP levels in beef steak and chicken breast are not significantly different (P=0.36). The extremely high PhIP levels reported in some studies of grilled chicken are not seen in home-cooked samples.Many studies suggest individuals may have varying susceptibility to carcinogens and that diet may influence metabolism, thus affecting cancer susceptibility. To understand the human metabolism of PhIP, we examined urinary metabolites of PhIP in volunteers following a single well-done meat exposure. Using solid-phase extraction and LC/MS/MS, we quantified four major PhIP metabolites in human urine. In addition to investigating individual variation, we examined the interaction of PhIP with a potentially chemopreventive food. In a preliminary study of the effect of broccoli on PhIP metabolism, we fed chicken to six volunteers before and after eating steamed broccoli daily for 3 days. Preliminary results suggest that broccoli, which contains isothiocyanates shown to induce Phases I and II metabolism in vitro, may affect both the rate of metabolite excretion and the metabolic products of a dietary carcinogen. This newly developed methodology will allow us to assess prevention strategies that reduce the possible risks associated with PhIP exposure.

Effects of dexamethasone on aryl (SULT1A1)- and hydroxysteroid (SULT2A1)-sulfotransferase gene expression in primary cultured human hepatocytes

To determine whether the dexamethasone (DEX)-inducible hepatic sulfotransferase gene expression that has been described in the rat is conserved in humans, the effects of DEX treatment on hydroxysteroid sulfotransferase (SULT2A1) and aryl sulfotransferase (SULT1A1) gene expression were investigated in primary cultured human hepatocytes. Hepatocytes were prepared from nontransplantable human livers by collagenase perfusion of the left hepatic lobe, and cultured in Williams' medium E that was supplemented with 0.25 U/ml insulin. As reported in the rat, DEX treatment produced concentration-dependent increases in SULT2A1 mRNA and protein expression, with maximum increases observed at concentrations of DEX that would be expected to activate the pregnane X receptor (PXR) transcription factor. In contrast to the rat, in which DEX-inducible SULT1A1 expression has been demonstrated, SULT1A1 expression in primary cultured human hepatocytes was not measurably increased by DEX. In transient transfections conducted in primary cultured rat hepatocytes, the PXR ligands DEX and pregnenolone-16 alpha-carbonitrile significantly induced transcription of human and rat SULT2A reporter gene constructs. Cotransfection of either the human or rat SULT2A reporter gene with a PXR dominant negative construct significantly reduced DEX-inducible transcription. These results underscore that while certain features of rat hepatic sulfotransferase gene regulation are conserved in humans, important differences exist across species. The findings also implicate a role for the PXR transcription factor in DEX-inducible rat and human SULT2A gene expression.

Biomarkers of exposure to heterocyclic amines: approaches to improve the exposure assessment

Various methods of exposure assessment, such as questionnaires, sometimes combined with pictures of cooked meat, have been employed in investigations on the relationship between heterocyclic amines (HA) and health effects. However, as the content of heterocyclic amines vary greatly with cooking conditions, it is difficult to obtain an accurate estimate of the exposure. To improve the exposure assessment, the use of biomarkers has been investigated. The metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is well characterised. In humans, the major part of the dose is excreted in urine within 24-48 h following a meal. A few percent is excreted as parent compounds, whereas the major part is metabolites. Urinary level of parent HA reflects only recent exposure. However, the pattern of excreted metabolites might indicate the capacity to activate or detoxify HAs. The excretion of glucuronide conjugates of N-hydroxy-PhIP and N-hydroxy-MeIQx could be a marker for the N-hydroxylation capacity and the dose of the proximate metabolites. Recently, we proposed 5-OH-PhIP as a marker for the ultimate reactive metabolite of PhIP, since it is formed from this compound as a by-product along with the formation of PhIP-DNA adducts. In a search for biomarkers reflecting exposure over some time, blood protein adducts with a longer lifespan have been investigated, and PhIP adducts of serum albumin and haemoglobin from meat-consuming humans were recently reported. Many compounds, like drugs, nicotine and narcotics, bind to melanin in hair and give information on exposure for longer time periods. In mice, PhIP is irreversibly incorporated in a dose-dependent manner into hair, and in humans exposed to an ordinary diet, it was found to vary from <50 to 5000 pg PhIP/g hair. The incorporation is also dependent on the content of eumelanin. The use of PhIP in hair as a biomarker of exposure is promising, but needs validation, using other methods of exposure assessment.

Increased bioavailability of the food-derived carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in MRP2-deficient rats

MRP2 is an apical transporter expressed in hepatocytes and the epithelial cells of the small intestine and kidney proximal tubule. It extrudes organic anions, conjugated compounds, and some uncharged amphipaths. We studied the transport of an abundant food-derived carcinogen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in vitro, using an MRP2 transfected epithelial cell line (MDCK II) and intestinal explants from Wistar and MRP2-deficient TR(-) rats in Ussing chambers. In the experiments with the transfected cell line, we could demonstrate more than 3-fold higher transport from basolateral to apical than vice versa, whereas the transport in the parent cell line was equal in both directions. These results were confirmed in studies using isolated pieces of small intestine from Wistar and TR(-) rats in the Ussing chamber. Subsequent in vivo experiments demonstrated that after oral administration, absorption of PhIP was 2-fold higher in the TR(-) rat than in the Wistar rat. Consequently, PhIP tissue levels in several organs (liver, kidney, lung, and colon) were 1.7- to 4-fold higher 48 h after oral administration. MRP2 mediated transport of unchanged PhIP probably involves intracellular GSH, because GSH depletion by BSO-treatment in Wistar rats reduced intestinal secretion in the Ussing chamber to the same level as in TR(-) rats. In accordance, BSO treatment increased oral bioavailability in intact Wistar rats. This study shows for the first time that MRP2-mediated extrusion reduces oral bioavailability of a xenobiotic and protects against an abundant food-derived carcinogen.

Lack of UGT1 isoforms in Gunn rats changes metabolic ratio and facilitates excretion of the food-derived carcinogen 2-amino-1-methyl-6-phenylimidazo

UDP-glucuronosyltransferases (UGTs) play an important role in detoxification of endo- and xenobiotics. Deficiencies of these enzymes can have serious consequences, for example, in Crigler-Najjar disease Type I. Recently it was shown that the activated form of the abundant food-derived carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is glucuronidated mainly by UGT1 isoforms. Therefore UGT1 deficiency may have an important impact on metabolism and excretion of PhIP in the body and consequently for the susceptibility toward carcinogenic effects through PhIP. To test this hypothesis we investigated fate and distribution of PhIP in the UGT1-deficient Gunn rat. In 2 h after intravenous injection of PhIP, Gunn rats excreted significantly more PhIP and metabolites than control animals, which were age- and weight-matched Wistar rats. In bile, both glucuronides of N-OH-PhIP were reduced but, in urine, only the N3-glucuronide was reduced while the N2-glucuronide was elevated. The metabolic pathway ratio between 4'-hydroxylation and N-hydroxylation was dramatically changed in the Gunn rat (five times higher in bile and doubled in urine, resulting in a four times higher ratio in total), mostly because of the doubled amount of 4'-PhIP-sulfate in Gunn rats compared to Wistar rats. Tissue levels of PhIP and metabolites were significantly lower in liver and colon of the Gunn rats. We conclude that, in Gunn rats, PhIP is alternatively metabolized through UGT2B enzymes and sulfotransferases, which adds another clue to the potential importance of sulfotransferases in detoxification of PhIP.

Carcinogenicity of the N-hydroxy derivative of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, 2-amino-3, 8-dimethyl-imidazo[4,5-f]quinoxaline and 3, 2'-dimethyl-4-aminobiphenyl in the rat

Heterocyclic amines and carcinogenic aromatic amines are similarly metabolically activated suggesting that they may have similar organ specificity. Three day-old male ACI/seg rats were injected, i.p., twice a week for 10 weeks with 50 micromol/kg of N-hydroxy-3, 2'-dimethyl-4-aminobiphenyl (N-OH-DMABP; Group II), N-OH-2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (N-OH-MeIQx; Group III) or N-OH-2-amino-1-methyl-6-phenylimidaza-[4,5-b]pyridine (N-OH-PhIP; Group IV). Animals in control group (Group I) were similarly injected with solvent alone. The animals were sacrificed at age 68 weeks, and 31, 30, 27 and 31 rats from Groups I, II, III and IV, respectively, were evaluated. Colon carcinomas were found in 0, 15 (P<0.001), 2 and 4 (P<0.06), and bladder transitional cell tumors in zero, two, two and four (P<0.06), in Group I, II, III and IV, respectively. The incidence of atypical hyperplasia of ventral prostate in Groups III and IV, and of anterior prostate and seminal vesicle in all treated groups was also significantly greater (P<0.05). These results suggest that N-OH-PhIP and N-OH-MeIQx may be potential carcinogens for the prostate. Since bladder tumor is rare in ACI rats, N-OH-PhIP may also be a potential carcinogen for the bladder.