2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine


PhIP (2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) is one of the most abundant heterocyclic amines (HCAs) in cooked meat. PhIP is formed at high temperatures from the reaction between creatine or creatinine (found in muscle meats), amino acids, and sugar. PhIP formation increases with the temperature and duration of cooking and also depends on the method of cooking and the variety of meat being cooked. The U.S. Department of Health and Human Services National Toxicology Program has declared PhIP as "reasonably anticipated to be a human carcinogen".[1] International Agency for Research on Cancer (IARC), part of World Health Organization, has classified PhIP as IARC Group 2B carcinogen (i.e., possibly carcinogenic to humans).[2] There is sufficient evidence in experimental animals, as well as in vitro models, for the carcinogenicity of PhIP.[3]

2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine
Names
IUPAC name
2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine
Other names
PhIP
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.159.539
UNII
Properties
C13H12N4
Molar mass 224.267 g·mol−1
Appearance Off-white solid
Density 1.3 gcm−3
Melting point 300 °C (572 °F; 573 K)
Boiling point 468.9 °C (876.0 °F; 742.0 K)
407.1 mg/L
Hazards
Main hazards T
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references
This article is about heterocyclic amines. For pleckstrin homology domain-interacting protein, see Pleckstrin homology domain.

Sources of PhIP

PhIP has been found in cooked beef, pork, chicken, and fish products. Exposure to PhIP depends on the eating habits of the individual and can vary up to 5000-fold. Exposure is also related to the type of meat, doneness, cooking method, and quantity consumed. Individual exposures can differ due to various anti-carcinogens in the diet. Different cooking methods for meat (broiling, grilling, frying, roasting, pan drippings) all contribute to formation of PhIP.

PhIP and cooking methods

Studies examining the amount of PhIP in cooked meats have shown that high levels of exposure are possible. Doneness levels of meat (rare, medium, well-done, and very well-done) are factors in the development of PhIP. Methods to reduce formation of PhIP in meats include decreasing the temperature at which the meat is cooked, decreasing the length of cooking time, pre-heating meat in the microwave oven (which reduces creatine), and marinating the meat.[4]

PhIP values for cooked meat[5]
Meat typeCooking variationPhIP ng/g ± SD
Beef (1.5 cm thick)Fried - medium rare (51 °C)0.29 ± 0.14
Fried - well-done (63 °C)0.73 ± 0.02
Fried - very well-done (74 °C)7.33 ± 0.11
Lamb chopFried - medium (75 °C)0
Fried - well-done (85 °C)2.4
Pork (2 cm thick)Fried - medium (63 °C)0.37 ± 0.06
Fried - well-done (83 °C)7.82 ± 1.13
Mince beef patty (2 cm thick)Fried - medium (51 °C)0
Fried - well-done (58 °C)3.96 ± 0.13
Chicken (2.5 cm, no skin)Fried - lightly browned (63 °C)0.2 ± 0.005
Fried - well-done (79 °C)17.54 ± 0.17
SausageFried - lightly browned (42 °C)0
Fried - well browned (70 °C)0.61 ± 0.06
Bacon, middleFried - lightly cooked0.11 ± 0.002
Fried - well cooked1.93 ± 0.37

Dietary intake of PhIP

Determining dietary intake of PhIP can be obtained by more or one ways. One method used is a Food Frequency Questionaries (FFQ) which surveys a population on their estimated consumption of cooked meats.[6][7][8][9] Another method directly measures the quantity of PhIP in a cooked meat sample.[10] However, because the formation of PhIP in cooked meat items is dependent on temperature, cooking time, and cooking method, variations do occur in the direct measurement method.[11] Direct measurement methods have determined dietary intake levels of PhIP to range from 0.07-4.3 ng/kg per day.

Metabolism

Metabolic activation is required for PhIP to function as a mutagen. Therefore, the cancer risk posed by PhIP depends on the extent at which PhIP is metabolized. After absorption, PhIP is converted to a genotoxic metabolite in the liver by Phase I enzyme N-oxidation by Cytochrome P-450 1A2 (CYP1A2). PhIP can be further metabolized into a more potent metabolite through O-acetylation by hepatic or colonic N-acetyltransferase 1 (NAT1) and N-acetyltransferase 2 (NAT2), or by sulfotransfereases. However, PhIP may also undergo a detoxification pathway through Phase II conjugation reaction via UDP-glucuronosyltransferases (UGTs) to form N-glucuronide conjugates.[3] PhIP's nitrenium ion intermediate is a powerful electrophile that has propensity to form C-8 guanine adducts with the DNA.[12] In addition, some of these metabolic enzymes are inducible and have polymorphic variation. CYP1A2 displays a 40-fold variation in expression among humans and can be induced by smoking, diet, and chronic hepatitis.[13] The expression of UGTs also displays inducibility; however, NATs do not. Individuals can be classified as either rapid or slow N-oxidizers and O-acetylators by assessing CYP1A2 and NAT2 activities.[14] Individuals with the rapid phenotype of either CYP1A2 or NAT2 metabolize PhIP more effectively and are therefore at greater risk of PhIP's carcinogenic metabolite and could be at a higher risk of cancer.

Animal studies and PhIP

Rats were fed PhIP at concentrations of 25, 100, and 200 ppm. The rats gained weight throughout the experiment, but feeding concentration of PhIP remained constant. Rats were fed PhIP ad libitum at concentrations of 12.5 and 50ppm. Rats developed mammary tumors at each concentration of PhIP administered.[15] An in vivo study found mice injected with 5, 10, 12, 18, 20, 24, 28, 32, or 36 mg/kg bw showed a strong correlation between consumption of PhIP and genetic damage.[16]

Risk assessments for PhIP

There is no dose [of PhIP] without effect. Therefore, a margin of exposure (MOE) based on the benchmark lower confidence limit (BMDL) reference has been developed for PhIP in relation to prostate and mammary carcinomas.[17]

Calculated MOE for prostate carcinoma based on dietary expsoure to PhIP from cooked meats
Exposure estimate (mg/kg-bw/day)MOE for model average BMDL 10
Average (50%) (0.000006)100,000
High Level (0.00002)40,000
Calculated MOE for mammary carcinoma based on dietary expsoure to PhIP from cooked meats
Exposure estimate (mg/kg-bw/day)MOE for model average BMDL 10
Average (50%) (0.000006)80,000
High Level (0.00002)20,000

Associated cancers

Numerous in vivo and in vitro studies have demonstrated that PhIP is a potent mutagen and can induce tumors of multiple sites in animal models. PhIP was positive in bacterial (Ames) test and induced chromosomal abnormalities in human and Chinese hamster cells in vitro. PhIP has also formed DNA adducts in vivo in both rats and monkeys.[18] PhIP has been tested for carcinogenicity in both mice and rats by oral administration. Increases in lymphomas were seen in mice and increases in adenocarcinomas of the small and large intestine in males and mammary adenocarcinomas in female were seen in rats.[18] Also, an increasing number of epidemiological studies have evaluated the association of well-done meat intake and HCA exposure with cancer risk in humans. In general, these studies have reported that high intake of well-done and/or high exposure to PhIP may be associated with cancer of the colorectum, breast, prostate, pancreas, lung, stomach, and esophagus.[19]

PhIP has been shown to induce DNA adducts and mutations. These adducts have been found in a wide variety of sensitive tissues and organs such as the colon. However, adducts also formed in sites that did not commonly form tumors, such as the kidneys. In humans receiving a dose of PhIP equivalent to that found in very well-done chicken, DNA and protein adducts were formed in the colon and blood. However, the adducts were unstable and declined over a 24-hour period.[20]

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References
  1. U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program. (2011). Report on Carcinogens, 12th ed., p. 222.
  2. a b International Agency for Research on Cancer (IARC) (1997) PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine)(Group 2B). Summaries & Evaluations. http://www.inchem.org/documents/iarc/vol56/08-phip.html
  3. a b Cross, A., & Sinha, R. (2004) Meat-Related Mutagens/Carcinogens in the Etiology of Colorectal Cancer. Environmental and Molecular Mutagenesis. 44:45-55.
  4. Felton, JS., et al. (1994) Effect of microwave pretreatment on heterocyclic aromatic amine mutagens/carcinogens in fried beef patties. Food Chem Toxicol. 158:59-68.
  5. Norrish, A.E., Ferguson, L.R., Knize, M.G., Felton, J.S., Sharpe, S.J., Jackson, R.T., 1999. Heterocyclic amine content of cooked meat and risk of prostate cancer. J. Natl. Cancer Inst. 91, 2038–2044.
  6. Sinha, R., et al. (1995) High concentrations of the carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) occur in chicken but are dependent on the cooking method. Cancer Res. 55:4516-4519.
  7. Sinha, R., et al. (1998) Heterocyclic amine content in beef cooked by different methods to varying degrees of doneness and gravy made from meat drippings. Food Chem Toxicol. 36:279-287.
  8. Sinha, R., et al. (1998) Heterocyclic amine content of pork products cooked by different methods and to varying degrees of doneness. Food Chem Toxicol. 36:289-297.
  9. Knize, MG., et al. (1995) Heterocyclic amine content in fast-food meat products. Food Chem Toxicol 33:545-551.
  10. Keating, G., et al. (1999) Factors determining dietary intakes of heterocclic amines in cooked foods. Mutation Research 443:149-156.
  11. Sinha, R., et al. (2001). Dietary Intake of Heterocyclic Amines, meat-derived Mutagenic Activity, and Risk of Colorectal Adenomas. Cancer Epidemiol Biomarkers Prev. 10:559-562.
  12. Felton, J., et al. (1997). Health Risks of Heterocyclic Amines. Mutation Research. 376: 37-41.
  13. Schweikl, H., et al (1993) Expression of CYP1A1 and CYP1A2 genes in human liver. Pharmacogenetics. 3:239 -249.
  14. Roberts-Thompson, I., et al. (1996)Diet, acetylator phenotype, and risk of colorectal neoplasia. Lancet. 347:1372-1374.
  15. Japanese Journal of Cancer Research (1996). "Dose-dependent Induction of Mammary Carcinomas in Female Sprague-Dawley Rats with 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine". Japanese Journal of Cancer Research. 87 (11): 1116–1120. doi:10.1111/j.1349-7006.1996.tb03120.x. PMC 5921009.
  16. Durling, L., et al. (2005) A comparison of genotoxicity between three common heterocyclic amines and acrylamide. Mutat. Res. 580:103-110.
  17. Benford, D., et al (2009) Application of the Margin of Exposure (MOE) approach to substances in food that are genotoxic and carcinogenic*. Food and Chemical Toxicology. 48 S2-S24.
  18. a b Carthew, P., DiNovi, M., & Setzer, W. (2010).Application of the Margin of Exposure (MOE) approach to substances in food that are genotoxic and carcinogenic Example: CAS No: 105650-23-5 PhIP (2-amino-methyl-6-penylimidazo[4,5-b]pyridine). Food and Chemical Toxicology. 48: S98-S105.
  19. Zheng, W. & Lee, S. (2009). Well-done meat intake, heterocyclic amine exposure, and cancer risk. Nutr Cancer. 61(4):437-446.
  20. Dingley, K., et al. (1999). DNA and protein adduct formation in the colon and blood of humans after exposure to a dietary-relevant dose of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. Cancer Epidemiol Biomarkers Prev. 8:507-512.
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