Toxicology of Benzo[a]pyreneCigarette smoke

Olaniyi Olayinka

Saint Louis University

Overview

• Introduction

• Constituents of Cigarette smoke

• Nicotine

• Tar

• Toxicology of Benzopyrene

Introduction

• Cigarette is a highly sophisticated nicotine delivery system primarily made from processed leaves of the tobacco plant (Nicotiana tabacum)

• Tobacco is commonly smoked.

• Can also be chewed, sniffed.

http://www.ikisan.com/links/ap_tobaccoClimate%20And%20Soils.shtml

https://docs.google.com/viewer?url=http://www.cdc.gov/nchs/ppt/hp2010/focus_areas/fa27_charts.ppt&pli=1

Health effects

• Causal effects of smoking largely based on observational studies

• American Cancer Society- 1M, 9yrs, 80K deaths

• British Medical Research Council- 34K, 40yrs, 20K deaths.

Corbin Grady Studio/Science Photo Library

Constituents of Cigarette smoke

• Mainstream – drawn through the tobacco column, exits through the mouthpiece during puffing (0.1 to 1𝜇m)

• Sidestream – emitted from the smouldering tobacco between puffs (0.01 to 0.1𝜇m)

• Products of incomplete combustion

• Particulate phase vs. Gas Phase

Constituents of Cigarette

• Nicotine

• Carbon-monoxide

• Tar, over 4000 chemicals- WHO 18 mainstream smoke constituents, 7 “Most hazardous.” (FCTC)

Nicotine,

• Natural product of tobacco, t1/2 1-2hours

• Addiction, tolerance, dependence,

• Vd > total BW (mean 88± 17L)

• Crosses BBB easily

• Effects of Dopamine, NE, E release and metabolism

• Liver- CY P450- Cotinine, Nicotine N-Oxide

http://lib.bioinfo.pl/meid:8527

Tar

• describes the particulate matter inhaled when the smoker draws on a lighted cigarette.

• Each particle is composed of a large variety of organic and inorganic chemicals.

• Condensate form is a sticky brown substance.

• Carcinogens: Polycyclic aromatic hydrocarbons, tobacco-specific nitrosamines, metals

Carcinogens

Responsible for Addiction

Poly-aromatic Hydrocarbon

• Polycyclic organic matter

• Epidemiologic studies have reported increase in lung cancer in humans exposed.

• Animal studies have reported Respiratory tract tumors from inhalation exposure to benzo[a]pyrene and stomach tumors, leukemia, and lung tumors from oral exposure to benzo[a]pyrene.

Benzo[a]pyrene (B[a]P)

• Physical Properties

• 5-ring PAH = Benzene ring fused to Pyrenemolecule with chemical formula C20H12, MW= 252.3 g/mol

• Melting point= 179 °C , BPt.= 495 °C

• Vapor pressure of 5.6 × 10-9 mm Hg at 25 °C.

• log octanol/water partition coefficient (log Kow) of 6.06, H2O solubiity= 0.11mg/L

Risk assessment

• Hazard Identification- IARC Group 1 carcinogen

• Cancer and Non-Cancer effect

• Epidemiologic studies- Humans, Animals

• Reproductive/Developmental

• Mathematical models: CalEPA- inhalation unit risk estimate of 1.1 x 10-3 (µg/m3)-1

http://www.epa.gov/ttnatw01/hlthef/polycycl.html

• EPA has not established a Reference Concentration or a Reference Dose for benzo[a]pyrene

• Low tar cigarettes- 10ng B[a[P/cigarette on average

B[a]P

• Exposure: Inhalational (Nose, Mouth)

• Exposure Sites: Lungs (size of particles, molecules), Nose-Alveoli

TOXICOKINETICS

Absorption

• Highly lipid-soluble, Lung, Gut and Skin

• Bronchial clearance, ciliated mucosa, penetration of cells where metabolism takes place.

• 85% initial dose cleared within 30mins.

Distribution

• Distributes readily in all body tissues

• Detectable levels in mins-hours highest in Liver

• Rapid metabolism occur preventing accumulation, even in fat cells

Metabolism

• Toxicokinetics

• Absorption by Lung epithelial cells

• Cellular transformation: Bioactivation

• Peroxidase Cycle: CY P450 Peroxidase, Air, CY P450 Reductase, NADH- CY b5 Reductase

• Yield: Radical Cation, Quinones, Peroxide (O22-

), Superoxide anion (O2·-) and Hydroxyl radical

(OH·)]

http://www.ncbi.nlm.nih.gov.ezp.slu.edu/pmc/articles/PMC2423818/

• Mono-oxygenation: NADPH- dependent CY P450 isoforms (1A1 and 1B1) to yield areneoxides.

• Rearrange 3,7, or 9-OH-B[a]P or

• Hydrated by Epoxide Hydrolase to yield B[a]P dihyrodiols

• Further mono-oxygenation to anti-B[a]P-7,8-diol-9,10 epoxide (anti-B[a]PDE)- Rat carcinogen

http://www.ncbi.nlm.nih.gov.ezp.slu.edu/pmc/articles/PMC2423818/

• In Humans – B[a]P-dihydrodiols can also be oxidized by aldo-keto reductase (AKR1A1, 1c1-1C4)

• Yields Ketol→ Cathecol (Unstable)

• Undergoes autooxidation (1e-) in air → Semiquinone anion radical → Michealacceptor, B[a]P-7,8-dione and ROS

• ↓B[a]P-7,8-dione → Cathecol →vicious circle

http://www.ncbi.nlm.nih.gov.ezp.slu.edu/pmc/articles/PMC2423818/

http://www.ncbi.nlm.nih.gov.ezp.slu.edu/pmc/articles/PMC2423818/

Metabolic pathways of B[a]P in humans

Excretion

• Hepatobiliary excretion and elimination in faeces irrespective of route of entry

• Minor quantities excreted in Urine.

TOXICODYNAMICS

• Inhaled dose depends on smoking behaviour, cig. type.

• Effect mediated by metabolites of B[a]P

• Local vs. Systemic

• Lipid Peroxidation, DNA Mutation

• Initiation and Promotion.

• Reversibility? Need > single mutation.

Carcinogenesis

• B[a]P-radical-cations- depurinating adducts-covalent binding to nucleic acid base (Guanine)

• B[a]P-1,6- and B[a]P-3,6-dione- activate EGF

• anti-B[a]PDE- stable bulky DNA adducts, activates ras proto-oncogene.

• Reacts with p53 tumor suppressor gene to form adducts in specific codons – Most mutated regions in Lung Cancer Patients.

Conclusion

• Benzo[a]pyrene, a Group 1 carcinogen, is a well established constituent of mainstream/sidestream cigarette smoke

• Animal studies have confirmed the carcinogenic effects of B[a]P especially in Respiratory system

• Convincing large human epidemiological studies in USA, Britain, Japan, Germany etc.