1. Emerging Evidence for the Role of Polymorphic Drug Metabolizing Enzymes in Smoking Behavior and Treatment
Caryn Lerman & Rachel Tyndale University of Pennsylvania & University of Toronto

2. Polymorphic Drug Metabolizing Enzymes CYP2A6 and CYP2B6
Genetic polymorphisms
Ethnic variation
Association with nicotine metabolism
Association with smoking behavior
Therapeutic approaches
Traditional NRTs
Novel treatments

3. Nicotine removal (i.e. metabolism)
Nicotine Dependent Individuals Adjust Smoking Behavior to Maintain Nicotine Levels
Nicotine intake
(i.e. smoking)
Nicotine removal (i.e. metabolism)
 80%
NICOTINE COTININE
CYP2A6

4. Chromosome 19 Gene cluster CYP2A and CYP2B 350bp Centromere Telomere
94% homologous

5. activity

6. Frequencies of CYP2A6 variant alleles vary among ethnic groups
WHO statistics
African
Americans
Caucasians
Canadian Natives
Chinese
Japanese

7. Genetic Variation in CYP2A6 alters nicotine and cotinine plasma levels
Nicotine 4 mg base, oral Japanese subjects (Xu et al., 2001)
Nicotine
inactive
Cotinine
*4/*4
reduced
*1/*1
plasma (ng/ml)
*1/*4
*1/*4
*1/*1
*4/*4
active
Time (min)
Time (min)
People with 1 or 2 inactive (*4) alleles have significantly
higher NIC (2.3x, 2.8x) and lower COT (7x, 12x)

8. CYP2A6 slow metabolizers are at lower risk for becoming tobacco dependent
Caucasians with an inactive alleles (*2, *4, *10) or low activity alleles (*7) are “slow” nicotine metabolizers
Allelic variants less frequent in smokers vs non-smokers:
Odds Ratio = 0.46
( 95% CI: )
indicates slow nicotine inactivators are less likely to become smokers
Caucasians
OR 0.46 ( )
P = 0.034
Slow metabolizer
frequency (%)
Non-smokers Smokers
TND (N=334) TD (N=365)
Tyndale et al, 2001

9. CYP2A6 Genotype Alters Smoking (n=296 Caucasians)
Carbon Monoxide Levels (ppm)*
Plasma Cotinine Levels (ng/ml)
P<0.05
P<0.05
*1/dup
(n=5)
*1/*1
(n=277)
*1/null
(n=14)
*1/dup
(n=5)
*1/*1
(n=277)
*1/null
(n=14)
Rao et al., 2000
*dose and timing not controlled

10. Summary of 2A6 Epidemiological Data
CYP2A6 ( activity variant)
Decreased nicotine metabolism
Decreased risk for tobacco dependence
Decreased smoking rate and exposure
Increased success quitting (Gu et al., 2000)

11. Human Brain CYP2B6 (and rat CYP2B1)
Alters Local Drug and Metabolite concentrations
Inactivate Drugs: Nicotine (central metabolic tolerance?)
Activate Drugs: Bupropion (greater efficacy?)
Endogenous Substrates: metabolizes Testosterone
Mutagenicity and Genotoxicity
Activate tobacco-smoke procarcinogens (i.e. NNK)
Inactivate: NIC, MDMA MA
Activate: Bupropion, phenytoin, cyclophosphomide
Others: propofol, ketamine, selegiline (deprenyl), aromatic hydrocarbons, pesticides,
Cannibinoids induce and inhibit

12. Nicotine Induces Increase in Brain Levels of CYP2B6 Enzyme
Brain Stem Protein
*** 2 4 6
Relative Density Units **
Brain Stem Protein **
Saline
mg/kg Nicotine
S Miksys et al., Biochem Pharmacol 59(12): , 2000.

13. CYP2B6 is found at higher levels in specific brain regions of smokers, compared with nonsmokers
Non-Smokers (n=10) Smokers (N=14)
Density Units 4 2
FC TC CG OC HC EC CD PT NA GP SN CV CH
Nonsmokers Smokers

14. Interactions between environment and genotype on Hippocampal CYP2B6
P=0.03 x3.9
P=0.07 x2.5 2
CYP2B6 Denisty
P=0.07x1.7 1
Miksys, Lerman, Shields, Mash, Tyndale, 2003
Wt Wt Mut Mut
NS SMK NS SMK

15. CYP2B6 and Smoking Cessation
Lerman, Shields et al. Pharmacogenetics, 2002
Placebo
Male Female
Bupropion
Male Female
n= n= n= n=123
P<.05 for sex x geno x trt

16. QUIT
P<.05

17. Summary: CYP2B6 CYP2B6( activity variant) Lower brain 2B6 levels
Decreased induction of enzyme by smoking
(Slower nicotine clearance &increased tolerance)?
Increased cigarette cravings
Decreased success quitting
Therapeutic application?

18. Therapeutic Implications

19. Mean of “desire to smoke”
CYP2A6 inhibitors & oral nicotine pills (4 mg) increase bioavailability & decrease desire to smoke in deprived smokers during 90 minute ad-lib period
Mean plasma nicotine
Mean of “desire to smoke” 8
p =
p = 7 -5 6
-10 5
-15 4
Mean (ng/ml), baseline adjusted
Mean Score, baseline adjusted
-20 3
-25 2
-30 1
-35
p = 0.007
p = 0.02
-40
Placebo
Methox-
salen
Tranyl-
cypromine
Placebo
Methox-
salen
Tranyl-
cypromine
Sellers & Tyndale, 2000

20. Latency Between Cigarettes
CYP2A6 inhibitors & oral nicotine pills (4 mg) leads to reduction in smoking
Latency Between Cigarettes
2A6 In
& Nic
& Plac
Plac 10 20 30
p = 0.01 40
Mean (±SEM) (min)
Number of cigarettes
smoked decreased
Total cigarette
puffs decreased
Time between
cigarettes increased
Sellers & Tyndale, 2000

21. Inhibition of CYP2A6 Increases Nicotine from Nicorette (4mg) n=11. 1/
Inhibition of CYP2A6 Increases Nicotine from Nicorette (4mg) n=11 *1/*1 genotype
52% Increase in Nicotine Concentration with Methoxsalen 35 * 30 * 25 * * * 20
Day 3 Nicotine (ng/mL) 15 10 5
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
Time
Methoxsalen
Placebo

22. Therapeutic Implications
Systemic Inhibition of 2A6*
Increase nicotine from tobacco & decrease smoking
Increase nicotine from NRTs & improve efficacy
Induction of Brain 2B6?
*Sellers & Tyndale, 2000

23. Acknowledgements!
University of Toronto: Rachel Tyndale, Sharon Miksys, Ewa Hoffman, Chun Xu, Yushi Rao, Bo Xu, Edward Sellers
University of Pennsylvania : Janet Audrain, Paul Wileyto, Angela Pinto, Vyga Kaufmann, Sue Kucharski, Mike Burdick, Freda Patterson
Georgetown University: Peter Shields
Brown University: Ray Niaura
UCSF: Neal Benowitz
Tyndale Lab:
Canadian Research Chair in Pharmacogenetics
NIDA: DA06889
Centre for Addiction and Mental Health
CIHR: MT-14173; MT-14719, training grant and doctoral awards
NCIC: , Ontario Mental Health Foundation, Nicogen Res.