1. CYP2B6 and Drug Interactions FDA Advisory Committee for Pharmaceutical Sciences Clinical Pharmacology Subcommittee November 18th, 2003.
Zeruesenay Desta PhD
David A Flockhart MD, PhD
Indiana University School of Medicine

2. Outline CYP2B6 expression CYP2B6 substrates CYP2B6 inhibitors
In vitro
In vivo?
CYP2B6 inhibitors
CYP inducers
Relevant in vivo data

3. CYP2B6 expression: Late 1990s
Low level of protein expression
Minor component of the total hepatic CYPs (<1% of total P450 in the liver)
Thought to play a relatively minor role in human drug metabolism

4. CYP2B6 Expression: Initial Studies
Detection (%)
Pmol/mg protein (range)
References
13/13 (100%)**
Yamano et al 1989
12/12 (100%)*
Forrester et al., 1992
12/14 (86%), Thai
Kirby et al 1993
12/50 (24%)
1.41.5 (0-7)
Mimura et al 1993
9/30 (30%) (Japanese)
0.25 (0-2)
Shimada et al 1994
26/30 (87%), Caucasian
1.0 (0-2.5)
2/10 (20%)
Baker et al 1995
?/40, Japanese
0.140.62
Guengerich et al 1995
?/32, Caucasian
1.41.8
?/16
3 (0-17)
Imaoka et al 1996
8/30 (27%)
Boobis et al 1996
High level in Thai
Kim et al 1997
7/10 (70%)
19.3623.9)
Tateishi et al 1997
2.655.94
6/30 (20%)
Edwards et al 1998

5. CYP2B6 Expression: Recent Studies
Detection (%)
Pmol/mg protein (range)
Variability
References
17/17 (100)
1521 (0.5-70)
247-fold
Code et al. 1997
24/26 (92)
10.99.5 (0-28)
Yang et al., 1998
19/19 (100)
9.7717.6 ( )
107-fold
Ekins et al 1998
28/28 (100)
24.9 18.2 (2-82)
41-fold
Stresser et al, 1999
43/48 (90)
1.7 (0.4-8, n=43 livers) 20
Gervot et al 1999
15/16 (94)
100-fold (n=15)
Faucette et al 2000
12/12 (100)
44.6 ( )
99-fold
Hesse et al 2000
136/136 (100)
16.916.6 ( )
191-fold
Lang et al. 2001

6. CYP2B6 Expression: 2003
New mono- and polyclonal antibodies of higher sensitivity and specificity for CYP2B6 protein immuno-quantification:
Greater frequency of detection (almost in all human livers tested)
Higher protein quantities:
Average: ~6% of the total liver CYP450 content
Absolute maximum amounts: up to 25 to 43.5%

7. The CYP2B6 gene CYP2B subfamily:
CYP2B6 gene (functional)
CYP2B7 gene (nonfunctional)
CYP2B7-like pseudogene)
Mapped to chromosome 19 [19q12 and 19q13.2]
Contains 9 exons - encodes a protein with 491 amino acids

8. CYP2B6: substrate drugs (in vitro)
HIV medications:
Nevirapine
Efavirenz
DPC 963
Anticancer drugs:
Cyclophosphamide
Ifosfamide
Tamoxifen
ThioTEPA
CNS acting drugs:
S-mephobarbital
Propofol
Bupropion
Ketamine
Selegiline
Methadone
S-mephenytoin
Other drugs:
RP 73401
Artemisinin
Benzphetamine
Cinnarizine
Diazepam
Clopidogrel
Lidocaine N-deethylation
Ticlopidine
Dextromethorphan
R-mianserin
Benzyloxyresorufin
Antipyrine
4-Chloromethyl-7-ethoxycoumarin
3-Cyano-7-ethoxycoumarin
7-ethoxycoumarin
7-Ethoxy-4-trifluoromethylcoumarin
Imipramine
methoxyflurane
Sertraline

9. CYP2B6: Xenobiotics CYP2B6: endogenous substances Testosterone Estrone
Styrene
6-aminochrysene
Dibenzo[a,h]anthracene
Aflatoxin B
Nicotine
Methylenedioxymethamphetamine (ecstasy)
2-dibromoethane
benzo[a]pyrene
Phenanthrene
Methoxychlor
1,3-butadiene
2-chloro-1,1-difluoroethene
azinphos-methyl (AZIN)
chlorpyrifos (CPF)
diazinon (DIA)
parathion (PAR),
CYP2B6: endogenous substances
Testosterone
Estrone
17beta-estadiol

10. Human metabolism of efavirenz (Ward et al., J Pharmacol Exp Ther 2003)

11. CYP2B6 catalyses the metabolism of efavirienz (1µM)

12. Effect of rifampin on efavirenz in healthy volunteers

13. CYP2B6 Catalyses metabolism of an efavirenz analog:
DPC 963
Drug Metab Dispos Jan;31(1):

14. CYP2B6 is a low affinity catalyst of S-mephenytoin metabolism to Nirvanol
Km = 564µM
Heyn et al Drug Metab Dispos 1996;24:

15. N-Demethylation of S-mephenytoin (200µM) by CYP2C9 and CYP2B6
Ko, Desta and Flockhart. Drug Metab Dispos 1998;26(8):

16. Human Metabolism of Mephenytoin

17. R-mephenytoin as CYP2B6 substrate probe?

18. Inhibitors of CYP2B6 Inhibitors:
Antidepressants (e.g. paroxetine and sertraline)
Antiretrovirals (e.g. nelfinavir and ritonavir)
Ticlopidine and clopidogrel
ThioTEPA

19. ThioTEPA is a specific cytochrome P450 inhibitor in vitro
IC50 5µM

20. Inhibition of CYP2B6 by thioTEPA
Ki=4.8 ± 0.3 µM (HLMs)
Ki=6.2 ± 0.7 µM (CYP2B6)
ThioTEPA therapeutic concentration: µM

21. Cyclophosphamide activation

22. Effect of thioTEPA on the PK of CPA
(Huitema et al. Cancer Chemother Pharmacol 2000;46: )

23. HMG-CoA reductase inhibitors
Inducers of CYP2B6
Rifampin
Hyperforin
Phenobarbital
Ritonavir
Phenytoin
Carbamazepine
HMG-CoA reductase inhibitors
Nevirapine
Efavirenz
Clotrimazole
Artemisinin

24. Conclusions
CYP2B6 is a significant contributor to hepatic CYP expression
The number of substrate drugs for CYP2B6 is growing rapidly
Efavirenz and buproprion are specific in vitro probes for CYP2B6 activity
ThioTEPA is a specific in vitro inhibitor of CYP2B6
No valuable, specific inhibitors of CYP2B6 in vivo have been demonstrated to date
Efavirenz is a potentially valuable in vivo probe for CYP2B6 activity

25. Effect of rifampin on efavirenz in healthy volunteers